HOUGH AND SEDGWICK 
 
ALBERT R. MANN LIBRARY 
 
 New York State Colleges of 
 
 Agriculture and Home Economics 
 
 AT Cornell University 
 
 Gift from the 
 CLIVE M. McCAY LIBRARY 
 
 OF 
 
 Nutrition and Gerontology 
 
Cornell University 
 Library 
 
 The original of tliis book is in 
 tine Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924003130972 
 
THE HUMAN MECHANISM 
 
 ITS PHYSIOLOGY AND HYGIENE 
 
 AND THE SANITATION OF 
 
 ITS SURROUNDINGS 
 
 BY 
 
 THEODORE HOUGH axd WILLIAM T. SEDGWICK 
 
 Profi;^sor of Plnj^UiliKjli in the i'lti- Professor of fiiologij in (he Mussa- 
 
 versidjof Virginia; somitime In- chusel/s Jns/itute of Technology ; 
 
 sfruc/or in Personal llijglenej Author of ^^ Principles of 
 
 Boston Normal ,'ichool Sanitari/ .Science and 
 
 of G>/m7tas(ics Public Health.^' etc. 
 
 The fundamental conception of the liv- 
 ing body as a physical mechanism . . . 
 is the distinctive feature of modern as 
 contrasted with ancient physiology 
 Huxley 
 
 GINN & COMPANY 
 
 BOSTON ■ XEW YORK ■ CHICAGO ■ LONDON 
 
35248^ 
 
 Enteked at Statio.neks' Hall 
 
 OOVVlillillT, 190G, BV 
 
 Theodore Hough and AVilliam T. Sedgwick 
 
 ALL KIGHTS RESERVED 
 914.G 
 
 QTfje ^tbenceum ^retftf 
 
 GINN i COMPANY ■ PRO- 
 PRILTOKS . BOSTON • U.S.A. 
 
PREFACE 
 
 The authors of this work believe that extensive and 
 fundamental changes must be made in the elementary 
 teaching of physiology, hygiene, and sanitation, if these 
 subjects are ever to occupy in the curriculum of education 
 the place which their intrinsic importance requires. This 
 text-book is a contribution toward effecting these changes, 
 and has been prepared both as a demonstration of what the 
 authors believe to be needed and as a practical aid toward 
 securing the end in view. 
 
 The health and efliciency of the human body have rarely, 
 if ever, been more highly esteemed than they are to-day, 
 and yet no subject of similar importance is so gener- 
 ally neglected in the schools, or, when taught, taught less 
 effectively. Several causes have contributed to this curi- 
 ous state of things, but undoubtedly one of the most 
 important is that the teaching has been too largely ana- 
 tomical and too remotely connected with the activities and 
 problems of daily life. 
 
 In the present text-book, anatomy has been reduced to 
 its lowest terms and microscopic anatomy or histology 
 touched upon only so far as seemed absolutely necessary. 
 Space has thus been gained for more physiology and, 
 especially, for more hygiene than is usual, and also for the 
 elements of sanitation, — a new and comparatively easy 
 subject, but one of the very first importance in all whole- 
 some modern living. 
 
 That point of view which regards the human body as a 
 living mechanism is to-daj^ not only the sure foundation 
 
iv PKEFACE 
 
 of physiology, hygiene, and sanitation, but is also surpris- 
 ingly helpful in the solution of many questions concerned 
 with intellectual and moral behavior. This view, therefore, 
 we have not hesitated to expound and emphasize. Avoid- 
 ing that form of physiology which looks chiefly at the 
 organs and overlooks the organism, we have constantly 
 kept in view the body as a whole, in order that physiology 
 may become the interpreter of the common physical phe- 
 nomena of the daily life and find in hygiene and sanitation 
 its natural application to conduct. 
 
 We believe with Matthew Arnold that " conduct is 
 three fourths of life," and that this is no less true of the 
 physical than of the moral and the intellectual life. We 
 therefore make no apology for fixing upon conduct as the 
 keynote of this work, and the right conduct of the i^hysical 
 life as the principal aim and end of all elementary teach- 
 ing of physiology, hygiene, and sanitation. 
 
 In those portions of the book devoted to public hygiene 
 and sanitation the authors have kept in view tlie importance 
 of this subject in all education for good citizenship. Sani- 
 tary science and the public health can be advanced only as 
 they are supported by an intelligent public opinion, which 
 appreciates the nature of the problems involved, the frequent 
 duty of subordinating personal liberty to the public good, 
 and the importance of rendering hearty support to public 
 officials in the discharge of difficult and often delicate tasks. 
 
 It has not seemed wise to include in the text exten- 
 sive directions for laboratory work. The opportunities and 
 facilities for such work vary to such an extent in different 
 schools that the largest discretion must here be left to the 
 teacher. Many demonstrations and experiments described 
 or referred to can, however, easily be performed, when ad- 
 visable, with comparatively little trouble or expense. 
 
PREFACE V 
 
 We are greatly indebted to Professor Werner Spalteholz 
 for his kind permission to copy certain figures from his 
 Hand Atlas of Anatomy} a book which we recommend as 
 a most useful reference work for anatomical study. Our 
 acknowledgments are also due to Professor Schottelius, 
 from whom we have taken many of the figures of bacteria 
 given in Part II. In the preparation of most of the original 
 figures we have had the assistance of Dr. Percy G. Stiles, 
 whose skill as a draftsman, combined with his appreciation, 
 as a physiologist, of our i)omt of view, has greatly facili- 
 tated this part of our work. 
 
 The index has been prejjared with special reference to 
 its use as a glossary of anatomical terms. 
 
 1 Werner Spalteholz, Hand Atlas nf Human Anatomy ; translated by 
 L. F. Barker. G. E. Stechert, New York, 
 
CONTENTS 
 
 PART r. PHYSIOLOGY 
 
 Chapter 
 
 T. The Human Mf.chanism ...... 
 
 II. The Structure (Anatomy) of the IIujian jMech- 
 ANISM ......... 
 
 III. The Finer Structure of Two Tyi'Ical Ohoans, 
 
 (Ilands and jMu.soi.E.'i. The Connective Ti.s- 
 sues. The Lyjipihtic Sv.-iiEji 
 
 IV. The "Work of the Human Mfchanis.-m the 1!e- 
 
 SULT.\NT of the ^^'oRK OF ITS ll|FFEI!FNT Ou- 
 
 GANS AND Cells ....... 
 
 V. Work, Fatigue, and Restoration . . . . 
 
 VI. The Interdei'ENDEnce of Organs and of (ells 
 VII. The Ad.tustmknt or Cooroin.'Vtion of the AN'ouk 
 
 OF (>RGANS and CfII.S 
 VIII. AliMENTAIKJN AND I )ll ; i:ST 1 1 pn; . 
 
 TliP Siip].ly (jf ]\l.-i1liM' and W.ni-r 
 
 Miuliiiic .... 
 
 Dige.sticn ill flu- Moutli. Tlie I't-ctli 
 ]>ig'e,stioii ill (lie Stoinarh 
 Digp.stiiiii and Absovptimi in tlu' Sm 
 
 ill the Large Intestine . 
 
 TX. I'llF ClRCUIAlIOX of THE lliociu 
 
 IMond and Lynijili 
 
 Mcehanirs id' ilie Cireiilatimi of tlie 
 
 Flow of Lymph 
 The Adjustment of the Circulation 
 Everyday Ijife 
 X. Resi'IRAtion .... 
 
 XL Excretion ..... 
 XII. Thermal Phenomena of the Bod^ 
 The Constant Teinperatuie 
 The Regiil.ation of the Body Tempera 
 
 Page 
 3 
 
 41 
 
 
 70 
 
 tu 1 lie Ililljiali 
 
 
 
 tiy 
 
 1. lM17\llle'S 
 
 its 
 
 
 km; 
 
 all liilesline and 
 
 
 
 11.". 
 
 Rln,-,d and "i i\,p 
 
 1 ;vj 
 1 '-'(-■ 
 
 to thip Needs rd 
 
 1 -ih 
 
 
 lift 
 
 
 16U 
 
 
 177 
 
 
 187 
 
 
 187 
 
 ature 
 
 199 
 
vni 
 
 CONTENTS 
 
 Chapter 
 XIIT. Nutrition ,.....•• 
 
 The Sources of the Power and Heat of the Human 
 
 Mechanism ..... 
 Special Effects of the Different Nutrients 
 Flesh, Fat, and Glycogen 
 The Choice of Food and Nutrients 
 XIV. Sense Organs and Sensations 
 XV. The Nervous System 
 Its Anatomical Basis 
 The Physiology of the Nervous System 
 
 Page 
 211 
 
 211 
 217 
 
 227 
 233 
 244 
 266 
 266 
 273 
 
 PAET II. THE HY(;iENE OF THE HUMAN MECH- 
 ANISM AND THE SANITATION OF 
 ITS SUKEOUNDINOS 
 
 XVI. Introductory . . . . . . .291 
 
 Hygiene the Right Use and Proper Care of the 
 Human Mechanism ...... 291 
 
 Health and Disease 293 
 
 The Three Great Factors of Disease . . . 296 
 
 Personal Hygiene 
 
 XVII. Muscular Activity ...... 304 
 
 The Ministry of Muscular Activity to the Body as a 
 
 Whole 304 
 
 General Muscular Exercise ..... 314 
 Muscular Exercises for Special Purposes. Corrective 
 
 Work. The Gymnasium . , , . .321 
 
 XVIII. The Hygiene of the Nervous Sy-stem. Rest 
 
 AND Sleep. ....... 334 
 
 XIX. The Hygiene of Feeding 347 
 
 XX. Food Accessories, Drugs, Alcohol, AND Tobacco 357 
 XXI. The Prevention and Care of Colds and So-me 
 
 Other Inflammations ..... 380 
 
 XXn. The Care of the Eyes and Ears . . . 395 
 
CO^'TENTS 
 
 LX 
 
 Chapter 
 
 XXIII, The Hygifnk of thf Feft 
 
 XXIV. Bathing . . , . 
 XXV. Clothing .... 
 
 Page 
 403 
 413 
 
 418 
 
 Domestic Hygienf and Sanitation 
 
 XXVI. The House.- Its Site, Construction, Furnish- 
 ings, and Care ...... 425 
 
 XXVI I. The Wakming and Lighting of the House . 434 
 
 XXVIII. The Air Supply of the House. Ventilation 442 
 XXIX. The Water Supply, Plumhing, and Drainage 
 
 of the House. Garbage and Rubbish . 451 
 
 PuHLIf HY(iIENK AND SANITATION 
 
 XXX. TuBLic Health. Inee(11(ks a.nu C<jntagioi' 
 
 DisEAKKs. Microbes .... 
 XXXI. Some iMk'Kobic Diseases anh theih Prkven- 
 TKiN. Vaccination and Antitoxic Serums 
 XXXII. Public Supplies of Food, AVater, and (Ias 
 Public Sewerage , . , , , 
 
 XXXIII, The Hy'Giene and Sanitation (.if TK.,iVELi.\o 
 
 Public Conveyances, Public Houses, etc 
 
 XXXIV. Public Protection of the Public Health 
 XXXV. The Health ov Na i io.n.s .... 
 
 INDEX .......... 
 
 4(13 
 
 477 
 
 505 
 
 520 
 529 
 5:)5 
 
THE HUMAN MECHANISM 
 
 Part I 
 PHYSIOLOGY 
 
CHAPTER I 
 THE HUMAN MECHANISM 
 
 1. The Human Body a Living Organism. — The human 
 body, as compared with bodies of water such as lakes and 
 seas, or with lieavenly bodies such as tlie sun, moon, and 
 stars, is a small mass of matter weighing on the average, 
 when fully grown, about 150 lb. and measuring in length 
 about 5 ft. 9 in. It is neither very hot, as is the sun, nor 
 warm in summer and cold in- winter, as are many bodies 
 of water, but in life and health has always almost exactly 
 the same moderate temperature, namely, 98.6° F. or 37.0° C. 
 The human body is not homogeneous, as is the substance 
 of a lake, that is to say, alike in all its parts, but very 
 unlike, the various parts — eyes, ears, legs, heart, brain, 
 muscles, etc. — being known as organs, and the whole body, 
 therefore, as the human organism. 
 
 The most remarkable peculiarit}' of the human bod}', how- 
 ever, is that it is a living organism. A watch has unlike 
 parts — spring, dial, hands, case, etc. — which are essen- 
 tially its organs, and the watch might, therefore, be called 
 an organism ; yet it never is so called. We speak of a 
 well-organized army, navy, government, society, church, or 
 school, but never of a well-organized automobile, type- 
 writer, printing press, or locomotive, — apparently for the 
 reason that in army, navy, or school living things play a 
 principal part, while in mere machinery life is wholly want- 
 ing. The highest compliment we can pay to a machine is 
 to say that it seems " almost alive," but it is not a com- 
 pliment to any human being to describe him as " a mere 
 
 3 
 
4 THE HUMAN MECHANISM 
 
 machine." What the vital property is, what we mean by 
 the terms " life " and " living," no one can exactly tell. 
 About all we know of it is that some of the commonest ele- 
 ments of matter — carbon, hydrogen, oxygen, and nitrogen, 
 with a little sulphur, phosphorus, and a few other elements 
 
 — frequently occur combined as living matter ; and that this 
 living matter has marvelous powers of growth, repair, and 
 reproduction, besides a certain spontaneity, originality, and 
 independence, which lifeless matter never displays. " While 
 there is life there is hope " for any plant or any animal, but 
 this saying does not apply to any lifeless machine, however 
 complex or wonderful. 
 
 2. The Human Body a Living Machine or Mechanism. 
 
 — By a machine we mean an apparatus, either simple or 
 complex, and usually composed of unlike parts, by means 
 of which power received in one form is given out or applied 
 in some other form. This power may be received, for 
 example, in the form of heat, or electricity, or muscular 
 effort, or as the potential energy of fuel ; and it may be 
 given out as heat, or electricity, or light, or sound, or as 
 mechanical work, or in any one of many other ways. One 
 of the simplest of all machines is a stove, an apparatus 
 composed of a few simple parts by means of which the 
 potential energy or power of fuel — -wood, coal, gas, or 
 oil — is liberated and applied as heat, for warming or 
 cooking. A lamp is a still simpler machine in which the 
 potential energy or power of gas or oil is liberated and 
 converted into useful light. A candle is a lamp so simple 
 that it almost ceases to be a machine, and yet the wick is 
 really an apparatus for securing proper combustion of wax 
 or tallow to provide good light. 
 
 Machines of greater complexity are watches or clocks, 
 pieces of apparatus composed of many unlike parts which 
 receive power in comparatively large amounts for a short 
 time during the process of winding, store it as potential 
 
THE HUMAI^ MECHANISM 5 
 
 energy in coiled springs or lifted weights, and liberate it 
 slowly in the mechanical work of moving the hands of the 
 timepiece over a dial. Still more complex is a locomotive 
 or an automobile, machines in which the power of coal, 
 oil, gasoline, or other fuel, or the electricity of a storage 
 batterer, is applied to swift locomotion. But the most won- 
 derful of all machines is the human body, a complicated 
 piece of apparatus in which the power stored in foods, such 
 as starch, sugar, butter, meat, milk, eggs, and fish, is trans- 
 formed into that heat by which the body is warmed, and 
 into that muscular, nervous, digestive, or other work which 
 it performs. 
 
 For delicate and intricate machinery the term " mechan- 
 ism " is often employed, and we may therefore describe 
 the human body either as the " human organism," or the 
 " human machine," or, perhaps best of all, as the human 
 
 MECHANISM. 
 
 The study and the science of the construction (structure) 
 of this mechanism is called its anatojiii/ ; of its ordinary 
 behavior, operation, or working, its pJti/siolo(/i/; of its 
 proper management, protection, and care, its hygiene. This 
 text-book is devoted chiefly to an account of its operation 
 and care, i.e. to its physiology and hygiene : but as any 
 true comprehension of these subjects depends upon some 
 preliminary knowledge of the parts of the mechanism 
 itself, we shall begin by considering briefly the structure 
 or anatomy of the human machine. 
 
CHAPTER II 
 
 THE STRUCTURE (ANATOMY) OF THE HUMAN 
 MECHANISM 
 
 Anatomy is studied partly by dissection, which reveals 
 chiefly those organs which are visible to the naked eye, 
 and partly by mieroscoinc examination, which gives a deeper 
 insight into the detailed arrangement of the cells and tissues 
 of which the organs of the mechanism are composed. The 
 present chapter is devoted to structures or organs shown 
 by dissection, — the gross anatomy of the body, — as dis- 
 tinguished from its tnicroscopie anatomy {histology)} 
 
 1 Further explanation of the structure of the human machine will be 
 given as it may be needed in subsequent chapters. At this point it is of 
 the utmost importance that the student thoroughly master the general 
 relations of the more important organs one to another ; this, however, 
 is not to be done by extensive reading, and still less by memorizing 
 verbal descriptions; the aim should rather be to acquire from, figures and 
 diagrams, or better yet from actual dissection, where that is possible, a cor- 
 rect mental picture of the structures involved. Far more can be learned 
 by constructing drawings or diagrams from memory than by the mere 
 memorizing of text. The drawings may lack finish and may be at first 
 difficult to execute ; but so long as they represent the relations of the 
 organs one to another they accomplish their purpose ; beyond this point 
 the more accurately they are drawn the better. 
 
 Moreover, drawing is a great aid to dissection. It not only fixes in the 
 memory what is seen but it compels close observation ; when one draws an 
 object he is forced to note details and relations of structure which would 
 otherwise escape observation. Nor is the freehand drawing which is 
 required for our purpose as difficult as is often supposed by those who 
 have never seriously used it. Let the student attempt to reproduce an 
 object from his memory of its picture; begin with one which is not too 
 complicated (such as the figure of the peritoneum and niesenU-ry on 
 page 14). Where he does not know how to represent a special structure 
 let him refer to the original from which he may get suggestions ; then 
 
 C 
 
STKUCTURE OF THE MECHAXLSM 7 
 
 The human mechanism is composed of different parts, 
 such as head, neck, trunk, arms, hands, legs, and feet, and 
 each of these in its turn is composed of lesser parts. Arms 
 and hands, for example, are covered by skin, winch may be 
 moved over underlying soft parts ; at the ends of the 
 fingers the place of the skin is taken by nails, while scat- 
 tered over and emerging from its surface are Jiairs. Through 
 the skin may be seen the veins, which may be emptied of 
 the purplish llood tliey contain by pressing one finger on 
 a part of the vein near tlie finger, and pushing another 
 finger along the vein towai'd the wrist ; so long as pressure 
 is maintained by both fingers the vein remains collapsed, 
 but on removing the first finger it fills again with blood. 
 Finally, through the soft parts (ffsJt) may Ijc felt the hard 
 bones. In general these various parts of which the body is 
 composed are known as its organs, and because it jiossesses 
 organs it is called an onjanism (p. 3). 
 
 1. The Skin. — The body is everj-where covered b\' a 
 complex protective and sensitive organ, the skin. Only 
 the eyes and nails seem to l.)e excc[itions ; but as a matter 
 of fact the exposed surface of the eye is covered by a 
 very thin, transparent portion of the skin, and the nails 
 are really modified portions of skin. 
 
 2. Subcutaneous Connective Tissue. — On cutting through 
 the skin we find that it is liound to tiie undeilying flesh 
 (chiefly meat or muscle) by what is known as connective 
 tissue, the structure of which we shall study in the next 
 chapter. jMeanwhile we may notice that it contains blood 
 vessels, that at some places it is more easily stretched than 
 
 close the book and draw from memory ; any completed part of the work 
 may be compiared with the original and possible improvements discovered. 
 Such practice may well precede drawing from an actual dissection and 
 will pave the way to the latter. At all events let the student understand 
 thoroughly thiit in the present chapter the figures, supjilemented if pos- 
 sible by actual dissections, form the main objects of study ; the text is 
 strictly subordinate to the figures. 
 
8 THE HUMAN MECHANISM 
 
 at others, and that when a flap of skin is pulled away 
 from the muscles, this subcutaneous tissue fills with air. 
 It often contains large quantities of fat. 
 
 3. Muscles and Deeper Connective Tissues. — The sub- 
 cutaneous connective tissue sometimes connects or binds 
 the skin directly to bone, as in parts of the head; usually, 
 however, in the neck, trunk, and limbs the underlying 
 tissue is the red flesh, or muscle, familiar to us as " lean of 
 meat." If the skin be removed from the forearm, it at once 
 becomes evident that this mass of meat or flesh is composed 
 of a number of muscles which may be separated from one 
 another more or less completely. In doing this it will 
 be found that the muscles are held together by connective 
 tissue in most respects quite similar to that immediately 
 under the skin. Further dissection will show that one or 
 another form of this tissue is the means of binding other 
 organs together; thus the muscles are joined to the bones 
 by a very dense, compact, and strong form known as tendon; 
 the bones are united by a somewhat similar form known 
 as ligament; and so on. The physical characters of the 
 tissue differ widely, according to its situation and the use 
 subserved ; but one form shades more or less into another, 
 and we have no difficulty in recognizing the general simi- 
 larity which leads us to group them all together in one class. 
 
 4. Muscles attached to Bones. — ^When a muscle is care- 
 fully dissected away from neigliboring muscles and other 
 organs, it is almost always found that it is attached to one 
 and usually to two bones ; this union is frequently made 
 by means of a tendon, as in the case of the large muscle of 
 the calf of the leg, which is attached at one end to the 
 bone of the thigh and at the other to that of the heel. A 
 good example of the direct attachment of muscles to bones 
 is furnished by those muscles which lie between the ribs 
 (see Fig. 141). In either case the shortening of the muscle 
 brings closer together the bones to which it is attached. 
 
STRUCTURE OF THE MECHANISM 
 
 9 
 
 5. Definition of Some Anatomical Terms. — Before pro- 
 ceeding further we must agree upon the exact meaning of 
 certain anatomical terms. We often speak of one part 
 of the body as being " above " or " below," " before " or 
 " behind," another. Such terms, however, are confusing, 
 because their meaning depends upon the position of the 
 body at the time they are used. For example, when one 
 is lying on his back the head is in front of, or before, the 
 trunk ; but when he 
 is standing on his 
 feet it is above the 
 trunk. 
 
 Now the body is 
 certainly divided 
 into right and left 
 halves, which are 
 much alike exter- 
 nally, though this 
 likeness is not so 
 marked in the inter- 
 nal parts. Right and 
 left then have their 
 ordinary meanings, 
 and that without re- 
 gard to the various -^ 
 positions the body 
 may take. 
 
 To indicate that 
 any part is nearer the head than another part, we say that 
 the former is anterior to the latter ; to indicate that the 
 latter is further away from the head, we say it is jyosterior 
 to the former. 
 
 Finally, the region popularly known as the back is called 
 dorsal (Latin, dorsum, back), that opposite the back being 
 called ventral (Latin, venter, belly). Thus the nose is on 
 
 Fig. 1. Cross section of arm 
 
 skin ; B, subcutaneous connective tissue, bind- 
 in^^ tlie slviu to the muscles D, and continuous 
 with the connective tissue wliicli binds together 
 the muscles ; C, blood vessels and nerves 
 
10 
 
 THE HUMAN MECHANISM 
 
 the ventral side of the head ; the toes are at the posterior 
 extremity of the foot. 
 
 6. The Body Cavities. — There is one striking and im- 
 portant structural difference between the trunk and the 
 
 limbs ; the former 
 contains a central 
 body cavity, com- 
 pletely tilled, how- 
 ever, with various 
 organs, while the 
 arms and legs are 
 each composed of 
 a continuous mass 
 of tissues, viz. mus- 
 cle, blood vessels, 
 nerves, bone, etc., 
 all bound together 
 by connective tis- 
 sue. 
 
 The cavity of the 
 trunk, or body cav- 
 ity, is subdivided 
 transversely by the 
 dome-shaped mus- 
 cle known as the 
 diaphragm into two 
 cavities, — an an- 
 terior, known as tlie thoracic or pleural cavity, and a poste- 
 rior, known as the abdominal or peritoneal cavity. Both 
 cavities are lined by a thin, smooth, shiny membrane, that 
 of the thoracic being known as the pleura, and that of the 
 abdominal as ihe peritoneum. 
 
 Filling tlie pleural cavity are found the heart, lunr/s, 
 wsophae/us, windpipe or trachea, and many great blood ves- 
 sels; filling the abdominal cavity, the stomach, the small 
 
 Fk;. 2. The thoracic or pleural, and the abdomi- 
 nal or peritoneal, cavities filled with organs 
 
STRUCTUEE OF THE MECHANISM 
 
 11 
 
 intestine, the large intestine, the liver, the pancreas, the kid- 
 neys, the spleen, and other organs together with numerous 
 large and important arteries and veins. In both cavities 
 the lining membrane {pleura or peritoneum) is folded back 
 over the organs ; that is to say, the organs do not really 
 lie in the cavities, but only fill them as the hand would 
 fill a bladder one wall 
 of which it pushes in 
 against the other. The 
 surfaces of the organs, 
 like the walls of the cav- 
 ity, are consec_[uently 
 smoothly covered and 
 glide over one another 
 with very little friction. 
 The preservation of 
 these ])leural and peri- 
 toneal linings in their 
 normal condition is a 
 matter of great impor- 
 taiice ; wlien inflamed 
 or otherwise injured 
 their surfaces become 
 roughened, and adlie- 
 sions of connective tis- 
 sue often develoji 
 between them which 
 
 fasten the organs together, or to the walls of the cavity, 
 so that surgical interference is sometimes necessary. Pleu- 
 risy is such an inflammation of the pleura, peritonitis of 
 the peritoneum ; and both are very serious conditions. 
 
 7. Attachment of the Organs to the Walls of the Pleu- 
 ral and Peritoneal Cavities. — The pleural cavity is com- 
 pletely divided by a median partition of connective tissue 
 (the mediastinum), within wdiich are found the trachea, the 
 
 Fif!. ?y. Cross seotion of flic thorax unterior 
 to the bifurcation of tlie tracliea 
 
 ^'1. a vertebra of tlie spinal coluinn; 7;, sjiinal 
 cord; ('. tlir pleur;ll (■a\"ity (\\hirli is exa^- 
 i;erate(l for the sake of i'leariiess), the sur- 
 face of the ]un<i- heinir actually in contact 
 with the body wall. Tlie oesciphauois, tra- 
 chea, together with se^-eral large arteries 
 and veins are shown in the mediastinum 
 ventral to the vertebra, and in the order 
 named 
 
12' 
 
 THE HUMAX MECHANISM 
 
 CBSophaffus, the great blood vessels, and — lying within a 
 special cavity of its own — the heart. AiDiDroximately 
 halfway from the anterior to the posterior border of the 
 mediastinum the trachea divides within that memJjrane 
 into two tubes or bronchi, which pass through the mediasti- 
 num outward to the ric/ht and hft lung respectively. The 
 
 Fig. 4. Cross section of thorax posterior to bifurcation of trachea 
 A, bronchus, entering the lung; Z?, the .inrta cut at its origin and again at 
 the descending part of its arch; C, tlie pericardial space; D, the pleural 
 cavity; P. A., the pulmonary artery 
 
 pleural lining of the mediastinum is pushed outward by 
 these tubes, and, as they end in the lungs, forms the pleural 
 covering of the latter (Fig. 5). Consequently the organs 
 of the pleural cavity either lie within the mediastinum 
 (heart, oesophagus, trachea, etc.) or else are covered by 
 extensions of the mediastinal pleura (bronchi and lungs). 
 
 The abdominal cavity is not similarly separated into right 
 and left halves ; but a membrane, the mesentery, passes 
 ventrally from the dorsal wall to the stomach and intes- 
 tine, which are slung in it somewhat as a man lies in a 
 hammock. The line of attachment of this mesentery to the 
 
STEUCTUKE OF THE MECHAlSriSM 
 
 13 
 
 small intestine is much longer than that of its attachment 
 to the body wall ; hence it has the general shape of a ruffle, 
 or flounce, — an arrangement which permits the suspension 
 of the very long intestine (20 to 25 ft.) from the compara- 
 tively short median 
 dorsal body wall 
 (see Fig. 136). The 
 creat arteries and 
 veins lie in tliu 
 mesentery near the 
 dorsal body wall 
 and branches are 
 distributed from 
 them to the intes- 
 tine within this 
 expanding mem- 
 brane (see Fig. 143). 
 The kidneys do 
 not lie movably 
 suspended in the 
 abdominal cavity, 
 as do the intes- 
 tines, but are 
 large organs, one 
 on each side, sit- 
 uated near the 
 spinal column 
 and dorsal to the 
 abdominal cavity 
 from which they are sep 
 arated by the peritoneum 
 
 , muscles, i-ihs, ftc. of the body wall ; B, 
 pleura, lining- the same; (', the pleural 
 space or cavity : i^ the pleural covering 
 of the luug : E, connective tissue of the 
 lung; F, alveoli of the lung; G, dia- 
 phragm; if, trachea; /, right bronchus, 
 branching ; K, the pericardial space in 
 which lies the heart. Note the division 
 of the lung into two lobes 
 
 Arteries and veins are sup- 
 plied to them from the large median artery and the median 
 vein already referred to {aorta and vena cava, Fig. 15), 
 and these renal arteries and veins are likewise outside the 
 abdominal cavity. 
 
14 
 
 THE HUMAN MECHANISM 
 
 The relation of the other organs to the peritoneum is more 
 complicated, notably in the case of the liver; but in all cases 
 
 the organs are in- 
 
 ,Kidneii 
 
 closed, or wrapped, 
 either in a fold of 
 the peritoneum, as 
 is the kidney, or in 
 a fold of the mes- 
 entery, as is the in- 
 testine; and their 
 bloodand nerve sup- 
 plies run to them 
 in similar folds. 
 
 8. The Axial 
 Skeleton. — The 
 bones and carti- 
 
 FlG. ( 
 
 Diagiammatic cioss section of the ab- 
 dominal cavity 
 Showing the relation of the kidneys and great lilood 
 vessels to the peritoneum. Tlie intestine has 
 been removed, the cnt border of the mesentery ][j^™gg q£ -^yhich the 
 being shown ^ 
 
 skeleton is com- 
 posed may be classified into an arial skeleton (of the head, 
 neck, and trunk) and an appendicidm- skeleton (of the arms 
 and legs). The axial skeleton comprises (1) the backbone 
 or vertebral column, (2) the ribs and breastbone, and 
 (3) the skull. 
 
 f 9. The Backbone or Vertebral (Spinal) Column. — This is 
 composed of separate irregular ringlike bones or vertehrw 
 placed one above another, and bound together by bands 
 of strong connective tissue known as litjaments. It is cus- 
 tomary to divide the backbone into the following regions. 
 
 a. Cervical. 7 vertehr?e of the neck. 
 
 h. Thorncic. 12 vertebr.'e of the chest, to which rihs are attached. 
 
 c. Lumhar. 5 vertebrcB of the "small of tlie back." 
 
 d. Sacral. 5 vertebras (fused together) to which the large 
 
 hip bones are attached. 
 
 e. Coccygeal. 4 or .5 very small simple vertebne (constituting 
 
 the skeleton of a rudimentary tail and corre- 
 sponding to the tail of lower animals). 
 
Fig. 7. The skeleton entire 
 15 
 
16 
 
 THE HUMAN MECHANISM 
 
 Fig. 8. Sixth thoracic 
 
 vertebra 
 
 SeerL from above 
 
 When one looks at the sjainal column from behind, the 
 vertebrte are seen to be placed one upon another, but all 
 in the median dorsoventral j^lane 
 of the body (see Fig. 7). Seen from 
 the side, however, several curves 
 come into view, as shown in Fig. 10. 
 On the ventral side, in the cervi- 
 cal and upper thoracic region, the 
 curvature is slightly convex, in 
 the thoracic region it is quite con- 
 cave, in the lumbar region slightly 
 convex, and in the sacral-coccygeal 
 region again concave. It may well 
 be asked how these separate verte- 
 brte, piled, as it were, one above another, maintain their 
 proper relative positions. This is partly due to the shape 
 of the individual vertebrje, partly to the ligaments (p. 17) 
 which pass from one vertebra to 
 another and limit the movements 
 of each, and partly to the action 
 of muscles which are placed upon 
 opposite sides of the vertebrte 
 and by their antagonistic action 
 hold them in place. The action 
 of muscles and ligaments upon 
 the bones may be illustrated by 
 two blocks of wood held to- 
 gether by two rubber bands {m,m', 
 Fig. 11) slightly stretched ; so 
 long as each pair of opposite 
 bands pulls with the same force, 
 the blocks are kept in what we may call their resting 
 position. Here the rubber bunds represent two of the 
 antagonistic muscles, which, by maintaining a steady and 
 equal pull on the opposite sides of the vertebrte, keep 
 
 Fig. 0. Sixth thoracic 
 
 vertebra 
 
 Seou from the side 
 
STEUCTUEE <)F THE MECHAXISM 
 
 17 
 
 Cervical 
 
 them in place. Should one pull harder than its antago- 
 nist, as when a muscle contracts (see Chapter IV), the 
 antagonist will be stretched and the bones become inclined 
 somewhat toward one another, as 
 shown in the figure. 
 
 This principle of muscular an- 
 tagonism is quite general in the 
 maintenance of the proper rela- 
 tive positions of bones in the 
 body. Almost every joint is the 
 theater of such plays of antago- 
 nistic muscles, which serve the 
 double function of keeping the 
 bones in proper position with 
 regard to one another, and of 
 producing movement at the joint, 
 the amount of this movement 
 being limited by the slack but 
 inextensiljle connective-tissue 
 ligaments which bind the bones 
 together. In Figure 11 both tlie 
 shortening of tlie muscle and tlie 
 slackness of the ligaments is pur- 
 posely exaggerated, in order to 
 represent more clearly the func- 
 tions of these tissues. Ligaments 
 may also guide the movement of 
 bones by preventing motion in 
 one direction or another. /' 
 
 10. The Ribs. — Each rib con- 
 sists of a hony and a cartilaf/iuous portion. The former 
 articulates (i.e. forms a joiut with) the vertebral column, 
 while the latter continues this bony portion to the ventral 
 median breastbone, to which it is directly joined. The ribs 
 form the framework for the thorax and may be lifted or 
 
 Fig. 10. The vertebral col- 
 umn 
 
 Seeu from the side 
 
18 
 
 THE HUMAN MECHANISM 
 
 Fig. 11 
 
 Model showing the action of muscles on two vertehra?, and of the ligaments 
 {I, V) in limiting the amount of movement. The contraction of the muscle rn 
 stretches its antagonist ni' . The amount of movement is greatly exaggerated 
 
 lowered by muscles which connect them with the vertebral 
 
 column and other parts of the 
 skeleton (see Fig. 12). 
 
 11. The Skeleton and the Cen- 
 tral Nervous System. — The skull 
 consists of the bones of the face 
 and those of the cranium, the lat- 
 ter holding the brain. It is sup- 
 ported on the spinal or vertebral 
 column whose ringlike vertebrse 
 inclose a bony canal continuous 
 with the cranial cavity. This' is 
 known as the spinal or vertebral 
 canal in which lies the spinal cord,i 
 — the continuation of the central 
 
 nervous system posterior to the brain. 
 
 1 The terms "spinal cord," "spinal column," and " .spinal canal " are 
 sometimes confnsed by beginners. The .spinal column is the entire bony 
 framework formed by the vertebrte, —the whole backbone ; it surrounds 
 the spinal canal which, iji turn, contains that part of the nervous system 
 known as the spinal cord. 
 
 Fig. 12. Dorsal view of ver- 
 tebrse and ribs 
 
 Showing some of the muscles 
 which lift or raise the ribs 
 
STKUCTUKE OF THE MECHANISM 
 
 19 
 
 Nerves, which pass through small openings in the cra- 
 nium and between the 
 
 vertebrffi, leave the brain ■ /e^^K'M^^\ Omnium 
 
 and cord and end in the 
 muscles, skin, glands, and 
 other organs of the body 
 (see Chapter VII). 
 
 12. The Appendicular 
 Skeleton. — The bones of 
 the arm, leg, hand, and 
 foot may readily be felt, 
 and are suificiently fa- 
 miliar. We may, however, 
 call attention to the simi- 
 larity in the number and 
 form of the bones of the 
 arms and legs, a similarity 
 which is not only helj^ful 
 in mastering their names 
 and arrangement, but is 
 also suggestive of the 
 similarity of function in 
 quadrupeds, both linrbs in 
 
 these animals being or- p^,, jg ^ejian dorsoventral section 
 Q'ans of locomotion. of the skeleton 
 
 Thorax 
 
 Veriebral 
 Canal 
 
 Sacmm 
 
 Pelvis 
 
 Arm 
 
 Humerus, single long bone of tlie 
 
 upper arm. 
 Radius and ulna, two nearly par- 
 allel bones of the forearm. 
 Eight small irregular bones of 
 
 the wrist. 
 Five parallel bones of the palm. 
 Thumb, two bones. 
 Other fingei-s, 
 I three bones. 
 
 Iiones of 
 fingers 
 
 Leg 
 
 Femur, single long bone of the 
 
 thigh. 
 Tiliiu anil fihula, two nearly 
 parallel hones of the lower leg. 
 Seven small irregular bones of 
 
 the ankle and heel. 
 Five parallel bones of the instep, 
 f Great toe, two bones. 
 Other toes, 
 three bones. 
 
 Bones of 
 toes 
 
20 
 
 THE HUMAN MECHANISM 
 
 The legs are attached to the vertebral column by the 
 laro-e }iip hones, which articulate directly and immovably 
 with the sacrum ^ ; but the Jmmerus, or bone of the upper 
 arm, articulates on each side with one of a pair of bones 
 which form the shoulder girdle, or skeleton of the shoulder 
 region ; this pair consists of the 
 eollar hone (clavicle) ventrally and 
 the shoulder hlade (scapula) dorsally. 
 The clavicle articulates with the 
 head of the breastbone ; otherwise 
 the shoulder girdle, with the arm 
 attached to it, is connected with 
 the axial skeleton by muscles only. 
 A wide range of movement is thus 
 secured at the shoulder joint. 
 
 13. Organs of Digestion. — The 
 digestive system consists essentially 
 of a long tube, the alimentary canal, 
 jjassing through the body.''' Into 
 this tube at various points ducts 
 Fig. 14. Diagrammatic from a number of glands pour di- 
 
 median dorsoventral .... rr^^ \- ^ i 
 
 gcstive juices. Lhe alimentary canal 
 begins with the mouth cavity and its 
 C, na.sai cavities; M, moutii familiar organs, the teeth, the tongue, 
 "^'7'- '^•J'^TJ-'' ^' etc. ; this cavity opens posteriorly 
 
 epi,!^lottis ; Gy glottis, or , j i^ jr j 
 
 opening from tiiepiiarynx into that of the phari/7ix, into wliich 
 into tjie^ trachea; t/, tiie ^igo opens the nascd cavitij, separated 
 
 end of the soft palate; 0, ^ .7' r 
 
 resophagus from the mouth only by the palate 
 
 (see Fig. 14). 
 
 On the ventral side of the pharynx just beyond the 
 root of the tongue is the slitlike opening of the windpipe 
 (see Section 14); posteriorly the pharynx is continued in the 
 long gullet, or cesophagus, a tube which passes downward 
 
 1 Tlie sacrum and the two hip bone.s together form the pelvis. 
 
 2 See Fig. 135 for the general arrangement of the organs of digestion. 
 
 section of the nasal and 
 throat passages 
 
STEUCTURE OF THE MECHANISM 21 
 
 through the neck and thorax (within the mediastinum) to 
 join the stomach, which it enters immediately after passing 
 through the diaphragm. 
 
 The stomach is a large pouch, with contractile walls per- 
 mitting adaptation of its size to the bulk of food it may 
 contain. Its situation is shown in Fig. 135, which also 
 shows how it opens on the right side of the Lody into the 
 very long, coiled, small intestine. The coils of this part of 
 the tube may be followed for from twenty to twenty-four 
 feet, to the large intestine, into one side of which it opens. 
 The large intestine, or colon, consists of three portions: the 
 first ascending on the right side to the general level of the 
 stomach, the second jaassing transversel y at this level from 
 right to left, and the third descending on the left side to tlie 
 rectum, the posterior terminal portion of tlie digestive tube. 
 
 Numerous glands pour secretions through ducts into 
 the digestive tube, the following being the more important 
 with their places of discharge : salivary glands (see Chap- 
 ter III) — mouth ; liver — beginning of small intestine ; 
 pancreas — beginning of small intestine (see Fig. 50). 
 Smaller glands empty into the stomach and intestines at 
 numerous places. 
 
 14. The Organs of Respiration. — The organs of respira- 
 tion consist of the right and left lungs (see Fig. 5) from 
 each of which a single bronchus (pi., hronclii) leads to the 
 trachea, or tvindpipe. The walls of the trachea and bronchi 
 are kept from collapsing by successive rings of cartilage. 
 Anteriorly the trachea opens into the pharynx through the 
 larynx, or voice box, the cartilages of which may be felt in 
 the throat at the root of the tongue. The familiar hoarse- 
 ness which accompanies inflammatory roughening of tlie 
 lining of the larynx shows how im^iortant is this organ in 
 the production of the voice. The respiratory and digestive 
 paths cross in the pharynx, the former reaching the exterior 
 through the nose, the latter through the mouth. 
 
22 THE HUMAN MECHANISM 
 
 15. The Organs of Circulation. — The iDosition of the 
 heart and the great blood vessels in the thorax has been 
 described on page 11. The heart is essentially a large mass 
 of muscle, containing a cavity which is divided into right 
 and left halves, wholly separate from each other. The 
 cavity on each side is divided into that of the large ventricle 
 with very thick walls, and that of the much smaller auricle. 
 The heart is thus composed of right and left auricles and 
 right and left ventricles. Valves are so placed in the 
 heart as to allow blood to flow in one direction only (see 
 Fig. 64). 
 
 The arteries are tubes which carry the blood to the 
 tissues, and from each side of the heart a single artery 
 takes its origin, — the fuhnonary artery from the right 
 ventricle and the aorta from the left ventricle. The pul- 
 monary artery supplies the lungs with blood, while all 
 other organs are supplied by the aorta. 
 
 The veins are tubes which conduct the blood from the 
 various organs to the heart. Beginning' in the tissues as 
 microscopic tubes, they unite to form larger and larger 
 tubes as they approach the heart ; those visible through 
 the skin of the hand may be regarded as of medium size ; 
 as the union goes on, the size of the vessels increases until 
 finally at the heart there are only two great veins on the 
 right side (superior vena cava and inferior vena cava) and 
 four on the left [indmonary veins). The venaj cavpe bring 
 blood back from those portions of the body which are sup- 
 plied by the aorta, that is to say, from all parts of the 
 body except the lungs; the pulmonary veins bring blood 
 back only from the lungs, that is to say, from the oro-ans 
 supplied by the pulmonary arteries. The venae cava3 empty 
 into the right auricle, the pulmonary veins into the left 
 auricle. The general arrangement of heart, arteries, and 
 veins is shown in Fig. 1,5, and the figures in Chapter IX 
 (especially 65 and 66) should also be consulted. 
 
STEUCTUEE OF THE MECHANISM 23 
 
 The blood flows in the following circuit. 
 Pulmonary circulation 
 
 Systemic circulation ^ 
 
 ' Right ventricle to 
 
 Pulnronary artery to 
 
 Lungs to 
 
 Pulmonary veins to 
 r Left auricle to 
 
 Left ventricle to 
 
 Aorta and its branches to 
 
 All organs of the body (except the lungs) to 
 
 Veins which unite to form the veruT' cavfe to 
 
 Right auricle to 
 
 Right ventricle. 
 
 Thus the blood which leaves the left ventricle flows to 
 the different organs of the body (except the lungs), and 
 returns by way of the veins to the right side of the heart ; 
 thence it passes through the lungs and again to the left 
 auricle and ventricle, thus completing the " circulation." 
 The term " circulation," strict!}' speaking, is applied to the 
 entire circuit which tlie blood must traverse mitil it returns 
 again to the point from wliich it started ; it is often con- 
 venient, however, to use it to denote the course from the 
 right ventricle to the left auricle, or from the left ventricle 
 to the right auricle ; in this case we speak of the former as 
 the pubnondri/ and of the latter as the si/stcmic or aortic cir- 
 culation. In this sense there may be said to be a " double " 
 circulation. 
 
 The veins have thinner walls than the corresponding 
 arteries, and those of the systemic circulation contain pur- 
 plish or even bluish blood, while the arteries of the same 
 circulation contain bright scarlet blood. The bright color 
 of the arterial blood is due to the fact that it contains 
 more oxygen. The change from purple to scarlet occurs 
 in the lungs, and the reverse change in the organs sup- 
 plied by branches of the aorta. Consequently the blood of 
 the pulmonary arteries is blue or venous in color, and that 
 of the pulmonary vein scarlet or arterial. 
 
R.A. 
 
 Fig. 15. Diagram of the circulation of blood 
 right aviricle; L.A., left auricle; R.V., right ventricle; L.V., left 
 
 ventricle; P. A., pulmonary artery; A, pulmonary artery and vein of 
 right lung; B, pulmonary artery and vein of left lung; C, carotid 
 artery to head, showing branch of left subclavian artery; D, portal 
 vein; E, hepatic vein; F, hepatic artery; G, jugular vein, bringing 
 blood from head and neck 
 
 24 
 
STRUCTUEE OF THE MECHANISM 
 
 25 
 
 16. The Course and Branches of the Pulmonary Artery 
 and Vein. — Soon after leaving the right ventricle the 
 pulmonary artery divides into two branches, one going to 
 each lung. Each of these further divides as it plunges 
 into the substance of the lung' alono-side the bronchus. 
 The course of the four pulmonary veins may be similarly 
 traced into the lungs, from which they bring the blood 
 back to the heart (Fig. 1.5). 
 
 17. The Course and Branches of the Aorta. — The aorta 
 passes anteriorly from the left ventricle, but very soon 
 arches dorsally and 
 posteriorly, forming 
 the a7'c}i of the aorta 
 (Fig. 16) ; the gen- 
 eral course of the 
 artery can be best 
 understood from the 
 figures, or from ac- 
 tual dissection. The 
 arch of the aorta is 
 continued in the 
 large dorsal aorta 
 which passes poste- 
 riorly on the left 
 side of the mediasti- 
 num near the spine, 
 through the dia- 
 phragm, to the lower 
 portion of the abdominal cavity, where it divides into two 
 large arteries which supply blood to the hips and legs. 
 From the arch of the aorta three large arteries pass to the 
 head, neck, shoulders, and arms ; from the thoracic dorsal 
 aorta arise a number of small arteries which supply the 
 muscles and other organs of the thoracic wall ; immediately 
 after passing through the diaphragm two large branches 
 
 FiG.Ki. A network of capillaries, with the 
 .artery a and vein ii (highly magnified) 
 
26 
 
 THE HUMAN MECHANISM 
 
 go to the stomach, spleen, liver, pancreas, and a large part 
 of the small intestine; jjosterior to these the renal arterien 
 pass right and left to the kidneys, and still further down 
 a large artery supplies the lower small intestine and the 
 large intestine. The supply to the legs has already been 
 mentioned. Other small arteries arise from the abdominal 
 
 aorta and are distributed 
 to the muscles and skin of 
 the back. The arteries to 
 the stomach and intestine 
 lie in the mesentery (Fig. 
 143) and their course may 
 be readily traced in a dis- 
 section. 
 
 18. The Course and 
 Branches of the Venae Cavae. 
 — The blood which has 
 thus been distributed from 
 the aorta returns to the 
 opposite side of the heart 
 through the veins which 
 ultimately form the two 
 vense cavse. In general, it 
 may be stated that the 
 veins of those organs which 
 are anterior to the dia- 
 phragm form the superior 
 vena cava, while those 
 posterior to the diaphragm 
 form the inferior vena 
 cava. The larger veins usu- 
 ally run near and approxi- 
 mately parallel to the larger arteries. This is the case with 
 those from the aims and legs, the kidneys, and the muscles 
 of the trunk. One notable and very important exception, 
 
 Fig. 17. The general arrangement 
 of the nervous system (dorsal 
 view) 
 
STRUCTUEE OF THE MECHANISM 
 
 27 
 
 however, is found in the venous supply of the stomach, 
 
 spleen, and intestines, the veins of which 
 
 unite to form a single large vein [■portal 
 
 vein) which passes to the liver, where 
 
 it breaks up into smaller vessels ; the 
 
 blood which has thus passed through the 
 
 liver is finally collected in the hepatic 
 
 vein and poured by this into the inferior 
 
 vena cava just before the latter passes 
 
 through the diaphragm on its way to 
 
 the right ventricle (Fig. 15). 
 
 19. The Capillaries. — The blood 
 which enters an organ throuarh the ar- 
 teries passes to its veins through a sys- 
 tem of microscopic tubes (Fig. 16), the 
 capillaries (Latin, capilla, a hair) ; these 
 may be readily seen under the micro- 
 scojDe in the web of a frog's foot. From 
 the foregoing description of the course 
 of the circulation it will be observed that 
 generally the blood must pass through 
 one set of capillaries in going from the 
 aorta to the venre caviB, or from the 
 pulmonary artery to the pulmonarj^ vein; 
 but the blood which flows through the 
 capillaries of most of the abdominal or- 
 gans (stomach, intestines, spleen) must 
 pass also through a second set of capil- 
 laries, viz. those of the liver, before it 
 can return to the heart. 
 
 20. Organs of the Nervous System. — 
 The skull and the spinal column (p. 18) 
 are chiefly occupied by the hrain and 
 the spinal cord respectively, and from j.^^, -^g -^^^^.^ trunks 
 each of these principal organs of the of the right arm 
 
28 THE HUMAN MECHANISM 
 
 nervous system branches consisting of cords of nervous 
 substance, the nerves, pass out through small holes in the 
 skull or spinal column and are distributed to all the other 
 organs, where they terminate in peculiar structures called 
 end organs. The optic nerve, for example, ends in the retina, 
 the auditory nerve in the inner ear, and motor nerves in 
 muscles, — the nerve endings in these different organs 
 differing materially in structure and arrangement. 
 
 Figure 17 gives some idea of the general arrangement 
 of the nervous system. The nerves to the shoulder, arm, 
 and hand will be seen to arise from the cervical region of 
 the spinal cord ; those for the trunk, from the dorsal and 
 lumbar regions ; those for the legs, from the sacral region. 
 The head and face receive nerves from the posterior por- 
 tions of the brain. The dissection of the arm in Fig. 18 
 shows more accurately the main nerve trunks to that 
 region. Further information with regard to the structure 
 of the nervous system will be given in Chapters VII, 
 XIV, and XV. 
 
CHAPTER III 
 
 THE FINER STRUCTURE OF TWO TYPICAL ORGANS, 
 GLANDS AND MUSCLES. THE CONNECTIVE TISSUES. 
 THE LYMPHATIC SYSTEM 
 
 In the previous chapter we have examined the general 
 construction of the human machine as regards its more 
 conspicuous parts or organs, and especially their location, — 
 whether internal or external, dorsal or venti'al, anterior or 
 posterior, on the right or on the left, — their relations to 
 certain important cavities, and their combination to consti- 
 tute the mechanism which we call the human body. We 
 must now push our examination further and investigate 
 the finer structure of some of the more important parts of 
 the machine. For this purpose we may select two typical 
 organs, a gland and a muscle, the one unfamiliar, by name 
 at least, to most people, the other well known in the form 
 of steaks, chops, roast beef, and other meats. 
 
 1. What is a Gland? — A gland is a mass of tissue, 
 generally softer than muscle, and of no special size or 
 shape, though often rounded or egg-shaped. The gland 
 most easily seen is the milk gland or udder of the cow. 
 This is a large mass of soft tissues devoted to manufac- 
 turing or secreting milk. In general, glands are manufac- 
 turing organs for the preparation of saliva, gastric juice, 
 bile, tears, sweat, or other secretions. Some have tubes 
 or ducts through which their secretions are carried away ; 
 others have no such outlets and hence are known as duct- 
 less glands. Glands vary in size from some which are 
 microscopic to the huge liver, which is the largest single 
 organ in the human body (see Fig. 2). The pancreas or 
 
 29 
 
50 
 
 THE HUMAN MECHANISM 
 
 " sweetbread " is an excellent gland for the beginner to 
 dissect or study. 
 
 2. A Typical Gland. — If we have before us the whole 
 or a part of any typical gland, we find that we are deal- 
 ing with a comparatively soft and sometimes even pulpy 
 mass held together by a loose mesh or network of harder, 
 tougher, and more or less fibrous materials. A pancreas 
 or a liver, if entire, shows conspicuous lohes, and in the 
 pancreas these lobes are plainly subdivided into smaller 
 lobes or lobules. In favorable specimens tubes may be seen 
 connected with the gland, some of which are blood vessels 
 supplying -blood to the gland, and one of which is a duct 
 draining away from it the liquid which the gland has man- 
 ufactured or secreted. After 
 a preliminary examination of 
 this sort of some edible gland, 
 we may consider in greater 
 detail one of our own sali- 
 vary glands, of which we 
 have three on each side of the 
 head, namely, one parotid, 
 one S'uhlingual, and one sub- 
 maxillary gland. 
 
 3. The Structure of the 
 Submaxillary Gland. — The 
 two submaxillary glands lie, 
 one on each side of the face, 
 embedded in the tissues be- 
 tween the lower jaw and the 
 upper portion of the neck. 
 From each gland a duct 
 passes forward in the tissues forming the floor of the 
 mouth, into which it opens by one of the small eminences, 
 or pajyillce, under the tongue. Through this duct the 
 gland pours into the mouth its secretion, saliva. 
 
 Fig. 19. Diagram of submaxillary 
 gland 
 
 D, itsduct; jV.itsnerve; J, itsartery ; 
 F, its vein ; T, tongue 
 
TYPICAL STKUCTUKE OF ORGANS 
 
 If the gland were to be cut in two in any direction with 
 a sharp knife, we should see at once that it is composed of 
 separate parts, or lobes, and that these lobes are still further 
 divided into smaller por- 
 tions, or lobules. The lob- 
 ules and lobes are bound 
 together with a rather 
 loose connective tissue 
 which is continuous with 
 a somewhat denser layer 
 surrounding the gland and 
 forming its capsule; the 
 connective tissue between 
 the lobes forms the j^ri- 
 mary septa (sing., septum), 
 and that between tlie lob- 
 ules the secondart/ septa. 
 The relation of these 
 structures is shown in 
 Fig. 20 
 the microscope ., „ 
 
 . into lobules: also tlK' nri;,Mii of the 
 
 that each lobule is still lar-er branches of the .lii.-t (l>) in the 
 
 further divided by con- '"'^'''^ •""' '"I'l'ies. The be-iiminss of 
 
 -, the duct are shown in FIl^s. lil and 'SI 
 
 nective tissue into nask- 
 
 shaped structures or alveoli (sing., alveolus) ; in these the 
 secretion, saliva, is manufactured, and from them it is dis- 
 charged into the duct of which the alveoli are the blind 
 ends (Fig. 21). 
 
 The whole gland may be compared to a large bunch of 
 grapes ; the main tubular duct of the gland branches (in 
 the septa of connective tissue) very much as the stem of 
 the bunch of grapes branches ; and just as the branches 
 and subbranches of the stem lead, when followed up, to 
 the grapes themselves, so the branches of the duct lead to 
 the alveoli of the gland. If now we pack the bunch of 
 
 20. Diagram of a cros 
 a slaiiil 
 
 section of 
 
 With the aid of showing its division by i.riuiary si-pta (.S') 
 we find '"*" l'-'''^'". -i'"' ''>' sci-ou.lary septa (.v) 
 
32 
 
 THE HUMAN MECHANISM 
 
 grapes in a small basket of sawdust or cork waste, as 
 Malaga grapes are packed, so that the sawdust fills up 
 loosely the spaces between the individual grapes and the 
 branches of the stem, we shall have something with which 
 
 Duct 
 
 ^^ Alveoli 
 
 Capillary Network 
 
 Fig. 21. The origin of the duct of a gland in alveoU, together with the 
 connective tissue and blood vessels 
 
 to compare the arrangement of the connective tissue in 
 relation to the rest of the gland, — the sawdust standing 
 for the connective tissue in which the ducts and alveoli 
 are embedded, and the basket for the capsule. 
 
 4. Minute Structure of Ducts and Alveoli. — The alveoli 
 are, however, neither empty shells like glass flasks, nor 
 solid masses like grapes, but rather hollow bags lined with 
 a layer of relatively thickish, closely set cells, all very 
 much alike. Each of these cells consists of two portions, — - 
 a small central body, the nucleus, and a larger surrounding 
 mass, the cytoplasm. And we may state at once that all 
 
TYPICAL STRUCTURE OF ORGANS 33 
 
 organs of the body are composed of cells, differing, it is 
 true, in different organs, or in different parts of the same 
 
 Fig. 22. Section of a portion of a salivary gland (magnified 500 diameters). 
 After Koslliker 
 
 The duct d divides into the two hr.anclies il' and </", our of whicli ends in the 
 alveoli, a, a. Nei.ghltoring alveoli, a', a^, whose duets are not in the plane 
 of the section, are also shown. In some cells the section does not include 
 the nucleus, which would be in the preceding or the succeeding section 
 
 organ (as in the duct and the alveolus of the gland) : but all 
 consisting of two never-failing parts, — iiucleus and eytoplasm. 
 The muscle and the gland consist of cells, just as all 
 the branches of the military service — the infantry, the cav- 
 alry, the artillery, the engineers, etc. — consist of men. The 
 cell is the anatomical or fundamental unit of these oro-ans, 
 as the soldier is the fundamental or anatomical unit of the 
 army ; in both cases the anatomical units, differing in 
 equipment and training, perform different kinds of work, 
 yet have the same essential structure ; and the cells are 
 combined into brigades, divisions, or corjjs, as tissues and 
 organs ; they make of the body an army organized to fight 
 its way through the vicissitudes and against the obstacles 
 of life.' 
 
THE HUMAN MECHANISM 
 
 5. The Structure of the Biceps Muscle. — The biceps 
 
 muscle is familiar as the mass of flesh lying on the front 
 
 of the upper arm and bulging somewhat when the arm is 
 
 bent at the elbow, especially when one "feels his muscle," 
 
 or when a weight is being lifted by the hand. Figure 23 
 
 shows this muscle with the 
 
 bones to which it is attached. 
 
 It consists of two portions : a 
 
 central, thick, red part, known 
 
 as the belly, soft when the 
 
 muscle is at rest, hard when it 
 
 is contracted ; and cordlike 
 
 strings, or tendons, two at the 
 
 upper end and one at the lower, 
 
 by means of which the muscle 
 
 is attached to two bones of the 
 
 shoulder girdle and to one of 
 
 _ .,., „, , . , , ,, the forearm. When the belly 
 
 Fig. %i. The biceps muscle of the ■' 
 
 arm t'f the muscle shortens the 
 
 The resting condition is shown by points a and h are brought 
 
 the solid lines, the contracting closer together and the arm is 
 
 condition Ijy the dotted lines n t i 
 
 bent, or flexed, at the elbow. 
 This drawing together, or contraction, is the special work, 
 or function, of muscles in general. 
 
 Every one has seen the cross section of a muscle in a 
 raw beefsteak. This shows that the muscle as a whole is 
 surrounded by a sheath of connective tissue which contains 
 more or less fat ; f;epta pass inwards, dividing the muscle 
 into lesser red masses known as fasciculi, or bundles, and 
 these are furtlier subdivided into secondary fasciculi by 
 secondary septa, very much as the gland is subdivided 
 into lobules. 
 
 A longitudinal section shows that the fasciculi run from 
 tendon to tendon, and microscopic examination proves that 
 the general connective tissues of the belly of the muscle 
 
TYPICAL STfiUCTUKE OF OKGAXS 
 
 o-j 
 
 are continuous with that of the tendon. The tendon itself 
 
 is a peculiarly strong and inextensible 
 
 variety of connective tissue, consisting 
 
 chiefly of parallel fibers which are sjoe- 
 
 cially fitted to transmit to the bone the 
 
 pull of the belly of the muscle. 
 
 6. The Muscle Fibers. — Examination 
 of the structure of one of the finer fascic- ~ 
 uli in the belly of the muscle shows that 
 it is composed of threads, or fibers, which ^ 
 at first sight differ greatly from the secret- == 
 ing cells of the gland. These are the ^ 
 muscle fihers. They are 1 to 11- inclies in 
 
 leng-th and frc 
 
 to 
 
 i "■ • ;; f, 
 in thickness, thus l)eing from 2.50 h 
 
 jr of an inch 
 
 ')()(} 
 
 Fii 24 Tendon 
 (hi^lih m i^iiiiied) 
 
 Sln>"\\'iii,u till' fiber 
 ImiiiUcs ^eiiurated 
 
 times as long as wide, and comparable in 
 shape to a long leather shoestring rather than tn a sausage. 
 ,,, ,||.i, Each fasciculus C(.>ntains hundreds or even 
 i'iilSi'l thousands of fibers. The fibers always run 
 lengthwise of tlie fasciculus, but, as a usual 
 thing, do not extend its entire lengtli, as ob- 
 viously follows from tlie fact that a single 
 fasciculus of the biceps is several inclies in 
 length. The libers are inclosed in a very thin 
 transparent meml^rane, the sarcolerinna, and are 
 bound into bundles, or fasciculi, by the same 
 fine connective tissues seen lietween the alveoli 
 of a gland. To the end of the sarcolemma are 
 attached fine fillers of connective tissue which 
 Showing the pass into the tendoii (Fig. '25). Indeed, the fibers 
 of the tendon o^ ^^^'^ tendon are the collected fibers from the 
 fibers to the sarcolemmas of all the muscle fibers. For this 
 
 sarcolemma , , , c ^i i j-i j- i • 
 
 reason the part ot trie muscle near tlie tendon is 
 "tough meat," while that in the belly of the muscle is tender, 
 owing to the smaller number of connective tissue fibers. 
 
 Fig. 25. One 
 
 end of a 
 muscle fiber 
 
36 
 
 THE HUMAN MECHANISM 
 
 7. The Muscle Fiber is a Cell. — The muscle fiber at first 
 sight does not seem like tlie typical cell already described, 
 with nucleus and cytoplasm ; for when examined in the 
 fresh condition the only obvious points of structure seen in 
 it are striking cross striations consisting of alternate dark 
 and liglit bands. It has been shown, however, by ingen- 
 ious and careful study, that the cross 
 striations are optical appearances pro- 
 duced by the peculiar shape of extremely 
 minute longitudinal rods in the cytoplasm 
 of the muscle fiber, and that, immediately 
 under the sarcolemma, there are numer- 
 ous characteristic nuclei which are easily 
 brought into view by suitable treatment. 
 Briefly, then, the muscle fiber is a cell 
 with many nuclei, in whose cytoplasm are 
 found peculiar structures, the myofibrils ; 
 upon superficial examination these myo- 
 fibrils not onl}^ obscure the nuclei but give 
 if to the whole fiber the appearance of cross 
 striation. 
 
 8. How far is the Structure of Glands 
 and Muscles Typical of All Organs? — 
 Both the gland and the muscle are thus 
 Specially prepared to composed of cells. Although differing 
 
 brinsr out the nu- . i i i • ^i , ,, n 
 
 merous nuclei considerably m the two organs, these cells 
 possess certain general and fundamental 
 features in common, for each one contains a nucleus (or 
 nuclei) and surrounding cytoplasm. Is the same thing 
 true of all other organs? The muscle and the gland are 
 examples of organs which do active work, but some other 
 organs perform purely passive functions. Such are the 
 bones, which do no work themselves, but upon which 
 the work of mechanical motion is done by the muscles ; 
 the tendons, which transmit the pull of muscles ; the 
 
 Fig. 26. Part of 
 muscle fiber 
 
TYPICAL STEUCTUEE OF OEGANS 
 
 37 
 
 ligaments, which limit and sometimes guide the motion 
 of bones ; and the connective tissues which bind together 
 other parts of the body. None of these is a working organ 
 in the sense that a muscle or a gland is a working organ, 
 and we are not surprised to find that their structure departs 
 from that of the muscle and gland in that, while nucleated 
 cells are present in all of them, the great mass of the organ 
 is cor)i2:>osed oflife- 
 lessmatter between 
 the cells. In the 
 tendon this con- 
 sists of very 
 strong parallel 
 fibers (Fig. 24); 
 the ligament 
 shows mucli the 
 same structure ; 
 a bone consists 
 chiefly of lifeless y 
 material contain- 
 
 ino- laree amounts Showing the branohiu 
 
 of mineral mat- 
 ter, with cells ly- 
 ing here and there 
 in spaces which 
 
 communicate with one another by means of minute chan- 
 nels. The connective tissues, like that which binds the 
 skin to the underlying muscles, or that which forms the 
 sheath and septa of glands and muscles, consists essen- 
 tially of lifeless fibers running in all directions and thus 
 ready to limit the extent of any pull tending to separate 
 unduly the adjacent organs. To organs and tissues of this 
 kind we may give the name of supporting organs and tissues, 
 and they form almost the sole exception to the general 
 rule that the essential part of a tissue consists of its cells. 
 
 Longitudinal {A) and transverse (B) sec- 
 tions of bone 
 
 :iinl commuiiicating canals — 
 in which are blood vessels and nerves — surrounded 
 by the lifeless bone sulistance. In this are spaces 
 connected "\\ith one another by very minute chan- 
 nels. Each of these spaces contains a living cell 
 shown in C 
 
38 
 
 THE HUMAN MECHANISM 
 
 The latter statement is true of all organs and tissues which 
 do work, — the active organs of the body. In the case of 
 the supporting tissues the cells which they contain are the 
 fundamental units of the organ, since they make the inter- 
 cellular lifeless substance. But the part which the organ 
 plays in the work of the body as a whole is performed by 
 
 the lifeless substance (fibers, etc.) 
 which the cells have manufactured 
 and keep in repair. 
 
 9. The Blood Vessels are Closed 
 Tubes in Connective Tissue. — The 
 arrangement of connective tissues 
 is fundamentally the same in the 
 gland and the muscle. As their 
 name implies, these tissues serve 
 the obvious purpose of binding 
 the anatomical units into organs, 
 but tliey also perform other func- 
 tions equally important. 
 
 We have seen (Chapter II, Sec- 
 tion 19) that each organ receives 
 blood through one or more arte- 
 ries, and that this blood flows away 
 from the organ through one or 
 T, no rpK 1 «i more veins. If a colored fluid mass 
 
 Fig. 28. Three muscle fibers 
 
 and an artery breaking up which would afterwards set, e.g. 
 
 into capillaries between ^ warm Solution of gelatin colored 
 '^"^^ with carmine, had been forced into 
 
 the arteries before we began our examination, we should 
 find that this mass would everywhere be confined in a 
 system of closed tubes which merely lie in the connect- 
 ive tissue. The artery entering the muscle branches into 
 smaller and smaller arteries in the general sheath of the 
 organ, or in its branches, the septa ; from these finer 
 arteries an exceedingly lich network of small thin- walled 
 
TYPICAL STRUCTURE OF ORGANS 
 
 39 
 
 tubes is given off to the finest connective tissue which 
 surrounds the cells themselves ; these tubes are the cajnl- 
 laries. They ultimately unite to form the larger veins, 
 which can be traced in the septa to those veins which 
 gross dissection reveals as leaving the organ (see Fig. 10). 
 Through these tubes — arteries, capillaries, and veins — 
 the blood flows ; and it is imjjortant for us to understand 
 that it is everywhere confined to them in its passage through 
 the organs ; nowhere does it 
 come into direct contact with 
 the living cells (save those 
 lining the vessels). Whatever 
 exchange of matter or energy 
 takes place between the blood 
 and the living cells must be 
 through the walls of the blood 
 vessels.^ These walls ai'e rela- 
 tively thick in the arteries, 
 usually somewhat thinner in 
 the veins ; in the capillaries, 
 however, they are very thin, 
 and it is through these thin 
 capillary walls that all inter- 
 changes of matter take place. 
 That the connective tissue sur- 
 rounding the capillaries bears 
 an important relation to the ^' 
 circulation we sliall now see. 
 ■> 10. The Lymph Spaces of the Connective Tissue. The 
 Lymph. — Careful examination shows that the fine con- 
 nective tissue within which the capillaries are embedded 
 
 29. Superficial and some deeper 
 lympliatics of tlie hand 
 
 ' The term " blood vessel '' is sometimes confusing to the beginner, since 
 it suggests a utensil for holding liquids, like a cup or kettle or barrel. In 
 anatomy the term "vessel" is applied to tubes, ducts, or canals through 
 which blood or lymph flows. 
 
40 
 
 THE HUMAN MECHANISM 
 
 is not a solid or continuous mass, but rather a mass or 
 
 mesh of extremely fine fibers or 
 bundles of fibers, with here and 
 there "connective- tissue cells" 
 which keep the fibers in repair. 
 The connective tissue, there- 
 fore, is everywhere channeled 
 by irregular spaces running 
 between the fibers and other 
 structures ; these spaces com- 
 municate freely with each other 
 and contain a colorless liquid 
 known as lymjA ; the spaces of 
 the connective tissue may thus 
 be conveniently described as 
 lymph spaces. They serve as 
 communicating channels be- 
 tween the cells and the walls 
 of the capillaries. 
 
 11. Origin of the Lymph 
 
 The lymph which the spaces 
 contain is derived partly from 
 water and soluble food mate- 
 rials which have passed through 
 the capillary walls from the 
 blood, and partly from material 
 produced by the neighboring 
 cells (see the next chapter) ; on 
 the other hand, the cells absorb 
 from the lymph substances 
 which the latter has received 
 from the blood, while the blood, 
 
 Fig. .so. The two main lymphatic 
 
 trunks (in white), with their 
 
 openings into the great veins 
 
 near the heart 
 The larger of these trunks — that 
 
 on the left side and known as the 
 
 thoracic cZucf — returns all the in turn, takes from the lymph 
 lymph except that from the risht gubstances discharged from 
 
 the cells. The lymph thus 
 
 side of the head, neck, and the 
 right arm and shoulder region 
 
TYPICAL STEUCTUEE OF ORGANS 
 
 41 
 
 becomes the means of communication, the middleman, be- 
 tween the living cells of the organs and the nourishing 
 blood, and forms the immediate environment of the cells 
 themselves. In other words, the cells of muscles, glands. 
 
 Fig. 31. Diagram of the relation of the cells of an organ to its blood 
 
 vessels, lymphatics, and connective tissue 
 
 y1, artery; F, vein ; i, lymphatic 
 
 and other organs live in lymjjh, just as the human body as 
 a whole lives in air, or a fish in water. 
 
 12. The Lymphatics. — Besides the veins, which convey 
 blood away from an organ, a second system of tubes or 
 vessels passes out through the capsule. These tubes arise 
 in the lymph spaces of the connective tissue and unite 
 with similar tubes from other regions to form larger and 
 larger trunks, known as lymphatics, which ultimately form 
 one or two great trunks and open into the great veins 
 near the heart (see Fig. 30). Through these direct outlets 
 the surplus lymph of the organ flows in a varying but for 
 
42 THE HUMAN MECHANISM 
 
 the most part continuous stream. This flow of lymph away 
 from an organ is of the very greatest importance in main- 
 taining the normal environment of the cells. 
 
 13. Function of the Lymph Flow from an Organ. — It is 
 clear from inspection of Fig. 31 that there is a steady flow 
 of liquid from the capillaries, through the lymph spaces of 
 the connective tissue, over the surfaces of the living cells 
 or of any intervening capillaries, to the lymphatics. The 
 cell is thus bathed not by a stagnant medium but by one 
 which is in gentle movement ; one which brings to all parts 
 of its surface the food which it needs and immediately 
 carries away from all parts of its surface to the adjacent 
 capillaries the products of its activity. By providing this 
 outlet from the lymph spaces the lymphatics render possible 
 the movement of lymph within the organ itself, whereby 
 material is readily transferred from the cell to the capil- 
 laries, and from the capillaries to the cell. 
 
 14. Distribution of Nerves to Muscles and Glands. — The 
 distribution of nerves resembles that of the arteries, the 
 larger nerve trunks being found in the septa and their fine 
 ultimate branches being distributed by way of the connect- 
 ive tissue which surrounds the cells, in whose neighbor- 
 hood or even within whose substance they finally end. As 
 we shall see in subsequent chapters, it is the function of 
 the nerves to arouse the gland cells or muscle fibers or 
 other cells to activity. 
 
 15. Summary. — Disregarding for the moment those 
 peculiarities of arrangement, shape, and structure of the cells 
 which are connected with the special work of each organ, 
 (e.g. the arrangement of gland cells to form a blind tube, 
 or of the connective tissue and fibers of muscle so as to 
 exert a pull on a bone), we may say that the typical struc- 
 ture of an organ would be represented in Fig. 31. The 
 whole is surrounded by a capsule, receives a blood supply 
 through a system of closed tubes, and contains the special 
 
TYPICAL STRUCTUEE OF ORGANS 43 
 
 cells upon whose activity its characteristic work depends. 
 These cells are held together by a fine connective tissue 
 whose numerous and freely communicating spaces contain 
 a fluid, the lymph, which is free to flow out through a 
 second system of tubes, the lymphatics. Nerves from the 
 brain or spinal cord are also distributed in the connective 
 tissue to the cells of the organ. 
 
 Before concluding this description of the finer structure 
 of organs, a word may be added with regard to the physical 
 nature of the cell substance. In its literal meaning the 
 word "cell" is a misnomer, since it suggests a hollow 
 space inclosed by solid partitions or walls. Plant cells do, 
 in fact, usually have such walls around their cytoplasm 
 (Chapter VIII), and this cytoplasm frequently contains 
 spaces (vacuoles) filled with a solution of salts, sugar, and 
 other dissolved material. But neither the cell wall nor 
 vacuoles are of universal occurrence, each being rarely 
 found in the animal cell, and often absent even in the 
 plant cell. Fifty years of thorough investigation has 
 reduced the number of essential cell constituents to the 
 cytoplasm and the nucleus. The ultimate structure of 
 these is still unsettled ; but it would seem that the cyto- 
 plasm is a mixture of higlily viscous, liquid materials, often 
 containing minute granules in suspension, and possibly 
 the nucleus is of a similar physical character. In these 
 fluid substances occur those "vital" or "life" processes 
 to the study of which we now proceed. 
 
 ^' 
 
CHAPTER IV 
 
 THE WORK OF THE HUMAN MECHANISM THE RESULT- 
 ANT OF THE WORK OF ITS DIFFERENT ORGANS 
 AND CELLS 
 
 From common experience we know that the work of 
 the body as a whole is the resultant of the work of its 
 various organs. We know that an act as simple as the 
 taking of food involves the cooperation of the arm, fingers, 
 eyes, teeth, tongue, and other organs. But we do not often 
 realize that the work of each organ is equally the resultant 
 of the work of its component cells. Ej^es, hands, and 
 tongue we can see and even watch at their work, but the 
 cells of muscles and glands and nerves are invisible. If 
 we could see them and delve into their recesses, we should 
 there discover activities no less remarkable. As we cannot 
 do this directly and with the naked eye we must do it 
 indirectly and make use of experiments, chemical analyses, 
 microscopic studies, and all other available methods. And 
 we may again choose for detailed consideration two typical 
 organs, muscle and gland, the structure of which we have 
 studied in the last chapter. 
 
 1. Physiology of the Salivary Glands. Working Glands 
 and Resting Glands. — The function of the salivary glands 
 is the secretion or manufacture of saliva for use in the 
 mouth, and one of the first things we notice about this 
 act of secretion is that it is not constant but intermittent. 
 Most organs have periods of activity, or tvork, followed by 
 periods of inactivity, or rest, and these glands are no excep- 
 tion. Physiologists frequently speak of " working glands " 
 
 44 
 
WORK OF ORGAl^S AND CELLS 45 
 
 and " resting glands." We all know that our own salivary 
 glands work more effectively at some times than at others. 
 The mouth " waters " at the sight of food ; when we are 
 in the dentist's chair the flow of saliva often seems exces- 
 sive ; and at other times our mouths are "parched" or 
 " dry." 
 
 2. The Chemical Composition of Saliva. — The saliva is 
 sometimes thin and flows readily, while at other times it 
 is thick and viscous, or glairy. This difference is caused 
 by the fact that the amount of water in it varies under 
 different conditions. At all times, however, it is a fluid 
 which consists of water containing certain solids in solu- 
 tion. The amount of these solids varies from five to ten 
 parts in a thousand of saliva, and they consist chiefly of 
 three groups of compounds. The first is 7rmcin, familiar 
 to us as the chief constituent of the phlegm or mucus 
 discharged from the nose and throat, and giving to the 
 fluid its viscous character ; the second group consists of 
 substances known as enzymes, which have the power of 
 chauo-ing starch to sugar; these we shall studv in detail 
 in the chapters on digestion ; the third group consists of 
 mineral or inorganic salts, of which ordinar}' table salt, or 
 sodium chloride, is the most important. As we shall see, 
 the salts and water are derived directly from the blood, 
 while the mucin and enzymes are mamifactured hy the 
 (/land. 
 
 3. Blood Supply of the Working Gland. — Whenever a 
 gland is actively working there is an increased flow of 
 blood through it. For this reason the resting gland is 
 slightly pink, while the working gland becomes distinctly 
 red. Since the secretion of saliva requires water, and this 
 can be obtained only from the blood, it is easy to see why 
 an abundant blood supply is essential to activity. Other 
 constituents of the saliva, such as the inorganic salts, 
 likewise come directly from the blood. 
 
46 THE HUMAN MECHANISM 
 
 4. The Relation of Nerves to Gland Work. Irritability. — 
 
 Nerves pass, as we have seen (p. 30), from the central nerv- 
 ous system to the salivary glands. These nerves are essen- 
 tially bundles of nerve fibers which are distributed to 
 the neighborhood of the gland cells, their final endings 
 not yet having been conclusively demonstrated. But it is 
 known that they are the means of conveying to the gland 
 an influence, called a nervous impulse, and that nervous 
 impulses cause the gland to secrete saliva. It is also a fact 
 that when these nerves are cut or injured in any way, so 
 that the gland is no longer in connection with the brain 
 and spinal cord, saliva is not secreted, even when food is 
 placed in the mouth. Evidently the activity of the gland 
 is normally aroused by nervous impulses from the brain 
 and spinal cord, just as the activity of a receiving instru- 
 ment in a telegraph office is aroused by the electric current 
 which comes to it over a wire, or as a mine is exploded by 
 the same means. The gland then stands ready for the act 
 of secretion, and is thrown into activity by a nervous 
 impulse from the central nervous system. We speak of 
 this action of a nerve upon the organ in which its fibers 
 end as stimulation, and that property of an organ in virtue 
 of which it may be aroused by a stimulus as irritability. 
 
 All the working organs of the body (in contradistinction 
 to the supporting organs, p. 37) are in this sense irritable, 
 and most of them receive nerves which set them to work. 
 Irritable tissues may, however, be artificially stimulated 
 by other means than by nervous impulses. Of these 
 means an electric shock is the most familiar ; others are 
 the sudden application of heat, the presence of certain 
 foreign substances in the blood, and even a sharp blow. 
 
 We have now to inquire wliat it is that happens in the 
 gland when it is stimulated by a nervous impulse. 
 
 5. The Response of the Gland to Stimulation by its 
 Nerve. — The visible result of stimulation of the gland is 
 
WORK OF ORGANS AND CELLS 47 
 
 the discharge of saliva into the mouth. Something must 
 have happened in the gland which has led to the passage 
 of water and other substances from the blood (and lymph) 
 through the gland cells into the duct. But something more 
 has happened, for the saliva contains several substances 
 which are not found in the blood. The gland has evidently 
 contributed something to the saliva. How were these con- 
 tributions to the secretion made ? 
 
 When a gland has been resting for some time micro- 
 scopic study shows that the cytoplasm of its cells becomes 
 loaded with small granules, at times so numerous as to 
 obscure the nucleus itself. As secretion goes on these 
 granules disappear from the cell, presumably contributing 
 something to the secretion. If the secretion continue for 
 several hours, it is found that the granules have disap- 
 peared and that the size of the cell is often distinctly 
 smaller than before secretion began. 
 
 The "resting" gland is therefore by no means an idle 
 gland, but gradually stores within its cytoplasm something 
 in the form of granules, which under the influence of 
 nervous impulses or other forms of stimulation more or 
 less rapidly disappear in the " secretion." 
 
 6. Activity of the Gland involves Chemical Change within 
 its Cells. — It might be supposed that the granules manu- 
 factured during rest are merely dissolved or washed out 
 of the cells in the copious stream of water and salts which 
 during secretion passes througli from the blood and lymph 
 to the duct. If this were so, it would be possible to dis- 
 solve from the gland a substance exhibiting in general the 
 same properties as the secretion itself. But this is not gen- 
 erally the case. Extracts of fresh glands commonly fail 
 to exhibit the characteristic properties of normal secretions, 
 and we are compelled to believe that the activity of a gland 
 means something more than the mere discharge of previ- 
 ously stored substances ; that is to say, the material of the 
 
48 THE HUMAN MECHANISM 
 
 granules in the resting cells is not simply set free when 
 the gland secretes, but is at the same time chemically 
 changed. In the digestive juices, for exami^le, we have ac- 
 tive substances called enzymes, which, it has been shown, 
 are derived from other substances called zymogens, in the 
 gland cells. The chemical change from the one into the 
 other is as essential to the process of secretion as is the vis- 
 ible flow from the duct. 
 
 These facts then present to us the picture of the cell as 
 the working or physiological unit, as we have already seen 
 that it is the anatomical unit of the gland fp. 32). The 
 work of the gland is the sum of the work of its constituent , 
 cells. During the period of rest these cells manufacture 
 from the blood zymogens or other substances which they 
 store away in the form of granules within their cytoplasm. 
 When they are stimulated by the nervous impulse a chem- 
 ical change takes place in them, the zymogens are changed 
 to enzymes and other substances, and these, together with 
 the water, salts, etc., derived from the blood, form the 
 secretion. 
 
 7. Physiology of Muscular Contraction. — At first sight 
 muscles and g-lands seem to differ in action or function 
 no less than in form and structure. No two acts are 
 apparently more unlike than lifting a weight by the mus- 
 cles of the arm and the secretion of saliva by the salivary 
 glands. But beneath obvious and important differences 
 there are profound and fundamental similarities in the 
 processes which occur in the two organs during activity. 
 Like the gland, the muscle is set to work or stimulated by 
 a nervous impulse ; its contraction is accompanied by an 
 increased blood supply; and, most important of all, the 
 work or contraction is accompanied — indeed preceded — 
 by chemical changes much more profound than that of the 
 transformation of zymogen into enzyme. These cliemical 
 changes supply the power for the work. 
 
WOKK OF ORGANS AND CELLS 49 
 
 Muscles retain the power of contraction for a time after 
 removal from the body. If such isolated muscles are in- 
 closed in an air-tight space and made to contract by elec- 
 tric stimulation, carbon dioxide (CO2) gas will be given off 
 and sarcolactic acid (CgllgOg) will be found in the muscle. 
 Thus the activity of the muscle, like that of the gland, 
 involves chemical change ; and just as glandular activity 
 produces an output called a secretion, so muscular activity 
 produces an output consisting of substances usually de- 
 scribed as waste products. 
 
 8. Combustion or Oxidation in Working Muscle. — The 
 source of the carbon dioxide, sarcolactic acid, and other 
 waste products, as of the mucin, enzymes, and other sub- 
 stances produced by the active gland, must in the long run 
 have been the matter taken into the body of the animal in 
 the food. When, however, we compare the intake of food 
 with the output of waste p)roducts of muscular contraction, 
 we find that whereas the food is poor in oxygen the waste 
 products are rich in that element. Somewhere, between the 
 entrance of the food and the oxygen into the body and 
 their final outgo as waste products, the two have chemic- 
 ally combined, very much as the coal and the oxygen 
 which enter the furnace of an engine combine to form the 
 gases which are discharged from the smokestack. This 
 combination of substances with oxygen always occurs 
 when anything is burned, a process which is called com- 
 bustion or oridation. The tvorlc of a muscle, like that of 
 the enf/iue, is the result of such comhustion or oxidation. 
 
 9. Storage of Fuel Substances in the Muscle. — In one 
 respect, however, the oxidation which gives rise to mus- 
 cular contraction differs from that which gives power to a 
 steam engine. At the time of the work the engine requires a 
 draft, i.e. a constant supply of free oxygen to its furnace ; 
 shut this off and the fire goes out, — the engine comes 
 to rest. 
 
50 THE HUMAN MECHANISM 
 
 With the muscle this is not so. An isolated muscle, 
 from which every trace of free ■' oxygen is removed, may be 
 made to work in a vacuum, and in doing so it produces 
 carbon dioxide and other waste products just as it does 
 when it is surrounded by air. The presence of free oxygen 
 at the time is not necessary to muscular work or to the 
 production of waste products by the muscle. 
 
 To account for this remarkable fact we must suppose that 
 the muscle has the power of storing oxygen within itseK, 
 not as free oxygen but in combination with other substances 
 which upon the application of a stimulus readily yield it 
 up to combine with the elements of the food, thus forming 
 the waste products. 
 
 We cannot attempt to explain in an elementary work 
 the details of this process ; but enough has been said to 
 show that during the period of rest the muscle, like the 
 gland, has been engaged in storing something within itself. 
 As the gland stores its zymogens ready to undergo chemical 
 change into the enzymes, so the muscle stores from the 
 food and oxygen what we may term fuel substances. It is 
 these which readily change into waste products when the 
 muscle is stimulated, thus completing the analogy so far 
 as chemical activity of tlie two organs is concerned. 
 
 10. Nature of the Stored or Fuel Substances of Muscle. 
 — The stored material of a muscle is not in the form of 
 granules, as is that of the gland, nor has it been extracted 
 from nmscles, as zymogens have been extracted from 
 glands. But it is clear that the material made by the 
 muscle fiber from the food and oxygen must consist of 
 unstable bodies, like gunpowder or dynamite, ready to go 
 off, as it were, at the slightest touch ; and, as in the case 
 of these explosives, the change to the stable substances 
 produced by their discharge gives the power for work. 
 
 1 i.e. oxygen uncombined with other elements, as the oxygen in the 
 atmqspliere. 
 
WOEK OF OEGANS AND CELLS 51 
 
 The comparison with gunpowder is so instructive that we 
 may enter into it in greater detail. Gunpowder is a mix- 
 ture of charcoal, sulphur, and saltpeter and contains no free 
 oxygen. The oxygen which the mixture contains is chemic- 
 ally combined with potassium and nitrogen in the salt- 
 peter (KNO3), and upon the application of an appropriate 
 stimulus, in the form of a spark for example, this oxygen 
 is given up to unite with the carbon of the charcoal accord- 
 ing to the following equation : 
 
 3 C -I- S -h 2 KNO3 = 3 CO. + N., + K,S 
 carbon sulphur saltpeter carbon nitrogen potassium 
 
 dioxide sulphide 
 
 Here again we may speak of the storage of oxygen during 
 the chemical processes by which the saltpeter was made, 
 just as the muscle has stored oxygen received from the 
 blood in the form of some substance wliieli readily gives 
 it up, upon the application of a stimulus, — whether a 
 nervous impulse, an electric shock, or a sharp blow, — to 
 form with the carbon and hydrogen derived fnjin the food 
 the highly oxidized and staljle waste products. 
 
 11. Similarity between the Work of a Gland and that 
 of a Muscle. — These two tissues (glandular and nmseu- 
 lar) comprise a very large j^art of the organs of tlie bodv. 
 Among glands are the salivary glamls, those in the lining 
 of the stomach and intestine, the pancreas, the liver, the 
 thyroid, the tear glands, and the kidney. Tlie skeletal 
 muscles comprise nearly half the total weight of the body; 
 while among the muscular or contractile tissues must he 
 classed the heart, half or more of the walls of the stomach, 
 the intestine, the arteries, and the veins. 
 
 In the present chapter we have described in broad out- 
 line the work of these two organs in order that the 
 student may see for himself that tliey present striking 
 points of similarity ; and these points of similarity are the 
 
52 THE HUMAIST MECHANISM 
 
 very points which are typical of the action of most living 
 cells. They may be summed up as follows : 
 
 1. Both gland cells and muscle fibers manufacttire from 
 raw materials of the blood something which they store 
 within themselves (zymogen, fuel substance). 
 
 2. When the o-land cell or muscle fiber which contains 
 
 o 
 
 these substances is set to work or stimulated by a nervous 
 impulse, or in other ways, a chemical change takes place, 
 and the stored substance disappears. At the same time 
 products of the chemical change made their appearance, 
 e.g. the enzymes or ferments of certain glands and the 
 "waste products" of muscular activity. 
 
 3. In all the cells of the animal body the general char- 
 acter of these working chemical changes is that of a com- 
 bustion or oxidation, certainly at times indirect, possibly 
 at other times direct. 
 
 12. Cells as Factories. — These and similar chemical 
 activities lie at the basis of the work of every cell of the 
 human mechanism. Not only in the cells of glands and 
 muscles, not only in all the other cells of higher animals, 
 but in those of lower animals and of plants as well, oxida- 
 tion and other chemical changes are invariable accompani- 
 ments of life. The exact nature of the change differs in 
 different cells, and is adapted to the work which the cell 
 must perform for the body as a whole. Hence the universal 
 need of carbon and hydrogen and nitrogen in the food, and 
 of oxygen obtained in one way or another. The dell is 
 fundamentally a chemical laboratory, or, better still, a 
 chemical mechanism fitted to employ chemical processes 
 which occur within it for definite and useful ends. 
 
 13. The Relation of Chemical Change to the Work of Mus- 
 cular Contraction. — The student will readily recall many 
 examples which show that the act of combustion supplies 
 the power necessary for doing work upon surrounding 
 objects. The explosion of gunpowder will shiver solid 
 
WORK OF OEGANS AND CELLS 53 
 
 rock ; a hogshead of water may be evaporated by the heat 
 of a fii'e and its vapor conveyed by tubes to other places 
 where it may be condensed again into water ; here the act 
 of combustion has just as truly done work by moving the 
 hogshead of water as if the water had been moved from 
 the one place to the other by human effont. In both these 
 cases it is comparatively easy to trace every step of the 
 process and thus see clearly the relation between the com- 
 bustion and the work done. In the case of the muscle 
 fiber, on the other hand, the relation is not so obvious, but 
 only because we do not so readily see the plan and mech- 
 anism of its constituent parts. 
 
 A man who has lived all his life as a hunter in the woods 
 and sees for the first time a shoe factory may not under- 
 stand how the combustion of coal in the furnace of the 
 engine docs so many and varied things, because he is not 
 familiar with the construction and operation of machinery. 
 We ourselves are in a somewhat similar position with 
 regard to the muscle fiber ; but, like the factory, it is a 
 mechanism so arranged that the power supplied by oxida- 
 tion of food is applied to accomplish a certain end, — the 
 shortening or contraction of the fiber in which it occurs, 
 and so the lifting of a weight. 
 
 It is now generally believed that the rodlike myofibrils 
 (p. 36) form an essential part of this mechanism. But we 
 cannot discuss this question more extensively at the present 
 time. Our only purpose in introducing it is to give to the 
 student that most fundamental conception of physiology, 
 — cells as living mechanisms capable of using the power 
 obtained from the oxidation of the food to do definite 
 kinds of work. 
 
 14. Recapitulation. — The present chapter is entitled 
 The Work of the Human Mechanism the Resultant of the 
 Work of its Different Cells. We have traced the character 
 of the work done in the case of the gland and the muscle, 
 
54 THE HUMAN MECHANISM 
 
 and have found that it is fundamentally the work of the 
 cells of which the organs are composed. The cells of other 
 organs are similarly constructed to do other kinds of work, 
 and the character of their chemical changes and of the 
 mechanisms for utilizing power varies accordingly; all, 
 however, showing the same fundamental plan of working 
 engines. We can now readily appreciate the meaning of 
 the heading of the chapter ; the body is a community of 
 groups of cells of different kinds, each kind doing some 
 work more or less peculiar to itself ; in addition to the two 
 groups (gland and muscle cells) which we have studied, 
 there are nerve cells in the brain, spinal cord, and else- 
 where ; cells which make blood corpuscles ; cells which 
 keep in repair the connective tissues, — bone, gristle, ten- 
 don, and ligament ; and many more, such as cells which 
 manufacture or themselves form the lining of free sur- 
 faces, like the skin, the alimentary tract, the air passages, 
 etc. The sum total or net result of the activities of these 
 and other cells makes up the work of the body as a whole; 
 the work of the body — the human organism, the human 
 mechanism — is thus the outcome or resultant of the work 
 of its different component cells. 
 
CHAPTER V 
 WORK, FATIGUE, AND RESTORATION 
 
 While it is true, as shown in the hist chapter, that 
 capacity for work is one of the principal characteristics of 
 the human body, no experience of daily life is more familiar 
 than that work is followed hy fatigue. This is true hotli 
 of individual orffans and of the organism as a whole ; for 
 fatigue may be either local, as when some one muscle is 
 tired from hard work, or general, as when weariness affects 
 all organs, — those which have been resting as well as those 
 which have been working. 
 
 We use the word " fatigue " in two different senses, and 
 it is important that a distinction be clearly made between 
 them. In the one sense the word means the diminution of 
 ivorking capacity due to work. In testing one's strength of 
 grip or of back a second test if made immediately shows 
 less work done than at the first test, and this is true 
 whether or not we are conscious of fatigue or of diminished 
 working power. If, however, a certain time be allowed 
 for rest, the second test will give as good results as tlie 
 first. 
 
 In the other sense the word refers to the feeling of fatigue 
 which frequently, though not always, accompanies the 
 diminution of working power. We may even " feel tired" 
 when we have been doing nothing, and conversely, under 
 the influence of excitement or other causes, we may experi- 
 ence no feeling of fatigue even when we are near the limit 
 of our working power. Often in an exciting game the 
 players do not know at the time that they are tired or even 
 
 55 
 
56 
 
 THE HUMAN MECHANISM 
 
 that their working power is lessened ; and stories are told 
 of soldiers in hasty retreat who feel that they must " drop in 
 their tracks " until the discharge of musketry close behind 
 stimulates them to move faster than ever. The report of 
 the rifles did not change the working power of the mus- 
 cles, but only removed for the time being the feeling of 
 weariness in the nervous system, which had interfered 
 with getting from these muscles their best work. 
 
 The consciousness of fatigue has its seat in the nervous 
 system, and its study must be postponed until we have 
 learned something of the physiology of the brain and 
 spinal cord. In the present chapter we are not immedi- 
 ately concerned with this side of the question, but rather 
 with the diminution of working power produced by work. 
 Such fatigue must be measured not by our sensations but 
 by the work accomplished, whether that work be physical 
 
 or mental. And as 
 we studied the physi- 
 ology of work in its 
 simplest form in a 
 single working or- 
 gan, such as a muscle 
 or a gland, so we can 
 best begin our study 
 of diminished work- 
 ing power or fatigue 
 in one of these same 
 organs, namely, the 
 skeletal muscle. 
 
 1. Fatigue of an 
 
 Isolated Muscle and 
 
 of a Muscle with 
 
 Intact Circulation. — The course of fatigue in a muscle 
 
 is best studied by causing the muscle to contract to its 
 
 utmost, at regular intervals of time, against the resistance 
 
 Fig 
 
 32. Diagram of apparatus for recording 
 successive muscular contractions 
 
FATIGUE AISTD KESTORATION 
 
 57 
 
 of a suitable spring. If now we record the height of each 
 contraction, we obtain a se- 
 ries which shows at once 
 the effect of the work on 
 the working power, i.e. the 
 course of fatigue. Fig. 32 
 gives a diagram of the ar- 
 rangement of such an ex- 
 periment with an isolated 
 muscle, i.e. a living muscle 
 detached from the rest of 
 the body. One tendon is 
 made fast in a rigid clamp, 
 while the other is attached 
 to the spring, which is 
 stretched by the contrac- 
 tion when the muscle is 
 stimulated. The length of 
 the line written by the 
 lever AB records what the 
 muscle is capable of doing 
 at the time; and if the 
 records of successive con- 
 tractions are made on the 
 smoked surface of a slowly 
 revolving di'um, as in the 
 figure, we have at once a 
 record of the course of 
 fatigue. 
 
 Such fatigue tracings 
 may also be taken from a 
 muscle within the body, 
 and hence witli its circu- 
 lation intact. Thus the 
 work of the biceps muscle 
 
 Fir.. :]3. Kecord of the successive con- 
 tractions of tlie flexor muscles of 
 tlie elbow joint 
 
 Sliowiiis tlie grarlual decrease in working 
 power to a fatigue level. The muscle 
 contracted once every three seconds 
 against the resistance of a strong 
 spring, which was stretched each time 
 as far as the strength of the muscle 
 permitted 
 
58 THE HUMAN MECHANISM 
 
 in flexing the arm at the elbow (Fig. 23) may be recorded 
 by instruments essentially similar to that used with the 
 excised muscle. And in Fig. 33 we have reproduced a 
 tracing of this kind. 
 
 It is evident that a continuous line joining the highest 
 points reached by the several contractions will represent 
 graphically the course of fatigue, and in Fig. 34 the line a 
 represents this so-called " curve of fatigue " in the experi- 
 ment whose results are given in Fig. 33. It falls off at 
 first rather rapidly, then more and more slowly, until at 
 last it becomes parallel with the base line. In other words, 
 the muscle in this case finally finds a constant level of 
 working power. This may be called the fatigue level. 
 
 The broken line b in Fig. 34 gives the result of a 
 fatigue tracing- with the isolated muscle. It will be seen 
 
 Fig. .34. Curves of fatigue 
 a, from a muscle with intact circulation; 6, from .an isolated muscle 
 
 that the fall in the heiglit of contraction continues until at 
 last the muscle no longer responds to stimulation. The 
 contrast thus brought out between the effect of work upon 
 muscles with and those without the circulation shows that 
 the circulation of the blood through the working organ in 
 some way maintains the working power. 
 
 The height of the fatigue level in the same muscle at 
 different times is very closely dependent on the rate at 
 
FATIGUE AI«rD EESTORATIOISr 59 
 
 which, the muscle works. Thus with a contraction every 
 four seconds instead of every three seconds the fatigue 
 level would be higher than in Fig. 33 ; with a contraction 
 every second it would be much lower. When the contrac- 
 tions come every nine or ten seconds there is usually no 
 falling off in the work done, the time between contractions 
 being sufficient for the complete recovery of working power. 
 
 This picture of fatigue hardly agrees with our feeling 
 of fatigue, for the decline of working power begins at 
 once, or at most after a very small number of contractions, 
 whereas we usually notice fatigue only after work has 
 gone on for a considerably longer time. One does not feel 
 tired from walking, for example, during tlie lirst ten or 
 twenty minutes of the walk. We need not discuss liere 
 just what makes us unconscious of the beginuings of 
 fatigue; but it is important to understand that whether 
 we are or are not aware of its presence, fatigue is the 
 invariable and immediate result of all muscular work. 
 
 Weariness is simply the conscious sensation of fatigue, 
 but fatigue is a physical condition of living cells and 
 organs. Moreover, its phenomena are b}' no means con- 
 fined to muscular work. When a gland is stimulated to 
 vigorous secretion a diminution is sooner or later noted in 
 the amount of the secretion, and there is good reason to 
 believe that nerve cells may also become tired from con- 
 tinued activity. Fatigue, then, in one word, is a natural 
 condition of an organ accompanying work, and we may 
 proceed to inquire into its exact cause. 
 
 2. Waste Products as a Cause of Fatigue. — When blood 
 which has been circulating through a fatigued muscle is 
 sent through a resting muscle, the resting muscle shows 
 signs of fatigue, even though it has itself done no work. 
 Apparently the blood has extracted from the working 
 muscle something which has the pow-er of lessening the 
 working capacity of a fresh muscle. 
 
60 THE HUMAN MECHANISM 
 
 The same tiling is illustrated by another experiment. 
 A muscle which is de23rived. of its circulation, e.g. by 
 clamping its arteries and veins, is fatigued by vigorous 
 work ; it is then found that although when left to itself 
 a slight recovery takes place, this recovery is much more 
 marked if we first pass through its blood vessels a weak 
 solution of salt. Here no food is supplied ; the salt 
 solution has only removed something from the fatigued 
 muscle, which in consequence of this treatment recovers 
 some of its working power. 
 
 Again, the mere exposure of a resting muscle to blood 
 containing sarcolactic acid, or to blood heavily charged 
 with carbon dioxide (COj), produces the condition of 
 fatigue. Now in the last chapter it has been shown that 
 both sarcolactic acid and carbon dioxide are waste prod- 
 ucts of muscular activity ; and these and other facts 
 have led to the view, now generally received, that the waste 
 products of the active organ interfere with the work 
 of the organ and so constitute one of the main causes of 
 fatigue. It is apparently for this reason that the injection of 
 an extract of worked muscle fatigues fresh muscle, for the 
 extract contains waste products. It is for the same reason 
 that washing out a fatigued muscle with salt solution pro- 
 duces partial recovery, for the waste products of activity 
 are in this waj' partially removed. We can also understand 
 why fatigue always accompanies vigorous work. Waste 
 products then necessarily accumulate and clog the living 
 mechanism, because they cannot be removed by the blood 
 as fast as they are formed by the muscle cells. No fatigue 
 occurs with only a single contraction every ten seconds or 
 more, because between contractions sufficient time is given 
 to insure the complete removal of wastes. 
 
 3. Loss of Fuel in the Working Muscle as a Cause of 
 Fatigue. — The blood, however, not only removes the 
 wastes but also brings new food and oxygen with which 
 
FATIGUE AND EESTOEATION 61 
 
 the muscle makes good the loss of explosive fuel ; and it 
 may well be — although it is not absolutely proved — that 
 recovery from fatigue depends upon both of these good 
 offices of the blood. We have certainly one well-estab- 
 lished cause of fatigue, namely, the presence of the waste 
 products of activity ; and we recognize the probability 
 that the depletion of explosive fuel may also contribute to 
 the result. But whether the first of these causes alone is 
 sufficient to explain it, or whether both work together, we 
 can understand that the maintenance of a good blood 
 supply is of the first necessity, and that undue fatigue 
 can be avoided only by working at a moderate rate. It is 
 an old and physiologically true saying that " it is the pace 
 that kills." 
 
 4. Explanation of the Fatigue Level. — In the experi- 
 ment with the isolated muscle no waste products were 
 removed, nor were new food and oxygen supplied ; hence 
 the wastes in the muscle increased with each contraction, 
 until at last their accumulation prevented all contraction. 
 In the normal muscle the wastes likewise accumulate for 
 a time ; and this is why the curve of work at first falls 
 (Fig. 33). It does not continue to fall, because as the 
 wastes within the muscle increase in amount the blood 
 carries more and more of them a^ay in a given time. The 
 quantity of waste removed thus continues to increase until 
 the same quantity is carried away from the muscle between 
 two contractions as the muscle produces with each con- 
 traction. When this happens no further accumulation of 
 waste is possible and the fatigue level is established. 
 
 5. General Fatigue resulting from Muscular Activity. 
 — Every one knows that after a day's tramp it is not simply 
 the worked muscles which are unfit for good work, but 
 that the brain, too, is tired, for hard mental work is then 
 difficult or well-nigh impossible ; and it is generally the 
 fact that long-continued muscular work fatigues the brain 
 
62 THE HUMAN MECHANISM 
 
 more than brain (mental) work itself. The obvious explana- 
 tion of this fact is that the waste products of muscular 
 activity have accumulated in the blood more rapidly than 
 the body can get rid of them, and so have fatigued the 
 other tissues, including the nerve cells of the brain, just 
 as the injection of the extract of a tired muscle lessens 
 the working power of a fresh muscle. No doubt these 
 same waste products may similarly fatigue gland cells ; 
 for experience seems to show that the secretion of digest- 
 ive juices is not so active when one is suffering from 
 muscular fatigue, and that it is not wise to eat heavy 
 meals when one is tired out. We can also understand why 
 long-continued, vigorous muscular action produces marked 
 fatigue in nerve cells and gland cells, while the activity 
 of the latter produces only inappreciable fatigue in the 
 muscles ; for the amount of chemical change and the pro- 
 duction of wastes is far greater in the case of muscular work 
 than in that of nervous or glandular activity. 
 
 6. The Analogy of the Engine. — In previous chapters 
 we have compared the living body with a machine or loco- 
 motive engine ; both do work, and both obtain the power 
 for work from the direct or indirect oxidation of food or 
 fuel. What we have now learned about fatigue suggests 
 an extension of the same comparison. Every locomotive 
 suffers impairment of its working power with use, and 
 special measures are taken to limit tliis impairment as 
 much as possible ; the gases and smoke are carried away 
 at once by the chimney or smokestack ; the furnace is 
 provided with a grate so that the ashes shall not accumu- 
 late and shut off the draft ; the bearings are oiled and 
 foreign matters removed; above all, as the consumption of 
 fuel goes on, the loss is made good by stoking. 
 
 The continuance of the work of the engine requires two 
 things, — fresh supplies of fuel and the removal of wastes. 
 Obviously the blood performs these same offices for the 
 
FATIGUE AND EESTOEATIOjST 63 
 
 cell. It supplies to the cell fuel (food) from the alimen- 
 tary canal and oxygen from the lungs, and it carries away 
 the waste. Provision is thus made to maintain the human 
 machine in working order and good condition during its 
 activity. If the blood flows too slowly through the muscle, 
 the same thing happens as in the locomotive when the fire- 
 man neglects to luke the fire or to put on new fuel ; the 
 efficiency both of the human engine and of the locomotive 
 may be impaired either by the undue accumulation of the 
 waste products of its own activity or by the neglect to 
 supply proper food or fuel. 
 
 7. The Cell a Living Machine. -. — The similarity of the 
 cell to the lifeless engine is more tlian a mere analogy. 
 The cell is not only like — it actually h — an engine, 
 machine, or mechanism, but it is also a liriuf/ mechanism. 
 This means that it possesses projjerties and })0wers not 
 shown by the locomotive. Both may be injured by over- 
 work or by accident, but only the living mechanism is 
 itself capable of repairing damage ; the locomotive must 
 be sent to the sliops and be repaired by work done upon 
 it by other machines ; if the boiler rusts, it must be taken 
 out and a new one put in ; if the wlieels wear unevenly, 
 they must be made true again by turning in a lathe, or 
 new ones must be substituted ; when the grate burns out 
 a new one must be put in its place. The living cell, on 
 the other hand, itself makes these repairs from the same 
 material that supplies its fuel. Indeed, the material for 
 the repair of the living cell is to some extent the same 
 thing as the fuel ; and when food is not supplied with 
 sufficient frequency this living machine actually consumes 
 its own substance for fuel, — as happens, for example, dur- 
 ing periods of starvation. Nothing of this kind, of course, 
 is seen in any locomotive. 
 
 These are merely suggestions of the striking diilerences 
 between the living engine and the locomotive. But we 
 
64 THE HUMAN MECHANISM 
 
 cannot pursue this subject further in an elementary work 
 like the present ; enough has been said to indicate the 
 difference, while the facts given in this and the preceding 
 chapter show that so far as the resemblances go, they are 
 complete. Both machines do work, both require power for 
 their work, both derive their power from the oxidation 
 of fuel, both are provided with special mechanisms which 
 transform energy into definite kinds of work, both are 
 clogged by the accumulation of waste products, and hence 
 both require care for their best efficiency. Yet the full 
 recognition of the fact that the cell is an engine, a 
 machine, a mechanism, should not make us forget that other 
 fact that it is more than this : that it is a livmg engine, a 
 living machine, a living mechanism, capable not only of 
 fatigue but also of self-repair ; and, perhaps most wonderful 
 of all, the oiganism which these cells compose is capable 
 of sensations of weariness in fatigue, and of refreshment 
 and new power after feeding and rest, — powers and sen- 
 sations utterly lacking in any lifeless machine however 
 perfect. 
 
CHAPTER VI 
 THE INTERDEPENDENCE OF ORGANS AND OF CELLS 
 
 1. Are Cellular "Waste Products" necessarily Harmful ? 
 
 — We have now learned that the active living cells of the 
 body are the seat of chemical changes which produce new 
 substances ; that the accumulation of these products of 
 activity tends to limit the working power of the cells in 
 which they are produced, and may even depress the activity 
 of other cells to which they are carried by the blood. In 
 the case of the skeletal muscles we have spoken of the car- 
 bon dioxide, the sarcolactic acid, etc., as "waste products," 
 meaning thereby that they are incapable of serving as 
 sources of power for the work of the muscle ; and this 
 term, toeether with the fact that they constitute one cause 
 of fatigue, is apt to mislead us into supposing that they 
 can be of no further use to the body, or, even more, that 
 they are necessarily harmful and that their presence in the 
 blood is objectionable. 
 
 These conclusions, however, do not necessarily follow 
 from the facts. It does not even follow that a substance 
 which produces fatigue is for that reason undesirable. 
 Most adults can recall times when because of long-con- 
 tinued application to mental work, or because of worry or 
 other nervous strain, they have become overexcitable and 
 restless, and have been unable to obtain the sleep of which 
 the body as a whole stands in need. At such times sleep is 
 often best secured by producing general fatigue through 
 muscular work. The waste products, by their very act of 
 fatiguing the overexcited nerve cells, may be of service to 
 
66 THE HUMAIT MECHAN'ISM 
 
 the body as a whole. And it is probably true that not only 
 in such abnormal conditions but also in the daily conduct 
 of life the fatigue of moderate muscular activity contrib- 
 utes its share toward inducing healthful and refreshing 
 slumber. 
 
 Thus far we have considered the chemical activities of 
 each organ as contributing to the work of the organ in 
 which they occur, and, because of the accumulation of waste 
 products, as the occasional cause of undue interference with 
 efficient activity, both in the working organ and elsewhere. 
 And yet the familiar case which we have just cited sug- 
 gests another view of the matter. The products of the 
 chemical activity of one organ may be of service to other 
 organs, and so to the body as a whole ; and while their 
 too rapid accumulation in the blood is undesirable, their 
 presence in moderate amounts may be beneficial, and 
 may contribute to the normal environment of the cells of 
 the body. 
 
 It must never be forgotten that the secretions of the sali- 
 vary glands, the stomach, and the pancreas as truly contain 
 products of cellular activity as does the blood which leaves 
 a working muscle. It is also probable, although it has not 
 yet been directly proved by experiment, that if such a secre- 
 tion were to accumulate within and around the gland cells, 
 it would limit their activity and produce a true fatigue. 
 The constituents of the saliva, for example, are waste prod- 
 ucts of the gland, in the sense that they do not serve for 
 the manufactvire of new saliva, but we do not call them 
 waste products chiefly because we know they are still use- 
 ful to the body. 
 
 2. The Thyroid Gland. — This view of the case is strik- 
 ingly emphasized in the physiology of the thyroid gland, — 
 a small organ in the neck, the two chief lobes of which 
 lie alongside the trachea. For a long time its use was 
 not understood, and at times it was even supposed that 
 
INTEKDEPENDJCNCE OF OEGAifS 
 
 67 
 
 it played an unimportant part in tlie life of the body as 
 a whole. It has been found, however, Ijy experiment 
 that removal of the thyroid is followed by a disease in 
 all respects similar to one 
 which had long been ob- 
 served in human beings, and 
 especially in children ; and 
 this fact sugafested that the 
 disease is due to the failure 
 of the thyroid to perform 
 its normal functions. 
 
 The subject was further 
 cleared up by the discovery 
 that after the removal of 
 the thyroid in a lower ani- 
 mal the disease in question 
 could be prevented by feed- 
 ing the animal thyroids or 
 even by giving to it a cer- 
 tain substance extracted 
 from them. Evidentl}' the 
 thyroid manufactures and 
 discharges into the blood a 
 
 peculiar substance necessary to the healthy life of the cells 
 of the body ; and when the gland fails to manufacture this 
 substance it can still be supplied artilicially by introducing 
 it into the blood by absorption from the alimentary canal. 
 
 3. Internal Secretions. — In our study of secretion in 
 Chapter IV (p. 44) we dealt only with glands which dis- 
 charge their principal products through a duct into some 
 part of the alimentarj^ canal ; such glands are the salivary 
 glands, the pancreas, and the liver. Other glands send ducts 
 to the surface of the body, — for example, the sweat glands 
 which discharge perspiration upon the skin, and the lachry- 
 mal glands -which discharge the tears on the eyeball. In the 
 
 Fi. 
 
 Cross section of the thyroia 
 rjland 
 
 The < 
 wl 
 
 ,h 
 
 st^iTete into tlie closed sacs, 
 tliev surround, the internal 
 secretion, "ix'hich then i)asses ont be- 
 tween the cells into the lyriiph spaces 
 of the connecti\'e tissue 
 
68 THE HUMAN MECHANISM 
 
 case of the thyroid, on the other hand, we have an example 
 of an organ which, like those just mentioned, manufactures 
 a special substance from the blood, but, having no duct, 
 contributes the products of its manufacture to the blood, for 
 • the use of other cells. This process is spoken of as internal 
 secretion, to distinguish it from ordinary secretion, in which 
 case something is discharged on a free surface like the skin, 
 or into the alimentary canal, the nasal cavity, or the air 
 passages. 
 
 4. Other Examples of Internal Secretion. — Certain other 
 organs similarly manufacture and contribute to the blood 
 material for use elsewhere. Lying immediately above the 
 kidney are two small bodies, the adrenal or suprarenal 
 bodies, which, like the thyroids, were until recently supposed 
 to be of minor importance. It is now known that these 
 also contribute to the blood an internal secretion which is 
 absolutely indispensable for the healthy life of other organs. 
 
 A still more remarkable discovery has shown that the 
 pancreas not only manufactures an important digestive 
 juice (pancreatic juice) which it discharges into the intes- 
 tine through its duct (pancreatic duct, see Fig. 50), but 
 also produces another substance which is necessary in 
 order that other organs may use the sugar which is in 
 their food. Here we have an example of an organ which 
 produces both an ordinary and an internal secretion, and 
 the same thing seems to be true of the kidney, as it cer- 
 tainly is of the liver. 
 
 Thus, through the medium of the blood the chemical 
 activity of one organ may affect the life of other organs 
 favorably or unfavorably. All the cells of the body help 
 to make the blood what it is, many of them contributing 
 to it something useful or even necessary to other cells. 
 The work of the body is not merely the sum total of the 
 work of its separate cells, each working for itself alone 
 and performing a single function. Between the cells an 
 
INTEEDEPEJSTDENGE OF ORGANS 69 
 
 exchange of products often takes place, so that cells become 
 both serviceable to and dependent upon one another for the 
 material needed to carry out their own special chemical 
 activities. And what is true of cells is no less true of 
 organs; these also are interdependent, ministering to one 
 another. 
 
 5. The Cooperation of Cells and Organs, and the Net 
 Result. — In the foregoing paragraphs we have repeatedly 
 spoken of the work of the cells (and organs) as if this 
 work were the sole end and object of their existence. If, 
 however, we turn back to Chapter IV and recall its prin- 
 cipal lesson, namely, that the net result (resultant) of the 
 work of the individual cells and organs of the body is the 
 work of the body itself, then we shall perhaps begin to 
 realize that no matter how important to the cell or to its 
 neighbors individual cell woik may be, this is even more 
 important as an indispensable constituent in the wotVl of 
 the hodj as a whole. Any one can observe the human 
 mechanism actively working at home or in field or factory ; 
 or running, lea^jing, or sleeping ; or engaged in professions 
 or business or trade ; but only the physiologist realizes 
 how these most various acts are merel}' the net result of 
 the lives and activities of myriads of individual cells, — lives 
 in which the cells }iot only minister and are ministered unto, 
 but also work toffether both for mutual benefit and for the 
 benefit of the body as a whole. 
 
 Least of all do we commonly realize that it is not merely 
 the operation, but the cooperation, — not merely the work, 
 but the combined, and especially the harmonious or orderly, 
 work of the myriad cells of the body, united as they are 
 into one grand army, — that underlies the effective work 
 of the human mechanism as a whole. To the study of the 
 mechanism of this cooperation we may now proceed. 
 
CHAPTER VII 
 
 THE ADJUSTMENT OR COORDINATION OF THE WORK OF 
 ORGANS AND CELLS 
 
 A great physiologist once said, " Science is not a body 
 of facts ; it is the explanation of facts." Some of the 
 most important chapters of science are those which seek to 
 explain facts so well known and obvious that we are apt 
 to forget that they need explanation. When anything irri- 
 tates the lining of the nasal cavity we sneeze ; when it 
 irritates the larynx we cough ; when it irritates the exposed 
 surface of the eyeball we wink. These three facts are 
 well enough known ; but it is safe to say that any one 
 considering the matter for the first time would find it dif- 
 ficult to explain how it comes about that anything going 
 " down the wrong way " does not make us sneeze or wink, 
 but sets us to coughing. The answer to the general ques- 
 tion thus raised is the subject of this chapter, which con- 
 siders the adjustment of the work of the individual cells 
 and organs of the body, each to do its work at the proper 
 time, and so to play its due part in the work of the organism 
 as a whole. 
 
 The more we think of it, the more wonderful does this 
 fact of adjustment appear. The millions of living cells are 
 in a way individual units, and communities of individuals 
 do not invariably work together. Let us compare the human 
 body in this respect with bodies or groups of men or boys. 
 In a game of football each team is a body of eleven indi- 
 viduals, and each individual is assigned to a definite posi- 
 tion to do definite things as occasion arises. Theoretically, 
 
 70 
 
COOEDINATION 
 
 71 
 
 under given conditions of the game it is the work, or func- 
 tion, of the " left tackle " to prevent a certain player of the 
 opposing side from making a certain play. But there is 
 always a doubt whether he will do this thing, or " lose his 
 head " and do something else, leaving his man free to do 
 what he pleases. In the latter case that organism which 
 we call a football eleven would act very much as the human 
 organism would act if it were to wink and not cough when 
 a foreign body lodges on the lining membrane of the larynx. 
 
 Evidently we have something here to explain. Why 
 are the actions of the body purposeful, that is, adapted to 
 accomplish the right thing at the proper time? and in the 
 more complicated actions, how is the work of the different 
 units — the organs and tl^e cells — adjusted, or coordinated ; 
 that is to say, how is each one made to do its [)roper share of 
 the work ? Let us begin with the study of a very simple 
 action, — that of winking. 
 
 1. Winking is caused by the contraction of muscle fillers 
 which run transversely across the eyelid in a curved course. 
 As they are attached most firmly at the regions A and /.' 
 (Fig. 36), their shorten- 
 ing- straightens their 
 arched course and so 
 brings the two edges of 
 the eyelid into contact. 
 The work of this muscle 
 is obviously purposeful, 
 for the wink takes place 
 when the eyeball needs 
 protection ; it is also co- 
 ordinated, since the act is executed by a number of fibers 
 working together, for if only those of the lower eyelid 
 were to contract the lids could not be closed. 
 
 The muscle fibers which work together to produce the 
 wink do not originate their own activity. They merely do 
 
 Fig. 36. The muscular mechanism of 
 winkhi tr 
 
72 
 
 THE HUMAN MECHANISM 
 
 what they are stimulated to do by the nervous impulse, 
 which acts upon the muscular fuel substance somewhat as 
 a fuse acts upon a charge of gunpowder. Even the amount 
 of contraction is determined by the strength of the nervous 
 impulse, a strong impulse producing greater contraction 
 than a weak impulse. In health the muscle fibers are the 
 
 
 Fig. 37. Cross section of a nerve 
 
 Showing Ave bundles of nerve fibers bound together by connective tissue con- 
 taining a few blood vessels. On the right are shown four fibers more highly 
 magnified, the dark center being the axon, around wliich is the "white or 
 fatty sheath, both axon and fatty sheath being inclosed within the fine 
 membrane, tlie neurilemma. Compare Fig. 38 
 
 obedient servants of the nerves, and if they act in a pur- 
 poseful and coordinated manner, it is because the nerves 
 stimulate them to act in this way. The explanation of pur- 
 poseful and coordinated action must therefore be sought 
 not in the muscles but, behind these, in the nervous system, 
 to the study of which we now turn. 
 
 2. Structure of a Nerve. — A nerve, like a muscle, may 
 be separated into long fibers (Fig. 38) which are bound to- 
 gether by connective tissue containing blood vessels, lymph 
 
COORDIisATION 
 
 73 
 
 spaces, and lymphatics. The 7ierve fiher,v/hich is the essen- 
 tialpart of the nerve, just as the muscle fiber is of the muscle, 
 differs somewhat in structure in different nerves ; it gener- 
 ally consists of a central threadlike 
 core surrounded by a fatty sheath, 
 the latter being, therefore, shaped 
 like a hollow cylinder, — which, 
 however, is interrupted at intervals 
 of about one millimeter, — and both 
 of these are enveloped in a delicate 
 membrane comparable to the sarco- 
 lemma of the muscle fiber. Such 
 
 fibers are from about ^q^q q- 
 
 to 1 
 
 ^" To "00 
 
 > Tour nerve fibers 
 (highly magnified) 
 
 of an inch in diameter (compare the 
 diameter of a muscle fiber, p. 35). 
 
 There are, however, nerve fibers i?, node of Ranvier at which 
 which have no fatty sheath, and tJie fatty sheath is discou- 
 others which are destitute of mem- 
 brane. The essential part of the fiber is the threadlike por- 
 tion in the center ; this is never absent from nerves and 
 is known as the rt.ro?;,, or axis ci/liiider. 
 
 3. The Axon of a Nerve Fiber is a Branch of a Nerve 
 Cell. — By suitable methods these axons may be traced 
 along the nerve of which they form part, and even into the 
 brain and spinal cord ; it is then found that they pursue 
 an uninterrupted course and ultimately become continuous 
 with the cytoplasm of a nerve cell. Nerve cells are found 
 in the brain, in the spinal cord, in enlargements (ganglia) on 
 certain nerves, and even alone in the connective tissue of 
 many organs of the body, as the heart, the intestine, etc. 
 By far the greater number are in the brain and spinal cord, 
 and in some cases the axons to which they give rise are of 
 very considerable length; those of the muscles of the foot, 
 for example, reach from cells in the sacral region of the 
 spinal cord to the extremity of the foot. Such fibers would 
 
74 
 
 THE HUMAN MECHANISM 
 
 be over a yard long and less than yoVo °^ ^^ ^^^^ wide, 
 and we may regard the cell whose main portion is in the 
 sacral cord as sending out a branch, or process, from this 
 region to the foot. 
 
 Furthermore, recent investigations have led to the gener- 
 ally accepted conclusion that each axon is a part of only 
 
 Fig. 39. Four nerve cells 
 
 A and C, from the cerebellum ; B, from the gray matter of the spinal cord ; 
 D, from the cerebrum; a, the axon. The cells A andD are stained so that 
 the main body and the dendrites (p. 76) are a uniform black ; B and C, so 
 as to show the nucleus and cytoplasm 
 
 one nerve cell; a single cell may give off more than one 
 axon, but the axon is never connected with more than one 
 nerve cell. Of these cells and of their connections with 
 nerve fibers we can get a more definite picture by an exam- 
 ination of the structure of the spinal cord. 
 
 4. Structure of the Spinal Cord. — When the vertebral 
 canal is opened a whitish cord is found within it, — the 
 
COOEDIXATION 
 
 75 
 
 Do; sal Root 
 
 Dorsal 
 Ganglion 
 
 spinal cord, — from each side of which arise thirty-one pairs 
 of nerves, or, in general, one pair for each vertebra. One 
 nerve of each pair arises on the ventral side of the cord, 
 the other on the dorsal side. These nerves are known as 
 the ventral and dorsal nerve roots ^ respectively. On the 
 dorsal nerve root some 
 distance from the cord 
 there is a slight enlarge- 
 ment, or ganglion. Just 
 outside this ganglion 
 the two roots unite, 
 and from their union 
 nerves pass to the skin, 
 the muscles, the blood 
 vessels, the viscera, etc. DotsoJ 
 
 The spinal cord itself 
 in cross section shows 
 a darker central core, 
 known as the gray mat- 
 ter, surrounded by an 
 outer lighter portion, 
 the u'hite matter. The 
 white matter consists essentially of nerve fibers which run 
 lengthwise of the cord and here and there send branches 
 into the gray matter ; it may be regarded as a large nerve. 
 The gray matter, on the other hand, contains a mesh of 
 fibers, and in addition numerous nerve cells. There is the 
 same difference everywhere between the white and gray 
 matter of the nervous system ; the arrangement in the 
 brain is not so simple as in the cord, but here also the 
 white matter consists of fibers running from one part of 
 the nervous system to another, while the masses of gray 
 matter always include collections of nerve cells. 
 
 1 The older anatomical terms, and those even to-day more generally 
 used, are anterior and posterior instead of ventral and dorsal. 
 
 Dorsal Root 
 
 GancjV< 
 
 Fig. to. The origin of the dorsal and 
 ventral nerve roots of a segment of 
 the spinal cord 
 
76 THE HUMAN MECHANISM 
 
 5. Fibers of the Ventral, or Anterior, Nerve Root. — 
 
 These fibers may be traced into the spinal cord. It is then 
 found that the nerve cells from which they arise lie in the 
 gray matter in the immediate neighborhood of the root to 
 which they belong ; i.e. the fibers of the roots do not come 
 from higher or lower parts of the cord, or from the brain. 
 It has also been found that when they are stimulated 
 they throw muscles into contraction and produce effects 
 on the blood vessels and glands, but they do not give 
 rise to sensations, nor produce other effects in the cord 
 itself. In other words, the fibers of the ventral root conduct 
 impulses from the cells of the spinal cord outivard ; they 
 do not conduct impulses from outside into the spinal cord. 
 Hence they are known as efferent fibers (Latin, e.c, out of ; 
 ferre, to carry). 
 
 The nerve cells from which these fibers arise consist of 
 a mass of cytoplasm around the nucleus and of one or 
 more outgrowths of this cytoplasm, usually more or less 
 branched. These outgrowths, or processes, divide and sub- 
 divide, ultimately forming in the gray matter exceedingly 
 fine terminal branches like those of a tree in the air. Such 
 processes are known as dendrites (Greek, dendron, a tree). 
 The nerve cells in question have numerous dendritic proc- 
 esses ; in other nerve cells there may be but one, and 
 still others possess no dendritic processes at all. In all 
 cases the general appearance of the cell depends largely 
 upon the number and manner of branching of these proc- 
 esses. Thus it happens that nerve cells differ from one 
 another in appearance just as a Lombardy poplar, an oak, 
 an elm, and a maple differ, although all show the funda- 
 mental characteristics of a tree (Fig. 39). 
 
 In subsequent portions of this work it is unnecessary 
 for us to go into the details of the form of the nerve 
 cells to any extent ; the student need only understand 
 henceforward that nerve cells consist of a central mass of 
 
COOEDIKATION 
 
 77 
 
 nucleated cytoplasm from which proceed outgrowths, or 
 processes, which are of two kinds, — (1) those which become 
 axons of nerve fibers, and whicli form an essential part of 
 all nerve cells ; and (2) the dendrites, which are usually but 
 not always present. The whole structure, including the 
 central cell body with its dendrites and axons, is an ana- 
 tomical unit, — a cell. To this entire cell the term neurone 
 is given. The neurone is the cellular unit of the nervous 
 
 Fig. 41. Semidiagraramatic longitudinal section of a ganglion 
 of the dorsal (posterior) root 
 
 system, just as the muscle fiber is the cellular unit of the 
 muscle, and the gland cell of the gland. 
 
 6. Fibers of the Dorsal, or Posterior, Roots. — The ven- 
 tral roots, as we have seen, are entirely efferent in func- 
 tion ; that is, they conduct impulses only away from the 
 spinal cord. The dorsal, or posterior, roots, on the other 
 hand, are found to be essentially afferent (Latin, ad, to; 
 ferre, to carry), i.e. they carry impulses from outside toward 
 and into the spinal cord. This is shown by the fact that when 
 
78 THE HUMAN MECHANISM 
 
 these roots are destroyed by disease muscles can still be 
 thrown into contraction, glands will still secrete, etc., — i.e. 
 there is no interference with efferent impulses, — but no 
 sensations are received from the part of the body to which 
 these nerves are distributed ; pinching the skin is not felt ; 
 the flesh may be burned and its owner be entirely uncon- 
 scious of it. Since these results never follow destruction 
 of the ventral roots, we must conclude that impulses enter 
 the cord solely by the dorsal roots precisely as they leave 
 the cord solely by the ventral roots. 
 
 It has been stated above (p. 75) that there is a ganglion 
 on the dorsal root. Microscopic study of this ganglion 
 shows that the fibers of the dorsal root pass through it, and 
 that each fiber gives off at right angles to itself a branch 
 which becomes continuous with a pear-shaped nerve cell 
 of the ganglion. These cells have no other processes. We 
 may express the relation between the pear-shaped cells of 
 the ganglion and the fibers of the dorsal root by saying 
 that the single axon from the main cell body divides into 
 two in the ganglion, one branch passing outward to the 
 periphery, and the other centrally into the spinal cord 
 (Fig. 41). 
 
 7. Endings of the Peripheral Branches of the Neurones 
 of the Dorsal Root in Sense Organs. — The peripheral 
 branch ultimately ends in some " sense organ," one of the 
 ijiost important of which, so far as the spinal nerves are 
 concerned, is the skin. The eye, the ear, the nose, the 
 mouth, are examples of other sense organs, and they all 
 contain the peripheral endings of afferent neurones. Each 
 is sensitive to some special influence from without, as the 
 eye to light, the ear to sound, etc. ; and when stimulated 
 they start nerve impulses moving inwards along the nerves 
 toward the brain or cord. 
 
 8. Ending in the Spinal Cord of the Central Branch of 
 the Neurones of the Dorsal Root The other or central 
 
COORDINATION 79 
 
 branch passes into the spmal cord. It does not, however, 
 like the neurones of the ventral root, there become con- 
 tinuous vcith the nerve cells of the gray matter,^ but divides, 
 on entering the cord, into an ascending and a descending 
 branch ( Fig. 42), each of which runs for a longer or 
 shorter distance in the white matter of the cord. Indeed, 
 many of the ascending branches extend as far anteriorly 
 as the lower parts of the brain. As shown in the figure, 
 these branches give off at right angles to themselves sub- 
 branches, the collaterals, each of which enters the gray 
 matter and ends there by breaking up into a tuft of 
 extremely fine fibrils, the synapse. The synapse is in close 
 proximity to and possibly in a kind of anatomical con- 
 tinuity with the dendrites, or the main body of nerve cell 
 of the gray matter. Each afferent neurone, then, is a cell 
 whose main body is in the ganglion of the dorsal root, 
 and whose branches, or arms, reach out, one of them to a 
 peripheral sense organ and the other to the gray matter 
 of the spinal cord and brain, where they end in synapses. 
 By means of the synapses the afferent neurone excites or 
 stimulates other neurones. 
 
 9. Anatomical Relation of Afferent to Efferent Neurones. 
 — We may now put together what wu have learned about the 
 neurones of the ventral and those of the dorsal root ; we then 
 obtain a plan like that shown in Fig. 42, and such, in prin- 
 ciple at least, represents the manner in which the afferent 
 neurone is brought into relation with efferent neurones. 
 
 Afferent and efferent fibers enter and leave portions of 
 the brain in much the same way, although the separation 
 into ventral and dorsal roots is not obvious. We may 
 therefore take the above scheme as typical of the relation 
 between these two kinds of neurones, — those of the brain 
 as well as those of the cord. 
 
 1 It is, as has already been pointed out (p. 78), part of a nerve cell 
 in the ganglion of the dorsal root. 
 
80 
 
 THE HUMAN MECHANISM 
 
 10. Application of These Facts of Structure in the Ex- 
 planation of Purposeful and Coordinated Action. — The 
 
 diagram in Fig. 43 readily explains why the sudden ap- 
 pearance of an object in front of the eye causes us to 
 wink and not cough; that is to say, it explains the pur- 
 poseful character of this so-called reflex action. The for- 
 mation of the image of the object on the retina, a sense 
 
 '^^^ 
 
 A B 
 
 Fio. 42. Relation of afferent (af) to efferent (e/) neurones of the 
 spinal cord 
 
 In A the single afferent neurone branches into six collaterals, each of which 
 ends in a synapse around an efferent cell. In B the connection is made 
 through the agency of the cell a, as explained in Section 13 
 
 organ, starts impulses along the fibers of the afferent optic 
 nerve ; these libers extend into the brain, and their synap- 
 ses end around and stimulate those efferent nerve cells 
 which stimulate the muscles of the eyelid. The action is 
 purposeful because the fibers of the optic nerve end 
 around these cells, and not around those which, for exam- 
 ple, innervate ^ the muscles which open the mouth or flex 
 the finger (Fig. 43). 
 
 1 i.e. supply with nerve fibers. 
 
COOEDINATION 
 
 81 
 
 Our diagram also gives the basis of coordination, — 
 the comljination of the work of different muscle fibers in 
 orderly harmonious action. The system of collaterals on 
 the central branch of the 
 
 afferent neurone is obviously 
 a mechanism to combine the 
 action of the efferent neu- 
 rones in this way. The dia- 
 gram also gives a clew, at 
 least, to the explanation of 
 another element of coordi- 
 nation : when two or more 
 muscles work together to ac- 
 complish a given act, one of 
 the muscles usually works 
 harder than another ; not 
 only must they work to- 
 gether, but the amomit of 
 force exerted by each must 
 be adjusted to the needs of 
 
 Fii;. 43. Diagram of the nervous 
 inechanism bywliichawink is pro- 
 duced by the sudden appearance 
 of an object in front of the eye 
 
 r, afferent ni'uroni' of tlie optic nerve; 
 )n, III', III", III'", efferent neurones 
 tu the muscles (it tlic eyelid 
 
 the movement as a whole. This adjustment is most proba- 
 bly effected by differences in the connection of the synapses 
 with their cells; thus those muscles which contract most 
 forcibly are innervated by neurones whose dendrites and 
 ittain cell body come into more intimate contact with the 
 synapses of the afferent neurone ; or the number of fibrils 
 of the synapse may be greater in their case than in the 
 others. These, iiowever, are only possibilities ; the whole 
 subject requires further elucidation. 
 
 11. Definition of Reflex Action. — An action such as we 
 have just been studying is known as a reflex ^ action. By 
 
 1 The word literally suggests the idea of reflection from the afferent to 
 the efferent neurones, as light is reflected from a surface; but the stu- 
 dent has already learned enough to understand that efferent impulses are 
 something more than mere mechanical reflections, or rebounds, of afferent 
 impulses. 
 
82 
 
 THE HUMAN MECHANISM 
 
 this we mean an action called forth hy the more or less 
 direct action of afferent wpon efferent neurones, and without 
 the intervention of the will. The afferent neurone may be 
 stimulated by some external agent, such as light, heat, 
 
 sound, pressure, etc., 
 or by some condition 
 within the body it- 
 self, as when dis- 
 eased or abnormal 
 conditions of the 
 stomach or some 
 other organ induce 
 vomiting. 
 
 It is a common 
 error to suppose that 
 all actions which are 
 not called forth by 
 the will are reflex. 
 The essential feature 
 of a true reflex is the 
 more or less direct 
 
 Fig. 44. Diagram of the nervous mechanism 
 represented in Fig. 43, with the addition 
 of the neurone b (see Section 12) 
 
 action of the afferent impulses on efferent neurones, and 
 not merely its nonvolitional character. There are, in fact, 
 involuntary actions in which the efferent neurones are 
 directly stimuhited not by afferent neurones but by the 
 condition of the blood, or in other ways. Such actions 
 are not reflex, though they may be either involuntary 
 or unconscious, or both. They are known, in general, as 
 automatic actions, and we shall meet examples of them as 
 we proceed with the study of the various functions of 
 the body. 
 
 12. Actions resulting from Stimulation by the Will. — 
 A wink is not always a reflex action. We can wink " on 
 purpose," or, otherwise expressed, a wink may be called 
 forth by the will, and entirely apart from the sudden 
 
COORDINATION 
 
 83 
 
 appearance of some object in front of the eye. Here the 
 muscles of the eyelid act in exactly the same manner as in 
 a reflex wink, which means that they are stimulated in 
 the same way by the 
 same efferent neurones. 
 Thus far the mechanism 
 is the same in the two 
 cases, but the source of 
 stimulation of the effer- 
 ent neurones must be 
 different. 
 
 In later chapters of 
 this book we shall brins: 
 forward evidence to 
 show that the exercise 
 of the will (volition) re- 
 quires the cooperation 
 of the highest portion 
 of the brain or cerebrum. 
 Nerve cells in the gray 
 matter of the cerebrum send off axons which pass down- 
 ward to those jjortions of the brain and spinal cord from 
 which the motor or eft'erent neurones arise ; ^'^■ith tlie neu- 
 rones of these nerves they make exactly the same kind of 
 connections (collaterals and synapses) as are made by the 
 afferent fibers from the retina which excite the rellex (see 
 Fig. 44, in which h is the cerebral neurone). 
 
 The collaterals and synapses of the cerebral neurone 
 (which, it will be observed, is entirely confined to the cen- 
 tral nervous system) simply duplicate those of the afferent 
 neurone; hence the two neurones produce the same result. 
 
 There is, however, still a third way in which winking 
 may be stimulated. When the cornea of the eye begins to 
 dry, a reflex wink spreads tears over the eyeball. In this 
 case we have to deal with a second reflex, the afferent 
 
 Fill. 45. The nervous iiieelianisni slifnvn 
 in Fig. 44, with the ailditioii nf the 
 afferent neurone f, from the cornea 
 (.see Section 12) 
 
84 
 
 TPIE HUMAN MECHANISM 
 
 neurones being not those in the optic nerve, but those in 
 what is known as the trigeminal, the sensory nerve of the 
 cornea. Our scheme thus becomes that shown in Fig. 45. 
 13. The "Master" Neurone. — The multiplication of 
 collaterals and arborizations which this scheme involves 
 would seem to be largely avoided by the presence of a 
 
 third neurone between 
 those which stimulate 
 the action and the effer- 
 ent neurones which 
 directly act on the mus- 
 cles (Fig. 46). 
 
 In this way, when a 
 wink is produced, 
 whether from the cere- 
 brum or from the retina 
 or from the cornea, the 
 single cell a is stimu- 
 lated; and this in turn 
 stimulates the groups 
 of efferent neurones 
 which immediately innervate the muscles of the eyelids. 
 Many of the nerve fibers of the cord and brain belong to 
 neurones which perform the same function as that attrib- 
 uted to the cell a in our diagram. They are entirely con- 
 fined to the l)rain or cord, and group together those 
 efferent cells which by working together produce a coor- 
 dinated action. 
 
 The organization of the nervous system is, in fact, much 
 like that of a large manufacturing establishment. The 
 nerve cells which send axons to the muscles, glands, 
 blood vessels, etc., may be compared with the operatives, each 
 with his special task to perform ; over these are foremen, 
 or " l)0sses," from whom they take their orders, or, in 
 physiological language, who stimulate them to do their 
 
 Fig. 40. The "master" neurone 
 
COORDINATION 85 
 
 work, and who would correspond to cells like a in Fig. 46. 
 The foremen in turn receive orders, now from one depart- 
 ment of the establishment, now from another, as the work 
 of their operatives is needed in making one or the other of 
 the products offered for sale. So the " master " neurones 
 receive stimuli from the brain, or from afferent nerves, as 
 the needs or the desires of the organism as a whole require 
 their activity. The comparison is instructive and may 
 easily be carried out in greater detail by the student 
 himself. 
 
 By this time the student has, no doulit, made the dis- 
 covery that a mere wink is after all an extremely comjjlex 
 event; and yet winking is very simple compared with 
 many of our actions, such, for example, as throwing a 
 stone. Here not only muscles wlii('li produce motion at 
 the shoulder, elbow, wrist, and linger joints are called into 
 play, but also muscles which maintain tlic erect position 
 and balance of the body as a -whole. Tlie entire mechan- 
 ism involved bafties the imaginatiim to cunceive : and yet 
 any bo}' can perform tlie act. He can do it, ho^^•ever, 
 because hi.s motor neurones are grouped together into a 
 perfectly well-organized army whicli executes at once the 
 bidding of its commander-in-chief, — tlie will. 
 
 We have given in this chapter only the merest glimpse 
 into the com^ilexity of one part of the wonderful human 
 mechanism. No watch, no macliine which man has ever 
 invented or constructed can for a moment compare with 
 this living machine in complexity or in perfection. Yet, 
 like all machines, this one can l)e abused, it can get out of 
 order, it can even break down. And we have already 
 learned enough to understand \\liy this is so. Some neu- 
 rones may be injured by overwork, or may degeneiate 
 from disuse ; indulgence in stimulants or narcotics may 
 poison the governing nerve cells ; above all, constant fail- 
 ure to lead a normal life may deprive these precious cells 
 
86 THE HUMAN MECHANISM: 
 
 of their sole means of repair. The human body is a machine 
 designed for use, even for hard use, and it thrives upon 
 right use; but it is a machine too delicate and too com- 
 plex to be abused with impunity. 
 
 When one thinks of the hundreds, perhaps thousands, of 
 movements which the body makes, and of the combination 
 of these movements into definite actions or work, and then 
 reflects that tire muscle fibers which execute any movement 
 are thrown into orderly contraction by nerve cells which 
 are themselves commanded by higher nerve cells ; that 
 these in turn are marshaled, as it were, by still higher cells 
 when the separate movements they evoke are to be com- 
 bined into a still more complicated action, — one begins to 
 appreciate the complexity of the organization of the nervous 
 system. The number of the nerve cells is measured by 
 hundreds of thousands, and their efficiency in directing the 
 working organs of the body, so as to meet the demands of 
 life, depends not only upon the integrity of the neurones 
 but also upon the perfection of their organizations, i.e. 
 their grouping into squads, comjjanies, regiments, brigades, 
 divisions, and corps, ready to yield instant and obedient 
 response to the command of the higher officers of the will, 
 or to the signals of those pickets, — the sense organs and 
 their afferent neurones, — which everywhere guard the out- 
 posts and give information of the need for action. 
 
 Moreover, this army of neurones, like any other army, 
 becomes efficient by work, l)y drilling, by practice, even by 
 battle. Like the soldiers of a regular army, the neurones 
 may be overworked, and the efficiency of the military body 
 may suffer thereby ; but they may also work too little ; the 
 perfection of their development, of their organization, de- 
 pends on the practice they get with reasonable activity. 
 To this point we shall return ; but meantime the student 
 can safely make the application for himself. Such com- 
 parison and such application are not only instructive but 
 
COORDINATION 87 
 
 intensely practical in their bearing upon the affairs of 
 everyday life, — upon that right conduct of life which is 
 the first duty of every man, every woman, every child. 
 
 14. The Activity of the Individual Neurones. — We have 
 thus far emphasized the grouping of the neurones and tlieir 
 organization into an army, and have said but little of what 
 takes place in the individual neurone during activity. We 
 have once or twice, to be sure, spoken of the discharge of 
 a nervous impulse from the cell, and the j^assage of a nerv- 
 ous impulse along the fiber, but have avoided any discus- 
 sion as to wliat these nervous actions really are. To go into 
 these matters in detail would take us too far afield. We 
 may simply say that there is every reason for believing that 
 the activity of the nerve cell, like that of the muscle fiber, 
 involves chemical changes in the cell, — a consumption of 
 stored material and the production of carbon dioxide, water, 
 acids, etc. It has been shown, in fact, that tlie nucleus and 
 cytoplasm actually change their form and ajjpearance after 
 work, somewhat as gland cells do after prolonged activity. 
 The general phj^siology of the individual nerve cells would 
 thus be similar to that of the individual muscle fibei's and 
 gland cells described in Clhapter lA^. 
 
 The nerve iJDjuiIse wliicli jjasses along the axon of the 
 nerve fiber, on the other hand, ap[)ears to be a physical 
 change rather than an explosive chemical decomposition, 
 — comparable, for example, rather to the conductiijn of a 
 current of electricity through a salt solution than to the 
 progress of chemical change along a fuse of gunpowder. 
 
 The nervous system participates in every act of human 
 life, and as we study further the functions of the body we 
 shall constantly extend our knowledge of its working. The 
 present chapter is intended to give only some indispensa- 
 ble preliminary information. To other of the more impor- 
 tant functions and relations of the nervous system we shall 
 return in Chapters XIY and XV. 
 
CHAPTER VIII 
 ALIMENTATION AND DIGESTION 
 
 A. The Supply of Matter and Power to the 
 
 Human Machine 
 
 1. Power and the Materials for Repair supplied sepa- 
 rately to Lifeless Machines. — Living and lifeless machines 
 are alike in that worn-out parts must be renewed and that 
 power must be supplied to do work. In the lifeless machine 
 these two requirements are supplied separately. A factory 
 and its ec|uipment of machinery are kept in repair and 
 enlarged (grow) by means of bricks, lumber, steel, belting, 
 new pieces of machinery, etc., which are brought into the 
 building ; while the pou'er which runs the machinery comes in 
 quite separately as fuel, or water power, or electric power. 
 
 2. Power and the Materials for Growth and Repair sup- 
 plied to the Human Machine in the One Form of Foods. — 
 With the human mechanism this is not so. Materials for 
 growth and rei:)air, and power for running, are introduced 
 from without not separately, but together, both being sup- 
 plied in the one form of food. As it does its life work the 
 human mechanism, like a lifeless machine, not only con- 
 sumes power but its parts deteriorate, and it is the double 
 function of the food we eat to make good this double loss. 
 Some foods possibly serve only as means of power ; others 
 merely make good the loss of essential parts of the mechan- 
 ism ; while still others may serve both purposes. 
 
 3. Food as a Source of Power. — Experiment and experi- 
 ence alike prove that foods are the source of power for 
 work. Bread, butter, starch, sugar, beef, and the like may 
 
ALIME^JTATION AND DIGESTION 89 
 
 be dried and then burned as fuel, giving power to an 
 engine. The occasional use of Indian corn or wheat for 
 fuel, in the West, the employment of hams and bacon as 
 fuel by steamers short of coal, the explosion of flour dust 
 in mills, and similar phenomena further illustrate by the 
 teachings of experience the fact that these foods are rich 
 in energy, or pou'er. 
 
 When we say that the food must supply power to the 
 body we mean that tlie power which it contains must he 
 available to the body. A lump of coal may be a source of 
 power, as is shown by its use in a locomotive ; but a lump 
 of coal would be of no use as food, because the body has 
 no such means of burning it as has the engine. Again, 
 nitroglycerin contains chemical elements needed in the 
 food ; but although when exploded in a dynamite cartridge 
 it may furnish power enough to sliatter heavy armor plate, 
 its energy is not available to the body. 
 
 Thus, to recapitulate, food (a) makes good the loss of 
 living substancij in the body, (/<) it supplies material for 
 growth, and (c) it supplies power for the work wliich the 
 bod}' is to do. It also performs one mure important func- 
 tion which will be more clearly understood hereafter ; 
 for (d) by its oxidation food provides the heat usiially 
 required to keep up the bridy temperature. The detailed 
 consideration of this subject, however, must lie postponed 
 to Chapter XII. 
 
 4. Chemical Composition of Foods ; Nutrients. — The hu- 
 man race has learned by long experience that certain things 
 do, and that other things do not, meet the demands of the 
 body for food. Perhaps no animal uses so many different 
 materials as man in satisfying sensations of hunger and 
 thirst. Some foods are taken from the animal and some 
 from the vegetable kingdom, and tlieir variety is greatly 
 increased by special modes of preparation. But however 
 numerous the foods from which we prepare the dishes 
 
90 THE HUMAN MECHANISM 
 
 served at different meals, chemical analysis shows that the 
 essential constituents of all foods belong to a compara- 
 tively small number of chemical groups. These classes, or 
 groups, may be called nutrients ; and as all the members 
 of the same group undergo practically the same processes 
 of digestion, and perform similar functions in nourishing 
 the body, it will be equally accurate and more convenient, 
 in treating of this part of physiology, to speak of the dif- 
 ferent nutrients, and not of beef, mutton, fish, eggs, bread, 
 milk, butter, etc. 
 
 From the point of view of digestion the most important 
 nutrients are the proteids, the albuminoids, the carbohy- 
 drates, the fats, the inorganic salts, and tvater; and the 
 student must at this point become thoroughly familiar with 
 what is meant l)y these fundamental terms. 
 
 5. The Group of Proteids. — We may obtain a working 
 idea of what a jjroteid is by recalling some of the foods in 
 which proteid preponderates or is easily seen. Such foods 
 are the white of egg, the lean of tender meat (muscle fibers), 
 the curd of milk, the tenacious gluten of wheat. Proteids 
 also exist in relatively large quantities, though not so readily 
 seen, in yolk of egg, beans, peas, oats, and other grains. 
 
 Proteids contain carbon, hydrogen, nitrogen, oxygen, and 
 sulphur. Some of them contain phosphorus, and some iron. 
 Chemically they are exceedingly complex substances, the 
 proteid molecule consisting of a very large number of atoms. 
 We cannot write the chemical formula for proteid as we 
 write H2O for water ; we know the proportions by weight 
 in which the elements are combined, but we have not yet 
 discovered liow many atoms of each element there are in the 
 molecule. It should be noted that the proteids are the most 
 important nutrients which contain nitrogen and sulphur. 
 
 Manjr proteids readily become insoluble. Examples of 
 this are the hardening of the white of egg or the lean of 
 meat by cooking, and of the casein or curd of milk by 
 
ALIMENTATION AND DIGESTION 91 
 
 rennet or " junket taljlets." Tliis change is known as coagu- 
 lation, and most of our proteid food is eaten after having 
 l)een coaguL\ted in the process of cooking. 
 
 Proteids occur only within tlie living cells of plants and 
 animals, or as the products of these living cells. They form, 
 as we shall more clearly see later, the basis of the living 
 cell, and are constantly disintegrating within the cell into 
 simpler substances. Hence there is a constant cellular loss 
 of proteid, which in the animal body can he made good 
 only from proteid in the food. Plants, on the other hand, 
 have the pjower of manufacturing proteids from sugars 
 and certain mineral salts, which latter supply the needed 
 nitrogen and sulphur. The plant kingdom is, tlierefore, in 
 the long run the sole source of proteid food for animals, 
 for while some animals (carnivores) get their j)rotcid entirely 
 by eating the flesh of other animals, the latter (lierhivoroiis 
 aiiiinals) in tui'u have ol)tained their proteid from jilants. 
 
 6. The Group of Albuminoids. — C'luscly related to the 
 proteids in chemical composition, aui.l indeed derived from 
 them, is a group of bddies known as the albuminoids. 
 Physiologically they cannot entirely leplace j)roteids in 
 our diet, but they can replace them partially. The most 
 important of these sul)stances in use as food is the fihrous 
 connective tissue, whose iibers in the uncooked state con- 
 tain the albuminoid substance colJa<iciu which by heating 
 in the presence of water is converted into the closely 
 related gelatin. Other albuminoids are of minor practical 
 importance. 
 
 7. The Group of Carbohydrates. — "Within the cells which 
 make up a potato or a grain of wheat one often sees a large 
 number of granules of starch. This starch was derived 
 originally, ^vith the aid of the sunlight and the green parts 
 of the plant, from carbon dioxide obtained from the air, and 
 water abs<irbed by the roots. It is a typical carbohydrate 
 (a term shortly to be explained) and constitutes the chief 
 
92 THE HUMAN MECHANISM 
 
 carbohydrate food of the world. It is readily changed into 
 the other members of the same group, — the gums, or dex- 
 trines, and the sugars. The latter are often formed in the 
 plant itself, as in sugar maples, sugar cane, sugar beets, and 
 in sucli fruits as apples, pears, and peaches. 
 
 Carbohydrates are compounds of the elements carbon, 
 hydrogen, and oxygen, the last two usually, but not always, 
 occurring in the proportions of two atoms of hydrogen to 
 one of oxygen. The carbohydrates actually used as foods 
 belong for the most part to three closely related groups. 
 The members of the simplest, the monosaccharides, have 
 the formula CgHj20g, and the most common are grape sugar 
 and fruit sugar ; the second group includes cane sugar and 
 milk sugar, each of which has the formula GjgHjgOjj ; the 
 third group includes the gums and starches which have 
 the formula (Ci^li-^QOr)^ where n is some factor which mul- 
 tiplies the numbers 6, 10, and 5. Thus wJiile the exact 
 formula for starch is not known it is safe to say that n is 
 as large as 25. This would make the above formula 
 Gi50^^250*-*i25- T^i® molecule of starch therefore contains 
 a very large number of atoms of carboyi, hydrogen, and 
 oxygen, hut no nitrogen. 
 
 8. The Group of Fats. — Fats are familiar to us in such 
 forms as butter, lard, olive oil, and the fat of meat. Like 
 the carbohydrates they are compounds of carbon, hydrogen, 
 and oxygen, although the oxygen is always present in small 
 quantities. The formula for one of the fats is CgjHg^Og, 
 and this composition is typical of all of them. 
 
 Fats may be split up into certain acids {fatty acids) and 
 glycerin (CgH^Og), and when treated with alkalis, like 
 caustic soda or caustic potash, they form soaps. They are 
 insoluble in water. Like the carbohydrates they contain 
 no nitrogen. 
 
 9. Oxidizable and Nonoxidizable Nutrients. — All the 
 above nutrients may and do combine with oxygen within 
 
ALIMENTATION AND DIGESTION 93 
 
 the cells of the body, although the way in which this 
 chemical union is brought about is one of the unsolved 
 problems of physiology. While all of the nutrients may 
 be burned after being dried, such comlmstion requires a 
 high temperature. Within the body they are not only 
 burned (i.e. combined with oxygen) at a temperature rarely 
 exceeding 39° C. (100° F.), but they undergo oxidation 
 while in a moist state, or even in solution. However this 
 oxidation may be effected within the cell, there can be no 
 doubt that it yields the heat for keeping the body warm 
 and the power for its work. 
 
 The remaining groups of nutrients, the inorganic salts 
 and water, are, for the most part, not oxidized in the body. 
 
 10. The Groups of Inorganic Salts and Water. — These 
 nonoxidizable nutrients are absolutely necessary for the 
 proper nourishment of the body. Their presence in the 
 blood and lymph, and in the living cells, is indispensable 
 to the processes of life. The salts are taken in small quan- 
 tities, partly as salt itself, partly as portions of the various 
 foods we eat. During growth they furnish much of the 
 mineral matter of bones, and since the body is daily losing 
 salt, it is necessarjr that salt be supplied in the food. 
 Salts, however, are not acted on to any large extent in the 
 alimentary canal by the processes of digestion ; they are 
 largely absorbed in the same form as eaten. Hence they 
 do not concern us at present to the same extent as do the 
 oxidizable nutrients which generally have to be chemic- 
 ally changed, or di<:^estcd, before they can be absorbed for 
 use in the body. And the same thing is true of water. 
 
 11. Indigestible Material in Food. — When we say that 
 a food is digestible we mean that when taken into the ali- 
 mentary canal, if not already in solution, it is chemically 
 acted upon by the digestive juices so as to be dissolved 
 and made capable of being ahso7-hed into the blood. The 
 greater part of the food we eat consists of digestible 
 
94 
 
 THE HUMAN MECHANISM 
 
 substances, but many foods contain a certain amount of 
 indigestible material, and some a very considerable amount. 
 The most conspicuous example of such material is cel- 
 Ailose, a member of the same group of carbohydrates to 
 which starch belongs. It occurs in almost all vegetable 
 foods, and since in the human alimentary canal cellulose 
 
 Fic 47. Part of the seed of the bean 
 
 ShoTving the larger starch and the finer 
 prciteid gi'annle.s inclosed within the 
 cellulcfse cell walls 
 
 Fig. 48. Section of potato 
 
 Sho\\'ing starch granules inclosed 
 within the cellnlose cell walls 
 
 is for the most part unaffected, it cannot be absorbed and 
 necessarilj' forms an iniporttint part of the feces. Other 
 indisrestiljle substances are the outer skin of animals, e.a:. 
 the skin of fowls, and certain portions of the connective 
 tissue of meat. 
 
 12. Animal and Vegetable Foods. — The classification of 
 foods into animal and vegetable not only describes the 
 origins of foods from the two great kingdoms of living 
 things but also defines important differences between them 
 with reference to digestion. These differences may be 
 summed up as follows. 
 
 Animal foods are generally rich in proteids and poor in 
 carbohydrates, while vegetable foods are generally poor in 
 proteids and very rich in carbohydrates, especially starch. 
 In the second place, animal foods contain relatively little 
 indigestible material, while vegetable foods, as they occur 
 
ALIMEisTATKJN AND DIGESTION 
 
 95 
 
 in nature, contain large amounts of indigestible cellulose. 
 Ill the third place, the digestible materials of vegetable 
 foods (the proteids, carbohydrates, and fats) are often con- 
 tained witliin a plant cell which is surrounded by a cellu- 
 lose membrane impermeable to the digestive juices ; before 
 they can be digested this membrane must be ruptured in 
 one way or another. In the case of many animal foods, on 
 the other hand, especially meat and fat, the cells (muscle 
 fibers and fat cells) which contain the essential nutrients 
 are held together by connective tissue made up largely of 
 libers of an albuminoid nature. These fibers are soluble 
 in the juices of the stomach, in which the cellulose which 
 holds together the vegetable foods is insoluble. The full 
 importance of these differences will be evident before we 
 have finished the study of digestion. 
 
 13. Composition of Some Common Foods. — The follow- 
 ing table gives the percentage composition of some c.if the 
 more common foods. 
 
 
 WATEI! 
 
 l"l:oT|.;ii. 
 
 Starch 
 
 Sl t: \l; 
 
 Fat 
 
 S.VLTS 
 
 )-!i-c:i,.l .... 
 
 ']7 
 
 8 
 
 17 
 
 :; 
 
 1 
 
 ') 
 
 Wlieat flour . . 
 
 l.-j 
 
 n 
 
 CO 
 
 t.2 
 
 ■:) 
 
 1.7 
 
 Oat meal . 
 
 1.3 
 
 12. i; 
 
 58 
 
 5.-t 
 
 5.0 
 
 ■} 
 
 Rice .... 
 
 13 
 
 c 
 
 71) 
 
 (1.1 
 
 0.7 
 
 0.5 
 
 Pi-as .... 
 
 l.j 
 
 2-) 
 
 ~>o 
 
 2 
 
 
 'J 
 
 Potatoes . 
 
 7 •'» 
 
 o 
 
 IS 
 
 
 0.2 
 
 (1.7 
 
 Milk .... 
 
 80 
 
 4 
 
 — 
 
 
 4 
 
 o.s 
 
 (Jiieese 
 
 •") i 
 
 ■'!:) 
 
 — 
 
 -- 
 
 24 
 
 5 
 
 Lean l>eef 
 
 7-2 
 
 10 
 
 — 
 
 — 
 
 b 
 
 1 
 
 Fat beef . . . 
 
 .31 
 
 It 
 
 — 
 
 — 
 
 '2U 
 
 1 
 
 .Glutton . . . 
 
 72 
 
 IS 
 
 — 
 
 — 
 
 5 
 
 1 
 
 \'eal .... 
 
 G3 
 
 IC 
 
 — 
 
 — 
 
 16 
 
 1 
 
 White fish . . 
 
 78 
 
 18 
 
 — 
 
 — 
 
 3 
 
 1 
 
 Saliuoii 
 
 7T 
 
 k; 
 
 — 
 
 — 
 
 5-0 
 
 1.5 
 
 Kgg .... 
 Butter. . . . 
 
 74 
 15 
 
 It 
 
 — 
 
 — 
 
 10.5 
 S3 
 
 1.5 
 
 b 
 
96 THE HUMAN MECHANISM 
 
 14. The Process of Alimentation. — Before corn, wheat, 
 meat, vegetables, and other food materials can be taken 
 into the body and made to yield up to it the material and 
 power which they contain, they must, in most cases, undergo 
 various preparatory or preliminary processes or treatments 
 which shall make them easier or better to eat, or more at- 
 tractive. The most familiar of these processes is cooking, 
 but it is by no means the only one. In the case of animal 
 food the animal must be captured, if wild, or raised, if 
 domesticated. It must be killed, skinned, dressed, cut up, 
 and the meat in many cases " ripened " by keeping, or 
 " cured " by smoking, salting, drying, or corning. So, also, 
 with plant food, such as cereals, vegetables, fruits, nuts, 
 and the like ; these must first be found, if wild, or grown, 
 if domesticated. They must then be separated from the 
 rest of the plant, — threshed, if wheat, rye, oats, or barley ; 
 husked and shelled, if corn ; dug up or removed from the 
 earth, if vegetables like potatoes, celery, radishes, or let- 
 tuce. Fruits and nuts must be separated or picked from 
 vine or tree ; milk, drawn from animals ; and even salt, 
 water, and condiments like mustard and pepper, separated 
 from the earth or the sea or from plants. After collection 
 and further preparation by winnowing, grinding, or clean- 
 ing, elaborate cooking is applied to many forms of food 
 before it is put upon the table; and even then, at the last 
 moment before it is eaten, a further separation, as of meat 
 from bone, must be made either by the carver or by the 
 eater himself. 
 
 To this entire process of the supply and preparation of 
 food for eating the term "alimentation" may be conven- 
 iently aj^plied. Reflection will show that it is largely a 
 process of food refining, the principal result being a con- 
 centration of the nutrients at every step. It is also a 
 separation of the comparatively useful from the compara- 
 tively worthless (as food) ; and just here, and in these two 
 
ALIMENTATION AND DIGESTION 97 
 
 points, - — refining and. the separation of c/ood from jxjor mate- 
 rials., — we may recognize a true process of digestion, but 
 one external rather than internal : a refining in the field, 
 the mill, and the kitchen rather than in the stomach ; in 
 the environment rather than within the organism. 
 
 15. Digestion, External and Internal. — The word " diges- 
 tion " comes from the Latin dis and gerere, to tear apart 
 or separate. A " digest " of the laws of any state or coun- 
 try is a compact, concentrated statement of those laws 
 from which the unimportant parts have been separated 
 and omitted. Now one of the principal objects and func- 
 tions of digestion is to separate the nutritious and therefore 
 important part of the food from the innutritious and 
 therefore unimportant. Another and no less important 
 function is to prepare this food that it may be more readily 
 absorbed and utilized by the body proper. A sui'vey of 
 the processes of alimentation and digestion shows that fdl 
 are tributary to these ends, and that we may with advan- 
 tage consider alimentation (including cooking) as a kind of 
 external digestion, which applies to the food a treatment 
 not only preliminary to Ijut also preparatory for ordinary 
 or internal digestion. Digestion as commonly understood 
 thus becomes only the last part of a much longer process. 
 
 16. The Alimentary Canal a Digestive Laboratory. — 
 Many animals low in the scale of life, and all plants, are 
 destitute of those tubes running through the body, which 
 are called alimentary canals, but all animals (except 
 some parasites) above the lowest are provided with such 
 tubes or canals. These are really no more than places, or 
 spaces, in which food is prepared for absorption, and they 
 always closely resemble chemical laboratories or refineries 
 in which raw materials are altered or refined by the 
 action of chemical reagents. The laboratories are of sev- 
 eral kinds, — mouth, stomach, intestine, — and the reagents 
 correspondingly different, — salivary juice, gastric juice. 
 
98 THE HUMAN MEUHANISM 
 
 pancreatic juice, bile, intestinal juice, etc. To the detailed 
 consideration of internal or ordinary digestion we may now 
 proceed. 
 
 B. Digestion in the Mouth. The Teeth. Enzymes 
 
 17. Digestion. — The food upon which the body depends 
 for its supplies of matter and power enters the body proper 
 through the lining membranes of the alimentary canal into 
 which it is received through the open mouth. Inasmuch, 
 however, as these lining memliranes contain no visible open- 
 ings, the food must somehow soak, or "diffuse," through 
 them, and in order to do this, solid foods must be greatly 
 changed and prepared for absorption by being brought into 
 solution. 
 
 But this is not all. Some foods are soluble but still not 
 suitable for the use of cells and tissues. Common cane 
 sugar {saccharose), for example, is very soluble ; but before 
 it can be used by the cells it must be converted into the 
 closely related but somewhat different sugars known as 
 dextrose and levulose. Again, proteid food is absolutely 
 indispensable for all animals, but ordinary proteids pass 
 through membranes like those of the alimentary canal 
 with great difficulty ; they are, therefore, before absorp- 
 tion, turned into other and more diffusible bodies known 
 as peptones. Starches and fats also require special treat- 
 ment to make them available or acceptable for absorption. 
 We shall begin our stud)r of the iirocesses of digestion 
 with those which take place in tlie mouth, and these are 
 the crushing or grinding of the food by the teeth and its 
 mixture with saliva. 
 
 18. Structure of a Tooth. — A tooth has three parts, — 
 the croivn, or exposed portion ; the neck, a narrow con- 
 striction at the edge of the gum ; and the root, or roots, by 
 which the tooth is fixed in the jawbone (sec Fig. 138). 
 
ALIMENTATKJN AXD DIGESTION 
 
 99 
 
 The tooth consists of a hard body surrounding a central 
 pnJjy cavity, filled with a loose connective tissue containing 
 blood vessels and nerves, which enter the pulp cavity by a 
 minute opening on the tip of each root. Elsewhere the 
 pulp is surrounded by the hard parts of the organ, which 
 consist of three different tissues. 
 Innnediately surrounding the pulp, 
 both in the root and in the cro\\'n, 
 is the denlim:, which makes up the 
 main bulk of the tooth ; this is a 
 Ijard structure, containing some 65 
 per cent of mineral matter, chiefly 
 carbonates and phosphates of cal- 
 cium. It is channeled, as shown in 
 the figure, l)y minute canals, the 
 dentinal tuhulcs, which run into the 
 pulp cavity. In the root the dentine 
 is covered with cement, which is vir- 
 tually bone in structure and compo- 
 sition. In the crown, or that part I''"-- -!'■'■ t'ection of an in- 
 of the tooth not covered by the gum, 
 the dentine is covered witli enamel, 
 the hardest substance in tlie luidy. 
 This contains in the adult from 95 
 to 97 per cent of very insoluble mineral matter, and is the 
 protective covering of the tooth. In structure, enamel con- 
 sists of columns, hexagonal in section, set together so as 
 to form a practically impenetrable mosaic co%'ering. It is 
 thus admiraljly fitted to protect the dentine, and indeed 
 the whole tooth, from meclianical injury, from chemical 
 erosion, and from bacterial action. 
 
 19. Care of the Teeth. — Too much stress can hardl}^ be 
 laid on the })reservation of tlie teeth, .\part from consid- 
 erations touching personal aiipearance, the teeth are of 
 great imjjortance in masticating the food. Mastication, or 
 
 cisor tooth. After Spal- 
 tehnlz 
 
 , ciiallii'I : B, dolltilio ; ( ', 
 uiiiii : JK pulp ; E, jaw- 
 Imnr ; /', cement 
 
100 THE HUMAN MECHANISM 
 
 chewing, is one of the many acts of digestion, and when 
 the power of chewing is impaired, the efficiency of the 
 whole digestive process is to that extent lessened ; other 
 portions of the alimentary tract, especially the stomach, 
 must then do, as far as possible, what should have been 
 done by the teeth ; digestion is hindered; some kinds of 
 food are never properly digested, and the opportunity for 
 bacterial decomposition of the food is greatly increased, 
 because of the prolonged exposure of the food to bacte- 
 rial action. Besides, there is always the possibility that a 
 decaying tooth will cause the formation of an abscess (an 
 ulcerated tooth), often a most painful and sometimes a 
 dangerous thing. 
 
 20. Decay of the Teeth is usually, if not always, due to 
 the action of bacteria, which produce acids by fermenting 
 food particles in the mouth. These acids dissolve away 
 the lime salts of the enamel and the dentine ; the enamel, 
 however, is acted upon very slowly and with great diffi- 
 culty ; so long as it is intact, the underlying dentine, 
 which is dissolved much more readily, is protected ; but if 
 for any reason the enamel becomes worn away, its absence 
 should be made good by filling the tooth, thereby prevent- 
 ing access of foreign substances, and especially of destruc- 
 tive bacteria, to the dentine. 
 
 The action of bacteria upon the enamel is favored by 
 the formation of a hard deposit known as tartar, a mix- 
 ture of lime salts precipitated from the saliva, and espe- 
 cially apt to be deposited between the lower teeth and in 
 the neighborhood of the gums. Sometimes tartar is even 
 deposited under the gums, in which case it is inaccessible 
 to the action of a brush. Because of this tartar crust, bac- 
 teria and the acids which they produce are not properly 
 rubbed away by the movements of tongue and cheeks, 
 and hence the importance of its artificial removal. This is 
 greatly facilitated by using a tooth powder which contains 
 
ALIMENTATION AND DIGESTION 101 
 
 some substance, like precipitated chalk, not hard enough 
 to injure the enamel but exerting friction enough to break 
 up the deposit. At least once or twice a week a good 
 tooth powder should be used in cleaning the teeth. At 
 other times the brush and water are sufficient. After using 
 powder, indeed always after brushing the teeth, the mouth 
 cavity should be very thoroughly rinsed out. 
 
 It is, however, very difficult, and at times impossible, 
 to remove the tartar entirely by the use of powder and 
 brush. For this reason the teeth should be examined by 
 a dentist at least once a year, the accumulated tartar 
 thoroughly removed, and the teeth polished. In this way 
 the beginnings of decay are detected and measures can be 
 taken to prevent its further progress. Further advice as 
 to the care of the teeth can and should be obtained from a 
 good dentist. 
 
 Decay of the teeth caused by bacteria is also prevented 
 by removing as far as possible the food supply of these 
 organisms, to whose growth and activity nothing is more 
 favorable than particles of food between, or otherwise in 
 contact with, the teeth. The ideal plan is to brush the 
 teeth with water and rinse out the mouth after each meal ; 
 in most cases this is perhaps more than is required ; it 
 may be suggested, however, that the teeth should be 
 brushed at night as well as in the morning, and that 
 brushing them at night accomplishes more toward re- 
 straining bacterial action than does brushing them in the 
 morning. 
 
 By thus exercising care the teeth may usually be pre- 
 served in efficiency until old age. The unpleasant neces- 
 sity of using false teeth is almost always the result of 
 neglect of the teeth in earlier life. 
 
 21. Chemical Action of Saliva. — We have already stated 
 that the main function of the saliva is to lubricate the 
 food and so aid mastication and swallowing. But if saliva 
 
102 THE HUMAN MECHANISM 
 
 is brought in contact with starch, a very characteristic 
 chemical action occurs, and tliis we must study in some 
 detail, chiefly because it is typical of many similar chem- 
 ical effects produced by other digestive juices. To demon- 
 strate the effect in question some starch paste should be 
 prepared. This is not a clear solution like salt or sugar, 
 but an opalescent liquid, which does not become clear 
 ujjon passing through an ordinary filter. A characteristic 
 test for starch — the blue color produced when a few 
 drops of a solution of iodine are added to it — may be 
 used to detect its presence in the course of the following 
 experiments. 
 
 Experiment I 
 
 Three tulies or small beakers ccnitaiiiiiig starch paste are pre- 
 pared; one of them should lie cooled nearly to the freezing point by 
 packing ice around it, and some filtered saliva should be cooled in 
 the same way. A second portion of the filtered saliva is to be heated 
 to the boiling point, while a third is to be used without further 
 treatment. To the cooled starch ]iaste add the cooled saliva, keeping 
 the cooled mixture surrounded by ice ; to the second portion of the 
 starch paste add the boiled saliva, and to the third the remain- 
 ing portion of saliva. ]\Iere obsHrvatimi of the three test tubes will 
 soon show that wdiile the first two remain opalescent, the last soon 
 becomes clear. After this change has occurred, a little of the third 
 mixture may be removed, diluted with water, and tested with iodine ; 
 the color now produced is no longer a pure blue, but purplish, i.e. a 
 mixture of blue and led ; some minutes later the iodine test gives a port- 
 wine color, and still later no color at all. This shows that under the 
 action of the saliva the starch has disappeared from the third test tube. 
 The inti-rmediate port-wine color is due to the formation of dcxiriiicx, 
 or animal r/iim.'f, into "which the starch is first tr.ansformed ; but sub- 
 sequently this, also, disappears. Sleanwhilc, the starcli in the first 
 two test tuVies shows no change either in its opalescent appearance 
 or in its original reaction with iodine. Keep these test tubes, the 
 cooled one still surrounded by ice, for the subsequent examination 
 described iu Experiment III. 
 
ALIMEXTATIOX AXD DIGESTION 103 
 
 Exi'EHIMEXT II 
 
 Let us now inquire what lias ijecoine of the starch in the thii'd 
 test tube. Tlic sohition looks clear auil has a sweetish taste. More- 
 over, if lioiled with a mixture of sodiiuu hydroxide and copper sul- 
 pihate, it gives a red precipitate indicating the presence of sugar. 
 These simple tests, then, prove that xidira Jir.-il chanyts slarc/i into dcx- 
 Irbies and ^nhseipwidly clidiiges these dextriiies into sugar. 
 
 22. The Salivary Ferment, or Enzyme. — This change of 
 starch first into dextrine and then into sugar is caused by 
 pti/alin, a constituent of the saliva which belongs to the 
 general group of enzymes or unorganized ferments. A 
 striking feature about the action of ptyalin is the fact that 
 it is not destroyed or consumed in producing the reaction. 
 This is unlike what usually occurs to suljstances taking- 
 part in chemical I'cactions. Wlicn wood combines with 
 oxygen in burning, l)oth the w 1 and the oxygen disap- 
 pear and in their place the products of comlnistion — 
 smoke, gas, etc. — make their ap[)earance. In the salivary 
 digestion of starch three sul)stanees enter into the reaction, 
 — starch, water, ami the eirzyme ; t\vo of these, starch and 
 water, disappear; the third, the enzyme, remains over after 
 the reaction is completed and ct»ntinucs capable of trans- 
 forming new starch ])aste into sugar. 
 
 We shall repeatedly meet willi other enzymes (pepsin, 
 trypsin, etc.) in our further studies of digestion. 
 
 ExPEinjIENT Til 
 
 Returning now to our experiments Avith the starch paste, we 
 find that neither test tube 1 noi- 2, Avhether tested for starch or for 
 su"'ar, shows any change. The student should now remove a por- 
 tion of the cooled mixture and allow it to rise to the room tempera- 
 ture ; it will soon be found that the starch gradually changes into 
 sugar, which shows that the cold has pre^"ented or inhibited the action 
 of the enzyme but has not destroyed it. If mixtures of saliva and 
 starch be kept at different tenjperatures, it will be found that as the 
 
104 THE HUMAN MECHANISM 
 
 temperature of the mixture approaches that of the body (98.6° F.), the 
 more quickly is the transformation into sugar effected. The activity 
 of the enzyme is dependent upon the temperature. If, however, we use 
 still higher temperatures, one will eventually be found which destroys 
 the enzyme. This temperature differs with different enzymes, but all 
 are destroyed before the boiling point of water is reached ; and no 
 matter how long we may keep a mixture of starch and boiled saliva, 
 no change of starch into sugar will occur. 
 
 Experiment IV 
 
 Still another instructive experiment may be made upon the action 
 of saliva on starch. Prepare five or more small beakers of starch 
 paste and add to the first one a drop of filtered saliva, to the second 
 two drops, to the third three drops, and so on ; then observe the time 
 I'equired in each case for the disappearance of the iodine reaction. 
 This experiment will show that while a very small amount of saliva 
 will transform an indefinite amount of starch to sugar, the more 
 saliva present the more rapidly will the transformation occur ; and 
 the same thing is true of all enzymes. 
 
 Experiment V 
 
 Finally, dilute some starch paste with an equal volume of 0.4 per 
 cent hydrochloric acid (this will, of course, make a 0.2 per cent solu- 
 tion of the acid). Now add a few drops of saliva. It will be found 
 that no reaction takes place. Saliva will not act in an acid medium 
 of this strength ; and it can be easily shown that it acts most vigor- 
 ously in a neutral or faintly alkaline solution. This result is of much 
 practical importance, because the gastric juice contains apjiroximately 
 0.2 per cent of hydrochloric acid, and may therefore be expected to 
 interfere with salivary digestion. 
 
 While we are eating, the food obviously stays too short 
 a time in the mouth to allow the conversion of any large 
 amount of its starch into sugar before it is swallowed. 
 Whatever actual work the saliva may do in bringing about 
 this chemical change must evidently be done chiefly in the 
 stomach, and this will be studied in the next section. 
 
 We have dwelt at length upon the enzyme action of 
 saliva not merely for its own sake but rather because 
 
ALIMENTATION AND DIGESTION 105 
 
 the behavior of the salivary juice is typical of the action 
 of other of the digestive juices and of enzyme action in 
 .general. All the other juices of the alimentary canal, with 
 the single exception of the bile, contain enzymes, and it 
 will greatly help our understanding of the digestive action 
 of these enzymes if that of the salivary enzyme be first 
 mastered. 
 
 Digestion in the mouth, then, consists first, of a mechan- 
 ical process of chewing, by which food is crushed or com- 
 minuted ; second, of a physical process of moistening, by 
 which dry foods are prepared for tlie act of swallowing ; 
 and third, of a chemical process, the chief part of which is 
 the conversion of starch into sugar by enzyme action.^ 
 
 ^ Those who have studied the elements of chemistry will find tlie fol- 
 lowing facts helpful to a fuller understaudiuf;- of the action of saliva on 
 starch. Saliva first causes the large molecule of starch to unite with 
 water, and in so doing to split up into one or more molecules of sugar 
 and a certain gum (dextrine). The saliva next causes the gum or dex- 
 trine thus formed to combine with water and break up into one or more 
 molecules of sugar and a different gum, or dextrine. Tlie repetition of 
 this process ultimately transforms all the dextrine to sugar. The process 
 may he conveniently represented as follows. 
 
 Stakch -I- water 
 
 sugar Dextrine (1) + water 
 
 Dextrine (3) -|- water 
 
 sugar sugar 
 
 (The original substance is given in capitals and the end products in 
 italics. ) Thus in the above diagram one molecule of starch by these suc- 
 cessive processes of cleavage would yield five molecules of sugar. In point 
 of fact, one molecule of starch undoubtedly yields much more sugar than 
 
106 THE HUMAiN' MECHANISM 
 
 Q. Digestion in the Stomach 
 
 According to popular ideas the stomach is the chief organ 
 of digestion ; in fact, however, it is an organ in whicli tlie 
 food wliich has been swalhiwed is temporarily stored while 
 undergoing a preliminary preparation for tlie more import- 
 ant digestive changes which are to take place in the intes- 
 tine. In this preparatory process, to be sure, some of the 
 
 this. Assuming as we did on page 92 that the formula for starch is 
 C100H250O125, the reaction would be 
 
 CisoHjsoOizs + 25H2O ^. 25CcHioO,. 
 starch water sugar 
 
 In reactions like the ahove, the sugar is known as the end product, 
 whUe the various dextrines are known as intermediate products. It is 
 also obvious that this cliange from starch to sugar involves a leduclion 
 in size of the molecule ; one large molecule is split up into many smaller 
 molecules. This reduction in size nf the starch inolecnle appears to be of 
 fundamental physiological importance, in that the carbohydrate food thus 
 acquires physical properties which remler it capable of passing through 
 the lining membrane of the intestine in the process of absorption. 
 
 As stated in the text, the chemical action of the enzyme at first sight 
 appears very unlike the ordinary chemical reaction where all substances 
 concerned disappear in contributing to the fi}ial result. For example, 
 
 HCl + KOtI = KCl + H2O. 
 
 Here for every molecule of hydrochloric acid which disappears, one 
 molecule of potassium hydrate lilvcwise disappears. The distinction, how- 
 ever, is only apparent. The reaction between hydrochloric acid and potas- 
 sium hydrate sliould be compared with the reaction between starch and 
 water, not between starch and saliva ; and there are numerous examples 
 in chemistry of this action of a third body in producing or accelerating 
 a chemical reaction between two other bodies. Thus hydrochloric acid 
 acts very .slowly, if at all, at ordinary temperatures on pure zinc ; if, 
 however, a drop or two of platinum chloride be added, vigorous union 
 takes place ; 
 
 Zua + 2IIC1 = 2ZnCl2+ II2. 
 
 Here again the platiimm chlnridc by its mere presence produces the 
 reaction without itself contributing anything to the compounds formed. 
 Such reactions are in general known as catalytic, and are by no means 
 confined to living things. Their frequent occurrence, however, in the 
 chemical processes of animal and plant life is a striking fact. 
 
ALIMEXTATIOX AIv^l) DIGESTIOX 
 
 107 
 
 food is incidentally changed into those forms in which it 
 is taken into the blood ; but this action is incidental and 
 subordinate to the main function. 
 
 23. Form and Structure of the Stomach. — The stomach 
 is a large jijouch into which open two tubes, — the cesopha- 
 gus (gullet) toward the left side and the intestine on the 
 right (see Fig. 50). The two regions into which these 
 
 Gall 
 
 'Bladder 
 
 Bile Duct 
 
 Jhtestine 
 
 CEsophagus 
 
 'Pancreatic Duct 
 
 Fig. 50. Stomach, beginning of small intestine, and entrance of 
 bile and pancreatic ducts 
 
 During digestion the bile flows directly from tlie liver into the intestine ; at 
 otlier times the opening of tlie bile duct is closed, aucl the bile passes into 
 the gall bladder, where it is stored 
 
 tubes open are different in structure, and are known as 
 the cardiac (left) and pijloric (riglit) portions of the stom- 
 ach ; the cardiac jDortion differs from the pyloric portion 
 in having greater diameter and thinner walls. The entire 
 inner surface is lined by a membrane some three or more 
 millimeters in thickness, crowded with comparatively sim- 
 ple glands which pour their secretion, the gastric juice, 
 
108 
 
 THE HUMAN MECHANISM 
 
 into the stomach, very much as sweat glands discharge 
 perspiration on the skin (see Fig. 61). 
 
 The glandular membrane is one of the two principal 
 components of the stomach wall ; the other is the mus- 
 cular or contractile tissue which 
 forms a second coat outside the 
 other and closely united to it by 
 connective tissue containing the 
 larger blood vessels, lymphatics, 
 nerves, etc. The muscular coat 
 is comparatively thin in the car- 
 diac, and comparatively thick in 
 the pyloric region, the thickening 
 in the latter rearion beings caused 
 chiefly by muscle fibers circularly 
 arranged. 
 
 24. The Gastric Juice. — The 
 gastric juice is a clear, thm liquid, 
 which contains, among other 
 things, about 0.2 per cent of hy- 
 drochloric acid and two enzymes, pepsin and rennin. The 
 latter changes casein, one of the proteids of milk, into an 
 insoluble body, — the curd, — which is subsequently acted 
 upon by other enzymes of the gastric and pancreatic juices 
 in the same manner as are other proteids.^ 
 
 The other enzyme of the gastric juice, pepsin, is of much 
 greater importance. It acts only in the presence of free 
 acids, and on entering the intestine is destroyed bj^ the alka- 
 line pancreatic juice ; hence its action is confined to the 
 stomach. It produces no effect upon pure fats (although it 
 plays an important part in the digestion of adipose tissue), 
 nor upon carbohydrates, such as starch and sugar. Its part 
 in digestion consists in its action upon two constituents 
 
 1 Rennin is the essential constituent of "junket tablets," which are 
 made from the stomachs of calves. 
 
 Fig. 51. The inner surface of 
 the stomach (magnilied 
 about 20 diameters) 
 
 Showing the openings of the 
 glands. Tlie lining glandular 
 membrane is thrown into 
 folds 
 
ALIMENTATION AND DIGESTION 109 
 
 of the food, — the proteids and the connective tissue of 
 animal foods. The fibers of connective tissue are closely 
 related to the proteids in chemical composition, and the 
 action of the enzyme is essentially the same on both ; their 
 complex molecules are split up into smaller molecules, and 
 the new substances formed pass into solution in the gas- 
 tric juice. By thus dissolving the connective tissue, which 
 holds together the muscle fibers, fat cells, etc., animal food 
 is considerably subdivided and made to present a greatly 
 increased surface to the further action of digestive juices. 
 It is also well to remember that the gastric juice dissolves 
 connective tissue much more rapidly than does any other 
 of the digestive juices ; and that this action upon con- 
 nective tissue is really more important than that upon 
 proteids, although the hitter is usuall}' more emphasized. 
 Proteids not acted on in the stomach are rapidly digested 
 by the pancreatic juice in the intestine ; connective tissue, 
 on the contrary, escaping solution in the stomach, is dis- 
 solved but slowly in the intestine. 
 
 The action of the gastric juice, as regards both proteids 
 and connective tissues, consists essentially in splittiiu/ vp 
 larger molecules into smaller ones. This recalls the action 
 of saliva in converting starch into dextrines and sugars 
 (p. 105). In the case of proteids the products formed are 
 known as proteoses and peptones, substances readily soluble 
 in the gastric juice. 
 
 The student is, however, warned against supposing that 
 because gastric juice is able to transform the proteids of 
 the food to peptones, it actually does exert this action 
 upon all the proteid eaten. In point of fact, as proteid 
 foods are divided into smaller and smaller particles in 
 the stomach, they are discharged into the intestine where 
 their digestion is completed by the pancreatic juice. In 
 man the 2>ancreat.{c, eincl not the gastric juice, is the main 
 agent of proteid eligestion. 
 
110 THE HUMAN MECHANISM 
 
 25. The Stomach at Work. — Having now gained a 
 general idea of the chemical changes which occur in the 
 stomach, we may next proceed to consider what actually 
 happens when food enters that organ. And here our 
 knowledge has been gained partly by examining the gas- 
 tric contents at different periods of digestion, partly by 
 observing the movements of the stomach by the aid of the 
 Rontgen rays,i and partly by other means. 
 
 As soon as food enters the stomach, and even while it 
 is still in the mouth, the gastric glands begin to discharge 
 the gastric juice, and continue to do so during the four 
 or more hours of gastric digestion. When the meal is fluid 
 or is small in amount this gastric juice is thoroughly 
 mixed with it ; when, however, the food is more or less 
 solid and bulky only the outer layers, which are in imme- 
 diate contact with the walls of the stomach, are mixed 
 with the juice. At least this is true at the cardiac end ; 
 the cavity of the pyloric portion is so small, and the 
 amount of movement there so great, that all portions of 
 the pyloric contents are thoroughly mixed with gastric 
 juice ; in the much larger cardiac portion the central mass 
 of the food may receive no gastric juice, and thus remain, 
 for an hour or more after the meal, neutral or alkaline 
 in reaction. Under these circumstances very considerable 
 amounts of starch may continue to undergo the salivary 
 digestion begun in the mouth. 
 
 Any chemical action is aided by agitation, since the 
 reacting compounds are thus brought into more intimate 
 union; and observation of the working stomach shows 
 that while the cardiac portion makes no movements, but 
 
 ' For this purpose food is swallowed whicli contains enough of the 
 harmless salt, bismuth subnitrate, to make the stomach contents imper- 
 vious to the Rontgen rays. The changes produced in the shape of the 
 food mass by the corresponding movements of the walls of the stomach 
 are then observed by means of shadows cast upon the fluorescent screen. 
 
ALIMENTATION AND DIGESTION 
 
 111 
 
 merely keeps up a steady contraction and thereby exerts 
 a moderate pressure upon its contents, tlie pyloric portion 
 executes, from a very early stage of digestion and through- 
 out the whole process, a series of movements which gradu- 
 ally bring about a thorough mix- 
 ture of the contents and rub 
 down the softened food into 
 smaller and smaller masses. 
 These movements consist of 
 waves of constriction which arise 
 at the beginning of the pyloric 
 portion and f)ass onward to the 
 pylorus itself, a new wave begin- 
 ning al)Out once every ten sec- 
 onds, and consuming from thirty 
 to forty seconds in passing to the 
 pjdorus. Consequently there are 
 always two or more slowly mov- 
 ing waves in the pyhiric end of 
 tire stomach at one time.-' 
 
 The pyloric end nf the stomacli 
 is tints the seat of <i comhitied Fic&l'. Outline of the contents 
 cliemiced and weehanieal action 
 on the food. The vegettdjle foods 
 are softened, while tlie connec- 
 tive tissue of the animal foods 
 is dissolved away; in addition, 
 tlie food is mixed with a consid- 
 erable amount of liquid supplied 
 by the secretion of gastric juice 
 pyloric end of the stomach thus ultimately come to con- 
 sist of ■minute solid masses suspended' in a licpdd, the 
 
 of tlie stomach of a cat at 
 three stages of the digestion 
 of ;i meal taken aljoiit 11 a.m. 
 
 Sliowini;' the peristaltii;' constrio- 
 tioiis which pass over the 
 py hn'ie port ion, audi he dint in u- 
 tion of the qttantity of food in 
 the cardiac end. (P^ull descrip- 
 tion ^iven in Section 1*5) 
 
 Th 
 
 jontents of the 
 
 1 These movements of the stomach and intestine are best shown in 
 zoetrope figures, which may be obtained from the Harvard Apparatus 
 Company, Boston, Mass. 
 
112 THE HUMAN MECHANISM 
 
 consistency of the whole being that of moderately thick 
 pea soup. This product of the work of the stomach is 
 known as chyme. 
 
 26. The Expulsion of Chyme into the Intestine. — The 
 
 openings of the oesophagus and intestine into tiie stomach 
 are usually closed; the former is opened, normally, only 
 during the act of swallowing, while the latter opens at 
 irregular intervals during the process of gastric digestion. 
 The opening of the pylorus allows the waves of constric- 
 tion moving over that region of the stomach to discharge 
 the semifluid chyme into the intestine. If, however, a 
 large mass of solid food arrives and is driven against 
 the walls, the pylorus reflexly closes, thus guarding the 
 entrance of the intestine from the passage of food not yet 
 ready for intestinal digestion. The pressure exerted by 
 the sustained contraction of the walls of the cardiac end 
 of the stomach adds to the food in the pyloric region 
 new portions from time to time, and the same combined 
 chemical and mechanical process already described is con- 
 tinued until the whole mass is reduced to chyme and 
 driven into the intestine. 
 
 This brief sketch of the working of the stomach sliows 
 that this organ serves the two main functions of storing 
 the food and of making it more accessible to tlie diges- 
 tive fluids of the intestine. When the chyme is delivered 
 to the intestine the mechanical difficulties in the way of 
 absorption are practically gone ; the surface of the food 
 exposed to digestive action is now immensely increased 
 by its subdivision, and the work remaining for the intes- 
 tine is almost wholly the chemical duty of changing the 
 constituents of the chyme into substances whicli are soluble 
 and ready for absoiption. 
 
 Serious troubles arise when, for one reason or another, 
 gastric digestion goes wrong, because the subsequent 
 processes of digestion are largely dependent upon the 
 
ALIMENTATION AND DIGESTION 
 
 113 
 
 preparation which the food receives in the stomach. Gas- 
 tric digestion may be impaired in one of three ways : first, 
 the gastric juice may not be secreted in proper amount 
 or proper strengtli ; or, second, the stomach may not 
 execute its movements efficiently ; or, third, the gastric 
 juice secreted may not be able to get at the food readily, 
 owing to improper cooking or insuificient mastication. The 
 study of the conditions which produce these troubles, which 
 taken togetlier constitute indigestion, or dyspepsia, will 
 be postponed to the chapter on the Hygiene of Feeding 
 (I^art II). 
 
 27. The Stimulus to the Secretion of the Gastric Juice. 
 — The first requirement for the W(irk of the stomach is 
 the secretion of sufficient gas- 
 tric juice. Of late years the 
 brilliant researches of physi- 
 ologists have shown that the 
 secretion of gastric juice, like 
 that of saliva, is excited by 
 impulses from the nervous 
 system, and that these im- 
 pulses are called forth in two 
 ways. 
 
 (a) The ^' Fsi/ehie'' Secre- 
 tio)i. — When agreeable or ap- 
 petizing food is offered to an 
 animal, and especially when 
 such food is taken into the i"i 
 mouth, a secretion of o-astric 
 
 1 
 
 9 
 
 3 
 
 4 
 
 5 
 
 6 
 
 7 
 
 H 
 
 n 
 
 /() 
 
 
 
 
 
 
 
 
 
 
 
 
 I 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 , 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 S 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 N 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 ^ 
 
 
 
 
 ' 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 N 
 
 '>0. The curve of the "psychic" 
 secretion of gastric juice 
 
 . . ,. ,, 1-1 yertie;il lines represent half-hour i)e- 
 
 juice follows, winch may con- ,.,„|^^ ^^tj^^, j^,-i„g ^j^,^ ^^^-^.^ ^^^^■_ 
 
 tinue for fifteen minutes or zontal lines, relative amounts of 
 
 m, . , • -^astrie iuiee secreted 
 
 more, ihis secretion occurs 
 
 when the food has been in the mouth only ten or fifteen 
 seconds, and even when it is merel}' offered to a hungry 
 animal and not taken into the mouth at all. Again, it 
 
114 THE HUMAN MECHANISM 
 
 occurs only when the animal is conscious ; for if food be 
 introduced into the stomach of a sleeping dog, it evokes 
 only the most scanty secretion of gastric juice after the 
 animal has awakened. Moreover, both the amount and 
 the efficiency of the juice secreted vary directly with the 
 enjoyment of the meal. When meat is given to a dog 
 which is not hungry, no such abundant secretion of gastric 
 juice occurs as during hunger. 
 
 It is clear that we have here to deal with a nervous 
 process more complicated than the simple reflex, and that 
 the efferent discharge to the stomach occurs as the result 
 of nervous processes taking place in the brain in connec- 
 tion with the i'lvjoymcnt of food. In otlier words, the more 
 the food is desired or enjoyed, the more efficient will be 
 this secretion of the gastric juice. 
 
 It is known that this " psychic secretion " will continue 
 for several hours after an ordinary meal, increasing in 
 amount during the first hour or more, and gradually 
 diminishing from that time onward (Fig. 53). 
 
 (I)) Reflex Secretion from Stimulntion of the Stomach itself 
 hy tJie Products of Digestion. — We have just seen that the 
 mere contact of most foods with the walls of the stom- 
 ach does not call forth a secretion of the gastric juice. 
 This is shown by the experiment of placing food in the 
 stomach of a sleeping animal ; such food, even when appe- 
 tizing and nutritious, has been known to remain in the 
 stomach for twelve or more hours after the animal had 
 awakened without evoking a secretion of gastric juice. 
 Yet it is also known that when digestion of food has been 
 begun by the action of the "i)sychic" secretion, the pep- 
 tone formed from proteids arouses a second secretion which 
 increases in amount as the first, oi' " psychic," secretion 
 diminishes. That the peptone formed from proteids is the 
 true cause of this secretion is proved by the fact that the 
 introduction of pej^tones into the stomach of a sleeping 
 
ALIMENTATION AND DIGESTION 115 
 
 animal evokes this secretion, although, as already stated, 
 ordinary food, which contains no peptones, does not. This 
 second secretion continues throughout the period of gastric 
 digestion. 
 
 The secretion of gastric juice must be kept up through- 
 out the entire period of gastric digestion, to make good its 
 loss by the discharge of chyme into the intestine ; and it 
 would seem that this great need is met only by tiie com- 
 bined action of the " psychic " stimulus and the stimidus 
 of peptones produced during the process of digestion. 
 Without the " psychic " secretion peptones are not formed 
 fast enough to induce sufficient subsequent secretion ; 
 without the " peptone " stimulus the psychic secretion 
 does not suffice to complete the digestion of a hearty 
 meal, a labor which ninj requii'e four or five hours. It is 
 also plain that proteids should form part of large meals, 
 since it is only from proteid that peptones can be made. 
 
 D. Digestion" and Absi^rptiox in the Small Intes- 
 tine AND IN THE LAEGE InTESTINB 
 
 Every few minutes during the process of gastric diges- 
 tion the pylorus opens and tlie stomach forces a few 
 cubic centimeters of chyme into the intestine. Chyme 
 consists of water holding in solution certain products of 
 digestion, and in suspension relatively large cpantities of 
 undissolved matter, and has the consistency of moderately 
 thick pea soup. The suspended matter consists, among other 
 things, of small bundles of muscle iibers (from meat), fat^ 
 
 1 The fat of meat consists of connective tissue whose cells are greatly 
 swollen with drops of fat. In typical adipose tissue the connective-tissue 
 cell becomes one large fat droplet surrounded by the thin layer of the cell 
 cytoplasm with its nucleus. Tliese ■' fat cells," like the muscle fibers of 
 meat, are thus held together by the fibers of connective tissue, and are 
 set free when the latter are digested and dissolved away by the gastric 
 juice (see Figs. 83-85). 
 
116 
 
 THE HUMAN MECHANISM 
 
 melted by the heat of the body and set free from adipose 
 tissue by the digestion of its connective tissue, bits of 
 coagulated proteid, such as casein from milk or the white 
 
 of egg, together with other 
 proteid s, starches, and fats 
 of animal or vegetable foods. 
 The work of intestinal 
 digestion is essentialli/ chem- 
 ical in nature, and consists 
 largely in the transformation 
 of the suspended constitu- 
 ents of chyme into soluble 
 substances which can be ab- 
 sorbed into the blood and 
 furnish nourishment for the 
 body; proteids are thus 
 changed into peptone-like 
 ■^ bodies, starches into sugars, 
 o and fats probably into fatty 
 J acids and soaps. These 
 a changes are accomplished 
 ;^ by digestive reagents, or 
 juices, secreted into the in- 
 FiG. fi4. Longitudinal section of the testine Somewhat as the gas- 
 small intestine (-j.jg j^^^g jg secreted into the 
 
 stomach, while the mixture 
 of food and digestive juice 
 is bcincr- driven along the 
 intestine by tlie action of its 
 muscular walls ; meanwhile the products formed are being 
 absorbed into the blood through the lining membrane of 
 the intestine. 
 
 28. The General Structure of the Intestine. — The main 
 functions of the intestine, like those of the stomach, are 
 indicated in the structure of two of its coats, the muscular 
 
 The submucous coat consists of connect- 
 ive tissue and contains the larger 
 blood vessels from which the raucous 
 and muscular coats are suiiplied with 
 blood 
 
ALIMENTATION AND DIGESTION 117 
 
 coat and the glandular mucous membrane. The fibers of 
 the former are arranged in two layers, an inner layer in 
 which thej^ are circularly disposed around the mucous 
 membrane of the gut (see Fig. 54), and a much thinner 
 outer layer in which they run lengthwise. The contraction, 
 or shortening, of the circular fibers constricts the bore, or 
 lumen, of the tube; and it is essentially this result that 
 the muscular work of the intestine produces. 
 
 In the structure of the inner or mucous membrane two 
 points are of importance to us. In the first place numerous 
 simple tubular glands discharge into the intestinal tube an 
 important digestive juice, the intestinal juice ; in the second 
 place, finger-like processes, or villi (0.5-0.7 mm. long by 
 0.1 mm. thick), arise from its surface and jDroject into the 
 intestinal cavity. These are important organs of absorption. 
 The entire surface of the villi, the glands, and the plane 
 surface of the intestine between these structures is lined 
 with a continuous membrane comjiosed of columnar cells, 
 which separates blood vessels and lymphatics in the intes- 
 tinal wall from the cavity of the intestine (see Fig. 55). 
 
 29. The Pancreas and the Liver. — Besides the intestinal 
 juice thus added from the glands of the mucous membrane 
 to the contents of the intestine as they are passed along 
 its length of twenty or twentj^-five feet, two other juices, 
 the pancreatic juice and the bile, are mingled with the 
 chyme from two large and important glandular organs 
 whose ducts open near the stomach. These glands are 
 the pancreas and the liver. It is not necessary for our 
 present purpose to describe the minute structure of these 
 organs ; it is enough for the student to understand that 
 they are glands (p. 29), which pour their secretions through 
 ducts into the intestine, very much as the salivary glands 
 pour their secretions into the mouth. 
 
 The food in the intestine is thus treated with three 
 principal secretions, or chemical reagents, — the pancreatic 
 
118 
 
 THE HUMAN MECHANISM 
 
 juice, the bile, and the intestinal juice ; and we may now 
 pass on to a study of these reagents and the chemical 
 reactions they cause in the chyme. 
 
 30. The Pancreatic Juice is a strongly alkaline liquid, 
 and consequently, when mixed with the acid chyme, it 
 
 neutralizes most if not all the 
 hydrochloric acid of the latter. 
 Thus it hapi^ens that while 
 the food in the stomach is 
 strongly acid, in the intestine 
 it becomes at once either neu- 
 tral or alkaline. Under these 
 circumstances the pepsin of 
 the gastric juice becomes in- 
 active (see p. 108), and is soon 
 destroj-ed by the pancreatic 
 juice, so that it jilays no fur- 
 tlier role in proteid digestion. 
 This is henceforward carried 
 on by an enzyme of the pan- 
 creatic juice, tri/psin, which 
 acts most vigorously in a 
 neutral or slightly alkaline 
 medium. It forms from the 
 proteids of the food the same 
 general class of peptone-like 
 substances produced by the 
 action of the gastric juice. 
 In addition to trypsin the 
 pancreatic juice contains at 
 least three other enzymes ; 
 one of which seems to be 
 closely related, if not identical with, the rennin of the 
 stomach, although tlie exact part it plays in digestion is 
 not yet clear. The other two enzymes are much more 
 
 Fig. 
 
 of 
 
 Longitudinal section 
 tlie tip of a, villus 
 
 Slir)wi]]t^ the colnmnar linin;^ cells 
 11, tlirnugh wliicli tlie products of 
 (li^i'stion must jiass on their way 
 tf) tlie blood vessels atiil lymphat- 
 ics. The connecti^'e tissue be- 
 tween the colnmnar c-ells and tlie 
 vessels is inilieated diai^ramiuati- 
 eaily and "\vit limit slioviii;^ its 
 structure. A, cidl wliicli niann- 
 factures mucus ; r,', capillaries ; 
 D, lacteal or lyni|ihatic 
 
ALIMENTATION AND DIGESTION 119 
 
 important. One of them, amijlopsin, is practically identical 
 with the ptyalin of the saliva, and changes starch into 
 sngar, much as happens in salivary digestion. The other 
 enzyme, lipase or steapsin, acts upon fats, changing them 
 into fatty acids and glycerin. We cannot go into the 
 details of the somewhat complicated digestion of fats. 
 The change, like that of proteids into peptones and of 
 starches into sugar, involves the formation of a smaller 
 molecule, either of fatty acids, or soaps, or both, and it 
 is probably in these forms that fats are received from the 
 intestine by tlie villi. 
 
 The pancreatic juice thus contains a special enzyme for 
 each of the three great classes of nutrients, — proteids, fats, 
 and carbohydrates, — and thoroughl}'' completes their diges- 
 tion, after they have undergone tlie preparatoiy processes 
 effected by cooking, mastication, and gastric digestion. 
 Pancreatic juice is hi/ fa r /he most inijiiirtinit of tlie digest- 
 ive juices ill piroiliieiiiji the eliemieal cJiaia/es of dii/estion. 
 In this respect, also, we may say it is ol primary impor- 
 tance in the work of intestinal digestion, tlie other two 
 juices, the bile and the intestinal juice, acting as aids in 
 its work. 
 
 31. The Bile contains no enzymes of importance in 
 digestion. It is in fact partly an excretirm, some of its 
 constituents being waste products v"hicli are poured into 
 the intestine only to lie ultimately discharged from the 
 rectum. Other constituents of the bile pla}- an important 
 r81e in the digestion and absorption of fats, as is sho^\n 
 by the fact that, if bile be prevented from entering the in- 
 testine, from forty to sixty per cent of the fat eaten fails of 
 absorption and is discharged with the feces. It is probable 
 that this is because certain soaps formed in pancreatic 
 dio-estion are not soluble unless bile is present. When 
 these soaps are not dissolved they are not only themselves 
 not absorbed, but, by being precipitated and adhering to 
 
120 THE HUMAN MECHANISM 
 
 other still undigested food, prevent the ready access of the 
 enzymes, and so greatly delay digestion. 
 
 32. The Intestinal Juice, like the bile, contains no enzymes 
 of primary importance, although it contains some which 
 play a subordinate role. It is, however, characterized by 
 its large content of alkaline salts, especially sodium car- 
 bonate. This is of importance, since two processes not 
 yet emphasized but constantly occurring in the intestine 
 produce acid ; these are (a) the splitting of the fats into 
 fatty acids and glycerin, and (b) the bacterial decomposi- 
 tions of carbohydrates and (to some extent) of proteids. 
 The sodium carbonate of the intestinal juice which, it will 
 be remembered, is being secreted along the entire length 
 of the intestine, neutralizes these acids and so maintains 
 the reaction of the contents at an approximately neutral 
 point. This reaction is most favorable for the action of 
 the enzymes present. 
 
 33. Action of the Muscular Coat of the Intestine. — Few 
 points in the structure and functions of the intestine are 
 more strilving than the provisions made for promoting 
 mechanically by muscular movements the effective mixture 
 of the food received from the stomach with the digestive 
 juices and abundant contact of the digested foods with ab- 
 sorbing surfaces. Consider, to begin with, the structure of 
 the intestine. The average length of the small intestine 
 is about twenty feet ; its average circumference is some 
 three inches ; this of itself would give an absorbing sur- 
 face of some five square feet. But this surface is increased 
 fivefold or more by the finger-like villi and by folds of 
 the mucous membrane ; so that we may estimate the total 
 area of the absorbing surface as probably not less than 
 twenty-five square feet, an area almost twice as great as 
 that of the skin. But this is not the whole story. If the 
 digesting mixture in the intestine were kept in constant 
 motion, it is clear that contact of all parts of it with the 
 
ALIMENTATION AND DIGESTION 
 
 121 
 
 villi would be greatly facilitated; and such a motion, or 
 agitation, is in fact maintained by contraction of the mus- 
 cles of the intestinal walls and the consequent movement 
 of the contents. 
 
 34. Divisive or Segmenting Movements. — The move- 
 ments of the intestine, like those of the stomach, are best 
 
 B 
 
 (t) 
 
 
 D 
 
 \ / \ / \ / \ / \ / 
 
 o o o 
 
 Fig. 5(5. The divisive or seH'meutin" 
 
 ovcmcnts of tlie small intestine 
 
 A, suvf.Tco view of a pitrtiou of the intestine, sliowiny- si.\ eonstrietions "which 
 divide tlic eontents into five se,Lrnients, as sliown in />' ; as tliese constric- 
 tions pass away new ones come in l)etween tlieui and di^'ide eacli segment 
 ot tlie <'onlcnts into two. the adjoining lialves of neighboring segments 
 fnsiiig to nialve tlie new segments sliown in C. Repetition of this process 
 results in the condition sliown in It 
 
 studied by means of the Rontgen rays. These studies 
 have shown that the food is not distributed continuously 
 along the entire length of the intestine, but is divided into 
 a number of separate portions which lie in different loops 
 of the intestine. This is partly due to the intermittent 
 character of the discharge of the chyme from the stom- 
 ach. A certain number, sometimes all, of these masses of 
 food will be seen to undergo division into small segments, 
 
122 THE HUMAI^r MECHANISM 
 
 obviously produced by a series of constrictions of the 
 walls, as shown in Fig. 56. The next moment these are re- 
 placed by a second series of constrictions between the first. 
 Each segment is thus divided into two, and the neighbor- 
 ing halves of these segments fuse. The next moment the 
 second series of constrictions is replaced by the first, and 
 this process continues at times for many minutes ivith no 
 change in the general 'position of the food mass. These 
 divisive, or segmenting, movements occur from twenty to 
 thirty times a minute, and it has been estimated " that a 
 slender string of food may commonly undergo division into 
 small particles more than a thousand times while scarcely 
 changing its position in tlie intestine." ,, 
 
 35. Peristalsis. — Every now and tlien a ring of con- 
 striction, instead of Ijeing confined to one place, moves 
 onward, pushing before it the contents of the tube. A 
 contraction of this kind is called peristaltic. The effect 
 Ijroduced is much the same as when the contents of a 
 rubber tube are emptied by squeezing it along between 
 the thumb and finger ; and it is by this means that 
 the contents of the small intestine are from time to 
 time moved onwards from the stomach toward the large 
 intestine. 
 
 Thus each consignment of chyme delivered from the 
 stomach immediately receives its share of pancreatic juice 
 and of Ijile, and tlie iiiial transformation of the digestible 
 foods takes place as the whole is driven from time to 
 time along the intestine Ijy peristaltic contractions, the 
 efficiency of tlie contact of tlie food with the digestive 
 juices, as well as its exposure to the absorbing surfaces, 
 being greatly enhanced by tlie agitation jiroduced by the 
 movements of constrictive division carried out by the cir- 
 cular muscles between pjeriods of peristaltic activity. TJie 
 efficiencij of digestion and ahsorption pjrohahlg depends as 
 'niaclt oil the inoi'cments carried out hg tlie muscular coat as 
 
ALIMENTATION AND L>iGE.STION 
 
 upon the clieviical processes effected hi/ the enzymes and tJie 
 t-tlier constituents of t lie d'ujestlee j uiees. 
 
 So far as is known, the stimulus to tliese movements is 
 afforded by the presence of mate- 
 rial within the intestine, and es- 
 pecially by the contact of solid 
 particles with the walls ; Ijut we 
 cannot enter into detail on this 
 point furtlier than to say that ner- 
 vous factors are known to play an 
 important part. 
 
 36. Absorption is the name given 
 to the passage of digested food 
 materials from the cavity of tlu 
 intestine into the IJood. The word 
 itself perliaps suggests that tin 
 products of digestion are received 
 into tlie blood without change, as 
 a sponge might absorb a solution 
 of peptone, sugar, fatty acids 
 soaps, and imirganic salts. Such 
 however, is by no means the case, 
 and the actual pliysical and chem- 
 ical processes of alisorption aic 
 extremely complicated, — far too 
 complicated to be discussed here. 
 Sulhce it to say that the intestine 
 is not lined by a dead mendjranc 
 but by living cells, and through 
 these guardian cells the products 
 of digestion must pass to enter the 
 blood (see figure of a villus, p. 118). 
 It is known that the jDcptone-like bodies are not allowed 
 to enter the blood (except in mere traces), and it is gen- 
 erally believed that they are again built up into certain 
 
 Fi'.. 57. Tliu iiite.stiiial. struc- 
 tures concerned in ab- 
 soriiti'iin 
 
 In one \Illiis is sliown lliecloso 
 nctwiirlv of hliiod vessels 
 iDuiieiiiately miller tlie lili- 
 in- nii'inlii-ane; in ilie otlier 
 Till lis, I he centra llympliatie 
 Oliserve that tlie 
 of (liLie-stiiin nurst 
 xposed to absorp- 
 -1 vessels 
 
 orlaeleal 
 
 proilllels 
 
 tirst 111 
 
 tioii liv the bill 
 
 befoi'i' they can enter the 
 lacteal 
 
124 THE HUMAN MECHANISM 
 
 definite proteids which can nourish the tissues. Again, 
 tlie fats, thougli tliey enter tire lining membrane as soaps 
 or fatty acids, appear in the blood largely as fats. The 
 grape sugar, on the other hand, formed in the digestion of 
 starch, probably passes into the blood unchanged. 
 
 The present state of our knowledge does not justify 
 further discussion of the full significance of these changes. 
 But what has been said is enough to show that the chemical 
 changes of digestion do not end with those produced by 
 enzymes or otlier agents in the cavity of the intestine, 
 but that these are succeeded by other important changes 
 effected during absorption. The object of the whole process 
 of alimentation, digestion, and absorption would seem to 
 be that of supplying food to the muscle fiber, the gland 
 cell, the nerve cell, etc., through the blood as an internal 
 medium, or middleman, in that form which is best fitted 
 for the use of the tissues. No matter how varied the forms 
 of proteid (meats of various kinds, beans, peas, white of 
 egg, curd of milk, etc.), all would seem to enter the blood, 
 after being worked over Ijy the processes of digestion and 
 absorption, in the form of a few definite compounds, pos- 
 sibly of proteid nature, which are utilized by the tissues 
 with comparative ease ; and something similar is true of 
 fats and carbohydrates. It is very instructive to find that 
 whereas grape sugar and fruit sugar, when introduced 
 directly into the blood, nourish the tissues of the body as 
 well as when they are received from the alimentary canal, 
 the closely related cane sugar when so injected is use- 
 less ; taken as a food into the alimentary canal, the latter 
 is there changed into grape sugar and fruit sugar, and 
 absorbed in these forms, it nourishes the body. Simi- 
 larly, the raw or unboiled white of egg is useless as a 
 food unless it first be changed into other proteids by the 
 processes of digestion. Indeed, it is in general correct to 
 look upon the chemical processes of internal digestion as 
 
ALIMENTATION AND DIGESTION 125 
 
 merely initiating a long series of successive chemical 
 changes, which, beginning in the intestine, are continued 
 in tlie process of alisorption and only completed in tlie 
 
 Fig. 58. The paths by which the prodiirt.s of dige.slion enter the general 
 
 circuhition 
 
 Those which are iiiisoriied by the ))1ci(k1 vessrls f' nf tlie intestine puss hy tlie 
 portal vein P.V. to the li\'er before^ they can I'liti-r tie- ri^ht. ;iurii'lr 
 It. A. through the hepatii- vein l[.V. ami the inferior \-ena I'ava l.V.I'. 
 Those products which are ahsorhed h\" tin; lactcals pass <iiroialy to the 
 superior vena cava S. VJ.'. llirou;,di the thoracic duct 
 
 tissues themselves. The last stage of these changes we 
 have yet to study in the chapters on nutrition. 
 
 37. Microbic Life in the Intestine. — < Iccurring sinniltane- 
 ously with the chemical changes produced hy the digestive 
 juices are others produced hy mierolies (Part II), which 
 are always found in tlie intestine in large quantities. The 
 acidity of the gastric juice keeps down the numbers of 
 these germs in the stomach, and under healthy contlitions 
 greatly limits their acti\'ity in that organ. We ]la^■e seen, 
 however, that some portions of the contents of the stomach 
 are not acid in reaction during certain perioils of digestion ; 
 and it not infreciuently happens for this reason that un- 
 healthy living, and especially improper feeding, may result 
 in serious gastric indigestion with excessive bacterial decom- 
 position of the food. The production of gas, leading to 
 flatulence or belching, is one of the most familiar results 
 of such bacterial action. 
 
126 THE HUMAN MECHANISM 
 
 In the intestine the approximately neutral or even alka- 
 line reaction is much more favorable to bacterial life and 
 growth, and we accordingly find that the number of microbes 
 gradually increases from the stomach to the large intestine. 
 It is not the microbe itself, however, which is of impor- 
 tance to the organism as a whole, but the substances which 
 it produces from the foods. Most of these are either harm- 
 less themselves or else are readily changed into harmless 
 substances, either before or soon after entering the blood; 
 it has even been suggested that some are useful and that 
 microbic life is a valuable aid to digestion, — an idea that 
 has no substantial liasis ; others are poisons, but are nor- 
 mally present in such minute quantities as to be entirely 
 negligible ; moic rarely tliey ai'e produced in large quanti- 
 ties and may cause various ill effects either locally or upon 
 the body as a wliole. 
 
 The production of undue quantities of such harmful 
 substances, most of which are derived from proteids, is 
 chiefly dependent upon the food supply of the bacteria. 
 This is normally kept low by the speedy and efficient 
 removal of the pepttmes. Native ^ proteids are acted on com- 
 paratively slo«'ly by l)actcria, and in any case must first 
 l)e changed into peptones before they can be further broken 
 down into harmful bodies. If, however, the processes of 
 absorption quickly and elficiently remove the pe[)tones 
 formed both by the Ijacteria and by the enzymes, subse- 
 quent harmful decomposition of the food is prevented, for 
 tliere are normally no bacteria in the blood. It is, there- 
 fore, of great importance to maintain the efficiency of 
 absorption. This can be done in general only by leading 
 a normal life, — by taking sufficient muscular exercise, by 
 
 1 A " native" proteid is a. proteid in its natural form, or as it occurs 
 ill nature before beinu' changed by digestion or otlier chemical action. 
 Tlie proteids in food are largely native proteids, or else, what amounts 
 to the same thing, as far as the action of bacteria is concerned, native 
 pjroteids coagulated by lieat. 
 
ALIMENTATION AND DIGESTION 
 
 127 
 
 proper habits of sleep and rest, by proper feeding, and so 
 on. The hygienic conduct of life tends to maintain all 
 functions of the body in proper working condition, those 
 of the dicfestive org-ans included; and nothino- else can be 
 depended on, in the long run, to do this. To tins suliject 
 we shall return in the chaptei'S on In-giene, when dealing 
 directly with the personal conduct of life. 
 
 We have thus far Ijcen dealing only 'witii those microbes 
 commonly found in the intestine. At times foreign mi- 
 crobes find entrance, and these may be the cause of such 
 diseases as typhoid fever, dysentery, ch(jlera, etc. The 
 action of these occasional intruders will be more fully 
 dealt with in Part II. 
 
 38. Digestion in the Large Intestine. — Tlie large intes- 
 tine contains no villi, and its glands secrete a sumewhat 
 different intestinal juice from 
 that of the small intestine, it 
 is a juice cliai'acterized ly a 
 larger percentage of the vis- 
 cous substance, mucin, familiar 
 in the nasal discharge during a 
 cold, and in the thiclv, glairy 
 saliva sometimes secreted. This 
 juice serves chielly to lubiicate 
 the walls of the intestine and 
 to prevent harmful friction 
 with the more solid contents. 
 
 In the small intestine tlie 
 amount of water added by se- 
 cretion balances that absorbed, V' 
 so that the consistency of tlie 
 contents undergoes l>ut little 
 change fronr the stomach to the beginning of the large in- 
 testine. Tliis consistency, it will be remembered, was (ap- 
 proximately) that of moderately thick pea soup. During 
 
 
 Vj. Liiiinitudinal .section of 
 
 the large intestine 
 Note the ahsence of villi 
 
128 THE HUMAN MECHANISM 
 
 the passage through the small intestine the digested por- 
 tions of the food are being removed by absorption, while 
 the indigestible elements are left behind. Among the indi- 
 gestible elements of food are certain connective tissues of 
 the animal foods, but especially tlie cellulose (p. 94), which 
 forms the cell wall of plant tissues. The large intestine 
 receives from the small this indigestible material, together 
 with a certain variable but usually comparatively small 
 proportion of the proteids, fats, and carbohydrates which 
 have thus far escaped digestion ; in addition there are cer- 
 tain constituents of the digestive juices which are not 
 absorbed, and some (e.g. certain constituents of the bile) 
 which are distinctly excretory products. 
 
 The digestion of proteids, fats, and carbohydrates may 
 to some extent continue in the large intestine, and the 
 products of digestion be absorbed. This process, however, 
 tends to be checked, owing to the fact that here the 
 processes of secretion fall behind those of absorption, 
 so that the contents of the intestine gradually become 
 more and more solid, being finally collected as the feces 
 in the lower colon and rectum. As the intestinal contents 
 Ijecome more solid, the peptones and other digestive prod- 
 ucts are less efficiently removed while bacterial action 
 greatly increases, giving to the feces their characteristic 
 odors. And while the substances which are responsible 
 for the offensive odors of the feces may themselves be 
 harmless to the body, it must be remembered that such 
 odors are an indication of l)acterial action capable of pro- 
 ducing other substances, which may be, and very often are, 
 the cause of headaches, general malaise, etc., or of even 
 more serious troubles. 
 
 39. The Elimination of Intestinal Waste. — Those who 
 are " blessed with a g-ood digestion " sometimes find it 
 hard to realize that the preparation of food for absorp- 
 tion through the delicate membranes lining the alimentary 
 
ALIMENTATION AND DIGESTION 129 
 
 canal is a difficult and complex process, requiring much 
 delicate physical and physiological apparatus and involv- 
 ing various and important chemical reactions. Even when 
 they realize this, they rarely appreciate the indispensable 
 cooperation and fine adjustment of the several parts and 
 processes concerned. It is just here, however, that a clear 
 understanding is important, for witliout this it is not easy 
 to see how disorders of digestion arise. 
 
 Let us then remember that the efficient handlino- of the 
 
 o 
 
 food in the stomach is aided by the preparatory crushing 
 it receives in the process of mastication ; that in the stom- 
 ach an adequate and efficient secretion of gastric juice 
 must take place, and that this begins as the result of nerv- 
 ous events connected with our enjoyment of the food 
 when eaten ; that the continued secretion of gastric juice 
 is secured, in turn, by stimulation of the mucous membrane 
 of the stomach by the peptones which the "psychic " secre- 
 tion has formed from the proteids of the food ; and finally 
 that the chemical action of the gastric juice is aided by the 
 peculiar contractions of the muscular coat of the stomach. 
 All these agencies ivorlcing toi/ether deliver the food to the 
 intestine in a finely divided state, well adapted and indeed 
 absolutely necessary to secure the proper contact of the 
 food with the pancreatic juice, the bile, and the intestinal 
 juice. 
 
 The flow of pancreatic juice, in turn, is partly the result 
 of the action of the hydrochloric acid of the chyme on the 
 walls of the intestine, while the efficiency of the action of 
 the pancreatic enzymes depends upon the simultaneous 
 action of the bile and the intestinal juice ; lastl}', the 
 chemical action of these juices, as well as the final act of 
 absorption, requires the cooperation of the muscular coat. 
 Healthy conditions with respect to bacterial action simi- 
 larly depend upon all else occurring as it should. Diges- 
 tion, in short, *'S « chain of events, each depending upon 
 
130 THE HUMAN MECHANISM 
 
 those which liave gone before, and to a large extent upon 
 those which are taking pLice at the same time. 
 
 Keeping these facts in mind, it is easy to appreciate the 
 possibility of diarrhea or constipation, the latter consist- 
 ing in the retention of wastes, the poisonous constituents 
 of wliicli may be absorbed into the body and cause dis- 
 comfort, headaches, and malaise. When all the digestive 
 processes work together properly there should be a per- 
 fectly natural and regular evacuation of the bowels. The 
 frequency of such evacuation varies somewhat and is 
 largelj' a matter of habit ; with some people it is twice a 
 day, with others once everv other day, hut with the vast 
 nidjoriti/ it is normalli/ once evert/ day and at ahout the same 
 time. Where this is not the case there is reason to believe 
 that some part of the work of digestion is not being prop- 
 erly performed. The trouble is not ordinarily in the mech- 
 anism governing the actual discharge of the feces from the 
 rectum, but in a derangement somewhere else; it may be 
 entirely the fault of the mechanism of peristalsis, or it may 
 be due to imperfect secretion. In all cases it means that 
 sometJdvg is wrong, and the remedy should l)e sought not 
 in drugs or pills l)ut in searcli for and removed erf the 
 eaase. A moment's consideration will show the reasonable- 
 ness of this position. If a watch loses time because it needs 
 cleaning, we do not seek a remedy in drugs, but in its 
 cleaning, better adjustment, and good care ; and the rem- 
 edy for diarrliea or constipation should in all cases be 
 sought for in the better conduct of life. Is enough mus- 
 cular exercise being taken '.^ Is the diet jiroperly chosen? 
 Are we drinking enough water? Especially, is the food of 
 sufficient liulk, and does it ciintain enough laxative mate- 
 rial (sucli as fruit)? And, al)ove all, are we getting enough 
 sleep? Are we overworl'cing, or do we work too long at a 
 time without resting? Is our clothing warm enough, or 
 are we over(dad? Such are the questions wliich should be 
 
ALIMENTATION AND DIGESTION 131 
 
 seriously asked. The student of personal hygiene cannot 
 lay to heart too seriously the truth that the man who goes 
 from day to day, from week to week, from year to year, 
 neglecting the warnings of diarrhea or constipation, only 
 reaps the harvest of his folly when in later years he suffers 
 loss of health and at times bodily discomfort ; and it is noth- 
 ing short of impiety to marvel under such circumstances at 
 the " mysterious " ways of a Providence which so " afllicts "' 
 his creatures. It is no exaggeration to say that the regular 
 discharge of the wastes is quite as important as the regular 
 feeding of the bod3', and that no less pains should be taken 
 to form good habits in the one case than in the other. 
 Many of the headaches, man}' of the Ijad feelings, and 
 many of the bad tempers of tlie world are due to neglect 
 of this simple fact. iS'o city, lio\\'ever well fed or l)eautiful, 
 the drains of which arc choked with liltli, can long remain 
 eitlier wholesome or attractive, — and tlie luinian body is 
 essentially a city jjopulous with living cells. 
 
CHAPTER IX 
 THE CIRCULATION OF THE BLOOD 
 
 A. Blood and Lymph 
 
 1. The Blood as a Common Carrier. — In previous chap- 
 ters some of the more general features of the circulation 
 have already been touched upon. In studying the parts of 
 the body the student has become somewhat acquainted 
 with the heart, the arteries, and the veins ; in considering 
 the typical structure of the organs (Chapter III) he has 
 seen how the arteries are connected with the veins by a 
 system of communicating tubes, the capillaries, through 
 the thin walls of which interchange takes place between 
 the lymph and the blood ; and in studying the interdepend- 
 ence and cooperation of the cells and organs (Chapter VI) 
 he has learned how the blood leaving each organ returns 
 to the heart, there to be mixed with that coming from all 
 other organs and thence pumped first to the lungs and 
 then to the rest of the l;)ody. The need of a circulation is 
 obvious ; for the food received from the alimentary canal 
 and the oxygen received from the lungs must somehow be 
 carried to the muscle fibers, the nerve cells, the gland 
 cells ; the cellular wastes must be taken away to the 
 organs of excretion, and the internal secretions of the 
 body must be transported fronr the organs in which they 
 are made to those in which they are to be used. In other 
 words, it is a necessary corollary to the fact that no cell 
 or organ " liveth unto itself " that there should be some 
 common carrier of matter and of energy from one organ 
 to another. Such a common carrier is the blood. The 
 
 132 
 
THE CIRCULATION OF THE BLOOD 
 
 analogy of the blood system of the body with the railway 
 system of a country is instructive. As different persons 
 and different communities in any country make different 
 products and have dif- 
 ferent needs, it becomes 
 more and more neces- 
 sary that the means of 
 communication between 
 them be extensive and 
 efficient. Hence the re- 
 markable growth of the 
 railroads, or " common 
 carriers," of any coun- 
 try in which industrial 
 development produces 
 increasing division of 
 labor. 
 
 The blood, which is 
 thus the common carrier 
 first between the various 
 organs, and second be- 
 tween each organ and 
 the outer environment, 
 is the net product of the 
 united work of all the 
 organs ; from the alimen- 
 tary canal it receives 
 water and the products 
 of digestion ; from the 
 lungs it receives oxygen ; 
 each organ contributes its share of waste products or of in- 
 ternal secretion, while some influence the composition of the 
 blood by removing from it certain things that it contains. 
 
 2. The Microscopic Structure of the Blood. — Examined 
 under the microscope the blood is seen to consist of a 
 
 (iO. Structure (..f ; 
 seen under tin; 
 
 L. drop of blood as 
 inicroscoiie 
 
 Alto\'n are slio^^'ii Tiiiio red corpuscle's, hi;^hly 
 ma^'Tiiiicd ; bcluw, less hiulily iiia.^iiiliod, 
 the apiicai-aiice of llie liliind snon after 
 liciiii^ drawn. T\\'<i white rorimsdcs are 
 shown, and ttie red corjiusides stick to- 
 gether, loi'niing " riadcaux." Size of red 
 c{irpnsele, 7.7m widi', L'^/^ thick. ])iam- 
 cter of wliite crtrpiisele, .VKI/^. Nunilfer 
 of red corpuscles, 4,r)tK:),(kKK-i,(ioo,(io(> per 
 citliic jnillinicter. Niitnlier of \Yhitc cor- 
 l>nscles, 4.10()-l;i.ii()0 ])er ciiliic iiiillinieter, 
 accordiiie to the state of dii^estion, etc. 
 Surface area of all the red corpuscles 
 of the hlood, 3IX» square meters (.'jCTOO 
 square feet or approximately four times 
 the size of a basel)all diamond. (If* or 
 micron = O.OOf millimeter) 
 
134 
 
 THE HUMAN MECHANISM 
 
 '^ 
 
 ■X? 
 
 liquid portion, the plasma, crowded with small solid bodies, 
 the corpuscles. These are of two kinds : the red corpuscles, 
 — biconcave disks containing a pigment, hemoglobin, which 
 gives the red color to the blood ; and the white corpuscles, 
 which are colorless, nucleated cells. 
 
 Important data on the number, size, and surface area of 
 the corpuscles will be found in connection witli Fig. 60. 
 
 3. The White Blood Corpuscles. — The white blood cor- 
 puscles really comprise several different kinds of cells, bav- 
 in o- different functions, 
 the study and explana- 
 tion of which belong to 
 advanced rather than ele- 
 mentary physiology. It 
 is enough for our pur- 
 pose to state that these 
 cells are not confined to 
 the blood, but work their 
 wdj out of the blood ves- 
 sels between the cells of 
 the capillary walls and 
 are often found in the 
 l3'mph spaces of the tis- 
 sues as wandering cells. 
 Tlie latter term refers to 
 their movement from place to place. The cytoplasm of the 
 white corjjuscle is a thick, viscous fluid without constant 
 or definite form. In locomotiou the cytoplasm flows slowly 
 from Some part of tlie surface in the direction of motion, 
 forming what is known as apsciidopodium (from the Greek ; 
 meaning a false fijot), as shown in Fig. 61 ; the rest of the 
 body of the corpuscle then flows into the pseudopodium. 
 By the continuation of this process the white corpuscles 
 make their way through the spaces of the connective tissue. 
 Locomotion by means of pseudopodia is frequently spoken 
 
 ^o^ 
 
 ^^ 
 
 , 61. 
 
 Amceboid movement of ii wliite 
 corpuscle 
 
 Showing four oonsecuti\'e positions anion 
 a group of red corpuscles 
 
THE CIECULATION OF THE BLOOD 135 
 
 of as amceboid, from the amceba, a unicellular animal wliicli 
 moves in the same manner. 
 
 4. The Red Blood Corpuscles. — The red corpuscles are 
 pigmented, biconcave disks, with no nucleus ; tliey are nor- 
 mally confined to the blood vessels and are carried around 
 passively in the blood current without active movements 
 of their own. The main function of these corpuscles is 
 to carry oxygen from the lungs to the tissues, a function 
 which will be further studied in connection with respira- 
 tion. They contain a pigment, Jtemoglohin, which gives to 
 the blood its red color and carries the ox_ygen. 
 
 5. The Blood Plasma is an exceedingly complex fluid 
 wliose general composition is represented as follows: 
 water, 90 parts; solids, 10 parts, — proteids, 8 parts; inor- 
 ganic salts, 1 part; extracti\'es, 1 part. 
 
 Under the extractives ar(>, included a very large num- 
 ber of substances which, thougli present in small (juantities, 
 are interesting to llie pliysiologist bei/ause tliey are largely 
 products of the chemical activities iif tlie body, and as such 
 o-ive information aljout the nature of the cliemical chanfres 
 occurring in the organs. 
 
 Finally, it should be remend)ered that the cells of the 
 body generally are bathed with lyniph, not with blood; 
 in (jther words, that the lymph and not the blood is the 
 immediate environment of tlie cells. Lymph is sometimes 
 described as blood minus its red t'orpuscles, but this state- 
 ment, thougli convenient, is not strictly correct, since the 
 amount of waste products in lymph must be greater than 
 in blood, while the amount of food material must be 
 less (see Chapter IV). Much as the blood is a product of 
 the united chemical activity of all the organs of the body, 
 so the lymph of each organ is derived from the cells of 
 that organ and from the blood flowing through it. Lymph 
 thus has a double origin, and of course shows very con- 
 siderable differences of composition in different organs. 
 
136 
 
 THE HUMAN MECHANISM 
 
 B. Mechanics of the Circulation of the Blood 
 
 AND OF THE FlOW OF LyMPH 
 
 The greatest discovery ever made in physiology was 
 that of the circulation of the blood. As late as the settle- 
 ment of the earliest English colonies in America it was 
 thought that the blood moved back and forth in the blood 
 vessels, as the waters in the sea ebb and flow ; but of any 
 circulation, in the sense of a steady stream returning to 
 its source, there was no idea; and it was not until 1621 
 
 that William Harvey, an Eng- 
 lish physician, proved beyond 
 the shadow of a doubt that the 
 blood in the body of all the 
 liigher animals flows like a 
 stream always in one direc- 
 tion, ultimately returning to 
 its source. 
 
 Tests made upon various 
 animals have shown that this 
 circulation is accomplished in 
 the surprisingly short time 
 of from twenty to thirty sec- 
 onds ; which means that the whole mass of the blood (in 
 man about twelve pints) passes between three and four thou- 
 sand times a day through the various organs of the body, 
 bringing to them their food, carrying away their wastes, 
 and in general helping to maintain normal conditions. By 
 what hydraulic machinery is this marvelous work done ? 
 
 6. The Motive Power of the Circulation as a Whole. 
 The Beat of the Heart. — Whenever a mass of liquid is 
 kept in motion we naturally look first for the motive power. 
 In answering the question, " What makes the blood cir- 
 culate ? " we shall find that while there are several causes, 
 
 Fig. 02. The circulation of the 
 blood as seen in the small ar- 
 teries and capillaries of the 
 web of a frog's foot 
 
THE CIECULATION OF THE BLOOD 137 
 
 one of these, namely the beat of the heart, is vastly more 
 important than all the others combined. This fact is now 
 so familiar that it is hard to realize that we owe to Harvey 
 not only the discovery of the circulation, but also the dis- 
 covery of the meaning of the heart beat. Before his time, 
 to be sure, tire living heart had been seen at work, alter- 
 nately shrinking in size and then swelling, the shrinking 
 being called systole and the swelling diastole. But these 
 changes in size were regarded as the results of the contrac- 
 tion and expansion of certain " vital spirits " which the 
 arterial blood was then supposed to contain, and not as 
 muscular contractions and relaxations. Harvey showed 
 that the heart is a 2}oiverful muscle, and that its systole is 
 a muscular contraction ; that during systole it becomes 
 hard, just as the biceps muscle does when it shortens, 
 and during diastole soft and flabby ; he also proved that 
 with each systole the heart drives or spouts blood into the 
 large arteries (the aorta and the pulmonary artery), and 
 that this blood is prevented from flowing back into the 
 heart during diastole by membranous valves at tlie very 
 beginning of the large arteries in cpiestion. 
 
 7. The Heart a Muscular Force-Pump. — The beat of the 
 heart, even to its most minute detail, is one of the most 
 important as well as one of the most interesting subjects 
 in physiology ; everything in the body hangs on its proper 
 efliciency and regulation, and it cannot be too thoroughly 
 studied. For our present purposes it will suflice to describe 
 the heart as composed essentially of a pair of muscular 
 force-pumps. Dissection shows that it is divided into 
 right and left lialves (see Fig. 65), completely separated 
 from each other, and that each half consists of two cham- 
 bers, — an auricle and a ventricle. The auricles, into 
 which the great veins open, have thin muscular walls and 
 are comparatively small in size ; the ventricles, on the 
 other hand, from which the great arteries arise, have thick 
 
lo8 
 
 THE HUMAN MECHANISM 
 
 aiiow tni 
 
 I 
 
 ^2^zj 
 
 muscular walls, esf)ecially the left ventricle. The ventricles 
 indeed constitute the principal part of the force-pump ; 
 the auricles merely facilitate the work of the ventricles, 
 and for purposes of elementary study may be mostly neg- 
 lected. The student should, if possible, examine for him- 
 self and actually handle the auricles, ventricles, and great 
 blood vessels of a sheep's heart, whicli in size and struc- 
 ture sufliciently resembles the human heart. Figures 15 
 and 142 should also l)e consulted. 
 
 8. The Mechanics of the Heart Beat. — All force-pumps 
 consist of two indispensable parts, — some device for press- 
 ing upon a liquid within a chamber, and valves at the 
 
 openings of the cham- 
 ber so arranged as to 
 allow the passage of 
 d in one di- 
 only. Each 
 ventricle of the heart 
 is reall}' such a pump, 
 and is provided with 
 two sets of valves, — 
 one set at the inlet, 
 between the auricles 
 and the ventricles, 
 and the other at the 
 arterial outlet. These valves permit blood to pass only 
 from the great veins through the auricles and on through 
 the ventricles to the great arteries. The contraction of the 
 nuiscular wall of the ventricles produces pressure on the 
 blood within their cavities ; this pressure Cjuickly and easily 
 closes the auriculo-ventricular valves, and finally forces 
 open the shut valves at the openings of the great arter- 
 ies. In this way the right ventricle drives venous blood 
 into the pulmonary artery, and the left ventricle arterial 
 blood into the aorta. With the relaxation of the ventricles 
 
 
 Fig. 03. Diagram of the action of a 
 force-pump 
 
THE CIECULATION OF THE BLOOD 
 
 139 
 
 (diastole) pressure falls within their cavities, and were it 
 not for the valves at the mouths of the aorta and the 
 pulmonary artery, blood would regurgitate, or flow back, 
 into the heart ; but 
 this " slip" (as it is 
 called in hydraulics) 
 the valves prevent, 
 and the ventricles 
 again fill through the 
 only open channel, i.e. 
 the one leading from 
 the great veins and 
 the auricles. Thus by 
 contractions rhyth- 
 mically repeated the Fi>-.. 04 
 heart continues to 
 
 On the left is shown the coudition during dias- 
 tnlo ; (in tlie right, during systole 
 
 The force-pump action of a veutricle 
 of the heart 
 
 spout or deliver blood 
 
 from the two sets of 
 
 great veins into the two sets of great arteries. It is plainly 
 
 a double force-pump, or, better, a pair of force-pumps lying 
 
 and working side by side. 
 
 9. The Arterial and the Venous Reservoirs. — To under- 
 stand the exact nature and result of the work of the heart 
 we must now consider the relation of this living pump to 
 the pipe system (arteries, capillaries, and veins) with which 
 it is connected. The student should first trace the general 
 course of the circulation in the simple diagrammatic repre- 
 sentation given in Fig. 65. This shows that the blood which 
 enters the aorta from the left ventricle must return to the 
 right side of the heart and pass through the lungs before it 
 can again reach the aorta. As the physical principles of the 
 circulation are the same for the systemic and the pulmonary 
 vessels, we shall confine our attention to the former. 
 
 In the first place we may observe that the heart pumps 
 the blood into what is practically a large reservoir (the 
 
140 
 
 THE HUMAN MECHANISM 
 
 Fig. 65. 
 
 Diagram 
 
 of the organs 
 
 of the circuhxtion 
 
 , puhiionury circulation; ^/^circulation tlirough 
 the organs suspended hy tlie uiesentcry, the 
 blood being carried to the liver P belore it 
 returns to the lieart. Tlie circuhil inn tlnough 
 other organs, such as Ijrain, mustdes, slviu, 
 and kidneys, is indicated. Lymphatics are 
 represented by dotted lines 
 
 larger arteries), and 
 that the blood flows 
 from this reservoir to 
 a second reservoir (the 
 larger veins) ly vari- 
 ous routes; for the 
 vessels of the different 
 organs represent many 
 alternative courses 
 which the blood may 
 tiike in flowing from 
 the arterial to the ve- 
 nous reservoir. The 
 blood stream indeed 
 may be compared 
 with a stream supply- 
 ing water power to a 
 series of mills in a 
 manufacturinof town. 
 The larger arteries 
 from the main source 
 of pressure (the heart) 
 corresiDond to the 
 headrace from above 
 the dam, while the 
 larger veins corre- 
 spond to the tailrace. 
 The water flows from 
 the one into the other 
 only through the 
 smaller sluices, or 
 penstocks, which sup- 
 ply the mills. So, in 
 the vascular system, 
 a part of the blood 
 
(Esoiihagua 
 
 To head and 
 
 To shoulder and 
 arm 
 
 To stomach, 
 intestine, etc 
 
 To kidney 
 
 Fig. 66. The aorta and its main branches 
 
 At the begiuning: are shown the three pocket valves, which prevent regurgi- 
 tation of blood during diastole 
 
 141 
 
142 THE HUMAN MECHANISM 
 
 pumped into the arterial reservoir, or aorta, finds its way 
 into the venous reservoir by way of the skin, another part 
 by way of the digestive organs, another by way of the 
 brain, still another by way of the kidneys, and so on ; but 
 the flow in every case is essentially the same, namely from 
 a reservoir of high pressure to one of lower pressure. 
 
 10. The Driving Force for the Flow of Blood from the 
 Aorta. Pressure in Arteries and Veins. — The hydraulic 
 conditions in the aorta may be illustrated by means of 
 the following simple piece of apparatus. To an ordinary 
 rubber syringe attach a piece of elastic rubber tubing, the 
 other end of which is closed by a detachable nozzle. If 
 now the nozzle be removed and water pumped into the 
 tube, it will be found that the flow from the open end con- 
 sists of squirts or sjoouts and continues only during the 
 stroke of the pump ; if, however, we attach the nozzle, 
 and again pump water into the tube, the resistance caused 
 by the small orifice of the nozzle prevents the water from 
 flowing out of the tube as fast as the syringe pumps it in. 
 The tubing becomes distended ivith water. Since, however, 
 the tube is elastic,^ and so tends to return to its original 
 size, it forces the liquid out through the nozzle even be- 
 tween the strokes of the pump. The immediate cause of 
 the steady flow from the nozzle is therefore the elasticity 
 of the rubber tube. The intermittent stroke of the pump 
 produces distention of the tube, and the elasticity of the dis- 
 tended tube constantly forces the water out of the nozzle. 
 
 Closely similar conditions obtain in the arterial reser- 
 voir. Here the outlet is also through very small tubes, 
 
 1 An elastic body is one lohich returns to its original shape when it has 
 been stretched, compressed, or otherwise deformed. Elasticity must not 
 be confounded with "extensibility," or the property of allowing stretch- 
 ing. Thus when we "pull" taffy we deal with a body which is very 
 extensible but which is practically inelastic. A body indeed may be exten- 
 sible only with diificalty, but possess a very high degree of elasticity ; 
 ivory is a good example of this kind. 
 
THE CIECULATION OF THE BLOOD 143 
 
 the small arteries, whose bore is not greater than gJ^- or 
 yl^ of an inch; which fact introduces the same condition 
 as does the nozzle of our apparatus, i.e. a resistance to the 
 outward flow of the blood. Consequently the blood can- 
 not flow out of the aorta as rapidly as it is driven in, and 
 the extensible and elastic walls are necessarily stretched. 
 The immediate effect of the heart beat is to keep the 
 arterial reservoir overfilled or distended, so that the elastic 
 reaction of its walls is brought into play ; and it is this 
 elastic reaction of the arterial walls wliich is the imme- 
 diate cause of the steady outflow through the small arter- 
 ies and capillaries. 
 
 The force of compression, or pressure, exerted by the 
 elastic arterial walls is primarily exerted upon the blood 
 within them ; and the more the arteries are distended the 
 greater will be the pressure exerted on the blood. A licpid 
 thus under pressure tends to find an outlet ; should any 
 part of the arterial wall be weak, as sometimes happens 
 in diseased conditions, it is Ijulged outward ; and, for the 
 same reason, a flow of blood will take place through such 
 outlets as are presented by the smaller arteries and capil- 
 laries. Moreover, the greater the pressure of the blood in 
 the arteries, the more rapid will be the flow into the capil- 
 laries. Hence it is customary to use the arterial hlood 
 pressure as a measure of the force of elasticity exerted 
 by the distended arterial wall. 
 
 The veins, on the other hand, are less elastic than the 
 arteries ; they are indeed more like mere conducting 
 tubes through which the blood can How back to the heart. 
 
 o 
 
 They are not overfilled (since, for one reason, there is no 
 resistance to the flow of blood out of them into the heart), 
 and hence venous Hood pressure is low. 
 
 Thus we have the conditions favorable for the flow from 
 the aorta to the great veins, — a Jiic/h pressure in the arte- 
 rial reservoir and a low pressure in the venous reservoir. 
 
144 
 
 THE HUMAN MECHANISM 
 
 It is the function of the heart, by continually pumping the 
 blood from the veins into the arteries, to keep the arterial 
 reservoir distended, thus maintaining a difference of pres- 
 sure in the two reservoirs. It is this difference of pressure 
 which drives the blood through the organs. 
 
 11. The Distribution of the Blood among the Organs. — 
 Some organs require more blood than others, and the same 
 
 organ often requires more 
 ^-^^S^E?v_,„5>,,-s!*^^P^ 1 blood at one time than at 
 
 another. Thus muscles and 
 glands, the seat of very 
 active chemical changes, 
 require more blood than 
 a tendon ; and a gland re- 
 quires more blood during 
 the process of secretion 
 than during rest. How is 
 the supply of blood to the 
 organs regulated to meet 
 their varying needs? In 
 the first place, some or- 
 gans are more vascular 
 than others ; those requir- 
 ing a larger supply of 
 blood receive a greater 
 number of arteries from 
 the arterial reservoir and 
 have a closer network of 
 capillaries. But in addi- 
 tion to this, these smaller 
 arteries contain circular 
 muscle fibers whose con- 
 traction diminishes the bore of the tube. Wlien an organ 
 needs. more blood, the muscle fibers of its small arteries 
 relax, thus permitting the arterial tubes to widen or dilate ; 
 
 Fifi. 67. Cross sections of portions of 
 tlie wall of a smaller artery (a) and 
 a smaller vein (p) 
 
 internal coat; B, middle coat, witli 
 muscle tiljers ; C, outer coat of connect- 
 ive tissue. Tlie contraction of the cir- 
 cularly disposed muscle fibers narrows 
 the bore of tlie tube 
 
 A 
 
THE CIECULATIOX OF THE BLOOD 145 
 
 just as when we want the water to flow faster from a 
 faucet, by turning the spigot a httle further we widen the 
 outlet from tlie pipe. When less blood is needed the 
 small arteries are caused to constrict, just as a spigot may 
 be partially turned off (see Sections 25-27). In this way 
 the flow of blood to any organ is regulated to meet the 
 varying needs of the organ in question. i 
 
 12. Secondary Aids to the Circulation. — In the preced- 
 ing discussion we have seen that the cause of the flow of 
 blood through the organs is the difference of j}re.ssure in 
 the two reservoirs. We have further seen that this differ- 
 ence of pressure is maintained by the heart beat in pump- 
 ing blood from the venous into the arterial reservoir. A 
 moment's consideration will show that anything which 
 hastens the flow of blood from the veins into the heart, 
 and so lowers pressure within the veins, would similarly 
 aid the circulation, since, with the same arterial pres- 
 sure, more blood will flow into an empty vein than into 
 one which is partially lilled. 
 
 (a) The Breathing Morements. — There are two factors 
 which thus tend to emptj' the veins. The first is the 
 suction exerted on the hlood within the veins hy breathing 
 movements. The exact mechanism liy whicli this is accom- 
 plished must be left for consideration in the chapter on 
 respiration. Suffice it to say here that just as the enlarge- 
 ment of the thorax, when we take in a Ijrcath, sucks air into 
 the lungs, so it also sucks blood from the large veins outside 
 
 ^ In order that the .student may become more familiar with these fun- 
 damental hydrattlic principles of the circulation, such questions as the 
 following should be answered: (1) What are the two principal factors 
 whose variations change the amount of arterial pressure 1 Illustrate 
 by an example or model. (2) How would the dilation of all the arteries 
 of the intestine affect the general arterial pressure :' (3) What would be 
 the effect upon the a»ioiint uf blood flowing through the skin under this 
 condition ? (4) How would dilation of the arteries of the skin affect the 
 blood flow throuErh the brain ? 
 
146 
 
 THE HUMAN MECHAISTISM 
 
 Fig. 68. The pocket valves in the veins 
 
 On the right is shown the external appearance 
 of the vein at the valves when the latter are 
 closed ; on the left a vein slit lengthwise and 
 opened ; in the middle a longitudinal section 
 of a vein 
 
 the thorax into those which lie within it ; because of 
 the thickness of the walls of the arteries, the same effect 
 
 occurs to only a very 
 slight extent in the 
 arterial reservoir. 
 During expiration, 
 on the other hand, 
 the reduction in size 
 of the thorax forces 
 air out of the lungs, 
 and we might expect 
 that it would simi- 
 larly force blood 
 from the veins within 
 the thorax into those 
 without. And this it certainly would do if the veins were 
 not provided with valves which allow the blood to flow 
 only toward the heart. In general, therefore, both inspira- 
 tion and expiration aid the circulation, the former by 
 sucking blood into the thoracic veins and so emptying 
 those outside, the latter by making this blood in the intra- 
 thoracic veins flow on more rapidly to the heart, whence it 
 is pumped into the arteries. In a word, deep breathing 
 greatly promotes a good circulation. 
 
 {h) Intermittent Compressioji of the Veins. — The other 
 secondary factor of the circulation is intermittent compres- 
 sion of the veins, and in ordinary life this is brought about 
 in two ways: (1) Whenever a muscle contracts it thickens 
 and hardens ; the veins and capillaries which are between 
 the fibers and fiber bundles, or in the connective tissue 
 between two contracting muscles, will thus have the blood 
 squeezed out of them into the large veins ; when the mus- 
 cle relaxes the empty veins and capillaries will readily 
 fill from the arteries, since the valves of the veins will 
 prevent any backward flow of the blood from the larger 
 
THE CIECULATION OF THE BLOOD 147 
 
 veins. Alternate contractions and relaxations of muscles, 
 therefore, aid the flow of blood through this so-called 
 " pumping " action on the veins. (2) A similar pumping 
 action on the veins is exerted by alternate flexions (bend- 
 ings) and extensions at any joint. In general, flexions 
 force the blood out of the veins, while extensions allow 
 them to fill. When we remember how largely most of our 
 usual muscular actions consist of alternate flexions and 
 extensions of joints, and alternate contractions and relaxa- 
 tions of muscles, as, for example, in walking and running, 
 we can at once ap>preciate how greatly muscular activities 
 must aid the circulation. When to the effect of these we 
 add the suction action of the deepened breathing move- 
 ments the effect uj5on the circulation becomes verjr great. 
 
 13. Massage. — The action of massage is only another 
 illustration of the same principle. Uy rubbing the legs and 
 arms in the direction of tlie heart, the blood contained 
 in their veins is forced onward and the circulation aided, 
 precisely as when a muscle contracts or one member of 
 a limb is flexed upon another. 
 
 14. The Lymphatics. — Important as are the suction 
 action of the breathing movements and the pumping action 
 of contracting muscles as aids to the circulation of the 
 blood, they are even more important as causes of the 
 flow of lymph along the greater lymphatic trunks toward 
 the heart. Reference to the general method of origin of 
 lymphatics, as described iu Chapter III, will show that 
 the lymph in the lymph spaces, unlike the blood in the 
 capillaries, has not behind it a high-pressure reservoir; 
 there is no such force from hcliind to send it onwards, 
 since the lymphatics arise blindly in the tissues. What, 
 then, makes the lymph flow along the l3-mphatics toward 
 the heart? 
 
 The lymphatics resemble the veins in structure, having 
 thin walls and pocket valves ; like the veins, most of them 
 
148 THE HUMAN MECHANISM 
 
 originate in extra-thoracic organs, and join or combine to 
 form larger trunks as they proceed toward the thorax. 
 All of them finally unite in two large lymphatics within 
 the thoracic cavity, and these open into the great veins 
 near the heart. (Figures 30 and 65 should be consulted 
 in this connection.) It is at once clear that the breathing 
 movements must exert on the lymph within these thin- 
 walled vessels exactly the same suction action as they 
 exert on the blood in the veins ; and anything which 
 increases this suction action, such as the deepened breath- 
 ing movements during muscular activity, must necessa- 
 rily increase the flow of lymph from every organ of the 
 body. On the other hand, a pumping action on the lymph 
 in the organs results from all rhythmic movements of parts 
 of the body with reference to one another, since each 
 change of position carries with it some change of external 
 pressure on Ijauphatics. Familiar examples are the move- 
 ments of arms and legs in locomotion, of the diaphragm in 
 breathing, and of the lungs in respiration. 
 
 It has also been supposed that a third cause of the 
 lymph flow is the passage of waves of constriction (peri- 
 stalsis, cf. p. 122) over the larger lymphatics. This, how- 
 ever, probably plays only a minor part. 
 
 Finally, in the formation of lymph from the blood, more 
 water generally passes from the capillaries to the lymph 
 spaces than from the lymph spaces into the capillaries. 
 Under these circumstances, at least at certain times, the 
 lymph spaces become distended and a certain low pressure 
 obtains in them. This we may speak of as the " active 
 force " of lymph formation, and it constitutes a fourth 
 factor in causing the lymph flow. 
 
 We have already pointed out the importance of the 
 lymph flow in maintaining the lympli currents about the 
 living cells ; we are now able to appreciate the importance 
 of those agents which secure this flow. As enumerated 
 
THE CIRCULATION OF THE BLOOD 149 
 
 above, they are four in number: (1) suction action of tlie 
 breathing movements ; (2) pumping action of muscular or 
 passive movements ; (3) active force of lymph formation ; 
 (4) peristaltic contractions of the large lymphatics. 
 
 Of these the fourth is at least doubtful and in no case 
 of great importance ; the other three may therefore be 
 regarded as the chief causes of the lymph flow, and of 
 these the first and second are brought into most effective 
 action by muscular activity; tliis deepens the breathing 
 movements and so increases their suction action on the 
 lymph, while the movements of the body exert on the 
 lymphatics a pumping action, which is hugely lacking 
 during com[)lete inactivity. The great practical impor- 
 tance of this aspect of the subject will be discussed beyond 
 in those chapters which deal with the liygiene of muscular 
 activity (Part II). 
 
 C. The Ad.tustment of the Cikculation to the 
 Needs of I^vekyday Life 
 
 The total quantity of lilood in the Ijody (ten to fourteen 
 pints) is not enougli to furnish a W()rkiiig supph- to all 
 organs at the same time ; and since, in general, whenever 
 an organ works it receives more blood, and when it is at 
 rest it receives less, our daily life witli its changes of activ- 
 ity among the organs makes necessary fie(juent adjustments 
 of the circulation to the needs of the organs at various 
 times. 
 
 Some of these adjustments are matters of familiar experi- 
 ence. The increased flow of blood to the skin on a warm 
 day makes the veins stand out and tlie face red, and we 
 are conscious of the more rapid heart beat during muscu- 
 lar activity, even in an act so simple as running upstairs. 
 Other adjustments are not so evident, but betray themselves 
 by their results, as happens after a heart}- meal when the 
 
150 THE HUMAN MECHANISM 
 
 demand of the digestive organs for blood lessens the sup- 
 ply to the brain and we feel disinclined to hard mental 
 work. We may begin our study of these adjustments by 
 learning what occurs in the circulation during some of the 
 more common activities and events of daily life. 
 
 15. The Circulation during Exposure to Heat and Cold. 
 — When the skin is exposed to cold its blood supply is 
 greatly diminished ; the veins no longer stand out promi- 
 nently on the hand, and if a small area of skin be made 
 pale by pressing upon it (thus driving the blood out of 
 its capillaries), the pallor passes off very slowly. This 
 simple experiment shows that blood is flowing but slowly 
 from the arterial reservoir into tire skin. Conversely, on 
 a warm day the veins stand out prominently and the 
 red color instantly returns upon the removal of pressure. 
 These variations in the supply to the skin are due, as we 
 have already seen (p. 144), to changes in the diameter of 
 the arteries of the skin, which changes serve like the spigot 
 of an ordinary water faucet to regulate the flow of liquid. 
 
 The changes in tlie blood-flow through the skin are 
 accompanied by corresponding but inverse changes in the 
 internal organs. On a cold day the stomach and intes- 
 tines, the pancreas, the liver, the kidneys, etc., are richly 
 supplied with blood, while on a warm day their blood 
 supply is diminished. In the former case the blood with- 
 held from the skin finds its way into the internal organs ; 
 in the latter case the skin draws upon these organs for 
 its needed supply. The circulation in the internal organs 
 compensates for that in the skin. 
 
 16. The Reason for Compensatory Changes. — We have 
 seen that it is the function of the heart to keep the arte- 
 rial reservoir adequately distended with blood, thus sup- 
 plying a steady driving force for the flow of blood through 
 the organs. When the small arteries of the skin widen 
 on a warm day blood escapes more rapidly into the skin 
 
THE CIRCULATION OF THE BLOOD 151 
 
 from the arterial reservoir. Tliis alone would diminish 
 the amount of blood in the reservoir unless the heart 
 pumped more blood, or unless the dilation or widening of 
 the cutaneous arterioles were compensated by a constriction 
 elsewhere, so that the total drain on the reservoir remained 
 the same. In the case in question it is the latter of these 
 alternatives which is adopted, and the reservoir is kept 
 filled without calling on the heart to pump more blood. 
 
 Conversely, on a cold day the diminution of the outflow 
 into the skin would lead to a backing up or accumulation 
 of blood in the great arteries, and so to their increased and 
 perhaps undesirable distention, if the dilation of the arte- 
 rioles of internal organs did not jjrovide an outlet for the 
 surplus blood. 
 
 Nowhere, perhaps, is this principle of compensatory dila- 
 tion or constriction of arteries in one region, to allow for 
 the effect of the opposite change in some other region, 
 so highly developed or so fully applied as in the reactions 
 of the body to changes in external temperature. 
 
 17. The Circulation during Muscular Activity. — Dur- 
 ing muscular activity the arterioles of the muscles and of 
 the skin are dilated, tlie former in order to supply more 
 blood to the working^ oro-an, the latter to aid in the dis- 
 charge of the excess of heat produced by the contracting 
 muscles. Tire heavy drain upon the arterial reservoir by 
 these two large areas (among the largest in the body) is 
 compensated to some extent by the constriction of the 
 arteries of the digestive and other internal organs. This 
 alone, however, would not suffice to keep the arterial reser- 
 voir filled; and we accordingly find that the heart beats 
 more rapidly and more powerfully, pumping more blood 
 into the aorta in a given time. 
 
 It is very important to remember that muscular activ- 
 ity is the one condition of life which materially increases 
 the work of the heart ; at other times the greater demand 
 
152 
 
 THE HUMAN MECHANISM 
 
 of blood for the working organ is met more or less success- 
 fully by withdrawing blood from a resting organ, while the 
 supply to the whole arterial system, and hence the work 
 of the heart, remains approximately unchanged. During 
 muscular exercise, and then only, is the heart called upon 
 for decidedly increased work ; and, like the skeletal muscle, 
 its strength, its ability to meet strain and emergencies, and 
 to withstand fatigue depend to a great extent upon the 
 trainmg given it in this way. 
 
 Muscular activity also influences the circulation indi- 
 rectly by increasing the action of its secondary driving 
 forces, — the suction action of the respiratory movements 
 and the pumping action of the contracting muscles on the 
 
 Fig. G9. Simple apparatus to illustrate the relation between the output of 
 the heart, the peripheral resistance, and the general arterial pressure 
 
 The amount delivered by the faucet represents the output of the heart, and is 
 one factor in keeping up arterial pressure; two alternative routes of out- 
 flow, each capable of regulation, represent the arterioles to different 
 organs. Compensatory constrictions and dilations and other hydraulic 
 conditions described in the text may readily be imitated 
 
THE GIRCULATIOX OF THE BLOOD 153 
 
 veins. These are among the most hnportant effects of this 
 agent upon tlie flow of blood, but they are too complicated 
 for detailed discussion here. 
 
 It is sometimes stated that muscular exercise "quickens" 
 the circulation. This is true in the sense that the heart 
 pumps more blood into the pulmonary artery and the aorta 
 than during rest. From this it follows that durinaf exercise 
 more blood flows through the lungs, and that blood flows 
 more rapidly out of the arterial reservoir ; but it does not 
 mean that blood flows more rapidly through all organs, 
 for the digestive and other internal organs at such times 
 actually receive less blood. Indeed, we may say that the 
 quickening of the circulation during exercise is chiefly con- 
 fined to three important organs, — the muscles, the skin, 
 and the lungs ; in other organs the change is relatively 
 slight, as, for example, in the brain ; while in still others, 
 notably those of the digestive system and the kidneys, the 
 speed is diminished. 
 
 18. The Circulation during Sleep. — An adequate blood 
 supply is necessary to the full activity of the brain ; when 
 the circulation in this organ is seriously interfered with, 
 imperfect mental action or even unconsciousness is a 
 result. Thus when all the arterioles of the body dilate, 
 or the heart beat is slowed down, in consequence of some 
 sudden " shock," so that pressure in the arterial reservoir 
 falls too far, the driving force for the flow of blood through 
 the brain, in common with other organs, is diminished, 
 and the person loses consciousness, or faints. Most cases 
 of fain tins' are traceable to one or the other of these causes. 
 
 The most familiar and most common example of uncon- 
 sciousness, however, is that of sleep, which in so many 
 respects resembles fainting as to suggest that the uncon- 
 sciousness in both cases is due to the same cause, namely, 
 a lessened blood supply to the brain. Unquestionably, 
 the amount of blood flowing through the brain is greatly 
 
154 
 
 THE HUMAN MECHANISM 
 
 lessened during sleep. The evidence for this statement 
 cannot be given here in full, but it is known that where 
 accident has destroyed a part of the rigid bone of the 
 
 skull, and the wound has 
 been covered over by con- 
 nective tissue and skin, 
 the scar sinks in during 
 sleep, — indicating less 
 blood in the brain, — and 
 returns to the level of 
 the general surface of the 
 head when the subject 
 awakens. 
 
 Upon this point of di- 
 minished blood supply to 
 the brain during sleep al- 
 most all physiologists are 
 agreed; there is also gen- 
 eral agreement that the 
 arm and the leg increase 
 in volume when we go to 
 sleep, and this is thought 
 to be due to a dilation of 
 the arteries of the skin. 
 It is very significant, on 
 the other hand, that the 
 arm shrinks in volume 
 when the brain is active 
 in mental work, and espe- 
 cially in mental work in- 
 volving the personal interest or mental concentration of 
 the subject of the experiment. 
 
 It is thought by some that other vascular areas, that 
 of the abdominal cavity, for example, behave in this 
 respect in the same way as the skin ; but on this point 
 
 Fig. 70. Showing the relation between 
 general arterial tone and the supply 
 of blood to the brain 
 
 In A the arterioles ot the organs m, n, s 
 are constricted, raising general arterial 
 pressure which forces a large amount 
 of blood through the brain 6. \n B the 
 arterioles of m, ?/, .and .5 are dilated, 
 general arterial pressure is low, and 
 less blood is forced through the brain. 
 //, heart 
 
THE CIECULATION OF THE BLOOD 155 
 
 the evidence is not conclusive. It is, indeed, not improb- 
 able that these other vascular areas play some part in the 
 regulation of the flow to the brain, but it is not likely that 
 they stand in the same intimate relation to it as does the 
 skin. 
 
 The fact is clear, however, that a close relation exists 
 between cutaneous circulation and the maintenance of 
 proper vascular conditions in the brain. Mental work, 
 for exarriple, is more difficult for most people in very 
 warm weather because at that time the cutaneous arte- 
 rioles are widely dilated ; and, on the other hand, it is 
 easy to understand why the constriction of the vessels of 
 the skin by cold makes it difficult to go to sleep with- 
 out sufficient bedclothing. 
 
 19. The Circulation during the Digestion of a Meal. — 
 After eating a meal, more blood is needed in tiie secret- 
 ing digestive glands (especially the stomach and pancreas), 
 and also in the intestinal organs of absorption, the villi. 
 And this need is greatest during the first hour or two, 
 when there is the largest amount of food to be worked 
 upon. We find, accordingly, that the arteries of these 
 organs then dilate to such an extent that the mucous 
 membrane of the stomach and intestine, which is pale 
 pink while those organs are at rest, now becomes very 
 red on account of the large amount of blood flowing 
 through them. 
 
 There is probably some compensation for this in other 
 organs, but it is an imperfect compensation. The 
 drowsiness which is apt to come on after a hearty meal 
 is probably an indication that these compensations are not 
 complete, and that owing to the fall of arterial pressure 
 the brain is not receiving its normal blood supply. 
 
 20. Some Practical Applications. — We may pause here 
 to consider some important practical applications of these 
 facts. While the most active secretion is in progress 
 
156 THE HUMAN MECHANISM 
 
 nothing should be done which will take blood away in large 
 quantities from the stomach. Muscular exercise, for exam- 
 ple, then, as always, dilates the arterioles of the muscles 
 and skin and constricts those of the digestive organs; this 
 is obviously an unfavorable vascular condition for the act 
 of secretion ; if the meal be a light one, so that compara- 
 tively little of the digestive juices are required, no harm 
 may be done by taking exercise after a meal ; but where 
 the meal is heavier it is almost always unsafe, especially 
 in warm weather. Similar considerations, which are like- 
 wise in full accord with experience, indicate that it is 
 unwise to eat as large meals in very warm weather as in 
 cooler weather ; the larger the meal the greater the amount 
 of gastric juice required to start its digestion ; but in 
 warm weather the arteries of the stomach and intestine 
 tend to be constricted (see p. 150), so that it is difficult 
 to secure an adequate blood-flow through these organs, 
 and their efficiency is to this extent impaired. 
 
 It is sometimes stated that mental work immediately 
 after meals causes indigestion by taking blood away from 
 the digestive organs and sending it to the brain. It is 
 very doubtful, however, whether the increased blood-flow 
 to the brain is secured largely at the expense of that to 
 the digestive organs. While instances might be cited of 
 indigestion among people who do mental work upon a 
 " full stomach," it must be remembered that these are 
 usually people who fail to take proper exercise or suffi- 
 cient sleep and rest ; the indigestion from which they too 
 frec^uently suffer is more often attributable to these causes 
 than to the fact that the digestive organs are deprived of 
 their proper l)lood supply. 
 
 21. The Mechanism of the Regulation of the Flow of 
 Blood. — Having thus considered exactly what takes place 
 in the circulation during some of the more important 
 events of daily life, we may next inquire briefly into the 
 
THE CIECULATION OF THE BLOOD 157 
 
 physiological mechanism by which these adjustments are 
 secured. Its most important features are the regulation of 
 the inflow from the heart into the arterial reservoir and 
 the regulation of the outflow through the arterioles and 
 capillaries of the organs. These two must be adjusted to 
 each other in order that the reservoir may remain full, 
 and thus the driving force for the flow through the organs 
 be maintained. We shall go into the details of this very 
 beautiful but complicated mechanism only far enough to 
 enable the student to appreciate certain principles of fun- 
 damental importance in the practical conduct of life. 
 
 22. The Regulation of the Pumping Action of the Heart. 
 — The amount of blood which the heart pumps varies 
 considerably from time to time. At times it ma}- be as 
 low as three quarts a minute, and at other times as high 
 as twelve quarts, the quantity Ijeing largely determined 
 by the drain made at the time upon the arterial reservoir. 
 It will be seen at once tliat this involves a wide range of 
 adjustment. 
 
 The beat of the heart is primarily due to events which 
 take place witliin the heart itself. We have seen that this 
 beat is a muscular contraction. But the cardiac muscle 
 differs from the skeletal muscle in that it does not require 
 an inqjulse from the central nervous system to throw it 
 into activity. When the heart is cut off from connec- 
 tion with the rest of the body it continues to beat for a 
 time, and if supplied with warm blood it may he kept 
 beating for hours. We express this by saying that the 
 heart beat is automatic, by which we mean that the heart 
 contains within itself a complete mechanism for doing its 
 own work. 
 
 23. The Augmentor and the Inhibitory Nerves of the 
 Heart. — Nevertheless, the heart receives from the central 
 nervous system two pairs of nerves, which are able to influ- 
 ence the rate and the force of the automatic beats. One 
 
158 THE HUMAN MECHANISM 
 
 pair of these nerves carries from the spinal cord to the 
 heart impulses which stimulate that organ to beat more 
 rapidly or more forcibly, or both. Hence these are known 
 as the augmentor or accelerator nerves. 
 
 The fibers of the other pair of nerves produce exactly 
 the opposite effect. liunning from the lower part of the 
 brain, they carry to the heart impulses which slow the 
 beat or lessen its force, or they may produce both effects 
 at the same time. They act, as it were, like a brake on a 
 wheel, checking the activity of the automatic beat. These 
 fibers are known as inliihitory fibers, and their action is a 
 case of inhibition. 
 
 24. Inhibition. — In the examples of nervous action 
 which we have thus far studied, the nervous impulse has 
 uniformly thrown some cell into activity. The stimulation 
 of muscle fibers to contract, of gland cells to secrete, and 
 of nerve cells in the execution of reflexes will be readily 
 recalled. To this same class of nervous actions must now 
 be added that of the augmentor nerves of the heart, for 
 they excite the heart to greater activity. 
 
 In the inhibitory nerves, on the other hand, the nervous 
 impulse produces exactly the opposite result. Instead 
 of setting organs to work or stimulating them to more 
 vigorous action, they diminish activity, and in extreme 
 cases check or stop it altogether. In our subsequent 
 studies we shall meet with many examples of this effect ; 
 but we may say at once that inhibition is as characteristic 
 and as important a feature of the nervous system as is 
 excitation (see p. 285). 
 
 25. The Regulation of the Outflow from the Arterial 
 Reservoir. Arterial Tone. — Wound around the walls of 
 the arterial tubes, especially the smaller arteries {arte- 
 rioles) which deliver blood from the arterial reservoir to 
 the organs, are peculiar muscle fibers. Their contraction 
 diminishes the size and bore of the tube, and when they 
 
THE CIRCULATION OF THE BLOOD 159 
 
 relax, the tube and its lumen become wider. As a usual 
 thing these smaller arteries are kept somewhere midway 
 between extreme constriction and extreme dilation by the 
 sustained moderate contraction of their muscle fibers. On 
 a day of moderate temperature, for example, the arterioles 
 of the skin are moderately narrowed by this action of their 
 muscle fibers. During colder weather these fibers contract 
 more than usual, and so lessen the size of the tube, while 
 during warm weather they relax somewhat and widen it ; 
 but ordinarily they are never contracted to their utmost, 
 nor are they often completely relaxed. 
 
 This condition of sustained activity of the arterial mus- 
 cles is known as arterial to>ie, and in general any sustained 
 activity of a living cell is spoken of as tonic activiti/, or tone. 
 Since, as we have seen, the total quantity of blood in the 
 body is not enough to fill completely and distend all the 
 blood vessels when they are widened to their utmost, it 
 follows that the maintenance of arterial tone is essential 
 to that overfilling of the great arteries which supplies the 
 driving force for the flow of blood through the organs. If 
 every arteriole were to lose its tone, blood would flow out 
 of the reservoir more rapidly than the heart could possibly 
 pump it in ; we should have somewhat the same con- 
 dition of affairs as if, in our artificial model (p. 142), the 
 small nozzle which affords resistance to the outflow were 
 removed. Arterial pressure would fall and, the driving 
 force being thus removed, the blood would remain at resr 
 in the capillaries and veins of the organs ; the circulation 
 would cease because blood would not return to the heart 
 to be pumped. The maintenance of arterial tone is con- 
 sequently no less essential to the circulation than is the 
 beat of the heart itself. 
 
 26. Vasoconstrictor Nerves. — The muscle fibers of the 
 arteries receive nerves which stimulate them to contract, 
 for if these nerves are cut, the arteries lose their tone 
 
160 THE HUMAN MECHANISM 
 
 (dilate). We conclude, therefore, that the ordinary main- 
 tenance of arterial tone is, in part at least, a function of 
 the nervous sj'stem. The muscle fibers of the arteries, in 
 other words, remain in tonic activity because the neurones 
 which supply them with nerve fibers are in tonic activity ; 
 and we can understand how general arterial tone may be 
 increased or decreased by the condition of the central nerv- 
 ous system, by reflexes, by the nervous "shock " of surgical 
 operations, etc. 
 
 Neurones which maintain the proper amount of arterial 
 tone are known as vasoconsti-ictor neurones. They obvi- 
 ously do for the muscles of the arteries what the motor 
 nerves do for the skeletal muscles, and the augmentors do 
 for the heart. 
 
 27. Vasodilator Nerves. — Arteries, however, receive 
 a second set of nerves, which have exactly the opposite 
 function, i.e. to make their muscle fibers relax, and so 
 lead to a widening or dilation of the artery. These nerves 
 do for the tonic contraction of the arteries what the in- 
 hibitory nerves of the heart do for the heart beat ; they 
 diminish or abolish an existing activity and thus give us 
 our second example of inliibitory nerves. They are known 
 as the vctHodilators. 
 
 The vasodilators are not regularly in tonic activity like 
 the vasoconstrictors. They are called into action, reflexly 
 or otherwise, when it is necessary tliat an organ receive 
 more blood than usual ; at other times the vasoconstrictors 
 are free to exert their tonic stimulation and so regulate 
 the flow of blood to the organ. 
 
 28. Importance of the Nervous System to the Vascular 
 Adjustments of Daily Life. — It is impossible within the 
 limits of the present work to enter further into the mode 
 of action of these arterial nerves, or to discuss other 
 minor influences which regulate the circulation of blood. 
 Our main purpose is to show the student that the proper 
 
THE CIECULATION OF THE BLOOD 161 
 
 coordinated working of the nervous system is as important 
 in adapting the work of the heart and blood vessels to the 
 hourly needs of daily life as it is in producing purposeful 
 movements of the skeletal muscles. Every change of occu- 
 pation and activity, every change of surrounding conditions 
 of temperature, moisture, wind, etc., necessitates some spe- 
 cial adjustment of the vascular system ; and this adjust- 
 ment is dependent upon the same sort of coordinating 
 nervous action which we have already compared with the 
 operations of a large army. In spite of the fact that we 
 are for the most part unconscious of it, it is none the less 
 a part of our daily life; and the fatigue induced within the 
 mechanism of vasoconstrictor and vasodilator neurones by 
 their continued activity probably contributes a large share 
 to that general bodily fatigue which leads us to seek recup- 
 eration in rest and sleep. 
 
 The apparatus, the operation, and the regulation of the 
 flow of blood and lymph afford an excellent illustration of 
 the fact that the human body, at least in this particular, is 
 a comjjlex machine. But while we of to-day look upon it 
 with somewhat less of awe than did our ancestors, and 
 while there is for us less of mystery and more of mechan- 
 ism in it, we gain, on the other hand, a wholly new revela- 
 tion of its intricacy, and a fresh sense of its marvelous 
 delicacy, beauty, and perfection of adjustment. The mere 
 fact that every one of us carries in his bosom a powerful 
 double force-pump of remarkable design, original construc- 
 tion, and extraordinary power, capable in many instances 
 of successful and unremitting service for more than three 
 quarters of a century, should be, in itself alone, enough to' 
 excite admiration and respect for the entire mechanism of 
 which it is only one part, and to awaken within us a desire 
 to use that mechanism " as not abusing it." 
 
CHAPTER X 
 RESPIRATION 
 
 1. Breathing is not the Fundamental Act of Respiration. 
 
 — We have found in studying the chemical changes which 
 underlie cellular activity (Chapter IV) that muscle fibers 
 and gland cells and, we may now add, nerve cells, take in 
 oxygen and give out carbon dioxide. This cell breathing 
 is the essential act of respiration, for respiration is only 
 another name for the oxidative processes of the living body. 
 Respiration of this kind (and of this kind only) is universal 
 among living things. The one-celled animal, for example, 
 takes its oxygen directly from the free oxygen of the water 
 in which it lives, and discharges its carbon dioxide into the 
 same surrounding medium. Every one of the thousands of 
 cells of which the human body is composed repeats this 
 same process, taking its oxygen from, and discharging its 
 carbon dioxide into its surrounding medium, — in this case 
 the lymph. The breathing movements, which renew the air 
 in the lungs, and the circulation of blood, which affords 
 the channel of communication between the lungs and the 
 tissues, are merely accessory mechanisms rendered neces- 
 sary by the distance of the cells and the lymph from the 
 surface of the body. Their principal function is to keep 
 the lymph supplied with oxygen and to remove from it 
 the carbon dioxide. In other words, breathing, though 
 ministering to respiration, is not respiration itself. 
 
 2. The Quantity of Oxygen and of Carbon Dioxide in the 
 Lymph Surrounding the Cells of the Body. — The cell is the 
 true seat of oxidation. Within its imperfectly understood 
 
 162 
 
EESPIEATION 163 
 
 mechanism are found the conditions which lead to the 
 union, direct or indirect, of oxygen with the proteids, the 
 carbohydrates, and tlie fats of the food. 
 
 The cell draws oxygen from the surrounding lymph very 
 much as a burning match draws oxygen from the surround- 
 ing air. Consequently the amount of oxygen dissolved in 
 the lymph is generally comparatively small, and would be 
 removed altogether were it not constantly renewed from 
 the blood. 
 
 For similar reasons the lymph must be relatively rich 
 in carbon dioxide, since it is this fluid which directly 
 receives the gas (in solution) from its source of manufac- 
 ture, the working cell.^ 
 
 3. The Quantity of Oxygen and of Carbon Dioxide in the 
 Plasma of Arterial Blood. — It is through the lungs that the 
 body as a whole receives its oxj'gen and discharges its 
 excess of carbon dioxide. Consequently arterial blood 
 contains more oxygen and less carbon dioxide than venous 
 blood. The actual figures are as folhiw's : 
 
 Oxij<jt?ii (.'t'rbun Iiio.riih' Nitrogen 
 100 cc. of arterial lilciod contain 20 cc. ISS cc. 1-2 cc. 
 
 100 cc. of venous lilood contain 8-12 cc. ir)-riO cc. 1-2 cc. 
 
 These figures apply to the whole blood, i.e. to plasma 
 and corpuscles ; but what is true of the whole blood is true 
 in a general way also of the circulating plasma, which 
 consequently enters the capillaries- relatively rich in oxygen 
 and poor in carbon dioxide, thus ^jresenting exactly the 
 
 1 The gases oxygen and carbon dioxide are, of course, dissolved in the 
 liquid lymph and blood plasma. A liquid exposed to a gas absorbs or 
 dissolves the gas. Thus, 100 cc. of water when exposed to atmospheric 
 air at O'^C. dissolves 1 cc. of oxygen and 2 cc. of nitrogen. 
 
 - The total time consiimed by the blood in passing from the capillaries 
 of the lungs through the heart to those of the rest of the body seldom 
 exceeds five or six seconds. Hence the amount of the gases in the blood 
 entering the capillaries, for example, of a muscle is practically the same as 
 in the blood leaving the lungs. 
 
164 
 
 THE HUMAN MECHANISM 
 
 reverse composition, in respect to these gases, of that found 
 in the lymph surrounding the living cells. 
 
 4. The Passage of Oxygen and of Carbon Dioxide be- 
 tween the Lymph and the Blood Plasma. — In the capillary 
 regions of all parts of the body except the lungs we have 
 two fluids, the lymph and the blood plasma, containing 
 
 very different amounts 
 of oxygen and carbon 
 dioxide, and separated 
 from each other by the 
 exceedingly thin mem- 
 brane of the capillary 
 wall. Under such con- 
 ditions both gases will 
 tend to equalize, and 
 each gas will pass 
 through tire membrane 
 from that liquid in 
 which it is more abun- 
 dant to that in which it 
 is less abundant ; that 
 is to say, the oxygen will pass from the blood plasma in 
 which it abounds to the lymph in which it is scarce ; and 
 the carbon dioxide in the other direction, fi'om the lymph 
 to the blood plasma (see Fig. 71). Hence the blood enters 
 the veins richer in carbon dioxide and poorer in oxygen 
 than it left the arteries. 
 
 5. The Red Corpuscle as a Carrier of Oxygen. — The blood 
 plasma under the conditions of temperature and pressure 
 to wldch it is exposed can Irold in mere solution only a 
 small amount of oxygen, far too little to meet satisfactorily 
 the demands of the resting tissues, and utterly inadequate 
 for the much greater needs of the working tissues. This 
 diihculty is naet and the oxygen-carrying capacity of the 
 blood vastly increased by the peculiar properties of the 
 
 Fig. 71. The passage of oxygen and car- 
 bon dioxide between the blood and the 
 lymph in the tissues 
 
E^SPIEATION 165 
 
 coloring matter, or pigment, of the red corpuscles. This sub- 
 stance, known as hemoglobin, readily forms with oxygen 
 a compound (oxyhemoglobin) whenever the amount of oxygen 
 is high in the ■inedium surrounding it; if, however, much 
 oxygen is removed from its surrounding medium, the oxy- 
 hemoglobin breaks up or dissociates into hemoglobin and 
 free oxygen. Applying this to the conditions in the capil- 
 laries, we find that 100 cc. of arterial blood contain less 
 than 1 cc. of free oxygen in the plasma, but about 19 cc. 
 of oxygen combined in the oxyhemoglobin of the red cor- 
 puscles. When the blood enters the capillaries of living 
 tissues, oxygen passes, as we have seen, from the plasma 
 into the lymph, so that the oxygen content of the plasma 
 is reduced. When this reduction goes below a point 
 which is quickly reached, dissociation of tlie oxyhemoglo- 
 bin occurs, and the oxygen thus set free in the plasma is 
 drawn away by the lymph, from which it is in turn drawn 
 by the cell, the real seat of oxidation. 
 
 It is important that the student should clearly un- 
 derstand that the cell is the master of the res[)iratory 
 processes, not their servant, — "\\'e may lead a horse to 
 water or fetch water to a horse, but we cannot make him 
 drink," — and tliat the cause of this movement of oxygen 
 step by step from the corpuscle to tlie plasma, thence to the 
 lymph, and thence to the cell, is the utilization of oxygen 
 l)y the cell for its life work, not the presence of oxygen 
 in the blood;- and the chemical activities within the cell, 
 not the amount of oxygen brought by the blood, pri- 
 marily determine how much oxygen shall pass to it from 
 the blood. This fundamental truth will be emphasized as 
 we proceed. 
 
 6. Gaseous Exchanges of the Blood in the Tissues. — How 
 much oxygen is taken from the blood by the tissues and 
 how much carbon dioxide is received? This question might 
 be answered for the body as a whole by comparing the 
 
166 THE HUMAN MECHANISM 
 
 amounts of these gases in the blood of the large arteries 
 and in that of the large veins. 
 
 Oxygen Carbon Dioxide Nitrogen 
 100 cc. of arterial blood contain 20 cc. 38 cc. 1-2 ce. 
 
 100 cc. of venous blood contain 8-12 cc. 45—50 cc. 1-2 cc. 
 
 It appears from these figures that there is always a very 
 considerable amount of carbon dioxide in arterial blood, and 
 a very considerable amount of oxygen in venous blood; in 
 other words, that the tissues do not remove all the oxygen 
 nor do the lungs remove all the carbon dioxide from the 
 circulating blood. Wliat the processes of respiration ac- 
 complish is to keep the gaseous content of the arterial 
 blood nearly constant at the figures given above; and except 
 in the case of suffocation (asphyxia), the gaseous content 
 of arterial blood does not vary widely from that given. 
 It was formerly assumed that in muscular exercise the 
 increased consumption of oxygen and production of carbon 
 dioxide by the working muscles made the arterial blood 
 more venous, i. e. poorer in oxygen, or richer in carbon 
 dioxide, or both ; but it is now known that the arterial 
 blood during exercise contains about tlie same amounts of 
 oxygen and carbon dioxide as duiing rest. 
 
 Not only is this true, but it has also been proved that 
 the amount of oxygen given above is almost all that can 
 be taken up by the blood, when it flows through the lungs 
 under the atmospheric pressures to which we are ordinarily 
 subjected. An elementary text-book is not the place for 
 the details of this subject ; but even the beginner should 
 understand that neither hy deeper nor by more rapid 
 breathing, nor by any change in the character of breathing, 
 is it possiUe to increase appreciably the amount of oxygen 
 absorbed by the same volume of blood flowing through the 
 lungs. The assertion, too frequently heard, that some 
 special form of breathing movements leads to more efficient 
 
eespieatio:n" 
 
 167 
 
 oxidation of the body wastes betrays lamentable ignorance 
 of this fundamental fact of physiology. This, however, is 
 not denying that one tyjoe of breathing movement may still, 
 for other reasons, be preferable to another, nor affirming 
 that deepened breathing may not sometimes be desirable. 
 Breathing movements accomplish other things than oxygen- 
 ation of the blood, and we may now proceed to study their 
 physiology. 
 
 7. Structure of the Lungs. — In Chapter II the anatom- 
 ical relations of the air passages (trachea, hroncM, etc.) and 
 lungs -have been described. The student 
 at this point should consult es[)ecially 
 Fig. 5 (p. 13) in order to obtain a, clear 
 idea of the structure of the lungs. The 
 bronchi wliich enter the lungs l)ranch, 
 much as the ducts of a gland, and their 
 ultimate branches end in the aloeoli, 
 which, like those of a gland, consist of a 
 single layer of cells, but in this case of 
 vcr?/ thin, fiattcned cells. Fig. 72 shows 
 two of these alveoli dissected, and Fig. 73 
 a section taken lengthwise through the 
 same. Connective tissue binds together the alveoli and 
 bronchial tulies, thus forming the lobes of the lungs. In 
 this connective tissue — and hence between the alveoli — 
 are the larger blood vessels, branches of the pulmonary 
 artery and pulmonary veins. Tlie arterioles supply an 
 exceedingly close network of capillaries (Fig. 139), which 
 are in direct contact witli the lining cells of the alveolus, 
 so that the blood in these capillaries is separated from the 
 air in the alveolus only ly the thin capillary wall and the 
 equally thin layer of flattened alveolar cells. Under these 
 circumstances the exchange of oxygen and carbon dioxide 
 takes place readily between the air in the lungs and the 
 blood in the capillaries. Finally the absorbing surface of 
 
 Fii;. 72. Two adja- 
 cent alveoli of the 
 lung 
 
 Showing tlie air cells 
 
168 
 
 THE HUMAN MECHANISM 
 
 the alveolar wall is greatly increased by being arranged in 
 the form of pits or air cells} as shown in Figs. 5, 72, 73, 
 and 139. 
 
 8. Purpose of Breathing Movements. — As the blood is 
 constantly giving up carbon dioxide to, and taking oxygen 
 
 ^^^''"'IF^Wn^s 
 
 Fig. 73. Diagram of a longitudinal section of two alveoli with their com- 
 mon bronchiole and showing, in black, the larger blood vessels in 
 tlie connective tissue 
 The capillary network lieloii^iug to these vessels is shown in Fig. 139 
 
 from the air of the lungs, this air would soon cease to be of 
 use in purifying the blood, were it not for the breathing 
 movements, whose function is to replace the vitiated air 
 within the lung with pure air from without. Breathing 
 is, accordingly, an act of ventilation of the lungs, and it is 
 the stoppage of this ventilation which produces suffocation 
 or asphyxia. 
 
 1 The word "cell" is here used to represent a hollow space, and not 
 with its usual histological meaning. 
 
EESPIEATION 
 
 169 
 
 r^K 
 
 D 
 
 9. Mechanics of the Breathing Movements. — A knowl- 
 edge of the mechanism of the breathing movements is of 
 much practical importance, especially in hygiene, and may 
 be understood without great difficulty by the study of the 
 model shown in Fig. 74. The trachea and the bronchi are 
 represented by the glass tube, and the lungs by an elastic 
 bag, L, at the end of the tube. The 
 lungs lie in the large air-tight thorax 
 which incloses the pleural or thoracic 
 cavity (p. 10). This thoracic wall is 
 represented in the model by a glass 
 bell jar closed beneath by a sheet of 
 thick rubber, D. The cavity of the 
 bell jar represents the pleural cavity, 
 and the rubber represents the dia- 
 phragm (see Fig. 1.34). The condition 
 of the lung in the pleural cavity may 
 be still further imitated in the model 
 by partially exhausting the air in the 
 bell jar. The distensible and elastic 
 rubber bags now inflate, because the atmospheric pressure 
 within them is greater than tlie pleural pressure witliout. 
 We shall not stop to explain why pleui-al pressure is less 
 than that of the atmosphere; but knowing the fact, it is 
 easy to understand why the lungs should be distended, so 
 as virh(.ally to fill those portions of the thoracic cavity not 
 occupied by the heart, great blood vessels, and other organs. ^ 
 
 If now the size of the pleural cavity of our model be 
 enlarged by pulling downwards the sheet of rubber at the 
 bottom, the pressure will be still further lowered in the 
 cavity and the lungs will expand. At the same time air 
 is sucked in throusfh the e^lass trachea and mixes with 
 
 Fic. 74. Model of the 
 action of the thoracic 
 walls and lungs in 
 rcsjiiration. See Sec- 
 tion 9 
 
 ^ The student is again warned against supposing tliat the pleural cavity 
 is a large space filled with air ; in this respect the model is misleading, 
 since the lungs and other organs completclt/ fill the thoracic cavity. 
 
170 THE HUMAN MECHANISM 
 
 that in the model lungs. When the pull is released, the 
 pleural pressure rises, the lungs grow smaller, and air is 
 forced out. In this way the mechanism of the ventilation 
 of the lungs may be imitated in essential particulars. 
 
 In life the pleural cavity is enlarged during inspiration 
 by the contraction of the diaphragm and the elevation of 
 the ribs. Both of these are movements effected by the 
 action of skeletal muscles. The understanding of the ele- 
 vation of the ribs need give no difficulty; muscles, some 
 of which are shown in Fig. 12, pull upwards on the ribs; 
 and the attachment of the ribs to the vertebral column 
 and the breastbone (sternum) is such that when they are 
 raised the diameter of the thorax is increased dorso-ven- 
 trally and from side to side. The diaphragm, on the other 
 hand, is a kind of circular muscle with a central fibrous or 
 tendinous portion from which the bundles of muscle fibers 
 radiate outwards to its edges. Any shortening of these 
 fibers evidently diminishes the diameter of the diaphragm ; 
 and because of its form (that of a dome directed upwards 
 into the thoracic cavity), contraction of this muscle must 
 increase the size of the lower thorax. '^ 
 
 There are three typical modes of breathing: (1) The pre- 
 dominantly costal or " rib " hreathing. Here the diaphragm 
 is but little used. It is the type characteristic of those who 
 impede movements of the lower ribs and abdomen with 
 constricting clothing, such as tight corsets. (2) The pre- 
 dominantly abdominal. Here the ribs are little used while 
 
 1 The action of the diaphragm is often described as increasing the 
 antero-posterior (liead to foot) dimension of the thorax ; but this can 
 happen only wlien the diaphragm is free to descend, and it can descend 
 only when, by displacing downwards the contents of the abdominal 
 cavity, it causes the well-known respiratory movements of the abdominal 
 walls. These "abdominal movements" may, however, be prevented by 
 the simultaneous contraction of the abdominal muscles. In this case the 
 diaphragm cannot descend, and its contraction can only raise the lower 
 ribs to which it is attached. The mechanism in these two methods of 
 using the diaphragm is clear from Fig. 75. 
 
EESPIEATION 
 
 171 
 
 the diaphragm does most of the work, the abdominal mus- 
 cles being relaxed so that the belly-wall has its maximum 
 of movement. This type of breathing involves great relax- 
 ation of tone of the abdominal muscles, which is a serious 
 disadvantage. (3) TJie lateral costal. Here the abdominal 
 muscles act at the same time as the ribs and diaphragm. 
 
 Pig. 75. Action of the diaphragm in " abdominal" and in 
 " lateral costal " breathing 
 
 Solid lines represent position of body wall, diaphragm, and ribs during ex- 
 piration ; dotted lines, the same during inspiration. Tlie left-liand figure 
 represents abdominal breathing, the diaphragm becoming more convex, 
 displacing downAvard the abdominal viscera, and forcing outward the ab- 
 dominal body wall. In the lateral costal type the diaphragm raises the 
 lower ribs and the abdominal walls may actually move inward, owing to 
 the contraction of their muscles 
 
 This form of breathing produces the highest pressures 
 on the contents of the abdominal cavity, and maintains the 
 tone of the abdominal walls, without diminishing the effi- 
 ciency of the oxygenation of the blood. It also forces the 
 use of the upper ribs to a much greater extent than does 
 the predominantly abdominal type of breathing (Fig. 75). 
 
172 THE HUMAN MECHAISTISM 
 
 It is seldom that one or another of these types is used 
 in its entirety, and the advantages of one form over another 
 are often greatly exaggerated. The following statements 
 may, however, be taken as summing up the essential 
 practical points. 
 
 1. The breathing movements should be such as to use all 
 portions of the lungs. In the abdominal type there is little 
 or no movement of the upper thorax. The result is that 
 the apical or upper lobes of the lungs do not share in the 
 enlargement and contraction of the lungs; they are poorly 
 ventilated, their lymph current — which largely depends 
 upon these movements — becomes sluggish, and because of 
 these unfavorable physiological conditions there is greater 
 liability to disease. More than sixty per cent (some observers 
 claim as many as eighty per cent) of the beginnings of the 
 lung ravages of pulmonary consumption are found in this 
 portion of the lung ; and this is believed to be due to the 
 lack of movement which results from the failure to use 
 the upper thorax. 
 
 2. Actual study of the breathing movements in people 
 who have not worn constricting clothing indicates that the 
 enlargement of the thorax in inspiration is effected by 
 the approximately equal action of the diaphragm and of 
 the muscles which elevate the ribs. 
 
 3. The abdominal muscles should to some extent con- 
 tract with the diaphragm. This is especially important in 
 those whose occupation is more or less sedentary, as it is 
 the most convenient means of giving to these muscles the 
 use which is essential to the maintenance of their strength, 
 and the consequent prevention of that loss of tone which 
 takes away from the organs of the abdominal cavity one 
 of their chief supports. (Consult Part II, Chapter XVIII.) 
 
 4. There are good reasons for thinking that it is impor- 
 tant to develop properly the muscles of the upper thorax 
 and especially those which lie in the triangle between the 
 
RESPIRATION 173 
 
 root of the neck, the collar bone, and the shoulder blade. 
 When these muscles are not developed, especially in thin 
 people, the wall of the thorax in this region sinks inward 
 during inspiration ; under these circumstances this portion 
 of the thorax is not enlarged during inspiration, the apical 
 lobes no longer share in the expansions and contractions 
 of the lungs, and imperfect ventilation of this part of the 
 lung results. 
 
 10. Secondary Effects of the Breathing Movements. — 
 The student will now be better able to understand the part 
 taken by the breathing movements in facilitating the return 
 of blood and lymph to the heart. The enlargement of the 
 thorax during inspiration sucks blood and lymph in toward 
 the great veins by the same process that it sucks air into 
 the lungs. 1 Especially in the case of the lymph flow is 
 this a most important factor. Moreover, in the lymphatics 
 of the lungs, situated as they are entirely within the thorax, 
 the movements of the lungs during respiration pump the 
 lymph onwards and are of special importance in this 
 respect. Much of the invigoratiiig effect of muscular 
 exercise, popularly ascribed to better oxygenation of the 
 blood and tissues, is really attributable to the greatly 
 improved lymph flow from all organs which results from 
 the deepened respiration in muscular activity. 
 
 11. The Nervous System as a Factor in Respiration. — 
 The muscles of the diaphragm and those of the ribs, like 
 the biceps and other muscles which act upon the skeleton, 
 are stimulated to contraction by nervous impulses from the 
 brain and spinal cord. Every movement of respiration is 
 called forth and regulated, in accordance with the needs 
 of the body at the time, by the coordinated action of a 
 number of nerve cells. Those which are most intimately 
 
 1 The model of the thorax (p. 169) may be made to show this action 
 in the veins and iymphatics by the addition of a second bag to represent 
 the blood vessels. 
 
174 THE HUMAN MECHANISM 
 
 concerned with respiration are found in different parts of the 
 central nervous system, from the lower portion of the brain 
 to the end of the first half of the spinal cord, inclusive ; 
 and there is good reason for thinking that nerve cells in 
 the lower portion of the brain send out the stimuli to those 
 of the cord, and through them excite the muscles to contract. 
 Their precise mode of stimulation, however, is too complex 
 for description in an elementary work on physiology. 
 
 12. The Circulation as an Essential Part of the Mechanism 
 of Respiration. — The consumption of oxygen and the pro- 
 duction of carbon dioxide thus involve an interchange of 
 these gases between the blood and the tissues (internal 
 respiration) on the one hand, and between the blood and 
 the air in the lungs (external respiration) on the other. 
 But to carry out these gaseous exchanges a third factor is 
 obviously necessary, namely, a means of communication 
 between the two, so that the oxygen absorbed in the lungs 
 may be carried to the tissues, and the carbon dioxide pro- 
 duced in the tissues be carried back to the lungs. This 
 communication is provided, as has been shown in earlier 
 chapters, by the circulation, which thus becomes an essen- 
 tial part of the respiratory mechanism. 
 
 We have already seen that under the most varying con- 
 ditions 100 cc. of arterial blood always contain approxi- 
 mately 20 cc. of oxygen and 38 cc. of carbon dioxide ; and 
 that this is practically all the oxygen this amount of blood 
 can hold. From this it follows that so long as the amount 
 of blood pumped by the heart in a given time remains 
 constant, no more oxygen will be carried to the tissues, 
 even if we breathe more deeply. In other words, increased 
 ventilation of the lungs uiithout any accompanying increase in 
 the rate and force of the heart beat ivill not supply more 
 oxygen to the tissues. The beat of the heart is as impor- 
 tant to proper tissue respiration as are the deepened breath- 
 ing movements ; and we find accordingly that these two 
 
RESPIEATION 175 
 
 events are closely coordinated. Greatly increased tissue 
 respiration invariably carries along with it increased work 
 on the part of the heart. 
 
 A large number of measurements of the respiratory 
 exchanges ^ under different conditions and activities of 
 our life has shown that these are increased by the taking 
 of food, by exposure to cold, by awaking from sleep, and, 
 above all, by muscular activity. The last three really be- 
 long under the one head of muscular activity ; for when 
 we are exposed to cold we move about more actively, or 
 if we do not, we " shiver " ; and the same thing is true of 
 awakening from sleep. The taking of food does not lead 
 to any remarkable increase in gaseous exchange, except 
 when one passes from a period of prolonged fasting to 
 one of feeding ; but this event is so abnormal that it may 
 be neglected. Consequently we have left the one condition 
 of muscular activity as the important means of increasing 
 tissue respiration. 
 
 And this increase is at times very great. Even the mus- 
 cular activity necessary to maintain the erect position in 
 sitting and standing, as compared with the complete relax- 
 ation of sleep, doubles the gaseous exchange ; gentle exer- 
 cise (a walk of three miles an hour) more than doubles 
 that of rest ; and vigorous, yet by no means excessive, 
 exercise will increase it tenfold. These increases mean 
 corresponding, though not absolutely i^roportionate, de- 
 mands on the heart, and emphasize the importance of 
 keeping that organ in an eflicient working condition. 
 Breathlessness, for example, usually indicates, in part at 
 least, that the heart fails to respond properly to the 
 demands made upon it, these demands being greater than 
 it can meet without undue fatigue ; it is a warning that 
 we are pushing the heart too hard, a warning which we 
 will do well to heed. Generally it is also a warning that we 
 1 i.e. oxygen absorbed and carbon dioxide discharged in a given time. 
 
176 THE HUMAN MECHANISM 
 
 are not getting sufficient muscular activity ; the heart fails 
 to meet the emergency of some unusual exertion because 
 all along it has not been kept in proper training ; so that 
 while we should, as stated, heed the warning not to push 
 the heart so hard for the time being, we should also act 
 upon the equally important warning that it needs practice 
 or training, — a training which can be given only by rea- 
 sonable, regular, muscular activit3^ 
 
 The training of muscular activity is therefore not only 
 a training of the muscles but also of the heart. But this 
 is not all. The work of the circulatory and reHjnratory mechan- 
 isms must he adjusted or coordinated, the one to the other. 
 When, for example, the deepened breathing movements 
 accompanying muscular activity rush the blood back more 
 rapidly to the heart (p. 145), it becomes necessary for the 
 heart to adjust the character of its beat to the new condi- 
 tions; and this adjustment is the work of the nervous sys- 
 tem. Time is, however, required to make the adjustment, 
 so that it is wise to " warm up " gradually to more vigor- 
 ous work. We can also understand how by physical train- 
 ing this process of adjustment comes to be shortened ; for 
 we have not only trained the heart by giving it more work 
 to do, but we have also trained those portions of the ner- 
 vous system which regulate its beat. 
 
CHAPTER XI 
 EXCRETION 
 
 We have seen that through the chemical activity of 
 the cells of the body waste products are produced from 
 the food, and this largely by a process of oxidation. We 
 have also learned that the presence of undue quanti- 
 ties of these wastes interferes with many normal activities 
 of the cells. In the organs of the body, as in the houses 
 of a city, wastes tend to accumulate, and both for health 
 and for decency, they must be removed. It is this removal 
 which is known by the general mime of ercretion. 
 
 1. The Waste Products of the Human Mechanism. — We 
 may first consider the nature of tlie more important wastes, 
 and then the means by which they are eliminated from the 
 body. 
 
 The oxidation of fats takes place according to the fol- 
 lowing equation : 
 
 CsiH.jgOe + 71 O., = 61 CO^ + 4G H^O. 
 
 In other words, they are changed to carbon dioxide and 
 water. The same tiling is true of the carbohydrates: 
 
 CgHi„Og + 6 O3 = 6 H.jO + 6 CO2. 
 
 In the case of the proteids and such allied compounds 
 as gelatin the chemical changes are more complex and 
 the pioducts of the change more numerous. These sub- 
 stances, unlike the fats and carbohydrates, contain nitrogen 
 in addition to carbon and hydrogen, and their waste prod- 
 ucts comprise both nitrogenous and non-nitrogenous com- 
 pounds. Of the former, urea (C0N2H^) occurs in largest 
 
 177 
 
178 THE HUMAN MECHANISM 
 
 quantity; indeed, it constitutes most of the nitrogenous 
 excretion of the body ; other nitrogenous substances are 
 found in small quantities, among which may be mentioned 
 uric acid (CgH^N^Og), the nearly related xanthin bases, 
 hippuric acid, and creatinin. The non-nitrogenous wastes 
 of proteids are chiefly carbon dioxide and water ; others 
 that we need not mention occur in small quantities. 
 
 The main excretions of the body are, therefore, carbon 
 dioxide, water, and urea, together with small amounts of 
 other nitrogenous and non-nitrogenous compounds. In 
 addition to these the body is constantly excreting inorganic 
 salts, sodium chloride being the largest in amount. 
 
 2. The Organs of Excretion. — Excretion, like absorp- 
 tion, must obviously take place either through the outer 
 surface of the body, — the skin, — or else in organs like 
 the lungs, the intestine, or the kidneys, which directly com- 
 municate with this outer surface ; and we actually find 
 that waste products leave the blood through all of these 
 channels. An organ may, however, be essential to the 
 proper removal of a given waste product, or it may remove 
 the waste product only incidentally in performing its essen- 
 tial functions. Thus the skin removes a small amount 
 of carbon dioxide from the body merely because a certain 
 amount of this gas diffuses from the blood as it flows 
 through the skin. It is not necessary to the health of the 
 body that the skm should excrete this carbon dioxide, for 
 the lungs are quite capable of doing the work, and would 
 do so if for any reason such excretion through the skin 
 were prevented. Without the lungs, on the other hand, 
 the carbon dioxide would rapidly accumulate in the blood 
 and cause death. The lungs are essential to the removal 
 of this waste ; the skin is not. Similarly, the perspiration 
 contains small amounts of urea and other wastes which are 
 removed in large quantities by the kidneys. It is not 
 necessary that the skin should remove any of these, for 
 
EXCRETION 179 
 
 the healthy kidney can and does, when necessary, remove 
 them. They appear in the perspiration because they are 
 in the blood from which the perspiration is formed, and 
 because the cells of the sweat glands allow them to 
 pass through, just as the skin allows the passage of carbon 
 dioxide. 
 
 These considerations are of practical importance in the 
 hygiene of the skin. It is not necessary to induce perspi- 
 ration merely to remove waste products from the body. 
 If the human skin, like that of the cat, and the dog, con- 
 tained no sweat glands, the waste products would be as 
 thoroughly removed as they are now ; and in cold weather, 
 when no perspiration is being secreted, the excretion of 
 waste is as complete as when in warm weather pcrsjjiration 
 is abundantly secreted. 
 
 The most important organs of excretion are the lungs 
 and the kidneys ; next to these should be placed the 
 intestine. Each of these is essential to the elimination of 
 one or more of the waste products from the blood. The 
 skin, on the other hand, as just shown, is only incidentally 
 an organ of excretion. The chief wastes leaving the body 
 by each of these channels are given in the follo^i'ing table, 
 the incidental excretions being in italics. 
 
 Lungs : carbon dioxide, tralcr. 
 
 Kidne3'S : virea, uric acid, and otlier nitrogenous com- 
 pounds, salts, water. 
 Intestine : bile pigments, iiilrogniniix iy-jnipouii/lx, etc. 
 Skin : urea, etc., sails, water. 
 
 The structure and action of the lungs and intestine 
 have already been described ; so that we have left for 
 study the kidneys and the skin. 
 
 3. Structure of the Kidneys. — Each kidney is a bean- 
 shaped gland whose duct, the ureter, runs to the urinary 
 bladder. As the ureter enters the kidney at the center of 
 the depression in that organ it expands to form a large 
 
180 
 
 THE HUMAN MECHANISM 
 
 — Vena. Cava 
 
 - Ureter 
 
 basin, known as the pelvis of the ureter. Into this basin 
 open the hundreds of glandular tubules of which the bulk 
 of the kidney is composed. Each tubule, like the alveolus 
 
 and ducts of the gland de- 
 scribed in Chapter III, con- 
 sists of a single layer of cells, 
 which separate the blood and 
 lymph from the lumen of the 
 tubule ; and the formation of 
 urine by the kidney is essen- 
 tially an act of secretion. 
 
 4. The Secretion of Urine. 
 — The urine is being con- 
 stantly secreted from the 
 blood, at one time more rap- 
 idly than at another, but un- 
 der normal conditions never 
 ceasing altogether. Passing 
 down the tubules, it collects 
 at the ujDper portion of the 
 ureter, and successive peri- 
 staltic waves carry it from this 
 point to the urinary bladder, 
 an orwan with muscular walls, in which the urine accumu- 
 lates and from which it is from time to time discharged. 
 Secretion by the kidney, however, presents some pecul- 
 iarities which make it unlike the secretion of the gastric 
 juice or the saliva. We have seen that the secretion of 
 tliese latter juices is called forth by nervous impulses ; but 
 in the kidney the main factor which determines tlie amount 
 of urine secreted is the amount of blood flowing through 
 that organ. 
 
 It has been shown that any increase of blood-flow 
 through the kidney is accompanied by an increased secre- 
 tion of urine, and vice versa. The same thing is seen in 
 
 Fig. 76. Dorsal aspect of the kid- 
 neys, ureter, urinary bladder, 
 and abdominal aorta and vena 
 cava 
 
EXCRETION 
 
 181 
 
 everyday experience. The three factors which increase 
 the activity of the kidneys in healthy people are (1) 
 exposure to cooler temperature, (2) drinking large quanti- 
 ties of water, and (3) the taking of proteid food. Expo- 
 sure to cold, or, more accurately, the cooling of the skin, 
 causes constriction of the cutane- 
 ous arterioles and a compensating 
 dilation of those of internal org-ans. 
 This means an increased hlood-flow 
 through the kidneys and an in- 
 crease in the amount of urine se- 
 creted. The absorption of water 
 or of proteid food also increases 
 the activity of the kidneys by in- 
 creasing the blood-flow through 
 them. 
 
 These changes in the quantity 
 of the urine secreted are, gener- 
 ally speaking, only changes in the 
 amount of water rather than in '''' 
 the amount of urea, uric acid, 
 salts, and other dissolved constit- 
 uents. Certain constituents of the 
 urine, liowever, are not very solu- 
 ble, so that it is not well to have water, the only solvent 
 of these substances in the urine, unduly diminished. A 
 very scanty secretion of urine during the day is, in general, 
 a distinct indication, especially in warm weather, that in- 
 sufficient water is being taken. ]\Iany persons drink too 
 little water rather than too much. 
 
 5. The Structure of the Skin. — The skin is an organ 
 which j^erforms several functions, the most important 
 being (1) that of protecting the underlying structures 
 from drying and mechanical injury ; (2) that of assisting in 
 maintaining the constant internal temperature of the body ; 
 
 :;. 77. Vertical section of the 
 kidney. Diagrammatic 
 
 Tlie tiiliulcs (J) n: tlie fflaiid 
 open, oil tlie iiapilUe (/>*, 11), 
 into the pelvis (C) of the 
 ureter (J>) 
 
182 
 
 THE HUMAN MECHANISM 
 
 and (3) that of receiving the external stimuli of pressure, 
 heat, and cold. Incidentally, as we have seen, the skin is 
 an organ of excretion. We may there- 
 fore describe its structure and ex- 
 cretory function in this connection, 
 reserving the study of its other func- 
 tions for Chapters XII and XIV. 
 
 The skin consists of an outer layer, 
 the epidermis, and an inner layer, the 
 dermis, cutis, or corium. The dermis 
 consists of connective tissue richly sup- 
 plied with blood vessels, lymphatics, 
 and nerve fibers, together with sense 
 organs of touch. The fibers of the der- 
 mal connective tissue are most dense 
 near the epidermis ; in the deeper por- 
 tions the network is loose and the 
 lymph spaces larger, the connective 
 tissue of the dermis passing insensibly 
 into that of the subcutaneous connect- 
 ive tissue. 
 
 The cells of the more open portions 
 of the dermal network, and especially 
 those of the subcutaneous tissue, store 
 up more or less fat within their cyto- 
 plasm. The subcutaneous tissue, in- 
 deed, is one of the most important 
 
 tXx&u^^ 
 
 
 T- 
 
 Fig. 78. Cross section 
 of skin 
 
 , horny layer of epider- 
 mis; B, deeper layer 
 
 of epidermis; C, duct organs in the body for the storage of 
 of sweat siand; Dthe ^^^_ Connective tissue in which large 
 
 dermis; _B, subcutane- ^ 
 
 ous connective tissue 
 (p. 7). The blood ves- 
 sels are injected to 
 show black. Compare 
 Fig. 82 
 
 amounts of fat are stored is known as 
 adipose tissue (see p. 230). 
 
 The outer surface of the dermis is 
 not flat, but contains mound-like pro- 
 jections known as papillw, which project into the overlying 
 epidermis. Some of these papillte contain nerve endings 
 
EXCKETION 183 
 
 of the sense of touch, while others contain capillaries, which 
 are found also in other portions of the dermis. The dermis 
 is the vascular organ of the skin, blood vessels being entirely 
 absent from the epidermis (see Figs. 81, 82). 
 
 The epidermis consists of many layers of cells, the num- 
 ber of layers being very great, a hundred or more on the 
 palms of the hands and the soles of the feet ; in other 
 places less exposed to pressure they may not exceed twenty. 
 The cells which rest immediately upon the dermis are 
 cylindrical or columnar in shape ; those immediately above 
 them are more or less spherical or polygonal, with minute 
 spaces between them for the flow of lymph. Both tlie 
 cylindrical and the spherical cells are alive, and it has 
 been shown that they grow and divide. As we pass 
 farther from the dermis with its blood supply and nearer 
 the surface with its exposure to drying, the cells show 
 signs of degeneration and are gradually transformed into 
 dead, flattened, horny scales, which, packed together and 
 known as the horny layer, form the outer portion of the 
 epidermis. These scales are being constantly rubljed off 
 and their loss made good by the growth and multiplication 
 of the spherical cells beneath. Such a covering or lining 
 is therefore well htted for surfaces whicli are exp{jsed to 
 friction or drying. The mouth, the part of the pharynx 
 used in swallowing, the cesophagus, and the rectum are 
 lined with the same tissue. Afferent nerves are found in 
 the lower layers of the epidermis. 
 
 The hairs, the sweat glands, and the nails are modified 
 portions of the epidermis. Of these the hairs and the sweat 
 glands are of sufficient importance for our purposes to 
 merit some description. 
 
 6. Structure of a Hair and a Hair Follicle. — A hair grows 
 from the bottom of a pit, the hair follicle, which extends 
 downward into the dermis or even into the subcutaneous 
 tissue. Microscopic examination shows that this follicle is 
 
184 
 
 THE HUMAN MECHANISM 
 
 lined with a continuation of the epidermis, just as a gland 
 of the stomach or intestine is lined by an ingrowth of the 
 cells of its surface. At the bottom of the follicle is a 
 P^pi!''^^' ^^^^ the hair which grows out 
 1/ from this papilla to the free surface 
 
 tjjf bears to the cylindrical and spherical 
 
 l| cells of the papilla the same relation 
 
 — ^ " that the horny layer of the epidermis 
 bears to the similar underlying cells. 
 We accordingly find that the hair is 
 composed of horny scales closely pressed 
 together into the well-known threadlike 
 structure. The scales of the outer layer, 
 which form the covering for the hair, 
 are, however, flattened and overlap some- 
 what like the shingles on the roof of a 
 house. 
 
 Opening into the hair follicle, one or 
 more sehaceous glands discharge an oily 
 secretion which lubricates the hair and 
 the horny layer of the epidermis, and so 
 prevents drying and chapping. 
 /7l. The Sweat Glands are tubular pro- 
 longations of the epidermis through the 
 dermis into the subcutaneous tissue. 
 Here the tube becomes much coiled, 
 forming the secreting recess, which is 
 richly supplied with blood vessels and 
 also receives nerves. It is a simple tubu- 
 lar gland formed as an ingrowth from 
 
 Fig. 70. Hair and 
 hair follicle 
 
 ^, root of hair ; i?,bulb 
 of the hair ; C, inter- 
 ual root sheath ; D, 
 external root sheath ; 
 E, external mem- 
 brane of follicle ; F, 
 muscular fibers at- 
 tached to the follicle ; 
 //, compound seba- 
 ceous gland with its 
 duct K; L, simple se- 
 baceous gland ; M, 
 opening of the hair 
 follicle 
 
 the epidermis (see Figs. 81 and 8^ 
 
 8. The Secretion of the Perspiration, like the secretion of 
 the gastric juice, is under the control of the nervous system. 
 When the nerves going to the sweat glands of a given 
 area of skin are cut or otherwise injured, the secretion of 
 
EXCRETION 
 
 185 
 
 perspiration ceases over that area; and tlie appearance of 
 cold beads of perspiration as the result of fright shows how 
 events taking place in the nervous system may excite these 
 glands to activity apart from the presence of their nor- 
 mal stimuli, — the application of heat to the skin and tlie 
 liberation of heat within the 
 body by muscular and other 
 activities. The distinction 
 should be made between the 
 so-called "sensible" and "in- 
 sensible" perspiration, the lat- 
 ter name being given to the 
 perspiration the water of which 
 evaporates as fast as secreted; 
 the former to that which does 
 not evaporate so i-apidly and 
 hence remains for a time on 
 the surface of the skin. When 
 the water evaporates, the dis- 
 solved solids (salts, urea, and 
 other compounds) remain be- 
 hind on the skin. 
 
 9. Value of Profuse Perspi- 
 ration in the Care of the Skin. 
 — While the skin is not pri- 
 marily an organ of excretion, 
 tlie perspiration contains a 
 certain amount of waste sub- 
 stances and salts which are left by the evaporation of the 
 water upon the surface, and, to some extent, in the mouths 
 of the ducts of the sweat glands ; this is especially the case 
 when evaporation takes place about as rapidly as the per- 
 spiration is discharged. When the secretion of perspiration 
 is more abundant, as during muscular work, or at very high 
 temperatures, or, in general, where it does not evaporate 
 
 Fr 
 
 ;. 80. INIagnilied section of the 
 lower portion of a liair and 
 liair follicle 
 
 meniltrane of the hair follicle, 
 cells with nuclei and pigmentary 
 granules; B, external lining of 
 the root slicath; C, internal lin- 
 ing of the root sheath; D, corti- 
 cal or fihrous portion of the hair 
 shaft; i?, niedul lary portion 
 (pith) of shaft ; F, hair hulh, 
 showing its development from 
 cells from ^f 
 
186 
 
 THE HUMAN MECHANISM 
 
 as rapidly as discharged, the accumulation of solids in 
 the ducts of the glands is washed out. For this reason a 
 vigorous perspiration followed by a bath 
 is a useful hygienic measure in the care 
 of the skin, although it is not necessary, 
 as is sometimes supposed, in order to 
 secure the efficient elimination of wastes 
 from the blood. 
 
 10. The Skin as an Organ of Absorp- 
 tion. — While it is true that water as 
 perspiration may readily find its way out 
 through the skin, such escape is effected 
 chiefly by the sweat glands which are 
 under the strict control of the nervous 
 system. Apart from this the skin is vir- 
 tually water-tight ; and, oiled as it is by 
 the secretion of the sebaceous glands, it 
 serves both to keep in the water, which 
 forms so important a part of the tissues, 
 and also to keep out water which might 
 otherwise soak into the body, as, for 
 example, during bathing. This water- 
 proof characteristic also makes it next 
 to impossible for us to absorb food mate- 
 rials by way of the skin. A "milk bath" may be at times 
 useful in the care of the skin, because the fat or oil of the 
 milk may supply any deficiency in the sebaceous secretion 
 and so insure lubrication of the epidermis ; but it cannot 
 be regarded as a means of supplying food to the body. 
 
 Fig. 81. Sweat {,'land 
 slightly magnified 
 
 Note tlie coiled form of 
 tlie tube in tlie sub- 
 cutaneou.s tissue. 
 Compare Fig. S-! 
 
CHAPTER XII 
 
 THERMAL PHENOMENA OF THE BODY 
 
 A. The Constant Tempeeatuee 
 
 1. The Constant Temperature. — No characteristic of the 
 human mechanism is more remarl^able than its constant tem- 
 perature. Whether we are awake or asleep, by night or 
 by day, at work or at rest, at home or abroad, in suumier 
 or winter, in the tropics or the polar regions, in suljterra- 
 nean caves or on lofty mountain peaks, the temperature of 
 healthy human beings is always nearly the same. So steady 
 is this temperature that an inciease or decrease of two or 
 three degrees gives just cause for anxiety, and a change 
 of seven or eight degrees is looked upon with alarm. 
 
 In many modern laboratories constant temperatures are 
 obtained by the use of a thermostat, the apparatus of which 
 is visible and easily understood ; but no such special appa- 
 ratus regulates the constant temperature of the human 
 body, and we have rather to seek an explanation in the 
 coordinated activities of organs already familiar, such as 
 muscles, skin, blood vessels, and especiall}^ the all-control- 
 ling nervous system. 
 
 2. Temperature and Chemical Changes. — Every chemi- 
 cal reaction takes place more readily under some external 
 physical conditions than under others, and among these 
 conditions none is more important than temperature. We 
 have already seen this fact illustrated in the case of the 
 enzymes. At the freezing point, saliva exerts no action 
 upon starch paste ; as the temperature rises, the activity 
 of the enzyme increases up to a certain point, and then 
 
 187 
 
188 THE HUMAN MECHANISM 
 
 diminishes more or less rapidly until a point is finally 
 reached at which its peculiar chemical properties ai-e 
 destroyed. (See Experiment III, p. 103.) 
 
 3. Temperature and Vital Activities. — When we come to 
 the activities of living cells, — activities which, it will be 
 recalled, depend on chemical changes, — precisely the same 
 thing holds true, and in so striking a manner as to create a 
 widespread but erroneous impression that this dependence 
 upon temperature is peculiarly characteristic of living 
 things. The one-celled animal, Amoeba, moves about more 
 actively and digests more food at 20° C than at 10° C. ; 
 bacteria grow more rapidly at the room temperature than 
 near the freezing point ; the pitch f)f the note made by a 
 cricket rises with the temperature, indicating that the 
 movements of the wing covers which' produce the sound 
 are being made more rapidly ; and in the winter sleep 
 of hibernating animals we have a beautiful example of 
 the decline of vital activities with the fall of external 
 temperature. 
 
 Nor are the living cells of the human body exceptions 
 to this rule. The rate of the heart beat varies directly with 
 the temperature of the blood, and the character of the 
 breathing movements is influenced by the same cause ; a 
 cooled muscle contracts more slowly, a cooled gland secretes 
 less abundantly. If the temperature of the bodjr itself falls, 
 every vital activity is depressed, and death itself may 
 result from undue cooling. 
 
 4. The Constant Temperature of the Body. — This depres- 
 sion of nervous, nuiscular, and glandular activity results, 
 however, only from a fall of the temperature of the hody, not 
 of that of the surrounding air or other medium. These two 
 things are by no means the same, as may be readily seen 
 from the fact that a clinical thermometer placed in the 
 mouth indicates almost the same temperature of the body on 
 warm and on cold days ; even while we are shivering with 
 
THEEMAL PHENOMENA 189 
 
 cold, the thermometer gives about the same reading as when 
 we are enjoying the warmest summer weatlrer. Tlie tem- 
 perature of the body remains nearly constant, regardless of 
 changes in tlie temperature of the air around it. 
 
 We have only to appeal to experience to see that this is 
 not the way in which lifeless matter generally behaves ; a 
 stone, the earth, a piece of iron is warmer on a warm day 
 and colder on a cold da}' ; in general, lifeless things take 
 the temperature of the medium in winch they are placed, 
 and tliis is one of the fundamental principles of physics. 
 Nor do most living things act differently; tlie temperature 
 of a plant or a tree, of an earthworm, a frog, a turtle, a 
 snake, does not differ greatly from that of its surroiuidings. 
 It is onli/ birds mid inaniiihds \\'hicli show tins remarkable 
 power of maintaining an approximately Cdustant body 
 tempera.turc notwitlistanding wide limits of cl^ange in that 
 of the surrounding air. Sucli arnmals are known as ivarin- 
 hlooded because tlie}' arc usually warmer than surrounding 
 objects; those animals wliirh dn not thus maintain a con- 
 stant temperature, on tlie other hand, are known as <Mld- 
 blooded} 
 
 It is clear that the power to maintain a constant liody 
 temperature is oi the utmost impoitance in enabling an 
 animal to conntcract the \aryiiig conditions of climate. 
 Were it not for this power, man would Ijc a hibernating 
 animal; with the coming of \vinter all his activities \\'(juld 
 gradually be slowed diiwn, and long before our rivers and 
 ponds had begun to freeze all Ijusiness, industrial life, 
 and intellectual life would come to a standstill ; it would 
 not be possible for the human race to people every zone 
 
 1 A cold-blooded animal exposed to a temperature of 99° F. is as 
 warm as a warm-blooded animal. Such animals are so called because 
 they usually feel colder when handled than do warm-blooded animals ; 
 bat this is merely because the temperature of the air (which is also their 
 temperature) is usually lower than the temperature of warm-blooded 
 animals. 
 
190 THE HUMAN MECHANISM 
 
 of the earth, — the shores of Alaska or Iceland as well as 
 the banks of the Ganges or the Amazon. 
 
 And yet here again we are dealing with a reaction which 
 is not limited to man, or even to mammals and birds. 
 There are some inanimate objects of human invention 
 which show similar power. The automatic valves which 
 are used in some of our house furnaces to regulate the 
 draft are so arranged that as the external temperature goes 
 down the draft is opened more widely and the tempera- 
 ture of the furnace remains the same or even rises. The 
 best incubators for hatching eggs are provided with simple 
 devices which maintain a temperature as constant as does 
 the hen sitting on her nest ; and we have already referred 
 to the laboratory thermostat. 
 
 5. The Temperature of the Body not absolutely Constant. 
 — The term " constant " as applied to the temperature of 
 warm-blooded animals is not, howaver, to be taken too 
 literally. No animal has an absolutely constant tempera- 
 ture. In the first place there are slight variations from 
 time to time under the changing conditions of life. The 
 temperature is higher by from one to four degrees during 
 muscular exercise than during rest; it varies during the 
 day, Ijeing highest in the afternoon and lowest in the small 
 hours of the morning ; it is often raised half a degree or 
 more by taking food, and marked changes of surrounding 
 temperature may cause a change of one degree or even 
 more in that of the body. These changes between 97.5° 
 and 99. .5° F. are of everyday occurrence and are entirely 
 normal ; so that when we speak of the temperature of the 
 body being constant we mean that it varies only within 
 narrow limits, or that it is constant in comparison with 
 that observed in cold-blooded animals. 
 
 6. The Temperature of Different Organs. — Nor is this all; 
 some parts of the body have a higher temperature than 
 others. Thus the temperature of the liver is often as high 
 
THERMAL PHENOMENA 191 
 
 as 107° F. ; that of the muscles varies between 99° and 
 105° F. ; that of the blood in the right side of the heart is 
 usually a degree or so higher than that of the blood in the 
 left side. But it is in the skin that we meet with the widest 
 variations from the general average. Every one knows that 
 on a very cold day the temperature of the skin may be far 
 below 98.6° F. ; indeed, the experience of "frosted" ears 
 or feet shows that at times cutaneous temperature may 
 descend to, or even below the freezing point itself ; and 
 it is very exceptional indeed when the skin temperature 
 is above 92° or 93° F., even on very hot summer days. These 
 variations are due to the fact that the skin is the organ 
 which is immediately exposed to the changing environ- 
 ment, and hence peculiarly subject to cooling influences. 
 It is therefore customary to distinguish between an outer 
 body zone of variable temperature and the more constant 
 temperature of internal organs. 
 
 7. Measurement of the Body Temperature. — The great 
 equalizer of the body temperature is the blood. That which 
 has flowed through the skin comes away cooled ; that 
 which comes from an organ like the liver or a working 
 muscle, in which active oxidations or other chemical clumges 
 have taken place, is heated. In the great veins and in the 
 heart the warmer blood is mixed with the cooler ; so that 
 theoretically what we mean bj' the body temperature would 
 be that of the blood which the left side of the heart pumps 
 to the organs. It is, of course, impracticable to measure 
 this ; and we must take instead the temperature of organs 
 which are well supplied with this blood, themselves pro- 
 ducing little or no heat, and at the same time not exposed 
 to cooling influences. The most accurate measurements 
 of temperature are made in the rectum, and there are times 
 when reliable readings can be obtained only in this way. 
 It is customary, however, to take the temperature in the 
 mouth, the bulb of the thermometer being placed under the 
 
192 THE HUMAN MECHANISM 
 
 tongue and the lips kept closed. For many purposes this 
 is sufficiently accurate, although at times it may intro- 
 duce a comparatively large error. Subject to the varia- 
 tions mentioned above, the rectal temperature is about 
 99.6° F., that of the mouth being 98.6°. 
 
 8. The Feeling of Cold or Warmth is not a True Test of 
 the Body Temperature. — It is well at this point to warn the 
 student against confusing the body temperature with sen- 
 sations of cold or warmth. Since sensations of heat and 
 cold originate only in tlie skin, the mouth, and perhaps 
 the upper part of the oesophagus, — parts which belong to 
 the zone of variable temperature, — it is clear that our 
 feelings give us no information as to the temperature of 
 the internal parts of the body. Tliis fact is strikingly illus- 
 trated in the case of a "chill," when the internal tempera- 
 ture is almost always really alxive, and not below, the 
 normal ; and the feeling of warmth produced by muscrdar 
 activity or l)y warming one's self at a lire merely indicates 
 a higher temperature of the skin, not a higher temperature 
 of internal organs. 
 
 Having now learned the more obvious facts about the 
 constant temperature of the body, we have next to inquire 
 by what means this constant temperature is maintained. 
 Such an inquiry is of the utmost importance in the study 
 of human physiology and hygiene, since the effort to keep 
 a constant temperature involves at times the introduction 
 of physiological conditions intimately related to the main- 
 tenance of the highest standard of health. 
 
 9. The Production and the Transfer of Heat. — The 
 temperatnre of any body of matter can remain constant only 
 as long as the amount of heat it produces or receives (from 
 objects warmer than itself) exactly equals the amount it 
 gives out ; the temperature rises when the beat produced 
 plus that received is greater than that lost ; and it falls 
 when the balance of the account is on the other side. 
 
THERMAL PHENOMENA 193 
 
 In applying this principle to the human body we must 
 first remember tliat llie body produces or liberates heat. 
 The chemical changes, largely oxidative in character, 
 which are at the basis of the work of its muscles, glands, 
 nerve cells, etc., liberate heat just as truly as the burning 
 of coal in the furnace of an engine liberates heat. Heat 
 production is therefore an indispensable result of cellular 
 and organic activity, and it is greatest in those organs, 
 like the muscles and liver, which carry out the most active 
 chemical processes. Tlie body is warm for the same reason 
 that a sti)ve is warm ; that is, hecause combustion (o.iicJatio)t) 
 is (joiiig oil' iritJiin it. 
 
 In the second place, tlic lieat ld;ierated within the body, 
 like that liheratcd in a stove, may lie transferred to sur- 
 rounding olijects. Ever}- one knows that wlien a warm 
 body is brought near a colder one, the former becomes 
 colder and the latter -warmer. Heat has been transferi'ed 
 from one to the other, and this transfer continues until the 
 two bodies reach the same tempeiature. The clothing is 
 warmed by contact with the body : so is the air in imme- 
 diate contact with the skin; and conversely the Ijody may 
 be wai-med liy contact with anything \\-arn-ier than itself, a 
 hot-water bottle, for example. 
 
 It is not, however, necessary that two solid bodies be in 
 actual contact in order that heat may [lass from one to the 
 other. A stove \\-arms all the objects in a room, although 
 few of them are touching it: and the luunan body may 
 lose heat to, or receive heat from, objects at a greater or 
 less distance. There are, indeed, tljri'e methods of trans- 
 ferring heat from one body to another, and before we go 
 farther it is important that tliese methods of heat transfer 
 be thoroughly understood. Tliey are known as conduction, 
 convection, and radiation. In the case of conduction and 
 convection the means of transferring heat, or the " vehicle " 
 of transfer, is some form of matter, either a solid, or a 
 
194 THE HUMAN MECHANISM 
 
 liquid, or a gas ; radiation, on the other hand, occurs inde- 
 pendently of the presence of ordinary matter between the 
 two bodies, though on our earth the air, at least, is almost 
 always present. 
 
 10. Conduction. — Whenever heat is transferred directly 
 from one mass of matter to another with which it is in 
 contact, such transfer is known as conduction. A good 
 example is the heating of a poker in a fire ; the heat of 
 burning coal is communicated directly to the outer par- 
 ticles of iron, and then from one particle of the iron to 
 another. The particles of iron do not move up and down 
 the length of the poker ; each one simply passes on to 
 the next the heat it has received, and finally those of the 
 handle communicate their heat to the hand. All transfer 
 of heat along solid objects, or from one mass of matter to 
 another with which it is in immediate contact, is by means 
 of conduction. 
 
 Solids and liquids are much better conductors of heat 
 than gases, and air when perfectly still is one of the 
 poorest conductors of heat with which we have to deal. 
 It is a familiar fact that the skin is chilled much more 
 rapidly by water than by air of the same temperature 
 (why ?) ; and we shall learn in hygiene that warm fabrics 
 owe their warmth mainly to the amount of poor-conducting 
 air stagnant within their meshes. 
 
 11. Convection. — When a warm body is surrounded by 
 a fluid such as water or air, heat is similarly conducted to 
 the adjacent layer of water or air, which thus becomes 
 warmer ; but, unlike the case of the solid, this heated 
 layer now moves off, carrying its heat with it to other 
 parts of the gas or liquid, and so communicating it to 
 other matter with which it subsequently comes in contact. 
 This method of heat transfer is known as convection, which, 
 it will be seen, depends at bottom upon conduction, but 
 which is at the same time conduction modified by the 
 
THERMAL PHENOMENA 195 
 
 movement of a heated gas or liquid. So long as the air 
 around us is at rest it does not remove heat readily from 
 the skin, since air is a poor conductor. Air in motion, on 
 the other hand (as in fanning), cools the skin more rapidly, 
 because as each part of the air is heated it is moved away 
 and replaced by colder air. In this case the air cools the 
 skin by convection (Latin con-, with; vehere, to carry). 
 
 The transfer of heat from the internal heat-producing 
 organs to the skin affords an excellent examjjle of the 
 difference between conduction and convection ; for some 
 of this heat passes Ijy direct conduction through the sub- 
 cutaneous tissue to the overlying skin, while some of it is 
 carried to the surface by convection in the blood stream. 
 When the arterioles of the skin are dilated, convection is 
 an important means of heat transfer to the surface ; when, 
 in the reverse case, the cutaneous arterioles are constricted 
 to their utmost, convection Ijecomes relatively imimpor- 
 tant and direct conduction alone remains as the chief means 
 of heat transfer to the skin. Moreover, when the subcuta- 
 neous tissue contains large amounts of fat, it is a poor con- 
 ductor of heat, and for this reason fat people when sitting 
 still on cold days often feci colder than lean people do. 
 
 12. Radiation. — Heat is tlms removed from the skin 
 by conduction, and at times to an even greater extent by 
 convection. Lut there is still a third method of heat loss, 
 known as radiation, by which heat can be transferred across 
 a space in which there is neither solid, liquid, nor gas, 
 and in which conduction and convection are consequently 
 impossible. The most familiar and striking example of 
 radiation is the transfer of heat from the sun to the earth, 
 since there is no atmosphere in the greater part of the 
 more than ninety millions of miles of space which separate 
 us from that intensely lieated body. 
 
 Any detailed consideration of radiation belongs to the 
 domain of physics rather than physiology, and would be 
 
196 THE HUMAN MECHANISM 
 
 out of place here. It is enough for our present purposes 
 to understand that, whether a solid body be in an atmos- 
 phere of air, or in a transparent liquid, or even in a vacuum, 
 it transfers or loses heat by direct radiation to colder 
 objects about it. From an open fire heat may be trans- 
 ferred by conduction to andirons or walls in direct contact 
 with it ; or by convection through heated air currents to 
 the chimney top ; or, finally, by radiation to persons stand- 
 ing in front of it. In the latter case the heating is chiefly 
 by radiation, since there is no contact with the fire, and 
 such air currents as exist are mostly composed of cool 
 air sucked towards and into the chimney by its draft. It 
 is for these reasons that open fires are said to "roast people 
 in front and freeze them liehind." Conversely, the human 
 body, if warmer than its surroundings, may lose heat by 
 conduction, convection, and radiation to cooler objects in 
 the vicinity. 
 
 The practical importance of these facts is seldom real- 
 ized. It often happens that the air in contact with the 
 skin is of the proper room temperature ; and yet, if one 
 is sitting too near a cold wall or window, enough heat 
 may be lost by radiation from the skin to the cold wall, 
 through the warm air, to chill the skin mateiially, causing 
 a loss of heat and a "cold." 
 
 13. Laws of Conduction and' Radiation. — For our pur- 
 poses the two most important factors which determine the 
 loss of heat by conduction and radiation are (1) the differ- 
 ence in the temperature of the two objects ; and (2) the 
 distance between them. In general, the gi-eater the dif- 
 ference of temperature the more heat will be lost from 
 the warmer to the colder object ; thus the skin loses heat 
 rapidly hy these means when surrounding objects are at 
 0° F., but only slowly when they are at 90°. It is also 
 clear that as soon as the temperature of surrounding ob- 
 jects and of the atmosphere is as high as that of the 
 
THERMAL PHENOMENA 197 
 
 body (98.6° F.) no further heat can be lost by conduction 
 and radiation ; and that above 98.6° F. heat is conducted 
 and radiated to tlie body, not from it. 
 
 Furthermore, the greater the distance of the colder ob- 
 ject from the body the less heat will the body lose to it. 
 Here heat loss takes place inversely as the square of the 
 distance ; i.e. when we are twice as far away from a cold 
 (closed) Avindow we lose only one fourth as much heat 
 through it by radiation ; if we are three times as far away, 
 we lose only one ninth as much, and so on. Consequently 
 we rapidly diminish radiation from our bodies by sitting 
 farther awaj' from the walls of a room ; and it is important 
 to have our living rooms large enough to make it unneces- 
 sary to sit near the windows or near a cold outer wall in 
 very cold weather. 
 
 14. Heat Output by the Evaporation of Perspiration. — 
 In one other way heat may be lost from the body. The 
 change from the liquid to the gaseous state {evaporation) 
 always involves absorption of heat from surrounding ob- 
 jects. This is readily illustrated l)y the simple experi- 
 ment of blowing a steady current oi dry air over the dry 
 hand, and comparing the cooling tlius produced by con- 
 vection with that produced by blowing the same current 
 against the moistened linger. ^ The cooling in the latter 
 case will be much greater than in tlie former, and is caused 
 hj the evaporation of the water. Liquids like etiier, which 
 evaporate more rapidly than water, will produce even 
 greater feelino- of cold on the skin. 
 
 In the secretion of perspiration we have this principle 
 applied to tlie uses of the body. The student should 
 understand clearl}' at the outset, however, that it is the 
 evaporation of the perspiration, not the secretion of it, 
 wliich abstracts heat from the body. Perspiration may be 
 
 1 In this experiment the temperature of the room and of the current 
 of air must be so low that they will not induce active perspiration. 
 
198 THE HUMAJN MECHANISM 
 
 secreted in large quantities, but if it does not evaporate, — 
 as happens on a very moist, humid, muggy day, when the 
 atmosphere already contains about as much aqueous vapor 
 as it can hold, — it takes little or no heat from the skin. 
 Nor is the efficiency of the perspiration as a cooling agent 
 measured by the amount of visible or " sensible " perspira- 
 tion, for tSRs is only the perspiration which has not evap- 
 orated ; the true measure of the cooling effect would be 
 the perspiration which has evaporated and of which we 
 are not conscious. 
 
 It is important to note that the evaporation of perspira- 
 tion (or of water from the lungs and air passages) is the 
 only means of cooling the body when objects around it 
 are warmer than the body itself. In this case conduction, 
 convection, and radiation only add heat to the body ; but 
 even their combined action may often be overcome by an 
 abundant evaporation of perspiration. Men have remained 
 for some time in rooms whose temperature was as high as 
 260° F., or 48° above the boiling point of water, without 
 any marked rise of the body temperature and without 
 severe discomfort, the temperature of tlie body being kept 
 down solely by the evaporation of perspiration from the 
 skin. In order to make this means of cooling possible, it 
 is absolutely essential that the air be dry and capable of 
 taking up moisture. No one can survive long at such tem- 
 peratures in moist air. 
 
 15. Summary. — In conclusion we may recapitulate the 
 ways in which the body may gain or lose heat. 
 
 It may gain heat by conduction, convection, or radiation 
 from warmer surroundings. The amount of heat it receives 
 from without is, however, very small compared with that 
 derived from the oxidations taking place within itself. 
 These oxidations constitute the chief sources of its heat. 
 
 It may LOSE heat (1) by conduction, convection, or radia- 
 tion to things external to itself. Here are included not 
 
THERMAL PHENOMENA 199 
 
 only the heat lost from the skin but also that lost in heat- 
 ing the expired air and the excretions ; (2) by evaporation 
 of water from the skin, the lungs, and the air passages. 
 
 It is quite impossible to give definite figures showing 
 just what percentage of the total heat-loss occurs by each 
 of these channels, because this differs at different times ; 
 on a very cold day, for example, none is lost in perspiration 
 because no perspiration is secreted, but much is lost by 
 radiation and convection ; on a hot day the conditions are 
 reversed. On a warm day when the wind is blowing con- 
 vection plays a more important r81e than it does when the 
 air is quiet. During vigorous exercise on a cool day we 
 lose heat by evaporation of perspiration ; when we are at 
 rest at the same temperature little or no heat is lost by 
 this means. A little reflection will show that most of the 
 output of heat from- the body is from the skin. The heat 
 which the body loses in the urine and feces is almost negli- 
 gible, and that which is lost by heating the expired air 
 and evaporating the water which this air carries off as 
 ac[ueous vapor probably never exceeds ten or twelve per 
 cent of the whole. From eighty-five to ninety per cent of 
 the total output of heat is by way of the skin ; so that it 
 need not surprise us to find that it is the skin which jjlays 
 hy far the most important role in checking or facilitating, 
 as the case may be, the output of heat. 
 
 B. The Regulation of the Body Temperatttee 
 
 In the preceding section we have learned the essential 
 facts concerning the production and output of heat by the 
 human body, and have also seen that a constant tempera- 
 ture is maintained because heat production and output are 
 kept equal. We have now to study the means employed 
 to adjust heat production to heat loss, and heat loss to heat 
 production. For this purpose we may begin by considering 
 
200 THE HUMAN MECHANISM 
 
 what happens during the two conditions of daily life which 
 most conspicuously threaten the heat balance, i.e. during 
 changes in external climatic conditions and during internal 
 conditions produced by muscular activity. The former 
 disturbs the balance by changing the rate of conduction, 
 convection, radiation, and evaporation from the surface; 
 the latter, by producing more lieat within tlie body. 
 
 16. Reactions of the Body at Rest to Changes of External 
 Temperature. — The more important climatic conditions 
 which affect the transfer of heat are temperature, atmos- 
 pheric moisture (or humidity), and wind. We may study 
 first the influence of changes of temperature, assuming for 
 the present that the air remains of moderate humidity and 
 that there is little or no wind. 
 
 On a warm day, when the tliermometer stands at 90° F. 
 or more, and one is lightly clad and sitting still, perspira- 
 tion is secreted abundantly and the arterioles of the skin 
 are widely dilated. Let us now suppose that the day grad- 
 ually grows cooler. The perspiration becomes less abun- 
 dant, and, as the falling temperature nears 70° F., ceases 
 to be of consequence ; but the blood-flow through the skin, 
 though lessened, is still considerable. Below 70° the per- 
 spiration ceases, so that heat is now lost almost entirely 
 by conduction, convection, and radiation. 
 
 As the temperature falls below 70° l*"., the veins are no 
 longer conspicuous on the hand and arm, which shows that 
 the loss of heat is being cliecked by limiting the blood 
 supply to the skin. At the same time the arterioles of 
 internal organs are dilating (see p. 150), so that the liver, 
 the kidneys, the mucous membranes of the alimentary 
 canal, and those of the air passages contain an increased 
 amount of blood. 
 
 By the time the temperature has fallen to 60° F., or 
 thereabouts, the cutaneous arterioles have constricted to 
 their utmost, the blood-flow through the skin has nearly 
 
THERMAL PHENOMEXA 201 
 
 ceased, and the organism has no means at command by 
 which to restrict tlie further output of heat. If in this 
 emergency heat production were to remain constant while 
 external temperature continued to fall, the temperature of 
 the body would be lowered, for conduction and radiation 
 not only continue but increase. That it is not usually 
 lowered is due solely to the fact that more heat is then 
 produced within the body; the oxidations (and hence heat 
 production) which have remained fairly constant in amount 
 between 90° F. and 65° F. external temperature now in- 
 crease to compensate the inevitable loss, and continue to 
 increase as the atmosplieric temperature continues to fall. 
 
 17. The " Dangerous Region " of Atmospheric Tempera- 
 ture. — It is important to understand that the methods of 
 regulating the temperature of the Ijody differ with differ- 
 ent external temperatures. Below 00° the only available 
 method is to vary heat production ; al.iove 98.6° the sole 
 method is to evaporate perspiration ; l)etween 70° and 98.6° 
 the evaporation of perspiration is comljined with variations 
 in the blood-flow through the skin ; and linally we have 
 left the region between 60° and 70°, when the vascular 
 changes in the skin play the most important role. 
 
 In this region between 60° and 70° three or four degrees 
 make all the difference between the ideal room tempera- 
 ture and one which is decidedly dangerous. At 06° or 68° 
 the blood is properly distributed between the skin and the 
 internal organs, and there is no excess in cither. At 60° 
 or 01°, on the other hand, the blood is forced back upon 
 the internal organs, thus threatening serious congestions 
 and other unhealthful conditions whicli we shall consider 
 at length in our study of hygiene (see Part II, Chap- 
 ter XXI). It is because the temperature of a room may 
 fall from 66° to 60° so gradually that we do not notice it 
 until the internal damage is done, whereas it could not fall 
 to 50° or 40° without our noticing it and correcting the 
 
202 THE HUMAjST mechanism 
 
 trouble, that more colds are taken in the former case than 
 in the latter. In other words, as the temperature goes 
 below 65° the body seems at first to rely wholly on the 
 vascular mechanism of temperature regulation, and does not 
 begin to produce more heat until this resource has been 
 utilized, not only to its utmost, but even to an extent incon- 
 sistent with health. The " dangerous region of room tem- 
 perature," then, may be placed between 60° F. and 65° F.^ 
 
 18. The Influence of Atmospheric Moisture or " Humid- 
 ity." — The amount of water vapor which the atmosphere 
 contains influences the output of heat in two ways. In the 
 first place moist air conducts heat from the skin more 
 readily than dry air. We do not notice this action of moist 
 air in very warm weather because the difference of tempera- 
 ture between the body and the air is then so slight that 
 neither moist nor dry air conducts any large amount of heat 
 from the surface. But when this difference of temperature 
 is greater, the effect of humidity in facilitating conduc- 
 tion becomes quite evident. Moist air at 65° F. is distinctly 
 chilly when one is sitting still, while dry air at this tem- 
 perature is comfortable. 
 
 As soon, however, as the evaporation of perspii-ation 
 becomes necessary to maintain the constant temperature, 
 the presence of moisture, so far from aiding the output of 
 heat, actually interferes with it ; for evaporation will not 
 take place readily into moist air. This is the reason why 
 a high degree of humidity makes us feel warmer on a 
 warm day, whereas, as stated above, it also makes us feel 
 colder on a cold day. In the latter case it acts by conduct- 
 ing heat more rapidly from the skin ; in the former by 
 interfering with evaporation, which is then the chief means 
 of heat output. 
 
 ' The student is reminded that in the above discussion the figures given 
 apply only when one is lightly clad and sitting still, when the humidity 
 is moderate, and when the air is comparatively quiet. 
 
THEEMAL PHENOMENA 203 
 
 19. The Effect of Stagnant vs. Moving Air ; the Aerial 
 
 Blanket On a perfectly still day the layer of air about 
 
 the body becomes warmed by the skin, and, so long as it is 
 not removed, forms an air-blanket which goes far to keep the 
 skin warm ; for air is a poor conductor of heat. As soon, 
 however, as a breeze springs up convection comes into play 
 and the skin is cooled more rapidlj^. In stagnant air, more- 
 over, the evaporation of the perspiration tends to saturate 
 this air-blanket with water vapor, so that further evaporation 
 is rendered difficult. Accordingly, when perspiration is not 
 being secreted, moving air cools the body by increasing 
 convection ; and when the skin is moist it cools the body 
 both by increasing convection and by facilitating the evapo- 
 ration of perspiration. 
 
 20. The Influence of Muscular Activity. — External tem- 
 perature, moisture, and wind tend to disturb the balance 
 of the heat account chiefly by changing the rate at wliich 
 heat is lost from the body. But the balance may also be 
 disturbed by changing the amount of heat liberated within 
 the body. This occurs most conspicuousl}' during muscu- 
 lar activity, for the chemical changes (or oxidations) which 
 then take place necessarily involve the liberation of large 
 amounts of heat within the muscles ; and this heat must 
 evidently be got rid of, if a constant temperature is to be 
 maintained. 
 
 The reactions which then occur are familiar to every one. 
 The arterioles of the skin are dilated (while those of internal 
 organs are constricted) and perspiration is secreted. These 
 are the same reactions which are noticed when the body is 
 exposed to external warmth, and their purpose is the same 
 in both cases, — to facilitate the escape of heat. But in 
 the one case they are made necessary by the fact that 
 climatic conditions interfere with the output of heat, in the 
 other by the fact that more heat is being liberated and 
 hence more must be got rid of. 
 
2(J4 THE HUMAN MECHANISM 
 
 Seldom indeed is so severe a strain imposed upon the 
 mechanism of heat dissipation as during vigorous muscular 
 exertion, and especially when the external conditions are 
 not favorable for radiation, convection, and the evaporation 
 of perspiration. Caution is then urgently indicated lest 
 we make the strain too great. It is a practical point to 
 remember in this connection that some forms of muscular 
 exertion introduce conditions for getting rid of the surplus 
 heat much more readily than others ; this is especially true 
 of those which involve movement of the body as a whole. 
 Bicycle or horseback riding, by creating a breeze, renders 
 the cooling of the body a much easier matter than does 
 sawing wood, or swinging Indian clubs, or gymnastic work 
 in general ; again, a particular form of exercise on a dry 
 day, when the perspiration can evaporate readily, may be 
 safe, while it would be decidedly inadvisable on a muggy 
 day, even though the temperature were somewhat lower. 
 Indeed, by this time the student must have learned that 
 the thermometer alone is no safe indicator of the difficulty 
 of heat elimination in warm weather. 
 
 21. Relations of Climatic Conditions to Mental Work 
 and Sleep. — During mental work the brain requires an 
 increased supply of Ijlood, and this is obtained partly by 
 diminishing the supply to the skin (constriction of cutane- 
 ous arteries) ; during sleep, on the other hand, the supply 
 to the brain is diminished, and this is ordinarily effected 
 by dilating the arteries of tJie skin (see p. 153). Meirtal 
 work is difficult on very warm days, partly because it is dif- 
 ficult to bring about cutaneous constriction ; and it is 
 especially difficult on warm, muggy days, since the mainte- 
 nance of the constant temperature then requires an exces- 
 sive cutaneous dilation, and the brain is quite unable to 
 command its needed blood supply. 
 
 During sleep, on the other hand, the arterioles of the 
 skin should dilate, and this they cannot readily do when 
 
THEEMAL PHENOMENA 205 
 
 the skin is exposed to cold. To " sleep warm " is good 
 advice, and is based on sound' physiological principles. 
 
 22. Digestion and the Regulation of the Constant Tempera- 
 ture. — During digestion, and especially during its earlier 
 stages, when secretion is at its maximum, a large supply 
 of blood is needed in the stomach, the pancreas, and the 
 intestine. This cannot readily be secured when the blood 
 is being sent in large qiiantities to the skin in order to cool 
 the body. We have seen all along that the two great vascu- 
 lar areas of the skin and digestive organs are more or less 
 antagonistic or compensating in their vasomotor reactions. 
 When the blood is present in large quan titles in the skin 
 it is present in smaller quantities in Uu; stomach, the intes- 
 tine, the pancreas, the liver; and, vice versa, these organs 
 can best obtain an adeciuate blood su[)ply when the demands 
 of the skin are not excessive. Cimseqiicntly digestion is 
 more diflicult in warm than in cold weather, and we should 
 then eat less at a time, even if we have to eat somewhat 
 more frequently. 
 
 During the digestion of a meal the chemical activities of 
 secretion, the peristaltic muscular movements, etc., some- 
 what increase heat production in the body ; and this in- 
 crease, though not great, is at times great enougli to make 
 us feel distinctly warmer. When one is slightl ij chilly, 
 for example, he often feels warmer after eating some- 
 thing, even thoirgh the meal be cold ; and on a very 
 warm, muggy day, when the bloocl-flow through the skin is 
 already excessive and its temperature unduly high, the 
 digestion of a meal often adds to the discomfort, because 
 the larger production of heat leads to further dilation of 
 the skin vessels. 
 
 23. Subcutaneous Adipose Tissue a Hindrance to the Out- 
 put of Heat. — In the last chapter we saw that heat may 
 be brought from the internal heat-producing organs to the 
 skin in two ways, — by the circulation (convection), or else 
 
206 THE HUMAN MECHANISM 
 
 by direct conduction through the tissues. How much will 
 take one course and how much the other depends partly 
 upon the conducting power of the tissues in question. Fat 
 is a poor conductor of heat ; and when it is stored, as is 
 usual in fat people, in large quantities in the connective 
 tissue immediately under the skin, it so interferes with 
 direct heat conduction that a larger proportion of the 
 transfer of heat to the surface must he by the circulation ; 
 the cutaneous arterioles must he more widely dilated and 
 the perspiration, if necessary, must be increased. Hence 
 muscular exertion and exposure to warm weather produce 
 more discomfort in fat than in lean peopjle because of the 
 larger quantity of perspiration and the heating of the skin 
 by the greater cutaneous blood-flow ; such people readily 
 become " overheated." On the other hand, contrary to pop- 
 ular belief, when fat people are exposed to cold weather 
 they are often more chilly when sitting still than lean 
 people, probably because, since the blood-flow through the 
 skin is lessened, their skin has not the same source of 
 warmth in the direct conduction of heat from subjacent 
 tissues. Fat people thus retain the heat of the body bet- 
 ter, and should maintain more easily the constant inter- 
 nal temperature ; but this very retention of heat within 
 the internal organs keeps heat away from the skin, which 
 consequently becomes cold and produces the feeling of 
 chilliness. 
 
 24. The Mechanism of Temperature Regulation. — The 
 preceding pages have shown us that temperature regula- 
 tion depends chiefly on three phj^siological mechanisms : 
 (1) the vasomotor system, which controls the distribution 
 of blood between the skin and the internal organs ; (2) the 
 sweat glands ; (3) the mechanism of heat production. The 
 first of these has already been described in the study of 
 the circulation. The heating of the skin stimulates afferent 
 nerves which reflexly dilate the arteries of the skin and 
 
THERMAL PHENOMENA 
 
 ^07 
 
 also simultaneously constrict those of internal organs. This 
 reflex, then, is dependent on the temperature of the skin ; 
 anything which heats the skin causes a reflex dilation of 
 its arterioles and lessens the supply of blood to internal 
 organs. 
 
 The secretion of perspiration is also under the control 
 of the nervous system. The sweat glands, like the salivary 
 
 Nerve Endings affected 
 by Warmth 
 
 Fig. 82. Diagram of the cutaneous reflexes of temperature regulation 
 
 Sbowing the epiilerniis, hlood ve.sscls of tlte deriiii.s, a sweat yland, and the 
 nervouM niechani.sni j^overiiing tilood-ve.ssels and s^\^eat glands 
 
 glands, receive nerves, and secrete only in response to their 
 stimulation. When the nerves going to the sweat glands 
 of any region are injured, exposure of these glands to 
 external warmth produces no perspiration ; stimulation of 
 their nerves, however, produces a copious secretion. 
 
 25. The Skeletal Muscles the Main Organs in the Regu- 
 lation of Heat Production. — The third mechanism of heat 
 regulation is that whereby the amount of heat produced is 
 increased as it is needed, and experiment has shown that 
 
208 THE HUMAN MECHANISM 
 
 the main organs here concerned are the skeletal muscles. 
 When these are in any way prevented from contracting, 
 the increase of heat production on exposure to cold does 
 not occur. 
 
 Certain drugs, for example, interrupt the connection 
 between muscles and their motor nerves, and when an 
 animal under their influence is exposed to a falling tem- 
 perature the increase of heat production below 60° F. is 
 not observed ; the temperature of the body rapidly falls. 
 Similarly, experiment has shown that if a human being, 
 exposed to a falling temperature, is careful to lie in as 
 relaxed a condition as possible, thus suppressing as far as 
 possible the activity of the skeletal muscles, the usual heat 
 production fails to appear. 
 
 From these facts we conclude that below 60° F. cold 
 stimulates cutaneous afferent nerves, which refiexly excite 
 .the skeletal muscles of the Ijody, and that the heat pro- 
 duced by their contraction tends to compensate the in- 
 creased loss of heat from the skin. 
 
 26. Shivering. — Tlie contraction of the muscles thus 
 produced does not necessarily result in movement. Almost 
 every joint at which motion ordinarily takes place is acted 
 upon by antagonistic muscles ; for example, the elbow, 
 linger, knee, and ankle jijints are moved by extensor and 
 flexor muscles, but only when one group is contracting to 
 a greater extent than the other. If the two groups con- 
 tract to the same extent, the action of the one neutralizes 
 that of the other, so far as movement is concerned ; but 
 both contract,'^ and in contracting produce heat. Tliis is 
 what happens when external cold stimulates all the nuis- 
 cles of the body to increased contraction. Movement does 
 not usually result because the extensors and flexors are 
 more or less I'qually balanced. There are times, however, 
 
 1 A muscle iiuiy nuiti-firt witliout shurtcning or producing movement, 
 as when we pull ou a vveijjlit we cannot lift. 
 
THEEilAL PHENOMENA 209 
 
 when this contraction of the antagonists is not well bal- 
 anced, and we " shiver." Shivering is the extreme expres- 
 sion of this heat-producing reflex. 
 
 We are often conscious of this increased muscular action 
 during cold, even when we are not shivering. Every one 
 knows the difference between the "bracing" effects of a 
 cool or cold day and the " relaxed," " slack-twisted " feel- 
 ing on a warm day ; and this is largely traceable to the 
 sensations which come from the contractino- muscles in 
 the former case and to tlie absence of such sensations from 
 the inactive muscles in the latter. To put it in another 
 way, cold increases the tone of the skeletal muscles (see 
 p. 159). A skeletal muscle on a cold day is never com- 
 pletely relaxed ; like the unstriped muscles of the arteries, 
 it is in a condition somewhere between extreme contraction 
 and extreme relaxation. 
 
 27. The Regulation of the Body Temperature a Function 
 of the Nervous System. — We may close this brief account 
 of thermal phenomena of tlie body by recalling to the 
 attention of the studeiit what must now be obvious at a 
 glance, namely, that a constant temperature is maintained 
 by the coordinating action of very many nervous reflexes. 
 The action of the vasomotors of tlie skin and of the 
 internal organs, of the nerves of the sweat glands and of 
 the motor nerves of the skeletal muscles, must all be so 
 adjusted with regard to one another that exactly the riglit 
 balance is preserved amid all the variations of heat pro- 
 duction and of climatic conditions which affect heat-loss. 
 Success in tliis adjustment depends upon the skill with 
 which the coordinating nervous system does its part. 
 With the single exception of muscular exertion, no con- 
 dition of life makes such far-reaching, or such imperious 
 demands upon the system as a whole as does the mainte- 
 nance of the proper internal temperature. Mental work 
 and the efficiency of digestion are examples we have 
 
210 THE HUMAN MECHANISM 
 
 already studied — and more could easily be cited — of 
 functions which, important as they are, are subordinated, 
 even sacrificed, to prevent a marked rise or fall in the 
 temperature of the blood. 
 
 To such an extent is the nervous system as a vi^hole 
 adapted to maintain the constant temperature that the 
 failure to do this, as shown by the presence of fever, or 
 by the even more serious subnormal temperature, becomes 
 one of the most important indications that something has 
 gone wrong. We know already how the nervous system 
 intervenes in every function of our lives, and how the well- 
 being of the body as a whole depends upon the adjust- 
 ments which it brings about. It is for these reasons that, 
 when it is no longer able to exercise that supreme control 
 of the constant temperature which is one of its most char- 
 acteristic features in health, the physician's orders usually 
 are to " go to bed and be perfectly quiet." The body is then 
 in no condition to make demands on the nervous system for 
 action ; and a person who refuses to heed the plain warn- 
 ing which his temperature holds out has nothing but his 
 own foolishness to blame if he suffers serious conseauences. 
 
CHAPTER XIII 
 
 NUTRITION 
 
 A. The Sources op Power and Heat for the 
 Human Mechanism 
 
 1. Food and Nutrition. — The one source of supply of 
 matter required for tlie growtli and repair of the liuman 
 machinery, and of power required for it,s work, is food ; 
 and the study of the various ways in which different kinds 
 of food serve the needs of the body, and the value of each 
 kind of food, under different circumstances or for different 
 purposes (such as athletic training, mental work, life in 
 cold climates, in hot climates, etc.), is the study of nutri- 
 tion. We shall limit our consideration of this great sul)ject 
 to a few elementary and practical to[)ics, heginnnig with 
 the supply of heat and power to tlie Ijody. 
 
 2. The Fuel Value of Food. — In any locomotive engine 
 the same amount of a given fuel will enable the engine to 
 pull a train of the same weight for the same distance over 
 the same track, provided, of course, the engine itself, tlie 
 bearings of the wheels, etc., are in the same CDudition. 
 When a ton of coal is put into the tender, it is with the 
 expectation that it will move the train a certain distance. 
 Thus there is a definite relation between the fuel burned 
 and the work done. Every engineer knows also that the 
 same weight of different fuels will cany the train different 
 distances ; a thousand pounds of wood, of bituminous coal, 
 and of anthracite coal have different /((fZ values. 
 
 Tlie same weight of a given fuel when burned will 
 always yield exactly the same amount of heat, as is proved 
 
212 THE HUMAN MECHANISM 
 
 by burning the fuel under conditions which enable us to 
 measure the heat given off. The simplest means of doing 
 this is perhaps with the iee calo7'wieter, — a metal box 
 witliin which the fuel is burned, the box being every- 
 where surrounded by a thick layer of ice. The heat pro- 
 duced in burning the fuel is measured by the amount of 
 ice melted. 
 
 In this way we may find the relative amounts of work 
 which can be done witli two different fuels ; for it has 
 been discovered by actual experiment that if one kind of 
 fuel produces twice as much heat as another, it will also 
 do twice as much work. 
 
 Now food is the fuel for the muscular work of the body 
 and also for the lilieration of heat. Consequently, if we 
 determine how much heat is liberated when a certain 
 amount of proteid, or fat, or carbohydrate is burned in a 
 calorimeter, we know how much work it may do in the 
 body ; or at least we know that it can do no more than the 
 amount indicated ])j the calorimetric experiment. 
 
 3. Units of Heat and Work. — In order to measure we 
 must have units of measurement. Common units of length 
 are the inch or centimeter ; units of area are the square 
 yard, the square meter, or the acre ; units of volume, the 
 quart or peck ; units of weight, the pound or kilogram. 
 And we express the results of these measurements by say- 
 ing that a thing is so many inclies long or of so many 
 pounds weight. What are the units of heat and work ? 
 
 Like all units, tliese are arbitrarily chosen. The unit of 
 heat, known as the caloric, is the amount of heat necessary 
 to raise one gram (one cubic centimeter) of water one degree 
 Centigrade. The vnit of ivork is the amount of work done 
 in lifting a kih)gram (2.2 His.) to tlie height of one meter 
 (•89.37 in.) from the surface of tlie earth against the attrac- 
 tion of gravitation. Tliis is known as the Mlogrammeter. 
 Thus when a man weighing sixty kilograms goes up a 
 
NUTRITION 213 
 
 flight of stairs ten meters high his muscles do 600 kilo- 
 grammeters of work.^ 
 
 Finally, it hus been found that the same fuel which 
 when burned will liberate one calorie of heat will supply 
 the power to do 0.423985 kilogrammeters of work. By 
 this we mean that not more than 0.423985 kilos-ram- 
 
 o 
 
 meters of work can be obtained from it. Not every engine 
 is so perfectly constructed as to get from a certain fuel 
 its full working capacity ; indeed, most engines transform 
 only a small fraction of the power of their fuel into work, 
 the rest escaping as heat, — in the smoke, oi- by radiation, 
 conduction, and convection from tire furnace, l^oiler, etc. 
 But by the metliod above outlined it is possible to find the 
 maximum amount of work which can be olitaiiaed from a 
 given weight of fuel. 
 
 Applying the same methods to food, we find that: 
 
 1 gram of dried pnileiil yii'lfls 110(1 calorii's. 
 
 1 gram of dried cdrholii/dnile yields 11(10 calories. 
 
 1 gram otj'iit yields !i:;00 calories. 
 
 These figures ai'c known as the fiii'l ctihics of proteids, 
 carbohydrates, and fats. 
 
 But the total possible power which maj' be dbtained ])j 
 actually burning a certain substance under the most favor- 
 able conditions is one thing, and the amount of power w hich 
 tlie nniscles may obtain from it is (.|uitc another. Wlien coal 
 is burned in an engine it does work; but tlie human body 
 would get no energy for its nmscular work from eating 
 coal. So that ^ve have now to inquire fiom wliat nutri- 
 ents (p. 89) the muscles get tlieir energy for work, and from 
 what nutrients the body derives its lieat. 
 
 4. The Power for Muscular Work. — Few questions in 
 physiology have been more thorouglily investigated than 
 
 1 Work may also be expressed in foot pounds. (How many foot 
 pounds equal one kilogrammeter ?) 
 
214 THE HUMAN MECHANISM 
 
 this. In the first half of the nineteenth century many 
 investigators, impressed with the fact that the muscle 
 fiber yields, on chemical analysis, large quantities of proteid 
 and only traces of carbohydrates and fats, believed that 
 the energy for muscular contraction comes entirely from 
 the consumption of the proteid of the muscle substance. 
 If this were true, it would necessarily follow that proteid 
 is the propjer food to j-ield the energy for muscular con- 
 traction, while fats and carbohydrates would simply be 
 oxidized to give heat. 
 
 This view was disproved by the following epoch-making 
 experiment of physiology. Two observers determined for 
 three successive days the nitrogen excreted by themselves ; 
 since almost all this nitrogen comes from proteid, this gave 
 the amount of jiroteid consumed by the body. On the first 
 and third days no vigorous muscular work was done ; on the 
 second day they climbed a mountain 19.56 meters (6415 ft.) 
 high. As one man weighed 66 kilograms and the other 
 76 kilograms, the work done in lifting the body to the top 
 of the mountain in the two cases was 129,096 and 148,- 
 656 kilogrammeters respectively. The jaroteid which was 
 oxidized in this time could in the two cases have yielded 
 power for only 68,690 and 68, -376 kilogrammeters of work. 
 In other words, the proteid did not begin to yield suflrcient 
 power for the muscular work done in lifting the body to the 
 top of the mountain ; something else than proteid must have 
 been oxidized for that purpose, and that something must 
 evidently have been carbohydrate or fat, or both. 
 
 Again, it was noticed that there was no increase of pro- 
 teid disintegration on the day of work ; this remained 
 practically unaffected by muscular contraction. Numerous 
 other experiments made since that time have shown the 
 same thing. Muscular exercise does not necessarily increase 
 proteid disintegration, and the power for it can be obtained, 
 in large part at least, from fats and carbohydrates. 
 
NUTEITIOK 215 
 
 In the experiment above referred to no determinations 
 were made of the excretion of carbon dioxide. Since then 
 numerous experiments have been made in which, on an 
 abundant mixed diet, both tire nitrogen and the carbon of 
 the excretions were accurately determined. These have 
 shown that while muscular exercise does not necessarili/ 
 increase proteid disititei]ration, it invariahltj increases the 
 profluction of carhon dioxide. If the carbon of the carbon 
 dioxide came from proteid, it would l)e accompanied bj' 
 increased excretion of nitrogen derived from the broken- 
 down proteid. The fact that it is not so accompanied 
 can only mean that it came from the oxidation of some- 
 thing which did not contain nitrogen, i.e. from fat or car- 
 bohydrate, or both. 
 
 But while muscular work does nrit necessarily or even 
 usually increase proteid decomposition, and the power for 
 the same may be derived largely, if not entirely, from car- 
 bohydrates and fats, it lias been shown conclusivelj^ that 
 under certain conditions this power may come entirely 
 from proteid. In one experiment a large and very lean 
 dog was fed for several weeks on an aljundant diet of lean 
 meat, containing practically no caiholn^drate or fat ; dur- 
 ing this time the dog did large amounts of \\ ork in a tread- 
 mill and in other ways ; and since it was found that this 
 work could be done for weeks at a time on the meat diet, 
 we conclude that the proteid must have been the sole 
 source of power for the work ; it must also have served 
 as the source of heat production, for the normal tempera- 
 ture of the animal was maintained. 
 
 5. Summary of Considerations on the Supply of Power 
 for Work. — These and other experiments show (1) that 
 the animal body can get its energy for mechanical work 
 and for the production of heat from proteid, or from car- 
 bohydrate, or from fat ; (2) that when the animal is on an 
 abundant mixed diet, even vigorous muscular work does 
 
216 THE HUMAN MECHANISM 
 
 not increase the oxidation of proteid,' but that it does 
 enormously increase that of carbohydrates and fats. The 
 probable meaning of this is to be sought in the fact that 
 proteid decomposition depends primarily not on muscular 
 M'ork but, as we shall see later (p. 222), on the amount 
 of proteid eaten ; while the oxidation of fats and carbohy- 
 drates dejiends entirely on the demands of the body for 
 energy, and is largely independent of the amount of these 
 foods eaten. 
 
 6. The Supply of Energy for Heat Production. " Heat- 
 ing " Foods. — In studying the phenomena of heat pro- 
 duction in the body w^e found that when the body needs 
 more heat in order to maintain its normal temperature, 
 this heat is supplied chiefly by greater chemical activity 
 in the muscles (p. 207). Tlie contraction or tone of the 
 muscles increases in response to stimuli from the same 
 motor nerves wliicli stimulate them to activity when they 
 do external mechanical work. Heat production in the 
 body, from the standpoint of nutrition, is therefore, as far 
 as we know, largely a case of increased muscular activity; 
 and here, as in the case of mechanical work, the energy 
 can be obtained from one kind of food as well as from 
 another. Contrary to popular ideas, we have no conclusive 
 evidence that one kind of food supplies heat more readily 
 tlian anotlier. What is required in cold weather is more 
 food, whether proteid or carbohydrate or fat. We shall 
 see that there are good reasons for not unduly increasing 
 the proteid of the diet under any conditions (p. 225), and 
 hence in this special case it is probably better to increase 
 the non-nitrogenous foods to a greater extent than the 
 proteids, though not because they are better " heating " 
 foods. 
 
 1 Under the abnormal conditions of excessive muscular work (e.g. six- 
 day walking matches or bicycle races) the proteid oxidation is often 
 increased. 
 
NUTRITION 217 
 
 There is another reason why the fats and carbohydrates 
 may well be increased in cold climates, and that is that 
 they are par excellence the foods wliich lead to the storage 
 of fat. The storage of fat in the subcutaneous tissue, we 
 have already seen, greatly facilitates the maintenance of 
 the body temperature by interposing a poor conductor 
 of heat between tlie skin and the more internal heat-pro- 
 ducing organs; and this gives to these foods a special value 
 in a cold climate. 
 
 It is often pointed out that inliabitants of cold climates 
 eat more fat, while the non-proteid food of those who live 
 in warm climates consists chiefly of carbohydrates, and 
 the conclusion is drawn that fat is a better food for cold 
 weather. This reasoning, however, neglects the fact that 
 vegetables, from which alone carboh^ydrates are obtained, 
 are not common in cold climates. The i)eople in such 
 climates get their non-proteid foods from the most con- 
 venient source, — the adipose tissue of animals. The 
 question is further complicated by cimsiderations of diges- 
 tibility of the two kinds of food, and cannot be answered 
 offhand. 
 
 B. Special Effects of the Diffekbnt Nutrients 
 
 Our study in the preceding section of the sources of 
 power and heat has led to the conclusion that while one 
 nutrient may be preferable to another, either proteid or 
 carbohjrdrate or fat may meet these two needs of the body. 
 In other words, no marked difference in use has thus far 
 developed for the oxidizable nutrients. In the present 
 section we desire to dwell upon the special uses of the 
 nutrients, i.e. the uses which each subserves and which 
 cannot be replaced by others. 
 
 The more important nutrients are, as we have seen in 
 Chapter VIII, proteicls, carbohydrates, fats, inorganic salts, 
 
218 THE HUMAN MECHANISM 
 
 and water. These may now be conveniently classified as 
 follows : 
 
 1. Proteids 
 
 2. Carbohydhates 
 
 J 
 
 Non- 
 ,3. Fats I P^'^'^^^s 
 
 A. Compounds of carbon which 
 
 undergo marked cliemical 
 change, by oxidation or 
 otherwise, into waste prod- 
 ucts within the body 
 
 B. Substances excreted from f 4 Inorganic Salts 
 
 the body largely in the -i 
 
 same form as absorbed [ 5. Water "■ 
 
 The proteids contain carbon, hydrogen, nitrogen, oxygen, 
 and sulphur ; some of them also contain phosphorus and 
 some contain small quantities of iron. The carbohydrates 
 and fats contain only carbon, hydrogen, and oxygen. All 
 three are oxidized in the body to the waste products with 
 which we have already become acquainted (see Chapter XI). 
 The usual inorganic salts of the food, on the other hand, 
 do not undergo oxidation ; and, while they may change 
 into other chemical forms during their stay in the body, it 
 is still true, on the whole, that they are excreted in the 
 same form as that in whicli they were absorbed. And the 
 same thing is true of the water we drink. 
 
 7. The Albuminoids and Extractives. — In addition to 
 the above, the connective tissues of animal foods contain a 
 sixth nutrient which is composed of the same elements as 
 proteids, i.e. carbon, hydrogen, nitrogen, oxygen, and some- 
 times sulphur. By far the most important member of the 
 group is the colhtr/cn of the fibers of connective tissues, 
 bone, etc., and the gelatin into which this collagen is con- 
 verted when it is boiled with water. These so-called albu- 
 minoids may do the work of proteids in part, but they 
 cannot replace the proteid entirely. 
 
 Finally, most foods contain small quantities of other sub- 
 stances, known as extractives^ which are of use to the body 
 
 1 Sec Chapter VIII fur definition ot the terms in this table. 
 
NUTRITION 219 
 
 chiefly because they contribute to the taste or flavor of the 
 food. Some of them are thought to exert a stimulating 
 effect, while still others are of importance in disease. For 
 our present purposes, however, they are of minor impor- 
 tance, and we shall confine our attention to the nutritive 
 value of the proteids, carbohydrates, fats, salts, and water. 
 
 8. The Inorganic Salts and Water. — We may begin 
 with these because, so far as they concern us in an ele- 
 mentary work like this, their uses are most easily under- 
 stood. Inorganic salts (chlorides, sulphates, carbonates, 
 and phosphates of sodium, potassium, calcium, etc.) are 
 found in all living cells, in the blood and the lymph, and 
 are constantly being removed from the blood in the urine 
 and the perspiration. This loss must be made good by the 
 food. While inorganic salts are not oxidized by the liody, 
 their presence in the blood and tissues is for various reasons 
 absolutely necessary. The craving of herbivorous animals 
 for salt, in which their food is deficient, is well known, 
 and in parts of India salt famines have occurred during 
 which the j^rice of salt was liigher than that of gold. 
 
 The loss of water from the lungs, skin, and kidneys is 
 made good from two sources. In the first place, proteids, 
 cai'bohydrates, and fats all contain liydrogen ; and when 
 they are burned within the body their hydrogen combines 
 with oxygen to form water, thus : 
 
 CJIi.O,. + 6 0„ = G CO, + 6 H,0 
 
 sugar oxygen carbon water 
 dioxide 
 
 In tliis way, on an average diet, almost one pint of water 
 may be produced within the body in a day ; and we are able 
 to understand how at times, despite the constant loss of 
 water and the comparatively small amount drunk or taken 
 in the food, the tissues do not dry up. Usually, however, 
 this supply is iusufiicient, and it is necessary to use water 
 as an article of food. 
 
220 THE HUMAN MECHANISM 
 
 9. The Proteid and the Non-Proteid Foods : Three Points 
 of Difference. — We may pass now to the study of those 
 food stuffs which are compounds of carbon, and which 
 undergo marked chemical change into waste products 
 within the body. We have already separated those food 
 stuffs — proteids, carbohydrates, and fats — into two divi- 
 sions, the proteids and the non-proteids, because in at least 
 three respects the members of the two divisions play 
 different roles in the nutrition of the body. They differ (1) 
 in their availability to serve in repairing losses of living 
 tissue ; (2) in their effects upon the chemical changes in 
 the body ; and (3) in their relation to the storage of fat. In 
 all these respects the fats and carbohydrates resemble each 
 other closely, while they differ from the proteids. 
 
 10. The Waste and Repair of the Living Cells. — Certain 
 organs of the body, notably the muscles and glands, con- 
 sist chiefly of living cells, thus presenting a striking con- 
 trast to other organs, like bones, tendons, ligaments, and 
 cartilage, in whicli the living cells are few in number, the 
 main mass of the organ consisting of lifeless matter between 
 the cells. If, therefore, we make a chemical analysis of 
 organs of the first kind, we can obtain some insight into the 
 composition of the living cell. The results of the analysis 
 of muscle (lean meat) will serve as a type of what is found 
 in such cases. 
 
 Composition of lean meat : 
 
 Water 75% 
 
 Solids 25 % 
 
 Proteid 21% 
 
 Salts 1% 
 
 Extractives 1 % 
 
 Fat, connective tissue, etc 2%, 
 
 We do not know in what form the proteid which we 
 find in dead muscle is present in living muscle ; but even 
 if it is not in the precise form which we know as lifeless 
 
NUTRITION 221 
 
 proteid (such as the white of an egg), it is still true that 
 this proteid constitutes the chemical basis of the living 
 substance. Hence we may say that the living cell is 
 largely composed of a proteid-like substance containing 
 among other elements carbon, liydrogen, nitrogen, 0x3'- 
 gen, and sulphur. 
 
 11. The Proteid-like Substances of the Living Cell are 
 constantly disintegrating. — Actual study of the excre- 
 tions of the human bod}^ shows that under all conditions 
 of its life nitrogenous substances and sulphates are given 
 off. Since the nitrogen and sulphur which these contain 
 are found only in the proteids of the food, it follo^\'s that 
 there is a constant disintegration of proteid going on in 
 the body. This is observed during fasting and even when 
 an abundance of fats, carljohydrates, salts, and water, but 
 no proteid, is taken in the food. The disintegration of 
 proteid seems to be as truly a part of the life of the body 
 and of its constituent cells as is the consumption of ox}-- 
 gen ; and all the facts show that whether we are doing 
 muscular work or mental work, whether we are asleep or 
 awake, whether the weather be warm or cold, in short, no 
 matter what maj' be the conditions of life, tliis disintegra- 
 tion of proteid is going on in every living cell. We shall 
 subsequently learn that it is greater at one time than at 
 another, but that it never ceases. Hence it follows that 
 proteid food, which alone contains the nitrogen and sul- 
 phur needed to make good this loss, is an indispensable 
 part of the diet ; and it has been shown that when their 
 diet contains no proteid, animals starve as surely as they do 
 when they eat no food at all. Fats and carbohydrates, on 
 the other hand, may be excluded from the diet without 
 fatal, and sometimes without harmful results. 
 
 12. The Effect of Proteids and Non-Proteids on the Con- 
 sumption of Material in the Body. — The amount of coal or 
 other fuel burned in an open fire depends to a large extent 
 
222 THE HUMAN MECHANISM 
 
 on the amount put on the fire, and is independent of the 
 need for heat in the room. Is this true of the food taken 
 into the body? 
 
 Actual study of the subject has shown that the proteid 
 and the non-proteid foods differ greatly in this respect. 
 When more proteid is eaten, digested, and absorbed, more 
 proteid is consumed in the body and more waste products 
 produced. Here there is some resemblance to an open fire. 
 Feeding more of proteid does actually lead to greater oxi- 
 dation of proteid in the body. 
 
 With the non-proteid nutrients this is not true, or at 
 least not to the same extent. The body uses only as much 
 of them as its needs require, or but little more than this, 
 and stores the excess for future use. It behaves toward 
 them somewhat as the fire would behave toward the fuel 
 if, independently of the amount of fuel fed, it burned 
 only so much as was necessary to heat the room to the 
 proper temperature. The behavior of the body toward 
 them suggests the action of the automatic regulator which 
 closes the draft of a furnace as the temperature of the 
 room rises, and so limits the consumption of fuel according 
 to the need for heat. The body, then, behaves toward pro- 
 teid somewhat as an open fire does toward the fuel put 
 upon it, while in its behavior toward fats and carbohydrates 
 it is more like a self-regulating furnace. 
 
 13. The Relation of Proteid Feeding to the Quantity of 
 Proteid-like Substance in the Living Cells of the Body. — We 
 have shown in the last paragraph that when more proteid 
 is eaten more is consumed in the body. We may now point 
 out another important relation of proteid to nutrition. It 
 has been found that if a man, whose food meets all require- 
 ments of his body so that he is in good health, does full 
 work, and is neither gaining nor losing in weight, increases 
 the amount of proteid in his food, not all the extra proteid 
 is disintegrated into waste products, but that some of it is 
 
NUTEITION 223 
 
 retained in the body, apparently increasing the total amount 
 of living cell substance. If now the increased proteid diet 
 be continued from day to day, it will be found that the 
 amount of proteid thus reserved or the amount of living 
 cell substance thus gained becomes less from day to day, 
 until finally the amount of proteid disintegrated exactly 
 equals that absorbed. The condition thus finally attained 
 is known as yiitroyenous equilibrium, which means that the 
 nitrogen of the intake (proteid food) exactly equals the 
 nitrogen of the output (urea, etc.); and so long as the same 
 amount of proteid is fed this state of nitrogenous equilib- 
 rium continues. Such experiments, therefore, prove that 
 increasing the proteid of the diet not only leads to increased 
 proteid consumption but also to an increase of the stock 
 of proteid-like material within the cells of the body. 
 
 Furthermore, if after the estal)lishment of nitrogenous 
 equilibrium the amount of proteid in the diet he decreased, 
 the body for a time excretes more nitrogen than it takes 
 in; tliat is to say, it consumes all the proteid of the food 
 and, in addition, some of the proteid-likc substance of its 
 own living cells. Obviously the amount of proteid within 
 the living cells of the bodj' depends on the amount of pro- 
 teid absorbed from the alimentary canal. We may, there- 
 fore, conclude that (1) the amount of jiroteid digested and 
 absorbed is a determining factor in the amount of proteid 
 consumed or disintegrated in the body ; and (2) it is a deter- 
 mining factor in the proteid contained within the body. 
 In other words, to maintain a given amount of proteid in 
 the living cells, a certain amount of proteid is necessary 
 in the food; less than this will lead to loss of proteid, 
 more will increase it. 
 
 With the non-proteid foods the case is different. Eating 
 more fat or carbohydrate does not necessarily change the 
 amount of these substances consumed or oxidized. The 
 body seems, on the whole, to use as much of them as it 
 
224 THE HUMAN MECHANISM 
 
 needs (see pp. 213-217), and to store up the rest chiefly 
 as fat. Here then is the second point of difference between 
 proteid and non-proteid foods ; the amount of the former 
 which the body disintegrates varies conspicuously with 
 the amount fed, while the disintegration of the latter is 
 chiefly determined by other causes. 
 
 14. The Effects of Proteids and Non-Proteids respectively 
 on the Storage of Fat within the Body. — The third point 
 of difference between proteid and non-proteid food is that 
 the latter readily lead to the storage of fat, — in other 
 words, that they are "fattening foods"; while the former 
 are converted into fat only to a much smaller extent, and 
 normally are possibly not turned into fat at all. This sub- 
 ject will be more fully discussed in the next chapter. 
 
 15. Summary. — In three respects, then, the proteid and 
 non-proteid foods exert different nutritional effects upon 
 the body : first, only the proteids can repair waste of liv- 
 ing tissue ; second, the mere act of eating more proteid 
 leads to increased proteid disintegration and to increased 
 chemical change in the body ; while the excess of non- 
 proteids over and above the needs of the body for power 
 and heat is stored away for future use ; and third, fat is 
 formed chiefly, if not entirely, from the non-proteid foods. 
 
 On the other hand, many requirements of the body may 
 Ije met by either class of food stuffs, though one may do 
 better than the other. Li the previous chapter we found, 
 for example, that both proteids and non-proteids may sup- 
 ply power and heat. In some respects it is a matter of 
 indifference whether we eat proteid or carbohydrate or fat ; 
 in otlier respects it is a matter of consideralile importance. 
 
 16. Other Considerations with Regard to Special Effects 
 of Proteids and Non-Proteids. — We have repeatedly spoken 
 above of the amount of a nutrient " eaten, digested, and 
 absorbed," instead of merely the amount " eaten." This 
 was done to emphasize the fact that the nutritional effects 
 
NUTRITKOT 225 
 
 we have described are produced by the material which 
 passes into the blood by absorption, and not merely by that 
 which is swallowed. For, in the first place, a part of the 
 food eaten is not digested at all, but leaves the body with 
 the bowel discharges ; the amount of food thus lost to 
 the organism varies with different individuals. Observa- 
 tions are on record which show that at times as much as 
 one third of the proteid eaten tlrus escapes the action of 
 the digestive juices. Nor is this all. We have seen that 
 more or less of tlie food eaten is consumed within the in- 
 testine, and sometimes even within the stomach, by bac- 
 teria. Proteids, carboliydrates, and fats thus destroyed l)y 
 Vjacterial action produce none of the effects descrilied above. 
 These effects are jiroduced only liy tliose foods whicli are 
 digested and absorbed into fJic blood. 
 
 These considerations partly explain the striking fact 
 that some people remain lean altliough they are " great 
 eaters," while others grow fat on a comparatively meager 
 diet. In other words, it is one thing to take food into the 
 alimentary canal and quite another thing to digest, absorb, 
 and in acceptable form distribute it to tlie living cells of 
 the body. 
 
 Anotlier point is of very considerable practical impor- 
 tance and should be reckoned witli in the choice of food. 
 The carl)on dioxide and water into which the fats and car- 
 bohydrates are oxidized in the cells are at once taken up 
 by the blood and carried directly to the lungs and other 
 excretory organs where they are got rid of. With the 
 waste products of proteid disintegration the case is differ- 
 ent. Proteid does not break down in the cells at once into 
 urea, etc., but largely into intermediate products ; tliese 
 are thereupon taken to other organs, chiefly the liver, 
 where they are chemically changed into urea ; and it is 
 only after this series of chemical changes that the proteid 
 waste is carried in its final form to the kidnej's for removal 
 
226 THE HUMAN MECPIANISM 
 
 from the body. Evidently it is a more serious task to elimi- 
 nate the waste products of proteids than those of fats and 
 carbohydrates. While the body is no doubt quite capable 
 of caring for large amounts of these proteid wastes, there 
 are reasons for thinking that tlieie is such a thing as 
 overburdening the liver and even the kidneys with them. 
 Consequently, although proteids yield heat and the power 
 for muscular work as readily as do carbohydrates, it may 
 be better to use carbohydrates for these purposes rather 
 than proteids. 
 
 Finally, questions of digestibility often complicate the 
 problem of meeting the nutritional demands of the bod)'. 
 It may well be that of two diets we niay choose one rather 
 than the other because it is more digestible, and not because, 
 when digested, its constituents more perfectly meet the 
 demands of the body. Thus 100 grams of pjroteid + 100 
 grams of fat + 300 grams of carbohydrate liave together the 
 same approximate nutritional value as 100 grams of jjro- 
 teid 4- 50 grams of fat + 41.5 grams of carbohydrate (since 
 50 grams of fat are equivalent to 115 grams of carbo- 
 hydrate) ; but a man whose digestive system cares for fat 
 with difficulty might well choose the latter diet rather than 
 the former. 
 
 17. Are there Special Foods for Special Organs or for 
 Special Work? — Tins question is raised by the flaming- 
 advertisements of " nerve " foods, " brain " foods, foods 
 " good for muscle," or for other parts of the body, which 
 are often seen in puljlic journals or public places. The stu- 
 dent who has mastered the foregoing chapters of this book 
 hardly needs to be told that in the swiftly circulating blood 
 all cells of all tissues have a common ration and practically 
 sit at one table, very much as officers and soldiers do while 
 engaged in actual warfare. No one then thinks of giving to 
 officers brain food and to comnaon soldiers muscle food, but 
 rather to both a good, general, " all-around " food supply. 
 
NUTKITION 227 
 
 Moreover, any one who will recall the complex chemical 
 processes of digestion cannot have forgotten that all foods 
 are greatly changed during that process, so that if a special 
 food is eaten, there is no evidence that it ever reaches any 
 special cells, such as those of brain or muscle, in or even 
 near its original form ; or, if it should reach the desired 
 organ or tissue, that it would be either wanted or used 
 there. Very likely some persons sometimes take special 
 foods or drinks with apparent or temporary advantage, due 
 to change of diet or to a real increase of food suppl}^ ; but 
 that their improved feeling or condition is due to any special 
 feeding of brain or muscle or other tissue is unlikely ; and 
 the same or even better effect could probalily ha\'e lieen 
 obtained b}' the same amount of attention paiil to a change 
 of diet, increased muscular activity, rest, sleep, or any one 
 of a number of tlie factors of hygienic living. 
 
 C. Flesh, Fat, and Glycogen 
 
 18. Flesh and Fat. — People of the same height differ 
 greatly in weight. We recognizA' this fact in tlie use of the 
 adjectives "stout" and "tliin," or "lean." We also recog- 
 nize the fact that a man of heavy build may owe his weiglit 
 to fat or to muscle. Sometimes we hear a man say, "I weigh 
 the same as formerly, but I have got less muscle and more 
 fat," or vice versa. This expresses a physiological distinc- 
 tion which is not always made in our conunon use of words; 
 for when we sav some one is "stout" or "fleshy" or "cor- 
 pulent," we do not state wiiether his greater weight is due 
 to muscle or to fat, although we more commonly mean that 
 it is due to fat. 
 
 It will l_)e convenient for our purposes if we agree upon 
 a definite use of terms to express this difference. In a 
 piece of beefsteak there are three well-known parts, — the 
 lean meat, the fat, and the connective tissue ; and this 
 
228 THE HUMAN MECHANISM 
 
 composition of one of the organs of the body corresponds 
 to the composition of the body as a whole, which consists 
 of three kinds of material : (1) the living cells, represented 
 by the muscle fibers, gland cells, nerve cells, etc.; (2) the 
 connective and supporting tissues, such as tendon, liga- 
 ment, lione, and cartilage ; and (3), stored away within cer- 
 tain cells, lifeless matter or food material, which may be 
 called upon to supply in part the needs of the body when 
 food is not being absorbed from the alimentary canal. Fat 
 is one of the two most important members of the third 
 class. 
 
 With the second of the above classes (connective and 
 supporting tissues) we are not concerned in the present 
 chapter, since they are not rapidly consumed in the work 
 of the body and hence do not suffer large loss of substance, 
 which must be made good from the food. With the first 
 and third groups, however, we are immediately concerned, 
 since they stand in intimate relation to the cellular activi- 
 ties, and so are subject to chemical change. 
 
 To the essential living substance of the cells, which 
 we have seen is built chiefly from proteid, we give the 
 name Jlmh^ and it is only with this meaning that we shall 
 subsequently use the term. Flesh, then, would include 
 the living substance of the mviscle fibers, of the cells of the 
 pancreas, kidney, liver, nervous system, etc.; in short, all 
 the material of the cell wliich is essential to its life. It 
 does not include the lifeless matter which may be stored 
 away within these cells. Thus a cell of the liver may con- 
 tain drops of lifeless fat in addition to its living flesh. 
 
 19. The Increase of Flesh. — The fat and flesh of the 
 body may vary in amount at different times; one of them 
 may change while the other remains constant, or both may 
 change simultaneously. The annmnt of each is, to a large 
 extent, independent of that of the other ; and we have 
 now to inquire what is known about the conditions which 
 
NUTRITION 229 
 
 determine how much flesh and how much fat the body 
 contains at any given time. 
 
 We have already described in the preceding chapter one 
 of the conditions of the storage of flesh in the body, namely 
 abundant proteid feeding. Simply feeding more proteid 
 certainly leads at times to the formation of more flesh. 
 The student will review what has been said on page 223. 
 But it is by no means certain that this is the only condi- 
 tion that leads to increase of flesh. The growth of muscles 
 by use suggests that under conditions of active muscular 
 work more of the proteid eaten is retained in the body as 
 flesh, and less broken down to waste products, than when 
 little muscular work is done. The whole subject needs 
 further elucidation. 
 
 20. The Hoarding of Inactive or Lifeless Food Material. 
 The Storage of Fat. — The ))(xly not only adds from time 
 to time to its stock of living flesh, but also stores lifeless 
 material for future use, and that, for the greater part, in 
 two forms, — fat and ijhji:o(jcii. Fat may be stored as drops 
 of oil in the cytoplasm of any cell of the body. Muscle 
 fibers, for exam[)le, contain at times large quantities of 
 this substance, and are then said to have undergone fatty 
 degeneration. Under normal conditions, however, the pres- 
 ence of considerable quantities of fat in muscle fibers or 
 nerve cells or most gland cells is unusual. In the cells of 
 connective tissue, on the other hand, fat is readily stored 
 under normal conditions, and the udipo^r- tissue or fat of 
 the body is simply connective tissue whose cells are loaded 
 with droplets of fat. Figs. 80-80, with their explanation, 
 will show how this takes place. But while fat may be nor- 
 mally stored in any of the more 0[ien connective tissues, it 
 is especially in the subcutaneous tissue, the great omentum, 
 the mesenter}', and some other situations that its chief 
 storage takes place. From these storehouses it is drawn 
 upon as a reserve food material when the immediate supply 
 
230 
 
 THE HUMAN MECHANISM 
 
 Fio. 85 
 
 Fk.s. 83-8-j. Tliivu successive stages in tlie 
 transformation of ordinary connective 
 tissue into adipose tissue 
 
 A portion of the cajiillary network is sliown, 
 surrounded by the fibers, anion^- wliiidi are 
 several cells. Tbe accumulation of fat 
 droplets in tbe cell cytoplasm is sbowu iu 
 Fij,'. H4:, and tlie fusion of these to form 
 one large droplet, surrounded by tbe cyto- 
 plasm, is seen in Fig. 80 
 
 of food from the alimentary 
 canal becomes inadequate for 
 the work of the body. The 
 exact mechanism by which it 
 is stored in a cell at one time 
 and discharged at another is 
 not fully understood. Some 
 of the conditions under 
 which it is accumu- 
 lated, and some of those 
 under which it disap- 
 pears, are known ; but 
 we do not know the 
 whole story. Some peo- 
 ple lay up fat in larger 
 quantities than others 
 on the same diet, and 
 apparently while doing 
 the same amount of 
 work; and some keep 
 lean under conditions 
 apparently the most 
 favorable for growing 
 fat. 
 
 It was formei'ly be- 
 lieved, and is still some- 
 times supposed, that 
 the animal body forms 
 fat only from the fat 
 of the food ; that to get 
 fat we must eat fat. 
 Tliis was disproved by 
 a numljer of experi- 
 ments, especially one 
 by Liebig, who kept 
 
NUTEITION 231 
 
 account over a long period of the fat in tlie food sup- 
 plied to a cow, and found tliat the fat given off in the 
 cow's milk far exceeded that in her food. In another 
 experiment four pigs out of a litter of eight were killed 
 and the total amount of fat in their hodies determined. 
 The other four pigs were fattened for a time, then killed, 
 and the fat in their bodies eventually determined. Assum- 
 ing that the second set of four pigs originally had the 
 same quantity of fat as the first four, the difference between 
 the two quantities of fat found would give the quantity of 
 fat the last four had stored up. IMeantime, strict account 
 had been kept of the fat supplied in the food of tlie last 
 four, and it was shown that for eA'erj' 100 parts of fat fed 
 to them these pigs had laid up 472 parts of fat. They had 
 evidently miuinfucturfd fat fro)it soim: suhstance other tlxui 
 the fat in their food. 
 
 21. Fats can be stored from Fats and Carbohydrates in 
 Food. — There is no doubt that fat may be both stored 
 away and iitaniifactured from the fats in the food. There 
 is also no doubt that large quantities of fat may be and 
 often are manufiicturcd and stured from the carbohydrates 
 (sugars, starches, etc.) of the food ; so tliat, while there is 
 some truth in the idea that one may get fat by eating fat, 
 it is equally true that we can get fat Ij}' eating otlier foods. 
 
 22. Are Proteids a Source of Fat ? — Whether fats are 
 normally made in the body from proteids is a more diffi- 
 cult question. There is no undisputed case on record of 
 such manufacture and storage ; and while the facts do not 
 yet justify us in denying the possibility, there can be little 
 doubt that such transformation does not take place to any 
 great extent, and it is possible that in the mamnralian body 
 it does not normally occur at all. 
 
 Fats, then, are manufactured readily from fats and car- 
 bohydrates, and sparingly, if at all, from proteids. Their 
 disappearance during starvation, when they are drawn 
 
232 THE HUMAN MECHANISM 
 
 upon to supply power and heat for the body, shows that 
 they serve as a true reserve food material. They are a 
 kind of food capital or hoard, saved and laid up by the 
 body against a rainy day. 
 
 23. The Hoarding of Inactive or Lifeless Food Material. 
 The Storage of Glycogen. — In many cells of the body, but 
 especially in those of the liver and to a less extent in those 
 of the skeletal muscles, there is found a carbohydrate sub- 
 stance known as glycogen. This substance belongs to the 
 same group of carbohydrates as starch and dextrines (see 
 Chapter VIII), and is sometimes called animal starch. Like 
 them it is changed into sugar by the action of saliva and 
 pancreatic juice, whence its name (jXyKW sweet ; -yevij';, 
 former). The same change occurs on the death of the 
 cells in which it is contained, the sugar thus formed giv- 
 ing to such tissues a sweetish taste. This is often noticed, 
 for example, in liver and in scallops (the shell muscle 
 of Pecten). The total amount of glycogen in the human 
 body may reach 100-200 grams (3-7 ounces), one half 
 of which is concentrated in the liver and the other half 
 scattered about in the other tissues of the body. 
 
 Experiments have shown that glycogen is not formed 
 from the fat in food ; that it is formed in small quantities 
 from proteid ; while its chief source is the carbohydrates of 
 the food. 
 
 The blood may be said to be always sweet, its constant 
 percentage of sugar (2-3 grams per 1000 cubic centimeters 
 of blood plasma) being a striking fact, and one that we 
 should hardly have anticipated. One might suppose that 
 the sugar in the blood would increase, as does the amount of 
 fat, during active digestion and absorption ; and that, after 
 digestion had ended, it would diminish. As a matter of fact 
 the amount of sugar in the blood remains practically constant 
 for many hours after the completion of digestion, and this 
 despite the fact that the tissues are constantly abstracting 
 
NUTRITION 233 
 
 sugar from the blood. Evidently the blood must be supplied 
 with sugar from some other source than the alimentary 
 canal, and there must be somewhere in tlie body a compen- 
 sating mechanism controlling the sugar supply of the blood. 
 
 Experiments have shown that sugar is absorbed from 
 the alimentary canal entirely by the intestinal blood ves- 
 sels. It must pass, therefore, through the liver by the 
 portal vein (see P'ig. 58, p. 125) before going to the rest 
 of the body. The liver, thus standing at tliis great gate- 
 way to the circulation, would seem to act as the carbo- 
 hydrate storehouse, or savings bank, of the bodj^. Any 
 excess of sugar in tlie portal Itlood is there transformed 
 into glycogen and deposited, saved until it is needed, and 
 then " paid out," as sugar, when the ready supjjly is ex- 
 hausted. Other tissues doubtless aid in preventing an 
 undue richness of sugar in the blood, acting likewise as 
 temporary storehouses for this form of food. If it were 
 not for some such provision as these together make, the 
 eating of a quantity of candy or of maple sugar might 
 easily add to the blood enough sugar to damage the tissues. 
 
 The body is thus not immediately dependent upon the 
 food which is being absorbed from tlie alimentary canal at 
 the time. In the glycogen and fat it has within itself a 
 stock of hoarded non-proteid material upon which it may 
 call to meet its requirements for power and heat, and by 
 which it is protected against temporary shortage of these 
 nutrients in the diet. When, on the other hand, the pro- 
 teid of the diet is lacking, the body can make good the 
 deficit only by yielding up its own flesh. 
 
 D. The Choice of Food and Nutrients 
 
 When we buy food for the table we do not, of course, 
 ask for proteids, carbohydrates, fats, salts, etc. Foods sold 
 in markets are mixtures of these nutrients in varying 
 
234 THE HUMAN MECHANISM 
 
 proportions. Moreover, most meals consist of mixtures of 
 several foods, and the foods used at one meal differ from 
 those used at another, so that the diet becomes what is 
 known as a " mixed diet." In this way two results are 
 secured : first, the food is rendered more appetizing, — an 
 important physiological and hygienic consideration ; and, 
 second, we avoid the undue preponderance of one nutrient 
 over another, which would almost surely follow the use of 
 a single kind of food. Lean meat, for example, contains a 
 large amount of proteid and little fat or carbohydrate; 
 potatoes, on the other hand, contain little proteid, but a 
 large amount of carbohydrate ; by using the two at the 
 same meal we obtain better proportions of proteid and 
 carbohydrate ; and if butter be used on the potatoes, we 
 add fat to the dietary. 
 
 These three nutrients play different roles in nutrition, 
 and it is not a matter of indifference whether any one of 
 them is absent, deficient, or present in excess. In other 
 words, while the proportions of the diffei'ent nutrients is 
 not the chief consideration in the selection of food, — such 
 things as digestibility, flavor, bulk, amount of indigestible 
 material, etc., also entering into the decision of the ques- 
 tion, — it is nevertheless an important consideration, and 
 there can be no doubt that some persons are poorly nour- 
 ished because they err in this respect. We have, therefore, 
 to consider when we may need more proteid, or more fat, or 
 more carbohydrate in the food in order to meet properly 
 the different conditions of daily life, such as prolonged or 
 hard muscular work, exposure to cold or warm weather, 
 mental work, and so on. 
 
 24. The Effect of Occupations and of External Conditions 
 upon the Consumption of Food. — Since it is the function 
 of food to make good the material losses of the body, we 
 naturally ask, first of all, how the various occupations and 
 conditions of life influence the excretions, for this would 
 
NUTRITION 235 
 
 give us some information as to the material whose loss 
 must be made good. Many experiments have been made 
 to determine this ; but tlieir results may be briefly sum- 
 marized by saying that only three things seem to affect 
 directly the consumption of food in the body, viz. mus- 
 cular activity^ changes of temperature, and the act of feeding 
 itself. The changes thus produced in the C|uantity and 
 quality of the chemical activities of the body have already 
 been fully described in previous chapters. Muscular work 
 increases the oxidation of fats and carbohydrates, and when 
 these are not present in sufficient quantities to yield energy 
 for the work, it also increases the disintegration of ijroteid. 
 Exposure to cold seems to have the same effect. As to 
 the influence of the third factor mentioned above, — the 
 act of feeding itself, — it will lie recalled that the quantity 
 of proteid disintegrated depends on the quantity fed, and 
 that the quantity of fat stored depends chiefly on the 
 quantity of fat and carbohydrate eaten. 
 
 25. Feeding for Muscular Activity. — The hard-working 
 man must be well fed if he is to do his best work. Not 
 only is his appetite greater, but he cannot continue hard 
 work successfully for any length of time on a meager diet. 
 No further argument is needed to show that the diet in 
 muscular work should be relatively abundant. But should 
 its character also he changed ? From the fact that when the 
 diet remains unchanged muscular work increases the oxi- 
 dation of fats and caiiiohydrates, while it affects that of 
 proteid only as the other nutrients fail, some physiologists 
 have concluded that the increased food taken should be 
 non-proteid rather than proteid. It is, however, actually 
 found that when those who are engaged in hard work 
 are free to choose their diet according to the dictates of 
 appetite, they use more proteids as well as more carbo- 
 hydrates and fats. Similarly it has been found that when 
 a race horse is being fed for the track he does better work 
 
236 THE HUMAN MECHANISM 
 
 when fed on those grains which (like oats) contain rather 
 large proportions of proteid than on those which (like 
 corn) contain small proportions of proteid. This prob- 
 ably means that any animal, man or horse, doing hard 
 work is better off for having more living flesh with which 
 to do that work easily, and this flesh can be gained (see 
 p. 222) only by eating more proteid. Moreover, when once 
 the flesh weight has been increased to the desired amount 
 it can be maintained only by continuing with the same 
 quantity of proteid food (p. 223). While, therefore, it is 
 probable that the greater part of the increase of the diet 
 for steady and continued muscular work should fall on the 
 fats and carbohydrates, it is reasonable to eat more proteid 
 and thus maintain a greater flesh weight. When, on the 
 other hand, the " greater work " consists of a day or so of 
 increased effort by one who is otherwise engaged only in 
 moderate muscular exertion, there is not the time for an 
 increase of flesh, and it is not clear that any nutritional 
 gain would accrue from eating more proteid after the day 
 of work ; at the same time there is no reason to think that 
 harm would result from a moderate increase of this nutrient. 
 
 In this connection it is easy to see why muscular work 
 should antagonize the accumulation of fat in the body, 
 since it involves the consumption of those foods (carbohy- 
 drates and fats) which, when present in excess of require- 
 ments, furnish the material for the manufacture and storage 
 of fat. Hence the most reasonable treatment of obesity is 
 to limit the use of non-proteid foods and to indulge in hard 
 muscular work. 
 
 26. Feeding for Cold and for Warm Weather. — The 
 appetite is normally better in cool weather than in warm, 
 and this corresponds to the greater need for food, one 
 function of which is to supply by its oxidation the heat 
 necessary to maintain the temperature of the body. More- 
 over, we tend to do less muscular work in warm weather 
 
NUTRITION 237 
 
 than in cold, and this is an important factor in the prob- 
 lem. The needed increase of heat on cold days may be 
 obtained from the increased use of fats and carbohydrates, 
 although there would seem to be no good reason why pro- 
 teids may not also be increased. We have seen that eat- 
 ing more proteid readily leads to the oxidation of more 
 proteid, and so to the production of more heat. Hence, in 
 warm weathei', when the body finds difficulty in securing 
 the needful output of heat, it is unwise to indulge too 
 freely in proteid foods, since their very presence within 
 the system necessarily increases the amount of heat to be 
 got rid of. There is a widespread popular impression that 
 it is not wise to eat as much meat in summer as in winter, 
 and the impression is probalily correct, although it applies 
 also to vegetable foods, like beans and peas, which con- 
 tain relatively large quantities of proteid. For a discus- 
 sion of the relative value of fats and carbohydrates in cold 
 and warm climates, see }). 21(3. 
 
 27. Feeding for Mental Work and Sedentary Occupations. 
 — Mental work requires special attention to diet chieify 
 for two reasons : in the iirst place it involves for the 
 greater part a sedentary life, and in the second place the 
 processes of digestion are lialjle to interfere more or less 
 with the most effective working of the brain. Thus far no 
 effect of mental work upon the character of the chemical 
 changes within the body has been conclusively proved 
 beyond those effects which are ol)viously the lesult of the 
 sedentary life which accompanies it. Hence feeding for 
 mental work is largely a question of feeding for the seden- 
 tary life. We do not then need so much food, and espe- 
 cially do not need so much non-proteid food. To eat as 
 much carbohydrate and fat as when we are leading an 
 active life would be to have these nutrients present in 
 excess, and thus to lead either to the storage of fat or else 
 to overtaxing the digestive organs which, in the absence 
 
238 THE HUMAN MECHANISM 
 
 of reasonable muscular activity, may not be able to do 
 their best work. 
 
 This being the case, it is evidently advisable to use 
 rather easily digestible food, especial attention being paid 
 to its proper preparation in cooking. Pork and beans, for 
 example, may do well enough for men working on a farm 
 in cold weather, but they are not the best foods for a 
 clerk who is confined to his office during the greater part 
 of the day, or for the college student. Moreover, special 
 attention should be paid to the eating of food. " Quick 
 lunches," inadvisable at all times, should then be avoided 
 altogether, and the food should be well chewed. These 
 matters will be fully discussed in the chapter on Alimen- 
 tation, or Right Feeding (Part II). 
 
 There is also every reason to believe that mental activity, 
 especially when it involves certain feelings or moods, may 
 exert no inconsiderable effect on the nervous processes of 
 digestion. We have seen that the secretion of gastric juice 
 depends to a large extent upon the enjoyment we obtain 
 during the act of eating ; and the man or woman whose 
 absorption in mental work, or in the cares and annoyances 
 of life, is so complete that the food is eaten with little 
 more attention to its taste and flavor than if it were so 
 much paper, is certainly laying the foundation of indiges- 
 tion, and so of bad nutrition. 
 
 The above considerations point clearly to the need of 
 special attention to diet during mental occupations, using 
 that term in its broader sense to include all occupations 
 which involve mental rather than muscular work. It must 
 not be supposed, however, that we can correct the effects 
 of a sedentary life by mere attention to diet. As we 
 shall show in Part II, the human mechanism is adapted 
 to a life of muscular activity, and on the whole cannot 
 remain healthy for a long time without reasonable use 
 of this factor of health ; it is also adapted to the use of 
 
NUTRITION 239 
 
 a reasonable amount of rest, and cannot thrive when 
 subjected to undue nervous strain. There are times in 
 every life when we cannot get all the muscular activity 
 and rest we need, and when we must necessarily be sub- 
 jected to severe nervous strain. When such is the case 
 the S2:)ecial measures we have outlined should be taken 
 with regard to diet ; but we should not be deceived into 
 thinking that attention to diet can in the long run replace 
 other essential factors of healthy living. In other words, 
 normal, healthy nutrition depends not alone on the amount 
 of proteids, fats, and carbohydrates we eat, not alone on 
 their preparation, not alone on the manner in which we 
 take them, nor alone on all these things taken together ; 
 it depends no less on the hygienic c<)nduct of life in all 
 respects. The best nutrition is in the end the same thing 
 as the best health, and this is assured only to those who 
 " keep the whole law." 
 
 28. General Conclusions as to the Choice of Nutrients. — 
 We may sum up as follows. The amount of fats and carbo- 
 hydrates needed depends primarily on the amount of mus- 
 cular work done, on the amount of heat needed to maintain 
 the constant temperature of the body, and (in cold climates) 
 on the quantity of subcutaneous fat desirable to assist in 
 preventing undue loss of heat from the internal organs. 
 The amount of proteid needed, on the other hand, is not 
 so easily determined. Increased proteid feeding increases 
 proteid destruction within the body and leads to a greater 
 flesh weight. The human machine will continue to live 
 and do its work on ^er}- different quantities of proteid in 
 the food ; that is to say, when it disintegrates very different 
 quantities of proteid and when it possesses very different 
 flesh weights. Just what quantity is best is a question to 
 which the facts at present at our disposal do not justify a 
 final answer. But we are within safe limits when we say 
 that it is well to avoid both extremes ; too much proteid 
 
240 THE HUMAN MECHANISM 
 
 probably throws undue work on the organs of excretion; 
 too little leads to an undesirable reduction in the living 
 flesh or working equipment of the body. 
 
 In the foregoing pages we have refrained from giv- 
 ing definite figures as to the amount of each nutrient 
 needed under different conditions of life, partly because 
 such figures mean but little to those who can go no more 
 extensively into the subject than the majority of those who 
 read this book, but chiefly because any figures we may 
 give are apt to be misleading. So much depends on in- 
 dividual peculiarities of height and general build of the 
 body, and so much on the occupations and other condi- 
 tions of life, that to deal with the subject effectively would 
 take us far beyond the limits of the present work. In 
 actual practice we cannot depend upon the chemical bal- 
 ance and the use of dietary tables to secure the correct 
 proportion of nutrients in our food; but knowing the 
 general composition of our foods, we can by judicious 
 selection vary the amount of nutrients, and thus, with the 
 help of appetite, adapt our diet more or less successfully 
 to the varying conditions of life. To change the proteid 
 we have only to change the quantity of those foods like 
 meat, eggs, and beans, «'hich contain relatively large quan- 
 tities of proteid ; and we similarly vary the quantity of 
 non-proteid by changes in the consumption of foods like 
 bread, potatoes, rice, butter, etc., which contain relatively 
 large amounts of carbohydrates and fats. 
 
 29. The Question of Vegetarianism. — Closely connected 
 with the matters discussed in the present chapter is the 
 question whether the human machine thrives best on a 
 mixed diet or on vegetalile food alone. No one would 
 seriously suggest that man should live on animal food 
 alone ; it is true the Eskimos do this, but with them it 
 is a case of necessity, and no one would recommend to 
 people at large the adoption of their diet. In animal foods 
 
NUTEITION 241 
 
 the non-proteid nutrients are represented chiefly by fat, 
 and we have every reason to believe that an undue amount 
 of fat in the diet severely taxes the digestive powers ; ani- 
 mal foods also contain too little indigestible residue to 
 give the proper stimulus to the movements of the alimen- 
 tary canal. 
 
 Vegetable foods, on the other hand, generally contain 
 enough indigestible cellulose to meet the latter require- 
 ment, and most of them also contain all three nutrients. 
 With few exceptions, however, the carbohydrates are 
 present in marked excess. In order to obtain from such 
 foods the proper quantity of proteid, it is generally neces- 
 sary to eat too much of carljohydrate. Some vegetable 
 foods, notably peas and beans, are not open to this objec- 
 tion since, although they have a very high percentage of 
 carbohydrate, they have also a relatively higher percentage 
 of proteid ; and it is possible b}' judicious selection and 
 mixture of foods to obtain a nutritious diet from vegetable 
 foods alone. 
 
 This is mucli easier to do if milk and eggs are added to 
 the diet, and many people who call tliemselves vegetarians 
 do, as a matter of fact, make extensive use of these animal 
 foods. With them vegetaiianism means simply the use 
 of a diet from which meat is excluded. Such persons are 
 not really inconsistent, as is often charged, for the question 
 of vegetarianism is practicallj' the question wliether man 
 should or should not eat meat. 
 
 Against the use of meat two different lines of a]-gument 
 are used. The first is tliat man has no right to take animal 
 life when lie can obtain his food without doing so. With 
 this argument we have here no concern, since we are dealing 
 solely with the pliysiological and hygienic aspects of the 
 subject. People who have conscientious scruples against 
 the use of meat may fulfill every requirement of nutri- 
 tion without going contrary to tlie dictates of conscience. 
 
242 THE HUMAN MECHANISM 
 
 The second argument is that animal food is not a healthy 
 or wholesome food, and to this we must give some atten- 
 tion. It is claimed that while the animal proteids and fats 
 may be unobjectionable, meat contains other substances 
 which exert an unfavorable action on the body. Although 
 it is true that meat contains small quantities (about one 
 per cent) of organic substances other than proteids, carbo- 
 hydrates, and fats, it has not been proved that any of them 
 are harmful to a healthy man or woman. Nor is it an answer 
 to this argument to urge that in gout and rheumatism meat 
 is often limited or even prohibited by the physician. No 
 one — not even a vegetarian — would suggest that starchy 
 foods should be given up because persons suffering from 
 diabetes must use them sparingly. In other words, it has 
 not been shown that the eating of meat is the cause of 
 rheumatism, nor even that it is one of the causes of the 
 disease ; on the contrary, it is more in accord with our 
 knowledge of the subject to regard the disease as originat- 
 ing in other causes ; and, only when it has once obtained 
 a foothold in the body, is it rendered worse, or its treat- 
 ment more difficult, by the use of meat. It does not seem, 
 therefore, that any valid hygienic reason has yet been given 
 for excluding all meat from the human dietary. 
 
 Nevertheless, as a standing jjrotest against some care- 
 less habits of feeding, vegetarianism has a certain value. 
 Too frequently the quantity of meat eaten during a meal 
 is out of all proper proportion to the bread and vegetables. 
 This means that the proteid of the diet is in excess, that 
 the food is too concentrated, and that there is an insuffi- 
 cient quantity of indigestible waste to exert a proper laxa- 
 tive effect on the movements of the intestine. Take, for 
 example, some of our "course" dinners, — oysters, soup, 
 fish, roast, game, dessert (perhaps mince pie), crackers and 
 cheese, coffee. Here nearly every course consists of some 
 form of proteid, wliile only a few contain, in addition, a 
 
NUTEITION 243 
 
 small quantity of vegetables. A very common method 
 of ordering meals at a restaurant is to order a heavy por- 
 tion of meat, — a sirloin steak or mutton chops, — and to 
 make the meal largely from this proteid food, while the 
 vegetables play only a minor role. It is against such things 
 as these that vegetarianism properly protests ; but there 
 seems to be no good reason why all should abstain from 
 meat because some people eat it to excess. In general, 
 poorly cooked vegetables are less digestible than poorly 
 cooked meat. Hence to attempt to live on a purely vege- 
 table diet, under the actual conditions of modern life, is 
 to run serious risk of impairing the digestion. We should 
 welcome all attempts to improve the preparation of vege- 
 tables and all other foods by better cooker}^, and vegetariiins 
 have done much to help forward this reform. But so long 
 as the actually available food is cooked as it is, it is wiser 
 to continue the mixed diet of meat and vegetal)le foods 
 which man lias freel}' chi:>sen in the past, and against whose 
 reasonable use no valid objections have yet been urged. 
 
CHAPTER XIV 
 SENSE ORGANS AND SENSATIONS 
 
 1. The Human Mechanism a Conscious Mechanism. — 
 
 Thus far we have repeatedly compared the human mechan- 
 ism with lifeless mechanisms, and the points of similai'ity 
 are most interesting and instructive. In the supplj* of 
 power, the elimination of wastes, the interdependence and 
 cooperation of parts, the adjustment to the changing con- 
 ditions of work, and in many other respects the resem- 
 blance holds good. But in one respect there is no likeness 
 whatever. When a human mechanism is not in good work- 
 ing order or is tired, it may be aware of the fact ; when 
 an engine is damaged in any way, the engine does not 
 know it. Events taking place in the living animal body 
 arouse in it, and in it only, conscious sensations. 
 
 Sensations are always called forth by the condition of 
 some organ or by the condition of the body as a whole. 
 When several hours have passed since the taking of food 
 we feel hungry ; or of drink, we feel tliirstj^ ; when any- 
 thino- touches the skin a sensation of touch is aroused ; if 
 it presses very hard, that part of the skin feels painful ; if 
 the tongue is acted upon by sugar or salt, we get a sensa- 
 tion of taste ; if light enters the eye, it produces conditions 
 in that organ which arouse in us sensations of color. In 
 all these cases the conscious sensation is due to the condition 
 of some part of the hodj/. 
 
 2. The Reference of Sensations. — Sometimes we refer 
 the sensation to the part of the body which is first affected, 
 or to the body as a whole, and sometimes we refer it to 
 
 244 
 
SENSE ORGANS 245 
 
 external objects. Thus, if in driving a nail the hammer 
 misses the nail and hits a finger, we refer the pain to the 
 finger and not to the hammer ; and we similarly refer sen- 
 sations of hunger and thirst to the body and not to exter- 
 nal objects. If, on the other hand, the skin is cooled by a 
 piece of ice, we do not say that the skin is cold, but that 
 the ice is cold ; we refer the sensation to the external object 
 which causes it, not to the skin in which it actually origi- 
 nates. In the case of the sense of sight, this reference of 
 the sensation to the external object which sends light into 
 the eye is so complete that unless we stop and reflect upon 
 it, we do not realize that it is the condition of the eye of 
 which we are conscious, rather than the condition of the 
 external object at which we are kioking. 
 
 3. Sense Organs. — A few sensations, like pain, are 
 aroused by the condition of most, if not all, parts of the 
 body ; there is no one organ set apart to produce them. 
 Some, like hunger, although at times more or less general 
 in origin, are commonly aroused by the condition of some 
 one organ ^ which ordinarily performs otlier functions. 
 Other sensations arise in organs set apart for the purpose 
 and constructed to react to only one kind of stimulus {spe- 
 cial sense orf/aiis, or organs of special sensation). To this 
 latter class belong the eye, the ear, the olfactory mucous 
 membrane of the nose, the touch organs in the skin, etc. 
 We therefore speak of general sensations and special senses, 
 although no sharp line of division can be drawn between 
 the two. 
 
 4. The Brain the Seat of Sensation. — In all cases how- 
 ever the sensation, although originating elsewhere, is de- 
 veloped in the brain and not in the sense organ. If the 
 optic nerve be cut, bliirdness ensues, although light falling 
 on the retina produces the same effect in the eye itself as 
 when the nerve is intact; it even starts nervous impulses 
 
 1 In the case of huncrer, the stomach. 
 
246 THE HUMAN MECHANISM 
 
 toward the brain ; but, since these impulses go no farther 
 tliau the cut, they excite no sensation of light. And the 
 same thing is true of other sensations. Conversely, after the 
 amputation of a limb it often happens that sensations are 
 felt, as if they came from the lost member. In this case 
 the stump of the cut nerve is stimulated in some way, and 
 the impulses thus sent to the brain excite the same sensa- 
 tions as if they came from the usual endings of the nerve. 
 When one hits his "funny" or "crazy bone" (i.e. directly 
 stimulates the ulnar nerve) the sensations developed in the 
 brain may be referred to the fingers in which the nerve 
 originates. 
 
 In the development of every sensation, therefore, we 
 have to distinguish between (a) what takes place in the 
 sense organ or end organ, (b) the passage of a nervous 
 impulse from this organ to the central nervous S3'stem, 
 and (c) the events which the arrival of the nervous impulse 
 excites in the brain. It is only the last (c) that, strictly 
 speaking, we can call sensation. The sense organs and their 
 afferent fibers are merely tributary mechanisms which serve 
 to excite the sensations in the brain. We are not aware 
 that it is the brain which is thus active, for we refer the 
 sensation either to the organ or to some external object. 
 
 5. The Sense of Sight. The Eye. — Sight is one of the 
 most highly specialized of the senses. The eye is the only 
 organ in which originate sensations of light or color, and 
 it is a wonderfully constructed apparatus, the function 
 of which is to stimulate tlie optic nerve by rays of light. 
 It is essentially a living camera in which, by means of a 
 lens, an image of things around us is formed upon the 
 retina ; just as in the photographer's camera the lens forms 
 an image on the ground glass or on the sensitive plate 
 or film. 
 
 6. Structure of the Eye. — The eyeball consists of three 
 concentric coats surrounding and inclosing transparent 
 
SENSE OEGANS 247 
 
 substances through which rays of light pass to the retina. 
 The outer or sclerotic coat (the white of the eye) is com- 
 posed of very tough, dense connective tissue, and forms 
 the protecting covering of the eye. Over a small area in 
 front this coat is transparent, and this part of it is known 
 as the cornea. Inside the sclerotic is the middle coat or 
 choroid, richly supplied with blood vessels and contain- 
 ing in its connective tissue large quantities of black pig- 
 ment, which prevents the j^assage of light into the eyeball 
 except through the cornea. The choroid lines the scle- 
 rotic everywhere except in front, where in the region of 
 the cornea it leaves the sclerotic and projects toward the 
 long axis of tlie eye as a kind of curtain, the iris, — that 
 part of the eye which is Ijlack or gray or blue. The j'fipH 
 is the dark round opening, or hole, in the iris. Immedi- 
 ately inside the choroid is the third and innermost coat, 
 the retina. This is a thin menilirane, not miire than one 
 eightieth of an inch in thickness, and lining the chamber 
 of the vitreous humor as far forward as the ciliary region 
 (Fig. 8G). The retina is the part of tlie eye sensiti^-e to 
 the stimulation of light. Here also betrin the fibers of the 
 optic nerve, which passes through and perforates the choroid 
 and sclerotic coats behind on its way from the retina to 
 the brain. These and other parts of the eye may be easily 
 seen by dissecting the eye of an ox or sheep. 
 
 7. The Lens and the Muscle of Accommodation. — Imme- 
 diately behind the pupil is the lens, a biconvex, transparent, 
 compressible, and elastic body fastened by a circular liga- 
 mentous sheet to the choroid, coat immediately above and 
 behind the iris. The lens and its suspensory ligamentous 
 sheet thus divide the eye into two distinct chambers : the 
 one, in front of the lens and behind the cornea, filled with 
 a watery fluid, the aqueous hnmor; the other, behind the 
 lens and surrounded by the retina, filled with a jellylike, 
 transparent substance, the vitreous humor (Figs. 86, 89). 
 
248 
 
 THE HUMAN MECHANISM 
 
 The elastic choroid, coat is not long enough to reach 
 around and inclose the vitreous humor without stretching, 
 and hence it constantly exerts a steady, elastic pull or ten- 
 sion on the ligament of the lens. This tension flattens the 
 
 compressible lens (i.e. makes 
 it less convex), and the lens 
 is always in this flattened 
 condition in the resting eye ; 
 for example, when one is 
 asleep. The same condition 
 should obtain, as we shall 
 learn, whenever we are look- 
 ing at distant objects. 
 
 The pull of the tense cho- 
 roid on the lens is, however, 
 overcome at times by the ac- 
 tion of the sheetlike ciliary 
 muscle. The fibers of this 
 peculiar muscle originate in 
 the sclerotic coat around and 
 just outside the cornea, and 
 diverge radially outward 
 and backward to end in the 
 choroid beyond the attachment of the suspensory ligament 
 of the lens. Figure 87 shows how the contraction of this 
 muscle, fixed as it is near the cornea, must draw the cho- 
 roid forward and so ease the pull of the latter on the liga- 
 ment of the lens. When this happens, tlie lens, owing to 
 its own elasticity, assumes its independent (more convex) 
 shape. 
 
 The curvature of the lens is thus variable, and is deter- 
 mined by the action of this muscle of accommodation. 
 When the ciliary muscle is at rest, the lens is kept flattened 
 by the pull of the choroid on the ligament; when the 
 muscle contracts, this pull is eased off (or slacked) and 
 
 Fig. 86. Vertical section through the 
 ciliary region of the eye 
 
 Showing the structures concerned in 
 accommodation (see Section 7) . This 
 should l)e compared with the per- 
 spective view into the hemisphere 
 of the eyehall, shown in Fig. \il 
 
"SENSE OEGANS 
 
 249 
 
 the lens becomes more convex. The entire operation is 
 known as aceommodation, and we may now inquire wliat 
 jjart accommodation plays in vision. 
 
 8. The Formation of an Image by a Lens. — The eye is 
 a camera, in that it forms on the retina an image of objects 
 in front of the cornea ; and it is 
 the first essential of clear vision, 
 just as it is the first essential of 
 photography, that this image be 
 sharp, or at least distinct. A sim- 
 ple experiment will show that clear 
 vision of near and of distant objects 
 cannot be had Ijy the eye at the 
 same time. Hold up a j>encil or a 
 pen about ten inches from the eye 
 and look first at it and then at 
 some object far away. Both can be 
 seen, but only one at a time clearly : 
 and often an effort is required to 
 shift from the far to the near ohject. 
 
 The change which occurs in the 
 eye in the act of accommodation is 
 illustrated in the following experi- 
 ment. A wooden or j^asteboard box 
 (api^roximately 8 by 5 by 4 inches) is fitted with a piece 
 of ground glass on one side and provided with a convex 
 lens on the opposite side. This is a rude camera, and 
 some object is now placed at such a distance that the lens 
 forms an image of it on the ground glass, which is now in 
 focus for the object. If, later, the object be moved nearer 
 to the lens, the focus is changed; the image on the glass 
 becomes blurred, and in order to make it distinct it will 
 be found necessary to use a more convex lens. 
 
 Essentially the same change occurs in the eye in accommo- 
 dating for near objects : the lens must he made more convex ; 
 
 Fic. 87. Diagram of the 
 meclianisiii of accommo- 
 dation 
 
 The ciliary muscle is repre- 
 sented as tliree fibers pass- 
 ing oltli<iuely from the 
 sclerotic to the choroid 
 
250 THE HUMAN MECHANISM 
 
 and this, it will be remembered, involves work on the 
 part of the muscle of accommodation (see p. 248). We 
 can thus understand why, in gen-eral, it is too much of 
 " near work," and especially near work necessitating very 
 distinct vision, that tires the eye. The ideal condition of 
 the eye, regarded merely as a camera, is that in which dis- 
 tant objects are focused on the retina when the muscle of 
 accommodation is completely relaxed and the lens is thus 
 flattened to its utmost by the elastic pull of the choroid coat 
 
 -B'^'-;: 
 
 
 Fig. 88. Action of a convex lens in bringing to a focus tlie rays of light 
 
 diverging from a single point 
 
 The rays from A are focused at a ; those from B, at b 
 
 (p. 248) ; for in this case the eye is rested by looking at dis- 
 tant objects, and works only when looking at near objects. 
 Such an eye is known as an emmetropic eye (Fig. 90, E). 
 
 Unfortunately, not all eyes meet this requirement. The 
 eyeball may be either too short or too long ; so that, with 
 the muscle of accommodation relaxed, the position of the 
 perfect focus for distant objects is either before or l)ehind 
 the retina ; the eye no longer sees distant objects distinctly 
 when it is at rest, because then the retinal image is blurred. 
 To understand more fully the undesirable consequences 
 of this condition, we must learn how convex lenses produce 
 Images of objects. 
 
 9. The Action of a Convex Lens on Rays of Light. — The 
 rays of light diverging from a shiyle f>oint and entering a 
 
SENSE OEGA_NS 
 
 251 
 
 convex lens are bent so that all come too-ether aeain in a 
 point behind the lens, or, as it is said, are brought to 
 a. focus. This is shown in Fig. 88, as is also the fact that 
 rays of light diverging from more distant points come to a 
 focus behind the lens sooner than those diverging from 
 nearer points. 
 
 Now a lens forms an image of an object because all the 
 rays of light from each point of the object are focused in 
 
 Fig. 89. Diagram showing (lie foniiatinn rif an iiiiage on the retina 
 
 1, 2, the oljJL^ct ; V, LI', tlie image of tlie sanu- ; c, cnrnea : i, iris; ^, lens; 
 ■/', vitreous Inunor ; ir, sclerotic; ch , choi-oicl ; on, optic nerve 
 
 corresponding points behind the lens. This is shown in 
 Fig. 89, where all the rays diverging from 1 are focused at 
 1', all those from 2 at 2', and those from intermediate 
 points of the object at intermediate points of tiie image. 
 
 If the raj-s from each point meet in front of the retina 
 and then diverge before reaching the retina, the retinal 
 image is blurred ; and the image is also blurred if the 
 retina is so near the lens that the rays from each point 
 have not yet come to a focus. The more convex the lens 
 the more will the rays of light be bent ; consequent!}' we use 
 the muscle of accommodation (which makes the lens more 
 convex) to get clear images of near objects (see Fig. 88). 
 
252 THE HUI»1AN MECHANISM 
 
 10. Myopia, Hypermetropia, and Presbyopia. — In the 
 
 emmetropic eye (Fig. 90, E) the distance between the 
 retina and the lens is such that light from distant points 
 
 comes to a focus on 
 ^/ \ the retina without 
 
 any active muscular 
 
 accommodation; to 
 
 see near objects the 
 lens is made more 
 convex. 
 
 When the retina 
 
 is so far away from 
 
 the lens that, with 
 
 the muscle of ac- 
 commodation com- 
 pletely relaxed and 
 
 therefore the lens 
 
 flattened to its ut- 
 most, light from dis- 
 tant points comes to 
 
 Fig. 90. Course of the rays of light from a a focus in front of 
 distant point the retina, the reti- 
 
 Through the emmetropic (E), the myopic (i¥), and ^.^^ imaceis blurred 
 the hypermetropic (/J) eye, the muscle of accom- . ^ , . ' 
 
 modation heing relaxed. (The rays diverging and it is impossible 
 
 from a distant point would enter the eye practi- £qj, such an eVC to 
 cally parallel) *'. 
 
 see distant objects 
 clearly. To correct such vision it would be necessary to 
 make the lens still less convex, and this the eye is unable 
 to do. (Why?) Such an eye is known as myopic, or near- 
 siffhted, and its defect must be corrected by the use of co7i- 
 cave glasses, which act as if the lens were made flatter, and 
 so throw the focus farther back upon the retina. A myopic 
 eye generally has clear sight for very near objects because, 
 as stated above, the nearer the object the farther back is 
 the image formed. 
 
SENSE OEGANS 253 
 
 On the other hand, the eyeball may be too short, fore 
 and aft (Fig. 90, //), so that, when the ciliary muscle is 
 relaxed, light from distant points has not yet been brought 
 to a focus when it reaches the retina (liypermetropia). 
 Such an eye must accommodate not only for near but also 
 for distant objects, and its muscle of accommodation can 
 never rest so long as the eye is being used. Moreover, 
 to see near objects the ciliary muscle must work much 
 harder than in the normal eye, and it often happens that, 
 even with its utmost effort, the rays are not sufficiently 
 bent to focus them on the retina ; so that a book, for ex- 
 ample, must be held at arm's length to be read. Persons 
 having such eyes form one class of those said to Ijc " far- 
 sighted," and their trouble can be corrected by the use of 
 convex glasses. 
 
 As old age approaches, changes occur in the lens, in 
 consequence of which it no longer becomes as convex as 
 formerly in response to the action of the muscle of accom- 
 modation (preshi/opia, from irpea/Su^, old). Some, though 
 not all, I'esults of this condition resemble tiiose of liyper- 
 metropia ; but the two differ in cause. Hypermetropia is 
 due to sho)-tncss of eijcball; presbyopia, to failure of accom- 
 modation. 
 
 11. Astigmatism. — We liave thus far been dealing with 
 those optical imperfections due to imjiroper distance be- 
 tween the lens and tlie retina. Another and frecjuently 
 more serious trouble, known as astigmatism, results when 
 the curvature of the cornea (and sometimes of the lens) is 
 not perfectly regular ; i.e. when these surfaces are not seg- 
 ments of perfect spheres, but resemble in curvature the 
 side of a lemon. In this case the rays of light from a point 
 are not brought to a focus again in a point behind the 
 lens; and remembering the importance of sharp focusing in 
 securing distinct retinal images, the student will see that 
 this defect must seriously interfere with clear vision. The 
 
254 
 
 THE HUMAN MECHANISM 
 
 optics of astigmatism are too complicated to be explained 
 ill an elementary work, but the defect reveals itself gener- 
 ally in an inability to see with equal clearness lines run- 
 ning in different 
 directions. Thus 
 some of the lines 
 in Fig. 91 will be 
 sharply defined and 
 black while one is 
 looking with one 
 eye at the white 
 center, and others 
 will be blurred and 
 lighter in color. 
 
 Astigmatism is 
 of special impor- 
 tance in reading, 
 because the lines of 
 printed letters run 
 in different direc- 
 tions. The effort to see clearl}^ the printed page is often 
 severe, and results in headaches and other general dis- 
 turbances of health, the true cause of which is often un- 
 suspected. The trouble may usually be corrected by the 
 use of so-called " cylindrical " glasses, i.e. glasses which 
 compensate the defects of curvature in lens and cornea. 
 
 12. Accommodation and "Near Work." — The above- 
 described defects of the eye as an optical instrument may 
 usually be successfully corrected by the use of proper 
 glasses, which sliould, generally speaking, be prescribed 
 by a good oculist and not by an optician. Glasses may be 
 used for various reasons, — as a matter of convenience, as 
 where a person with slight myopia wears them merely to 
 see distant objects clearly ; or of necessity, as when the 
 myopia is more pronounced ; or they may serve the much 
 
 Fig. 91. A test for astigmatism 
 
SEXSE ORGANS 255 
 
 more important purpose of relieving the muscle of accom- 
 modation of undue work in reading or sewing, and thus 
 of avoiding " eye strain." A hypermetropic eye should 
 always be provided with glasses, since otherwise its muscle 
 of accommodation cannot be rested by looking at distant ob- 
 jects. But since it is near work which requires the greatest 
 effort of accommodation, it is in reading, writing, drawing, 
 sewing, etc., that the eye strain is apt to be greatest. As 
 this kind of work is constantly increasing in modern life, the 
 need for the complete correction of such defects Ijecomes 
 more and mpre necessary. Those whose occupations require 
 long-continued use of the eyes should see to it that these 
 precious organs are used only under the most favorable 
 conditions, and that all strain is as far as possiljle relieved. 
 
 13. The Importance of Sufficient Illumination, etc. — 
 When we try to read in a poorly lighted room we natu- 
 rally hold tlie printed page closer to the eye in order to get 
 from its bright parts the greatest possible amount of light, 
 and also because the images thus formed are larger. But 
 this clearly involves more severe work on the part of the 
 muscle of accommodati(_)n in order to give the lens its 
 unusual convexity. Sucli work frequently results in injury 
 to the parts concerned, and sliould be avoided as fur as 
 possible. ]\Iany an eye has been injured, especiall}^ in early 
 life, by reading too late in tlie waning liglit of evening 
 before ligliting a lamp. 
 
 It is also of the highest importance tliat printed matter 
 be held steady, as the effort of accommodation is othei'wise 
 rendered more difficult. Consequently it is in general a 
 bad tiring to read, and especially to read fine or poorly 
 printed matter, on any but the steadiest railroad train. 
 For the same reason, for all kinds of near work an un- 
 steady or flickering light is very objectionable. 
 
 14. Visual Sensations. — We have shown (p. 245) that 
 the sensation of sight does not develop in the eye, but in 
 
256 THE HUMAN MECHANISM 
 
 the brain, as the result of nervous impulses sent thither 
 over the fibers of the optic nerve from the retina. Just 
 how the light falling upon the retina originates these 
 impulses cannot be discussed here ; suffice it to say that 
 the character of the impulse differs according to the color 
 of the light ^ stimulating the retina ; the lens focuses upon 
 the retina a flat, colored picture of the objects at which it 
 is looking, just as a photographic camera does, or as the 
 painter represents a scene on canvas. One part of the ret- 
 ina is thus stimulated by light of one color, and another 
 part by light of another color, or by another shade of the 
 same color ; and the different kinds of impulses started in 
 the fibers of the optic nerve ultimately, upon their arrival 
 in the brain, excite in consciousness what we know as 
 vismal sensations. The sensations which we get from the 
 retina are therefore primarily sensations of color. 
 
 15. Visual Judgments. — But when we look at an object 
 we get more than mere sensations of color. The world 
 does not appear to us as a flat surface, of different colors, 
 like the painter's canvas. When we look at the wall of a 
 room we know that it is a flat surface, and when we look 
 at a box we know that it has not only length and breadth 
 but also thickness. If we were dependent entirely upon 
 the retinal image for our idea of the box, it would look 
 as flat as the wall ; that it does not appear so is because 
 we receive other information about the box than that 
 which comes from the retina. We have to accommodate 
 the lens differently for the near and the far edges, and we 
 have learned by experience that this necessity indicates 
 depth, or different distances of different parts of the object. 
 Again, we see the box with both eyes, and the images 
 formed on the two retinas are not exactly the same. One 
 eye sees more of one side, the other eye more of another 
 
 1 In this and the following paragraphs white, black, and gray are 
 regarded as colors. 
 
SENSE OKGAjSTS 257 
 
 side ; and while we are not conscious of this fact, we have 
 really learned by experience and by the actual handling of 
 objects that these slightly different sensations from the 
 two eyes are produced only by solid objects. Again, when 
 we look at any point on the near edge of a box the two 
 eyes are converged by their muscles to a greater extent 
 than when we look at a point on the far edge, and we 
 have learned that these different pulls of muscles and posi- 
 tions of eyeballs indicate that the object is not flat, but lias 
 depth. The importance of binocular vision in the estima- 
 tion of depth or distance from the eye is most strikingly 
 illustrated by attempting, with one eye closed, to bring 
 together the points of two pencils held in the hands and 
 moved from side to side at arm's length. 
 
 Consequently when we look at anything we get a num- 
 ber of sensations ; from the retina, those of color and the 
 position of the color spots ■\\ith reference to one another; 
 from the muscular efforts of accommodation and cif con- 
 vergence of the eyeballs, those which reveal the property 
 of depth in what we see. And from all of these, fused 
 together and interpreted in the light of experience, we 
 construct a visual judgment of tiie natuie of the object. 
 
 16. Optical Illusions. — That our vision is essentially the 
 result of unconscious judgments is strikinglj- shown by 
 the fact that these sometimes deceive us. Thus the par- 
 allel vertical lines in Fig. 92, when crossed by the oblique 
 lines, seem to be inclined toward each other. The retinal 
 images of the lines are parallel, and we falsely judge them 
 inclined, this error of judgment arising from the presence 
 of the oblique lines. In other words, our final idea of the 
 lines does not correspond to their image on the retina. 
 
 Many other examples might he given showing that our 
 visual idea of the world around us is not a simple sensa- 
 tion or impression, but an unconscious inference, judg- 
 ment, or conclusion built up from a number of simple 
 
1^58 
 
 THE HUMAN MECHANISM 
 
 sensations taken separately or blended together, and com- 
 pounded with results of lifelong experience. In looking 
 at a piece of fine silk or cloth we seldom stop to think 
 that its tissue may he resolved into many simple component 
 threads ; and in quite the same way we fail to realize that 
 
 even our quickly formed 
 
 5=' 
 
 i 
 
 ^ 
 
 r 
 
 j»^ 
 
 \ 
 
 ;; 
 
 ¥ir.. 92 
 
 judgments of the size, dis- 
 tance, form, or color of ob- 
 jects are likewise tissues 
 woven out of many threads, 
 most of which we have been 
 slowly and laboriously spin- 
 ning since childhood's days 
 in the hidden factory of in- 
 dividual experience. 
 
 17. Sound and Hearing 
 
 When the string of a violin, 
 piano, or harp "sounds," one 
 can observe that it is in 
 rapid vibration ; and the same thing is true of all sounding 
 bodies. These vibrations are imparted to the air, water, or 
 other surrounding medium, and through this medium they 
 are transmitted as waves of sound. It is these waves or 
 vibrations which, on entering the ear, excite the sensa- 
 tion of sound. The more rapid the vibrations the higher 
 is the pitch of the note, and the greater tlieir amplitude 
 the louder the sound. 
 
 The ear is an organ specially adapted to receive these 
 vibrations of air and to transform them into nervous im- 
 pulses. It is subdivided by anatomists into the outer ear, 
 tlie middle ear, and the inner ear. 
 
 18. The Outer Ear. — The outer ear consists of the 
 expanded pinna (or that part which we commonly call 
 " the ear ") and a tube along which the vibrations of 
 sound pass inward to the tyhipaidc membrane, or drum. 
 
.SENSE OEGANS 
 
 259 
 
 Glands along this canal secrete wax which guards the 
 approach to the drum. It is a had hahit to pick at this 
 wax, and especially to dig into the ear with any pointed 
 
 Fir,. 93. Diagram of the e.ar 
 
 A, the auditory oanal, Icailiui,^ to the tyiiipaiiir inoinlirane II : C, r^\\\y of the 
 tympanani, onittmmticatiiii^ b}' Iho Kiistachian tiihe with tlie pharyii.x I) \ 
 E, semicirruhir canals; F, cochlea; <r, auditory nerve 
 
 instrument, for there is always danger of perforating the 
 drum. If trouble is susjiected, a pliysician should be 
 consulted. 
 
 19. The Middle Ear. The Eustachian Tube.— The tym- 
 panic membrane separates the outer from the middle ear, 
 or ti/mpanum, a small cavity lying in the temporal bone of 
 the skull and communicating witli the throat or jiharynx by 
 means of the Eustachian tube. The air which it contains 
 is consequently under the same pressure as that of the at- 
 mosphere without, and the tympanic membrane is not nor- 
 mally bulged inward or outward In- inec[uality of pressure 
 
260 
 
 THE HUMAN MECHANISM 
 
 on its two sides. The opening of the Eustachian tube into 
 the pharynx is, however, closed except when one swallows, 
 and hence swallowing often relieves the drum from undue 
 pressure of air in the middle ear. 
 
 The cavity of the tympanum also communicates with a 
 network of spaces, or sinuses, in the temporal bone. Because 
 
 of these connections of the 
 middle ear with the throat, on 
 the one hand, and with the 
 temporal sinuses on the other, 
 inflammatory processes in the 
 nose and throat during a cold 
 sometimes extend into the 
 Eustachian tube, the tympa- 
 num, and even into the tem- 
 poral sinuses, causing serious 
 trouble, and occasionally 
 deafness. 
 
 Passing directly across the 
 tympanum, from the drum on 
 its outer side to the cochlea 
 on its inner side, is a chain 
 of three very small bones, the 
 ear ossicles (hammer, anvil, and stirrup). These bones are 
 bound together and attached to the walls of the tympanum 
 by ligaments, and are so arranged that when sound waves 
 set the tympanic membrane in vibration this motion is 
 transmitted by the ossicles to a portion of the inner ear 
 known as the cocJilea. 
 
 20. The Inner Ear. — The structures of the inner ear lie 
 in the temporal bone, on the side of the tympanum opposite 
 the drum. They consist of a system of small bony spaces 
 and tubes, the bony lahyrinth, within which lies a cor- 
 responding membranous labyrinth. Forming part of the 
 lining of the membranous labyrinth are very sensitive 
 
 B 
 
 Fig. 04. The bony labyrinth, its 
 actual size being shown in the 
 smaller figure 
 
 C, B, the semicircular canals : 
 A, the oval window, by means ot 
 which the vibrations of the stirrup 
 bone are transmitted to the cocli- 
 lea; E, F, G, the whorls of the 
 cochlea. Compare Fig. 95 
 
SENSE OEGANS 
 
 261 
 
 Vestibule with Openings 
 ofSemicircularCanals 
 
 \\ 1 l<r 
 
 Incus, 
 Malleus 
 Stapj 
 
 Cochlea 
 
 cells, and between these cells are the endings of the nerve 
 fibers which connect the ear with the brain. The cells of 
 the inner ear are sensitive to the vibrations which have 
 been transmitted 
 across the tympa- 
 num by the os- 
 sicles, just as the 
 retina is sensitive 
 to light ; and as 
 the retina is the 
 
 origin of the fibers Membrana 
 
 of the optic nerve, 
 so the inner ear 
 
 is the origin of ^^'^- f'5- Diagrammatic representation of the mem- 
 ,, n 1 ° ■,. branous labyrinth of the cochlea in relation to 
 
 tnose Ot the audi- ^^^ structures shown in Figs. 93 and 94 
 tOiy nei ve. jj^g ^j.^;,, ■oestibuU. and scala tympuid are the two por- 
 
 21, Taste and tlous of the bony cochlea which inclose the meni- 
 o 11 T^i 1 branous cochlea 
 
 Smell. — ilie end 
 
 organs of taste are small rounded eminences, or papilla-, 
 on the dorsal surface of the tongue, and from these the 
 fibers of the nerves of taste pass to the brain. The end 
 organs of the nerve of smell are situated in the upper por- 
 tion of the nasal cavity and consist of delicate cells very 
 sensitive to the presence of odnrs. Sensations of taste are 
 frequently confounded with those of smell. An onion, for 
 example, has little or no taste, as can be shown by placing 
 a bit on the tongue when one is holding the breath ; none 
 of the flavor of the onion is perceived. On the other 
 hand, sour, sweet, hitter, and salt are true taste sensations. 
 The unconscious blending of tastes with odors in forming 
 our ideas of the nature of objects recalls the formation 
 of visual judgments by the combination of retinal with 
 other sensations (p. 256). 
 
 22. Cutaneous Sensations. — The skin is the place of 
 origin of at least three sensations, — touch, cold, and 
 
262 
 
 THE HUMA.N MECHANISM 
 
 warmth. These sensations are distinct, as is shown by 
 the observation that on certain points of the skin some 
 of them may be felt, but not others. This fact is usually 
 interpreted to mean that each sensation has its own set 
 of end organs and nerve fibers. Especially striking is the 
 fact that warmth and cold are never felt by the same spot 
 of skin, which seems to prove conclu- 
 sively that they are separate sensations. 
 The afferent nerves of cold and 
 warmth not only carry into the brain 
 those impulses wliicli give rise to the 
 corresponding sensations but also serve 
 as one important means of stimulating 
 the reflexes which heljj to regulate heat 
 production and heat output (see Chap- 
 ter XII). 
 
 23. The Sense of Position. — The ex- 
 pression " tlie five senses " has become 
 proverbial, and comes from the time 
 when sight, hearing, taste, smell, and 
 touch were the recognized special senses. 
 To-day, however, we must add to these not only warmth 
 and cold but still others, most conspicuous among which 
 is the sense of position. When the eyes are closed we are 
 aware of the position of the various parts of the body. We 
 know whether the arm is bent at the elbow or straight ; 
 whether the head is looking forward or is turned to one 
 side or the other. And while we are aware of these things, 
 partly from tactile sensations, there is conclusive evidence 
 that afferent impulses from the muscles, tendons, and 
 joints also play an important part in the result. 
 
 When one is blindfolded and lies flat on a revolving 
 table which can be turned noiselessly in one direction or 
 the other, the subject of experiment can form fairly correct 
 judgments as to tlie angle and direction through which 
 
 Fig. 90. A tactile cor- 
 puscle in one of the 
 papillae of the der- 
 mis ; an end organ 
 of the sense of 
 touch 
 
SENSE OEGANS 263 
 
 the table is turned. Here there is no change of character 
 either in the tactile impulses or in those from the muscles, 
 tendons, and joints, for the subject of experiment lies still 
 and is only passively moved. It is believed that in this 
 case the sensations in question come from the movements 
 of the lymph in portions of the inner ear. One part of 
 this, the cochlea, is undoubtedly concerned with the per- 
 cejjtion of sound; but another part, the three semicircular 
 canals, are now believed to be end organs of this sense of 
 position. 
 
 The impulses which make us aware of tlie position of 
 parts of our bodies also play a very important lole in " 
 reflexly guiding our movements. Upon this we shall dwell 
 at greater length in subsequent chapters (see especially 
 Chapter XV). 
 
 24. Sensations of Pain. — I\Iost organs of the body may 
 also give rise to impulses which, on their arrival in the 
 brain, cause sensations of pain. It is still, perhaps, an 
 open question whether this sensation, like sight, smell, and 
 hearing, is aroused by its own mechanism of end organs 
 and afferent nerves, or whether it is called forth by the 
 excessive stimulation of the nerves of the other senses; 
 but for the discussion of tliis question the reader must 
 consult more advanced works on physiology. 
 
 Pain is a useful danger signal, since it effectively calls 
 attention to abnormal conditions and incites us to the 
 adoption of active remedial measures. Remedies, how- 
 ever, should not be confined to the abolition of unpleasant 
 sensations, but should be directed to the removal of their 
 cause. A toothache from a decaying tooth may often be 
 stopped, for a time at least, by the use of chloroform or 
 other anesthetic drugs, but the drug onl}^ stops the pain ; 
 it does not check the progress of decay or repair the 
 damage. Again, a bronchial cough may be unpleasant, 
 and even painful, but we should not rest content with the 
 
264 THE HUMAN MECHANISM 
 
 use of some drug or cough medicine which merely lessens 
 the irritability of the inflamed surface of the air passages, 
 and so, perhaps, stops the cough without curing the disease. 
 
 Pain is a warning that some abnormal condition needs 
 attention. Sometimes that attention may be supplied by the 
 sufferer himself, or by his friends, but often skilled medical 
 advice is needed. Too frequently, for the sake of economy 
 or from feelings of modesty, or even because of an unwilling- 
 ness to acknowledge illness either to the world or to one- 
 self, the mistake is made of postponing the visit to the 
 physician, the patient meanwhile bearing discomfort and 
 perhaps actual suffering in the hope that he will soon be 
 better and that the trouble will " cure itself." Sometimes, 
 of course, it does cure itself ; but sometimes it does not ; 
 and remediable disease has too frequently been allowed to 
 run on in this way until some vital spot is attacked or the 
 trouble has become too grave for medical skill to overcome. 
 Many diseases, like a fire, may be extinguished at the start, 
 but if not attended to, grow rapidly into a conflagration 
 beyond control. Pain is one of the most trustworthy warn- 
 ings that attention to the mechanism itself or to our opera- 
 tion of it is necessary; and we have no right, either for our 
 own sake or that of our friends, to neglect its warnings. 
 While there are times when it is an act of heroism to 
 enduie suffering and to keep the knowledge of it to 
 oneself, there are other times when to do this is not only 
 foolish but wrong. 
 
 25. Hunger and Thirst. — No account of the physiology 
 of sensations would be complete without some reference to 
 those very common experiences of life, — hunger and thirst. 
 We have already spoken of them as sensations which are 
 referred to the body and never to external objects, thirst 
 usually being referred to the mouth and throat, and hunger 
 frequently to the stomach ; but hunger and even thirst 
 may sometimes affect us as sensations coming from the 
 
SENSE ORGANS 265 
 
 body as a whole, in which case they are usually indistin- 
 guishable from certain forms of general fatigue. 
 
 It is not yet fully understood how hunger and thirst are 
 excited, though it is probable that they arise partly from 
 the conditions of the stomach and throat respectively, and 
 partly from conditions of the body as a whole. But they 
 are as definite sensations and as truly adapted to guide us 
 in the choice of food as sight is adapted to picture to us 
 the world in which we live. So long as the body is normally 
 occupied and healthy they may usually be trusted ; but 
 there are abnormal conditions of sedentary life, in the midst 
 of a superabundance of tempting food, when they become 
 less trustworthy ; and in some forms of dyspepsia the sensa- 
 tion of hunger is never absent, no matter how often one 
 eats. In such cases the very effort to satisfy hunger only 
 aggravates disease. Conditions of this sort should not pre- 
 vail if proper attention be paid to the general hygienic 
 conduct of life. Broadly speaking, appetites, like fire and 
 dynamite, are good servants but bad masters. 
 
CHAPTER XV 
 
 THE NERVOUS SYSTEM 
 
 A. Its Anatomical Basis 
 
 In the preceding chapter we have repeatedly emphasized 
 the fact that sensations of all kinds are developed in the 
 brain from nervous impulses coming from the sense organs ; 
 
 and in a previous 
 chapter (VII) we 
 have seen that 
 without reaching 
 the brain, or at 
 least without af- 
 fec ting conscious- 
 ness, these affer- 
 ent impulses may 
 give rise to reflex 
 action. A reflex 
 action or a con- 
 scious sensation, 
 or both a reflex 
 action and a con- 
 scious sensation, 
 may therefore 
 result from the 
 entrance of a ner- 
 vous impulse into 
 the central ner- 
 vous system, and we have now to inquire what is known 
 of the mechanism by which these results are brought about. 
 
 266 
 
 Fin. 97. The human brain viewed from above. 
 The cerebral hemispheres completely cover the 
 rest of the brain 
 
THE NEKVUUS SYSTEil 
 
 267 
 
 For this purpose we must first learn something more of the 
 anatomy of the spinal cord and brain. 
 
 1. Fundamental Structure of the Nervous System. The 
 Brain of a Frog. — The human spinal cord and brain are so 
 
 Forehrain 
 
 ^Tweenbraia 
 Midbrain 
 
 Iliadbraiu 
 
 Spinal 
 Curtl 
 
 Fig. 98. The brain and spinal cord of the frog 
 
 On the left i.? a longitudinal, right to left seetion, showing the central canal and 
 the ventricles of the hrain ; on the right the dorsal ^dew of the hraiu and cord. 
 *4, the cerebral herai.spheres; B, the optic lobes ; C, the cerebellum ; Z>, the 
 bulb ; £, the spinal cord 
 
 complicated that it is best to study first the nervous system 
 of a simple vertebrate like the frog ; for the fundamental 
 
268 
 
 THE HUMAN MECHANISM 
 
 plan of structure is the same in both. The spinal cord is 
 a relatively thick-walled tube, the walls of which are com- 
 posed of white and gray matter, the minute bore, or lumen, 
 of the tube being known as the central canal. The arrange- 
 ment in the brain is similar, but here the central space is 
 no longer a small tube of even bore, but consists for the 
 
 Fig. 99. Diagrammatic median longitudinal section of a mammalian brain. 
 After Edinger 
 
 For convenience the cerebrum with its lateral ventricle is represented as a 
 single organ in the median plane, instead of two hemispheres on either side 
 of this plane and eaph with its own lateral ventricle. The division into fore- 
 hrain, 'tweeubrain, midbrain, and hindbraiu is marked by the In-oken lines 
 
 greater part of irregular cavities known as the ventricles of 
 the brain, while the walls consist of masses of gray and 
 white matter varying in size and shape. 
 
 Figure 98 will assist the student in understanding this 
 plan of structure. Anteriorly the spinal cord is continued 
 in the hulh ^ whose central cavity is the fourth ventricle. 
 Part of the dorsal wall of this ventricle forms the cere- 
 helium, which in the frog is only slightly developed, but 
 which in higher vei'tebrates (birds and mammals) becomes 
 a large and conspicuous organ. Anteriorl)- the fourth 
 ventricle is connected with the third by a tube, the aque- 
 duct of Sylvius. The thick walls of this aqueduct contain 
 
 1 The older term for the bulb is the medulla oblongata, to distinguish 
 it from the medulla spinalis, or spinal cord. 
 
THE NERVOUS SYSTEM 
 
 269 
 
 various masses of gray matter whose names need not 
 detain us ; the walls of the tfiird ventricle are similarly 
 composed of large masses of gray matter scattered among 
 
 Fig. 100. The base of the human brain, showing the cranial nerves 
 
 A, cru.'i cerebri, composed lar,t;"ely of nerve fillers which connect the liindbrain 
 with the 'tweenbrain and foreiirain ; B.pnns Varolii, the anterior floor of 
 the fourth ventricle, connected laterally witli the cerebellum; C, bulb; 
 D, cerebellum ; JS, spinal cord 
 
 the fibers of the white matter. Still farther forward two 
 openings from the third ventricle, one on the right and 
 one on the left side, lead iirto the large lateral ventricles, 
 the nervous tissue of whose walls is the cerehrtmi, or the 
 
270 THE HUMAN MECHANISM 
 
 cerebral hemispheres. It is convenient to divide the brain 
 into the forebrain, surrounding the lateral ventricles ; the 
 'tweenbrain, surrounding the third ventricle ; the midbrain, 
 surrounding the aqueduct of Sylvius ; and the hindbrain, 
 surrounding the fourth ventricle. 
 
 2. The Brain of the Mammal is built on the same funda- 
 mental plan as that of the frog, and differs from it mainly 
 
 Fig. 101. Median longitudinal section of the human brain 
 
 A, r>, C, Tl, L, convolutions of the median surface of the cerehrum ; E, F, the 
 cerebellum, showing in the plane of section the inner "white matter and 
 the outer gray matter; //, th^.pons Varolii; K, the l)ulh 
 
 in the greater number of neurones and in the complex- 
 ity of their connections with one another. This results in 
 great thickening of the ventricular walls and the forma- 
 tion of a very complicated anatomical structure. Mammals 
 are especially characterized by an enormous development 
 of the cerebral hemispheres, which in man grow to such 
 proportions upwards and backwards as to overhang and 
 completely cover the other structures on the dorsal side. 
 
THE NEEVOUS SYSTEM 271 
 
 But even these large masses of nervous tissue, no less than 
 the smaller cerebrum of the frog, are composed entirely of 
 the gray and white matter forming the walls of the lateral 
 ventricles. 
 
 By comparing the brain of a frog (Fig. 98) with those 
 of the rabbit, cat, and monkey (Fig. 146), and finally with 
 the human brain (Figs. 97, 100, 101), a fairly good idea 
 may be had of the increasing complexity of the brain as 
 we pass from the lower to the higher animals. Especially 
 noteworthy is the greater relative prominence of the cere- 
 brum. In the frog this organ is small and inconspicuous ; 
 in the rabbit it is much larger, but its surface is smooth ; 
 in the cat there is a further increase in size, and the 
 surface is thrown into folds, or convohitimiis ; and this in- 
 crease in size and surface folding — carried yet farther 
 in the monkey — readies its highest development in the 
 human brain. 
 
 3. The Cranial Nerves. — Nerves enter the 'tweenbrain, 
 midbrain, and hindbrain somewhat as they enter the spinal 
 cord ; and, although their separation into dorsal and ventral 
 roots is not obvious, the neurones to which their nerve fibei's 
 belong are in all resjjects analogous to the neurones of the 
 spinal nerves. They may serve as the paths of reflexes 
 (e.g. a wink is a reflex from the optic or the trigeminal 
 nerve to the facial nerve), and their relation to the cells of 
 the cerebrum and other higher portions of the brain is essen- 
 tially the same as that of the spinal nerves. Figure 100 
 will give the points of entrance or exit of these nerves 
 from the human brain. 
 
 4. Histological Structure of the Brain. — Microscopic 
 study of the brain shows an aggregation of neurones sim- 
 ilar to that seen in the spinal cord. These neurones differ 
 greatly in shape (see Chapter VII, p. 74), in the number 
 of their dendrites, and in the abundance of their connec- 
 tions with other neurones. The regular arrangement in 
 
272 
 
 THE HUMAN MECHANISM 
 
 the cord of central gray matter surrounded by white mat- 
 ter is wanting ; instead, masses of gray matter occur here 
 
 and there among the 
 bundles of nerve fibers 
 of which the white mat- 
 ter is composed. In the 
 cerebrum and cerebel- 
 lum the external sur- 
 face consists of gray 
 matter and is known 
 as the cortex of the cere- 
 brum and cerebellum 
 respectively. These 
 corticalstructures form 
 the most complicated 
 system of nervous tis- 
 sue in the body, and 
 the cerebral cortex is 
 intimately concerned 
 with the highest func- 
 tions of the brain. (See 
 Figs. 102, 103, and 
 104.) 
 
 The figures give 
 some idea of the vari- 
 ety and complexity of 
 the neurones of the 
 
 Fifi. 102. A portion of the gray matter (cor- brain. But however 
 
 tex) of the cerebrum (highly magnified). 
 
 different, at first sight. 
 
 After Kollilcer 
 .,,,,, u * 1 1 i Ti, the brain may be from 
 
 Note the large number of dendrites. The axon.? ^ -' 
 
 are the fibers of unilorm diameter running the Spinal COrd, the 
 
 lengthwise of the drawing. One of these auatomical plan of Or- 
 ganization is the same 
 
 cells is shown in Fig. .39, D 
 
 in both ; the brain as well as the cord does its work be- 
 cause the connections of its neurones with one another 
 
THE NEKVOU.S SYSTEM 
 
 273 
 
 bring about coordinated action. The secret of the struc- 
 ture of the brain, as of the cord, lies in the nature 
 of the connections of its units, the neurones, one with 
 another. 
 
 B. The Physiology of the Nervous System 
 
 Whenever through accident, disease, or otherwise, some 
 portion of the nervous system is destroyed, functions 
 dependent upon 
 it are no longer 
 performed, or at 
 least are not per- 
 formed normally. 
 A very large 
 number of obser- 
 vations have been 
 made upon both 
 animals and men 
 in this condition, 
 and these have 
 made it possible 
 for us to obtain 
 some idea of the 
 part played in 
 normal life by 
 each part of tlie 
 brain and cord. 
 We shall attempt 
 here to sketch only a few of the more important outlines 
 of the picture, which the reader may complete by more 
 extensive study of physiology and psychology. 
 
 We shall choose for study the case of a single animal, 
 the froff, tlie anatomical structure of whose brain has been 
 given in the last chapter. The phenomena shown by the 
 
 Fig. 103. Transverse section of a convolution of 
 the cerebellum. After Ramon y Cajal 
 
 The figure represents only a few of each kind of nerve 
 cells and nerve endinc^s. A, J), E, cells; B, C, 
 nerve endings (syn.apses) 
 
274 
 
 THE HUMA]N" MECHANISM 
 
 frog are, however, as far as we shall describe them, in, 
 
 general true of higher vertebrate animals. 
 
 We shall therefore study (1) the behavior of a frog 
 
 whose brain has 
 been destroyed, 
 i.e. a frog which 
 possesses no part 
 of its central ner- 
 vous system ex- 
 cept the spinal 
 cord ; (2) the be- 
 havior of a frog 
 with spinal cord 
 and bulb intact, 
 but destitute of 
 midbrain, 'tween- 
 
 FiG. 104. Section of the cortex of the cerebeHum brain, and cere- 
 (at right angles to that shown in Fig. 10."). ■|-|j.^^ji^. /g) ^{^g 
 After Eamon y Cajal .' p . 
 
 behavior of a frog 
 
 with spinal cord, bulb, midbrain, and 'tweenbrain, but des- 
 titute of the cerebrum. 
 
 The behavior of these incomplete animals will each be 
 compared with that of a normal frog, which, of course, pos- 
 sesses a complete nervous system. 
 
 5. The Behavior of a Brainless Frog, i.e. a frog which 
 possesses of its nervous system only the spinal cord. Such 
 a frog can carry out only reflex actions of a comparatively 
 simple character. It lies flat upon its belly and, like the 
 normal frog, bends its hind legs under its flank, but does 
 not sit erect Ijy supporting the head and upper trunk on the 
 fore legs. There are no resjjiratory movements ; the vaso- 
 constrictor tone of the blood vessels is impaired or absent, 
 as are also many other of the most important reflexes. 
 
 But if one leg be j)ulled gently backward, the animal 
 will bend it again to its normal position under the body. If 
 
THE NEEVOUS SYSTEM 275 
 
 the toe be pinched, the leg will suddenly be drawn away ; 
 and if the skin of the flank be irritated by a bit of filter 
 paper moistened with acid, the paper will be kicked off by 
 the leg of the same side. 
 
 These are all purposeful^ and coordinated actions, and 
 make upon the inexperienced observer the impression that 
 the frog is aware of the stimulus and acts intelligently. 
 But the mere fact that an act is purposeful and coordinated 
 does not show that it is a conscious act ; our movements 
 of respiration, winking, coughing, and sneezing are pur- 
 poseful and coordinated, but we know well cnougli that 
 the}', as well as more complicated actions, may and often 
 do occur in the complete absence of consciousness. One 
 of the first lessons that the student of animal behavior 
 must learn is not to make the mistake of regarding an 
 action as conscious merely Iiccause " it looks so," or is 
 purposeful and more or less highly coordinated. 
 
 The s[iinal cord alone, then, and without the help of the 
 brain, is capable of maintaining a small part of the normal 
 posture of tlie resting frog, and also of executing some of 
 the simple reflexes, esjiccialh' those involving movements 
 of the hind legs ; but it does not seem to be capable of origi- 
 nating actions or of doing any except reflex actions. 
 
 6. The Behavior of a Frog with Spinal Cord and Bulb 
 only. — In this case there is no new feature in tlie main- 
 tenance of posture ; the frog lies on its belly and executes 
 the same reflexes as before. The respiratory movements, 
 however, go on in a normal manner; the vasomotor tone 
 of the arteries is maintained, most vasomotor reflexes maj' 
 be produced with ease, and tlie heart may be reflexly in- 
 hibited. As compared with the brainless frog, the number 
 of actions which the animal can execute is increased, and 
 the reflex movements become somewhat more complicated; 
 
 1 The word purposeful is used here in tlie same sense as in Chapter VII 
 (p. 71) and does not include conscious purpose in its meaning. 
 
276 THE HUMAN MECHANISM 
 
 but the differences are slight, as compared with those seen 
 in the animal which has the 'tweenbrain and midbrain in 
 addition to the hindbrain and cord. 
 
 7. The Behavior of a Frog with Spinal Cord, Bulb, 
 Midbrain, and 'Tweenbrain ; that is to say, a frog with 
 the entire nervous system exclusive of the forebrain, or 
 cerebrum. The following points are especially noteworthy : 
 («) the sitting posture maintained at rest ; (h) balancing 
 movements ; and (c) more complicated movements of 
 locomotion. 
 
 (a) Such a frog, unlike those already described, sits erect 
 exactly like a normal frog ; and this fact shows that com- 
 plete maintenance of the normal posture requires the coop- 
 eration of higher portions of the nervous system than the 
 bulb and spinal cord, but does not involve the cooperation 
 of the cerebrum. 
 
 {b) If the frog be placed on a rectangular block of wood, 
 and the block slowly turned so that the frog tends to slip 
 off backwards, it will crawl up and over the descending 
 edge, keeping itself perfectly balanced. By continuing 
 to turn the block the frog can be made to creep around it 
 almost indefiniteljr. Thus it not only maintains the erect 
 position but also corrects loss of equilibrium by appropriate 
 balancing movements. 
 
 (<;) If the frog be stroked upon its belly, it will croak ; if 
 its lips be touched with a blunt pin, it will brush the 
 pin away with its forefoot. Most important of all, if it be 
 thrown into the water, it will swim ; and when it reaches a 
 solid object it will crawl out upon it and come to rest. In 
 short, the animal will carry out almost any movement of 
 which a normal frog is capable, provided the ■proper stim- 
 ulus is ap)p)lied ; but without this it will do nothing, though 
 capable of doing sa much. 
 
 The facts thus far brought forward show that the neu- 
 rones of the 'tween-, mid-, and hindbrains, and of the spinal 
 
THE NEKVOUS SYiSTEM 277 
 
 cord, constitute nervous mechanisms which can maintain 
 tlie normal posture, correct loss of balance, and even carry 
 out the usual acts of locomotion. The more of the nervous 
 system which the animal retains the more complicated are 
 the movements, as we should expect when we remember 
 the increase in the number of neurones, and the greater 
 complexity of coordination thereby rendered possible. 
 
 8. Comparison with the Normal Frog. — The behavior 
 of a frog lacking only the forebrain (or cerebrum) differs 
 from that of a normal frog in two most significant respects. 
 In the first place, the animal rarely makes any movement 
 without obvious external stimulation ; if protected from 
 drying, it will often sit motionless for days, or even weeks. 
 Such is not the conduct of an animal which is aware of 
 what is going on around it, or of its own sensations or 
 feelings, i.e. of a conscious animal. In the second place, 
 the frog shows tlie most remarkable regularity and persist- 
 ency in making repeatedly the same response to tlie same 
 stimulus ; if its lips be touched tJdrty times ■\\ ith a blunt 
 needle, it will brush at the offending object everj- time in 
 the same way with the same forefoot. We should certainly 
 not expect a conscious animal to do this ; for, after trying 
 one plan of action a few times, it would realize that its 
 efforts were unavailing, and would try something else, 
 such as jumping away. This same peculiarity is met with 
 in all animals deprived of the cerebrum. They act like 
 mere complicated and faithful machines; they do not act 
 as if they were thoughtful, original, or wise. 
 
 Especially striking is the avoidance of objects during 
 locomotion. This fact looks at first sight as if the animal 
 were aware of the presence of the obstacle in its path ; 
 but a dog without a cerebrum, even when it has been 
 without food for a day or more, will go to one side of a 
 piece of meat and pass it by. He acts as if unaware of the 
 nature of the object, of its use as food, etc. The image of 
 
278 THE HUMAN MECHANISM 
 
 the piece of meat formed on his retina seems to generate 
 nervous impulses wliich pass to the brain by way of the 
 optic nerve and reflexly guide the movements of the dog ; 
 but these impulses do not inform the animal of the nature 
 of the object, and we have no reason to believe that the 
 dog is aware of the existence of the meat. 
 
 When we consider our own experience we find that we 
 too, as we walk along a crowded street, avoid objects, not 
 only without noticing them but without even being aware 
 of their presence. Here again the afferent impulses from 
 the retina pass to the nervous system and refiexly guide 
 our walking without affecting consciousness at all. And 
 the wonderful feats of somnambulism, where the "eyes are 
 open" but "their sense is shut," where the sleeper main- 
 tains his balance and avoids stumbling in situations where 
 he would almost inevitably fall if he were aware of his 
 surroundings, show how perfect is this very complicated 
 mechanism of locomotion, which seems to be complete 
 even in the absence of the cerebrum. 
 
 We are, indeed, so accustomed to regard our actions as 
 volitional and conscious that we rarely consider the large 
 part which reflexes from the eye, the ear, the skin, the 
 muscles, and the joints play in guiding them. We will to 
 do a certain thing, to walk to a certain point, for example ; 
 perhaps the first step is a volitional act, but subsequent 
 steps, the suiting of these steps to slight unevennesses of the 
 path, the avoidance of many obstacles, the maintenance of 
 the balance of the body as a whole, — for we walk not only 
 with the legs but with the entire body, — all these things 
 take place apart from any exercise of the will, and, for 
 the greater part, in the entire absence of consciousness, 
 although consciousness may, of course, at any time inter- 
 vene. Reflex actions thus play a most important part even 
 in the execution of those movements which we think of as 
 distinctly conscious acts. 
 
THE NEEVOUS SYSTEM 279 
 
 9. Connections of the Cerebrum with Lower Portions of 
 the Nervous System. "The Way Out." — Granting that 
 the nervous events at the basis of consciousness occur 
 witliin the cerebrum, liow do these events influence the 
 muscles, the glands, and other organs which do the bid- 
 ding of the will ? What is the way out from this seat of 
 consciousness? This path has already been referred to 
 in Chapter ^"11 (p. 82). C'ells in the gray matter of the 
 cerebrum give off axons which pass downward through 
 the structures of the 'tween-, mid-, and hindbrain into the 
 white matter of the spinal cord. These axons give off 
 along their course collaterals which end in arborizations 
 around nerve cells of the lower portions of the nervous 
 system, and Ijy Ijringing groups of these cells into coordi- 
 nated activity produce delinite volitional movements. The 
 student should review carefully in this connection what 
 has already been said with reference to these neurones 
 (see Fig. 145, v). 
 
 10. Connections of the Cerebrum with Lower Portions of 
 the Nervous System. "The V/ay In." — The fact that affer- 
 ent impulses from our sense organs of sight, hearing, etc., 
 may affect consciousness indicates that there must be some 
 connection between afferent rjcurones and the cerebral 
 hemispheres, since only when the latter are present does a 
 nervous impulse produce a conscious sensation. Tlie con- 
 nection is not, however, so direct as in the case of efferent 
 impulses. Tlie neurone of the dorsal root ma}' be traced 
 as far as the bulb, but no farther : from this point the im- 
 pulse can find its way to the cerebrum only by new neu- 
 rones, and of these it would seem that there are several. 
 These relations are indicated in Fig. 146, where the efferent 
 neurones are represented in black and the afferent in red. 
 
 This diagram brings out the fact of increasing complexity 
 of reflexes as we proceed to the more anterior portions of 
 the nervous system. In the spinal cord the collaterals 
 
280 THE HUMAN MECHANISM 
 
 of the afferent neurone act upon the efferent neurones ; in 
 the structures of the midbrain and the 'tweenbrain the 
 afferent tract makes connection with more and more com- 
 plicated and extensive systems of these efferent neurones 
 or motor mechanisms. The range of possible movement 
 is increased to include most of the usual actions of the 
 animal, and some of these actions represent a very high 
 degree of coordination. Finally, in the cerebrum the highest 
 of all these connections is made ; here take place those 
 events of whose nature we have thus far been quite unable 
 to form any conception, but which play some part in the 
 genesis of conscious sensations and in the closely related 
 dispatch of volitional impulses. We can now understand 
 why it is that removhig this highest portion of the nervous 
 system leaves untouched not only the simpler reflexes but 
 even the more complicated reflexes of locomotion, of 
 swimming, of flight, etc. 
 
 11. Walking an Endless Chain of Reflexes. — The reflex 
 mechanism of walking may not Ije clear at first sight, 
 although the facts we have already given in this chapter 
 (p. 278) leave no doubt that the movements of locomotion 
 are guided to some extent by afferent impulses. 
 
 In a previous chapter (p. 262) it was shown that afferent 
 impulses from the skeletal muscles, the joints, etc., enable 
 us to form judgments of the position of parts of our body 
 with lef erence to each other ; we have also seen that affer- 
 ent impulses from the semicircular canals contribute to 
 this same sense of position. The afferent neurones, how- 
 ever, which serve as the path of these impulses to the 
 cerebrum, send off collaterals to the lower portions of 
 the brain, as shown in Fig. 145, and in this way make the 
 reflex connections which execute the act of locomotion. 
 
 With each position which the body assumes, a certain 
 combination of these afferent impulses passes into the 
 central nervous system, and each position gives rise to its 
 
THE NEE.VOUS SYSTEM 281 
 
 oion combination of impulses. It is clear (Fig. 145) that 
 these impulses, having entered the central nervous system, 
 have before them one or the other or both of two destina- 
 tions : they may proceed onward to the cerebrum and 
 there give rise to the knowledge of the position of parts 
 of the body ; or they may go by way of collaterals to the 
 cerebellum and other lower nerve centers, and there influ- 
 ence the efferent or motor mechanisms of the more com- 
 plicated reflex actions. 
 
 It is further necessary to understand that the result of 
 the arrival of the afferent impulses at the cerebrum or the 
 lower centers depends on the condition of those portions 
 of the nervous system at the time. Thus the impulses 
 affect consciousness only when we heed them ; when one 
 is sitting still and thinking of other things they help to 
 maintain equilibrium by their influence on certain lower 
 nerve centers which control the muscles, but they do not 
 stinmlate new movements ; finally, during locomotion they 
 not only maintain the balance of the bod}' but also call 
 forth the next movement, because at that time the efferent 
 motor mechanism of the lower centers is free to act. 
 
 In walking, for example, as we take the forward step 
 with one foot, the combination of these afferent impulses 
 is constantly changing ; this change of combination ob- 
 viously changes the stimuli acting upon the efferent or 
 motor cells, and the movement progresses, changing in 
 character each instant until the foot taking the step is 
 planted firmly on the ground and the step with the other 
 foot begun. Here again the same kind of changing reflex 
 movement goes on until the second step is completed and 
 the parts of the body are in the same position with refer- 
 ence to one another as when the first step was begun ; we 
 return in this wai/ to the original comhination of afferent 
 impulses and so begin again the same cycle of movements ; 
 the third step is like the first. Locomotion is thus an 
 
282 THE HUMAN MECHANISM 
 
 endless chain of reflexes. The character of the act may be 
 modified by new afferent impulses from the eye (as when 
 we unconsciously avoid objects), or from touch when we 
 tread upon some obstacle ; or volition may modify it, as 
 when we give attention to what we are doing. But so 
 long as the character of the path or of surrounding objects 
 remains the same and volition does not intervene, the act 
 progresses as an endless chain of reflexes. 
 
 Essentially the same thing is true of swimming, of run- 
 ning, and of many other actions. They are fundamentally 
 reflexes from the sense organs of position, though their 
 character may be modified either reflexly, by new afferent 
 impulses, or voUtionaUi/, by the intervention of new impulses 
 from the cerebrum. Such an act as dancing differs from 
 the others only in the greater extent to which the volitional 
 imjjulses play upon the chain of reflexes of which the 
 action is fundamentally composed. 
 
 12. Actions resulting from Nervous Processes originating 
 within the Cerebrum. — A very large part of the activities 
 of the body are thus fundamentally reflex actions ; they 
 do not require tlie aid of consciousness for their execu- 
 tion. And it is fortunate for us that this is the case; one 
 has only to imagine a human being who has to give his 
 attention, or " his mind," as we often say, to every adjust- 
 ment of the digestive, respiratory, and vascular systems 
 required to meet the changing necessities of life ; who 
 has to keep his thouglits on every movement of walking 
 or running ; who has to be constantly on guard against 
 loss of balance even when sitting still. Such a being is 
 almost inconceivable ; he would " go crazy " in a single 
 day ; Ijut we can in this way realize to what extent the 
 reflex mechanisms of the body perform the menial offices 
 of life, leaving the mind free for liigher things. 
 
 Speech is the result of movements in which the muscles of 
 respiration — those of the larynx, those of the tongue, and 
 
THE NEEVOUS SYSTEM 283 
 
 those of the lips — cooperate to produce articulate and intel- 
 ligible sound. The act of writing also consists of a series 
 of movements in which the muscles of the arm and hand 
 cooperate to make thought visible ; performing on a musi- 
 cal instrument, modeling a figure in clay or marble or 
 bronze, painting a picture, — all these things occur to us 
 as examples of movements which are not fundamentally 
 reflex, and from which the character of an endless chain 
 is absent. Such are the highest actions of the body, and 
 the movements of whicli these actions are made up are 
 chosen and directed by the will. 
 
 These higher actions, like consciousness, depend upon 
 the presence of the forebrain. When a certain area of the 
 cerebrum is destroyed by disease, the power of speech is 
 lost; when another part is destroyed, the skilled use of 
 the hand is lost ; destruction of other portions affects in 
 the same way others of these skilled movements. In such 
 cases locomotion, the maintenance of balance, the move- 
 ments of respiration, etc., may Ije and usually are unaffected ; 
 the patient merely loses the power of doing one or more 
 of those things which, in no sense reflexes, involve the 
 selection of disconnected and to some extent independent 
 movements giving expression to some original thought, 
 sentiment, or idea. 
 
 The neurones of the cerebrum and their connections 
 thus constitute nervous mechanisms whose activity is 
 essential to consciousness, — to our seeing, our hearing, 
 our smelling, and, more than this, to our understanding 
 of what we see, or hear, or smell, — nervous mechanisms 
 whose activity is also necessary to the expression of our 
 thought in action. It is because of this fact that, when 
 the cerebrum is removed, the animal becomes merely a 
 complicated reflex machine, acting only as it is immedi- 
 ately stimulated from without, or by events taking place 
 within its own body. 
 
28-4 THE HUMAN MECHANISM 
 
 13. Effects of Anesthetics on the Nervous System. — When 
 a person passes under the influence of an anesthetic the 
 first function to disappear is consciousness ; the ether or 
 the chloroform first paralyzes this highest and most com- 
 plex connection between the afferent and the efferent sides 
 of the nervous system. In this condition the patient may 
 groan and struggle, for he is in somewhat the same state 
 as the animal without cerebral hemispheres. The use of 
 the surgeon's knife will still produce movements ; resjjira- 
 tion may be affected so as to result in groans and other 
 movements which the inexpert observer, perhaps in alarm, 
 attributes to severe suffering ; and yet when the patient 
 awakes he tells us he knew nothing of what passed and 
 felt no pain. It is important to realize that the signs of 
 pain are never reliable evidence of its existence. 
 
 If the anesthesia be pushed further, even these more 
 complicated reflexes disappear. In the ordinary major 
 operations of surgery, the ether or the chloroform is given 
 until it interrupts not only the cerebral connections between 
 the afferent and efferent paths, but also those of the lower 
 portions of the brain ; it is even administered until only 
 a few reflexes ^re left, such as the wink when the cornea 
 is touched, the contraction of the pupil when the eye is 
 exposed to light, etc., — these serving as useful tests of the 
 condition of the patient. If, for example, the pupil no 
 longer contracts to light, it is an indication that the anes- 
 thesia is going too far, — too near the point where the nerv- 
 ous mechanism of respiration, etc., will be paralyzed. The 
 giving of ether is then susjDended until these reflexes are 
 again well established. 
 
 After the operation, as the ether or chloroform is elimi- 
 nated from the system, the reflexes return in the reverse 
 order ; and the unconscious movements, groans, incoherent, 
 or even more or less coherent, talking (comparable with 
 talking in one's sleep) are sometimes most harrowing to 
 
THE NERVOUS SYSTEM 285 
 
 the feelings of those who do not understand that they are 
 all unconscious acts. The physician and nurse who remain 
 unmoved may even be wrongly charged with lack of feel- 
 ing because they do not waste sympathy where they know 
 there is neither suffering nor consciousness. 
 
 14. Inhibitory Phenomena in the Nervous System We 
 
 have learned that some nerves excite organs to activity, 
 while others diminish activity or abolish it altogether 
 (p. 158). The beat of the heart is quickened by one set of 
 nerves and slowed by another ; the circular muscular fibers 
 of the arterioles are excited to contract by vasomotor nerves, 
 their tonic constriction is paralyzed or inhibited by vaso- 
 dilators, and many other examples might lie drawn from 
 the action of neurones on peripheral organs of the body. 
 
 Precisely the same thing is true in the lirain and spinal 
 cord. Afferent impulses may not only reflexly excite 
 neurones to activity but may also inlribit the existing or 
 threatened activity of other neurones, as wlien a sneeze is 
 stopped by biting the upper lip or by pinching the nose ; 
 or an action may be inhibited by a volitional impulse from 
 the cerebrum, as when the breathing movements are volun- 
 tarily stopped for a while, or when we similarly stop a 
 wink or a sneeze. These are all examples of inhibition, 
 not of the skeletal muscles concerned but of the neu- 
 rones Avhich innervate them, — in other words, of the inhibi- 
 tion of one neurone by another. 
 
 It must be understood that inliibition is as essential a 
 part of the activity of the nervous system as is excitation. 
 Just as- the diiver of a team must urge on one horse while 
 he restrains another, so in all more complicated actions, 
 probably in all actions, reflex or volitional, the orderly 
 movement is as much the result of holding one neurone 
 in check as of stimulating another one to work, or to 
 work harder. Consciousness proves its presence most con- 
 clusively by suppressing reflexes which would otherwise 
 
286 THE HUMAN MECHANISM 
 
 inevitably occur, and by bringing about new movements 
 to meet the desired end. Even in the highest processes of 
 the most highly organized of nervous systems, viz. those 
 in w^hich human action originates, the man reveals his char- 
 acter and influences the world around him by what he does 
 not do, — by what he refrains from doing, sometimes at the 
 cost of severe struggle against impulse, instinct, or passion, 
 — quite as much as by what he does. Education, even, 
 has been defined as the " training of inhibitions and the 
 control of reflexes." 
 
 15. Use and Disuse as Factors in Individual Develop- 
 ment, Training, and Efficiency. — When we consider the 
 marvelously complicated character of the nervous mechan- 
 isms which control our actions, we naturally wonder how 
 this intricate machinery can be built, and why it does not 
 more frequently get out of order. We cannot say that a 
 simple and comprehensive answer will not some day be 
 given to these questions ; but to-day we have no adequate 
 answer whatever. The neurones with which we must 
 work in life are born with us, but in most cases efficient 
 connections must subsequently be made between them, 
 thus perfecting the mechanisms they compose ; and this 
 perfecting of the nervous machine comes with use. The 
 use of a nervous mechanism is generally essential to its 
 proper development, just as the use of a muscle is essential 
 to its strength. If the child never tried to walk, the neu- 
 rones which carry out the movements of walking would 
 not develop; not only do the muscles of an arm strapped 
 down to the side of the body waste away and become 
 practically bands of connective tissue, but the neurones 
 concerned in the actions which the arms should execute 
 degenerate and are ultimately irreparably injured. 
 
 Provision is made from earliest life for the proper devel- 
 opment of these neurones, and the establishment of irri- 
 table connections between them by use ; out of the first 
 
THE NEEVOUS SYSTEM 287 
 
 aimless movements of the head and eyes and hands and 
 legs of the baby the simpler coordinating nervous mechan- 
 isms are one by one brought to perfection ; then comes the 
 training of those reflexes which maintain the erect position, 
 and of those which govern locomotion ; then j;Z(r^ comes in, 
 with its ceaseless activity, increasing still further the num- 
 ber of movements which the nervous system can make, 
 and correspondingly enlarging the possibility of human 
 achievement. As the child grows older the family calls 
 upon him to contribute some share to its life or support ; 
 new activities, in the shape of chores about the house 
 or the farm, now share with play the work of the nerv- 
 ous system ; activity becomes less general, more special. 
 Finally, the youth settles down to some definite occupa- 
 tion or pursuit, and the more strictly this is adhered to 
 the narrower becomes the range of activity ; the more con- 
 stantly a few systems of neurones are used, the more 
 rarely are others called into play. 
 
 16. The Physical Basis of Habits. — All this indelibly 
 writes its history in the nervous system. No fact is more 
 significant, or of greater physical and moral import, than 
 that the doing of any act so affects the connections of 
 neurones with one another as to make it easier to do the 
 same act again under the same conditions ; that refrain- 
 ing from doing something toward which we are inclined 
 similarly renders more easy the inhibitory processes con- 
 cerned when the same conditions impel us toward it again. 
 We are largely what we make ourselves by the training 
 which our actions give to the nervous system. 
 
 And what activity thus does for the development of 
 power it does also for the maintenance of power. An 
 efficient nervous mechanism of any kind once acquired 
 does not remain efficient without use. The man who has 
 developed a rugged constitution in colder climates and 
 then lives for years in the tropics, constantly exposed to 
 
288 THE HUMAN MECHANISM 
 
 warm climate, finds on return to the home of his youth that 
 the mechanism of heat regulation does not readily adjust 
 itself to cold damp winds and blizzards ; the athlete who 
 has learned to execute the greatest variety of " tricks " 
 in the gymnasium and then settles down to a sedentary 
 life finds after some years that he is almost as helpless as 
 the man who gave no attention to such training. It is 
 unnecessary to multiply examples. Efficiency in any direc- 
 tion is the result of continued use of organs, and especially 
 of continued training of the nervous system. As we fit 
 ourselves to do some few things, and to do them well, we 
 have not time to conserve by use the efficiency of all the 
 nervous mechanisms we have acquired ; we must to some 
 extent sacrifice the more general actions for those which 
 are more special and useful. But it must not be forgotten 
 that this can be carried too far ; that « certain amount of 
 general activity is a condition of healthy living, and that 
 one of the problems of life to solve, and to solve aright, is 
 how to distribute our activity between the two. To the 
 consideration of these questions we shall return in our 
 study of personal hygiene. 
 
THE HUMA:?^ mechanism 
 Part II 
 
 THE HYGIENE OF THE HUMAN MECH- 
 ANISM AND THE SANITATION OF 
 ITS SUKROUNDINGS 
 
CHAPTER XVI 
 
 INTRODUCTORY 
 
 A. Hygiene and Sanitation : the Right Use and 
 Pkoper Care of the Human Mechanism 
 
 In Part I of this work we have examined in some detail 
 the normal structure and workings of the human mechan- 
 ism. We have now to consider that care and use of the 
 mechanism wliich constitute a wise and proper conduct of 
 life, to the end that we may possess and enjoy the greatest 
 possible measure of bodily health, strength, and usefulness. 
 
 The owner of any valual)le lifeless mechanism, such as 
 a watch, a yacht, or a piano, or of any valuable living 
 mechanism, such as a prize-winning dog or horse, gives 
 close attention to tlie proper care, management, and use 
 of his property; and if, as often happens, great sums are 
 invested in such property, this care is naturally all the 
 greater. Comparatively few persons own such things, but 
 every one of us — every man and woman, every boy and 
 girl — begins life possessing a far more valuable mech- 
 anism whicli we call the human bod}-. Some of these 
 mechanisms are of marvelously strong and perfect con- 
 struction ; they seem to be equal to any amount of work 
 and to suffer but little from careless operation or unfavor- 
 able surroundings. Many are not so strong, but yet with 
 reasonable care give excellent service. Others require con- 
 stant, intelligent attention, without which they readily get 
 out of order, but with which they do fair and even good 
 work. Much of the best work of the world has been done 
 by persons in poor health ; Darwin never had robust 
 
 291 
 
292 THE HUMAN ME(JHANISM 
 
 health ; Heine was an invalid in his later years ; Milton 
 was blind; Sir Walter Scott was always lame; Pasteur 
 was partially paralyzed during much of his life. On the 
 other hand many, originally robust, have not only broken 
 down and failed to do good work for themselves and their 
 fellows, but have actually become a burden to the world, 
 simply because they have refused to give to their bodies 
 the care which they would freely bestow on a watch or 
 a bicycle. 
 
 The proper management and operation of the human 
 mechanism requires not only care, but intelligent care. A 
 locomotive is intrusted only to an engineer who knows its 
 construction, who can detect the evidences that something 
 is wrong, who knows how much steam to apply at different 
 times, what to do on various grades, how to start his 
 engine safely, and how to bring it to rest. By lessons on 
 anatomy and physiology, in Part I, we have endeavored to 
 impart to the student the same preliminarj' knowledge of 
 the construction and workings of the human mechanism 
 which any one intending to be an engineer must have 
 of machiner}' before he can master the practical opera- 
 tion of his engine. The chapters immediatelj^ following are 
 concerned with the proper care and management of the 
 mechanism under the various conditions of daily life. 
 
 The principles governing the proper care and right use 
 of the human mechanism and its surroundings form the 
 subject-matter of hygiene and mnitation ; and practical 
 hygiene and sanitation consist in the application of the 
 principles of physiology and sanitary science to the con- 
 duct of physical life. Their object is the preservation and 
 promotion of health, the jirevention of premature death, and 
 the establishment and maintenance of the highest possible 
 working efficiency of the human mechanism. 
 
HYGIENE AND SANITATION 293 
 
 B. Health and Disease 
 
 All things vary from time to time in respect to their con- 
 dition. They may be cold or hot, wet or dry. Machines 
 may be in good condition, — in good working order, — or 
 the reverse ; and the human body, the human machine, is 
 no exception. It may be cold or hot, wet or dry, in good 
 condition or in bad condition. 
 
 HeaWi is a condition of the liuman mechanism in which 
 all the organs are sound and in good working order. 
 " Perfect health " is much the same thing, since imperfect 
 health suggests lack of soundness, i.e. disease. " Robust 
 health," " strong health," " sound health," " good health," 
 and all similar terms are easily luiderstood as signify ing 
 various states or cvmUtuois of the human mechanism. The 
 common use of such a term as " broken health " testifies 
 to the popirlar recognition of the fact tliat health is the 
 normal or sound condition of the machine. The terms 
 " poor health," " weak health," " feeble health," etc., are 
 obviously contradictory in tlicmselves, and really refer to 
 states or conditions in which liealth is either imperfect or 
 altogether wanting. 
 
 1. Degrees of Health. — There are, nevertheless, degrees 
 or variations in health as in other conditions. A stone may 
 be very wet or very dry, or only slightly wet or half dry. 
 A watch may be in tolerably good condition or in excellent 
 condition. Likewise the human body may be in fair health 
 only, or in excellent health, or in " splendid " health. 
 
 2. It is the Aim of Hygiene and Sanitation to secure the 
 Best Health Possible under Any Given Conditions. — In 
 youth fair health is not enough to expect, but in extreme 
 old age anything beyond this may be impossible. Some 
 trades or industries are, from their very nature, unhygienic 
 and correspondingly dangerous. Moreover, the degree of 
 health, as of other things, is largely determined ui each 
 
294 THE HUMAN MECHANISM 
 
 individual by personal ambition, resolution, and effort. 
 Many persons go through life on too low a plane of health 
 simply because they are too unambitious, too careless, 
 or too indolent to make the effort needed to rise to any 
 higher plane. Without strong desire for improvement, 
 improvement rarely comes. 
 
 One characteristic of the human body in health is that 
 it does its work with ease ; but when it passes into an 
 unsound or abnormal condition this characteristic tends to 
 disappear, and dis-ease, disturbance, or difficulty in opera- 
 tion takes its place. A "jumping" toothache, the "sore" 
 throat of diphtheria, the " uneasiness " of dyspepsia, the 
 " pains " of rheumatism, the " racking " cough of advanced 
 consumption, the " splitting " headache of incipient typhoid 
 fever are good examples. 
 
 Disease is a condition of the human mechanism in which 
 one or more of the organs is unsound or abnormal, or in 
 such poor working order as to interfere seriously with the 
 welfare of the entire mechanism. 
 
 3. Degrees of Disease. — We speak of " mild disease," 
 "severe disease," and even of "malignant disease," using 
 phrases which testify to the fact that there are degrees of 
 disease or disturbance of the human mechanism, no less than 
 of its health, and the attention which we give to disease is 
 largely determined by its severit}^. When we have a cold 
 in the head we regard the disease as a triflino' matter and 
 expect it to pass away of itself in a day or two. We are 
 apt to summon a physician only when we find that we 
 have a fever, or when some trouble, which at first seemed 
 of small consequence, " hangs on " or seems to be getting 
 worse instead of better. We are especially apt to call in 
 the doctor when we are actually suffering pain and are 
 unable to find relief, — all of which facts bear witness to 
 our practical recognition of the truth that there are various 
 degrees of disease as well as of health. 
 
HYGIENE AND SANITATION 295 
 
 4. Different Attitudes assumed towards Disease. — Vari- 
 ous attitudes are assumed by different persons towards 
 health and disease. One attitude, represented perhaps by 
 the practice of tlie majority of people, is to go about one's 
 work, whatever that may be, giving no thought whatever 
 either to the maintenance of health or to the avoidance 
 of disease ; in other words, to pay no attention to the mech- 
 anism and to do nothing to keep it in order ; to wait 
 until something happens, some breakdown occurs, some dis- 
 ease has clearly developed, and then hastily to take a dose 
 of medicine, or, finally, to call a physician. This we may 
 call the attitude of heedlessness. 
 
 A second attitude is that of neglecting any active culti- 
 vation of health, but carefully attempting to avoid those 
 things which are liable to produce disease. In this case 
 persons often give great attention to the choice of diet, 
 to jjrotection against cold, to the purity of their drink- 
 ing water, their food supplies, etc., fixing their attention 
 wholly on the agents of disease and assuming that, if these 
 be kept at a distance, the body will take care of itself. This 
 may be called a half-Iu/i/ienic attitude. 
 
 A third attitude — the reverse of the second — consists 
 in actively cultivating abounding health by attention to 
 those things which are believed to build up a strong con- 
 stitution, in the belief that no disease can attack a strong 
 and vigorous body. Such persons concentrate attention 
 on health and underestimate the possibilities of succumb- 
 ing to attacks of disease. This also is a half-hygienic atti- 
 tude, although in practice perhaps somewhat safer than 
 the second ; very man}', perhaps all, diseases are less likely 
 to appear in a strong and vigorous body than in one which 
 is not in sound health. But if the experience of the race 
 teaches anything, it is that strong men, seemingly in per- 
 fect health, often succumb to attacks of disease. It is not 
 safe, even for a healthy man, to swallow the germs of 
 
296 THE HUMAN MECHANISM 
 
 Asiatic cholera; it ia not safe, even for a healthy man, to 
 prick his finger with a knife which has been used in 
 lancing a boil. Without in the least undervaluing the 
 importance of maintaining health and physical vigor as 
 preventives of disease, we cannot too strongly affirm that 
 these are not absolute preventives, that they are not reli- 
 able preventives, and that in some cases they are not pre- 
 ventives ^ at all. 
 
 A fourth (and the only right) attitude toward health and 
 disease is that which actively seeks to maintain in the 
 mechanism the highest possible degree of health under 
 all conditions, and at the same time constantly takes all 
 reasonable precautions to ward off attacks of the external 
 agents of disease. This is the true hygienic attitude, as 
 indicated by reason and modern science ; and this atti- 
 tude of mind we shall endeavor in the following pages to 
 encourage, justify, and strengthen in students or readers 
 of this work. 
 
 C. The Three Great Factors of Disease 
 
 5. The Three Great Factors of Disease. — Keeping 
 always in mind the truth that the human body is a machine 
 or mechanism, and agreeing to regard any condition as 
 one of dis-ease in which the body does not do its work 
 smoothly or with ease, we perceive that there are three 
 great causes of disease of the body, just as there are three 
 
 1 The truth of this fact is illustrated when there appears among a 
 people some disease to which neither they nor their ancestors have regu- 
 larly been exposed ; the ravages of epidemics of Asiatic cholera in Europe 
 and America, and the history of the great plague in Europe in the seven- 
 teenth century, or of yellow fever in our own southern states, being cases 
 in point (see Daniel Defoe, Journal of the Plague Year ; James Ford 
 Rhodes, Bistory of the United Staiefs, I, p. 400). The North American 
 Indians, who were p}'esumably strong and healthy, were decimated by 
 measles, — a comparatively mild disease, — when this was brought jmong 
 theni by the early settlers of this country. 
 
HYGIENE AND SANITATION 297 
 
 chief causes of trouble in the running of a locomotive. 
 These are (1) imperfections in the mechanism itself; 
 (2) unskillful operation and care; and (3) unfavorable exter- 
 nal conditions. Let us consider carefully the part played 
 by each of these in the maintenance of health and the pre- 
 vention of disease. 
 
 (a) Imperfections in the Mechanism. — The wheels of an 
 engine may not be j^erfectly true, some of its valves may 
 leak, some bearing may he unduly exposed to dust. So 
 is it with the human Ijody. Wonderful as is the human 
 mechanism, it is never perfect. A valve in the heart may 
 leak and permit " regurgitation " of the blood ; a defect in 
 the structure of the spine may make it hard to hold the 
 trunk in its normal posture ; the glands of the stomach or 
 pancreas may be made of poor material and so secrete an 
 ineffective digestive juice ; in short, any organ may be of 
 poor construction and so have imperfect capacity for work. 
 Such constitutional defects may be l)orn with us, or they 
 may be acquired b}' some accident or other circumstance 
 which leads to irreparable and permanent injury. Where 
 they exist they must be recognized and reckoned with in 
 what we attempt to do, although their cure or compensa- 
 tion is by no means hopeless. The deaf mute adapts him- 
 self to a lack of hearing, and in spite of it communicates 
 with his fellows ; and men and women with serious organic 
 troubles may often lead useful and, on the whole, healthy 
 lives. 
 
 Again, every human body possesses as the outcome of its 
 construction or constitution more or less capacity to endure 
 hardship and to struggle for continued existence. In the 
 strong this capacity, loosely called vital resistance, may be 
 very great, and in the weak or feeble very small; but in 
 order that life shall continue at all, every human body 
 must have more or less of it. It is required to withstand 
 heat and cold, underfeeding and overfeeding, the attacks 
 
298 THE HUMAN MECHANISM 
 
 of parasites, the work and the play of life, the infirmities 
 of age. If it be very great, almost all hardships can be 
 endured, almost all diseases avoided or overcome ; if it be 
 very small, as it often is in old age, even the grasshopper 
 may become a burden. 
 
 As we pass middle life and old age creeps over us we 
 find this power of vital resistance lessened. Of all people 
 who enter their seventieth year, a much larger percentage 
 die before reaching their next birthday than is the case 
 with those entering their twentieth year. This can only 
 mean that the ability to cope with unfavorable conditions 
 is lessened as age advances. The body shows by growing 
 feebleness that it is wearing out, and ultimately succumbs 
 to disease which in earlier life would have been a matter 
 of small consequence. Hence it follows that old people 
 must reckon with a poorer constitution, and must give 
 greater care than the young to the bodily machine. 
 
 (b) Unskillful Operation and Care. — The most perfect 
 engine will behave badly in the hands of an ignorant, un- 
 skillful engineer or fireman. There is a proper method of 
 firing, a proper method of starting ; and when a grade is 
 to be ascended it must be taken in the proper way. When 
 these things are not done rightly the engine is very apt to 
 suffer damage, even to acquire structural or constitutional 
 defects ; and in no such case can it be expected to do its 
 best work, or to do any work, with perfect ease. Human 
 life involves the operation of a much more delicate engine 
 or mechanism. The human body is a machine calculated to 
 do work, and when we say that it is alive, we mean that it 
 makes use of the potential energy of foods to accomplish 
 ends which no lifeless machine can accomplish ; but it does 
 not do this life-work without management or operation. It 
 is the faithful servant of an intelligent will, and it may be 
 worked or used wisely or unwisely, skillfully or unskillfully. 
 This engineering, management, direction, or operation of 
 
HYGIENE AND SANITATION 299 
 
 the human mechanism constitutes the physical conduct of 
 life, and is one of the most fundamental and important 
 elements in the maintenance of health. 
 
 (c) Unfavorable External Conditions. — Again, the best 
 work of an engine requires more than good construction 
 and skillful operation ; it also requires favorable conditions 
 and surroundings. If the road-bed be poorly ballasted or 
 the rails rusty and uneven ; if the weather be so cold as to 
 make it impossible to keep up full steam in the boiler ; if 
 the water tanks be not kept supplied with water, or the 
 coaling stations with fuel, poor work and often actual 
 injury to the mechanism itself — constitutional injury — is 
 the result. Finally, if by chance a stone has rolled upon 
 the track, or a signal has been wrongly set and a collision 
 results, a good locomotive may be disabled or even ruined. 
 
 So with the human mechanism. Like all other living 
 things it cannot continue its work under certain external 
 conditions. It cannot live without food in a desert ; it can- 
 not endure exposure to extreme cold without protection ; 
 it cannot keep sound in a room witli leaky gas fixtures, or 
 in a cell which admits no sunshine. It must have pure 
 drink and pure food, and it must avoid exposure to the 
 contagion of diseases against which it has no sure defense. 
 
 6. Definition of "Environment." — We have alreadyused 
 and shall hereafter often use the terms " environment " and 
 "environmental," and a word in explanation of them may 
 be serviceable at this point, although they have been defined 
 briefly in Part I and will be further considered beyond. 
 They are used in this work, as in biology in general, for 
 that portion of the universe not ourselves, and generally for 
 those portions of it comparatively near to us. A man's 
 home, for example, is a part of his normal environment, 
 and so are his relatives, friends, and neighbors, his horses, 
 dogs, and pets, his clothing, and his surroundings generally. 
 In the same category come, however, more remote things, 
 
300 THE HUMAN MECHANISM 
 
 such as the village, town, city, or state ia which he lives. 
 Even the sun, the intermittent light from which causes day 
 and night, with their corresponding alternate sleeping and 
 waking, working and resting, though so far away, is a part 
 of man's environment. We have referred in Part I to the 
 blood and lymph as the environment of the cells of the 
 body, and such they are ; but this internal environment (of 
 the cells) need not be confounded with the external envi- 
 ronment (of the body). This latter may be concisely defined 
 as everything outside ourselves. 
 
 A close relation always exists between the body and its 
 environment, by which the former is, so to speak, in har- 
 mony ivitli or adapted to the latter. Just as the ship is 
 adapted to the sea ; the punt, the shell, and the canoe to 
 quiet waters ; the bicycle to good roads, — so also man's 
 body reaches its best development and does its best work, 
 not in the tropics or at the poles, but in the more temperate 
 zones to which it appears to be better adapted. A perfectly 
 healthy organism might, perhaps, be defined as one perfect 
 in construction or constitution, and perfectly adapted to its 
 environment. 
 
 7. Man and His Environment. — Man is absolutely 
 dependent on a favorable environment. He cannot, like 
 a fish, live long in the sea ; or, like a bird, in the air. He 
 cannot even make his way to the poles of the earth ; and 
 at an elevation of six or seven miles in a balloon or on a 
 mountain he perishes. Fire quickly destroys, and molten 
 iron or boiling water quickly kills him. A constant tem- 
 perature of even 100° F. causes him great discomfort, if 
 not danger. Intense light is painful to him, intense noise 
 disagreeable. The same thing is more or less true of other 
 animals. They flee from fire, shudder or cry at great noises, 
 are often dazzled by intense light. 
 
 But while the environment thus narrowly limits and 
 controls human life and activities, man may, to a great 
 
IIYGIEKE A^'D SANITATION 301 
 
 extent, choose or control his immediate environment, — 
 a power which he shares with many other animals. The 
 beaver and the muskrat build rough shelters or houses, 
 many birds build nests, foxes dig burrows, and some birds 
 and other animals migrate from one environment to another. 
 J\Ian also seeks or makes for himself the shelter of caves, 
 huts, wigwams, houses, or hotels. These he heats if cold, 
 or cools if hot. He lights them by windows by day, and by 
 artificial light by night. He provides for himself, instead 
 of the ground, couches to lie upon, chairs to sit upon, and 
 tables upon which to place his food. All this is a kind 
 of artificial adaptation of the environment to man, and, if 
 wisely done, is an important aid to health, since it 'tends 
 to secure for the body that favorable environment which 
 is one of the greatest factors of health. ]\Ian can, more- 
 over, modify his surroundings at pleasure and make them 
 what they should be, or at least more nearly what the}' 
 should be. Fires are used in winter, shades and fans in 
 summer; ventilation replaces bad air with good air; by 
 proper drainage, filth and damjjuess are avoided ; by arti- 
 ficial lighting, darkness is done away with ; by cultivating 
 tlie soil and by raising cereals and cattle, the food supply 
 is maintained. Thus in many ways the environment is 
 controlled and healthful conditions are promoted or main- 
 tained. Failing all else, man may travel for a while and 
 find benefit in temporary change ; or, like thousands arriv- 
 ing in America, may abandon one environment and emigrate 
 to another. 
 
 8. Scope and Subdivisions of Hygiene and Sanitation. — 
 The considerations dwelt upon in the foregoing pages indi- 
 cate the scope and possibilities of the science of hygiene. 
 Given the constitution of any individual as it is at any one 
 time, we must seek to maintain or place that constitution 
 in a condition of health, or efficient working order, in two 
 ways : first, hy the proper care and operation of the mechanism 
 
302 THE HUMAN MECHANISM 
 
 itself, including the proper direction of its activities ; and 
 second, hy providing for it favorable surroundings or envi- 
 ronment. The former we call hygiene, the latter sanita- 
 tion. Each of these efforts reacts on the constitution ; 
 improper operation of the muscles in muscular work, or 
 improper use of the nervous system in mental work, may 
 " undermine " a strong constitution and lower its vital 
 resistance ; similarly, a bad climate, a neglect of the ven- 
 tilation of living and sleeping rooms, the use of polluted 
 water or milk or other food, exposure to the contagion of 
 disease or to excessive cold without proper clothing, — all 
 such failures to provide a proper environment may injuri- 
 ously affect the constitution or structure of the body. 
 
 It is impossible to draw any sharp line between the care, 
 management, and operation of the body mechanism, and the 
 care and control of the environment, and it is neither neces- 
 sary nor desirable to do so ; but we shall begin our detailed 
 study with those things which concern chiefly the care, use, 
 and operation of the mechanism itself, — such, for exam- 
 ple, as the proper direction of muscular and nervous activ- 
 ity; alimentation or right feeding; the use and abuse of 
 stimulants, narcotics, and other drugs; bathing, clothing, the 
 care of the eyes and ears, etc. Tliese matters which concern 
 chiefly the individual or the person constitute that part of 
 our subject known as personal hygiene. 
 
 We shall then proceed to consider those matters of 
 health which concern not only individuals but communities 
 of individuals, such as families, cities, states, and nations, — 
 for example, the site, ventilation, heating, aiid plumbing 
 of the dwelling house ; the control of food supplies, as to 
 their purity ; public supplies of water and milk ; sew- 
 age disposal ; the infectious and contagious diseases. All 
 these things require the cooperation of many individuals, 
 eitlrer as families, or as citizens of an entire town, city, or 
 nation. Hence they are classed under domestic hygiene and 
 
HYGIENE AND SANITATION 303 
 
 sanitation and public hygiene and sanitation. The exact 
 meaning of these terms will be explained in Chapters 
 XXVI and XXX. 
 
 It should be understood that the following statements 
 on the hygienic conduct of individual and social life are 
 not equally applicable to all who read them. For one, 
 those on muscular exercise are more important than those 
 on nervous strain ; for another, those on domestic hygiene 
 are more important than those on the hygiene of the per- 
 son. No attempt has been made to indicate the relative 
 importance of any part of the subject, either by the order 
 in which it is treated or by the amount of space devoted to 
 it. The application must be made by each reader for him- 
 self, with strict reference to himself and to the conditions 
 of his own life and environment. The principal desire 
 and aim of the authors is to persuade every one who 
 reads this book, not merely to study and to know himself 
 as a physical mechanism with the same earnestness and 
 thoroughness with which he would study a valuable watch, 
 a bicycle, a yacht, or an automobile, but also to use that 
 mechanism scientifically — i.e., intelligently, carefully, and 
 skillfully, — to the end that life may be longer, more use- 
 ful, and more enjoyable. 
 
PERSONAL HYGIENE 
 
 CHAPTER XVII 
 
 MUSCULAR ACTIVITY 
 
 A. The Ministry of Muscular Activity to 
 THE Body as a Whole 
 
 It is sufficiently obvious that it is through muscular 
 activity that we do many necessary, useful, or otherwise 
 desirable things ; and it is also a matter of common expe- 
 rience that muscular activity is required in order to build 
 up strong muscles. A very considerable amount of it is 
 required in order that the laborer may do his work, and a 
 similar amount is necessary in order that one may become 
 an athlete. But the effects of muscular activity on the 
 body as a whole are not so obvious ; while a large num- 
 ber of people think that it is " a good thing " and a smaller 
 number are convinced that it is absolutely necessary to the 
 best of health, yet we not infrequently hear men and 
 women seriously question the latter proposition and even 
 venture to doubt the truth of the former. 
 
 Now there is nothing in hj-giene more clearly estab- 
 lished than that muscular activity is absolutel)' essential 
 to healthy living. The effects of a sedentary life may not 
 show themselves at once, but almost without exception they 
 will assert themselves in the end. Muscular work, in other 
 words, not only enables us to influence our surroundings, 
 not only builds up strong muscles, but in other and equally 
 important though unseen ways ministers to the health of the 
 
 304 
 
MUSCULAR ACTIVITY 305 
 
 hody as a whole. It is the purpose of this section to present 
 this, the most important hygienic side of our subject, by 
 describing some of the physiological effects which muscu- 
 lar activity produces in the body, and the hygienic value 
 of each of these effects. 
 
 1. The Present Use of the Term "Muscular Activity." 
 — In the present chapter the term " muscular activity " is 
 used in a somewhat general sense, and without attempt- 
 ing to set sharp limitations upon it. Strictly speaking, 
 of course, muscular activity would include all work done 
 by the muscles of the body, and this is of various kinds. 
 Even those persons who do no manual labor unconsciously 
 perform muscular work ; the heart works on, the breath 
 comes and goes through orderly muscular contractions ; 
 sitting and standing, speech, gestures, mastication, — all 
 tliese things involve muscular activity, and do, as a matter 
 of fact, contriljute something to the maintenance of the 
 healthful conditions of the body. It is not improbable that 
 they are the physical salvation of thousands of people lead- 
 ing sedentary lives. At the other extreme are those wlio 
 perform severe manual labor, or who engage in vigor- 
 ous exercises or purposely cultivate exceptional physical 
 strength. 
 
 We are not, however, direct!}' concerned at present with 
 either of these extremes, nor with those forms of muscular 
 activity so common to-day in workshops where, hour after 
 hour, the workman performs the same task over and over 
 again. We are ratlier concerned with those forms of mus- 
 cular work which are seen in a lumber camp or on the 
 farm ; which jiresent the characteristic of variety and in- 
 volve the use of the musculature of the body as a whole ; 
 in short, those forms of activity by which until very 
 recently the human race has supported itself in its daily 
 life. Sucli things as brisk walking, running, rowing, wood- 
 chopping, swinnning, ten)us playing, would thus be placed 
 
306 THE HUMAN MECHANISM 
 
 in the same class, since they involve a use of the muscles 
 similar to those which we have mentioned. 
 
 2. The Physiological Effects of Muscular Activity and 
 their Hygienic Value. — We may now turn to the hygienic 
 value of the more important physiological effects of these 
 general muscular activities, leaving for subsequent consid- 
 eration exercises designed for special purposes, such as 
 much of our gymnasium work. 
 
 (a) The physical and chemical changes in the ivorking organ 
 are greater than those accompanying any other bodily activity. 
 The output of carbon dioxide by the body per minute is 
 increased at once from three- to tenfold with what would be 
 termed moderate or vigorous exertion,while digestion seldom 
 increases it more than one fifth, and mental work shows 
 practically no effect upon it. Large quantities of heat are 
 likewise liberated and the temperature of the muscle rises 
 several degrees. These physical and chemical changes are 
 mentioned first because the hygienic effects upon the body 
 as a whole are to be traced to them as the primary cause. 
 
 (b) As the result of these changes in the muscles new 
 physical and chemical conditions are introduced into the 
 blood and lymph. The excess of carbon dioxide is entirely 
 excreted by the lungs, so that the blood carried to the 
 other organs by the arteries shows no increase in this sub- 
 stance ; but other waste products (such as salts of sarco- 
 lactic acid), whose elimination requires the cooperation of 
 other organs than the lungs, are found in the arterial blood 
 in larger quantities than during rest. The chemical and 
 physical characteristics of the immediate environment of 
 every cell of the Vjody is thus changed, and profoundly 
 changed. Let us now consider the reaction of other organs 
 to these changes in the muscles and in the blood and 
 lymph. 
 
 (c) Some of the most striking effects of muscular work are 
 those which are connected with the heat-reyidating mechanism. 
 
MUSCULAR ACTIVITY 307 
 
 The large liberation of heat by the working muscle neces- 
 sitates active measures to get rid of that heat and main- 
 tain the constant temperature. The small arteries of the 
 skin dilate, while those of internal organs constrict, per- 
 spiration is secreted, and all these processes are carried 
 out in a coordinated manner. The nervous mechanism of 
 heat regulation is given a new form of activity, and thus 
 receives valuable training in adjusting itself to the chan- 
 ging conditions with which it has to cope in daily life. 
 
 (d) Closely connected with the fo7-egoing is the (t.emporary) 
 relief afforded to any congestioyi of blood in the internal 
 organs. Sedentary occupations usually involve more or 
 less overfUlLng of the blood vessels of the stomach and 
 intestine, the pancreas, the liver, the spleen, and the kid- 
 neys ; they also involve the absence of those movements 
 of the trunk whose pumping action affords a marked assist- 
 ance to the flow of blood through the abdominal organs 
 (p. 147). The congestion thus caused is not a good thing ; 
 it almost certainly renders the organs concerned more lia- 
 ble to inflammatory processes (see Chapter XXI), and, if 
 there has been established any tendency to catarrhal con- 
 ditions (see p. 381), it aggravates that tendency. Popular 
 experience has long associated with health a good color of 
 the skin ; and, while it is not safe to make such an infer- 
 ence in all cases, pallor very frequently means internal con- 
 gestion, unhealthy digestive functions, and greater liability 
 to cold in the head or the chest. 
 
 (f) Muscular activity is the only thing which can he 
 depended ttpon to increase the ivork of the heart. While this 
 fact makes caution and moderation necessary for persons 
 having certain forms of heart disease, yet for the vast 
 majority of people it is of the greatest hygienic importance 
 to accustom the heart to reasonably hard work. Only in this 
 way does it receive the training necessary for its proper 
 development and for the maintenance of its strength. 
 
308 THE HUMAN MECHANISM 
 
 Emergencies will arise when the heart is called upon for 
 severe effort, brief or prolonged. The familiar example 
 of the sudden " sprint" for a car is a case in point; and 
 there are times, as in pneumonia, when the issue in sick- 
 ness is largely determined by the endurance of the heart. 
 In too many such cases, if the patient escapes the fatal 
 issue, it is only with a permanently weakened heart. 
 
 It is important not only that the heart should be kept 
 ready for emergencies but also that it be kept in condition 
 for vigorous work as a regular duty of daily life. One of 
 the worst of " vicious circles," as physicians call them, is 
 the acquirement of a weakened heart by abstention from 
 proper muscular exertion, and, as a consequence of this 
 weakened heart, increasing disinclination to exertion of 
 any kind whatever. The failure to take proper exercise 
 leads to deterioration in strength and endurance on the 
 part of the heart; and this cardiac deterioration, with the 
 resulting discomfort of breathlessness, leads in turn to 
 abstention from muscular activity. 
 
 (/) Muscular exercise is the one agent zvhich increases the 
 depth and frequency of the respirator)/ movements. The 
 hygienic importance of this does not lie in the better oxi- 
 dation of wastes, since, so far as we have any accurate 
 knowledge on the subject, it would seem that the processes 
 of respiration during sedentary life more than supply the 
 existing demands of the tissues for oxygen. The increased 
 respiration is rather of importance because of the secondary 
 effects of the respiratory movements in promoting the flow 
 of blood, and especially the flow of lymph (see p. 147). 
 
 It is probable that the " freshening effects " of muscu- 
 lar exercise are to a very large extent attributable to the 
 improved lymph circulation in the tissues, and this effect, 
 it will be remembered, is felt in the immediate environ- 
 ment of almost every cell in the body. The suction action 
 of inspiration quickens the lymph flow from all organs 
 
MUSCULAR ACTIVUIY 3UU 
 
 outside the thorax (p. 173), and the increased pumping 
 action of the respiratory movements themselves aids the 
 lymph flow from the lungs and other organs within the 
 thorax. Waste products are more completely removed 
 from the lymph spaces surrounding all cells, and thus one 
 of the most important of fatigue conditions is relieved (see 
 p. 59). Where lymphatics are subject to the pumping 
 action of contracting muscles and of the alternate flexion 
 and extension of joints, the suction action of the respira- 
 tory movements is reeaforced. This pumping action espe- 
 cially affects the lymphatics of the arms and legs, and 
 those of the abdominal cavit)- (through the action of the 
 diaphragm and the trunk movements). 
 
 The increased respiratory movements also contribute to 
 greater mobility of the ribs and to the better ventilation 
 of the lungs. During vigorous exercise all lobes of the 
 lunos are used, and tlie dangers attendant ui)On disuse of 
 the apical lobes (p. 172) are largely obviated. 
 
 (g) 3Iodcrati' exercise e.rerts a favorable cjTect vpon the 
 dif/estive organs, aUliougli the precise action inv(.)lved is 
 very complicated. Here also it improves the lymph flow, 
 thus promoting absorption and producing better conditions 
 in all digestive glands and in the muscular apparatus of 
 the digestive tract ; it prevents continued congestion and 
 the unfavorable attendant conditions. It is probabl}' also 
 a direct stimulus to jieristalsis, for unquestionably the 
 exercises which involve movements of the trunk often 
 prove a peculiarly eiTicient remedy for constipation. 
 
 The above summary is very far from a complete enumer- 
 ation of tlie effects of muscular exercise upon the organ- 
 ism, but it will suffice to show how essential an element 
 such exercise is in the life of the body. The training of the 
 heat-regulating mechanism, the training of the heart, the 
 improved lymphatic environment of every cell resulting 
 
310 THE HUMAN MECHANISM 
 
 from increased breathing movements and from the pumping 
 action of mechanical motion, the relief of internal conges- 
 tions and the favorable influence upon digestive functions, 
 — all these things are fundamental to healthful cell life. 
 
 3. Muscular Activity a Necessity for All. — We often 
 hear of men and women who live to old age and do 
 large amounts of mental work with seemingly little or 
 no muscular activity ; and it is sometimes suggested that 
 the experience of these people proves that exercise is un- 
 necessary. There are also on record a few cases of men 
 who can drink large quantities of whisky without getting 
 drunk ; but it will not be contended that most men can 
 do likewise. As to any line of right hygienic conduct 
 there are some among the hundreds of millions inhabiting 
 the earth who can do the reverse with impunity ; but they 
 are not to be taken as safe guides. The cases are very 
 few indeed where abstinence from muscular activity per- 
 sisted in as the rule of life is without disastrous results ; 
 the bad effects do not always come in a day or a week or 
 a year, but sooner or later they almost invariably show them- 
 selves. We must never fail to distinguish carefully between 
 the immediate and remote effects of any line of conduct ; and 
 nowhere is this caution more needed than in observing the 
 effects of a sedentary life, the evil results of which, though 
 sometimes long postponed, usually appear sooner or later. 
 
 Some muscular exercise is a hygienic necessity for every 
 period of life ; it belongs to no one age. Youth is the time 
 when athletic sports, games, and all kinds of activity are 
 most agreeable, most necessary, and most enthusiastically 
 pursued. In old age the changes which take place in the 
 arterial walls necessitate caution as to severe exertion. 
 But these are only the extremes. Rarely indeed do we 
 meet with people who would not be benefited by a walk 
 of several miles a day, at a rate of three or four miles 
 an hour ; and it cannot be too strongly insisted that the 
 
MUSCULAR ACTIVITY 311 
 
 inability to do this with enjoyment and profit is in almost 
 every case because the habit of taking exercise is not kept 
 up. The heart is not as strong as it once was ; the con- 
 nective-tissue elements of the muscles, the ligaments, etc., 
 become sore upon taking exercise, not because of any inev- 
 itable " old-age change," but because the ability to do the 
 work easily has not been maintained by constant practice. 
 
 It would be amusing, if it were not sad, to see how the 
 average adult American will try almost everything which 
 holds out the slightest promise of maintaining some sort 
 of health rather than take muscular exercise, — alcoholic 
 drinks (to dilate cutaneous vessels), Turkish baths, mas- 
 sage, patent medicines, — anything rather than a horse- 
 back or bicycle ride, or a brisk walk, or some other simjjle 
 and perfect remedy which stands within easy reach. It 
 is not to be expected that when these exercises are first 
 tried after years of sedentary life, they will be enjoyed ; 
 and too often the man or woman, instead of persisting 
 patiently, draws the conclusion that the time for such 
 things has gone and only resignation to old age is in order. 
 When young men and women begin life, it should be with 
 a clear conception of the danger of falling into habits of 
 muscular inactivity, and with a conscious and strong deter- 
 mination to avoid this danger. 
 
 4. The Conservation of the Enjoyment of Muscular 
 Activity. — Muscular activity is so necessary for health, 
 for the enjoyment of life, and for usefulness, that the ability 
 to take it should be conserved at all costs. We should 
 not only keep " in practice " by making it as much a daily 
 habit as eating or sleeping, but we should also avoid those 
 unfavorable conditions which interfere with our enjoyment 
 of it. Some will not walk a step more than necessary 
 because, by the use of improper shoes, they have acquired 
 deformed feet, unable to support the weight of the body ; 
 sometimes a sunstroke, following incautious exposui'e to 
 
312 THE HUJIAN MECHANISM 
 
 the hot sun, leaves the heat-regulating mechanism so 
 injured that muscular exertion except in cool weather 
 becomes unsafe or even dangerous ; exposure to dampness 
 often brings rheumatism, an almost insuperable barrier to 
 pleasurable movement of any kind ; some infectious dis- 
 eases leave their trace in the form of an incurable organic 
 weakness which makes muscular activity inadvisable. 
 These things should, of course, be avoided for tlieir own 
 sake ; they should be avoided also because of their seri- 
 ous indirect effects on health. 
 
 5. General Character of the Most Useful Exercises. — 
 To specify the exact forms or amounts of muscular 
 exercise advisable would take us beyond the scope of 
 the present work. Here as elsewhere the student must 
 work out his own salvation. In the following cliapters we 
 shall discuss, as far as possible, the characteristics of some 
 special exercises; for the present a few general sugges- 
 tions may prove useful. The muscular activity which 
 formed part of the life of our ancestors may be described 
 as generally moderate, though at times vigorous or hard ; 
 only exceptionally did it involve extreme endurance or 
 great muscular strain. Our ancestors were not, as a rule, 
 given to " tugs of war," or to putting up heavy dumb- 
 bells, or to making inordinately long runs, or to " giant 
 swings " in the gymnasium ; nothing like a hundred-yard 
 dash or a four-mile boat race was a common occurrence 
 among them. Where work of this kind had to be done it 
 was left to those who, by reason of exceptional strength, 
 were especially fitted for it ; mankind as a whole did no 
 such work, and it is not necessary (or even advisable) for 
 most of us. 
 
 Nor can it be claimed that the cultivation of great mus- 
 cular strength was a common practice. There was a much 
 higher average of strength than among us, and we should 
 probably be better off were our average higher than it is ; 
 
MUSCULAK ACTIVITY 313 
 
 but if we can judge at all from the history of mankind, 
 sucli training as that required to break some college 
 strength test is not demanded for hygienic purposes. Nor 
 does our own experience tell a different story. Very strong- 
 men are no healthier nor longer-lived than those of only 
 average strength, and, in general, the athletic ideal is not 
 the hygienic ideal. It is not necessarily unhygienic, but it 
 is not required for purposes of health. 
 
 It is not desirable that exercise taken for general 
 hygienic purposes shall be unduly fatiguing. A moderate 
 amount of fatigue is not unwholesome, since fatigue brings 
 with it the desire for rest ; nor is fatiguing exercise neces- 
 sarily harmful. But exei'cise need not necessarily be of 
 this character ; and, in view of the other work of life, it is 
 certainly better to avoid undue fatigue, especially when 
 we cannot rest well afterwards. A walk of six or eight 
 miles will do more good than one of forty or hfty. 
 
 6. Exercise for Women. — Muscular exercise is no less 
 essential to the health of women than of men. Fortunately 
 the day is past when false standards misinterpreted tlie 
 truth that woman's most natural sphere in life is the home 
 to mean that, tied down to the confining duties of house- 
 hold life, she should never know the joy of movement, 
 except in dancing (and sometimes not even in that) ; and 
 then proceeded to make sure of the result by clothing 
 her in narrow, pointed, high-heeled shoes, heavy skirts, 
 and ticrht-lacin"- corsets. The reaction from this state of 
 affairs, at times going to the opposite and undesirable ex- 
 treme, has unhappily at times produced in women exhib- 
 itions of mannishness which once led a lady to speak of 
 " that terrible thing called muscular exercise." But dis- 
 gust with these grotesque but avoidable consequences 
 should not be allowed to blind us to the fact that a reason- 
 able enjoyment of daily muscular activity is as much a 
 necessity for women as for men. 
 
314 THE HUMAN MECHANISM 
 
 B. General Muscular Exercise 
 
 The present section deals with the use of muscular activ- 
 ity for its hygienic effect upon the body as a whole ; the 
 next section with its employment for special purposes. 
 Exercises undertaken for their general good effect are 
 frequently spoken of as "hygienic"; the term is, how- 
 ever, objectionable, and we shall speak of them as general 
 muscular exercises. 
 
 General muscular exercise is of hygienic value because 
 it produces the physiological results which have been 
 enumerated in the preceding section, — results which have 
 been shown to constitute essential elements of the normal 
 internal environment of the cells of the body. To review 
 the separate oifices of this ministry to the normal condi- 
 tions of the body : 
 
 1. General exercises should produce to a considerable 
 extent those physical and chemical changes which accom- 
 pany muscular contraction, with the resulting effects upon 
 the physiological condition of the muscle itself and upon 
 the general internal environment, the blood and lymph. 
 
 2. They should exercise, and so train, the heat-regulat- 
 ing mechanism. 
 
 3. They should tend to relieve vascular congestion in 
 internal organs, bringing the blood in larger quantities to 
 the skin. 
 
 4. They should afford training to the heart. 
 
 5. They should increase the ventilation of the lungs. 
 
 6. They should increase the flow of lymph in the lym- 
 phatics, and thereby improve the environmental conditions 
 of all the cells in the body (see Chapter IV). 
 
 7. They should exert a favorable influence upon the 
 digestive processes, promoting proper secretion and ab- 
 sorption, and tending to prevent unhealthful conditions 
 leading to constipation. 
 
MUSCULAR ACTIVITY 315 
 
 Such being the physiological ends sought for, we may 
 conclude, as to the character of such exercises : 
 
 1. They should consist of rhythmic rather than of sus- 
 tained coyitr actions. These involve less fatigue, are more 
 enjoyable, and especially facilitate the flow of blood and 
 lymph. 
 
 2. They should be vigorous, somewhat prolonged, and 
 should usually be continuous. A brisk walk or a run meets 
 most demands ; so do bicycling and many games. The 
 strolls or saunters which are too frequently mistaken for 
 exercise do not meet the reasonable hygienic demands 
 of the body; they involve only an insignificant increase of 
 chemical activity in the muscles, they hardly affect res- 
 piration, they do not train the heart ; in short, they do 
 not produce adequate physiological effects to accomjjlish 
 hygienic ends. 
 
 3. Tliey should involve considerable movetnent on the part 
 of the trunk as well as the limbs. Many excellent forms 
 of exercise, such as bicycling, are somewhat deficient in 
 this respect. It is not meant that sudden and violent 
 trunk movements are called for, but that hygienic ex- 
 ercise should bring full change and relief from the con- 
 strained positions of the trunk imposed by the sedentary 
 occujDations of modern life. A vigorous walk, with its ac- 
 companying increase of breathing and trunk movements, 
 fencing, and games which involve the throwing and catch- 
 ing of a ball are especially good in this respect. 
 
 4. TItey should he accompanied by full and free respira- 
 tion. The importance of this recjuirement needs no com- 
 ment. Constricting clothing should not be allowed to 
 interfere, and, as far as possible, the trunk should be held 
 erect, the neck and shoulders back, so as to permit the 
 freest movement of the upper ribs. 
 
 5. It is advisable not to confine oneself wholly to one 
 form of exercise. Similar considerations to those which 
 
316 THE HUMAN MECHANISM 
 
 hold in the choice of food apply to some extent to exer- 
 cise. At the same time it must be admitted that per- 
 fect health can frequently be maintained to old age by 
 using only one kind of general exercise, such, for example, 
 as walking. 
 
 7. Considerations concerning Fatigue. — The relation of 
 general exercise to fatigue is a matter of considerable 
 interest and also of importance in correlating muscular 
 work with the other work of life. Fatigue of the whole 
 organism is a very complicated matter and involves much 
 more than the total amount of chemical change in the 
 muscles and of the resulting waste products in the system 
 as a whole. We may, for example, be made very tired by 
 unpleasant sensations from the joints and tendons, or by 
 walking in shoes which do not permit free play to the 
 bones, ligaments, tendons, and muscles of the foot; and 
 this even when the amount of muscular exertion involved 
 may have been slight. It is well known that merely stand- 
 ing still for a time will frequently cause more fatigue than 
 will a longer time spent in walking. 
 
 Again, some forms of exercise throw a relatively large 
 share of the total work on some muscle or small group of 
 muscles, while others distribute the total work more evenly 
 over larger groups. Walking and running are very unlike 
 in this respect; in the former the weight of the body must 
 be lifted from the ground with each step, — especially 
 when we walk very erect, — by the extensor muscles of 
 the leg, and chiefly by the extensors of the ankle joint ; 
 running, on the other hand, consists in a continual falling 
 forward and the I'estoration of equilibrium by a more gen- 
 eral action of the muscles of the body as a whole. A walk 
 of four and a half miles an hour is much more fatiguing 
 to a person in good training than a run of four and a half 
 miles an hour, because in the former case a few muscles 
 are thrown into very vigorous contraction and so give rise 
 
MUSCULAE ACTIVITY 317 
 
 to severe local sensations of fatigue, sometimes accom- 
 panied by cramps in the muscles. 
 
 8. Some Examples of General Exercises : Cycling. — 
 
 Bicycle riding is remarkable for distributing the total 
 work over large numbers of strong muscles, so that the 
 amount done by each is relatively small ; consequently, 
 where there is but little hill-climbing' or no strono- head 
 winds, local fatigue is but slight, although the total work 
 done by the body is considerable. Actual measurements 
 of the carbon dioxide excreted have shown that this is 
 much greater per minute in a ride of eight miles an hour 
 on a smooth level track, than in walking three and a half 
 miles an hour ; in other words, the total work is greatei-. 
 The well-known increase of perspiration brought about 
 by such moderate riding points in the same direction ; the 
 chemical changes in the body are greater and so is the 
 associated heat production ; and yet any cyclist knows 
 that the conscious fatigue of the ride is as nothing com- 
 pared with that of the walk. Moreover, in wlieeling tlie 
 weight of the body is not supported on the feet, and we 
 are thus to a large extent relieved from the unpleasant 
 sensations produced by pressure and jar in the ankle and 
 knee joints. It is a characteristic of moderate or even 
 fairly vigorous bicycle riding that it produces a maximum 
 of chemical chano-e with a minimum of fatigfue. This is 
 of great practical importance. The larger production of 
 carbon dioxide involves deeper breathing, and, as the 
 student now well knows, increased work on the part of 
 the heart. 
 
 Within proper limits this is, of course, good for the 
 heart ; there is some danger, however, that in the absence 
 of conscious fatigue we may throw upon that organ more 
 work than is good for it, and medical experience leaves no 
 doubt that many cases of injury to the heart have resulted 
 from injudicious cycling ; that is to say, from " scorching " 
 
318 THE HUMAN MECHANISM 
 
 against strong head winds and in " showing grit " by refus- 
 ing to get off and walk up very steep hills. There are 
 occasions when it is not wise to be too ambitious, and when 
 " discretion is the better part of valor." 
 
 9. Some Examples of General Exercises : Games. — Some- 
 what similar considerations apply to most of our more 
 active games, such as basket-ball, football, tether-ball, 
 hockey, polo, etc. They are perfectly safe for healthy 
 people when not played more vigorously than the training 
 of the heart justifies ; the fact that there is an element of 
 danger in them is no reason why they should not be used, 
 but it is a very good reason why they should not be worked 
 to extremes, and especially why we should be sure, from 
 competent medical advice, that there is in those who play 
 them no organic trouble to begin with, and that players 
 are in good training when they play most intensely. 
 
 The choice of the kind of muscular work and exercise 
 involves so many considerations other than those which 
 are strictly physiological and hygienic that it is impossible 
 to give in an elementary treatise like this any detailed 
 discussion of the special merits and defects of each. We 
 often have other aims in view besides the purely hygienic ; 
 thus the group games, such as football, baseball, basket- 
 ball, hockey, etc., train the spirit of cooperation and may 
 be made useful means of moral training. In camping in 
 the woods canoeing is not simply a means of exercise, but 
 also a means of transportation ; and under other conditions 
 the same thing is true of horseback riding, rowing, etc. 
 "Woodchopping, digging, porterage, and plowing are valu- 
 able means of livelihood. It is believed, however, that 
 the principles here given will help the individual to form 
 a correct judgment as to whether his work in life supplies 
 him incidentally or inevitably with the needed general 
 muscular activity for hygienic purposes, and, if it does 
 not, to plan to meet the want intelligently. 
 
MUSCULAE ACTIVITY 319 
 
 The combination of muscular exercise with some other 
 pursuit is highly desirable, and when practicable often 
 simplifies the hygienic conduct of life. But it is nothing 
 short of a hygienic misfortune to lose the youthful love of 
 activity for its own sake. It is well as we glow older to 
 have golf, or a horse to be exercised (!), or a fishing pre- 
 serve in the woods, to " take us out in the open air " and 
 make us use our muscles. But a human being who is 
 dependent upon something of this kind to drag him into 
 activity cuts a sorry figure from a moral standpoint. 
 Man's highest distinction is the fact that his actions may 
 arise so largely from processes of jjsychic life within rather 
 than from some immediate stimulus from without. The 
 l^roper hygienic conduct of life involves moral fiber as 
 well as physical fiber, and tliis is especially true of that 
 absolutely essential part of hygienio conduct which de- 
 pends upon the use of organs like the skeletal muscles, 
 which are so largely subordinate to the commands of 
 the will. 
 
 10. Importance of Walking as a Means of Exercise. — 
 In their enthusiasm for athletic games and outdoor sports 
 in youth, and for other outdoor activities in middle life, the 
 American people are always in danger of losing their love 
 for the various forms of walking, such as tramping and 
 mountain climbino-. Walking- is the one form of general 
 exercise for sound people which can always be had for the 
 taking. For this reason, if for no otlier, it should ever be a 
 part of all sound physical training to conserve the love of 
 tramping and the abilitj' to walk. Apart from the obvious 
 fact that it is in this way that we can get closest to nature 
 and the real beauty of the world in which we live, the 
 possession of the love of the activity involved is one of the 
 most precious possessions of our hygienic life. The man 
 or woman who does not keep and improve this power by 
 use must look forward to the same fate as the servant in 
 
320 THE HUMAN MECHANISM 
 
 the parable who hid his talent in a napkin, only to have 
 it taken from him in the end. 
 
 11. Fresh Air not a Substitute for Muscular Activity. — 
 
 A word of warning is needed against the folly of sup- 
 posing that fresh air is a substitute for muscular activity. 
 Fresh air is one of our greatest hygienic blessings, and it 
 is very desirable to live an outdoor life as far as possible. 
 But too many think that lounging in the shade, or riding 
 in the open air in an automol)ile, a carriage, or an electric 
 car, does for them what muscular exercise alone can do. 
 Especially as age creeps over us and the love of activity 
 wanes from its disuse, more and more does the idea grow 
 upon us that " fresh air " is everything. To many the 
 possession of a comfortable carriage and a pair of thorough- 
 breds is a misfortune. At one of our most beautiful sum- 
 mer resorts some one said to a local physician, " Medical 
 practice at such a place as this must be very unremunera- 
 tive." "By no means," replied the man of experience; "peo- 
 ple come here where they are tempted to overeat ; in the 
 place of exercise they lie back on the cushions of their car- 
 riages wliile they are driven about ; their adipose tissue 
 increases rapidly, and very soon it is true that to no class of 
 people is the doctor so absolutely essential as to them." The 
 student can easily make the application for himself. Indi- 
 gestion, fatt}' degeneration, insomnia, loss of appetite, nerv- 
 ous prostration, and kindred ills rarely come to those who 
 labor with their hands ; and these ills can be largely pre- 
 vented, even in those who must engage in sedentary occu- 
 pations, by a wise and intelligent conduct of the physical 
 life, and especially by the daily employment of an hour or 
 so of vigorous general muscular activity properly corre- 
 lated with the other work of life. 
 
MUSCULAR ACTIVITY 321 
 
 Q. Muscular Exbecises for Special Purposes. 
 Corrective Work. The Gymnasium 
 
 The muscles may be used not only to produce those 
 general influences which are necessary to the maintenance 
 of health but also to produce desirable special effects, 
 among which the prevention and correction of faulty car- 
 riage and action are of great importance. In considering 
 the use of muscular work for this purpose our subject 
 naturally groups itself under two main divisions : first, 
 faults of form or carriage of the body at rest, — in other 
 words, a bad figure ; and second, faults of handling the 
 body while it is in motion, — in other words, a\vkT\ardness 
 or clumsiness. 
 
 12. The Shape or " Figure " of the Body. — The human 
 body may be chiseled in marble or molded in bronze, 
 and the statue thus formed may recall to the mind the 
 shape or figure of the person it represents. But the shape 
 of the living body is not rigidly fixed, as is that of the 
 statue. The bony skeleton is sometimes called a frame- 
 work, which supports the nuiscles, viscera, skin, etc. While 
 this is to some extent true, the organs are not rigidly sup- 
 ported by the skeleton, as the canvas is supported by the 
 poles and ropes which constitute the framework of a tent. 
 In other words, the bones of the skeleton are not i-igidly 
 joined togetlier ; they do not of themselves make a self- 
 supporting framework; the strong ligaments which pass 
 from one bone to another simply limit or guide the move- 
 ment of the l3ones (p. 16) ; they do not, strictly speaking, 
 bind them together. If all organs save the bones and liga- 
 ments were removed, the skeleton would collapse. It is 
 itself held upright by the muscles, which determine what 
 position the bones shall have with regard to one another; 
 and it is more correct to say that the muscles support the 
 skeleton than that the skeleton supports the muscles. 
 
322 
 
 THE HUMAN MECHANISM 
 
 D- 
 
 FiG. 105. Diagram 
 showing the action 
 of antagonistic 
 muscles which Iteep 
 the body erect. 
 After Haxley 
 
 Arrows indicate the di- 
 rection of tlie pull, 
 the feet serving as 
 a fixed basis of sup- 
 port. The muscles 
 A, B, II, and Ckeep 
 the l.iody from fall- 
 ing forward; D, K. 
 F,HT\i\ G keep it from 
 falling backward 
 
 13. Round Shoulders as a Type of 
 Faulty Carriage. Their Cause The 
 
 carriage of the shoulders well illustrates 
 the closing statement of the last para- 
 graph. Some people have square, while 
 others have sloping, shoulders ; in some 
 the shoulders are held back so that the 
 upper portion of the back is approxi- 
 mately flat, while in others they droop 
 forward, thus causing the upper chest to 
 be more or less contracted and the back 
 "round." To some extent these differ- 
 ences maybe due to hereditary structure; 
 but they result, for tlie most part, from 
 causes which are largely if not entirely 
 under individual control. There is little 
 or no excuse for round shoulders in 
 healthy people, and the marked effect of 
 training is evident in the fine bearing of 
 well-trained soldiers. The truth of this 
 statement is seen when we consider how 
 the deformity is usually acquired, the 
 chief causes being the following. 
 
 (a) Faulty Posture. — Round shoul- 
 ders are uncommon among people whose 
 work requires an erect carriage of the 
 body; for example, among those who 
 carry things upon the head. With most, 
 however, the occupations of daily life 
 lead to bending forward over work ; 
 writing, drawing, sewing, lifting, gar- 
 dening, paving, machine and tool work 
 at once occur as examples. The trunk 
 is held in such a position that the 
 shoulders tend to fall forward of their 
 
MUSCULAR ACTIVITY 
 
 323 
 
 own weight. This tendency is aided by the wrongly curved 
 backs of most chairs, — which seem as if planned especially 
 
 Fig. 106. Some of the muscles of the back 
 
 On the left side are shown tliose immediately under the skin ; hy dissecting away 
 this first layer, there are exposed the muscles shown on the right side 
 
 to force the shoitlders forward, — and in boys by the use 
 of many forms of suspenders. 
 
 (b) Improper Balance in the Phii/ of Antac/07nstic Muscles. 
 — The position of the shoulders with reference to the ribs, 
 
324 
 
 THE HUMAN MECHANISM 
 
 vertebral column, and breastbone is largely dependent upon 
 the action of several groups of antagonistic muscles, the 
 most important of which are those of the breast and those 
 of the back. Figures 106 and 107 show the general antag- 
 onistic action of these muscles. The contraction of the 
 great breast (or jjectoraJ) muscle pulls the shoulder for- 
 ward and nearer the breastbone ; the contraction of the 
 back muscles (rhomhoideus, trapezius, and others) pulls them 
 backwards and nearer the backbone. Both groups of mus- 
 cles are kept in a state of sustained moderate contraction 
 (or tone) by the nervous system ; but if the back muscles 
 relax, while those of the pectoral group remain in tonic 
 contraction, the shoulder will be pulled forward and the 
 back will be round. Obviously tlie maintenance by the 
 nervous system of the proper balance in the action of these 
 and other antagonistic groups of muscles is essential to 
 correct carriage of the shoulder. 
 
 (c) Deficient Use of tJie Back 31uscles, with or without the 
 Excessive Use of the Breast 3Iuscles. — Most occupations and 
 
 activities involve greater 
 use of the breast muscles 
 than of the back muscles. 
 (Striking a blow with a bat 
 or an ax, throwing a ball, 
 and similar actions are 
 jnore usual than acts, like 
 pulling taffy, which extend 
 tlie arms and draw the 
 shoulder blades closer to- 
 ge the r. Movements of 
 the first kind ol)viously 
 strengthen the breast and 
 stretch the back muscles ; 
 those of the second kind have the opposite effect. Conse- 
 quently any marked preponderance of pectoral action tends 
 
 Ftc. 107. The .skeleton of the trunk 
 .seen from above. After Demeny 
 
 .Showing the ant:ii,^niiistii- phiy iipnii llie 
 shoulder of the muscles of the hreast 
 (a) and Ijack {b) 
 
MUSCULAR ACTIVITY 
 
 325 
 
 to elongate the back muscles; and unless this is counter- 
 acted by movements of the opposite character, which stretch 
 the breast muscle, the pectoral and Imck groups become 
 " set," as we may express it, in improper relative lengths. 
 
 Fill. 108. Correct and incorrect pusiliuns of the shoulder girdle. 
 After Denieny 
 
 The result is round shoulders. Consequently one of the 
 most important things to have in view in gymnastic work 
 is the use of movements wliich train the back muscles and 
 stretch the pectorals, thus counteracting the effect of the 
 one-sided use of these two groups of muscles in ordinary 
 occupations. 
 
 14. The Period of Growth Especially Favorable for the 
 Acquisition of Round Shoulders and Other Deformities. — 
 The length of a growing muscle is determined largely by 
 the distance between its origin and insertion^ during the 
 period of growth. Tlie breast muscle will grow to be a 
 longer muscle when the shoulders are held back by the back 
 muscles than when they are habitually allowed to droop 
 forward. In the former case the pectorals grow to sufhcient 
 length and do not tend to pull the shoulders forward and 
 downward ; and we avoid the excessive length of the back 
 muscles, which makes it necessary for them to take xfp their 
 own slack before they can keep the shoulders in position. 
 
 1 Where a muscle is attached by its two tendons, the point of attach- 
 ment against which it usually pulls or is tixed is known as its origin, 
 while the one it usually moves is known as its insertion. Thus the origin 
 of the pectoral muscle is the breastbone and ribs, its insertion the shoulder 
 and the upper arm. 
 
326 THE HUMAN MECHANISM 
 
 The student can now appreciate the fact that it is in 
 youth, during the period of growth, that deformities are 
 most readily acquired and most easily corrected; for the 
 muscles, the ligaments, the bones, are then in their forma- 
 tive stage. In the case in point, if the boy or girl holds 
 the shoulders properly, the pectoral and back muscles of 
 each side adjust themselves to their proper length ; and the 
 shoulders grow into the correct form, just as the sapling 
 which is not bent nor deprived of proper sunlight grows 
 into the symmetrical, beautiful tree. During the period of 
 growth, then, — say up to at least the twentieth year, — 
 we can hope to accomplish most in correcting and especially 
 in preventing deformities. The correction and prevention 
 of round shoulders evidently depends upon the proper 
 training and use of the muscles which play upon the 
 shoulder ; it is therefore a legitimate part of gymnastic 
 training, for gymnastic training is largely the art of learn- 
 ing to use the muscles properly. 
 
 Where there is a special defect to remedy or prevent, 
 special exercises are required. These are of the general 
 character of the " setting-up " drill of the soldier, and in 
 the case in point we accomplish our purpose by using 
 movements which in the first place stretch the pectorals 
 and even overextend them ; in the second place, give to 
 the back muscles the exercise which they fail to get in 
 our ordinary occupations, and so bring up their strength, 
 their ability to withstand fatigue and to maintain the tonic 
 contraction demanded of them ; and which, in the third 
 place, give us the knowledge of the correct position of the 
 shoulders. 
 
 15. Education of the Consciousness of Correct Posture. — 
 In explanation of the last point we may say that when 
 one habitually carries the shoulders properly he feels that 
 he is taking an awkward position when he allows the 
 shoulders to droop ; on the other hand, the man who 
 
MUSCULAE ACTIVITY 327 
 
 habitually allows the shoulders to droop forward feels that 
 he is in an unnatural position when he holds his shoulders 
 back. This is largely because in the first case the back 
 muscles and in the second the pectorals must be put on a 
 stretch ; it is also due to the fact that the sensations 
 derived from the habitual posture, whether it be correct 
 or incorrect, have impressed themselves on consciousness 
 as signs of the normal conditions ; to take any other posi- 
 tion is to experience the feeling of something unusual or 
 abnormal. 
 
 We learn of the position of jsarts of our body with 
 reference to one another by sensations derived from the 
 muscles, tendons, joints, etc. (see Chapter XIV, p. 262); 
 and the sensations of position which result when we 
 assume the habitual j^osture fix themselves in our thought 
 as signs of the normal posture. Our jtractical, working 
 idea of normal posture, indeed, is nothing more nor less 
 than our exjjerience of the sensations of position resulting 
 from habitual posture. The man wlio never carries his 
 shoulders back really knows nothing of their correct posi- 
 tion, because the sensations from correct posture are lack- 
 ing ; he knows no more about them than a man blind from 
 his birth knows of the color of a landscape. One of the 
 first steps in correcting this and similar faults must be to 
 experience the muscular sensations which come from cor- 
 rect carriage ; and the more frequently these sensations are 
 experienced, the better does the subject become acquainted 
 with them, the more likely are they to replace his erro- 
 neous judgment. 
 
 It is only throngli the sense of position that we can hope 
 to acquire the practical working knowledge of correct 
 carriage. What we learn by reading about the matter or 
 by looking at pictures or statues of the correct figure is 
 of little use ; for such ideas come to us only through the 
 eye, and we obviously cannot depend on our sense of vision 
 
THE HUMAN MECHANISM 
 
 to inform us whether we are carrying ourselves properly 
 or not. We do not " see ourselves as others see us " ; gen- 
 erally we do not " see ourselves " at all. It is only the 
 sense of position that is capable of reminding us the instant 
 we go wrong; and this sense can be trained pioperly only 
 by actually assuming the correct posture. 
 
 16. The More Important Faults of Form and Carriage. 
 — We may now pass to the consideration of the more 
 important deformities, which it is the aim of sf)ecial mus- 
 cular exercises to prevent or correct. 
 
 («) The failure to hold the neck erect (allowing it to bend 
 forward). — Tliis results naturally from the fact that the 
 weight of the head will do this, pro- 
 vided the tendency is not corrected 
 by the proper training of the muscles 
 of the liack of the neck and trunk. 
 The position of the head usually 
 taken in reading, sewing, etc., is 
 another cause of this bad habit. 
 
 (/') Rouiul or stoop shoulderss. — 
 These defects have already Ijeen suf- 
 ficiently dwelt upon (p. o'-'l). 
 
 {(■) Too great hacktvard (dorsal) 
 
 ^ ,„, , , „ convexitii of the s/iiite in the thoracic 
 
 -Fig. 109. The results of . ' ; ^ , 
 
 proper (1) and improper ^effion. and too (jreat forivard {ventral) 
 
 (2) carriage of tiie ver- coneexity of the spine in the ahdomlned 
 
 tebral column. After region. — A certain amount of such 
 
 Demeny . , • i 
 
 curvature is normal m these regions 
 (see Chapter II) ; but there is usually a tendency to exces- 
 sive curvature because of the weight of the parts of the 
 body which the spine must support. Every one knows 
 that it is an effort to sit erect ; and this feelino- of effort 
 comes from the fact that the spine is straightened, or rather 
 its curvature kept normal, by the action of a rather com- 
 plicated group of muscles, — the erectors of the spine. To 
 
MUSCULAE ACTIVITY 329 
 
 sit, or stand, or walk erect involves the activity of these 
 muscles ; when they cease to act the faulty curvature be- 
 comes more pronounced. Hence the value of all exercises 
 which tend to straighten the spine, — exercises, for exam- 
 ple, in which, while standing on the feet, we try by our 
 own muscular effort to make ourselves as tall as possible. 
 They train and strengthen the muscles in question ; they 
 stretch their antagonists, just as throwing the shoulders 
 back stretches the pectorals ; and they impart to us by 
 actual experience the sensation of being erect. 
 
 (d) Lateral curvature of the spine. — When the spinal 
 column and its attached ligaments and muscles are prop- 
 erly developed there is little or no lateral curvature of the 
 spine ; the two halves of the Ijody are symmetrical with 
 regard to the median plane of the body, although a con- 
 siderable amount of bending of the spine as a whole to 
 one side or the otlier is possible. It is, however, quite 
 possible, by maintaining incorrect positions, to acquire a 
 more or less pronounced lateral curvature in which the 
 muscles and ligaments of the concave side become short- 
 ened and those of the convex side lengthened. Perhaps 
 nothing is so responsible for all these faults of curvature 
 of the spinal column as improper positions at the school 
 desk, and much can be done to prevent them by properly 
 constructed school furniture and careful attention to cor- 
 rect position. But it is not wise to depend on these alone. 
 No desk has been constructed in which correct posture 
 can be indefinitely maintained with ease, and we have still 
 in any case to contend with the force of gravity. Active 
 exercises which straighten the spine should supplement 
 the other measures. Experience has well established the 
 fact that the true preventive and remedy lies in move- 
 ments which elongate the spine. 
 
 (e) We have elsewhere (p. 172) pointed out the im- 
 portant action of the muscles of the abdominal wall in 
 
330 THE HUMAN MECHANISM 
 
 supporting the abdominal viscera, especially those, like 
 the stomach, the spleen, and the intestine, which are sus- 
 pended from the dorsal wall of the abdominal cavity. 
 Fig. 137 will at once make clear how the relaxation or 
 elongation of the abdominal muscles, by removing support 
 from these viscera, permits their weight to pull unduly 
 upon the mesentery, and so to stretch this support. It is 
 also not improbable that the tense mesentery at times, by 
 pressing upon thin-walled veins and lymphatics, interferes 
 with the circulation of blood and the flow of lymph in 
 some organs, and so leads to trouble. A pot-belly is not 
 a thing of beauty, and there is every reason for thinking 
 it to be undesirable from the hygienic point of view. It 
 is prevented, in the first place, by every movement which 
 prevents undue lumbar curvature of the spine, and, in the 
 second place, by exercises of the abdominal muscles, which 
 result in their improved tone. These, however, like all 
 corrective exercises^ must be followed up by maintenance of 
 the correct position of the trunk. 
 
 17. Special Exercise for the Training of Nervous Coordi- 
 nation. — A man or woman may possess none of the de- 
 formities noticed above, — the anatomical form of the body 
 may conform to tlie best ideals, — and yet the positions 
 and movements of the body may be awkward, inexpert, 
 ungraceful. In other words, the muscles may be well devel- 
 oped but the individual may be deficient in the power of 
 easily coordinating their action in the accomplishment of 
 desired ends. After what has been learned of the part 
 which the nervous system plays in directing our actions, 
 the brevity of any reference to this purpose of physical 
 training will not mislead the student into thinking that it 
 is of little importance. We have learned that the mainte- 
 nance of equilibrium, when the body is at rest and when 
 it is in motion, and the execution of complicated move- 
 ments, both require training of the nervous system by use. 
 
MUSCULAR ACTIVITY 331 
 
 The range of activities for which we can train is very exten- 
 sive ; playing upon musical instruments, the execution of 
 gymnastic feats on the parallel and horizontal bars, the 
 traveling rings, or the trapeze, are only a few examples of 
 what can be clone by the training of the nervous system 
 by practice. 
 
 A large part of gymnasium Avork consists in this sort of 
 training, and there is almost no limit to the forms of exer- 
 cise to which we may train, — vaulting, jumping, balan- 
 cing the body on one foot while various movements are 
 made, the tricks of the parallel and horizontal bars, trapeze, 
 etc. Is there any principle to guide us in the choice of 
 what we shall do ? In reply to this question we may say 
 that the leading principle should undoubtedlj^ be that of 
 training for what will be useful, and while we need not 
 discard all training which cannot be justified on this 
 ground, that which is useful should not be sacrificed to 
 that which is not useful. A large amount of skill is 
 required to walk upon the hands with the feet in the air, 
 and the thing can be done very gracefully by training ; 
 but it is certainly better to cultivate the habit of walking 
 gracefully upon the feet. And yet one may see profes- 
 sional gymnasts who are extremely graceful while perform- 
 ing their tricks, but whose gait is clumsy and awkward. 
 
 18. Balance Exercises. — It is evident that bjr far the 
 greater number of our customary coordinated movements 
 are made on the feet. Hence the value of so-called balance 
 exercises in the widest sense, whether they consist in 
 the execution of difficult movements while standing on 
 one foot or on the " walking beam," or in making a proper 
 landing from a jump or a vault ; all of them afford training 
 of those reflexes by which we retain control of the body 
 in motion, thus securing grace of posture and carriage. 
 
 The general purpose of training these reflexes is the 
 same as the purpose of those exercises which correct 
 
332 THE HUMAN MECHANISM 
 
 deformities ; they do for the nervous mechanism of the 
 movement what the others do for the skeletal parts and 
 the muscles wrhich play upon them ; they give the training 
 of use and prevent atrophy from disuse. 
 
 Both these ends, the corrective and the so-called coordi- 
 native, are best secured by the use of gymnastic move- 
 ments ; and the increasingly sedentary character of much 
 of our modern life correspondingly increases the value of 
 gymnastic work, especially in the period of youth. It is 
 well to learn and understand the most useful exercises, 
 and even in adult life to have resort to them two or three 
 times a week in order to hold fast what has been gained. 
 
 19. The Gymnasium as a Means of General Muscular 
 Exercise. — Under the conditions of city life, especially in 
 winter time, the gymnasium is also useful in supplying 
 general exercise in the form of running, gymnastic games, 
 etc. It is better to seek outdoor work as far as possible 
 for this exercise, but there are times when those living in 
 the heart of crowded cities cannot get to the country, and 
 outdoor exercise in town is not all that is to be desired. 
 While there is sometimes a tendency to extol unduly the 
 value of gymnastic work, there is equallj' marked igno- 
 rance in other quarters as to what the gymnasium may 
 accomplish. Our cities are vastly better off for their Y. M. 
 C. A. and other gymnasia, and we cannot afford to discour- 
 age any means of properly directed physical training. 
 
 20. Hygienic Value of Corrective Work. — Before leaving 
 the subject of corrective and coordinative work we may 
 answer a question which is frequently asked : Has it, after 
 all, any hygienic value ? All will readily grant that this 
 part of physical training has an ajsthetic value, and that 
 the cultivation of the taste for correct form and carriage 
 in one's own person is to be commended. But is a 
 man less healthy for being round-shouldered ? The answer 
 is that he may or may not be less healthy. The deformity 
 
MUSCULAE ACTIVITY 333 
 
 of round shoulders carries with it the lessened use of 
 the upper ribs in breathing ; and while one man or woman 
 may escape dangerous consequences, another may not, — 
 indeed we know does not, and it is the part of wisdom 
 to avoid the danger as far as possible. In one a pot-belly 
 ma}' be consistent with perfect healtli, while in others it is 
 not. C)ne may go through life with some faulty curvature 
 of the spine and not suffer from it ; but thousands of per- 
 sons have to consult physicians every year because of such 
 faults. Many a man wears improper siioes without bad 
 results ; hundreds pay for it with flat foot and suffering 
 which at times amounts to torture. There is not a single 
 deformity enumerated ahove whicli may not prove a seri- 
 ous matter ; and when it is so easy to avoid most of them, 
 it would seem from a h}'gienic point of view well worth 
 while to do so. 
 
 The hygienic value of corrective and eoiirdinative work 
 is justified, however, still more effectively on another 
 ground. The tendency to take general exercise is directly 
 proportional to the excellence of the ncurojuuscular 
 mechanism of the bodj'. The man wlio is awkward and 
 clumsy, who can make but few movements, does not enjoy 
 general exercise as does tlie man a\'1io has good control of 
 his muscles and can make many movements. It is proba- 
 bly not too much to say tliat a very large proportion of the 
 people who settle down to a sedentary life with the coming 
 of their thirty-fifth or fortieth year do this because they can 
 do so little with the body, and because exercise is conse- 
 quently monotonous and distasteful. We can undoubtedly 
 jDreserve more readily the love of movement for its own 
 sake when we have a body which can move freely and 
 easily, skillfully and joyously, than when, we have one- 
 which is never so much at home as in an easy-chair or 
 upon a soft bed ; and we have shown above (p. 311) how 
 valuable is this joy of movement to the body as a whole. 
 
CHAPTER XVIII 
 
 THE HYGIENE OF THE NERVOUS SYSTEM. 
 REST AND SLEEP 
 
 In no respect do the conditions of modern life stand in 
 more striking contrast to the life of former times than in 
 the increasing importance of mental work in contrast with 
 muscular work as a means of support. Not only are there 
 more professional men, — such as lawyers, editors, physi- 
 cians, teachers, and the like, — but the character of modern 
 business life involves no less the use of the nervous system 
 both on the part of those who direct large enterprises and 
 of those who occupy subordinate positions. The clerk in 
 a bank, as well as the president or cashier, is "living by 
 his wits " and is using his brain to an extent rarely seen 
 until within the last century. Never was competition so 
 keen ; never has it been so necessary to inform oneself 
 minutely as to market conditions of demand and supply ; 
 never before has the margin of profits been so small ; never 
 before has it been so necessary to avoid waste ; and never 
 before has it been so difficult to protect oneself against 
 novel and unforeseen conditions. Truly the modern busi- 
 ness man must be ever awake, ever alert. 
 
 Nor is this all. With the introduction of the telegraph 
 and telephone, communication between man and man is 
 facilitated ; the widespread employment of stenographers 
 results in an increase of letters received and sent; and 
 in other ways the number of matters demanding atten- 
 tion is multiplied many-fold. Moreover, the increase of 
 wealth has enlarged the possibilities of life ; concerts, art 
 
 334 
 
HYGIENE OF THE NEKVOUS SYSTEM 335 
 
 exhibitions, books, crowd upon us ; social engagements 
 are multiplied ; so that as a result we are kept ever on 
 the alert, and the man or woman who does not firmly 
 decline invitations, engagements, and efforts wiiich would 
 overcrowd life to no good purpose, experiences elements 
 of distraction, or fatigue, or worry, wliich tell upon 
 health and too often lead to what we call nervous pros- 
 tration. 
 
 For no student of the practical problems of h3rgiene 
 can shut his eyes to the marked prevalence of nervous 
 prostration and even insanity, or fail to recognize the 
 evident connection between these tilings and the intensity, 
 the hurry, the unrestful character of the lives we lead. 
 Probably there is no more pressing problem of practical 
 hygiene than that which is here presented. 
 
 And even where there is no question of nervous prostra- 
 tion or insanity, a large number of people suffer from 
 nervous troubles of one kind or another whicli interfere 
 seriously with their work and with the legitimate enjoy- 
 ment of life. Wc have seen how close is the connection 
 between all parts of the nervous system, and also how condi- 
 tions of the nervous system may and must influence nutri- 
 tive and other functions of the body. The two are most 
 intimately bound together, and many a man or woman 
 fails to secure the blessing of good health because in- 
 tense, unremitting work is demanded of the nervous sys- 
 tem, such as would never be imposed on the muscles, or 
 the stomach, or the skin. Consequently the avoidance of 
 actual nervous prostration is but a small part of what 
 must be accomplished by the hygienic conduct of life ; 
 a far more pressing practical problem is the lessening of 
 daily strain, worry, and fatigue wliich are the precursors 
 of the more serious troubles, and the avoidance of which 
 affords the only sure means of defense against the all too 
 common and distressing breakdowns of useful lives. 
 
336 THE HUM AX MECHANISM 
 
 Christian nations have always had in Sunday one day 
 in seven when most of the work of life may be suspended 
 and the strain relieved ; and the preservation as far as 
 possible of Sunday as a day of rest is a matter of sound 
 hygienic policy. Recent years have also shown encour- 
 aging signs of relief from steady strain in shorter hours of 
 labor, in the early closing of shojDS on Saturday, and in the 
 more general use of vacations in summer. These are all to 
 be welcomed and encouraged, but a large amount of strain 
 still remains ; the value of the Sunday rest is largely nullified 
 when we go back on Monday morning to an intense appli- 
 cation which continues without break until Saturday night. 
 
 1. What causes Nervous Strain? — The nervous system, 
 in common with all other organs of the body, is unfavor- 
 ably affected by unwholesome conditions or acts of life. 
 Improper feeding, sedentary occupations, bad ventilation, 
 or overheating of houses tell upon its working capacity ; 
 and the effects of such conditions upon the nervous sys- 
 tem are often wrongly attributed to mental work or over- 
 work. Especially is this true in the matter of muscular 
 activity. Many break down with nervous prostration, not 
 because the brain and sjjinal cord have been overworked, 
 but because, in an excessive devotion to business, or sci- 
 ence, or literature, or art, or pleasure, or eveii because of sheer 
 idleness, the muscular system has been neglected and has 
 accordingly failed to minister to the rest of the body. At 
 the same time it often happens that despite proper mus- 
 cular activity, proper feeding, etc., the element of strain 
 is still present. Muscular activity itself entails extensive 
 nervous work, and the nervous activity which participates 
 in muscular work may be only a part of the sum total 
 which produces overstrain, and even leads at times to 
 nervous prostration itself. 
 
 2. Misdirected Nervous Activity. — It is very important 
 to understand cleaily that it is misdirected nervous activity, 
 
HYGIENE OF THE NEEVOUS SYSTEM 337 
 
 and not mental work in itself, or the concentration of 
 attention which mental work requires, that leads to bad 
 results. It is a part of our normal life to do mental work 
 and to cultivate the power of close application to that work ; 
 it is a part of education to develop the power of concentra- 
 tion and attention against resistance and inclination, and 
 experience shows that this may be entirely consistent with 
 the maintenance of health. But when a student " crams " 
 for an examination for two or three days, with the minimum 
 of sleep during the period, and breaks down after it is 
 over, it is not merely mental work which should be blamed 
 for the result ; for he would probably have broken down 
 if he had attempted to work a typewriter during the same 
 time, with no more relaxation or rest. Tlie real cause of the 
 trouble is the too long continued use of the nervoiiii si/stcm. 
 
 3. Mental Work and Overwork. — Much nonsense is said 
 and written about " working the brain too hard." If by 
 this is meant working it too long at a time without rest, 
 or without stopping for muscular exercise ; if it means the 
 attempt to do moi'e sums in arithmetic, to read more Latin 
 or German, to write a longer composition, or to master 
 more science than the hours of study justify, and so pro- 
 long these hours of study to the neglect of other hj^gienic 
 demands, no objection can be made to the phrase ; hut if 
 it refers to the hard mental work and close application 
 required for a reasonable time by a sum in mathematics or 
 a passage in Latin, we may well hesitate to regard such 
 work as in any degi'ee dangerous. The world is overflow- 
 ing with people who have never acquired liabits of mental 
 concentration and hard thinking ; and yet their general 
 health is no better than that of persons who have acquired 
 such habits, while their mental powers often suffer severely 
 by comparison. 
 
 In the physiological portion of this work the anatom- 
 ical and physiological aspects of the nervous system were 
 
338 THE HUMAN MECHANISM 
 
 carefully described. One of the chief reasons for doing this 
 was to impress upon the student the extreme complexity 
 of the mechanism, the great number of parts (neurones) 
 which are concerned in our actions, and the natural diffi- 
 culty, as well as the necessity, of proper coordination. In 
 the work of a muscle it is not so important if some of the 
 fibers fail to do their work, provided the remaining fibers 
 work harder, and so exert the same pull on the tendon, 
 for the work will still be done. But it is not so with the 
 nervous system ; if ten or twenty neurones of a given 
 nervous mechanism fail to work, the work will not be 
 done at all, or will at best be done imperfectly. In any 
 mechanism of interdependent parts, weakness of one part 
 means weakness of the whole. The secret of efficiency in 
 the nervous system as a whole is the maintenance of the 
 efficiency of each and every unit. 
 
 4. The Care of the Nervous Machinery. Rest and Sleep. 
 — If a locomotive is to be kept in the state of high effi- 
 ciency, it must not be worked without ceasing until some- 
 thing goes wrong. When a train is to be pulled three 
 hundred miles it is customary to change engines two or 
 three times on the run; and these changes are made, not 
 because the first engine cannot pull the train to its desti- 
 nation on schedule time, but because heating occurs, or 
 dust finds its way into the bearings, or the strains and jars 
 impair adjustment ; and it prolongs the life of the macliine 
 and its good working to remove the dust, cool the parts, 
 and otherwise frequently put the engine in perfect order. 
 When an engine breaks down, it is usually because some 
 one part has given way. With proper care a good machine 
 should wear out but not break down. 
 
 The central nervous system, although infinitely more 
 complicated than the steam locomotive, is far less durable 
 as a mechanism. Its bearings are not made of hard steel, 
 but of living, irritable protoplasm keenly susceptible to 
 
HYGIENE OF THE EEEVOUS SYSTEM 339 
 
 fatigue. In the numerous connections between neurone 
 and neurone there is the same chance as in the steam 
 engine that some one part will fail to do its work ; and the 
 main principle of its hygienic care is to oil the hearings and 
 clean and repair the machinery^ hy repose and sleep, before 
 the danger of a breakdown is imminent. Rest, and especially 
 the rest of sleep, is the one preventive for these unfavor- 
 able conditions ; hy these alone is the fatigued neurone 
 withdrawn from work and given the chance to repair itself 
 and to return to its normal condition. 
 
 5. How Much Sleep is Advisable? — Different people 
 undoubtedly require different amounts of sleep ; but it is 
 safe to say that the vast majority of adults require from 
 seven to eight hours a day ; children and young people 
 require more. It is, however, an interesting question 
 whether all of this sliould be taken at one time or not. 
 Since the nervous life of to-day is more intense than was 
 that of our ancestors, it is all the more needful that we 
 keep the nervous system in a continuous state of liigh 
 efficiency. To go about the duties and pleasvu'es of life 
 from early morning until late at night without a moment's 
 rest is a great mistake ; we are then doing what the 
 engineer would do who should run his engine all day, 
 feeding it with coal but without giving it a drop of oil, 
 without tightening a nut, without cleaning a liearing. As 
 the play of nervous activity goes on, now calling upon one 
 combination of nerve cells, now upon anotlier combination, 
 those nerve cells whicli belong to more than one mechan- 
 ism are called oir for more than their share of work, and 
 every mechanism to which they belong may be to that 
 extent impaired. The stimulus of the will must be more 
 vigorously applied, and as this becomes ineffective, the 
 individual is tempted to use stimulants, as the whip is 
 applied to tii-ed and straining horses, or as blows were 
 showered upon galley slaves in time of battle. 
 
340 THE HUMAN MECHANISM 
 
 Contrast with this the benefit of brief sleep during the 
 day in facilitating night work. Some persons, it is true, 
 do not seem to be thus benefited, but the vast majority 
 are. And the benefit is out of all proportion to the time 
 spent asleep. We are tired and work is difficult, not so 
 much because the whole nervous system is exhausted, but 
 because unfavorable conditions of fatigue, etc., have come 
 in at important points ; during even a short nap, with its 
 marked muscular and nervous relaxation, normal condi- 
 tions are restored and the whole mechanism then works 
 on with less effort, less general fatigue, less local injury. 
 
 6. Nervous Rest in Change of Work. — Sleep is the very 
 best means of insuring local nervous repair, because it is 
 the only condition which involves complete relaxation. 
 There is, however, some rest, or at least some refreshment, 
 in mere change of employment ; as when, for instance, 
 we pass from mental work to physical exercise. Calling 
 into play a new group of nerve cells gives a chance 
 for rest to many cells which have previously been active. 
 And at times we feel tired after mental work because we 
 need muscular activity rather than sleep. The tired feel- 
 ing may come not from tired nerve cells but from the 
 want of what the muscles might furnish (see Chapter 
 XVII). At such times muscular exercise to some extent, 
 perhaps to a great extent, refreshes us ; and in general we 
 maintain a higher degree of working power by judicious 
 variety of activity. But it must be remembered that, in 
 the long run, neither muscular exercise nor any other 
 change of occupation can take the place of the complete 
 relaxation and refreshment found in sleep. It is, indeed, 
 doubtful whether there is any chauge of employment which 
 lii'ings with it an entire change of nervous activity. A 
 certain number of the same cells, already weary, are still 
 kept at work, as has already been explained above ; and it 
 is ly deep alone that every cell has its natural opportunity 
 
HYGIENE OF THE NEfiVOUS SYSTEM 341 
 
 for repair. Those who would define rest as "selected 
 excitement " should bear this fact carefully in mind. 
 
 The cardinal principle in the care of the nervous system 
 is thus the same as that in the care of the steam engine. 
 Do not often call upon it for activity of any kind when 
 conditions of undue fatigue are likely to be present. Go 
 to tire performance of every physiological activity, to diges- 
 tion, to study, to muscular work, to social life, — for all 
 these mean nei'vous activity, — as far as may be with a 
 rested nervous system. Of course to do this is not alwaj^s 
 possible ; there are times when we nmst drive the body to 
 mental work despite tlie fact that it is physically tired ; 
 but this ought to be the exception, never the regular order 
 of life. 
 
 7. Examples. — Let us suppose that some one, man or 
 woman, after applicatiiin at setlentar^y A^firk for six or 
 eight hours, has some time free before the evening meal, 
 and that, tired and peibaps nervous, relaxation is sought 
 in a brisk walk, which is almost immediately folhiwed by 
 dinner. The effort which the digestion of this, perhaps 
 the heaviest meal of the day, costs the nervous system 
 shows itself in a stupid, almost sonuiolent condition which 
 often follows. The body is trying to do hard ^\ork with a 
 tired nervous system, some of whose bearings need oiling ; 
 its owner is making the mistake of continued activity 
 without ojDjDortunity for the rest and repair ^'\"hich a nap 
 of fifteen or twenty minutes, or even absolute idleness and 
 complete muscular relaxation without sleep, for half an 
 hour or so before the meal, might have given him. 
 
 Again, there are times in every one's life when some 
 unusual strain must be borne ; when, for example, after 
 the day's work watch must be kept at a sick bed during 
 the greater part of the night. Too often people will 
 undertake this strain, expecting to " make up " the loss of 
 rest when it is over, even when it is possible to prepare 
 
342 THE HUMAN MECHANISM 
 
 for it by an hour or so of sleep beforehand. We seldom 
 work steam engines in this way. Should we treat the 
 nervous system less carefully than a steam engine? 
 
 These examples must suffice. The application must be 
 made by each individual according to his work in life. If 
 work is undertaken which requires constant activity from 
 early morn until late at night, the case is hopeless, and 
 the only remedy is a change of occupation. Only gross 
 ignorance of the plainest facts of human experience, as 
 well as of physiological science, can excuse such conduct. 
 
 8. Muscular Relaxation in Sleep. — Sleep and rest involve 
 muscular relaxation. All have noticed, when falling off to 
 sleep, the feeling of relief from strain ; the framework of 
 the skeleton seems to be held together less rigidly, and 
 finally, as we lose consciousness, relaxation seems com- 
 plete. And at times when sleep will not come, many 
 have felt the inability to relax ; when, as it has been well 
 expressed, we seem to be afraid that the bed will slip 
 away from under us and we must hold on to it. We have 
 seen that during waking life the nervous system is con- 
 tinually sending out impulses which keep the muscles in a 
 state of moderate contraction, and thus among other things 
 liberate heat for the maintenance of the body temperature. 
 Usually this tonic activity of the motor neurones must be 
 more or less relaxed before sleep will come, and the inabil- 
 ity to release it is one of the danger signals of the nervous 
 system. There can be no doubt that when nervous work 
 is pushed too hard against unfavorable conditions, the 
 nerve cells develop a condition of excessive irritability, 
 so tliat they are discharged by afferent impulses or other 
 stimuli which would ordinarily not affect them ; and they 
 maintain this irritable condition even in the presence of 
 general bodily fatigue. Normal rest is, of course, ex- 
 tremely difficult or quite impossible under these condi- 
 tions, which for this reason alone should be attended to at 
 
HYGIENE OF THE NERVOUS SYSTEM 343 
 
 once. The trouble may be in some general or special 
 unhygienic condition of life, — impaired digestion, insuffi- 
 cient muscular exercise, the presence of undue anxiety, 
 etc. ; these should be inquired into and remedied if pres- 
 ent ; but the trouble is usually the res\dt of pushing 
 activity of different kinds for too long periods without 
 cessation. In other words, we have lost the ability to relax 
 because we have not practiced relaxation. 
 
 9. Conservation of the Ability to relax. — The ability 
 to relax is something which, like all phenomena of nervous 
 life, depends on practice. Indeed, it is not improbable that 
 it is something more than a mere process of desisting from 
 activity, and that direct active processes of inhibition (see 
 Chapter XV) are concerned in it. All have known people 
 who can go to sleep the instant they lie down ; and they 
 can do this — it would almost seem by an act of the will 
 — because they have long done it. It is a power which 
 can indeed be cultivated too well ; by too frequent repeti- 
 tion of the jorocess of taking a nap, and by sleeping too 
 long at night, there may be aec[uired a diminished irrita- 
 bility of the nerve cells, which makes attention to work a 
 very difficult matter, and long-sustained attention almost 
 impossible. Those in this condition may escape the danger 
 of nervous pirostration, but they impair their usefulness 
 in life. 
 
 The true path, as in other matters of personal hygiene, 
 is that between these extremes. When one rises at seven 
 or eight in the morning, a short period of rest in the after- 
 noon is sufficient ; the persistent practice of the act of 
 relaxation every hour or less is apt to lead to loss of mus- 
 cular tone and of nervous efficiency in general. At the 
 same time, the habit of momentary relaxation in the midst 
 of the day's vrork is a valuable aid, partly in bettering 
 conditions at the time, but chiefly in retaining the power 
 to relax when it is wanted for longer periods of rest. 
 
344 THE HUMAN MECHANISM 
 
 10. Drugs are Delusive and Dangerous. — The physiolo- 
 gist cannot condemn too strongly the substitution of stim- 
 ulants for the proper regulation of work and rest. The 
 reader will see at once what this course of action may 
 be expected to accomplish ; the stimulant is an antagonist 
 of relaxation ; the nerve cell becomes more and more irri- 
 table as it is pushed harder and harder ; finally it reaches 
 either the condition of excessive irritability or else that of 
 being unable to work without the stimulant. It has adapted 
 itself to the presence of the stimulant in its environment, 
 it is trained to work under those conditions, and it cannot 
 work without them. It may be safely asserted that, in gen- 
 eral, the time above all others when stimulants should not 
 be used is when we are tired out ; to use stimulants regu- 
 larly, day after day, in place of rest is shown by experience 
 to be one of the most dangerous of mistakes. 
 
 Nor, on the other hand, can we condemn too strongly 
 the use of narcotics to produce sleep. Probably none of 
 these drugs are capable of producing normal sleep ; and 
 while in times of emergency the physician must have 
 recourse to them, they should never be relied upon in 
 place of the hygienic conduct of the whole life. Many of 
 them, and some of those in common use, are very danger- 
 ous, and none of them is known to be above reproach. 
 
 11. The Influence of Mental and Moral States. — Finally, 
 it must also Ije remembered that psychical processes exert 
 a profound influence upon the well-being of the brain and 
 spinal cord. It is a matter of common experience that 
 emotions, feelings, moods, etc., profoundly influence human 
 conduct, and so indirectly affect health, especially the 
 health of the nervous system. It is also certain that they 
 exert a more direct physiological influence on the bodily 
 functions ; the changes which emotions produce in the 
 heart beat are good examples of other changes which are 
 none the less important because they do not lend themselves 
 
HYGIENE OF THE NEKVOUS SYSTEM 345 
 
 so readily to observation. The bestowal of a healthy atten- 
 tion upon the moral aspects of conduct is a legitimate and 
 essential part of personal hygiene ; and it is not too much 
 to say that much of the ill health from which men and 
 women suffer is to be traced primarily to the absence of 
 sound moral sense or to its abnormal or perverted devel- 
 opment. Care and worry often cause weariness and loss of 
 sleep which even diversion and muscular exercise cannot 
 overcome. They seldom trouble the 3"oung, but as age 
 advances they are sometimes inevitable. Efforts should be 
 made to avoid them, as far as possible, by a wise order- 
 ing of life, by forethought, thrift, economy, sobriety, hon- 
 esty, and the like, which tend to " a light heart " and " a 
 clear conscience." A heavy heart and a clouded conscience 
 tend to unhappiness, anxiety, wakefulness, and other phys- 
 ical ills. 
 
 12. '< Mental" Cures of Disease. — It has been shown that 
 mental conditions are far from being without influence 
 upon the activities of the body, even leaving out of 
 account the voluntary muscles. The effect of emotions 
 upon the heart has been referred to, and so has the psychic 
 secretion of gastric juice. It is known that the movements 
 of the alimentary canal are readily modified b}^ events in 
 conscious life. In the hypnotic state the effect of sugges- 
 tion upon functions which we habitually regard as invol- 
 untary is even more striking. Facts like these have led 
 many to the rash assumption tliat there is no limit to the 
 domination of the mind over physiological processes. In 
 numerous cases the ascendency which some have gained 
 over certain forms of disease has been as surprising to 
 others as it has been gratifying to themselves. Bej'ond 
 question the righting of disordered functions and the sup- 
 pression of pain have been frequently attained, and this 
 fact makes it easy to see why so great a following has been 
 drawn to a belief in the universality of mental power. 
 
346 THE HUMAN MECHANISM 
 
 But certain dangers are always involved in the attempt 
 to overcome disease by resolutely forgetting it and denying 
 its existence. The feeling of pain may at times be ban- 
 ished by believing that it does not exist, but this may be 
 quite as undesirable as self-inflicted blindness or deafness. 
 While relief from pain may frequently favor recovery by 
 promoting rest and nutrition, it may at other times sim- 
 ply mean the loss of warnings which deserve to be heard. 
 Where there is grave organic disease, this may move on 
 to a fatal issue even while the deluded subject consist- 
 ently ignores its course. It is not wise to try to annul 
 the effects of a disease in consciousness when botli cause 
 and effect can be removed by rational medical treatment. 
 Hypnotism may relieve a toothache, but it is not claimed 
 that it will mend a decaying tooth. The dentist's filling, 
 which does both, is the tyjje of medical as contrasted with 
 psychical methods in dealing with acute disease. Espe- 
 cially foolish is it to ignore or deny the actual presence of 
 infectious or contagious disease, for here delay menaces 
 not only the patient but those about him. The conse- 
 quences of this folly, when confined to its deluded victim, 
 may end in virtual suicide ; when they extend to others, 
 they may fall little short of manslaughter. 
 
CHAPTER XIX 
 THE HYGIENE OF FEEDING 
 
 The present chapter deals with certain hygienic con- 
 siderations connected with the taking of food into the 
 body, — its preparation, its cooking, its quantity, the fre- 
 quency of our meals, and the adjustment of our habits of 
 feeding to the other work of life.^ 
 
 Mankind as a whole was probably never better fed than 
 it is at present. The opening up of the New World with 
 its vast fields of corn and wheat and its enormous pas- 
 tures ; the introduction of improved methods of agriculture, 
 agricultural machinery, and education in agriculture ; and 
 especially the improvements in transportation facilities 
 and in arts of food preserving (such as refrigeration and 
 canning), — all these have immensely increased the avail- 
 able food supply of the world and made famine and star- 
 vation much more rare than formerly. It is now only in 
 inaccessible places, such as the central parts of India, that 
 great famines still occur. 
 
 And yet in the midst of abundance it is still true that 
 many men and women are poorly nourished ; for it is the 
 absorption of food by the blood and not merely the eating 
 of meals which supplies the needs of the tissues. Hence 
 the problem of alimentation in its widest sense involves 
 not only the growing of food on farms or in gardens, and 
 
 1 Many practical points connected with alimentation have already been 
 considered in Part I (see chapters on digestion and nutrition). Special 
 reference may be made to Chapter XIII, p. 233, on the Choice of Foods 
 and Nutrients. 
 
 347 
 
3-48 THE HUMAN MECHANISM 
 
 the preservation of this food so that it may be delivered in 
 proper form to the consumer, but also the eating of it in 
 such form and quantities and at such times as will insure 
 its proper utilization, by the processes of digestion, for the 
 needs of the body. 
 
 1. Appetite as a Guide in Feeding. — Nature herself has 
 provided us with guides in the choice of food, and these 
 guides are the sensations of hunger and thirst, and what 
 we sum up in general under the term " appetite." So long 
 as these remain normal and unperverted, they are to be 
 largely trusted ; and, like all physiological functions, they 
 are kept normal and unperverted, in the first place, by 
 attention to the general health of the entire body. Appe- 
 tite is apt to fail or become untrustworthy in the case of 
 men or women who are suffering from lack of muscular 
 activity or from mental worry. The care of the appetite 
 is never a matter of direct attention to the appetite itself, 
 but of maintaining the bodily conditions in which it nor- 
 mally acts. Consequently the basic principle in securing 
 proper nutrition is attention to the general health. A pa- 
 tient suffering from indigestion once consulted a wise 
 old doctor and began recounting the foods that agreed or 
 disagreed with him, together with his innumerable symp- 
 toms, until the doctor interrupted him by saying, " The 
 first thing you must do is to forget that you have a 
 stomach," The present chapter is not written for people 
 like this patient, or for invalids, or for others suffering 
 from indigestion in any one of its thousand forms. It is 
 written for those who can, and will, first of all, take the 
 needful muscular exercise and the needful rest; who will 
 pay proper attention to clothing and bathing, to the heat- 
 ing and ventilation of the home, to the avoidance of 
 dampness and other unfavorable conditions ; who will not 
 abuse themselves by stimulants and narcotics. Those who 
 pi'efer not to belong to this class, or who because of some 
 
HYGIENE OF FEEDING 349 
 
 constitutional disease cannot, must seek and depend upon 
 medical advice as regards their habits of feeding. 
 
 At the same time, to insure proper digestion and nutri- 
 tion, more is required than attention to general hygiene. 
 What additional precautions are required in the taking of 
 food by persons leading an otherwise healthy life? It is 
 in answer to this c[uestion that we shall attempt to give 
 some suggestions. 
 
 2. Good Cooking as an Aid in Nutrition. — It has already 
 been pointed out that digestion begins with the prepara- 
 tion of the food by cooking, which serves three purposes. 
 
 1. It destroys parasites and disease germs. The impor- 
 tance of this will be shown and emphasized elsewhere 
 (Chapter XXXII). 
 
 2. It renders the food more apijctizing (see p. 113). 
 
 3. It makes some foods more digestible l)y making them 
 accessible to the action of the digestive juices ; thus the 
 connective tissue of animal foods, when heated in the pres- 
 ence of water, swells, and is more easily acted on by the 
 gastric juice, so that tough meat in this ^\■ay is often 
 made tender by boiling. The cellulose walls of the vege- 
 table foods, on the other hand, are softened by cooking, 
 the starch granules are swollen, and their envelopes burst 
 (see p. 94). 
 
 At the same time it is possible to render food less diges- 
 tible by improper cooking. A piece of meat may " have 
 the life cooked out of it" : and egg albumen, which in the 
 raw state mixes rather easily with the gastric juice, may 
 sometimes be boiled to a leathery consistency which ren- 
 ders the action of the digestive juices a slow process. 
 
 3. Chewing of Food an Aid to Digestion. — It is unneces- 
 sary to dwell at length upon the importance of chewing, or 
 mastication. We have already seen that the word "diges- 
 tion " is derived from the Latin words dis and gero. to tear 
 apart or separate ; and our studies of physiology have 
 
350 THE HUMAN MECHANISM 
 
 shown how the division of food into smaller and smaller 
 masses is prerequisite to reasonable rapidity of solution 
 and absorption. The student is also reminded of what has 
 been said (p. 100) concerning the importance of caring for 
 the teeth. 
 
 Vegetable foods especially should be well chewed, partly 
 because the cellulose which holds them together is not 
 readily acted on by the gastric juice, and partly because 
 the thorough mixture with the saliva facilitates the gastric 
 digestion of starch (p. 110). Meats also should be well 
 masticated. The fact that a dog bolts his food with im- 
 punity is no guide for civilized man, since, for one rea- 
 son, human gastiic juice contains much less acid and so 
 acts less readily upon connective-tissue elements. It is true 
 that the " quick lunch'' thrives in busy places, but no one 
 considers it hygienic. 
 
 4. Feeding in Relation to Gastric Digestion. — In order 
 that gastric digestion may be efficient it is, of course, neces- 
 sary that gastric juice shall be secreted in proper amount, 
 and we have learned that the first step toward this secre- 
 tion consists in the pleasurable sensations connected with 
 the satisfaction of appetite. Consequently it is one of the 
 first hygienic requisites of gastric digestion that the food 
 shall be appetizing, and that the condition of the body and 
 especially of the digestive system shall be such that the 
 food shall be eaten with relish. This is not the same thing 
 as saying that food which is appetizing will be digested; 
 it merely means that food is more digestible for being 
 appetizing, and that, when it is not enjoyed, its stay in 
 the stomach is apt to be unduly prolonged. For this and 
 other reasons the appetite should not be impaired by eat- 
 ing candy, or by visiting the pantry between meals for 
 something to eat ; on the other hand, a good appetite 
 should be encouraged by healthy living, by proper prepa- 
 ration of the food, and even, as far as possible, by agreeable 
 
HYGIENE OF EEEDlNd 351 
 
 table appointments. There was wisdom as well as pleas- 
 ure in the old custom of having a jester at the dinner table, 
 and there is reason in the saying, "Laugh and grow fat." 
 
 5. Excessive Quantity of Food. Overfeeding. — It is 
 furthermore important that the amount of food eaten at 
 one time be not excessive, and that the stomach under no 
 circumstances be unduly distended. A large proportion of 
 those cases of dyspepsia which have their origin partly or 
 entirely in the conditions of feeding are due to overeating, 
 which may take various forms. Too large a proportion of 
 the total food may be taken at one meal, usually dinner ; 
 or too many meals may be taken, — three should suffice ; 
 or each of the three may be full-sized meals, — a very unde- 
 sirable custom among those engaged in sedentarj^ pursuits. 
 We have seen that the one condition of life which calls 
 for heavy feeding is that of muscular activity, whether in the 
 performance of external work or for tlic production of heat 
 in cold weather ; a person who is engaged in some occu- 
 jjation which involves large amounts of muscular work 
 can and should have three full meals daily ; with others 
 the habit is attended with considerable risk. 
 
 Gluttony has always been a vice of the idle and luxu- 
 rious. As the world has grown wiser it has become less 
 common, because a larger intelligence makes it plain that 
 gluttony defeats its own ends, and that the secret of the 
 greatest pleasure in eating, as in everything, lies in temper- 
 ance, not in excess. 
 
 Many persons, however, without any desire or even any 
 thonght of gluttony, regularly overeat. These are usually 
 healthy persons leading sedentary lives, " blessed," as they 
 say, " with a good appetite," and because of quiet or even 
 indolent disposition giving but small heed to muscular 
 activity. As the years go by, such persons are apt to gi-ow 
 fat, and by and by to find themselves suffering fiom a weak 
 heart, or shortness of breath, or something worse ; seldom 
 
352 THE HUMAN MECHAlSriSM: 
 
 realizing, until it is too late, that overeating is the princi- 
 pal cause of their undoing. If sufficient manual labor or 
 other exercise of the skeletal muscles is practiced, trouble 
 from overeating rarely comes. It is the sedentary, inactive, 
 and indolent who suffer most from this source ; for them 
 a good appetite often proves to be a curse rather than a 
 blessing, and a jDoor appetite, by preventing overeating, 
 has often been a blessing, though a blessing in disguise. 
 
 6. Fried Foods. — Caution is required in the use of fried 
 foods. When a layer of fat varnishes over a particle of 
 food the digestive juices do not readily penetrate the mass, 
 and digestion is to that extent impaired. This is not of 
 so much importance in intestinal digestion, since in that 
 portion of the alimentary canal the layer of fat is itself 
 digested and removed : the stomach, on the other hand, 
 does not digest fat, and we can easily see how, because of 
 its interference with the fii'st processes of digestion in this 
 organ, the use of too much fried food is unwise. 
 
 ]\loreover, in frying, care should be taken to have the 
 temperature of the fat high enough to coagulate promptly 
 the surface layers of tlie food, thus preventing the pene- 
 tration of tlie fat into the food, which, moreover, should 
 not be served swimming in fat, but as dry as possible. 
 The frying pan is still used far too extensively in some 
 parts of America. Most of our foods should be roasted, 
 broiled, boiled, or baked, rather than fried. 
 
 7. Perspiration in Relation to the Hygiene of Feeding 
 
 The secretion of gastric juice is seriously impaired by 
 excessive perspiration, especially when the loss to the 
 system is not made good by drinking sufficient amounts 
 of water. This is probably true of the secretion of all of 
 the digestive juices, but it is especially important in the 
 case of the gastric digestion, upon the proper performance 
 of which the subsecpient Avork of intestinal digestion so 
 largely depends. Therefore, in general, smaller meals 
 
HYGIENE OF rEEJ)ING 353 
 
 should be eaten in hot weather, — we have seen that we 
 need less food at that time, — and heavy meals should not 
 be taken immediately after vigorous exercise involving 
 profuse perspiration. Indeed, it is a general rule that ex- 
 cessive loss of water by perspiration should be made good, 
 as far as possible, by drinking water more freely. 
 
 8. Digestion and Bodily Fatigue. — Digestion, like all 
 other functions of the body, involves to a very considerable 
 extent the intervention of the nervous system ; and we 
 may repeat here the advice already given (p. 341) not to 
 go tired to tlie digestion of a heavy meal. It is one of the 
 objections, probably the chief ol)jection, to evening dinners 
 that they so frequently follow immediately upon a hard 
 day's work, when the nervous system is in a poor condi- 
 tion for its share in digestive work. A rest of half an 
 hour before dinner is, however, generally all that is needed, 
 and usually prevents the mental heaviness which so often 
 follows a full meal. 
 
 9. Mental Work after Meals. — An exaggerated impor- 
 tance has probably been given at times to the danger of 
 mental work after meals. There is no proof whatever that 
 the demand of tlie brain for greater blood supply will 
 seriously interfere with that to tire digestive organs. 
 Wliile it is true that indigestion often affects people who 
 go straight from their meals to hard mental work, it is 
 also true that these are usually people who take insuffi- 
 cient muscular exercise, rest, and sleep. The relation of 
 the circulation in the brain to that in the digestive organs 
 is too imperfectly understood to justify some of the glib 
 but shallow utterances frequently met with on this subject, 
 especially when the statements in question are not clearly 
 supported by experience. (See page 155.) 
 
 10. Muscular Activity after Meals. — Vigorous muscular 
 activity immediately after meals is quite another matter. 
 Here we know that blood is taken away from the digestive 
 
354 THE HUMAN MECHANISM 
 
 organs and sent through the muscles and skin ; this fact 
 suggests caution, and experience amply confirms the need 
 of the caution thus suggested. Even here it is vigorous 
 exercise, and especially after lieavy meals, that is to be 
 condemned. 
 
 11. The Use of Water as a Drink. -^ Many people, and 
 especially many women, drink too little water. Water is 
 constantly being lost through the lungs, skin, or kidneys, 
 and this loss is only partially made good by the oxidation of 
 the hydrogen of the jjroteids and fats.^ No rules as to the 
 amount can be given, since it varies so much with tem- 
 perature and the amount of muscular activity ; but the 
 habit of drinking no water between meals and but little at 
 the table, in spite of popular opinion on the subject, is to 
 be strongly deprecated. We have already shown that the 
 abstraction of undue amounts of water by perspiration 
 may seriously interfere with the secretion of the gastric 
 juice, and there is every reason to believe that a deficiency 
 in the supply of water to the blood similarly interferes 
 with the secretion of the other digestive juices, and so, by 
 impairing intestinal digestion, favors constipation. 
 
 Undue emphasis has been laid upon the danger of drink- 
 ing water with meals. The reasons given — that such 
 water unduly dilutes the gastric juice or takes the place 
 of a normal secretion of saliva — are questionable. As a 
 matter of fact, the water thus taken is soon discharged into 
 the intestine and absorbed. It is true, however, that the 
 use of too much fluid with the meal is apt to lead to 
 insufficient mastication because it makes it easier to swal- 
 low the food ; and from this point of view caution is 
 advisable. It is probably also true that much drinking 
 with meals tends to overeating, by facilitating rapid eating ; 
 
 1 Tlie water excretert from the body comes partly from the water drunk, 
 but also partly from that formed by the union of the hydrogen of the 
 food with oxygen. 
 
HYGIENE OF FEEDING 355 
 
 and it may be that this is one reason why fat people are 
 usually great drinkers. 
 
 A further point in the hygiene of gastric and intestinal 
 digestion is the avoidance of those inflammatory conditions 
 of the bowels which follow exposure to cold. This sub- 
 ject will be dealt with in Chapter XXI. The student 
 will also recall what has been said in Chapter XVII with 
 regard to the importance of general muscular exercise, and 
 especially of exercises involving the use of abdominal 
 muscles. 
 
 12. The Importance of Coarse Foods. — In treating of 
 the physiology of digestion it was pointed out that the 
 presence of a certain amount of indigestible material in 
 the food is helpful as a stimulus to the muscular action of 
 the intestine. Food may be, and nowadays often is, espe- 
 cially in " delicate dining," too largely composed of very 
 digestible substances, which leave an insufficient residue 
 of solid material in the lower intestine to stimulate proper 
 peristalsis. Our main reliance for the needed coarse or in- 
 digestible part of our diet is the cellulose of many of our 
 vegetable foods ; oatmeal and fruit are t\'i^o of the more 
 important foods which serve the purpose. Indian corn is 
 also an important laxative, although its action is probably 
 dependent on other things than cellulose. The laxative 
 action of prunes and figs is well known. Graham flour, 
 bran, "whole wheat," etc., which retain more or less of the 
 hull of the grain, have the same action. In selecting the 
 diet care should be taken, especially in winter, to include 
 in it a sufficient quantity of such foods, and a judicious 
 addition of coarse foods to any diet is probably wise. 
 
 13. The Individual must study his own Needs. — In thus 
 sketching the broad outline of hj-gienic feeding little or 
 no attention has been given to what we should or should 
 not eat; and this has been done in order to discourage 
 looking at the subject from this popular but entirely 
 
356 THE HUMAN MECHANISM 
 
 misleading point of view. It may be true that "what is one 
 man's meat is sometimes another man's poison," but only 
 in a very limited sense. Each individual in the course of 
 his experience will learn that there are some things he 
 cannot eat with impunity, and, if he be wise, will gov- 
 ern himself accordingly. But it must be remembered that 
 man enjoys a wide latitude in the choice of his food. The 
 vast majority of people, if they will but lead otherwise 
 hygienic lives, can eat almost anything ; and the inability 
 to digest something which we have always eaten, or which 
 others eat with impunity, should lead not so much to 
 its exclusion from the diet as to an inquiry whether the 
 trouble does not have its origin in the general unhygienic 
 conduct of life. Those who treat such conditions by con- 
 structing a table of the things they can eat and another 
 of those they cannot eat, and confine their diet to the 
 former, usually find that as life advances the size of the 
 latter table increases at the expense of the former. It is 
 the same fallacy of dealing with the symptom instead of 
 the disease, which leads others to treat constipation with 
 cathartics, and still others to treat a bi'onehial cough with 
 so-called " cough medicines." 
 
CHAPTER XX 
 FOOD ACCESSORIES, DRUGS, ALCOHOL, AND TOBACCO 
 
 1. Food Accessories and Drugs. — Through the alimentary 
 and respiratory tracts tliere are received into the blood not 
 only substances such as proteids, gelatin, fats, carbohy- 
 drates, salts, and water, which we have described as sup- 
 plying the material for power and for growth and repair, 
 but also other substances capable of modifying in one way 
 or another the course of events within the body. The 
 flavors which contribute to the enjoyment of foods play 
 an imp)ortant r61e in the secretion of tlie gastric juice ; 
 and yet the substances which cause these flavors are negli- 
 gible as sources of power. Salt belongs under the same 
 head ; for we use in cooking more salt than is needed 
 to make good the daily loss from the body, and we do this 
 to develop an agreeable flavor in our food. Substances 
 of this kind are spoken of as food accessories, and among 
 them must be included coffee and tea, for their effect is 
 not chiefly a matter of nutrition ; certain constituents of 
 tea and coffee absorbed into the blood affect the ner- 
 vous system, and it is largely for this reason that we use 
 them. 
 
 We may pass in this way from the necessary food acces- 
 sories thi'ough those, like coffee and tea, which, while 
 not essential, may still be regarded as part of the food of 
 a large portion of mankind, to the great number of chem- 
 ical compounds known as druffs, which also act by chan- 
 ging the course of events within tlie body; and it is difficult 
 to draw an}^ sharp line of distinction between those which 
 
 357 
 
358 THE HUMAN MECHANISM 
 
 occasionally serve as medicine or "stimulants" and those of 
 which daily use is made as food accessories. 
 
 Animals as a rule take substances into their bodies only 
 to satisfy hunger or thirst or appetite ; man alone takes, 
 in addition to his nutriment, food accessories and drugs 
 for the sake of their special effect upon the nervous sys- 
 tem or other organs. Many of the numerous food acces- 
 sories which human ingenuity has discovered or devised 
 are harmless enough in the form used ; but others con- 
 tain substances which are capable of poisoning the body. 
 It is an important part of the study of personal hygiene to 
 learn of what these substances consist, what is their ac- 
 tion on the human organism, and wherein lie their special 
 dangers. 
 
 2. The Drug Habit. — It is a lamentable fact that large 
 amounts of drugs are swallowed by men and women apart 
 from any medical need which compels their use. In a sub- 
 sequent chapter we shall show reasons for avoiding an 
 undue dependence upon drugs as a remedy for various 
 minor ills. Bad as this practice is, with its tendency to 
 rely upon the uncertain action of a drug instead of taking 
 proper hygienic care of the body, it is far worse to make 
 habitual use of drugs for their special effects upon the 
 healthy body, for the habit is one which is only too easily 
 cultivated. There is no reason why a healthy human 
 being, living a normal life amid healthful surroundings, 
 should need to use drugs habitually, and a little consider- 
 ation will show that the practice is dangerous. 
 
 3. Dangers of the Drug Habit. — When we eat meat, or 
 vegetables, or when we breathe air, we take into the body 
 materials needed for normal living. These things have 
 always formed part of the food of the race, and, unless 
 wrongly taken, do good and not harm. When, on the other 
 hand, we take a drug, suuli as chloroform, or cocaine, or 
 opium, or alcohol, or coffee, or tea, we take something 
 
DRUGS, ALCOHOL, ANT) TOBACCO 359 
 
 which is foreign to the body, in so far as it has not been a 
 regular constituent of animal food in the past. It is not 
 needed, as proteid and salt and water are needed ; there 
 is no special preparation for its reception ; and, while it 
 may do good, there is danger that it may do harm. 
 
 In the second place, the exact action of many drugs is 
 only imperfectly understood. In an emergency the physi- 
 cian uses them temporai'ily^ for some effect which he desires 
 to produce, thus tiding over a difficulty. He uses the drug 
 only a few times at most, and is consequently not greatly 
 concenred about unfavorable attendant effects ; it accom- 
 plishes some needed purpose, and if it does any harm, the 
 organism may be trusted to recover from it. It is very 
 different, however, with the liahltual use of any drug. 
 The very fact that it gives some new direction to the 
 events taking place within the body means that abnormal 
 conditions of life are being maintained ; and we have 
 already learned that abnormal conditions of life are apt to 
 be unhygienic. 
 
 Again, the use of drugs is onl}^ too apt to be substituted 
 for the liygienic conduct of life. We may, for example, 
 take drugs to accomplish something which the healthy 
 body should accomplish iov itself without outside hel]j. 
 When any one drinks a cup of black coffee to facili- 
 tate mental work which his fatigued condition would not 
 otherwise allow him to do, he is trying to get from a drug 
 the power which he could and probably should secure 
 by noi'mal sleep. The coffee acts like a whip to a tired 
 horse ; the same work is done as might have been done 
 had the horse been allowed a little rest ; but the horse 
 is not as well off when he does the work under the lash 
 as when he does it in a properly rested condition. Similarlj^, 
 persons suffering from sleeplessness often take drugs used 
 to produce sleep (hypnotics), and, superficially at least, the 
 sleep thus secured resembles normal sleep ; but experience 
 
360 THE HUMAN MECHANISM 
 
 shows that few if any hypnotics can be used for any length 
 of time without bad effects. Here again a drug is being 
 depended upon to do what the normal body should do for 
 itself. Pepsin tablets may be taken to aid digestion, and 
 thereby an attack of indigestion may sometimes be pre- 
 vented or relieved ; but a healthy stomach should furnish 
 its own pepsin ; and the fact that it does not do so is a sure 
 warning that something is wrong in the conduct of life. 
 It is irrational to neglect the duty of attending to the 
 cause of the ailment, and it is foolish to substitute tem- 
 ■ porary relief for permanent cure. Perhaps if the drug did 
 all that the proper care of the body does, and did no more, 
 no serious olijection could be made to its use ; but there is 
 probably no drug of which this is true, and for this reason 
 it is foolish and rash to try to substitute the use of drugs 
 for the hygienic conduct of life. 
 
 Lastly, if the drugs do not accomplish in the long run 
 what should be done by the hygienic conduct of life, their 
 extensive use becomes all the more dangerous in view of 
 the unquestioned fact that we are apt thereby to become 
 their slaves. Every man is the slave, broadly speaking, of 
 the habits he forms, and it is only a question as to whether 
 he will be the willing slave of good habits or the abject 
 slave of bad liabits. The man who leads a hygienic life is 
 the slave of muscular activity, of correct feeding, of proper 
 clothing, of rest, etc. ; that is to say, these things become 
 necessary to his life ; he cannot get along without them. 
 If for these proper agents of health he persistently sub- 
 stitutes some drug, whether it be alcohol, or tobacco, or 
 coffee, or tea, or chocolate, or opium, the habit of using 
 the drug is substituted for that of maintaining normal 
 conditions. But since drugs cannot entirely take the place 
 of such conditions, the constitution goes from bad to worse, 
 and increasing dependence must be placed upon the drug. 
 It is a safe rule that whenever we are uncomfortable or 
 
DKUGS, ALCOHOL, AND TOBACCO 361 
 
 unhappy without the use of a certain drug we should cease 
 using it until, with the help of hygienic living, we can get 
 along without it. 
 
 There are people who are slaves of coffee, of tea, of 
 chocolate, of patent medicines, of candy, and of soda water, 
 just as truly as there are slaves of tobacco, or of alcohol, 
 or of opium. It is worse to be the slave of alcohol than of 
 coffee, because the evil consequences of alcohol are greater 
 than those produced by the corresponding use of coffee ; 
 but it is by the same process in both cases that the man 
 or woman becomes a slave to the drug, and that process is 
 the formation of bad habits. 
 
 With these practical considerations about the use of 
 drugs, — by which term it will be seen that we mean, not 
 simply the medicines purchased from the apotliecary, but 
 all those substances which are taken into the body in order 
 to give some new or abnormal direction to the course of 
 events in the organism, — we may pass on to the discussion 
 of those in common use. 
 
 4. Tea and Coffee. — Different as are these drinks in 
 taste and appearance, their most important physiological 
 effects are due essentiall}' to the same substances, viz. 
 caffeine (or theine) and tannic acid (or tannin). Caffeine 
 is a very powerful stimulant, especially of the nervous 
 system, and also of the heart, although probably to a lesser 
 degree ; tannin, on the other hand, is a bitter, astringent 
 substance, which may considerably hinder digestion and 
 directly injure the mucous membrane of the stomach. Tea 
 contains about twice as much tannin as an equal weight of 
 coffee, but as coffee is frequently made much stronger than 
 tea the actual amount per cup may often be more nearly 
 equal in the two drinks than these figures indicate. The 
 amount of tannin dissolved in tea varies greatly with 
 the method of preparation, and largely for this reason tea 
 should not be boiled, nor allowed to steep too long. The 
 
362 THE HUMAN MECHANISM 
 
 proper method of making tea is to pour over the dry leaves 
 water which has been brought just to the boiling point, 
 and then to allow the infusion to stand, without further 
 heating, for not more than a few minutes. 
 
 Both tea and coffee seem to have a slightly retarding 
 influence upon gastric digestion. In healthy people this 
 is of little consequence, but when the digestive powers are 
 in any way impaired the use of these beverages may be 
 inadvisable. The more important effect, however, of both 
 tea and coffee is in their stimulating action on the nervous 
 system. No satisfactory explanation has yet been given 
 of the fact that some people can use tea and not coffee, 
 while with others the reverse is true. It is probably safe 
 to say that when used in moderation, tea and coffee are usu- 
 ally harmless to those leading an otherwise hygienic life. 
 They should be used sparingly by nervous people and by 
 those in whom digestion is feeble and slow (Hutchinson). 
 Even by the perfectly healthy they should not be used 
 to excess, nor should the habit be acquired of using them 
 as the whip to the tired horse. Drinking strong coffee in 
 order to keep awake for evening study is objectionable, and 
 the substitution of afternoon tea for a little rest or sleep 
 is also unwise. 
 
 5. Cocoa is made from the seeds of trees of the genus 
 Theobroma, and chocolate is prepared from cocoa. In the 
 solid form both are highly nutritious, as shown by the 
 following average results of analyses. 
 
 ProteUl Fat Carbohydrate 
 
 Cocoa 21.6% 28.0% 37.7% 
 
 Chocolate 12.9% 48.7% .30.3% 
 
 When used as a beverage, however, the nutriment derived 
 from them is small. In addition, cocoa and chocolate both 
 contain theobromine, a substance closely related chemically 
 to caffeine and possessing much the same stimulating prop- 
 erties. In general, the same hygienic considerations which 
 
DRUGS, ALCOHOL, AND TOBACCO 363 
 
 apply to the use of tea and coffee should guide us also in 
 the use of chocolate and cocoa. 
 
 6. Soda Water and Similar Beverages Of these little 
 
 need be said. In general they are harmless enough, espe- 
 cially to those enjoying perfect' digestion. The large 
 amount of sugar which they contain is apt to make matters 
 worse in many cases of dyspepsia; by taking them fre- 
 quently between meals the appetite for wholesome food 
 is impaired, and excessive indulgence in them under any 
 circumstances is needless and foolish. 
 
 7. Alcoholic Beverages. — In the case of an alcoholic 
 drink we have to deal with something which, like tea and 
 coffee and cocoa and " temperance drinks," is used as a 
 beverage and to that extent must be classed in the same 
 group. Alcoholic drinks are, however, taken as stimulants 
 and so resemble tea and coffee and cocoa, but they differ 
 from all of these in their action upon the body. Moreover, 
 their abuse gives rise not only to degraded moral and 
 social conditions but is also attended with bad hygienic 
 effects. Every one sliould be informed of their nature and 
 of the dangers attending their use. 
 
 The common alcoholic beverages consist of (1) malt 
 liquors, including beer and ale ; (2) wines, such as hock, 
 claret. Burgundy, sherry, and champagne ; (3) distilled 
 liquors, including brandy, whisky, rum, and gin ; and (4) 
 liqueurs and cordials. These groups are distinguished 
 from one another largely by the method of preparation 
 and by the amount of alcohol they contain. Malt liquors 
 are fermented liquors which contain from three to eight 
 per cent of alcohol ; wines are also fermented liquors, but 
 contain from seven to twenty per cent of alcohol ; dis- 
 tilled liquors, on the other hand, are first fermented and 
 then concentrated by distillation, and contain from thirty 
 to sixty-five per cent of alcohol. In all these the most 
 important constituent, so far as their physiological action 
 
364 THE HUMAN MECHANISM 
 
 upon the body is concerned, is the chemical compound 
 known as ethyl alcohol (CgHgO or CgHg . OH). 
 
 8. Fermentation. — The ethyl alcohol in each of these 
 beverages is produced by the action of yeast on sugar, and 
 this action is known as 'alcoholic fermentation. Yeast is a 
 
 unicellular plant, and when a small 
 amount of it is added to a solution of 
 grape sugar or fruit sugar it breaks 
 up these substances, chiefly into al- 
 cohol and carbon dioxide gas. The 
 ^^x-v,^ ^— O latter passes off, while the alcohol 
 yi-^ remains behind in the solution. In 
 
 Vi,' addition to these chief products of 
 
 Fig. 110. Yeast cells fermentation there are always formed 
 other products in small quantities, 
 and to these, in part, the flavor of the fermented mixture 
 is due. Different varieties of yeast produce different kinds 
 of fermentation. Thus one variety (domesticated yeast) is 
 used in making beer, and another (wild yeast) in making- 
 wine. The amount of alcohol produced differs with the 
 yeast used, as do also the character and quantity of the 
 secondary products. The growth of yeast, like that of all 
 living ferments, is checked by the accumulation of the 
 products of its own activity. C'onsec[uently when the alco- 
 hol produced reaches a certain percentage (usually less 
 than ten per cent) the fermentation ceases. Alcoholic 
 drinks which contain higher percentages of alcohol are pre- 
 pared by special processes which will be described later. 
 
 9. Malt Liquors. — Malt consists of sprouted grains 
 (chiefly barley). The grains contain a large amount of 
 starch which during the process of germination is converted 
 into sugar by diastase, an enzyme produced by the living 
 cells of the plant, — the action of diastase being essentially 
 similar to that of the ptyalin of the saliva. The germinat- 
 ing plant thus comes to contain considerable quantities of 
 
DRUGS, ALCOIiOL, AND TOBACCO 365 
 
 sugar, together with salts, j)roteicls, and other substances. 
 The watery extract of malt is known as icort, and it is this 
 which, after l)cing l)oiled with hops, is acted upon by the 
 yeast. The liijuid tlius produced from wort by fermenta- 
 tion is known as ale, beer, stout, porter, etc., according to 
 the conditions under wiiich the fermentation takes place 
 and the character of the malt and the yeast emploj-ed. 
 Cxerman beers contain from three to four per cent of alco- 
 hol ; ale contains from four to six per cent. 
 
 10. Wines. — Wine is produced by the fermentation of 
 the juice obtained by crushing grapes, and the yeast comes 
 from the " bloom " on the skin of tlie grapes. The juice, 
 or " must," thus extracted is all()weil to luidergo fermenta- 
 tion, and the fermented liquid is wine. Most wines, how- 
 ever, are subjected to subsequent treatment. Some are 
 allowed to ripen in wooden casks, during which process 
 there take plai;e chemical changes which give to each 
 wine its pecidiar flavoi-. In other cases the wine is 
 " fortified '" by the diicct addition of alcohol. Wines differ 
 from one another accor;ling to the variety of the grape 
 used in making tlie must, according to the variety of 
 yeast used for fermentation, and according to other cir- 
 cumstances. 
 
 11. Distilled Liquors and Spirits. — This group of alco- 
 holic bevei'ages contains the highest percentage of alco- 
 hol, and includes whisky, brandy, rum, and gin. In the 
 making of all of these the essential procedure is the same ; 
 namely, first to produce fermentation in some sugary 
 liquid, and afterwards to distill from the products of this 
 fermentation its alcohol and some other volatile constitu- 
 ents. Whisky is made by distilling fermented corn or 
 rye ; brandy may be spoken of as distilled wine ; rum is 
 distilled from fermented molasses, and gin from a fer- 
 mented mixture of rye and malt, — juniper berries and 
 other substances being added to the distilled product. In 
 
366 THE HUMAN MECHAJSfISM 
 
 general, distilled liquors contain from thirty to sixty per 
 cent of alcohol. 
 
 With these differences of preparation, alcoholic bever- 
 ages differ greatly among themselves, independently of the 
 quantity of alcohol they contain, and some of their special 
 effects are due to other constituents. The chief danger 
 of most of them, however, lies in the action of the ethyl 
 alcohol upon the system, and we shall confine our dis- 
 cussion to the effects of this substance. The problem is by 
 no means a simple one, because these beverages are used 
 in so many different ways by different people. Moreover, 
 the results of their use differ according to the constitution 
 of the person using them, and according to his other habits 
 of life. Sweeping assertions are too frequently made, in 
 good faith, only to be found false by experience in special 
 cases, and in this way harm is done where good was in- 
 tended. For example, it is often asserted that alcohol used 
 in any amount whatever is a poison to the healthy organ- 
 ism. If this be so, it is the only known drug of which 
 this is true. Dr. John J. Abel, from whom we shall ex- 
 tensively quote, says on this subject: "All poisons are 
 capable of being taken without demoyistrahle injury in a 
 certain quantity, which is for each of them a special 
 though sometimes very minute fraction of their toxic or 
 lethal dose. There is no substance which is always and 
 everywhere a poison." Alcohol is a drug, and, like many 
 drugs, may be and frequently is used in poisonous doses ; 
 but it must not be supposed that its real danger lies in the 
 fact that it always exerts a poisonous effect on the body. 
 
 12. The Physiological Action of Alcohol. — As to the 
 immediate action of alcohol on the body we may say that 
 it belongs in the same general class of drugs as the ether 
 and chloroform used for anesthesia ; in other words, its 
 general action is that of an hyjmotic or anesthetic. To 
 
DEUGS, ALCOHOL, AND TOBACCO 367 
 
 quote again from Dr. Abel : " An exhilarating action is 
 an inherent property of these substances in certain doses. 
 Occasionally the jihysician meets with persons who have 
 formed the habit of inhaling chloroform from the palm of 
 the hand or from a lightly saturated handkerchief. The 
 inhalation is usually carried on for a short time only, and 
 its object is to induce a pleasant form of mental stimula- 
 tion. Only occasionally is the inhalation of chloroform 
 carried on until helpless intoxication occurs." And again : 
 " That alcoliol can produce as profound anesthesia as any 
 of the substances named is also well known. In the days 
 before anestliesia it was the custom of bone setters to ply 
 their patients with alcohol in order to facilitate the reduc- 
 tion of difficult dislocations. . . . The anesthesia produced 
 by alcohol is, however, not commendable, since it cannot 
 safely be induced in a short time and is too prolonged. 
 The quantity needed for sru-gical anesthesia would in many 
 cases lead to a fatal result." 
 
 13. Is Alcohol a Stimulant ? — The view of the action of 
 alcohol just stated is, of course, borne out by the condition 
 of a thoroughly intoxicated person ; but it is opposed to 
 the veiy general idea that alcohol, except in large doses, is 
 to be regarded as a stimulant. Whether we shall call it a 
 "stimulant" or not depends upon how we use that term. 
 Some of the exhilarating effects of alcoholic drinks might 
 lead us to speak of it in this way. People Avho have drunk 
 wine often become moi-e talkative, so that the first effects 
 of intoxication often resemble those of stimulation. There 
 is, however, stroncr reason for thinkino- that this action is 
 only superficially, and not fundamentally, a case of stimu- 
 lation, as we shall now see. 
 
 In studying the physiology of the nervous system we 
 found that processes of inhibition are as important in its 
 operation as are those of excitation ; and in mental opera- 
 tions the course of our thinking is constantly checked or 
 
368 THE HUMAN MECHANISM 
 
 inhibited by the knowledge of facts opposed to the con- 
 clusions towards which we are tending. ProhaUy it is this 
 essential feature of all accurate and valuable mental work 
 which is the first to be jyarali/zed by alcohol. The man who 
 takes alcohol becomes fluent, not because he is stimulated, 
 but because of the removal of checks whose presence may 
 make him talk less fluently, but which at the same time 
 make him speak more accurately. He may become witty, 
 and may say some brilliant things ; but he will almost 
 always do and say some very erratic things. 
 
 Tlie following (by Dr. Abel) appears to be a sound state- 
 ment of our present knowledge of this important subject: 
 " Alcohol is not found by psychologists to increase the 
 quantity or vigor of mental operations ; in fact, it clearly 
 tends to lessen the power of clear and consecutive reason- 
 ing. In many respects its action on the higher functions 
 of the mind resembles that of fatigue of the brain, though 
 with this action is associated a tendency to greater motor 
 energy and ease. 
 
 "In speaking of a certain type of individual James says: 
 ' It is the absence of scruples, of consec^uences, of consider- 
 ations, the extraordinary simplification of each moment's 
 outlook, that gives to the explosive individual such motor 
 energy and ease.' This description aptly applies to the 
 individual who is under the influence of a ' moderate ' 
 quantity of alcohol. It tends to turn the inhibitive type 
 of mind into the 'hair-trigger' type. We have said that 
 the speech and the bearing of men, the play of their fea- 
 tures, all bear witness to the action of alcohol on the lirain; 
 that it removes restraints, blunts too acute sensibilities, dis- 
 pels sensations of fatigue, causes a certain type of ideas 
 and mental images to follow each other witli greater rapid- 
 ity, and gives a ' cerebral sense of richness.' 
 
 " Larger quantities, such as are for most individuals 
 represented by one or two bottles of wine (ten per cent 
 
DKUGS, ALCOHOL, AND TOBACCO 369 
 
 of alcohol), may, according to the resistance and type of 
 individual in question, cause a lack of control of the emo- 
 tions ; noticealily affect the power of attention, of clear judg- 
 ment and reason ; and decidedly lower the acuteness of the 
 several senses. In many individuals such quantities will 
 develop so marked an anesthetic action that all phenomena 
 of intoxication may be seen to follow each other ia due 
 sequence, finally to end in the sleep of drunkenness. 
 
 " There has been much discussion as to whether alcohol 
 is in any sense a stimulant for the brain. We have seen 
 that pharmacologists of high repute deny that it has this 
 action, holding that alcohol is a sedative or narcotic sul>- 
 stance which belongs to the same class as paraldehyde and 
 chhjroform ; that its stimulating action is but fictitious ; 
 and that even the earlier phenomena of its action are to 
 be referred to a paralyzing action on cereljral (inhiljitory) 
 functions. This theory assumes an une(j^ual action on 
 cerebral functions in the order of time. Kraepelin, how- 
 ever, holds that this is a purely subjective analysis, and 
 that in the early stages of its action alcoliol truly stimu- 
 lates the motor functions of the brain ; that a state of men- 
 tal exhilaration, of ' motor excitability," may coexist with 
 undiminished power c>f perception and judgment. His 
 psychological experiments on the action of alcohol, taken 
 all in all, do not, however, entirely prove his position." 
 
 Some cases of apparent stimulation are really due to the 
 fact that alcohol, when taken in tlie form of ^^dnes and 
 distilled liqnors, sets up an irritation in the mucous mem- 
 brane of the mouth, oesophagus, and stomach, which re- 
 fie.rly excites the heart to greater activity or for the time 
 being refle.rh/ stimulates the nervous system. Such stimu- 
 lation is, however, transient and, as the alcohol is absorbed 
 into the blood, gives way to depression and even stupor. 
 
 It is neitlier possible nor necessary to state here in full 
 the reasons which have led to what seems to the authors 
 
370 THE HUMAN MECHANISM 
 
 the erroneous view that alcohol in small doses is a stimu- 
 lant and only in larger doses a depressant and hypnotic. 
 Enough has been said to show that there are at least two 
 opinions about the matter : that even if alcohol is at 
 times a stimulant, it is an uncertain stimulant; and that 
 its excitation is liable to give way at any time to de- 
 pressing effects. A critical examination of the literature 
 on the subject has failed to demonstrate to us a direct 
 stimulating action of alcohol on any of the functions, such 
 as the beat of the heart, respiration, digestion, etc. At 
 times, especially in sickness, alcohol may be useful ; but 
 the evidence tends to the conclusion that where it exerts 
 any physiological action on the healthy body at all, that 
 action is usually depressing. This is notably true as to 
 the beat of the heart, as to respiration, and as to the 
 ability to do muscular work. 
 
 We have dwelt at length upon this question in order to 
 disabuse the student's mind of the idea that alcoholic drinks 
 can be safely depended upon as an aid in the perform- 
 ance of work. Few causes are more effective in leading 
 to the abuse of alcohol than the idea that when one finds 
 difficulty in doing a thing it may be accomplished more 
 easily by having recourse to beer or wine or whisky for 
 their "stimulating" effect. In general, so far is this from 
 being the truth that the person seeking such aid is really 
 using a hypnotic and depressant. Obviously he would be 
 acting more wisely to adopt other methods of accomplish- 
 ing his end. Nor is this conclusion merely theoretical. 
 Brain workers who wish to " keep a clear head " almost 
 universally avoid alcoholic drinks, at least until work is 
 over. And even among those who do drink it is customary 
 to avoid drinking until the day's work is done. 
 
 14. Alcohol in Muscular Work.— That the general effect 
 of alcoholic drinks is to depress rather than stimulate the 
 powers of the body is furthermore indicated by the results 
 
DEUGS, ALCOHOL, AND TOBACCO 371 
 
 of experiments on men doing heavy work, as, for example, 
 soldiers on forced marches. In the Ashanti campaign the 
 effect of alcohol as compared with beef tea was tested. 
 " It was found that when a ration of rum was served out, 
 the soldier at first marched more briskly, but after about 
 three miles had been traversed the effect of it seemed to be 
 worn off, and then he lagged more than before. If a second 
 ration were given, its effect was less marked, and wore off 
 sooner than that of the first. A ration of beef tea, how- 
 ever, seemed to have as great a stimulating power as one 
 of rum, and not to be followed by any secondary depres- 
 sion " (Lauder Brunton). The results of these and other 
 experiments lead us to the conclusion that alcohol cannot 
 be depended upon to increase the capacity for hard mus- 
 cular work, and that in the great majority of cases it actu- 
 ally diminishes it. 
 
 15. The Dilation of Cutaneous Arteries by Alcohol. — 
 One of the most important effects of alcoholic drinks is 
 the dilation of the arteries of the skin, thus sending more 
 warm blood to the surface. It is a common experience 
 among persons not accustomed to alcoholic drinks that 
 even a small amount " makes the face hot " and flushed, 
 and the red face of the toper is jjroverbial. The result 
 of this dilating effect is that the temperature of the skin 
 rises and the individual feels warmer. Congested states of 
 internal organs may thus be relieved, and this is probably 
 one reason why men leading an exclusively sedentary life 
 often use alcoholic drinks apparently to some advantage. 
 But even these would do infinitely better to secure the 
 same result by proper muscular activity. 
 
 Even if a temporar}'- advantage appears to be gained 
 in some cases or at some times, this has often to be paid 
 for by bad secondary effects, such as impaired capacity for 
 good work some hours later ; and in mental work of the 
 highest kind, such as original writing or composition, the 
 
372 THE HUMAN MECHANISM 
 
 after effects of alcoholic drinks are sometimes prolonged 
 and easily detected by the subject of the experiment. 
 
 16. Alcohol as a Defense against Exposure to Cold. — Be- 
 cause of this effect upon the cutaneous circulation alco- 
 holic drinks are frequently used by men exposed to cold, 
 with the mistaken idea that the conditions within the body 
 are thereby improved. The student has, however, learned 
 (p. 192) that a feeling or sensation of warmth does not 
 necessarily indicate greater heat production within the 
 body ; and he also knows that bringing the blood to the 
 skin when the body is exposed to cold serves to increase 
 the loss of heat. As a matter of fact, the internal temper- 
 ature often falls when alcohol is taken under these condi- 
 tions. The story is told of some woodsmen who were 
 overtaken by a severe snowstorm and had to spend the 
 night away from camp ; they had with them a bottle of 
 whisky, and, chilled to the bone, some imbibed freely while 
 others refused to drink. Those who drank soon felt com- 
 fortable and went to sleep in their imjjrovised shelter ; 
 those who did not drink felt very uncomfortable through- 
 out the night and could get no sleep, but in the morning 
 they were alive and able to struggle back to camp, while 
 their companions who had used alcoholic drinks were 
 found frozen to death. They had purchased relief from 
 their unpleasant sensations of cold at the cost of lowering 
 their body temperature below the safety point. This, if 
 true, was, of course, an extreme case ; but it accords with 
 the universal experience of arctic travelers and of lumber- 
 men and hunters in northern woods, that the use of alcohol 
 during exposure to cold, although contributing greatly to 
 one's comfort for the time being, is generally followed by 
 undesirable or dangerous after effects. 
 
 17. Alcohol as a Food. — There has been much discus- 
 sion as to whether alcoliol is or is not a food, i.e. whether 
 its oxidation within the body maj' supply energJ^ This 
 
DKUGS, ALCOHOL, AivL TOBACCO 373 
 
 question must now be answered in the aiSrmative, although 
 whether it can do more than supply heat to maintain the 
 body temperature, i.e. whether it can also supply the power 
 for muscular work, as do fats and carbohydrates, we cannot 
 in the present state of our knowledge positively say. In 
 many cases of sickness the oxidation of alcohol is probably 
 a useful source of heat production, since it is absorbed 
 quickly and without digestion ; but the healthy man does 
 not and should not use it in this way. The amounts which 
 would be recjuired to be of any considerable service as food 
 are far beyond those in which it may be used with safety. 
 In other words, in using alcohol for food, one would be 
 obtaining heat at the cost of direct injury to many organs, 
 and also at the cost of impaired working power. iMoreover, 
 men do not use alcohol as a food ; they use it as a drug. 
 So that while the action of alcohol as a food is of practical 
 importance to the pliysician, wlio must deal with the 
 abnormal conditions of disease, its action as a food is not 
 a matter of practical importance to healthy people. 
 
 18. Pathological Conditions Due to the Use of Alcohol. — 
 When alcoholic beverages are taken in excessive amounts 
 we have the sad and degrading spectacle of a '• drunken 
 spree." Whetlier or not the drinker at first appears bright 
 or witty, sooner or later there is presented the pitiable pic- 
 ture of complete loss of nervous coordination and control. 
 The man becomes silly, or maudlin, or pugnacious, as the 
 case may be, but always in-ational ; he staggers, stumbles, 
 or falls ; and finally passes into a drunken stupor. In this 
 event the victim of his own indulgence is said to be "dead" 
 drunk, or "intoxicated," being as it were thoroughly 
 poisoned. If such intoxication is frequently repeated, there 
 is a complete bi'cakdown of the nervous system ; the vic- 
 tim of alcoholic indulgence becomes a raving maniac, and 
 with disordered vision tliinks he sees all about him snakes 
 or foul vermin (delirium tremens). The silly or foolish stage 
 
374 THE HUMAN MECHANISM 
 
 of this poisoning sometimes provokes smiles or laughter 
 in thoughtless observers, but none can witness the more 
 serious consequences of repeated intoxication by alcoholic 
 drinks without disgust and horror. 
 
 Many steady drinkers, even though they have never been 
 drunk in their lives, are apt ultimately to acquire various 
 diseased conditions of the body, into which we cannot 
 enter in detail. The heart may be injured, or the arteries 
 become diseased ; the repeated irritation of the stomach 
 may produce chronic gastritis ; or the connective tissue of 
 the liver and kidneys may increase, thus crowding upon 
 the living cells and ultimately throwing a large part of 
 them entirely out of use. While it must not be supposed 
 that drinking alcohol is the sole cause of these troubles, — 
 for some or all of them may come from other causes, — the 
 frequency of their occurrence in steady drinkers is sus- 
 piciously high, and this has led to the very strong con- 
 viction among medical men that alcohol plays a large r81e 
 in producing them. 
 
 19. Summary of the Action of Alcohol as a Drug. — In 
 small doses alcohol may be completely oxidized within the 
 body without exerting anj^ pharmacological action. In the 
 forms and amounts usually employed in alcoholic beverages 
 it exerts, m general, a hypnotic or anesthetic action ; the 
 result on the system as a whole depends on the amount 
 taken, and varies from the jiaralysis of inhibitory processes 
 to the depression of all nervous functions, ending in 
 drunken stupor. Continued excess may produce exagger- 
 ated forms of temporary insanity, among which delirium 
 tremens may be mentioned. There is, moreover, good rea- 
 son for believing that steady drinking is very frequently 
 an important agent in preparing the way for many other 
 diseases, and is hence a serious menace to health. 
 
 20. The Seat of the Danger in Alcoholic Drink. — The 
 regular use of alcoholic beverages is dangerous for the 
 
DRUGS, ALCOHOL, AND TOBACCO 375 
 
 same reason that the regular use of any drug is dangerous. 
 We are too apt to rely upon the drug to do for us what we 
 ought to accomplish only by the hygienic conduct of life ; 
 the drug never satisfactorily does the work, and we go from 
 bad to worse, and become its slave. But there is certainly 
 greater danger in hypnotic drugs, like alcohol, than in true 
 stimulants, like coffee, and cocoa, and tea. We need to 
 have ourselves well under control when v,e use any drug ; 
 the highest faculties of the mind must keep tight rein or 
 we may lose control of ourselves. With hypnotic drugs — 
 to which class belong not only alcohol but ether, chloro- 
 form, opium, chloral, etc. — there is special danger that 
 these powers of control (inhibition) may be stealthily para- 
 lyzed before we know it. Of course thousands of people 
 use alcohol in moderation and never become drunkards ; 
 but thousands also, with no intention of using it to excess, 
 do unconsciously let the reins drop, and before they know 
 it the drug gets the better of them. Experience shows 
 that it is with the hypnotic drugs that this most frequently 
 happens. 
 
 Again, if we make a habit of taking alcoholic drinks, we 
 are specially exposed to temptation from our fellow-men to 
 go too far. For the most part, people take coffee and tea 
 or do not take them, as they please ; no one urges them 
 to use these drinks when they are disinclined to do so. 
 To a less degree the same thing is true of tobacco, although 
 here the force of fashion and example is stronger. But 
 with alcoholic beverages the custom of "treating" makes 
 the exercise of self-restraint more difficult than it would 
 otherwise be ; for here we are dealing with a drug which 
 is capable of impairing self-control. Some one " treats " a 
 friend to a drink; the friend wishes to return the compli- 
 ment, and so they drink again; the person with deficient 
 self-control — and what little he has now lessened — 
 insists upon a third, and so on, perhaps to intoxication. 
 
376 THE HUMAN MECHANISM 
 
 This, of course, does not always happen ; thousands are 
 strong and escape the danger, but thousands are weak or 
 do not know better, and many a week's wages has gone in 
 this way, leaving behind poverty and misery and imi^aired 
 capacity before the close of Saturday night. 
 
 21 . Concluding Remarks on the Use of Alcoholic Beverages. 
 — In the foregoing pages we have stated the salient facts 
 concerning the physiological action of alcohol and alcoholic 
 drinks. It only remains to point out for the student the 
 obvious conclusions to be drawn from them and from the 
 long and on the whole very sad experience of the race 
 with alcoholic drinks. The first is that, except in sickness 
 and under the advice of a physician, alcoholic drinks are 
 wholly unnecessary, and much more likely to prove harm- 
 ful than beneficial. The last is that their frequent and 
 especially their constant use is attended with the gravest 
 danger to the user, no matter how strong or self-controlled 
 he may be. 
 
 It is true that history and romance and poetry contain 
 many attractive allusions to wine and other alcoholic 
 drinks, and it may also be true that such drinks, by loosen- 
 ing tongues and breaking down social, political, or other 
 barriers (removing inhibitions), may tend towards conviv- 
 iality and good-fellowship ; but it is no less true that the 
 path of history is strewn with human wreckage directly 
 due to alcohol ; that many a promising career has been 
 drowned in wine ; and that indescribable misery follows 
 in the trail of drunkenness. The only absolutely safe atti- 
 tude toward alcoholic drinks is that of total abstinence 
 from their use as beverages. 
 
 22. Opium, Morphine, and the Opium Habit. — The danger 
 of the use of drugs as a regular habit of life is perhaps 
 most painfully illustrated by what is known as the opium 
 haljit. Among the most valuable remedies at the phj-sician's 
 disposal is opium or its active principle, morphine, which 
 
DEUGS, ALCOHOL, AND TOBACCO 377 
 
 possesses remarkable power to produce insensibility to pain. 
 It sometimes happens, however, that by incautiously using 
 this drug for this purpose men and women become addicted 
 to the habit. They finally cannot do without the drug, 
 and its constant use causes an appalling moral and physical 
 degeneration; so far indeed does this often go that the 
 victim will commit crime in order to obtain the drug. It 
 should be clearly understood that it is unsafe for any one 
 to use opiates to relieve pain ; indeed, these should never 
 be used except when prescribed by a careful physician. 
 
 23. Chloral, Cocaine, etc. — Men and women may become 
 slaves to the use of other drugs, and in much the same 
 way as they become slaves to alcohol and morphine. 
 Among these drugs are chloral aud cocaine. They belong 
 in the same general group of hypnotics or anesthetics, and 
 the habit acquired is perhaps no worse tlian the opium 
 habit. It is certainly very little better. Let the student 
 remember that tlie root of the evil here, as elsewhere, is 
 the substitution of the use of the drug for normal habits 
 of healthful living. 
 
 24. Tobacco. — The jjhysiological effects of tobacco are 
 quite complicated, so complicated that it is difficult to 
 make general statements with regard to them. The effects 
 of chewing are quite different from those of smoking, and 
 those of smoking, no doubt, vary according as the smoke 
 is or is not drawn into the lungs (inhaled). 
 
 The leaf of tobacco contains a poison, nicotine, which 
 exerts a powerful action on the heart and on nerve cells. 
 It is not, however, proved that the bad effects of the use 
 of tobacco are due entirely or even chiefly to this substance ; 
 but it unquestionably contributes to the physiological 
 effects. 
 
 The smoke from tobacco also contains ammonia vapor 
 which locally irritates the mucous membrane of the mouth, 
 throat, nose, etc., and this irritating action at times acts 
 
378 THE HUMAN MECHANISM 
 
 as a stimulant to the whole system in much the same 
 manner as do " smelling salts." 
 
 It has been recently suggested that, owing to the incom- 
 plete character of the combustion, tobacco smoke contains 
 a small amount of the poisonous gas, carbon monoxide (CO), 
 and it is quite possible that some effects of smoking — 
 especially where the smoke is drawn into the lungs 
 (inhaled) — may be attributed to this gas ; but the sug- 
 gestion has not yet been submitted to the test of actual 
 experiment. 
 
 Indeed, the physiological action of tobacco probably not 
 only varies with the form in which the tobacco is used but 
 is in any case the result of a combination of a number of 
 factors partly physiological and partly psychical. We must, 
 here, however, confine our attention to the purely hygienic 
 aspects of the matter. 
 
 Human experience shows that the unwise use of tobacco 
 may unfavorably affect digestion, cause serious disorders 
 of the heart, and impair the work of the nervous system. 
 Those training for athletic events are usually forbidden the 
 use of tobacco because it " takes the wind," i.e. makes 
 impossible the most efficient training of the heart. Many 
 employers have found that youths who smoke cigarettes 
 are less reliable in their work ; and this is only one instance 
 of the effect upon the nervous system already referred to, 
 the same result being observed in a diminished steadi- 
 ness of the hand, often amounting to actual tremor. 
 
 These effects do not, of course, manifest themselves in 
 their extreme form whenever tobacco is used ; but it is 
 probable that they are always present in some degree. 
 Whether they are noticeable or not depends largely upon 
 the ability of the constitution to resist them. Tobacco is 
 thus often used without demonstrable bad effects when one 
 is leading a hygienic life ; but very often the habit, formed 
 under these conditions, persists after the increasing intensity 
 
DEUGS, ALCOHOL, AND TOBACCO 379 
 
 of occupation, and the attendant cares and responsibilities 
 of life result in neglect of muscular exercise and improperly 
 directed nervous activity. And as this neglect begins to 
 tell on general health it is found that the unfavorable 
 effects of tobacco become more pronounced. 
 
 Especially to be condemned is its use by those who 
 have not attained their full growth. During youth noth- 
 ing should be allowed to interfere with the best devel- 
 opment of the heart and nervous system, and the use 
 of tobacco endangers the proper development of both of 
 these most important parts of the human mechanism. It 
 can hardly be doubted that many a young man has failed 
 to make the most out of life because the habit contracted 
 in youth has struck in this way at the foundations upon 
 which he had subsequently to build. 
 
CHAPTER XXI 
 
 THE PREVENTION AND CARE OF COLDS AND SOME 
 OTHER INFLAMMATIONS 
 
 1. Hygiene and Physical Efficiency. — A most important 
 aim of personal hygiene is the maintenance of the highest 
 working efficiency of the body. We should not be content 
 with the avoidance of serious maladies like smallpox, 
 diphtheria, and consumption, but should try also to avoid 
 those minor ills which, though temporary and rarely fatal, 
 may seriously interfere with our capacity for usefulness and 
 enjoyment. The importance of avoiding constipation has 
 already been pointed out (p. 128). The present chapter 
 will be devoted to the practical consideration of such com- 
 mon complaints as colds, rhenniatmn, and diarrhea, all of 
 which are accompanied by inflammatory conditions in some 
 internal organ or organs, and are favored by exposure to 
 cold, drafts, or dampness, which chill the skin and drive 
 the blood into the internal organs. 
 
 2. Some Common Complaints and the Conditions which 
 favor them. — We shall not give any extended account of 
 the nature of the complaints mentioned in the preceding 
 paragraph, for their exact causes are still obscure. Two 
 points, however, should be emphasized for all of them. 
 
 1. The exposure to cold is not usually the cause of these 
 ■diseases hut only favors their development. It is the general 
 experience of arctic travelers that they suffer very little 
 or not at all from " colds." Nansen and his men were away 
 in the Fram for more than three years, and during a large 
 part of that time Nansen and Johaunson journeyed on sleds 
 
 380 
 
PREVENTION AND CARE OF COLDS 381 
 
 or afoot, exposed to the worst rigors of an arctic climate. 
 At times, after getting into their sleeping bags, they had to 
 thaw out their frozen clothing by the heat of their own 
 bodies before they could go to sleep ; and yet not one of 
 them had " a cold " until their return to Norway, when 
 an epidemic of colds l>roke out among them. This and 
 numerous similar experiences of others suggest strongly 
 that colds are largely infectious diseases ; but we must not 
 forget that dampness and drafts are favoring conditions 
 for their development. The experience of the race on this 
 point is abundant and conclusive. 
 
 2. Each of these dini/Kses is characterized by a condition 
 qfinflariuiiation. We shall not attempt to describe the exact 
 nature of inflammation ; it is sufficient to recall features 
 of it familiar to every one. The sting of a bee or hornet 
 or the bite of a mosquito results in local inflammation of the 
 skin ; a severe case of sunburn presents a similar condition 
 over larger areas ; a wound of any kind often shows more 
 or less of the same inflammatory process. The part becomes 
 7-ed, indicating the presence of an increased amount of 
 blood ; it is swoUe^i, partly because of the greater quantity 
 of blood and partly because of the greater quantity of 
 lymph present in the tissue ; it is usually hot ; and it is 
 often jjainful. At times, as in the case of a wound or boil, 
 j^m, or " matter, " may be formed. 
 
 One or more of these conditions is present in an inflamed 
 organ during the diseases mentioned. When we have a 
 cold in the head (rhinitis) the vascular membrane lining 
 the nasal cavity is the seat of trouble ; in a sore throat it 
 is the pharynx and larynx {phcn-yngitis and Jarijngitis) ; 
 in a cold on the chest (bronchitis) it is the ciliated mem- 
 brane of the trachea and bronchi ; similarly in catarrhal 
 attacks of the stomach and intestine it is the mucous 
 membrane of these organs ; and we must think of these 
 inflamed tissues of the respiratory and alimentary tracts as 
 
382 THE HUMAN MECHANISM 
 
 presenting somewhat the same condition as that seen in the 
 skin during a bad case of sunburn. They all have an exces- 
 sive amount of blood within them ; they are more or less 
 swollen, — as when one's "nose is stopped up " ; there is 
 an unusual amount of fluid in the tissue ; and there is, 
 besides, generally a transudation of this fluid to the surface, 
 as in the " running of the nose." 
 
 3. Congestion during Inflammation. — The presence of an 
 excessive quantity of blood in the capillaries of an organ 
 is known as " congestion" ; and this may be of two kinds, 
 — active (or arterial), due to an excessive supply from the 
 arterial reservoir; or passive (or venous), due to some inter- 
 ference with the outflow into the veins.-' 
 
 In a cold, congestion of the inflamed area begins as an 
 active congestion ; the arteries are widened, the pressure in 
 the capillaries is increased, and the blood flows much more 
 rapidly. This is essentially the same thing — only in greater 
 degree — that occurs when the arterioles of the stomach 
 dilate during digestion, or those of the skin during exposure 
 to warmth. This initial vascular stage is succeeded by one 
 of passive congestion, caused by the adhesion of white blood 
 corpuscles to the capillary walls, thereby diminishing the 
 bore of the tube, and so making difficult the outflow into the 
 veins ; the velocity of the blood through the capillaries is 
 lessened, pressure within them is increased (why?), and they 
 become gorged with blood. Such is the vascular condition 
 in an organ when an inflammatory process is at its height ; 
 the characteristic feature is the narrowing of the outlet of the 
 capillaries and the consequent excess of pressure ivithin them. 
 
 4. Dangers connected with Congestion. — A decidedly con- 
 gested condition is undesirable because it is a predisposing 
 
 1 The artificial model described on page 142 may easily be used to show 
 the difference between arterial and venous congestion. With the nozzle 
 in the far end of the rubber tube, the tube may be congested (or swollen) 
 with water by more rapid pumping (active congestion) or by narrowing 
 the outlet (passive congestion). 
 
PREVENTION AND CAKE OF COLDS 383 
 
 cause of these inflammatory diseases. It is not the only cause 
 nor the exciting cause of the disease ; but a congested organ 
 may succumb to an attack of disease and so become readily 
 inflamed where it would have escaped had its vascular con- 
 dition been normal. For example, the normal intestine may 
 be the seat of some unusual bacterial action (see Chapter 
 VIII, p. 125) and suffer no damage therefrom, while the 
 same bacterial action may give rise to catarrhal inflamma- 
 tion, accompanied by diarrhea, if it occurs when the in- 
 testinal blood vessels are congested. Or again, whatever 
 the cause of an ordinary cold may be, bacterial or other- 
 wise, it is probable that its attack upon the jierfectly nor- 
 mal organism may be and frequently is resisted ; while at 
 another time a congested condition of the nose, the throat, 
 or the bronchial tubes may permit the disease to gain a 
 foothold at that point. In other words, the congestion 
 alone will not cause colds in the liead or on tlie chest, 
 or diarrheal troubles in the intestine ; something else is 
 needed. We may have the congestion without the cold, 
 and we may also succumb to a cold without the preliminary 
 congestion ; but the presence of congestion often presents 
 to an infecting agent the weak spot which is needed in order 
 that the latter shall secure a foothold and do damage. 
 
 5. The Avoidance of Congestion during Colds, etc. The 
 Care of Catarrhal Conditions. — Again, whenever an inflam- 
 matory process is established, there is, as we have seen, more 
 or less of passive congestion ; under these circumstances 
 everything should be done to avoid arterial dilation in the 
 inflamed area. Suppose there is catarrhal inflammation of 
 some part of the small intestine, accompanied by diarrhea; 
 the outlet into the veins is narrowed and there is conse- 
 quently more or less " backing up " of the blood in the 
 capillaries (passive congestion). This congestion is kept 
 within moderate limits so long as the arterioles maintain 
 a ofood tonic constriction and so limit the amount of blood 
 
384 
 
 THE HUMAN MECHANISM 
 
 which can flow in; if, however, they are made to dilate 
 widely, — by eating a hearty meal for example, — this 
 check is removed, blood flows in under high pressure, and 
 the congestion is increased. Hence in all such catarrhal 
 
 Fig. 111. Experiment to show the effect of arterial constriction in relieving 
 capillary congestion 
 
 In the upper figure, constrictiou of the tuhe at ^1 results in distention (con- 
 gestion) of the tube between the pump aud the fingers; if, however, the 
 tube he also constricted at -E, as in the lower figure, pressure falls between 
 A and B, and the congestion is relieved. 
 
 attacks the diet should be very light and preferably con- 
 fined to those things which are easily digested and absorbed. 
 6. The Care of Colds, etc. — Again, when suffering from 
 any of these inflammatory diseases of internal organs the 
 greatest care should be taken to avoid chilling the skin, 
 because this means (Chapter XII) compensating dilation in 
 the inflamed area, and therefore increase of congestion there. 
 One should be warmly clad (not overclad) ; the living and 
 sleeping rooms, though well ventilated, should not be cold ; 
 when a cold sleeping room cannot be avoided some cover- 
 ing for the head is often useful, as this part of the body is 
 not protected by the bed covering; cold baths should be 
 
PEEVENTIOX AND CARE OF COLDS 385 
 
 discontinued; and, above all, dampness should be avoided. 
 In severe cases it is often necessary for the patient to go 
 away from a damp climate to a dry one. The key to the situ- 
 ation, so far as the management of the circulation is con- 
 cerned, consists in keeping in the skin its full sJiare of hlood. 
 A brief chilling of even a comparatively small area of the 
 skin (e.g. cold feet) may produce a congestion in the inflamed 
 organ capable of undoing the healing work of hours or days. 
 
 A word must be said in this connection about the " fresli- 
 air" cure for colds, etc. There is no doubt that being in 
 the fresh, dry air, even if it is cold air, and preferably out 
 of doors, is better for a cold or any other catarrhal condition 
 than remaining in a closed room. But this should never 
 involve the chilling of any portion of the skin ; one should 
 be warmly clad, even the head and neck l^eing well pro- 
 tected. It makes little or no difference tliat we breathe 
 cold air ; but it makes a very great difference whether or 
 not the skin is exposed to cold air. 
 
 In taking care of colds and similar troubles it is well 
 to remember that the inflammation is only one of the un- 
 favorable conditions against whicli the system is struggling. 
 Consequently we should not expect the disease to yield in 
 all cases to our measures for keeping tlie skin warm. At 
 times a hot bath, a drink of hot lemonade, or other meas- 
 ures for bringinof the blood to the skin checks a threatened 
 cold; but none of these measures is of great value after 
 the disease has once obtained a foothold. It is then a 
 struggle between the body and the disease ; and we can 
 do more by merely avoiding the chilling of the skin than 
 by taking measures to produce marked cutaneous dilation. 
 The true j^olicj-, in other words, is to give the living body 
 every chance to cure itself, and this is best done by not 
 calling on it to do too many other things at the same 
 time. Thus muscular exercise, ordinarily one of our best 
 means of keeping the blood in the skin, is not usually 
 
386 THE HUMAN MECHANISM 
 
 advisable when a cold is at its height, because an added 
 strain would thereby be imposed on the already sorely 
 taxed system. Later, when the worst is over, it is a valuable 
 aid, though it should not be too vigorous until one is on 
 the road to complete recovery. 
 
 "Stuff a cold and starve a fever" is one of those pithy 
 sayings whose very pith may be poisonous. A full meal 
 when we have a cold in the head often clears up the nasal 
 congestion for a time (probably by drawing the blood to 
 the stomach and intestine) and so deludes us into suppos- 
 ing that our " stuffing " has done good. It may also, and 
 doubtless often does, support and reenforce the body in its 
 battle with the disease. What it may do, however, is to 
 overtax the body with the digestion of a heavy meal ; the 
 meal may not be properly digested; bacterial processes in 
 the excessive mass of food may produce abnormal and 
 poisonous substances (see Chapter VIII) which gain admis- 
 sion to the blood, and the " last state " of the patient may 
 be "worse than the first." 
 
 7. Measures for stopping Colds. — When one "feels a 
 cold coming on," that is, early in the struggle, active 
 measures should first be taken to dilate the blood vessels 
 of the skin. A hot bath before going to bed, and hot drinks 
 such as hot lemonade, may be tried. If the cold does not 
 promptly yield to these measures, rest in bed is usually the 
 best treatment. The nervous system is frequently in no 
 condition to sustain hard work of any kind, and hence, 
 until the cold begins to clear up, it is well to confine the 
 diet to easily digestible foods in moderate quantity, and 
 to remain very quiet. Few people, unfortunately, act on 
 this principle. " It 's only a cold " is made the excuse for 
 meeting every engagement that may have been made, or 
 for attempting to do full work. Sometimes, perhaps gener- 
 ally, no serious results follow, but at other times the penalty 
 is heavy. 
 
PREVENTION AND CARE OF COLDS 387 
 
 Very often a cold is a more serious matter than we 
 suppose. Only physicians appreciate how often it is the 
 sign of more serious disease.^ While we cannot say that 
 one should always stop work until the cold is overcome, 
 we do say that limiting work to the minimum and secur- 
 ing all the rest possible is always advisable and should be 
 the rule rather than the exception. We may, unknown 
 to ourselves, be nursing more than a cold ; and, even if 
 we are not, we always hasten the cure by taking care of 
 ourselves. 
 
 8. The Use of Drugs for Catarrhal Conditions. — A remedy 
 very frequently resorted to for colds and other inflamma- 
 tory troubles is the taking of some drug. Large fortunes 
 have been made by the sale of " cough medicines " and 
 the like. Some of these " remedies " are worse than use- 
 less ; others may do no harm, and some may be useful. 
 But none of them are cures. The cure of the cold is effected 
 not by the drug but by the system of the patient ; tlie drug 
 can do no more than remove some condition which stands 
 in the way of the healing effort of the organism. The 
 severe coughing of a bad case of bronchitis is often irritat- 
 ing to the inflamed surface of the air jjassages, and may 
 stand in the way of clearing up the inflammation. Here 
 a drug may do good, though it should be taken only on 
 the advice of a physician, and never on the strength of 
 newspaper testimonials to its wonderful virtues. But the 
 use of these medicines does not render unnecessary the 
 measures we have outlined as the proper treatment. It 
 is worse than foolish to dose oneself with drugs when a 
 cold is coming on, and then attempt to do full Avork ; often 
 the only result of such folly is a complete " knocking out 
 of the stomach " by the drug. The average " cough medi- 
 cine " is especially likely to do this. 
 
 1 In typhoid fever a ' ' cold on the chest ' ' is frequently the first outward 
 indication of the disease. 
 
388 THE HUMAN MECHANISM 
 
 9. The Belief in Drugs. — A century ago the attitude 
 of men and women toward practical hygiene consisted 
 largely in living in ignorance of the workings of the body, 
 taking little or no care of it, and then whenever bad feel- 
 ings appeared "taking something simple" to cure them. 
 This course of conduct was persisted in until something 
 happened, — and something usually did happen, sooner 
 rather than later, — when recourse was had at once to drugs. 
 The doctor was the man who knew what drug to " give " 
 for each disease. He was expected to "prescribe"; and if he 
 did not prescribe something, he failed to satisfy his patient, 
 who concluded that the physician did not understand the 
 disease. The attitude of the public was largely that of 
 neglecting personal, individual care of the health and mean- 
 time implicitly believing that, no matter what hapjjened, 
 some drug could be swallowed which would set matters 
 right. 
 
 Medicine, and especially personal hygiene, have now 
 advanced beyond this crude condition. To-day we realize 
 as never before that the individual is responsible for the 
 intelligent care of his health. The time is probably coming 
 when he will be held as responsible for the care of his body 
 as he is to-day for the care of his morals. At the same 
 time, drugs are being much less used by the best physicians. 
 It is not true that all drugs are useless ; quite the con- 
 trary ; but it is true that careful nursing often counts for 
 more than does the use of drugs. Typhoid fever is to-day 
 often treated with ]io drugs at all, and the tendency to use 
 drugs in other diseases is distinctly lessening. 
 
 The wise physician is often hampered in his work by the 
 survival of this old-fashioned belief in the all-suificiency 
 of drugs. Instances of it are sometimes encountered even 
 among otlierwise intelligent people. We should under- 
 stand that in all cases of illness the one treatment which 
 should be applied is good nursing, whether by a trained 
 
PEEVENTION AND CAEE OF COLDS 389 
 
 nurse, or by one's familj', or by oneself. If medicine is 
 given, it is usually subsidiary to the main procedure, al- 
 though sometimes, as in the antitoxin treatment of diph- 
 theria, it is the main thing. But in no case should we be 
 so foolish, so unreasonable, as to distrust or lose confidence 
 in a physician because he gives few drugs or none at all. 
 
 10. The Avoidance of Colds, etc. General Principles. — If 
 the care of these slight ailments is of importance, their 
 prevention is of much greater importance. And iirst of 
 all among preventive measures must be placed not the 
 avoidance of drafts and other chilling of the skin, not 
 clotliing, but the 'projier hi/(/ieiiii: care of the hody, — 
 regular and sufficient muscular exercise, the avoidance 
 of improjDcr feeding (for colds are often due to digestive 
 disturbances resulting from overfeeding), and good habits 
 of sleep and rest. When these things are properljr attended 
 to, one may usually suffer considerable chilling of the skin 
 without ill effect. It is not true, as is often asserted, that 
 by attention to these general matters the protection of 
 the body from exposure to cold becomes unnecessary, 
 but it is true that without this attention such protective 
 measures are apt to be of little avail. 
 
 Among general measures none is more important than 
 the avoidance of e.rposure to cJiiUiiii/ influences when the 
 nervous system is depressed by marked fatigue. We take 
 cold more readily, much as we are more susceptible to any 
 disease, when we are tired. It is a question of a struggle 
 between the organism and unfavoraljle external or internal 
 conditions, and, in general, the greater the fatigue of the 
 organism the less is its chance of success in the struggle. 
 
 11. The Avoidance of Colds. Special Measures. — As to 
 measures specially concerned with the avoidance of con- 
 gestion in internal organs, let us iirst state clearly the 
 principle involved and then pass to its practical applica- 
 tions. The condition to avoid is the undue constriction of 
 
390 THE HUMAN MECHANISM 
 
 the blood vessels of the skin, produced by chilling. The 
 danger is not in the mere exposure to cold; people may 
 be comparatively lightly clad on a cold dry day, when 
 there is no wind, without chilling the skin, because the 
 layer of air in contact with the skin becomes warmed, and 
 in the absence of wind even light clothing, if dry, suffices 
 to keep this warm air in contact with the body. But if 
 the air is damp, so that it readily conducts heat, or if the 
 wind is blowing, so that convection becomes important, or 
 if the body is near cold objects to which it can radiate its 
 heat, the skin may be easily chilled, especially if we are 
 making no muscular exertion (pp. 193-198). 
 
 Again, during muscular exertion exposure to cold is 
 usually harmless, even if the clothing be light, because 
 the increased heat production within the body results in 
 an adequate flow of warm blood through the skin. We 
 seldom take cold during vigorous muscular work on a 
 cold day. It is when we are sitting still or, even more, 
 when we are lying down and the muscles are relaxed, that 
 we should be on our guard. 
 
 Finally, and most important of all, is the fact that the 
 dangerous region of atmospheric temperature is confined 
 to the narrow limits of a few degrees just below the proper 
 room temperature. This proper room temperature for light 
 clothing is about 66° F. with low or normal atmospheric 
 humidity, and about 69° or 70° F. for high humidity. 
 Above these points there is no chilling of the skin. Five 
 or ten degrees below these points we feel so cold that we 
 become uncomfortable and take steps to remedy matters, 
 either by putting on heavier clothing or by heating the 
 room. It is when the temj^erature is only slightly below 
 what it should be that we are apt to be unaware of the 
 insidious increase of arterial constriction and chilling in 
 the skin, until, after an hour or more of it, we suddenly 
 awake to the true condition of affairs. This is apt to 
 
PREVENTION AND CAEE OF COLDS 391 
 
 happen when the fire in the stove or in an open grate 
 goes down. It also happens at times when we come from 
 a walk out of doors into a room of this " dangerous 
 temperature," say 63° F. on a day of high humidity ; the 
 skin is warmed by the exercise we have been taking, so 
 that, as we enter the room, it does not seem cold (for the 
 temperature we really notice is that of the skin, not that 
 of the room at all) ; on sitting still in the room the cuta- 
 neous dilation of muscular exercise passes off so gradually 
 that we do not notice the change, and, before we are aware 
 that we are chilly, marked internal congestion may have 
 been produced. 
 
 It is also well to remember that not all parts of the 
 room have the same temperature. The floor is colder than 
 the ceiling ; it is colder nearer exposed walls and windows 
 than away from them, and the common habit of sitting near 
 a window on a cold day while reading or sewing is not wise. 
 
 12. Cooling off suddenly. — It is an old saying that it is 
 not well to " cool off suddenly." While tliere is some 
 truth in this, it is not true in general, nor in the form 
 stated. It is perfectly safe for most healthy people to 
 take a cold bath after exercise, or to pass directly from a 
 hot bath into a cold one (see Chapter XXIV). The sudden 
 cooling which experience has found to be harmful is where 
 the clothing has been saturated with perspiration and one 
 cools off by sitting still in a breeze or in a cool place. Here 
 the clothimj remains damp and so conducts heat readily 
 from the skin ; and the danger lies not in the cooling off 
 but in the prolonged chilling process which follows it. 
 Consequently it is a general rule that clothing made damp 
 by rain or perspiration should be changed as soon as pos- 
 sible, or else that drafts and cold rooms should be avoided 
 until the clothing is dry. 
 
 It is unnecessary to multipl}' examples. In all the prin- 
 ciple is the same, — the avoidance of conditions which 
 
392 THE HUMAN MECHANISM 
 
 produce marked constriction of cutaneous blood vessels, 
 with the accompanying congestion of internal organs ; and 
 the student is again reminded that by this course we do 
 not always secure immunity from internal inflammations ; 
 we merely remove one of the conditions which favor their 
 development. 
 
 13. " Hardening " the System to Cold. — We must refer 
 briefly to the importance of what is popularly known as 
 hardening the system to cold. Cold unquestionably pro- 
 duces its effects in some people more readily than in 
 others, and these differences are largely dependent upon 
 habit or training. When the living rooms are kept above 
 70°, and heavy clothing is always worn out of doors, the 
 skin is constantly subjected to a tropical climate and 
 becomes more sensitive to external cold. Internal conges- 
 tion will then be produced at 67° or 68° F. which would 
 not take place above 60° or 62° F. in persons who have 
 been accustomed to cold. In other words, it is possible to 
 overdo the matter of protection from external cold. For 
 this reason, overheated rooms and the use of heavy wraps 
 while walking in moderately cold weather (30°- 50° F.) 
 are very objectionable. 
 
 We should thus harden ourselves to cold ; but it should 
 never be forgotten that this process of hardening may be 
 carried too far. To harden oneself does not mean that the 
 temperature of the living room should be kept below 65° F., 
 nor that sleeping rooms should be cold enough to freeze 
 water at night. Severe colds and rheumatism have been 
 contracted by this folly. 
 
 Many people fail to realize that because a little will 
 do good, it does not necessarily follow that more will do 
 better. One person is impressed by the undoubted fact 
 that it is possible to eat too much meat, and thereupon 
 abstains from meat altogether; another discovers that a 
 sedentary life is a bad thing, and hastens intemperately 
 
PREVEI^^TION AND CAEE OF COLDS 393 
 
 to take " century rides " on a wheel. One finds that he has 
 been overclad, and, discarding all warm clothing, shivers 
 throughout the winter ; another, on learning the possi- 
 ble value of cold bathing, enthusiastically but unwisely 
 plunges into the coldest water he can get, and stays in it 
 until his skin is blue. Very likely any one of these exam- 
 ples can be duplicated from the reader's own circle of ac- 
 quaintances. It is important to remember that " nothing 
 too much " is always a good rule, and nowhere is it more 
 essential than in the hygienic conduct of life. 
 
 14. Reasons for avoiding Colds and Other Inflammatory 
 Troubles. — We may conclude this chapter with some facts 
 showing the hygienic importance of the prevention of colds 
 and other inflammatory diseases, such as sciatica, lumbago, 
 and rheumatism in its various forms. In all these diseases 
 we find the same close connection between the chilling 
 of the skin and the onset of the disease, so tliat what has 
 been especially urged with regard to colds applies in large 
 measure to the entire group. But some may say, "These 
 are slight ailments; why not ignore and disregard them?" 
 
 1. The first and sullicient reason is that these ailments 
 interfere seriously with our working power and with our 
 capacity for usefulness and enjoyment. Every one knows 
 from experience that the body is not as good a machine 
 dirring the progress of a cold or a diarrheal attack, or 
 wliile suffering from sciatica or slight attacks of rheu- 
 matism. We should strive not only to live, but to live 
 well ; not merely to do things, but to do them with our 
 might ; not merely to live and work, but to live happily 
 and to work cheerfully. 
 
 2. The popular impression as to the frequency with 
 which pulmonary consumption, jjneumonia, etc., are pre- 
 ceded by common colds is much exaggerated. It is never- 
 theless probable that in some cases a cold is the means 
 of lowering the power of resistance to the more serious 
 
394 THE HUMAN MECHANISM 
 
 disease, and we should take every reasonable precaution 
 which will maintain the ability of the body to cope success- 
 fully with the inroads of diseases, especially of those for 
 which there is no certain cure. 
 
 3. Colds and similar troubles have a well-known tendency 
 to become chronic. Probably no sufferer from nasal catarrh, 
 or chronic bronchitis, or chronic diarrhea, if he had his 
 life to live over again, would neglect measures tending to 
 avoid the occurrence of these conditions. Only those who 
 do not know from experience the capacity of such troubles 
 to produce annoyance and discomfort can regard their pre- 
 vention as unworthy of serious attention. We cannot too 
 strongly emphasize the fact that chronic troubles are very 
 frequently the result of the repetitions of the neglected 
 inflammations which accompany the acute attack ; they are 
 due not so much to inherent weakness of the tissue or 
 organ as to the carelessness of the individual about avoid- 
 ing them, or the failure to give them the attention they 
 deserve when they occur. One of our leading physicians, 
 a man of the widest experience and soundest judgment, 
 writes, concerning chronic nasal catarrh, " It is sad to 
 think of the misery which has been entailed upon thou- 
 sands of people owing to the neglect of nasopharyngeal 
 catarrh by parents and physicians." 
 
CHAPTER XXII 
 THE CARE OF THE EYES AND EARS 
 
 The visual apparatus (eye, optic nerve, nerve endings, 
 etc.) furnishes one of tlie most important paths from the 
 world without to the brain within, and it is of the utmost 
 importance to the exercise of the highest functions of the 
 human mechanism that this path he kept as smooth as 
 possible. Unfortunately, however, the path is seldom either 
 straight or smooth, and it frequently presents serious ob- 
 stacles. The curvature of the cornea or of the lens may 
 be irregular ; the muscle of accommodation may be weak ; 
 the retina may be too near or too far from the lens, or 
 its sensitive cells may too readily become fatigued by the 
 stimulation of light ; finally, the path into the brain may 
 be made of poorly constructed nervous tissue, or in the 
 brain itself the coordinations upon which depend our 
 visual judgments (p. 25G) may be imperfect. The simplest 
 act of vision is the end result of a most complicated series 
 of events, difficulty with any one of which may make quick 
 and accurate seeing impossible. J\Iany a child has been 
 considered stupid simply because an unrecognized condi- 
 tion of myopia or astigmatism renders it impossible to read 
 clearly the printed page or the distant blackboard ; and 
 many people, adults as well as children, suffer from head- 
 aches and other troubles because of the strain thrown on the 
 nervous system in the effort to work with defective vision. 
 
 When one is leading an outdoor life, occupied in the 
 work of the farm or the lumber camp, and doing but little 
 reading, the eyes usually give little trouble, because it is 
 
 395 
 
396 THE HUMAN MECHANISM 
 
 only when looking at near objects (three feet or less away) 
 that the mechanism of accommodation is called into vigor- 
 ous action. Eye strain is usually produced by prolonged 
 near work with eyes incapable of enduring without un- 
 due fatigue what is demanded of them. Hence it is that 
 defects of vision are more common to-day than they were 
 a hundred years ago. Both the vocations and the avoca- 
 tions of modern life, with their large amount of reading, 
 writing, and other forms of near work, impose upon the 
 eye the most trying and difficult task it can be called ujDon 
 to perform. The use of glasses is more common than for- 
 merly, and the care of the eyes is forced upon us as an 
 important factor in the hygienic conduct of life. 
 
 1. The Necessity of Expert Advice. — In the care of 
 the eyes expert advice is indispensable. The detection of 
 defects of vision frequently demands the best skill of those 
 who are thoroughly acquainted with the physiology of the 
 entire visual apparatus, including its relation to other bodily 
 functions, and who are also provided with every means for 
 gaining an insight into the conditions which are giving 
 trouble. The selection of the proper glass, for example, 
 when lenses are needed is more than a mere matter of test- 
 ing vision with test cards ; and eyes may be seriously 
 injured by using glasses prescribed on the basis of informa- 
 tion gained by imperfect methods. 
 
 First of all, then, let us insist upon the necessity of 
 competent medical advice whenever there is reason to sus- 
 pect something wrong with the eyes. If vision is not dis- 
 tinct, if the eyes tire quickly when used for near work, 
 and even ivhen one suffers from headaches, " nervousness" 
 and other forms of malaise without apparent cause, it is 
 wise to find out whether some remediable defect of vision 
 is not at the root of the trouble. 
 
 On first thought it may seem unreasonable to consult an 
 oculist with regard to headaches or other troubles with 
 
CAEE OF EYES AXD EAES 397 
 
 organs having no obvious connection with the eye; but when 
 we remember the fact that all parts of the central nervous 
 system are connected with one another, it is easy to see 
 how undue strain of one part in the effort to see with 
 astigmatic or otherwise defective eyes may, by injuriously 
 affecting other parts of the brain or spinal cord, unfavor- 
 ably influence organs which themselves have nothing to do 
 with vision. Over and over again it happens that head- 
 aches and other troubles are relieved, as by magic, when 
 vision is made perfect by the use of proper glasses. 
 
 With these remarks as to the importance of skilled 
 advice in the care of the eyes, we may pass to those prac- 
 tical measures which should he under the intelligent indi- 
 vidual control of every man and woman. Suppose vision 
 is perfect, or as nearly perfect as the best of medical skill 
 can make it; what precautions in the use of the eyes 
 favor the maintenance of their best working condition ? 
 
 2. Resting the Eyes. — Fii-st of all we would suggest 
 the importance of resting the eyes now and then while 
 engaged in near work. This is accomplished Ijy the sim- 
 ple expedient of looking for a few moments at some dis- 
 tant object (p. 250). The brief relaxation of the effort of 
 accommodation does for the neuromuscular mechanism 
 involved exactly what a brief relaxation of the body in 
 sleep accomplishes for the body as a whole. 
 
 3. Illumination of the Object. The Importance of Con- 
 trast. — The ease with which the details of an object are 
 seen depends chiefly on the contrasts of shade and color 
 which these details present to the eye, and nothing so 
 influences this contrast as tlie amount of illumination. 
 Thus as the light fades in the evening, the white paper of 
 a printed page becomes darker and darker, until finally it 
 reflects to the eye little more light than the black ink of 
 the printed letters, which consequently no longer stand 
 out clear and distinct. In order to admit all the light 
 
398 THE HUMAN MECHANISM 
 
 possible, the pupil enlarges, and in so doing lessens the 
 distinctness of the retinal image (spherical aberration); 
 more important than this, we hold the page closer to the 
 eye, thereby enlarging the retinal image and increasing the 
 intensity of stimulation, but throwing far more work upon 
 the ciliary muscle to focus for the near object. All of these 
 unfavorable conditions taken together place undue strain 
 upon the mechanism of accommodation. 
 
 Hardly less objectionable is excessive illumination of 
 an object. After a certain intensity of light is reached, the 
 retina no longer responds to increase of stimulation with 
 increase of visual reaction. If there were in addition to 
 our sun a second sun which sent into the eye twice as 
 much light, the second sun would seem no brighter than 
 the first because the effect of the first upon the eye has 
 already passed the point which calls forth the greatest 
 possible reaction in the retina. To apply this principle to 
 the case in point, we have only to remember that a printed 
 letter is not absolutely " dead black," but reflects some 
 light. When the illumination is moderate this reflected 
 light hardly affects the retina at all, and the contrast 
 between the black letter and the white paper is marked. 
 As the intensity of illumination increases, however, the 
 effect upon the retina of the light coming from the letters 
 increases more rapidly than the effect of that coming from 
 the paper. Contrast is lessened and sharper accommoda- 
 tion as well as closer attention is needed to see distinctly. 
 Added to this, no doubt, is the fatigue and lack of sensi- 
 tiveness in the retina, resulting from overstimulation. 
 
 4. The Size of Type. — The use of fine type should be 
 reduced to a minimum, because it necessitates greater 
 effort of accommodation and intensifies all the evils of 
 improper illumination. Any printed matter which must 
 be held less than eighteen inches from the eye in order to 
 be seen clearly is undesirable for long-continued reading. 
 
CAEE OF EYES AXD EAES 399 
 
 Especially is this true in youth, since then the eye is more 
 plastic, and excessive strain of the muscle of accommoda- 
 tion, pulling as it does on the sclerotic and the choroid 
 coats, may lead to permanent deformation of the curved 
 surfaces. The marked increase of myopia within the past 
 forty or fifty years is explained in this way. 
 
 5. Highly Calendered Paper Objectionable. — Closely con- 
 nected with the size of the type is the character of the paper 
 on which it is printed. This should be as dull as possible 
 in order to avoid the confusing effect of a glossy surface. 
 The use of highly calendered paper in many books and 
 serial publications, because such paper lends itself more 
 readily to the reproduction of pictures in half tone, is a 
 sacrifice of hygienic considerations to cheapness. 
 
 6. Importance of a Steady Light. Reading on Railroad 
 Trains. — The source of illumination for near work should 
 be as free as possible from unsteadiness or flicker, since a 
 flickering ligfht necessitates the most accurate accommo- 
 dation. A "student's lamp," " Rochester burner," or incan- 
 descent electric lamp is preferable in this respect to candles, 
 gas jets, and arc lights for near work. 
 
 For the same reason caution is demanded in the matter 
 of reading on railroad trains. American railway trains 
 have recently become so heavy, and the roadbed, rails, 
 etc., have been so much improved in various ways, that 
 the danger of reading or writing while traveling by rail is 
 much less than formerly. At the same time the danger 
 still exists, and reading on many railway and trolley cars 
 is still to be done with caution, or, better still, avoided 
 altogether. 
 
 7. Microscopes, Telescopes, and other optical instruments 
 require close and sometimes continuous use of one or both 
 eyes, and are popularly supposed to be "hard on the 
 eyes." But this is not necessarily the case, except for 
 beginners and investigators ; for beginners, because they 
 
400 THE HUMAN MECHANISM 
 
 try to see clearly by focusing with the eye rather than 
 with the use of the focusing apparatus of the instrument ; 
 for investigators, because the eyes are used for too long 
 periods at a time. Optical instruments are easily focused, 
 and, if care be taken to provide good lighting, routine work 
 with them need not be specially trying to the eyes. 
 
 8. The Removal of Cinders. — Particles of dust, cinders, 
 etc., are often washed away from the surface of the eyeball 
 by the copious secretion of tears which they call forth. 
 Sometimes, however, they must be removed directly from 
 the eyeball or the inner surface of the eyelid. In the case 
 of the lower lid this operation presents little diiiiculty, for 
 the eyelashes of this lid are easily seized, the lid drawn 
 forward away from the eyeljall, and the surfaces of the 
 eyelid and eyeball readily inspected. If any foreign body 
 is there located, it may be removed by the corner of a 
 handkerchief. Successful manipulation of the upper lid 
 is more difficult, because a piece of cartilage immediately 
 above the eyelashes interferes with turning back the lid. 
 The gaze of the patient should be directed downward, a 
 small pencil or other cylindrical object pressed against the 
 Uffer portion of the lid, above the cartilage, the eyelashes 
 seized, and the lid turned upwards and backwards over the 
 pencil. 
 
 9. Recapitulation. The Care of the Eyes. — To summa- 
 rize, we may remind the student that the eyes, no less 
 than other organs, should be kept sound and strong by 
 attention to the general health and welfare of the body. 
 Work, play, rest and sleep, muscular exercise, wise feeding, 
 and regular removal of the wastes, — these and all other 
 general hygienic haljits help to keep the eyes sound and 
 strong ; but besides these, posture in work, lighting, paper 
 (not forgetting wall paper), printing, dust, cinders, smoke, 
 acid fumes, traveling, sight-seeing, and many other con- 
 ditions have their effect. Finally, it must not be forgotten 
 
CAEE OF EYES AND EARS 401 
 
 that the eyes are too precious to be trifled with, and that 
 if one has sore or weak eyes, or pain in the eyes, or cannot 
 see clearly to read or to write, or cannot plainly distinguish 
 things near or at a distance, then it is always best to con- 
 sult an oculist or the family physician for advice. Remedies 
 or doctors puffed in high-sounding advertisements should 
 be carefully avoided. 
 
 10. The Care of the Ears. — Besides good care of the 
 general health, which common sense dictates and which we 
 have repeatedly urged as the fundamental requirement in 
 the hygiene of all organs, there is but little which the 
 individual can do for the ears. Deafness, especially total 
 deafness, is a defect or injury perhaps no less serious than 
 blindness. Acute hearing is probably as valuable as acute 
 vision, and a partial loss of hearing is a handicap often 
 harder to overcome than are some defects of vision. 
 
 Keej^ing in mind the auditory apparatus and its connec- 
 tions (Chapter XIV), it is easy to see that the drum may 
 be pierced or otherwise injured by slender objects thrust 
 in from without ; that catarrh of the throat may easily 
 extend into the Eustachian tube, inflaming it or choking 
 its lumen or outlet; and that any thickening of the drum 
 must tend to make its vibrations slower and more diffi- 
 cult. In these possibilities we have some of the actual 
 causes of deafness, and none of them is of a kind to be 
 treated by the patient. Any recognition of incipient deaf- 
 ness in oneself should be regarded as cause for consulting 
 a good physician. No attention should be paid to adver- 
 tisements promising to relieve deafness, for these are usu- 
 ally traps calculated to catch the ignorant, unwary, or 
 credulous. It is dangerous to explore the outer ears with 
 hairpins or other pointed objects, as the drum may thus 
 be broken or other harm done. 
 
 11. Noise, though delighted in by savages, who beat 
 tom-toms, blow conch shells, or otherwise tickle the sense 
 
402 THE HUMAN MECHANISM 
 
 of hearing, and though in moderation often found stimu- 
 lating and enjoyable by persons who have been living in 
 solitude or isolation, is by adults among the most highly 
 civilized peoples more and more regarded as a necessary 
 evil, or even as a nuisance. Children, on the other hand, 
 often delight in noise, and horn blowing, firecrackers, and 
 pistol firing on holidays like the Fourth of July appear 
 to give to them as much pleasure as to their elders pain. 
 Adults also on occasions of rejoicing still ring bells, beat 
 drums, blow horns, and fire cannon in order to express 
 their emotions. Loud noise, like strong light, is unques- 
 tionably stimulating and exciting, and for these reasons, 
 though justifiable at times of rejoicing, is something to 
 be ordinarily avoided as far as possible in city life, itself 
 already much too stimulating and exciting. One can, in- 
 deed, often learn to sleep even in the presence of dis- 
 tracting noises such as those of a busy city street ; but 
 such sleep cannot possibly be as wholesome as that enjoyed 
 in quiet places. The constant whistling of locomotives, 
 which was formerly a great nuisance in many American 
 cities and towns, has been largely done away with, and the 
 tendency of the times is to cultivate quiet, not only as a 
 private luxury but also as a public necessity. 
 
CHAPTER XXIII 
 
 THE HYGIENE OF THE FEET 
 
 The hygienic care of the feet consists essentially in 
 maintaining the ability of those organs to bear easily and 
 without discomfort the weight of the body. " Weak feet " 
 are to blame for many unhealthful conditions ; the dis- 
 comfort or pain which they cause as one goes about the 
 ordinary occupations of life subjects their possessor to 
 
 Fkj. 112. Bones of the right foot 
 Seen from the outer side 
 
 nervous strain and often prevents the enjoyment of that 
 muscular activity which the maintenance of health requires. 
 Nor is it generally known that this state of affairs may be 
 very laigely avoided by intelligent care. In the majority 
 of cases weakness of foot is the result of maltreatment of 
 the foot, and not the result of inborn structural defects. 
 
 Each foot consists of no less than twenty-six small bones 
 joined by ligaments and held in proper position relative to 
 one another by the action of a number of muscles. The 
 key to the understanding of the hygiene of the foot is the 
 fact that it is upon the proper performance of the work 
 of these muscles that the strength of the foot primarily 
 
 40.3 
 
404 THE HUMAN MECHANISM 
 
 depends, and that the weakening of the foot is due to 
 interference with their action, chiefly by the use of wrongly 
 shaped slioes. 
 
 1. The Arches of the Foot. — The bones of the foot should 
 form two well-marked arches. One of these is the con- 
 spicuous arch of the instep and the other a less conspicuous 
 but important transverse arch immediately back of the toes. 
 Not only is the preservation of these arches important 
 because they help to relieve the joints above them of jar, 
 but also because under them lie nerves, blood vessels, lym- 
 phatics, and other tissues which are injured when the arch 
 
 Fig. 113. Longitudinal section through the bones of the foot 
 
 Showing the aix-h of the in.step and the attaehnient of the tendon of the calf 
 muscle to the heel bone 
 
 gives way and permits pressure upon them from above. 
 Figure 144 shows the action of one of the groups of mus- 
 cles which maintain the arch of the instep, and illustrates, 
 in principle, how muscular action keeps the bones in proper 
 relative positions. The muscles shown in the figure (the 
 short flexors of the toes) act like the string of a hoiv, and 
 by contracting resist the tendency of the weight of the 
 body to break the arch down. Other groups of muscles 
 are concerned, but it is unnecessary that we go into the 
 details of their action. Enough has been said to show 
 the importance of keeping these muscles strong, so that 
 they may do the work imposed upon them. 
 
HYGIENE OF THE FEET 405 
 
 The groups of muscles specially concerned are those 
 which move the toes, and these, like other muscles, can be 
 kept strong only by use. Consequently interference with 
 the freedom of action of the toes must lead to the dis- 
 use and partial degeneration (weakening) of the muscles 
 in question. The fundamental jyrinciple in the care of the 
 foot is none other than the maintenance of the freedom of 
 motion of the toes, together with the use of the toes as ivell as 
 the ankle in locomotion. 
 
 2. The Foot of the Infant and the Adult Foot. — Every 
 human being begins life with a foot possessing wide range 
 
 ¥ii:.. 114. Ligaments of the foot and ankle 
 
 of movement, amounting almost to a grasping power. It 
 is most instructive to watch a baby use its toes; not only 
 are they bent downward or upward (plantar and dorsal 
 flexion) and spread apart (abducted) with the greatest ease, 
 but in walking the toes fairly grasp, or dig into, the ground. 
 The adult foot of civilized man usually presents a painful 
 contrast to this. Generally the toes are crammed together, 
 their power of spreading apart is wholly lost, and their 
 movements take no j^art whatever in walking. The foot, 
 in other words, is reduced almost to the condition of a 
 shoemaker's last. 
 
 Nor is this a natural change due to growth and develop- 
 ment. It is produced by the use of shoes which permit no 
 
406 THE HUMAN MECHANISM 
 
 movement of the toes and therefore lead to disuse of the 
 muscles in question. Walking thus comes to be performed 
 almost entirely with muscles which act upon the ankle 
 joint, the one articulation of the foot at which movement 
 is still possible ; whereas had the toes been allowed perfect 
 freedom of action, the work of lifting the weight of the 
 body from the ground with each step would have been 
 shared by bath groups of muscles, — those which raise the 
 heel and those which flex the toes. That this is true is 
 shown by the feet of people who have not worn constrict- 
 ing shoes, for in their case the toes are moved freely and 
 perform an important slmre in locomotion.^ 
 
 If it be asked why the flexors of the toes as well as the 
 extensors of the aukle should take part in the act of walk- 
 ing, the answer is that it is precisely the disuse of the 
 former which leads to their degeneration, so that they are 
 no longer efficient in opposing the tendency of the weight 
 of the body to break down the arches of the foot. It is 
 true that "flat foot" is not the invariable result of this 
 disuse, because some people are so fortunate as to possess 
 ligaments of sufficient strength to hold the bones together 
 despite the pressure of this weight, and also because tightly 
 fitting shoes often assist in holding the bones in position. 
 But it is also true that many others are not so fortunate ; 
 one or both arches give way, and some suffer agony as the 
 result. Even if the arches do not break down, the foot is 
 generally unable to stand the strain of prolonged walking 
 
 1 The action of each of these groups of muscles may be made clear as 
 follows : with the bare feet take a step forward by first raising the heel 
 and then pushing off by bending the toes downward as far as possible. 
 It will be found that this second movement is capable of assisting to a 
 very considerable extent in pushing the body forward. The student 
 should thus make himself practically familiar with the difference between 
 (1) walking when only the heel is raised and the toes passively bent up- 
 ward as the step is completed, and (2) walking when the raising of the 
 heel is followed by the active contraction of the plantar flexors of the toes. 
 
HYGIENE OF THE FEET 
 
 407 
 
 without marked discomfort, and it is not too much to say 
 that this weakness of the foot is one of the chief reasons 
 why most people regard a walk of ten or twelve miles as 
 a great task. 
 
 The hygienic care of the foot in actual practice consists 
 (1) in the use of properly fitting shoes, (2) in avoiding all 
 
 A J! c 
 
 Fir,. 115. Correct and incorrect shapes of the sole of the shoe 
 
 t>utliiie of the sole in solid lines: of tlie natural shape of the foot in dotted 
 lines. A, correct shape; in B the shape is correct excejit that the median 
 line of the sole is not straight in the re,i;ion of the toes, thus piressing the 
 great toe over toward the other toes; C has not only this defect hut is too 
 narrow 
 
 interference with the circulation of blood in the foot, (3) in 
 maintaining proper conditions of temperature and moisture 
 within the shoe, and (4) in the training and use of the 
 muscles of the foot, so as to keep them functionally strong 
 and active. 
 
 3. Shoes. — Among the most important requirements of 
 a good shoe are the following : (1) The sole of the shoe 
 
408 THE HUMAN MECHANISM 
 
 sliould everywhere be as loide as the sole of the foot when 
 one is standing and the feet are warm. (2) The heels 
 should be loiv and broad. (3) The sole and uppers should 
 be sufficiently flexible to permit without great resistance 
 the bending of the foot at the transverse line of articula- 
 tion of the toes with the instep. Many shoes, otherwise 
 correct, are faulty in that the sole or the upper from the 
 heel forward is too stiff to permit the efficient action of the 
 toe movements. (4) The inner (or median) side of the shoe 
 should be " straight," i.e. the prolongation of the median 
 line of the great toe should touch the heel. Figure 115 
 shows the proper and improper shape of the shoe in this 
 respect. Unless the foot is already greatly deformed, no 
 shoe should be tolerated which does not permit the great 
 toe, and for that matter all the toes, to point straight 
 forward, since otherwise it is not easy to flex them. Not 
 only is the " toothpick " shoe a hygienic abomination, but 
 any shoe in which the inner or median side slopes outward 
 toward the toe is highly objectionable (see Fig. 115, B). 
 (5) In the region of the toes the shoes should have sufficient 
 room to permit perfect freedom of motion in the toes. 
 
 4. Shoes for Deformed Feet. — It must be frankly admitted 
 that shoes which fill all the above requirements are uncom- 
 fortable to many feet. But this is only because such feet 
 have already become deformed. In such cases the attempt 
 should be made to bring the foot back toward its normal 
 shape by gradually approaching the lines above indicated. 
 With some hopelessly deformed feet this is, of course, 
 impossible ; but with many others great improvement is 
 possible. 
 
 Upon one point there can be no yielding : children should 
 wear only properly shaped shoes. It is a pitiable sight to 
 see the foot of a child, broadening out as it does toward 
 the mobile toes, forced into a shoe which seems to be con- 
 structed on the assumption that nature ought to have made 
 
HYGIENE OF THE FEET 409 
 
 the human foot wedge-shaped and that it is man's part to 
 improve on nature. 
 
 Recent years have witnessed marked improvement in 
 the shape of shoes. Fortunately it is now possible in many 
 places to buy properly made shoes ; but further improve- 
 ment is still possible, both among those who make shoes 
 and those who buy them. As a matter of common sense 
 nothing could be more absurd than the custom of changing 
 the shape of shoes each season merely to bring out a new 
 style; nor would this be done if people were moie gener- 
 ally informed as to the requirements of a good shoe and 
 insisted on having only those which meet these require- 
 ments. In so far only as there is a genei'al demand for 
 such shoes in any community, will manufacturers sujjply 
 them. The remedy lies with the public rather than with 
 the manufacturers. 
 
 And this same public must learn that neither from the 
 hygienic nor from the Eesthetic standpoint is a small foot 
 with a pointed toe and high heel the ideal foot. Such is 
 not the foot of the Apollo Belvedere nor that of the Venus 
 of Melos. It is simply a deformity, belonging in the same 
 category with the constricted waist, and far more harm- 
 ful to its possessor than the ear or nose ornaments of the 
 Hottentot. No hygienic lesson is more important than 
 that clothing should fit the bodj^ and not the Ijody the 
 clothing. 
 
 5. Interference with the Circulation in the Foot. — By 
 lacing the shoe too tightly, especially around the top, 
 and by the use of tight garters the superficial veins which 
 bring blood back from portions of the foot are often com- 
 pressed. More or less of passive congestion results, and 
 this not only produces discomfort but introduces in other 
 ways conditions highly unfavorable for the free action of 
 the foot; consequently it is part of the hygiene of that 
 organ to avoid these congestions at all times. Garters 
 
410 THE HUMAN MECHANISM 
 
 should always be adjustable in length to the size of the 
 leg, and shoes should not be laced tightly. 
 
 6. Proper Conditions of Temperature and Moisture with- 
 in the Shoe. — Although the best of shoes are but poorly 
 adapted to care for the perspiration and to maintain an 
 equable temperature of the foot, some shoes are preferable 
 to others in these respects. Thus any "patent" or "enamel" 
 leather is objectionable for walking because it is almost 
 impervious to moisture. In such shoes the foot becomes 
 overheated while one is walking, because the perspiration 
 does not evaporate from its surface ; and if one afterwards 
 sits still, the feet are apt to become cold because the wet 
 stockings make a good conductor of heat. Because their 
 surface radiates heat with such ease these shoes are cold 
 in cold weather ; and because they prevent the evapora- 
 tion of perspiration they are hot in hot weather. While 
 unobjectionable generally for dress occasions, they are 
 wholly unsuitable for ordinary wear. 
 
 The " russet " shoe for summer wear is a great boon. 
 The leather of which it is made is as porous as any on the 
 market, and, because of its color, absorbs less heat in warm 
 weather. The failure of the attempt a few j-ears ago to 
 retire these shoes from sale is good evidence that people 
 can get a certain shoe if only they insist upon having it. 
 
 In brief, the feet should be dry and neither distinctlj' 
 warm nor cold ; and anything which interferes with these 
 conditions should be attended to. Shoes and stockings 
 should be changed as frequently as necessary and only 
 such foot wear used as maintains as far as possible the 
 ideal conditions given aljove. 
 
 7. The Proper Physical Training of the Foot. — It is 
 quite possible to meet all the above hygienic conditions 
 and yet have feet which are incapable of doing the work 
 which we have a right to demand of them. As was shown 
 at the outset, the action of the foot in bearing the weight 
 
HYGIENE OF THE FEET 411 
 
 of the body is not a passive but an active one. Muscles 
 must assist in holding the bones in place when one is 
 standing still, and they must operate the foot during the 
 act of locomotion. The physical training of the foot, there- 
 fore, consists (1) in securing adequate strength of these 
 muscles, and (2) in establishing right habits in using them. 
 
 Since the muscles in which strength is especially needed 
 are those which produce plantar (downward) flexion of the 
 toes, we may strengthen these muscles by such exercises 
 as pressing as hard as possible with the toes against the 
 floor or the footboard of a bed, by attempting to "stand 
 on tiptoe," and by the familiar gymnastic movement of 
 " heels raise, knees bend," etc. 
 
 Among the haljits wluch should be cultivated may be 
 mentioned, first of all, walking and running with the foot 
 straight forward instead of toeing outward. The bones 
 wliich form any hinge joint, like that of tlie ankle, should 
 move in a plane perpendicular to the axis of motion in the 
 joint, and this is jjossible in the case in question only when 
 the feet are pointed forward. It is absolutely wrong to 
 teach children to toe outward in walking, and they would 
 never do so were they left to themselves and their feet 
 clothed in proper shoes. 
 
 In addition to this, the habit should be cultivated of 
 completing each step by " digging into the ground " with 
 all the toes. This cultivates the use of the foot muscles 
 in locomotion along with the use of those which raise the 
 heel, and the habit once acquired and regularly practiced 
 keeps these muscles strong. 
 
 Finally, it must be remembered that the training of these 
 muscles, like the training of all others, must be a gradual 
 process. Where they have been weakened by improper 
 use, one must proceed to strengthen them little by little 
 from day to day, and in no case make the mistake of 
 imposing upon them work which they are unable to bear. 
 
412 THE HUMAN MECHANISM 
 
 Most cases of " weak ankles " can be cured if taken in 
 time and their muscles gradually trained. But these mus- 
 cles can never be trained by imposing upon them sudden 
 and severe work which, in their weakened condition, they 
 are unable to perform. The fatigue thus induced too often 
 prevents their working at all, thus leaving the weight of 
 the body free to strain ligaments and do other damage 
 which may leave the foot in a worse condition than before. 
 
 We have already insisted (p. 319) upon the importance 
 of walking as a means of general muscular activity ; and 
 we may urge in concluding this chapter that the chief 
 hygienic importance of the care and training of the feet 
 lies, not so much in the fact that the danger of acquiring 
 flat foot is thereby lessened, as in the fact that we thereby 
 maintain in good working order this essential part of the 
 mechanism of locomotion. American men, and especially 
 American women, compare very unfavorably with their 
 English cousins in the ability to enjoy walking and tramp- 
 ing ; and while this is partly due to the general disuse of 
 walking as a means of exercise, it is equally attributable 
 to the deformation of the feet, which robs those organs of 
 the power and even the possihility of performing with ease 
 their proper function. 
 
CHAPTER XXIV 
 BATHING 
 
 1. The Hygiene of Bathing. — The principal hygienic 
 purpose of batliing may Ije stated in one word, namel}', 
 cleanliness. A bath is often stimulating and refreshing, 
 and special kinds of baths may l)e used upon occasions for 
 good and useful ends; their vidue in the treatment of 
 many diseases is coming to be widely recognized, and 
 even in health they may be useful as aids to the best work- 
 ing power. But experience shows that it is not necessarj^, 
 even if it be refreshing, for a- healtliy person leading a 
 healthy life to use bathing ior any other purpose tlian 
 cleanliness. 
 
 The sweat glands and the sel)aceoTis glands pour out 
 upon the skin secretions which primarily serve the useful 
 purposes of regulating the temperature of the body and 
 keeping pliable the horny layer of the epidermis. Each of 
 these secretions contains solid material, which, as the water 
 of perspiration evaporates, is left on the surface of the 
 skin or in the ducts of the sweat glands ; some of the solids, 
 too, are either themselves odorous or else are putrescible, 
 giving rise to offensive decomposition products ; conse- 
 quently it is a duty which every one owes to his fellow-man 
 to bathe so as to be clean and to render that hatliing effective 
 hy wearing clean clotJies. A clean person, clean clothing, a 
 clean house, clean premises, clean streets, a clean town are 
 so many forms of that habit of cleanliness which is one of 
 the characteristics of high civilization, one of the funda- 
 mental elements of self-respect and proper living. 
 
 413 
 
414 THE HUMAN MECHANISM 
 
 Besides this, filth and dirt are effective carriers of dis- 
 ease ; consequently bathing and the use of clean clothing 
 dirainisli the chance of infection. Finally, personal clean- 
 liness also keeps the skin in a healthy condition, and this 
 alone is a sufficient reason for making it a rule in the 
 hygienic conduct of life. 
 
 2. The Indifferent Bath. — A hath which is neither dis- 
 tinctly cold nor hot may be said, in general, to answer all 
 purposes of cleanliness. The temperature of such a bath 
 varies between 80° and 90° F. with different individuals. 
 When soajj is used, water of this temperature removes the 
 waste products from the skin sufficiently for all practi- 
 cal purposes, especially when such a bath is taken daily. 
 Indifferent baths are, however, without any stimulating 
 (or depressing) physiological effect, provided they are not 
 taken in a cold room ; and for some people they are the 
 most advisable form of bathing. 
 
 3. The Hot Bath used alone is not as a rule advisable. 
 It has a well-recognized enervating effect, and experience 
 shows that after it great caution is required as to exposure 
 to cold. These effects, however, are generally obviated by 
 following the hot bath with a cold needle bath, a cold 
 shower, or a cold plunge, and possibly this procedure may 
 be recommended as the most useful and beneficial form of 
 bathing for the great majority of people. The hot bath 
 serves the purposes of cleanliness more effectively than 
 the indifferent bath, and the shock of the cold bath is not 
 so trying to many people when taken immediately after 
 the skin has thus been thoroughly warmed. Too frequent 
 and especially too prolonged hot bathing, however, is apt 
 to remove too much oil from the skin. 
 
 Hot baths, either of the body as a whole or at times a 
 hot footbath, are often useful in bringing the blood to the 
 skin and thus checking a threatened cold or other inflam- 
 matorjr process. Special care is needed, however, in this 
 
BATHING 415 
 
 case to avoid subsequent exposure to cold. It should also 
 be remembered that a very hot bath is a strong stimulus to 
 the nervous system as a whole. 
 
 4. The Cold Bath is a powerful stimulus to the nervous 
 system. When the irritability of the latter is low, as when 
 we awake from slumber, it " wakes us up," and immedi- 
 ately after it we feel distinct exhilarating effects. In 
 addition to this it probably serves as a training to the 
 heat-regulating mechanism of the body, "hardening" the 
 body to the effects of cold. Undoubtedly its influence 
 with a large proportion of healthy people is beneficial, 
 though, as we shall see, this is not the case with all. 
 Before dealing with this side of the question we may give 
 some rules which are always applicable in the use of such 
 baths. 
 
 First, they should not be prolonged. To stay in a cold 
 bath longer than one minute is undesirable save in a very 
 few exceptional cases ; thirty seconds is the usual time, 
 while with some people ten seconds is the maximum. 
 
 Second, a cold bath should be taken when the skin is 
 warm. Immediately on rising in the morning, immediately 
 after muscular exercise, or immediately after a hot bath it 
 is most beneficial and least likely to produce bad after 
 effects. 
 
 Cold bathing should always be followed, except in warm 
 weather, by a good rub-down with a rough towel. This 
 promotes a good flow of blood through the skin and adds 
 to the tonic effects. 
 
 A cold bath should not be taken in a cold room. Many 
 profit by its use in summer, but experience undesirable 
 effects in winter. 
 
 Third, cold bathing should not be used unless it is fol- 
 lowed hj what is called the "reaction"; that is, unless it 
 produces a distinct glow in the skin. The persistence of 
 pallor in the skin after the rub-down is proof that the 
 
416 THE HUMAN MECHANISM 
 
 system does not react properly, and is a warning that this 
 form of bathing should be given up or at least modified. 
 This does not mean that the bath necessarily agrees with 
 us if it does produce the "glow," for this is only one of its 
 after effects, and we must judge of its usefulness not by one 
 but by the sum total of the effects produced. 
 
 Fourth, no bath, unless it be possibly the indifferent 
 bath, should be taken within an hour or more after a meal. 
 The evidence of experience on this point is so unmistakable 
 that nothing more need be said about it. 
 
 It would be a mistake to discourage all bathing except 
 that which is used for purposes of cleanliness ; and when 
 we insist that both hot and cold baths are an artificial ele- 
 ment introduced into the environment, it is only to enforce 
 the need of carefully observing the effects of their use. 
 No one is justified in saying that these baths are neces- 
 sarily good for all healthy people ; no one is justified in 
 recommending them as essential elements in the hygienic 
 conduct of life. They must be judged by their effects, and 
 when submitted to this standard it would appear that 
 while they are beneficial to some people they are harmful 
 to others. 
 
 We must furthermore distinguish between the immediate 
 results, those noticed later in the day, and the remote 
 results. The immediate effects may be exhilarating; we 
 may " feel splendid " afterward, and yet this feeling may 
 be succeeded by one of depression. At times cold bathing 
 on rising in the morning results in constipation, although 
 the bath itself may be enjoyable. This may be exceptional, 
 but it shows that every one must determine for himself 
 the value of the bath by the sum total of its after effects, 
 and not alone by those which accompany or immediately 
 follow it. 
 
 5. Swimming and Salt-Water Bathing. — When one is 
 swimming, the heat produced within the body by muscular 
 
BATHING 417 
 
 activity counteracts to some extent the effect of the cool 
 or cold water applied to the skin. Hence it is possible 
 to remain in the water a longer time with safety and 
 even with profit than in the ordinary cold bath. It is 
 quite impossible, however, to give definite rules as to the 
 length of time one should remain in the water, since this 
 depends on the amount of muscular activity, on the tem- 
 perature of the water, and on the condition of the bather. 
 But the hygienic value of swimming and sea bathing must 
 be determined by the same tests as have been urged in the 
 case of cold bathing in general. 
 
 It is also important to remember the danger of going 
 into cold water when one is fatigued from muscular activ- 
 ity. The fatigued muscles seem especially liable to go 
 into cramps under these conditions, and persons have been 
 drowned in this way before help could reach them. 
 
 When one takes vigorous daily exercise the best time 
 for the bath is immediately after the exercise. One is then 
 in a perspiration and it is best to change the clothing. 
 The skin is most readily cleaned in this condition, and 
 most persons find a hot bath, with or without the use of 
 soap, followed by a short, cold needle bath, shower, or 
 plunge, preferable to other forms of bathing. The time for 
 bathing, however, like the time for eating, must depend 
 on one's work in life. We do not live to bathe, any more 
 than we live to eat. 
 
CHAPTER XXV 
 
 CLOTHING 1 
 
 1. The Hygienic Object of Clothing. — Even in the sav- 
 age state some races clothe themselves thoroughly. The 
 Eskimos, for example, go warmly clad in furs, and the 
 wild Indians who once inhabited the northern United 
 States wore, at least in winter, the skins of animals. In 
 the tropics, on the other hand, as in northern Africa or 
 the islands of the South Seas, very little clothing is worn, 
 and that more for the sake of decency or ornament than 
 for warmth. In these facts we find the liijgieme reason 
 for the iise of elothing, namely, to aid the body in maintam- 
 ing its constant temjyerature. In cold weather, clothing is 
 a kind of portable house, a close and intimate shelter, an 
 indisi)ensable aid to the skin in preventing undue loss of 
 lieat; on the other hand, summer clothing should interfere 
 no more than is unavoidably necessary with the dissipation 
 of heat from the skin. If, in winter, warm days come, or 
 if the body becomes heated by muscular activity, or if (as 
 too often happens) houses or public places are overheated, 
 then winter clothing may not only become a burden but 
 may be actually unhygienic. Conversely, if in a change- 
 able climate cold days oi' nights come in summer, or sea 
 winds blow damp as well as cold, then ordinary summer 
 clothing may prove to lie insufficient. Here, as always, 
 the individual must be the watchful guardian of his own 
 welfare. 
 
 ' The student is advised to review Part I, Chapter XII, before studying 
 this chapter. 
 
 418 
 
CLOTHING 419 
 
 Clothing affects the temperature regulation of the body 
 chiefly through its influence upon three processes by which 
 heat is taken from the skin. These processes are (1) con- 
 vection by wind or other currents of air, (2) conduction, 
 and (3) the evaporation of perspiration. We shall deal with 
 each of these in some detail. 
 
 1. (Jlotliing and the Convection of Heat. — Any fabric 
 whose texture permits the air warmed by contact with the 
 skin to be I'ejjlaced readily by colder air from without will 
 obviously favor the cooling of the skin by convection ; and 
 conversely, any garment which lessens or altogether pre- 
 vents these currents of air through it is to that extent a 
 warm garment. The leather hunting jacket lined with 
 wool or fur is especially warm, and a newsjiaper under 
 one's coat or jacket similarly affords a large measure of 
 protection against cold. On the other hand, a rubber coat 
 may be very uncomfortable on a warm day, although the 
 effect in this case is due to its interference with the evap- 
 oration of the perspiration as well as to the prevention 
 of the passage of air through the garment. 
 
 2. Clothing and the Conduction of Heat. — Even when there 
 is no passage of air through the clothing, heat may, of course, 
 be transferred from the skin to the outer air hj conduction, 
 and some fabrics conduct heat more readily than others. 
 Other things being equal, tlie rate at which clothing con- 
 ducts heat depends on the amoiDit of air within its meshes. 
 Thus wool is warmer than cotton, not because of any differ- 
 ence in conductivity of the two kinds of fibers, but because 
 when wool fibeis are made into yarn their stiffness and 
 elasticity keep them apart, so that garments woven from 
 this yarn always contain spaces filled with air, which is a 
 poor conductor of heat. Moreover, the same properties of 
 the fibers prevent their being pressed and felted together 
 in laundering, as ordinarily happens with cotton and linen 
 fabrics. We shall see that cotton and linen may be so 
 
420 THE HUMAN MECHANISM 
 
 woven as to avoid this result, as in many "meshwork" 
 fabrics, but they are not usually so woven. 
 
 A moment's thought will show that the warmth of a 
 dry garment will deiDend on the size of its meshes. These 
 may be so fine and close as to inclose an insufficient quan- 
 tity of the non-conducting air, or they may be so large as 
 to permit- too free circulation. In the latter case heat is 
 carried away from the skin by convection. It is also clear 
 that the warmth of a garment is not determined by its 
 weight or thickness alone. 
 
 3. Clothing and the Perspiration. — So long as the meshes 
 of a fabric contain air, heat is conducted but slowly from 
 the skin. When, however, this air is partially or entirely 
 rejjlaced by water, the fabric conducts heat from the skin 
 much more rapidly ; and if the surrounding atmosphere 
 is distinctly colder than the body, the skin becomes chilled 
 and internal organs congested ; hence the danger of wet 
 clothing. 
 
 More important still is the relation of clothing to the 
 evaporation of perspiration. We have learned that per- 
 spiration is useful to the body only as it evajyorates. Con- 
 sequently the clothing should be such as will permit the 
 p)erspiration to evaporate almost as fast as it is secreted. 
 The skin will thus be cooled at the time that the needs 
 of the body require such cooling, and the clothing will not 
 remain wet after the secretion of perspiration has ceased 
 and the need for cooling the skin no longer exists. Or, 
 if it is not possible to secure this rapid drying, the fabric 
 should contain, even while moist, a considerable quantity 
 of air within its meshes, thereby checking the loss of heat 
 from the skin. 
 
 2. The Clothing worn next the Skin and the Outer Cloth- 
 ing. — Consideration of the above relations of clothing 
 to convection, conduction, and the evaporation of perspi- 
 ration shows at once that the clothing worn next the skin 
 
CLOTHING 421 
 
 must fulfill requirements not demanded of the outer clotL- 
 ing. The sole hygienic purpose of the latter is warmth 
 and the fabric should be chosen accordingly. In warm 
 weather, in well-heated rooms, and during muscular activ- 
 ity, warm outer clothing is undesirable ; on the other hand, 
 when the body is exposed to cold and is not at the same 
 time eno-aged in muscular exertion, the outer clothing- 
 should be chosen for warmth ; and for this jjurpose woolen 
 fabrics are superior to all others. 
 
 The clothing worn next the skin must, in addition, care 
 for the perspiration. For tliose forced by age or other 
 physical disability to lead sedentary lives, woolen under- 
 wear is very useful in cold weather. Since in the case 
 of such persons the blood is not brought to the skin by 
 muscular activity, it is necessary that the skin be kept 
 warm and internal congestions prevented. For such per- 
 sons woolen fabrics are probably superior to all others. 
 Moreover, during exposure to extreme cold, when little or 
 no perspiration is secreted even during vigorous muscular 
 work, woolen underwear is superior for every one because 
 of its oTcater waiiuth. 
 
 For healthy people, however, in the full vigor of life, 
 taking daily muscular exercise but not exposed to extremes 
 of cold, woolen underwear presents many serious draw- 
 backs. In the first place its very warmth is objectionable 
 during muscular activit}-, because it makes more difficult 
 tlie discharge of the surplus heat. In the second place, 
 wool absorbs the perspiration very slowly and so prevents 
 its evaporation from the outer surface of the garment ; 
 the perspiration does not cool the body as it should, but 
 remains between the skin and the garment, — an unhealtli- 
 ful condition for the skin. In the third place, when the 
 garment has once become "wet through," i.e. the air 
 within its meshes has been largely displaced by water, it 
 dries more slowly than a linen or a cotton garment. 
 
 & 
 
422 THE HUMAX MECHANISM 
 
 It is better, in other words, for healthy people to depend 
 upon the outer clothing, including overcoats, etc., for 
 warmth, when protection against cold is needed, rather 
 than upon even moderately heavy underwear. In this way 
 it is possible readily to relieve the bodj' of its heavier 
 clothing when it becomes necessary to get rid of surplus 
 heat, i.e. in warm rooms and during muscular activity in 
 only moderate cold weather, and yet to protect oneself 
 against cold when such protection is necessary. 
 
 Of late years the attempt has been made with consid- 
 erable success to weave linen, and even cotton, so as to 
 contain fairly large meshes between the threads. The 
 perspiration is rapidly brought to the surface of the gar- 
 ment tlirougli the threads by capillary attraction and so 
 evaporates Cjuickly ; for this reason the garment dries 
 readily, and even while wet usually retains a considerable 
 quantity of air within its meshes. 
 
 The thickness of underwear, as well as of the garments 
 worn immediately over it, should be determined by the 
 amount of exposure to cold ivhen at rest. When our houses 
 or offices are properly heated (65°-70° F.) in winter, heavy 
 clothing is as much to be condemned as the too common 
 overheating of our rooms, and for the same reason. When, 
 on the other hand, our work is out of doors in cold weather 
 but involves only a small amount of muscular activity, 
 warmer clothing should be worn ; in this case the use of 
 heavy woolen underwear is advisable. 
 
 It is unnecessary to go further into details. The stu- 
 dent can solve special problems for himself, always remem- 
 bering that proper clothes are such as will prevent undue 
 loss of heat and consequent chilling of the skin (with 
 accompanying internal congestions) when the body is at 
 rest. 
 
 3. The Outer Clothing. — Of this little need be said. By 
 varying the thickness of the outer clothing we adapt it 
 
CLOTHING 423 
 
 to the conditions of life. It must also be chosen with ref- 
 erence to its permeability to air. In hot summer weather 
 it should be as thin and porous as possible ; in winter it 
 should protect from wind. When still further protection 
 is needed, it may be obtained by the use of overcoats, 
 gloves, muffs, lap robes, or other wraps. 
 
 Some people do not use sufficiently warm clothing in 
 cold weather, but most adults make the opposite mistake. 
 The custom of using very thick clothing in cold weather 
 appears to have been inherited from the time when houses 
 were poorly heated, when transportation from jjlace to 
 place was in cold cars or carriages, and when, in general, 
 the human race was more exposed to cold than it is to-day. 
 Where these conditions prevail, as tliey still do in many 
 country districts, heavy elotliing should no doubt be worn 
 in winter. The same may be said of driving in open vehi- 
 cles, such as sleighs, etc. But in cities, where houses are 
 more likely to be overheated than underheated, Avliere steam 
 and electric cars are far from being chilly, where, in short, 
 we need not generally l)e exposed to cold except when 
 walking or taking other muscular exercise, the main 
 dependence for protection against cold should be upon 
 the outer wrappings rather than upon the underwear, the 
 coat and trousers, or the dress. We do not change to 
 heavy clothing in summer when the thermometer falls to 
 65° or 70° F., and there is no reason why we should use 
 such clothing at these temperatures in winter. The pre- 
 cautions which many take against sudden changes of 
 weather are often excessive. 
 
 4. Clothing not the only Protection against Cold. — It 
 must be i-emembered that we have another means of pro- 
 tection against cold besides clothing, and that is muscular 
 activity. Even if, as often happens, a balmy morning passes 
 into a chilly afternoon, most peojjle, especially those living 
 in cities, should be able to keep warm by a brisk walk 
 
424 THE HUMAN MECHANISM 
 
 when going home ; a little exposure to cold will not harm, 
 but will rather harden, a healthy man or woman. If we are 
 tired out and ought not to walk, we can usually ride in 
 a heated car. To wear heavier clothing than the probable 
 necessities of the case demand, merely because there is a 
 chance that suitable weather for such clothing may over- 
 take us, is in general unwise. Oppressed with its weight 
 and warmth, the usual result is a disinclination to any 
 vigorous muscular activity while out of doors, and this in 
 the long run is more dangerous than a comparatively brief 
 chilling of the skin. 
 
 5. Clothing should not be Heavy. — The reference in the 
 last paragraph to the burden of heavy clothing deserves 
 further consideration. The terms " warm," " thick," and 
 " heavy," as applied to clothing, are often used as if they 
 were synonymous, although a thick garment is not neces- 
 sarily a heavy garment, and a thinner but more loosely 
 woven coat may be warmer than one which is thicker but 
 more closely woven. In the selection of clothing it is 
 always advisable, not only as a matter of personal comfort 
 but also as a matter of practical hygiene, to avoid heavy 
 fabrics. While this holds especially for invalids and elderly 
 people, to whom the burden is more oppressive, it also 
 holds for the young and strong. The clothing should be 
 such as will interfere in the least degree with the freedom 
 of bodily movements. Not only should every one avoid 
 such fashions as tight lacing and liigh-heeled boots, — so 
 senseless as to be beneath the contempt of those who 
 respect the human body and care for its physical well- 
 being, — but care should be taken to have the clothing 
 everywhere loose enough to be comfortable and, above 
 all, light enough so that its weight is not a burden. For 
 this reason a very close weave is objectionable except in 
 windy weather, since it gives great weight of fabric with 
 but small air contents. 
 
DOMESTIC HYGIENE AND SANITATION 
 
 CHAPTER XXVI 
 
 THE HOUSE: ITS SITE, CONSTRUCTION, FURNISHINGS, 
 AND CARE 
 
 1. The Family a Private Community. — Every human 
 being has not only individual or personal relations with his 
 environment, but also various other, and j^j((ft/ff relations, 
 since the life of an individual is always more or less closely 
 connected with the lives of other human beings. Each 
 individual or person is a member of some family, and also 
 of some village, town, city, state, or nation. Connections 
 of this kind constitute kinship, relationship, and fellow- 
 ship, and are commonly described as social relations {nocms, 
 a fellow). They are nowhere more conspicuous than in 
 matters of life and death, health and disease. The human 
 infant is absolutely dependent upon parental care, and 
 among civilized people the sick, the aged, the dying, and 
 the dead must be tenderly cared for by those who are alive 
 arid well. But this is not all, for sickness is frequently 
 " catching," and plagues, pestilences, and epidemics have 
 often run like wildfire through families or communities, 
 leaping from person to person, and from village to village, 
 very much as a forest fire leaps from tree to tree. 
 
 A fundamental feature of all social relations is the fact 
 that persons in families, villages, towns, cities, states, 
 and nations have and use many things in common. This 
 has caused such groups of human beings to be known as 
 
 425 
 
426 THE HUMAN MECHANISM 
 
 communities {communis^ common). Of all communities the 
 simplest, the most fundamental, and the most important is 
 the family or household, in which the various individual 
 members share a common shelter, a common fireside, a 
 common table, and a common interest, based upon the all- 
 powerful ties of blood or marriage. In these and many 
 other respects the family is not only a community but a 
 peculiar kind of community, namely, a private community. 
 But precisely as the individual necessarily has relations 
 to the world outside himself and is by nature not merely a 
 man and an animal, but a social man and a social animal, 
 so the civilized family or household, although essentially a 
 private establishment, has certain public relations. It must 
 draw its air supply from the aerial ocean common to all 
 mankind ; it must form a component unit in some village, 
 township, state, or nation ; it must buy sugar or salt, or 
 tea, coffee, or spices, from overseas. 
 
 Midway between the more purely public relations which 
 we shall presently discuss under puhlic hi/c/iene and sani- 
 tation, and those individual relations which we have con- 
 sidered in the foregoing chapters on jyersoiial hygiene, stand 
 the hygiene and sanitation of the house and the family, 
 subjects neither altogether public nor altogether personal. 
 These we may describe as domestic hygiene and sanitation. 
 
 2. Housing and the House. — The chief function of 
 clothing (p. 418) is to protect the body from cold by main- 
 taining about the skin fairly constant temperature condi- 
 tions, and accordingly clothing is of least importance in 
 the tropics, where conditions of temperature are both con- 
 stant and warm. The housing problem is very similar, 
 for the principal function of tlie house is likewise to fur- 
 nish for the body a favorable environment, and especially 
 a fairly constant temperature ; here also the problem is 
 simplest in the tropics. The house, in fact, is a kind of 
 outer clothing or protective shell, although usually designed 
 
SANITARY HOUSING 427 
 
 not for a single individual but for an entire family, or, as 
 in the case of tenement houses, apartment houses, or hotels, 
 for many families, or for the public. 
 
 Savages and the lower races of men content themselves 
 with caves, trees, and other ready-made shelters, or with 
 huts, hovels, wigwams, and similar poor and primitive 
 dwellings. The most highly civilized races, on the other 
 hand, build carefully planned and sometimes elaborate 
 houses, furnished with various devices, simple or complex, 
 for heating, lighting, ventilation, water supply, drainage, 
 cookery, and other necessaries or luxuries of a comfort- 
 able, as well as a uniform and favorable, environment. 
 
 Houses may be separated and detaclied, as on farms or 
 in villages, or massed in groups or blocks, as in towns or 
 cities ; and owing to the fact that they are comparatively 
 durable and costly, most people live in dwellings already 
 built. But although very often a family cannot build its 
 dwelling, but must take to some extent wliat it can get, it 
 usually has, sooner or later, some choice ; and even if it 
 has not much choice, it may modify more or less from 
 time to time the domicile wliich it must occupy. 
 
 3. The Site of the House is often determined more by 
 necessity, taste, or convenience than by liygienic considera- 
 tions ; but in general it may be said that a human dwell- 
 ing should be so situated as to afford good air, good light, 
 good drainage, and good neighbors. If, in addition, beauti- 
 ful, charmincf, or attractive surroundings can be had, these 
 are of great importance, since beauty and charm often have 
 a distinct hygienic value. A certain seclusion or privacy 
 is also to be desired, for a c^uiet, retired, aud restful home, 
 removed from the distractions of publicity, is soothing 
 to tired nerves as well as conducive to normal and whole- 
 some family life. On the other hand, extreme isolation, 
 such as is sometimes found in farmhouses, often produces 
 a morbid feeling of loneliness. 
 
428 THE HUMAN MECHANISM 
 
 When possible the house should be placed upon open, 
 po7'ous, or gravelly soil, because such soil is less likely to 
 be water-logged and is more easily drained. In the United 
 States in general a southerly or soutJiwesterly slope is usually 
 preferable, as affording more sunshine in winter and more 
 breeze in summer. It is also wise, of course, to have the 
 principal living rooms on the side exposed to dry rather 
 than to cold and damp winds. 
 
 Good air for a house is to be sought for in a clean neigh- 
 borhood, a clean, dry cellar, and a free circulation, the 
 latter impeded as little as possible by other buildings or, 
 in the country, by too many trees. An elevation, therefore, 
 rather than a depression is obviousl}^ desirable as an aid in 
 securing these things, although the very top of a hill should 
 usually be avoided because of its bleakness. A dry cellar 
 and, if possible, a dry, open, and porous soil beneath the 
 house are highly important, no matter where the house is 
 placed. Cellar habitations are very objectionable and ought 
 to be avoided by even the poorest family. Such dwellings 
 were long since forbidden by law in England, and although 
 sanitarians are not yet agreed as to all the reasons why cellar 
 habitations are injurious, the principal reasons seem to be 
 the well-known unwholesomeness of dampness, and want of 
 sunlight. Undue dampness in air is believed to favor rheu- 
 matism and other disorders, and the absence of sunlight not 
 only favors dampness and microbic life but also tends to 
 mental depression. House cellars should be well drained. 
 
 Good light and, if possible, abundant sunshine are hj^gienic 
 conditions of great importance, both as aids to cheerful- 
 ness and happiness and as powerful sanitary agents. Sun- 
 shine tends to remove dampness and to destroy the germs 
 of infectious diseases. In winter, sunshine is valuable also 
 for warmth. 
 
 Good draiyiage is no less (and perhaps no more) necessary 
 for human habitations than good air and good light. With 
 
SANITARY HOUSING 429 
 
 the abundant use of water in recent times for washing, 
 bathing, cleaning, sewage disposal, and other purposes, it 
 becomes necessary in modern houses to get rid somehow 
 of a great deal of soiled and dirty water, and the possi- 
 bility of easy and safe " drainage," or lemoval of such 
 water, must be kept in mind in considering the sanitary 
 aspects of the situation of any house, old or new. Here 
 again tlie advantage is jjlain of some elevation of site. 
 
 4. The Construction of the House. — As the first object 
 of any house is shelter from rain, snow, wind, dampness, 
 and excessive heat or cold, its materials should be water- 
 proof, windproof, and non-conducting for heat, as far as is 
 consistent with a proper circulation of air. Wigwams or 
 tents, at least in temperate latitudes, are clearly defective 
 in some of tliese particulars. Houses built of glass or India 
 rubber would answer most of these requirements but would 
 still be most unhygienic, chiefl}' because glass houses would 
 be too light and too hot, while both glass and India rubber 
 would interfere seriously witli that free circulation of air 
 which takes place through relatively porous materials 
 such as wood and brick. Buildings of wood, stone, brick, 
 or steel and briek, rightly built, answer all rec^uirements. 
 A " double wall," i.e. a hollow wall, by providing a non- 
 conducting air space is of great value for preventing 
 rapid changes in the temperatures of houses under sudden 
 changes of climate, as well as for protection against damp- 
 ness and noise. 
 
 Much circulation of air usually takes place even through 
 walls or partitions of plaster and wood, and a knowledge 
 of this so-called " natural ventilation " helps us to under- 
 stand how it is that many people live, and even thrive, in 
 seemingly unventilated rooms and houses. It also helps 
 us to understand how the damp air of a cellar finds its 
 way upward into a house, and why a double floor (with 
 air spaces between) is especially useful immediately above 
 
430 THE HUMAN MECHANISM 
 
 the cellar. Blinds or shutters, and shades or curtains, for 
 darkening rooms, are of great hygienic value, since sleep 
 is deeper in darkness than in light, and in summer these 
 tend also to keep the house cooler. 
 
 5. The Furnishings of the House. — The ivalh of rooms 
 may be of wood, — bare, painted, or varnished, — or of plas- 
 ter, — either bare or covered by textiles such as burlap or 
 tapestry, or "papered" by pasting upon them with thick 
 paste, or "sizing," sheets of paper upon which designs, 
 often in color, have been printed. Sometimes, instead of 
 being papered, walls and partitions are painted, either with 
 white paint or in colors, and sometimes simply a hard 
 finish is given to j)laster, which is afterwards " white- 
 washed " or " calcimined." 
 
 A good feature of painted walls is the fact that they 
 may be washed, and of walls smoothly calcimined that 
 they ma}^ be easily done over. In general, a smooth and 
 washable surface is preferable to a rough one or one in- 
 jured by washing, for these not only collect more dust but 
 are harder to keep clean. 
 
 The most serious charge, from the hygienic point of 
 view, thus far brought against u'cdl ^Japers, is that of the 
 danger of poisoning for persons living in rooms papered 
 with such papers as contain arsenic. The subject has 
 been much in dispute, but the evidence of such occasional 
 poisoning seems now convincing, especially since the work 
 of Gosio, an Italian investigator, showed that molds or 
 other microorganisms which grow in the paste used to 
 stick the paper to the walls are capable of attacking the 
 arsenic of some coloring matters, thereby producing volatile 
 compounds of arsenic readily diffusible into the air of the 
 room. 
 
 There is similar danger from arsenical poisoning in some 
 tapestries or furniture coverings, and grave disorders have 
 been attributed, apparently with reason, to tliis source. 
 
SANITARY HOUSING 431 
 
 The iro7i bedstead, light, firm, cheap, and easy to keep 
 clean, is a marked improvement upon the heavy wooden 
 bedsteads formerly used. Curtains, canopies, valances, etc., 
 either above or below beds, are objectionable, as they inter- 
 fere with the free circulation of air. The modern oj^en 
 bedstead is an improvement upon the old-fashioned "four- 
 poster " with its hangings, almost as great as is the modern 
 " open," over the earlier concealed, plumbing. 
 
 Single bedn possess many advantages over double beds. 
 They are more easily cared for and ke|)t clean ; the amount 
 of covering can be more accurately adapted to the indi- 
 vidual needs of their occupants, who are also less exposed 
 in cases of infectious disease ; and the use of such beds is 
 more conducive to undisturbed sluml)er. 
 
 Folding beds, mantle beds, safa, beds, and all similar 
 devices for concealment of beds and bedding are subject to 
 the olijection that they are likely to be closed too soon 
 after having been used, and before the bedding has been 
 sufficiently aired or freshened. Certain forms of folding 
 beds are further olijectionable from the fact that by their 
 closing automatically and unexpectedly the occupant has 
 sometimes been im^uisoned and even killed. 
 
 Floors are in America usually wooden and made of 
 boards, " matched " or otherwise laid tight. Formerlj- the 
 material used for inex[>ensive floors was of pine, spruce, 
 hemlock, or other soft woods, oak being reserved for the 
 more costly Jiard floors. Nowadays hard pine (Southern 
 pine) is much used and many cheap yet good floors are 
 made of this material. Softu'ood floors are apt to become 
 dented and splintered unless covered and protected by 
 carpets or mattings ; but, if made of good stock and well 
 cared for by frequent painting, such floors answer very 
 well for a long time, especially in rooms, such as cham- 
 bers, not subject to hard usage. Bare floors possess the 
 immense sanitary advantage of being easy to clean and 
 
432 THE HUMAN MECHANISM 
 
 also of revealing dust and dirt. But they require, for the 
 latter reason, more care, and are also open to the objection 
 that they are comparatively noisy. 
 
 Fixed mattings are useful for the deadening of sounds, 
 and fixed carpets not only for this but also for warmth ; but 
 both hold dirt and are hard to clean, while light, movable 
 mattings, carpets, or rugs readily lend themselves to cleanli- 
 ness, because they can be removed and, in their temporary 
 absence, both they and the otherwise bare floors upon which 
 they rest can be thoroughly cleaned. 
 
 6. The Care of the House. — The house, the outermost 
 clothing of man and the family, like the dress of the body 
 or the clothing proper, is suliject to the wear and tear of 
 time, season, and weather, and likewise requires care, con- 
 sisting chiefly of cleaning and mending, or repair. Paint- 
 ing in the case of wooden houses (and for the steel parts 
 of steel-and-brick structures), and pointing (or the renewal 
 of mortar between bricks or stones) in the case of brick or 
 stone houses, helps to make them waterproof and wind- 
 proof and tends to keep out dampness. 
 
 TJie cellar, especially, requires watchful care, and should 
 be kept not only dry, by windows or other ventilating 
 devices, opened wide in favorable weather (and never 
 wholly shut), but also clean and free from rubbish, decay- 
 ing vegetables, or anything tending to dirt, dampness, or 
 uncleanness. 
 
 The walls of the rooms, if papered, should from time to 
 time be wiped with damp cloths, or if painted, they may be 
 washed. Floors and the hard parts of furniture are thor- 
 oughly cleaned only by washing, but may also be well 
 cared for by wiping with damp (not wet) cloths which are 
 easily washed and wrung out. Brushes with long handles 
 are also useful, though less effective, for ceiling's and bare 
 floors, and brooms for fixed mattings and carpets. Dust 
 cloths and the little feather duster may be used upon books. 
 
SANITARY HOUSING 433 
 
 fabrics, and other surfaces which would be injured by the 
 damp cloth; but such "dusting," in so far as it lifts the 
 dust into the air only to allow it to settle again some- 
 where in the same room, merely transfers it from one place 
 to another, and, while perhaps better than nothing, does 
 not produce true cleanness. The large feather duster often 
 used to clean (?) things which should be wiped with damp 
 cloths, easily washed or renewed, is simply a sanitary 
 abomination. It makes a show of cleanliness but lacks 
 the substance thereof. It is the favorite implement of the 
 superficial, indolent, or shiftless, and especially of janitors 
 of this class in public buildings. 
 
 Even when closed, houses quickly become dusty or dirty, 
 because the air which finds its way into them through 
 cracks or crevices is almost always more or less charged 
 with dust, while the occupants of inhabited houses bring 
 in upon clothes, shoes, and all kinds of articles more or 
 less dust or dirt. Fires, such as those in stoves, fireplaces, 
 or furnaces, also add greatly to the dust of houses. Dust 
 and dirt are composed largely of inorganic or lifeless mat- 
 ters such as particles of sand, or coal, or ashes, or fibers 
 of cotton or woolen, but also partly of microbes. j\Iost of 
 the latter are harmless, and some kinds of dust and dirt 
 are of little sanitary importance, — a fact which helps us 
 to understand why some people seem to have health even 
 in dirty surroundings. But dust and dirt sometimes con- 
 tain the germs of dangerous diseases, and the way of safety 
 is the way of cleanliness. 
 
CHAPTER XXVII 
 THE WARMING AND LIGHTING OF THE HOUSE 
 
 1. The Warming of the House. — The earliest method of 
 warming human dwellings was the open fire, in hut, cave, 
 or wigwam, and when chimneys were added to carry off 
 smoke and improve combustion by creating drafts, the 
 open fire still remained for a time the sole resource of 
 mankind for heating purposes. It is still the most attrac- 
 tive and most cheerful method of heating, and has been 
 well called " the eye of the room." It is a coveted luxury 
 in all tasteful homes, not so much for the heat it furnishes 
 as for its cheerful glow and the constant interest which it 
 excites. The home and the f reside have become everywhere 
 almost equivalent terms. 
 
 2. The Open Fire may be of either coal or wood. In 
 England it is almost always of coal, and in that country is 
 still the principal means of heating. In some other coun- 
 tries, and especially in the United States, it is made of 
 either coal or wood, but is less depended upon for heating 
 purposes. The open fire may be on a hearth in a fireplace, 
 or in an open grate or an open stove. In all modern cases 
 of true open fires, a chimney rises above the fire to carry off 
 the smoke, and the draft of the chimney (caused by the rising 
 of the column of lighter heated air) constantly sucks away 
 the air of the room and produces considerable ventilation by 
 removing vitiated air. The air thus removed is replaced 
 by air from adjoining rooms or from outdoors, driven in 
 by the atmospheric pressure through open doors or win- 
 dows or through the walls themselves which, if of wood or 
 
 4.",4 
 
WARMING AND LIGHTING OF HOUSES 435 
 
 plaster, or even of stone or brick, are to some extent 
 porous. But while such ventilation has great advantages 
 and is one of the best things about open fires, such fires 
 are wasteful of heat and often do not effectively warm the 
 entire room. This is because the warmed air is not returned 
 to the room, but is drawn up tlie chimney, and because the 
 movement of the cold air which is pressed in from the 
 outside tends to make the room " drafty." Radiation from 
 the fire itself, rather than convection by air currents, tlius 
 becomes the chief means of warmth ; and the complaint 
 against open fires that those gathered about them, wliether 
 indoors or out, are " roasted in front and frozen behind," is 
 undoubtedly well founded. Open fires, nevertheless, serve 
 admirably to " take the chill off " from a room in those 
 days of late spring or early autumn when the temperature 
 is only a few degrees below the proper jjoint (sec p. 201). 
 
 3. Stoves are superior to open fires as sources of warmth, 
 but far inferior in attractiveness and as aids to ventila- 
 tion. A stove is usually placed in a room at some dis- 
 tance from the wall, and connected with tlie chimney by a 
 stovepipe to carry off the products of combustion. There 
 is no such thing as an " air-tight "' stove, a term often used 
 because some stoves seem tightly closed, only enough air 
 being allowed to enter to supply the actual need for com- 
 bustion. A stove warms a room by the mixture of currents 
 of heated air around the stove with the cooler air in other 
 parts of the room, i.e. by convection, and also by direct 
 radiation from the fire itself. 
 
 4. Hot-Air Furnaces are usually inclosed stoves placed 
 in the basement or cellar. They are provided with smoke 
 pipes and surmounted by a space, the hot-air chamber, to 
 the lower portion of which a pipe, or " air box," conducts 
 cold air, while a second pipe or system of pipes leading 
 off from the upper portion of the chamber supplies the 
 various rooms with the warmed air. This is a convenient. 
 
436 THE HUMAN MECHANISM 
 
 economical, and popular method of heating a house, and 
 possesses the great advantage of bringing constantly into 
 the various rooms supplies of fresh air. If this air has 
 not been overheated while passing by the furnace, little 
 objection can be brought against it on any ground. It is 
 true that, having been warmed, its relative dryness has 
 been increased ; but this condition may be corrected to 
 some extent by always keeping in the hot-air chamber of 
 the furnace a vessel of water for evaporation. 
 
 If, however, the air supplied to the furnace is not fresh 
 and drawn from the outer atmosphere, but is simplj' taken 
 from the cellar in which the furnace stands ; or if the fur- 
 nace is not tight, but cracked or loose-jointed, so that the 
 gases of combustion may escape and mingle with the air 
 as the latter flows through the pipes and rises into the 
 rooms of the house ; or if, as often happens, the air is over- 
 heated and greatly overdried, then furnaces of this kind 
 may, and do, become objectionable. In very cold and 
 windy climates, and for houses in bleak or exposed places, 
 furnaces are less satisfactory than steam or hot-water 
 heaters. As they usually deliver warm air under very 
 small pressure, it is often impossible, especially in windy 
 weather, " to put the heat where it is wanted," a difficulty 
 not encountered in the use of steam or hot water. Another 
 objection to hot-air furnaces is the fact that much dust 
 finds its way in with the warm air ; but fresh air without 
 dust, at least in towns and cities, is rare. 
 
 A simple combination of stove and furnace is much used 
 in some places, where a stove (usually in or against the 
 fireplace) on the first floor heats not only the room in which 
 it stands but also, Ijy means of a pipe, or pipes, and regis- 
 ters, one or more rooms overhead. Unfortunately the air 
 thus supplied to tlie upper rooms is not always pure air 
 from out of doors ; sometimes it is the already vitiated 
 air of the room below. 
 
WAEMING AND LIGHTING OF HOUSES 437 
 
 5. Warming by Steam and by Hot Water. — It is very 
 common in the United States to find houses (and other 
 buildings) heated by steam or hot water. Through the 
 " radiators " or " coils " placed in the various rooms there 
 is maintained a circulation of steam or of hot water from 
 a " heater " below. Here the room is warmed jjartly by 
 direct radiation and partly by convection currents, very 
 much as in the case of the stove. The chief objection to 
 this method of heating is that the heating and the ventila- 
 tion of the room are not effected by the same process ; the 
 room must be ventilated by opening windows, and the air 
 thus introduced is apt to be cold and to produce undesirable 
 drafts. On the other hand, steam and hot water are both 
 superior to hot air in convenience and efficiency. They 
 can be carried anywhere and are free from disturbances by 
 atmos2:)heric conditions, wind-})ressure, natural ventilation, 
 etc., which greatly interfere with the proper distribution of 
 hot air.i Sometimes a combination of direct and '• indirect 
 radiation"^ is employed, the latter being used for all ordi- 
 nary heating, and the former kept for aid in extremely cold 
 or windy weather. On this plan fresh air drawn from out- 
 side is first passed over coils of pipes placed in a basement 
 or cellar and containing steam or hot water, and then car- 
 ried (as in the hot-air furnace) to the various rooms which 
 it is desired to warm. In addition, radiators are placed in 
 these rooms, often near doors or windows, and in extreme 
 cold weather are charged with steam or hot water to fur- 
 nish supplementary lieating by direct radiation. 
 
 6. Oil Stoves, Gas Stoves, and Electric Heaters. — These 
 do not greatly differ from other stoves except in the sources 
 
 1 The hot-water system is rapidly coming into favor to replace steam 
 heat, because a given volume of water will carry a larger amount of heat 
 than the same volume of steam ; consequently the water can be sent from 
 the heater at a lower temperature than the steam, the supply pipe is not 
 so hot, and the heat is more evenly distributed through the house. 
 
 - ''Indirect radiation " is, of course, really convection. 
 
438 THE HUMAN MECHANISM 
 
 of the heat which they provide, and in the important fact 
 that in the first two the products of combustion are not 
 usually carried off by chimneys but, as in oil or gas lamps 
 (which are also powerful room warmers and really only 
 luminous stoves), escape directly into the room and thus 
 tend to vitiate its atmosphere without causing any com- 
 pensating ventilation. In the case of gas stoves special 
 care must be taken to see that the unburnt gas does not 
 escape into the room from leaks in the connections or 
 elsewhere. Here the same considerations apply as in the 
 case of gas used for lighting. The flexible rubber tubes 
 often used for supplj'ing gas stoves deteriorate with age 
 and then frequently permit the escape of gas directly into 
 the room. Whenever possible, permanent (metallic) con- 
 nection of the stove with the iron gas pipe is advisable. 
 Where rubber connections are used, the gas should always 
 he turned off at the cock on the mam pipe, never at that on 
 the stove. 
 
 Electric stoves, like electric lights, are heated by elec- 
 tricity, and even electric lights, though inferior in this 
 respect to oil or gas lights, are often noteworthy factors 
 in the warming of houses. Stoves used for cooking add 
 materially to the warmth of houses, and hence gas or oil 
 stoves may be used with advantage when, as in summer, 
 heat is undesirable. 
 
 7. Solar Heating. Glass Verandas. — Less use is made 
 of the direct heat of the sun than is often advisable or ad- 
 vantageous. Rooms flooded with sunshine are always more 
 economically warmed than those without it, and a solarium, 
 or glass-covered room or veranda, on the south side of a 
 house is often useful as well as agreeable in winter. If 
 provision is made for heating it at night, and in cold and 
 cloudy weather, it may be made to answer also as a plant 
 conservatory, or greenhouse, and thus become a source of 
 added interest and pleasure. 
 
WARMING AND LIGHTING OF HOUSES 439 
 
 8. Overheating. — If the temperature of the house is too 
 high, we suffer from many of the objectionable conditions 
 of hot weather ; mental work is more difficult and we are 
 disinclined to muscular exercise. It is probably unwise 
 to keep the temperature of the house above 68° or 70° F. 
 A good rule is to keep it between 65° and 70° F. (see 
 pp. 389-391). Those who, by reason of infirmities of age, 
 cannot enjoy regular muscular activity often find rooms 
 of this temperature too cold ; but the aged should be en- 
 couraged and even urged to keep up at all hazards the 
 habit of doing some muscular work every day. With care- 
 ful attention to muscular exercise and outdoor life they 
 can not only endure but also enjoy lower room tempera- 
 tures than is generally supposed, and thus permit the 
 younger members of the household to live under more 
 wholesome temperature conditions. A])petite is also im- 
 proved by this practice and old age made in general more 
 comfortable and more cheerful. Youth should remember, 
 however, that the aged, largely because they cannot " get 
 warm from the inside," not only desire but actually require 
 warm clothing and often very warm rooms. 
 
 9. The Lighting of the House. — The fire light and the 
 light of the pine knot, with which the hut, the hovel, or 
 the wigwam were lighted, were objectionable chiefly because 
 of their inconvenience, smoke, and flare or flicker. The 
 invention of the lamp without chimney, and of the candle, 
 marked a step forward, though their light was weak and 
 flickering-. A much g-reater advance was the invention of 
 the lamp chimney, as it provided what nothing else had 
 done, steadiness of flame, and avoided flare and flicker. 
 Once the latter was overcome, it became easy to improve 
 the fuel, until now the oil lamp with chimney not only 
 illuminates and decorates the home of wealth but also 
 brig-htens and cheers the hut of the fisherman and the cabin 
 of the sailor; it aids and comforts the seamstress in her 
 
440 THE HUMAN MECHANISM 
 
 toil, in the humblest lodging ; it warns the mariner by 
 night from dangerous coasts by lighthouses, and throws 
 about the student a warm and cheerful radiance as he 
 "burns the midnight oil." Candles are still much used 
 both in churches and in houses, but chiefly because of sen- 
 timent or for decoration. They still furnish the softest 
 and most beautiful light, especially for quiet places ; but 
 they are unfit for reading purposes because of their flicker- 
 ing and their feebleness. 
 
 The introduction of gas lighting was a great advance 
 over lighting by lamps, owing to its convenience and clean- 
 liness and the intensity of the light afforded. But gas 
 lights, unless provided with chimneys, are generally un- 
 steady and therefore objectionable for use in reading. Gas 
 lights (and oil lamjos) also produce much heat, and by 
 this as well as by their products of combustion may greatly 
 vitiate the air of rooms in which they are used. 
 
 While some kinds of illuminathig gas are more poison- 
 ous than others, all manufactured — as distinguished from 
 " natural " — gas contains a considerable percentage of poi- 
 sonous constituents. When tlie gas is burned, these are 
 oxidized and form harmless substances, and hence there 
 is little or no danger from the products of its combustion. 
 But the greatest care should be taken to avoid the entrance 
 in any way of unburned gas into the air of a room. This 
 may happen by the gas escaping through leaky fixtures, or 
 after " blowing it out " instead of turning it off. It may 
 also occur when the light has been turned down very low in 
 the sleeping room and is afterwards blown out by a draft, or 
 goes out because of lessened pressure in the main, and the 
 unburned gas escapes freely when the pressure is restored. 
 Still another source of danger exists when the gas cock 
 used to turn off the gas works too easily in its socket, and 
 so is capable of being turned on by slight jars, touches, 
 etc. The student is referred to Chapter XXXII for a full 
 
WAKMING AND LIGHTING OF HOUSES 441 
 
 description of the dangers of inhaling unburned illuminating 
 gas. Illuminating gas is also explosive when mixed with 
 air in certain proportions. 
 
 Electric lighting is in many respects an ideal method, 
 giving a convenient, steady, and powerful light ; but, as is 
 stated in the next paragraph, care must be exercised that 
 such light is not too bright. 
 
 10. The Best Light. — Probably there is no one kind of 
 light which is best for all purposes. For general illumi- 
 nation of public squares and public buildings the electric 
 light seems to be generally preferred. The same thing 
 is probably true of private houses. For reading and for 
 microscopic work, on the other liand, the electric light 
 may easily be too bright ; but this objection can be over- 
 come by using lamps of proper candle power, by having 
 the lamp at a suitable distance, or by using bulbs with 
 ground glass. The same thing may be true of the light 
 yielded by any incandescent solid, such as tlie " lime " 
 (oxyhydrogen) light and the various " mantles " made 
 from incombustible earths, such as that in the Welsbach 
 light. In general, for reading a " soft " light is best, and 
 it is desirable to have the larger part of the light come to 
 the book by reflection from the walls of the room rather 
 than solely and directly from any source of light near by. 
 For this reason, dark-colored walls are objectionable for 
 rooms in which a number of people do much reading, 
 sewing, or other near work. 
 
CHAPTER XXVIII 
 THE AIR SUPPLY OF THE HOUSE. VENTILATION 
 
 Besides the relatively permanent furnishings and fixtures 
 of the house there are other necessaries of civilized domestic 
 life, such as air, water, oil, gas, coal, and provisions, which 
 come into the house only to be consumed, their waste ma- 
 terials being cast out again. These are commonly called 
 the domestic sujrplies, — air supply, water supply, gas sup- 
 pl)^ etc., — and they are usually derived from much larger 
 public supjdies which are used in common by many fami- 
 lies. All such public supplies, although convenient, may, 
 under certain circumstances, become dangerous to human 
 life and health. 
 
 1. The Air Supply of the house is probably more neg- 
 lected than any other. Water, gas, coal, and provisions are 
 costly and often difficult to get, but air is always abundant 
 and cheap. The familiar saying, " as free as air," applies 
 best, however, to outdoor air; for, as we shall see, good 
 air in houses is not always very abundant, nor always cheap 
 and easy to provide. 
 
 Inasmuch as the adult human body requires for its 
 regular uses about five hundred cubic inches of air per 
 minute, the air in the immediate vicinity of the nose is 
 quickly used up ; and as an equal amount of exhausted 
 or vitiated air is discharged per minute at the same place, 
 tlie need is obvious of a constant streaming of air about the 
 body which shall remove vitiated air and supply fresh air. 
 This circulation or flow of air is just as necessary as is the 
 circulation of the blood ; but, as the movement always 
 
 442 
 
VENTILATION 443 
 
 goes on unseen, through the diffusion of gases and by 
 other natural and invisible agencies, it is harder to realize 
 the need. Out of doors the air supply is ordinarily sufli- 
 cient and of good quality, especially while the body is 
 in motion, in walking, driving, riding, wheeling, skating, 
 sailing, rowing, or in doing much of the manual work on 
 farms, forests, ships, in fishing, etc. 
 
 2. Stagnant Air. — Indoors, conditions are very different. 
 Life is more sedentary, the body is more quiet, and nat- 
 ural wind currents or drafts are intentionally j^revented ; 
 the air of houses tends to become stationary and even 
 stagnant, and with it the air supply about the bodies of 
 the house dwellers. Since it is this stagnant air which is 
 steadily exhausted or vitiated by the air discharged from 
 the nose and mouth, a blanket of increasingly stagnant 
 and impure air tends to accumulate about the body of a 
 sitting or sleeping person. To prevent this stagnation, 
 and the consequent impurity of the air supply, movement 
 of the body or, better, movement of the air is a prime 
 necessity. 
 
 3. Ventilation (Latin, ventns, wind) is the name usually 
 given to any circulation or movement of the air of rooms 
 or buildings by which fresh, pure air, preferably from out- 
 doors, is introduced and vitiated air is removed, the move- 
 ment of the air being rapid enough to meet all the needs 
 of the body, but not so rapid as to cause dangerous cur- 
 rents or drafts. This movement or circulation may be 
 either intermittent and occasional, as when a window is 
 opened for a little while and then shut, or more or less 
 regular and constant, as in all efficient " systems " of ven- 
 tilation or even in such primitive methods as that of the 
 chimney above an open fire. 
 
 4. Natural Ventilation. — The walls of most houses are 
 more or less porous and permeable for gases. Cracks and 
 crevices around doors and windows also allow gases to 
 
444 THE HUMAN MECHANISM 
 
 leave and enter. In an experiment made by one of the 
 authors, four ordinary gas jets in a small room were left 
 open (but not lighted) all night, and after the gas had 
 poured in for eight hours it was found that the room con- 
 tained only three per cent of gas, the remainder having 
 escaped by natural ventilation. It is largely because of the 
 cracks, crevices, and pores in the walls that human beings 
 get on as well as they do in rooms and houses seemingly 
 wholly unventilated. Wood, brick, stone, and plaster are 
 more porous than glass, iron, or glazed brick, and dry walls 
 more porous than those wet or damp. Painted and papered 
 walls are less porous than those left bare, and accordingly 
 the walls of summer houses are often loosely made, prefer- 
 ably of "natural" woods. 
 
 5. What do we mean by Air Good or Bad, Pure or 
 Impure ? — Air is not a chemical compound of fixed com- 
 position, but a mixture of gases containing, even when 
 pure, varying amounts of nitrogen, oxygen, carbonic acid, 
 ammonia, and water, — this last in the form of invisible 
 (aqueous) vapor. Moreover, the density of the air varies 
 not only at different places, but also at the same place at 
 different times. Impure air contains all these gases, and 
 may contain in addition any other gas capable of mixing 
 with them, such as hydrogen sulphide, carbonic oxide, 
 marsh gas, etc. The terms " good " and " bad " air, 
 "moist," "fine," "dry," "bracing," "muggy," "humid," 
 "heavy," " fouh" "fetid," "stagnant," "dead," "thick," 
 or "lifeless" air, and all similar terms, are popular de- 
 scriptions of atmospheric conditions real or imaginary, 
 testifying to the wonderful variety of this part of the 
 environment of mankind. 
 
 We may define " pure " air as any portion of the atmos- 
 phere free from noxious gases or vapors and from infec- 
 tious microorganisms. Such air may, however, be unfit 
 for breathing, as is the case with those higher portions 
 
VENTILATION 445 
 
 of our atmospheric ocean into which aeronauts have some- 
 times ventured. At the height of three miles above the 
 sea the air no doubt is very " pure," but yet too thin to 
 support Iiuman life readil}'. 
 
 Air may be considered as polluted or " impure " when 
 it contains noxious gases, or floating particles in large 
 numbers (as in smoke), or disease-producing germs or 
 microorganisms (perhaps derived from dust). 
 
 6. The Sources of Discomfort and Danger in Air. — We 
 must be careful to discriminate between discomfort and 
 danger in atmospheric conditions. Positive danger comes 
 chiefly from deficiency of oxygen, excess of carbonic acid, 
 admixture of poisonous gases, or from infectious micro- 
 organisms. Aeronauts, explorers on high mountains, and 
 persons living at great altitudes are apt to suffer from 
 oxygen deficiency. Miners, cliarcoal burners, and well 
 cleaners sometimes suffer from carbonic acid excess. 
 Laborers in gas works and consumers of illuminating gas 
 may be poisoned by carbon monoxide ; and workers in 
 sewers, by various gases, especially I)}- illuminating gas 
 wliich may have leaked in. Air may also contain, and 
 thus convey, germs of infectious diseases such as smallpox, 
 scarlet fever, measles, etc. 
 
 On the other hand, air that is not dangerous, and even 
 perfectly "pure " air, may be a source of great discomfort, 
 simply because of its temperature or moisture, or its tem- 
 perature and moisture taken together. The air in the 
 " dog days '" of August is no less pure than that of June 
 or October ; yet it is often oppressive because it is both 
 too warm and too moist. It has been shown in Chapter XII 
 how greatly the regulation of the temperature of the body 
 depends upon the capacity of tlie atmosphere to take up 
 moisture, and it is plain that any atmosphere saturated or 
 nearly saturated with aqueous vapor must seriously inter- 
 fere with the cooling of the body. A careful review of that 
 
446 THE HUMAN MECHANISM 
 
 chapter will greatly help the student to an understanding 
 of the sources of discomfort in the atmosphere of houses 
 or rooms. 
 
 A shut and uninhahited room often becomes " musty " 
 or " damp," because of a want of circulation to remove air 
 containing traces of odoriferous gases and excess of mois- 
 ture ; the former perhaps derived from carpets or furniture, 
 the latter from the basement or cellar. 
 
 The air of an inhabited room may prove a source of dis- 
 comfort to its inmates, and therefore deserve to be called 
 had for any or all of the following reasons: (1) the air 
 may be overheated or underheated; (2) it may contain an 
 excess of moisture due either to its dampness of location 
 or to the breath of its inmates ; (3) it may be both over- 
 heated and overmoist ; (4) it may contain odoriferous gases 
 which cause displeasure or discomfort. 
 
 What such rooms do not often suffer from is oxygen defi- 
 ciency or carbonic acid excess; for experiments have proved 
 that unless the oxygen falls below twelve per cent, or the 
 carbonic acid rises above three per cent (conditions which 
 are very rarely met with in ordinary human habitations), 
 no marked discomfort ensues. 
 
 7. Ventilation replaces Bad Air with Good Air and causes 
 
 Aerial Movement or Circulation It is now easy to see 
 
 precisely how ventilation aids us in securing comfortable 
 and agreeable atmospheric conditions. It removes bad air 
 and supplies good (that is fresh) air and, by causing move- 
 ment, favors evaporation from the skin and consequent 
 cooling on muggy days. It is also easy to see why ventila- 
 tion is at times ineffective. No system of ventilation can 
 wholly overcome the " mugginess " of a " close " room in 
 August, because the pure outer air is itself unpleasant 
 and uncomfortable ; but active A'entilation, by producing 
 a breeze, can do more than anything else to make the con- 
 ditions tolerable (see Part I, p. 203). 
 
VENTILATION 447 
 
 8. Fans and Fanning. — It is an old point of dispute 
 whether or not a person who " fans " himself grows cooler 
 or warmer. However this may be, there can be no question 
 that persons who use fans feel cooler, and there is no doubt 
 that any one fanned by another or by a breeze not only 
 feels, but actually is, cooled thereby. The great and grow- 
 ing use of electric fans in hotels, houses, etc., testifies to 
 the same fact. 
 
 9. A Room may be well Ventilated but Oppressive from 
 Overheating. — This fact, though perfectly obvious, and fa- 
 miliar to all who have been in well-ventilated boiler rooms, 
 or who have lived in the tropics, is too little attended to. 
 Many public halls, Pullman and other railway cars, steam- 
 boats, and private houses, especially in the northern United 
 States, are rendered almost intolerable and very unhygienic 
 by simple overheating. Elderly people and infants require 
 higher room temperatures than do active persons in jouth 
 and middle life, but in general any temperature above TO^F. 
 must be regarded as excessive, and GS'^F. to GS'^F. is a better 
 temperature (pp. 201, 390). Housekeepers, at least in the 
 northern United States, would do well to try to keej) the 
 mercury in their houses between these lo^^■er limits. When 
 the outdoor air is moist, as iir rainy weather, a somewhat 
 higher temperature is often required than when it is dry. 
 
 10. A Room may be Comfortable in Temperature but 
 Defective in Ventilation. — This fact is less obvious than 
 that just considered, but nevertheless true. It may be 
 because of excessive moisture, or because of odors, or for 
 other reasons ; but those entering such rooms from out of 
 doors are often able to remark and deplore the fact. It is 
 perhaps oftenest exemplified in warm countries or in warm 
 seasons when people close and darken rooms to " keep out 
 the heat." Such closing and darkening interferes with a 
 good circulation of air and thus hinders ventilation. It 
 also favors dampness by excluding sunshine. 
 
448 THE HUMAN MECHANISM 
 
 11. Some Practical Hints about the Ventilation of Rooms. 
 
 — We have already referred to the great value of chimneys 
 and open fireplaces as ventilators, and to the "natural" 
 ventilation by porous walls, and by cracks above and below 
 doors and windows. The simplest and most usual artificial 
 method of ventilation is the opening more or less widely 
 of windows or doors, and this is a means which should 
 never be disregarded. The great drawback associated with 
 it is the fact that uncomfortable and frequently unwhole- 
 some drafts are likely to ensue. It should be remem- 
 bered, however, that the existence of a decided draft is 
 usually an indication that the amount of ventilation is 
 greater than is necessary. When the wind is blowing 
 against a window it is enough to open this an inch or 
 less ; if the wind is blowing very hard, the natural ventila- 
 tion may be sufficient. Moreover, the amount of natural 
 ventilation secured depends quite as much on the ease of 
 egress as of ingress of air. It often happens that if the 
 window be closed or very slightly opened on the wind- 
 ward side of the house, enough natural ventilation will 
 be secured by opening other windows on the side away 
 from the wind. 
 
 It is often possible, especially in warm or temperate 
 weather, to secure satisfactory ventilation by opening win- 
 dows both at the top and the bottom, the warm air passing 
 out above, while cooler, fresh air comes in below. This, 
 however, is not advisable when the temperature of the 
 incoming air is too low, since the air then sinks at once 
 to the floor and chills the feet. Another good plan is to 
 raise the lower sash three or four inches and place under 
 it a board made to fit the space. Air now enters between 
 the sashes and, the air being directed upward, the occu- 
 pants of the room are protected from drafts. Where elec- 
 tricity is available, an electric fan placed in one of the 
 upper sashes is frequently very effective in hastening the 
 
VENTILATION 449 
 
 i-emoval of vitiated air. Fans for this purpose are now read- 
 ily obtainable, and have often proved to be serviceable. 
 
 In winter, fresh air and good ventilation cost something, 
 as the air must be heated ; but it is poor economy to use 
 stagnant air for the sake of saving fuel. The keen edge 
 of capacity for good work is dulled by bad air, the vital 
 resistance is lowered, and the susceptibility to disease in- 
 creased. On the other hand, there is such a thing as too 
 much ventilation ; for if it causes dangerous drafts or 
 leads to actual chilling of the body, it may do almost as 
 much harm as too little ventilation. Here again each in- 
 dividual must study and determine his own needs. 
 
 The hot-air furnace is capaljle of supplying fresh air in 
 abundance and, if the air be not overheated or overdried, 
 gives an admirable method of heating and ventilation, 
 combined in a single device, — the jacketed stove, — pro- 
 vided always that the air supplied to its heating chamljer 
 is unobjectionable. 
 
 12. Mechanical Systems of Ventilation. — Buildings 
 larger than dwelling houses, such as large schoolliouses or 
 public halls, are, or should be, ventilated by some meclian- 
 ical system. These are of two piincipal types, known as 
 the " vacuum " and the " jilenum " systems. In the former 
 an attempt is made to effect good ventilation by sucking- 
 out the air from the building by an exhaust fan or blower 
 attached to one main pipe or duct, to which are led tribu- 
 tary ducts connected with the various rooms. To supply 
 the air thus removed, fresh air is supposed to make its 
 way in, either by " natural " ventilation (p. 443) or through 
 inlet ducts specially provided, in either case being pressed 
 in by the outer atmospheric pressure. In the plenum 
 system this arrangement is reversed, and air, previously 
 warmed if need be, is driven by a fan into a main duct 
 or space, from which smaller ducts carry it to the various 
 rooms, circulation being favored by outlet ducts through 
 
450 THE HUMAN MECHANISM 
 
 which the air flows away. Sometimes an effective combi- 
 nation of the two systems is used, in which case the air is 
 not only pumped in but also at the same' time sucked out. 
 
 In favor of the plenum system it may be said that, 
 instead of currents of (often) cold air pressing in by the 
 paths of natural ventilation, about doors, windows, etc., 
 the direction of the aerial current at these places is re- 
 versed, owing to the pressure to which the air is subjected, 
 so that it is more often possible to sit very near such win- 
 dows and doors. 
 
 The combination of the two sj^stems offers many prac- 
 tical advantages, but is obviously relatively costly. It is 
 sometimes forgotten that air, quite as truly as water, pos- 
 sesses inertia and moves along paths of least resistance ; 
 but experience has shown that in order to govern the 
 direction of flow of the aerial stream it is often necessary, 
 as well as advisable, to control the outgo as well as the 
 income of air. 
 
CHAPTER XXIX 
 
 THE WATER SUPPLY, PLUMBING, AND DRAINAGE OF THE 
 HOUSE. GARBAGE AND RUBBISH 
 
 1. Water Supply. — The water supply of tlie house 
 should be first pure, and second, alnnidant. No exact 
 figures can be given as to the amount required, but for 
 kitchen and laundry use, bathing, and good drainage, it 
 is safe to say tliat thirty gallons per day per capita are 
 ample. An ordinary barrel holds this amount. Most fam- 
 ilies get on with very much less; but for the greatest 
 convenience and cleanliness some such quantity, if not ab- 
 solutely needed, can be used to advantage, and no domes- 
 tic supply is a greater luxury than abundant water. 
 
 The furity of the domestic water supply should be 
 above suspicion. In a following chapter we shall empha- 
 size (Chapter XXXII) the requirements as to purity of a 
 proper public water supply from which the domestic sup- 
 ply may be drawn ; but we may here consider briefly those 
 private supplies, such as wells, springs, and brooks, from 
 which many houses, especially in the country, must obtain 
 their Avater. It is worth remembering that all water in 
 or upon the earth was originally rain water (i.e. distilled 
 water from the atmosphere). This, when it flows over the 
 surface of the earth, or percolates through the ground, is 
 known later as surface water, or ground ivater. Streams, 
 such as brooks, creeks, and rivers, are composed largely, 
 but by no means wholly, of surface water; deep wells, 
 dug or driven, and many springs, contain a mixture of sur- 
 face and ground waters. Surface waters are particularly 
 
 451 
 
452 THE HUMAN MECHANISM 
 
 exposed to pollution by dirt and filth from roads, manured 
 fields, and the surface of the ground generally. Ground 
 waters, on the other hand, although subject to pollution 
 by percolating through buried filth, and by surface waters 
 mingling with them through cracks or fissures in the earth, 
 are, in general, subject to great purification hy filtration 
 during their percolation through earth, which often acts as 
 a porous filter (Fig. 116). 
 
 2. Domestic Wells. — The well of water has an ancient 
 reputation and has long been celebrated in song and story. 
 As a supply more or less public it has often served as a 
 meeting place and a social center, and has frequently been 
 ornamented with decorated curbs or covers testifying to 
 popular esteem. Until 1854 the common well was, with 
 rare exceptions, regarded as a perfectly safe and satisfac- 
 tory method of securing water for public as well as pri- 
 vate water supplies ; but in that year all wells began to 
 be regarded with suspicion, because of a disastrous out- 
 break of Asiatic cholera in l^ondon, which was conclusively 
 traced to a polluted public well on Broad Street in that 
 city. It was found on investigation that a jirivy vault, 
 probably infected by the discharges of a cholera patient, 
 had leaked directly into the well ; and immediately all 
 wells, especially tliose near an}^ source of pollution, fell 
 under suspicion. 
 
 The truth, in brief, appears to be that many wells 
 are absolutely innocent of all contamination and yield 
 excellent water. Some wells undoubtedly contain water 
 originally impure, but coming from a long distance through 
 the soil before reaching the well and thoroughly purified 
 by filtration. Others, however, are in more direct con- 
 nection with cesspools, privy vaults, barnyards, stables, 
 or similar objectionable and perhaps dangerous sources, 
 and are utterly unfit to serve as water supplies for domes- 
 tic uses. Still others, though receiving good water from 
 
WATER SUPPLY AND DEAINAGE 
 
 453 
 
 the earth about them, yield bad water because objection- 
 able matters find their way in at the top. Poultry should 
 be prevented from walking over loose planking which only 
 partially covers the well, and farmers whose boots have 
 
 I ~ J 
 
 Fig. 116. A domestic well badly situated in a farmyard 
 
 Observe that the water which it yields is partly dr:iiiia;;e from the barnyard, 
 privy, pigpen, etc. It is also exposed to pollution at the tO}) 
 
 become fouled by walking in barnyards, or on fields heav- 
 ily manured with stable manure, should be careful to avoid 
 doins: likewise. The danger of even worse contamination 
 of wells from manure or other surface dirt washed in at 
 the top during heavy rains is also very great (Figs. 116 
 and 117). 
 
 3. Springs are usually sources of pure water, but a spring 
 in a barnyard or a cemetery would be plainly objection- 
 able, and care should always be taken to ask whence comes 
 the water which the spring yields. Springs often occur on 
 hillsides ; in such cases they should be protected from the 
 possibility of surface pollution, while sources of pollution 
 of any sort higher up the hill should not be tolerated, 
 since from these the spring may become contaminated. 
 
454 THE HUMAN MECHANISM 
 
 4. Cisterns of Rain Water are often used for domestic 
 supply in country houses and in some places, such as 
 New Orleans and the Bermuda Islands, where wells are 
 not available. There is no objection to this practice, which 
 should secure a pure and very soft water, provided the 
 roofs, cisterns, reservoirs, and other receptacles employed 
 in collection or storage of the water are clean and suitable. 
 Painted roofs, and pipes, roofs, or reservoirs containing 
 any exposed metallic copper or lead should be avoided, 
 since rain water may attack these metals, forming with 
 them soluble and poisonous salts. 
 
 After a long dry period in summer, roofs are often dusty 
 and dirty, and the first washings of dirty roofs should be 
 allowed to go to waste. Rain water collected in winter 
 or spring, and subjected to long storage, is probably the 
 purest and most desirable cistern water. 
 
 5. Streams, such as brooks and creeks, are sometimes 
 used as sources of private domestic water supply, and if 
 the places which they drain are wooded and entirely unin- 
 habited, i.e. not manui'ed in tillage or pasturage, the sur- 
 face water which they j'ield may answer fairly well for 
 house use. But, even at its best, such water is exposed 
 to pollution by wild animals and by passing tramps, fisher- 
 men, or gunners, and a carefully protected well or spring 
 is, as a rule, a safer supply. Well water is often more 
 palatable, especially in summer, but is sometimes very 
 hard. For washing, surface or rain water is generally 
 softer and of course unobjectionable. 
 
 6. Hard Waters and Soft. — Rain water contains few or 
 no salts in solution, and is therefore called "soft" water. 
 Many surface waters and some well and spring waters 
 are also soft. All such waters readily dissolve soap, and 
 because soapy waters are sticky and easily form air bubbles, 
 a " lather " or " soapsuds " is easily made in soft waters 
 with a very little soap. 
 
WATER SUPPLY AND DRAINAGE 455 
 
 Other waters, and especially ground waters, contain 
 salts in solution, some of which, notably those of calcium 
 and magnesium, form compounds and even precipitates 
 with soap, thus removing it from the water in which it 
 is placed. Such waters, therefore, require more soap to 
 make them soapy, lathery, or sudsy, and are known as 
 " hard " waters, because they feel less bland, or soft, to the 
 skin. 
 
 In some parts of the United States the well waters (and 
 sometimes even the surface waters) are so hard as to be 
 almost or quite useless for washing, and even for drinking. 
 It has never been shown that moderately hard waters are 
 necessarily any more harmful for drinking than soft waters. 
 Persons used to either kind are apt to suffer temporary dis- 
 turbances, such as diari-hea, when they change suddenly 
 from one to the other ; but otherwise no great or perma- 
 nent harm ordinarily happens. If, however, a drinking 
 water is very hard and heavily charged with mineral salts 
 so that it becomes essentially a mineral water, it may be 
 unht for regular use. 
 
 7. House Filters for water are not needed if the water 
 supply is pure and colorless, but in many places this is 
 not the case. If the water supply is impure, it should 
 either be carefully filtered by a germ-proof filter (several 
 kinds of which are on the market, but all of which are 
 costly), or else boiled for a few minutes and cooled before 
 it is used for drinking. If the water is pure but colored 
 or turbid, it may be made bright and attractive by filtering 
 throug-h a charcoal filter ; but this also, if durable and 
 effective, is sometimes costly. 
 
 8. The Ice Supply of the house is one of the greatest 
 of modern conveniences. Ice in summer was formerly a 
 luxury, but in northern latitudes ice is now generally har- 
 vested in winter and stored for the following summer. In 
 warmer climates the so-called artificial or manufactured ice 
 
456 THE HUMAN MECHANISM 
 
 brings the same luxury within reach of persons of mod- 
 erate means. Provided the water from which it is made 
 is pure, manufactured ice is as wholesome as the best 
 natural ice. The economical value of ice in preserving 
 foods is very great, as is also its sanitary importance in 
 hindering harmful decomposition and decay, for example, 
 in milk. 
 
 Ice water, so generally used as a beverage in America, 
 is probably harmless enough when not drunk in too large 
 quantities or too rapidly ; although, as a matter of fact, 
 thirst is normally slaked by cool water more effectively 
 than by very cold water. The ice added to drinking water 
 should be pure ; that is, ice obtained from ponds, streams, 
 or other waters unfit to serve as sources of domestic water 
 supply, should never be used in water intended for drink- 
 ing purposes, and all ice should be carefully washed before 
 being so used. 
 
 9. The Plumbing of the House. — Almost all houses 
 have a sink of some sort; from this there runs a drain- 
 pipe, which should be tight, and large enough to carry off 
 readily the drainage from the sink. Many houses have in 
 addition more or less complex systems of water supply 
 and drainage, requiring piping and plumbing. 
 
 The plumbing for water calls for brief comment only. 
 Lead service pipes should, as a rule, be avoided, for expe- 
 rience has shown that if the water passing through lead 
 pipes happens to contain an excess of free carbonic acid 
 (CO2), this may attack the lead and form with it a very 
 soluble bicarbonate which is a dangerous poison. In Massa- 
 chusetts there have been several epidemics of lead poison- 
 ing due to this cause. 
 
 Service pipes, i.e. pipes leading from street mains into 
 houses, may often be made of lead without any bad con- 
 sequences, and are usually preferred by plumbers because 
 they are more easily worked; but it is much safer to use 
 
WATEE SUPPLY A]ND DRAINAGE 457 
 
 iron for all water pipes, although iron pipes are less con- 
 venient to install and sometiroes get badly clogged with 
 rust. 
 
 The plumbing for ch-ainage should aim to provide against 
 escape or leakage of both liquids and gases. As drain- 
 pipes are not usually filled with liquids or gases under 
 pressure, leaky joints and even small holes may, and often 
 do, occur without detection. If under such circumstances 
 any stoppage happens, pressure may arise and the liquid 
 or gaseous contents escape. It was formerly believed that 
 great danger existed in defective plumbing, owing to the 
 escape of sewer gas or gases by leakage, and particularly 
 from the pressing backward, or " rising," of sewer gas into 
 bath rooms, or sleeping rooms piovided with set bowls, etc. 
 The present view is that while such gases may, and prob- 
 ably sometimes do, escape into houses, they are usually 
 greatly diluted before they are breathed and, at the worst, 
 are much less harmful than was formerly supposed. They 
 are, nevertheless, highly objectionable, and it is likely that 
 they occasionally produce serious poisoning. If breathed 
 for a long time, even in small amounts, they probably 
 lower vital resistance and increase susceptibility to infec- 
 tious disease, and are thus not merely objectionable but 
 also dangerous. 
 
 Pains should be taken to ventilate thoroughly all places, 
 such as sleeping rooms, bath rooms, and water-closets, into 
 which sewer gases may find their way, and it is advisable 
 and customary to seal up the various drainpipes by water 
 seals, or trwps. If, in addition, the main drainpipes are pro- 
 vided with vents to allow the escape of any gases accu- 
 mulated in the pipes, the essentials of sanitary plumbing 
 have been secured. Good workmanship is, however, indis- 
 pensable in all water and drain pipes, as well as in all 
 gas pipes, in the house, to prevent serious damage from 
 breaks or leaks. 
 
458 THE HUMAN MECHAI^ISM 
 
 The main drainpipe in tlie house is called the soil pipe. 
 This usually empties into an underground drain or sewer 
 outside the house, which discharges its contents, — now 
 known as setvage, — into a cesspool, or a stream, or upon a 
 sand bed, a sewage filter, a cultivated field, or some other 
 place of sewage disposal. 
 
 10. Drainage and the Disposal of Household Wastes. — 
 The consumption of the solid and liquid supjjlies of the 
 house — water, ice, coal, food, etc. — is accompanied by the 
 formation of various wastes which for sanitary as well as 
 sesthetic reasons must be promptly got rid of. Waste water 
 and melted ice necessitate drainage ; the dust and ashes of 
 fuel remain to be disposed of, and from food, putrescible 
 remnants, known as garbage. Dirt, bottles, papers, boxes, 
 tin cans, old clothes, worn-out mattresses, broken furniture, 
 crockery, and glass must also be removed. Among all the 
 wastes of the house, however, the discharges of human 
 bodies are of the first importance, not only because of their 
 putrescible and disagreeable character, but also because 
 they frequently contain the germs of dangerous diseases. 
 
 Drainage is often necessary for a house merely to carry 
 oii rain water from the roofs, and to keep the cellar dry. 
 It is very important to remove all surplus water from the 
 house and its vicinity in order to prevent dampness, — this 
 being one of the most unfavorable conditions in the envi- 
 ronment of mankind. 
 
 If the drains of houses or lands carry water only, they 
 keep the name of drains, and the water in them is called 
 drainage; but if such drains carry household wastes, and 
 especially human or animal excreta, they are more often 
 called sewers, their contents being known as sewage. The 
 process or act of removing sewage from a house or a city, 
 and the systems of sewers, are both known as .sewerage, 
 although this same term is sometimes popularly applied 
 to the sewage itself. 
 
WATER SUPPLY AND DEAINAGE 459 
 
 11. The Disposal of Drainage and Sewage. — Cellar drains, 
 and drains for the removal of roof water, usually discharge, 
 especially in the country, upon the surface of the ground 
 at some distance from the house, and give little trouble ; 
 but si7ik drains, since they contain dish washings, soap- 
 suds, and the liquid wastes of the kitchen, are apt to become 
 choked with grease. Grease is dissolved by alkalies, and 
 common lye (potash), allowed to dissolve and flow down 
 the sink waste pipe, will often remove greasy obstructions 
 and give at least temporary relief. The final disposal of 
 sink water, however, is more difficult, and a greasy, slimy, 
 malodorous, and unsightly channel or area behind a country 
 house too often tells of trouble. The only complete remedy 
 is a large waste pipe, as straight as possible, going to an 
 equally large or larger (underground) drain, which ends 
 in a covered pit or tank placed in porous or gravelly soil. 
 This pit must be cleaned out from time to time, and if 
 no open porous soil is available, a tight tank or pit must 
 be used and frequently emptied. 
 
 Sexvage disposal is a more difficult matter, for sewage 
 contains not only the sink wastes just mentioned but also 
 washings from the human body, human excreta, and other 
 putrescible matters, all in comparatively large volume. We 
 shall discuss beyond the problem of the disposal of the 
 mixed sewage of numerous houses combined into commu- 
 nities (p. 519), and therefore at this point need consider 
 only the disposal of the sewage of separate houses, such as 
 country homes or farmhouses. If these are so placed as 
 to be readily drained or sewered into the sea, or into some 
 large lake or stream nowhere used for drinking purposes, 
 the solution is simple. If not, the cesspool or disposal 
 upon land, as described in the next following paragraphs, 
 are among the best expedients. 
 
 12. The Cesspool is a receptacle or tank in the earth, at 
 some distance from the house, — not less than one hundred 
 
460 
 
 THE HUMAN MECHANISM 
 
 feet away, and the farther off the better, — into which 
 sewage is conveyed by a drain or sewer directly connected 
 with the soil pipe of the house. The cesspool may be 
 either water-tight to prevent leakage, or loosely built to 
 favor it. A common construction is one in which the cess- 
 pool is water-tight and has an outlet pipe just below the 
 surface. This outlet pipe may run into a drain, loosely laid 
 to facilitate leakage from its joints and thereby the escape 
 of liquid sewage into the earth. 
 
 Fig. 117. Disposal of household sewage by means of a cesspool 
 Observe that the loosely laid drainpipe, or sewer, allows some sewage to leach 
 
 away before reaching the cesspool, 
 distinct advantage 
 
 If no well were near, this would be a 
 
 It was formerly held that the cesspool was a sanitary 
 abomination ; that it favored putrefaction and the develop- 
 ment of sewer gas ; and that it ought always to be avoided 
 if possible. Experience has shown, however, that thou- 
 sands of houses have been drained into cesspools, not only 
 conveniently but without the slightest discoverable sani- 
 tary injury ; and sanitary science now recognizes in the 
 septic tank (a special form of cesspool) a useful and popu- 
 lar means of sewage disposal (Fig. 117). 
 
WATER SUPPLY AND DEAINAGE 461 
 
 13. Irrigatioa and Subsurface Disposal of Sewage. — 
 
 Another method often successful, esi^ecially in soil which 
 is open and porous, i.e. in sandy soil, is that of simply 
 discharging the sewage upon the surface of land set apart 
 for the purpose at a distance from the house. This method 
 may be recommended in many cases, but is often less sat- 
 isfactory than the use of a well-regulated cesspool. A mod- 
 ification of it, in which the sewage is distributed tinder, 
 instead of upon, the surface by means of a system of 
 branching pipes loosely laid, is frequently preferable, even 
 in the same porous or sandy soil ; but neither of these 
 methods is to be recommended as compared with the cess- 
 pool, if the soil is impervious or clayey. 
 
 14. The Care and Disposal of Garbage is a matter of 
 much importance both to the housekeeper and to the com- 
 munity. Garbage consists chiefly of the more solid refuse 
 from the kitchen and, since it is composed of the remnants 
 of food, — bits of meat and fish, either cooked or raw, bones, 
 vegetables, fruit, and the like, — it is highl}- putrescible. On 
 the farm, garbage may be fed to swine, and in many towns 
 and cities it is collected by farmers and used to maintain 
 large (and often offensive) piggeries, in or near a city or 
 town. More rarely, garbage is fed to milch cows, the milk 
 from such cows being known as sivill milk ; but this use of 
 garbage is rightly forbidden by boards of health. 
 
 In the house, garbage is simply a nuisance, to be got rid 
 of as quickly and as completely as possible. It should be 
 either burned (in which case disagreeable odors are often 
 produced), or kept as short a time as possible in a cletm 
 receptacle in or near the kitchen. The garbage recepta- 
 cle is usually the dirtiest and foulest smelling household 
 utensil. Nothing about the house requires more careful 
 attention. The receptacle itself should be of metal rather 
 than wood ; it should be no larger than is necessary, because 
 a small can is easier than a large one to keep clean ; it 
 
462 THE HUMAN MECHANISM 
 
 should have a tight^fitting cover; and it should be kept 
 where dogs and cats cannot overturn or open it. Above 
 all, it should be frequently emptied and cleaned. 
 
 15. The Disposal of Ashes, Dirt, and Refuse is perhaps 
 as much a question of good taste as of sanitation. Nothing 
 is more unsightly than a dump, especially if its papers, 
 boxes, and other combustible materials are set on fire, as 
 often happens, and left to smolder. In towns and cities 
 ashes and rubbish (as well as garbage) are usually removed 
 periodically by public carts, but isolated householders must 
 ordinarily look after their refuse disposal themselves ; and 
 of all methods, burial in pits, when possible, is the least 
 objectionable. For garbage, especially, when a piggery is 
 undesirable and cremation not possible, burial in sandy 
 land at a distance from the house has much to recom- 
 mend it. 
 
PUBLIC HYGIENE AND SANITATION 
 
 CHAPTER XXX 
 
 PUBLIC HEALTH. INFECTIOUS AND CONTAGIOUS 
 DISEASES. MICROBES 
 
 1. The Public Health. — The environment of man con- 
 sists of two principal, and very different, parts : one near, 
 and chiefly personal, domestic, or privatf, including his 
 clothing, house, family, and estate ; the other more remote, 
 impersonal, and public, including his neighborhood, village, 
 town or city, state, and country (p. 299). 
 
 In sparsely settled districts little attention is paid to 
 any health beyond that of the person or the family ; and 
 the family, as we have sliown (p. 425), is really a small and 
 private community ; but in thickly settled regions, such 
 as cities and towns, conditions and problems arise involv- 
 ing numbers of families, and puhlic hygiene and sanitation 
 become necessities. In the country each family generally 
 has its own private water supply, milk supply, food sup- 
 ply, and drains; but in cities and towns mutual conven- 
 ience, economy, and safety recjuire public sup)plies and 
 public drains. Instead of private roads we find public 
 streets ; instead of private estates, public parks. Public 
 gardens and public markets furnish flowers and vegetables ; 
 public conveyances, such as cars, steamboats, and car- 
 riages, serve public needs ; public institutions arise, such 
 as hospitals, schools, almshouses, and jails ; and public 
 buildings, such as halls, hotels, churches, schoolhouses, 
 shops, factories, stores. 
 
 463 
 
464 THE HUMAN MECHANISM 
 
 In all such cases numbers, groups, or masses of individ- 
 ual families — called collectively the people, or the public — 
 are at times and as a whole exposed to unfavorable condi- 
 tions, such as a general want of muscular exercise, lack 
 of sleep, a too sedentary life, and overwork ; or to germs of 
 infectious and contagious diseases in public supplies of 
 water, milk, etc. ; or to foul air, overheating, defective 
 lighting, gas poisoning, noise, dust, smoke, or impure food ; 
 some of which conditions are chiefly personal, affecting 
 more or less directly the bodies of the people, while others 
 are more remote, or environmental. 
 
 By the public health we mean the health of the public, 
 i.e. of the people as a whole ; and the health of the pub- 
 lic depends — just as the health of the individuals who 
 compose that public depends — on a great variety of con- 
 ditions, some of which, as just stated, are chiefly internal, 
 or in intimate relation with the persons of the people, and 
 may conveniently be called hygienic; while others are 
 chiefly external, or at least not in intimate relation with 
 the persons of the people but rather in their environment, 
 and may be described as sanitary (pp. 301, 302). 
 
 The applications of the various branches of science, such 
 as physiology, chemistry, bacteriology, vital statistics, clima- 
 tology, medicine, engineering, etc., to the control of these 
 various hygienic and sanitary conditions, and thereby to 
 the protection and promotion of the public health, consti- 
 tute the science of jniblic health ; and of this, as indicated in 
 the last paragraph, there are two grand divisions, namely, 
 hygiene and sanitation. 
 
 2. Public Health Rules and Regulations. — For the regu- 
 lation and control of those conditions which are personal and 
 domestic we must look, even in large communities, chiefly 
 to individuals and families ; but even if individuals and 
 families always obeyed the laws of personal hygiene and 
 domestic sanitation, the protection of the public health would 
 
INFECTIOUS DISEASES. MICROBES 465 
 
 still require special supervision and control of public sup- 
 plies, public drains, public vehicles, public institutions, and 
 the like, because these things are outside and beyond the 
 control of private individuals or families and stand in a 
 class by themselves. In point of fact, however, private 
 persons and families are often negligent in matters of this 
 kind, inflicting damage upon their neighbors by maintain- 
 ing nuisances of one kind or another, or else by their care- 
 lessness in respect to filth or in respect to the spread of 
 infectious or contagious diseases. Hence it has come to 
 pass that sanitary and hygienic rules and regulations have 
 been adopted by the citizens of most civilized communi- 
 ties for mutual benefit. 
 
 3. Public Health Authorities. — For the enforcement of 
 these rules and regulations {sanitary laws) special public 
 oflicials are usually elected or appointed, such as boards 
 of health, health officers, city physicians, sanitary in- 
 spectors, medical inspectors, quarantine officers, school 
 nurses, sanitary police, vaccinating physicians, etc. By 
 common consent of the majority of the citizens these 
 officers are authorized and required under the laws to 
 prepare, publish, and enforce needful sanitary rules and 
 regulations for the protection and promotion of the public 
 health. 
 
 4. Public Health Problems. — In this and the following 
 chapters we can touch upon only a few of those more 
 elementary and important problems of the public health of 
 which every educated citizen should have some knowledge. 
 Such problems are almost all fundamentally concerned with 
 the control of infectious diseases, to the nature of which 
 we shall therefore at once turn our attention. 
 
 Much of what follows was formerly the exclusive pos- 
 session of the medical profession and has only recently 
 become a part of the common knowledge of mankind. 
 Much of it also is comparatively new, and among the best 
 
466 THE HUMAN MECHANISM 
 
 fruits of the splendid advances of the last half century in 
 the sciences of pathology, hygiene, and sanitation. 
 
 5. Plagues, Pestilences, and Epidemics are the most strik- 
 ing examples of influences aifecting both personal and 
 public health. Only wars, riots, and great conflagrations 
 are capable of throwing communities into such terror as 
 has often been caused by plagues or pestilences of some 
 swiftly fatal disease. Such was the plague in London 
 described by Defoe in his Journal of the Plague Year, 
 a story which has been well called " that truest of all fic- 
 tions." History is full of similar instances. Even as late 
 as 1892 the rich and powerful city of Hamburg, Germany, 
 was terroi'ized by a severe epidemic of Asiatic cholera due 
 to a polluted public water supply, while still more recently 
 the cities of Ithaca in New York and Butler in Pennsyl- 
 vania have been ravaged by the plague of typhoid fever. 
 
 Plagues and pestilences are simply older names for great 
 epidemics of much dreaded diseases, such as smallpox, jg\- 
 low fever, Asiatic cholera, or the bubonic plague, and the 
 pesthouse which formerly existed in many towns and cities 
 was a remote and isolated shelter, or primitive hospital, 
 often of the crudest and poorest kind, to which the vic- 
 tims of pestilence were taken (or driven) by a frightened 
 public. The true sources of epidemics, plagues, and pesti- 
 lences have only recently become known. Savages often 
 attribute these to supernatural causes, such as evil spirits 
 or demons, and even for civilized people pestilences were 
 until recently mysterious in origin and incomprehensible 
 in behavior. It is now known, however, that such out- 
 breaks are simply extensive epidemics of contagious or in- 
 fectious diseases, wliich may often be controlled and even 
 prevented ; but in order that control or prevention shall he 
 effective, the intelligent cooperation of all good citizens is 
 essential. It is one of our first duties to acquaint our- 
 selves with the nature and the methods of prevention of 
 
INFECTIOUS DISEASES. MICROBES 467 
 
 contagious and infectious diseases, and thus at the same 
 time of plagues, pestilences, and epidemics. 
 
 6. What are Infectious and Contagious Diseases? — The 
 
 discoveries of Pasteur, Koch, and their successors in 
 the last half of the nineteenth century have brought to 
 light the remarkable fact that those "fevers" — typhoid 
 fever, malarial fever (malaria), diphtheria, smallpox, cholera, 
 tuberculosis, etc., and probably also measles, chicken pox, 
 scarlet fever, and many " colds " — which attack apparently 
 healthy persons and cause a severe but brief sickness that 
 seems to run its course and then cease, are due to invasions 
 of the body by microjiarasites^ called microbes. Each of 
 these contagious or infectious diseases has its own special 
 microbe to which it owes its origin ; and it is customaiy^ 
 to speak of the microbes of diphtheria, of typhoid fever, 
 of the bubonic plague, of Asiatic cholei'a, etc., as tlie cause 
 of these diseases. Although in some contagious and infec- 
 tious diseases the microbe has not yet been discovererl, all 
 these diseases are nevertheless so nuich alike, and causa- 
 tive microbes have been found in so many cases, tliat all 
 are believed to have a similar inicrohic orii/iit. 
 
 The view or theory just outlined is known as the [p'rm 
 theoj-y of infectious and contagious diseases, and the causa- 
 tive microbes are known as disease </erms. It is easy, on 
 this theory, to see why these diseases are "catching." It is, 
 
 1 fflicroparasites. — A parasite is a plant or animal which feeds upon 
 another plant or animal (called its host), and renders it no services in 
 return. Some parasites, like fleas, lice, the i^ork vi'(jrm (Trichina) and 
 " ringworm," are visible or nearly visible to the naked eye ; but many 
 others are invisible and may be called nncreparasites. Of these the most 
 important belong among the microbes (see p. 4(18) ; but, as the microbes 
 form an enormous group of plants and animals, most of which are in no 
 way parasitic or harmful to mankind, but on the contrary are highly use- 
 ful, we must be careful not to regard as parasites more than a very few of 
 the microbes. Those that do not lead a parasitic life are usually scavengers 
 and lead a saprophytic life ; that is, they feed upon dead organic matters, 
 often helping greatly to clean and to keep clean the surface of the earth. 
 
468 THE HUMAN MECHANISM 
 
 of course, not the disease but the parasitic microbes which 
 can be " caught " or " taken," as fleas can be " taken " from 
 a dog, or bedbugs carried from place to place in bedding 
 or clothing, or lice " caught " by children from lousy play- 
 mates. It is easy, also, to understand how destructive epi- 
 demics, plagues, and pestilences can occur, if public food 
 supj)lies, water supplies, milk supplies, carriages, steamers, 
 caravans, cars, or other conveyances have become infected 
 with infectious microbes or disease germs. 
 
 An infectious disease is one in which the disease germs 
 infect (that is, invade) the body from without. Such are 
 diphtheria, typhoid fever, tuberculosis, trichinosis, scarlet 
 fever, smallpox, measles, chicken pox, and all the more 
 common " fevers." Among these some are ordinarily con- 
 veyed quite directly and quickly from person to person, 
 and to such infectious diseases the term " contagious " is 
 often applied. Formerly a sharp line was drawn between 
 infection and contagion, but to-day it is recognized that 
 no such line exists. Typhoid fever, for example, is still 
 sometimes said to be " infectious but not contagious." If 
 by this is meant that it is not as often spread through 
 the air or by mere " contact " as are smallpox and some 
 other diseases, then it is true. But if the saying means 
 that it cannot be transmitted by mere contact with the 
 patient or his excreta, then it is false (see p. 485). It would 
 be better to drop altogether the term " contagious," for its 
 use is antiquated and misleading. 
 
 7. Microbes. — Brief references have frequently been 
 made on previous pages to microbes and their work, but 
 we must now give them special consideration. 
 
 As the word implies, microbes {micros, small ; bios, life) 
 are little living things, and they have been described as 
 " all forms of life, whether animal or vegetable, invisible or 
 barely visible to the naked eye." It is customary to regard 
 them as the smallest of all living things, and sometimes 
 
INFECTIOUS DISEASES. MICROBES 
 
 469 
 
 as identical with microorganisms, germs, or bacteria. All 
 bacteria, however, are plants, so that a broader term, such as 
 "germs," " microorganisms," or " microbes," is required if the 
 lowest forms of animal life are also to be covered. In these 
 pages we shall use the term microbes for tJtose foi-ms of life, 
 either plant or animal, ivhich are invisible or barely visible 
 
 ■-V 1/ ,'l 
 
 N / 
 
 f^/ 
 
 ^'^Z I 
 
 V3\ 
 
 
 ^ 
 
 '0 0^ 
 
 oO 
 
 G> 
 
 Fig. 118. Microbes (rod-shaped bacteria, or bacilli). (All very 
 highly magnified) 
 
 1, bacilli, some very large and some very small; 2, other forms of I)aoilli ; 
 .3, bacilli forming threads or filameuts ; 4, dead or dyiug bacilli (invulu- 
 tion forms) 
 
 to the naked eye, and of interest or importance in physiology, 
 hygiene, and sanitation. 
 
 For our purposes microbes may be divided into bacteria, 
 or vegetable microbes, and protozoa, or animal microbes. 
 The bacteria are unicellular plants of the simplest struc- 
 ture and of three principal forms, viz. rods, berries or balls, 
 and spirals. The rods form the group bacilli (Fig. 118), the 
 balls the cocci (pronounced cock's eye) (Fig. 119), and the 
 spirals the spirilla (Fig. 120). Bacteria often grow and 
 multiply (by simple cell division) very rapidly, and some are 
 
4V0 
 
 THE HUMAN MECHANISM 
 
 capable of producing within themselves smaller cells, called 
 spores, which have thick walls and possess great powers 
 of resistance (see Fig. 121). 
 
 The protozoa are unicellular animals, also of the simplest 
 structure, and among them one group, the sporozoa, is of 
 
 mp 
 
 W 
 
 .sl 
 
 a /sP 
 
 Q 
 
 "XF" 
 
 0© 
 
 'i % 
 
 % 
 
 %9 
 
 oo 
 
 # 
 
 em 
 
 
 Fig. 119. Microbes (ball-shaped bacteria, or cocci). (All very 
 highly magnified) 
 
 1, cneei single, and cocci united in a jelly mass or zoijglaa; 2, in twos and 
 fours {diplococci and tetrads) ; S, in clusters (staphylococci) ; 4, in chains 
 or necklaces (streptococci) 
 
 especial interest because it certainly includes the microbes 
 of malaria, and probably also those of smallpox and of 
 scarlet fever (see Fig. 126). 
 
 Microbes are of interest and importance to the physiolo- 
 gist, hygienist, and sanitarian, first, because they are na- 
 ture's scavengers, i.e. removers of organic waste matters ; 
 second, because they are the ordinary agents of the decom- 
 position, putrefaction, and decay of foods and other valu- 
 able organic matter ; and third, and especially, because 
 among them are found many microparasites, and especially 
 those germs which cause infectious and contagious diseases 
 such as consumption, typhoid fever, diphtheria, and malaria. 
 
INFECTIOUS DISEASES. MICEOBES 
 
 471 
 
 1. Microbes as Scavengers. — Whenever the dead body of 
 a plant or animal, or any part of it, is left upon the ground, 
 or in water, or is buried in the earth, it soon crumbles 
 or decays and disappears, turning, as we say, to dust and 
 ashes. It was formerly universally believed that this 
 
 Fig. 120. Microbes (spiral or screw-shaped bacteria, or spiVii/a). (All very 
 highly magDified) 
 
 1, spirilla from the human mouth; 2, microbes of Asiatic cholera; .3, spirilla 
 of relapsing' fever; 4, large spirilla from ditch water 
 
 change was a slow combustion or oxidation caused by the 
 direct action of the oxygen of the atmosphere. It is now 
 known, however, that this oxidation is due to the influence 
 of microbes which abound in the upper layers of the earth, 
 and to a less extent in air and water. These oxidize the 
 waste organic matters to carbon dioxide, water, etc., much 
 
472 THE HUMAJSr MECHAISTISM 
 
 as the muscle fiber oxidizes the food brought to it by the 
 blood (see Chapter IV). 
 
 It is now established that scavenging is one of the prin- 
 cipal functions of microbes, for they abound in sewage, 
 which they readily decompose and, under favorable cir- 
 cumstances, completely purify ; in excrement, which they 
 work over and change to harmless, inoffensive, and even 
 useful mineral matters ; and in many organic wastes, which 
 they reduce to simple and harmless chemical compounds 
 (Figs. 120, 4, and 122, 4). 
 
 2. Microbes as Agents of Decomposition and Decay. — 
 The peculiar property which makes microbes destroyers of 
 waste organic matters, and therefore useful as scavengers, 
 makes them also troublesome, if not dangerous, agents of 
 the decomposition, decaj^, and consequent destruction of 
 useful organic substances, such as foods. Milk, for exam- 
 ple, may be spoiled by lactic acid microbes which feed 
 upon its sugar and, by producing lactic acid in the course 
 of their feeding, cause the milk to turn sour ; but, on the 
 other hand, this very change wrought by the microbes, 
 though dreaded by the milkman, may be desired by the 
 cheese-maker, in whose work the souring of the milk is 
 necessary. The spoiling of meat, fish, fruit, and many 
 other forms of food is due almost wholly to the vital activ- 
 ity of microbes, and we have to use cold and heat for 
 protection against their inroads. Cold, by chilling and 
 benumbing them, checks their growth and multiplication ; 
 while heat, if sufficiently intense, destroys them altogether. 
 Upon the former fact are based the important arts of 
 refrigeration and cold storage ; upon the latter the great 
 modern industry of canning. 
 
 Microbes have a wide and useful employment in the 
 arts and industries, — such as the souring of milk in cheese- 
 making, the flavoring of cheese and butter, the preparation 
 of hides for tanning, the ripening of manures, the fixation 
 
INFECTIOUS DISEASES. MICROBES 
 
 473 
 
 1 
 
 2 
 
 3 
 
 4 
 
 a 
 
 b c 
 
 o 
 
 d c 
 
 f 9 
 
 of free nitrogen in agriculture, and many other processes 
 depending upon their vital activity. But, on the other 
 hand, spoiled foods — especially meat, eggs, and fish — 
 may be not only disagree- 
 able but also dangerous, 
 owing to the formation by 
 microbes of poisonous 
 by-products known as jo^o- 
 maines, to whose agency 
 have been attributed severe 
 outbreaks of acute disease. 
 Ptomaines are bodies of un- 
 certain chemical composi- 
 tion, which cause intense 
 general prostration and 
 
 , , J . Fig. 121. Diagram of the growth, 
 
 sometimes deatn. It is a multiplication, and spore forma- 
 
 good rule to avoid carefully tion of a bacterial microbe 
 
 all meat or fish which is l-l, srowth ami multiplication liy cell 
 
 "tainted," or suspected of rtivisio„(fission) ; a-j, fonuation.lib- 
 
 '^ ^ eration, and germmation of a spore 
 
 putrefactive decomposition. 
 
 At the same time there is reason to believe that some out- 
 breaks of obscure poisoning have been charged to this 
 rather uncertain source largely because no other explana- 
 tion could be as easily given. 
 
 3. Microbes as Disease G-erms. — But it is as disease 
 germs that microbes are of the greatest hygienic impor- 
 tance. Long before the nineteenth century it had been 
 suspected that infectious and contagious diseases were 
 caused by invisible "germs " of some kind, but it was not 
 until the last half of that century that the responsibility 
 for some of the worst diseases that afflict the human race 
 was clearly and specifically fastened upon certain microbes. 
 For this great discovery we are indebted chiefly to Louis 
 Pasteur, a French mineralogist and biologist, and Robert 
 Koch, a German physician and bacteriologist. Thanks 
 
474 
 
 THE HUMAN MECHANISM 
 
 mainly to their genius and their patient labors, we now 
 know that many of the infectious and contagious diseases 
 of animals and plants are due to the entrance into the 
 living body of specific microbes which, growing in it or 
 upon it, poison it, and cause it to sicken or even to die. 
 
 But if infectious and contagious diseases are thus due 
 to the attacks of microbes coming from the environment, 
 
 «w 
 
 Fig. 122. Some motile bacterial microbes, showing flagella or cilia 
 (all very highly magnified) 
 
 1, 4, large water bacteria; 2, microbes of Asiatic cholera; 3, microbes 
 of typhoid fever 
 
 we may hope to prevent these diseases, either by warding 
 off the microbes or by making the body competent to re- 
 sist or overcome them, or both ; and it is one of the chief 
 le.tsons of sanitation and hi/(/iene to shoiu hoiv the zvarding off, 
 the resistance, and the overcoming, of infectious and conta- 
 gious microbes may be most effectively accomplished. 
 
 8. The Biology and Control of Microbes. — It must never 
 be forgotten that microbes are living cells, and as such 
 subject to the laws which govern all living things. As a 
 rule, they work best at about the blood heat. They feed 
 
INFECTIOUS DISEASES. MICEOBES 475 
 
 and grow and multiply by cell division. Like muscle fibers 
 and gland cells, they decompose their food materials and 
 produce secretions and by-products, some of which may 
 be harmless and some harmful. Microbes, or germs, are 
 killed by strong acids, strong alkalies, and various other 
 substances, which, for this reason, are called germicides or 
 disinfectants. Sterilization is complete removal of microbic 
 life. It may be effected by germicides, or by intense heat, 
 such as boiling or burning, or by various other means. 
 Pasteurization is incomplete sterilization by heat, at a 
 temperature sufficient to destroy most, but not all, germs. 
 Antiseptics inhibit, but do not necessarily destroy, microbic 
 activity. 
 
 9. The Prevention of Microbic Diseases. — For the occur- 
 rence of any infectious or contagious disease two conditions 
 must be fulfilled, viz. (a) a specific disease germ., micro- 
 parasite, or microbe, and (/<) a person susceptible to the 
 disease in question. 'Without the microbe the disease can- 
 not arise, no matter how favoraljle for it the condition of 
 the person may be ; and, on the other hand, tlie microbe 
 appears to be powerless to produce the disease, unless the 
 condition of the person is favorable for its reception, life, 
 and activity. This being tlie case, we obviously have at 
 our command two principal lines of defense against the 
 attacks of infectious and contagious disease ; on the one 
 hand we must seek to obtain {a) control of diseaseprroducing 
 microbes, and on the otlier to secure (b) insusceptihilify or 
 inmdnerability of the human mechanis)n to their attacks. 
 
 Boards of health are constantly seeking to destroy or 
 control dangerous microbes, by requiring the reporting of 
 all cases of infectious diseases, by isolation of such cases, 
 by the placarding of houses, bjf disinfection, by the in- 
 spection of food and other materials, and by quarantine. 
 Cities are purifying their water supplies, their sewage, and 
 their milk supplies with a view to warding off the attacks 
 
476 THE HUMAN MECHANISM 
 
 of disease germs. Individuals also, if wise, will take all 
 reasonable care to avoid exposure to infection by such 
 germs, and will endeavor, as far as is practicable, to secure 
 food and drink free from microparasites capable of causing 
 disease. 
 
 To measures of this kind, devoted to the destruction or 
 avoidance of the active agents of infectious disease, should 
 be added efforts calculated to lessen personal susceptibility 
 to their influence if, as sometimes happens in spite of all 
 precautions, they find entrance into the body. The wise 
 individual will study himself and learn from experience 
 how to avoid colds and other slight ailments ; he will 
 regulate his diet and his exercise, his sleep and his bath- 
 ing, his work and his play, in order to build up and main- 
 tain abounding health with which to meet any microbic 
 invasions which may chance to occur. The wise and 
 enlightened community will also provide parks and play- 
 grounds, gymnasia and baths, and other means which will 
 facilitate the cultivation of this abounding health among 
 its citizens, young and old. 
 
CHAPTER XXXI 
 
 SOME MICROBIC DISEASES AND THEIR PREVENTION. 
 VACCINATION AND ANTITOXIC SERUMS 
 
 1. Tuberculosis. — One of the first diseases of which 
 microbes were conclusively proved to be the cause was 
 tuberculosis, a disease to which more deaths are annually 
 attributed than to any other. For the latter reason it has 
 
 
 'I f 
 
 '1 
 
 
 
 
 ^"^ ^.<C:> ' 
 
 
 
 
 
 Q ^ 
 
 / 
 
 7/ ' 
 
 
 
 
 .- '^^ 
 
 ■' 
 
 c7> 
 
 r- rO. 
 
 Fig. 123. The microbe of tuberculosis {Bacillus tiibercuU>sis). (Very 
 highly magDified) 
 
 A minute amount of the spit (sputum) of a consumptive has been spread out 
 thin and photographed. Tlie tubercle bacilli are the little sticks or small 
 rods scattered all about among larger irregular epithelial cells, mucus, etc. 
 
 been called the "great white plague," and under the com- 
 mon name of " consumption " one form of it is only too 
 familiar. The name " tuberculosis " was given to the disease 
 because of certain characteristic cheesy nodules or tuber- 
 cles (little tubers) found in the lungs and other tissues of 
 
 477 
 
478 THE HUMAN MECHANISM 
 
 persons affected with it, and until 1882 it was generally 
 regarded as a constitutional disease, readily inherited and 
 often "running in families." In that year, however, Koch 
 announced the great discovery that in the tubercles could 
 be found peculiar and apparently characteristic microbes 
 belonging among the bacteria ; also, that by special meth- 
 ods he had cultivated these microbes pure, or free from 
 all others ; and, finally, that with them he had inoculated 
 healthy guinea pigs and actually produced tuberculosis in 
 the infected animals. 
 
 Intense interest was everywhere felt in Koch's splendid 
 discovery, which was quickly confirmed, and soon became 
 an established and universally accepted fact. It is now 
 known that tuberculosis is not ordinarily an inherited or 
 constitutional, but an acquired, disease, infectious, conta- 
 gious, and environmental in origin, and due to the rav- 
 ages of a special microparasite, a bacterial microbe named 
 Bacillns tuherculosis. It is true that the disease often runs 
 in families, but infectious diseases frequently do this ; and 
 the reason why it affects some families so much more than 
 others is believed to be simply that some families are more 
 susceptible to it, more adapted to its growth, than others; 
 precisely as some soils are better suited than others for 
 growing wheat or grass or corn. 
 
 2. How Tuberculosis is spread. — As this disease is 
 caused by the invasion of a specific microbe, it can only 
 he caused hy the entrance of that microbe into susceptible 
 bodies. Sanitarians generally Ijelieve that the principal 
 ways in which tuberculosis is conveyed from one patient 
 to another are the following : (a) by contagion, that is, by 
 personal contact of tuberculous with non-tuberculous per- 
 sons, and especially by kissing. A consumptive mother or 
 sister or friend, fondling a baby and " smothering it with 
 kisses," may thus transmit the germs of the disease to the 
 child ; (b) by objects handled or mouthed (such as food. 
 
MICROBIC DISEASES. PEEVENTION 479 
 
 forks, drinking cups, pencils, or towels) first by consump- 
 tives and tlien by susceptible non-consumptives ; (c) by chist 
 containing sputum expectorated upon streets or floors, and 
 then dried and carried in the air ; (d) by milk or meat of 
 tuberculous animals ; and (e) by the mouture of the breath 
 thrown off not as vapor but as fine droplets or spray, in 
 coughing or even while talking. It is not certain just what 
 part any or all of these j^lay in the dissemination of the 
 disease. At one time it was thought that the principal 
 vehicle of tuberculosis was the spit, or sputum, of tuber- 
 culous patients expectorated upon floors, sidewalks, or 
 streets and afterwards dried, pulverized, and driven about 
 as dust particles, which might readily find access to healthy 
 throats and lungs. There is no doubt tliat the spit of con- 
 sumptives may, and often does, contain the germs of the 
 disease, and for this reason " spit cups " and destructible 
 handkerchiefs (e.g. of paper) should always l:)e used by 
 them; but further investigations have shown that drying 
 and exposure to sunlight both tend to weaken and to de- 
 stroy microbes, so that to-day, while dust is still regarded 
 by all as a vehicle of very great importance in the spread 
 of tuberculosis, more significance than formerly is attrib- 
 uted to other factors. 
 
 It is an open question how far milk is a carrier of tuber- 
 culosis. Cows undoubtedly often suffer from a form of 
 the disease not readily distinguishable from that which 
 occurs in man, and the frequency of tuberculosis in children 
 certainly suggests that infected milk may have been the 
 cause of it. But, on the other hand, it is far from certain 
 that bovine tuberculosis is readily transferable to man ; 
 and at present, while it may be that milk is an important 
 vehicle of the disease, the whole question is much in doubt. 
 The same thing is true of meat from tuberculous animals, 
 although in this case thorough cooking must destroy most, 
 if not all, of the germs. The safest plan is to use for food 
 
480 THE HUMAN MECHANISM 
 
 no milk or meat which is in the least doubtful in quality, 
 at least not without first subjecting it to thorough cooking. 
 At present the dissemination of the bacilli through the air 
 by means of finely divided droplets of mucous or moisture, 
 as suggested above, is regarded as of special importance. 
 
 3. The Prevention of Tuberculosis. — No successful steps 
 have yet been taken toward the prevention of tuberculosis 
 by vaccination, or in any other ways than by visarding off 
 the microbe and by helping the patient in his struggle 
 with it by giving him good air, good food, rest, and all 
 other favorable conditions which shall aid the body in 
 resisting infection and the ravages of the disease. 
 
 It has been proposed to isolate cases of tuberculosis and 
 in general to deal with them very much as the more con- 
 tagious diseases are ordinarily dealt with. There is some- 
 thing to be said in favor of this plan, but the general 
 opinion is that such isolation is a hardship to the patient 
 and not often necessary for the safety of the community. 
 Consumptives should, however, be expected, and even 
 required, to be especially cleanly in their habits, and to 
 collect and destroy their sputum in cheap paper cups or 
 in paper handkerchiefs, which can readily be burned. They 
 should never spit upon floors or streets, or cough into the 
 faces of friends or attendants, and they should wash their 
 hands and mouths frequently and thoroughly. 
 
 Milk or meat derived from tuberculous animals should 
 not be used, and dust, which may at any time contain the 
 germs of tuberculosis, should be kept down as far as 
 possible, both in houses and in streets. Above all, every 
 means of direct conveyance of the fresh virulent microbes 
 from persons having the disease to new victims should 
 be carefully avoided. Some of these means are kissing 
 and coughing, by which latter minute infectious particles 
 may be thrown to a distance and caught upon the face or 
 hands of friends, or upon food, tableware, or linen ; and 
 
MICROBIC DISEASES. PREVENTION 481 
 
 any lack of absolute cleanliness in washing dishes, cups, 
 spoons, napkins, etc., recently used by consumptives, is to 
 be scrupulously avoided. Those, also, who do the washing 
 need to be on their guard against infection, by exercising 
 extreme care and cleanliness. 
 
 There are many other forms of tuberculosis besides con- 
 sumption, but this is the form of principal interest to 
 students beginning the study of hygiene and sanitation. 
 
 4. Hygiene in the Treatment of Tuberculosis. — While 
 consumption is the cause of so many deaths, it is not neces- 
 sarily a fatal disease. The Bacillus tuberculosis is usually 
 of very slow growth and low virulence ; it does not, like 
 some microbes, produce large quantities of poisonous tox- 
 ins which, uj)on entering the blood, cause rapid and exten- 
 sive injuries to most organs. On the contrary, its action 
 is at first largely confined to the spot in the lungs where 
 it has gained a lodgment, and at the outset the constitu- 
 tional disturbances are slight. So insidious is the attack 
 and growth of the germ that the patient does not at first 
 even suspect its presence, and merely feels " out of sorts " 
 or "run down." Only later, when the pathological processes 
 have spread over a considerable area of tissue, are the symp- 
 toms serious ; and frequently the disease is not recognized 
 until irreparable damage is done. 
 
 It is chiefly for this reason that consumption claims so 
 many victims ; for the inroads of the disease are by no 
 means unresisted by the living cells of the body. From 
 the outset a struggle between these cells and the invading 
 microbes takes place ; and it should be better known than 
 it is that in the majority of cases the human mechanism is 
 the victor in the struggle. This is shown by the fact that 
 autopsies on persons who have died of other diseases dis- 
 close in a surprisingly large percentage of eases healed 
 tuberculous lesions, where the presence of the disease had 
 not been suspected. In other words, the disease moves on 
 
482 THE HUMAN MECHANISM 
 
 to a fatal issue only when the vital resistance proves une- 
 qual to the defense, and the mortality from consumption 
 would undoubtedly be exceedingly low were sufficient 
 attention paid to the hygienic care of the body and the 
 sanitation of its surroundings, by both of which the vital 
 resistance is powerfully reenforced. 
 
 Tlris is in itself a powerful argument for attention to 
 general hygiene, and it points out unmistakably the hygi- 
 enic treatment of the disease when once recognized. No 
 reliance wluitever should be placed in drwjs. On the con- 
 trary, the patient, his family, and friends should recognize 
 that the one hope lies in tlie hygienic conduct of life. The 
 patient should live and sleep out of doors, if possible ; he 
 should fearlessly breathe cold air, but should protect the 
 skin from chilling by warm clothing ; if he cannot live out of 
 doors, the windows of the living and sleeping room should 
 be kept wide open, even in winter weather; the sleeping 
 room should have light walls, and all curtains and draper- 
 ies which limit the amount of sunlight should be dispensed 
 with ; the furniture of the room should be reduced to a 
 minimum and should be such as can be easily cleaned ; rest 
 from anything but very moderate muscular activity, and 
 from nervous strain, is al)solutely essential ; and all these 
 measures should l)e reenforced by abundant feeding with 
 appetizing and easily digested food ; the feeding, indeed, 
 should be pushed to the extreme, should even he forced 
 feeding, but only with easily digestible foods. In brief, 
 rest, fresh and cool air, sunshine, and almndant food are 
 the cures for tuberculosis, and, unless the disease has gone 
 too far Ijefore it is recognized, they are almost certain 
 cures. 
 
 While some climates are more favorable for the treat- 
 ment of tul)erculosis than others, — a cold, dry climate 
 being preferable, — it should be understood that the treat- 
 ment we have outlined has been used with excellent results 
 
MICROBIG DISEASES. PREVENTION 483 
 
 even in the patient's own home, wherever that may be ; 
 and this hj^gienic treatment should be employed whether 
 or not it is possible to go away from home. 
 
 Continuous high temperature and high humidity, or 
 dampness, are the main conditions which make change of 
 climate desirable ; and where these are not present it is 
 generally better to treat the patient at his own home, 
 where family and friends may not only minister to his 
 needs but may also maintain those cheerful surroundings 
 which count for so much both in sickness and in health. 
 
 5. Typhoid Fever. — Tliis disease is now believed to be 
 due to the bacterial microbe known to bacteriolog-ists as 
 
 Fic. 124. Typhoid fever germs {Bacillus typhi). (Highly magnified) 
 
 Above, swimming free in normal lilood serum ; helow, " ehimped " or "agglu- 
 tinated" in the serum of a tyjilioid fever patient 
 
 Bacillus typlii. The latter was observed by Koch, Eberth, 
 and others about 18T9, but first thoroughly worked out 
 by a pupil of Koch, Di-. Gaffky, in 1884. 
 
 Typhoid fever has been well known since about 18-iO, 
 previous to which time it was confused with typhus {jail, 
 spotted, or sJiip) fever, — a disease now not often met with. 
 It is one of the worst maladies that afflicts mankind, for 
 
484 THE HUMAN MECHAJSTISM 
 
 although not generally fatal in more than about ten per 
 cent of the cases, it is a slow fever, disabling the patient 
 even when it does not kill, and requiring weeks and often 
 months for its course and for convalescence. It is widely 
 distributed, probably all over the world ; and, although less 
 widespread than tuberculosis, it is still one of the chief 
 causes of death. It is a disease which seriously damages 
 the intestine and is one form of what is sometimes called 
 " inflammation of the bowels." The microbe finds its way 
 into the alimentary canal with food or drink, and is believed 
 to multiply in the intestine and to invade the body proper, 
 producing a poisonous substance {typhotoxin) which causes 
 the fever and otherwise injures the whole organism. The 
 germs are probably cast off in abundance with the various 
 excreta, and as diarrhea is a frequent (though not invari- 
 able) accompaniment of the disease, typhoid fever is called 
 a diarrheal disease. At its beginning and, in mild attacks, 
 throughout the whole disease, the victim may not be con- 
 fined to his bed, but may "keep about." Such cases are 
 known as " walking" typhoid. 
 
 6. How Typhoid Fever is spread. — Since the germs of 
 this disease are present in the excreta, — both urine and 
 faeces, and even in the saliva, — it has been well called a 
 " filth " disease. The old idea of a filth disease, however, 
 was that filth bred disease, and that almost any heap of 
 dirt or rotting material might generate disease, especially 
 typhoid fever, and inflict it on persons in the vicinity. 
 This idea is now abandoned, for it is held that the germs 
 of any one kind or species of disease can come only from 
 other germs of the same kind; that is to say, typhoid 
 fever can come only from a j)erson or persons now having, 
 or having recently had, that disease. The excreta of such 
 persons may, nevertheless, readily convey it ; and if food 
 or drink are polluted in any way by such excreta, then the 
 germs readily find access to fresh victims. 
 
MICEOBIC DISEASES. PREVENTION 485 
 
 Unfortunately, food and drink are oftener polluted than 
 most persons realize. Water may be contaminated by sew- 
 age; milk, by the dirty hands of careless or unclean milkers ; 
 oysters growing in harbors or estuaries, by city sewers 
 discharging therein ; vegetables, by manure ; and fruits 
 or berries, by filthy hands. When we stop to think that 
 filth may readily find access to food and drink in these and 
 many other ways, it is clear that typhoid fever may still be 
 called a filth disease, even if we understand by the term 
 that it is a disease conveyed by infected filth, hut not bred or 
 generated by filth alone. 
 
 7. The Prevention of Typhoid Fever. — This disease can 
 be vastly reduced in amount and destructiveness in any 
 community in which it abounds, by careful attention to the 
 avoidance and destruction of its microbes, and by main- 
 taining high vital resistance through h3rgienic and whole- 
 some living. No means are yet known of vaccinating 
 against its attacks, as is done for smallpox, nor has any 
 antitoxic serum yet been discovered effective for its cure, 
 as is antitoxin for diphtheria. 
 
 The microbes of typhoid fever may generally be avoided 
 by the use of pure drinking water, pure milk, clean vege- 
 tables and fruits, raw oysters derived only from harbors 
 and estuaries free from sewage, and, in general, by the use 
 of pure foods and drinks. Microbes are readily destroyed 
 by cooking at a high temperature and, in the case of the 
 excreta of patients suffering from typhoid, by disinfection, 
 which, under the direction of an attending physician, should 
 always be thoroughly carried out. 
 
 It should never be forgotten that, contrary to the gen- 
 eral impression, typhoid fever is really contagious, i.e. may 
 be "taken" by contact, not necessarily with the patient, 
 but very readily with liis excreta, or with his linen, or 
 with any of his belongings soiled with his excreta. Even 
 trained nurses sometimes seem to forget this fact, for not 
 
486 THE HUMAN MECHANISM 
 
 infrequently a trained nurse contracts the disease from 
 her patient. Similar secondary cases of typhoid fever, espe- 
 cially in families in which the mother or sister attends the 
 patient and at the same time prepares food for the rest of 
 the family, are painfully common. 
 
 It has been proved again and again that persons over- 
 vporked or otherwise in poor condition are those most sus- 
 ceptible to this disease, and we could hardly have a better 
 instance than typhoid fever affords of the double duty 
 which a sound hygiene imposes upon us, namely, (a) the 
 duty of avoiding infection and (b) the duty of keeping 
 the mechanism in such good condition that if by accident 
 infection occurs, the microbes shall not be able to over- 
 come its vital resistance. 
 
 8. Diphtheria. — This disease has long been known under 
 the name of "membranous croup" and "malignant sore 
 throat," but it was not until 1884 that Loefller, one of the 
 pupils of Koch, detected the microbe now generally agreed 
 to be its sole and only cause (Fig. 125). 
 
 It is believed that this microbe, Bacillus diphtheriw, 
 finding lodgment in the throat of a susceptible person, 
 grows and multiplies there, secreting meanwhile a poison- 
 ous substance, or toxin, which injures the tissues of the 
 throat and causes the formation of the ivhite spots and false 
 membrane so characteristic of the disease, and also damages 
 the rest of the body after its absorption into the circula- 
 tion, by producing fever, paralysis of particular parts, and 
 sometimes death. 
 
 9. How Diphtheria is spread. — Inasmuch as diphtheria 
 is a disease of the throat especially, it is easy to see that 
 it must be chiefly conveyed from one person to another 
 by sputum and lij objects which come in contact tvith the 
 mouth or lips. Kissing and fondling among children, or 
 parents and children ; fingers, which, especially in children, 
 are too often in the mouth; books, handkerchiefs, pencils, 
 
MICEOBIC DISEASES. PREVENTION 487 
 
 playthings, and food bitten and passed from hand to hand, 
 — all these may be vehicles of contagion and infection in 
 this microbic disease. It may be spread by dried sputum 
 
 «" - . ;•. -' If 
 
 
 
 
 ■ ^^^^— ' — ___ 1 ;^ 
 
 
 Fig. 12.5. Microbes of diphtheria and croup (Bacillus diphtherke). 
 (Highly magnified) 
 
 On the left, a.s they appear under the microscope in discharges from the throat 
 of a diphtheritic patient. On the right, after cultivation in the hacteriologi- 
 cal laboratory 
 
 in dust, by milk infected by milkers suffering with the dis- 
 ease, by infected food, and possibly by pets, such as cats 
 and dogs and even birds, suffering from the disease. 
 
 10. The Prevention of Diphtheria Diphtheria being pri- 
 marily a disease of the throat, and therefore distributed 
 both by personal contact and by spittle, it would seem to 
 suffice for its prevention to isolate patients having the dis- 
 ease as long as they are capable of communicating it to 
 others, and thus cut off the escape and distribution of the 
 germs. Unfortunately, however, in this, as in many other 
 infectious diseases, persons often have the disease for some 
 time before they or their friends discover the fact ; and 
 some mild cases of this malady (as also of typhoid fever 
 and other infectious diseases) probably occur and run their 
 
488 THE HUMAN MECHANISM 
 
 course without ever having revealed their true character. 
 Hence arises the difficulty of accounting for the origin of 
 some apparently inexplicable cases, and also the difficulty 
 of stamping out a disease of this kind. Diphtheria is not 
 an eruptive disease, and ought, therefore, to be more readily 
 controlled than smalljjox, scarlet fever, or measles, which 
 are doubtless most often disseminated by means of scales 
 shed off from the skin during the " peeling " which follows 
 the eruption. 
 
 It should be clearly understood that all kissing by per- 
 sons having sore throats, or the mouthing of pencils or 
 other objects by children, is a dangerous practice ; and that 
 fingers, which so readily find their way into mouths, may 
 as easily as not carry infection to books, playthings, food, 
 letters, or other objects which are " handled." Letters sealed 
 or handled by diphtheritic patients, or by persons attend- 
 ing them, have probably at times conveyed the germs of 
 disease to persons at a distance in whom the appearance 
 of the illness seemed quite unaccountable. Here also, as 
 in tuberculosis, care in the disposal of sputum is of great 
 importance. 
 
 Within a decade the discovery of an antitoxic serum, 
 antitoxin (see p. 499), has given us a novel and invaluable 
 means of defense against the microbes of diphtheria by 
 increasing the resistance of the human body so that it 
 shall be no longer susceptible to the disease. But as this 
 discovery means much more to hygiene than the control 
 of this one disease, we shall devote to its careful consid- 
 eration an entire paragraph beyond. 
 
 11. The Spitting Nuisance, as the habit of public spitting 
 is often called, is not only a disgusting nuisance but a real 
 menace to the public health, because, as will now readily 
 be seen, it may l)e the means of spreading abroad diseases, 
 such as tuberculosis and diphtheria, with which many 
 persons — incipient cases, or "walking" cases — may be 
 
MICEOBIC DISEASES. PEEVENTION 489 
 
 moving about. Fortunately, the habit is chiefly confined 
 to one sex, and this fact shows how easily it might be con- 
 trolled if custom demanded. 
 
 12. Malarial Fever, or <' Malaria," is a world-famous 
 disease, especially common in warm climates but also fre- 
 quently occurring in the more temperate zones. It is prob- 
 ably by far the most important of all tropical diseases, for 
 while it does not kill as do yellow fever and Asiatic chol- 
 era, it is much more common and disables a far greater 
 number of victims. 
 
 Malaria has long been associated in the popular mind 
 with low grounds and swamps. Experience has shown, 
 however, that it cannot be caused by swamps alone, for 
 many swamps and marshes are entirely free from malaria. 
 Sometimes it has seemed to go with the digging up of 
 earth ; yet the earth has very often been opened and thrown 
 about without causing any malaria whatever. 
 
 The true source' of this disease remained an absolute 
 mystery until, in 1880, Laveran, a French investigator 
 resident in Algiers, discovered in the blood of malarial 
 patients a microbe of a very peculiar kind, — not a bac- 
 terium, nor even a plant, like the microbes of tuberculosis, 
 typhoid fever, and diphtheria, but an animal known as a 
 hcematozoon, or sporozoon, belonging among the simplest 
 animals, or protozoa (see p. 469). Figure 126 gives illustra- 
 tions of some of the various forms of the microbe, and its 
 life history is outlined in the description of the figure. 
 
 But even after the microbe of malaria had been detected 
 no one knew, for ten years longer, whence it came, or 
 whether it lived outside of man at all, or, if not, how it was 
 conveyed from victim to victim. When, at last, in 1899, as 
 if for a final benediction upon mankind by a century won- 
 derfully rich in human and especially in medical progress, 
 the whole subject was cleared up, the solution of the riddle 
 was found to be as simple as it was unexpected, for the 
 
fa® 
 
 ® *<g 
 
 ®@® 
 
 6 
 
 Fig. 126. The malaria microbe (Plasmodium malariw) and a malarial 
 mosquito. After Leuokart-Chun's wall diagram 
 
 1, the microhe; S-.i, its growth in a red blood corpuscle; 6, its multiplication 
 and escape from the corpuscle (it is now ready to iufect fresh blood cor- 
 puscles) ; 7, 8, crescentic forms of the microbe. For further development 
 the microbe must be transferred from man to mosquito (by a biting, i.e. 
 bloodsucking, mosquito). 
 
 In the stomach cavity of Anopheles : 9, female stage of the microbe; 9a, 95, 
 male stage; 10, union of 9 with one of the vibratile arms of 96; 11, the 
 microbe resulting from 10. 
 
 In the body proper of Anopheles : 12, The microbe (11) has penetrated the 
 stomach wall of the mosquito and embedded itself on the outer (body) side 
 of the stomach. Here it undergoes a process of growth, cell division, and 
 multiplication (1.1), eventually forming " tumors " on the outside (body side) 
 of the mosquito's stomach, as shown in 1.5. From these tumors the microbes 
 escape into the body cavity of the mosquito and find their way into the 
 salivary glands (shown in section in 16). From these they are readily trans- 
 ferred (with the saliva) into a human body bitten by Anopheles. 
 
 14, female malarial mosquito (head of male below). 
 
MICROBIC DISEASES. PREVENTION 491 
 
 vehicle proved to be a common insect, a special mosquito, 
 long known as a pest, but never hitherto suspected or 
 dreaded as a bearer of disease. At once it became clear 
 why malaria is a disease of some warm climates and some 
 seasons, and why it "hangs about" some swamps and not 
 about others. 
 
 13. How Malaria is spread. — The malarial microbe is 
 a microparasite (p. 467), spending a part of its life in man 
 and a part in certain mos- 
 quitoes, which are thus its 
 "hosts." A mosquito of the 
 right kind bites and sucks 
 the blood of a man having 
 malaria, and having thus 
 become infected bites other 
 persons, injecting into them 
 germs of malaria, along witli 
 that poison which causes the 
 familiar swelling often fol- 
 lowing a mosquito bite. 
 
 It is important to note that 
 only one genus of mosquito 
 (Anopheles), and that not the 
 commonest in most places, 
 seems capable of conveying 
 the disease. Moreover, it is 
 only the female Anojiheles — and even that only after it 
 has become infected by biting a person having the disease 
 — which can transmit malaria. Hence many mosquitoes, 
 even if Anopheles, are harmless, as are all mosquitoes in 
 regions in which no Anopheles or no malarial microbes 
 exist. For the causation of malaria three things are re- 
 quired, namely, (a) malarial microbes, (h) female Anopheles, 
 and (c) susceptible victims. Fortunately the first two do 
 not always coexist, and in case either is missing malaria 
 
 Fid. 127. Anopheles punrtipennis 
 (female). After a photograph 
 from life by W. Lyman Under- 
 woi.>d 
 
 C'omiiioii in tlie northern United States 
 
492 
 
 THE HUMAN MECHANISM 
 
 is absent. Many places abounding in mosquitoes have no 
 malaria, either because there are no Anopheles, or be- 
 cause the Anopheles pres- 
 ent have never become in- 
 fected. 
 
 14. The Prevention of 
 Malaria. — Beyond a gen- 
 eral reenforcement of the 
 body by wholesome living, 
 the only means yet known 
 of avoiding this disease is 
 the avoidance of mosqui- 
 toes in those regions in 
 Fig. 128. The malaria mosquito (Anoph- which they abound, and in 
 eles) above, and tlie common mos- ^j^igj^ malaria also occurs, 
 quito (Culex) below. After photo- 
 graphs from life by W. Lyman Un- -l± a region contains no 
 derwood malaria, the mosquitoes in 
 
 Showing a characteristic difference in the it cannot produce the dis- 
 resting attitudes tj! j_i • • 
 
 " ease. It the region is ma- 
 
 larious but has no malarial mosquitoes, no fresh cases, so 
 far as known, can occur. But if malarial fever and malarial 
 mosquitoes coexist, then the only hope is to remove one 
 or the other, and if possible both. For relief from malaria 
 already fastened upon a patient, application should be made 
 to a physician. For the extermination of mosquitoes from 
 a neighborhood, all swamps and marshes must be drained, 
 and pools of stagnant water either treated with crude petro- 
 leum or stocked with fishes that will feed upon and destroy 
 the mosquito larvse.^ 
 
 15. Yellow Fever is an infection greatly dreaded in the 
 tropics. It is now attributed to a microbe somewhat similar 
 
 1 For further information on this subject see Dr. L. 0. Howard, Mos- 
 quitoes, How they live, How they carry Disease, etc.. New York (McClure), 
 1901 ; W. L. Underwood, Mosquitoes and their Extermination, Boston 
 (W. B. Clarke), 1903; Celli, Malaria according to the New Researches, 
 New York (Longmans), 1900. 
 
MICEOBIC DISEASES. PEEVENTION 493 
 
 to that of malaria, and which is believed to be conveyed, 
 like the malarial microbe, by a mosquito, though of another 
 kind, Stcgomyia by name. If this view is correct, yellow 
 fever is to be prevented only by avoiding these mosquitoes. 
 Havana, which was formerly cursed by yellow fever, has 
 been virtually freed from it by 
 preventive action based wholly on 
 the mosquito theory of its causa- 
 tion ; and the suppression of yellow 
 fever in Cuban ports, from which 
 formerly the disease was frequently 
 exported to the United States, 
 means much to the southern states 
 of the Union. 
 
 In 1905 a disastrous outbreak ^ ,„^ „„ 
 
 Fig. 129. I he yellow fever 
 
 of yellow fever occurred m New mosquito (Stegomyia). 
 Orleans and . vicinity. Its germs After a drawing by L. 0. 
 were probably brought in from Howard 
 some infected district nearer the tropics by mosquitoes 
 (Stegomyia) on ships and on fruit (bunches of bananas). 
 The epidemic was fought wholly upon the mosquito theorjr, 
 and Avith results entirely satisfactory to the sanitarians in 
 charge. 
 
 16. Some Other Infectious or Contagious Diseases, and 
 how they are supposed to be spread. — In the case of some 
 of the commonest infectious or contagious diseases we are 
 still in the dark both as to their precise causation and 
 the ways in which they are scattered abroad. Concern- 
 ing measles, chicken pox, and whooping cough, for exam- 
 ple, we are still awaiting such discoveries as have already 
 been made for tuberculosis and the other diseases described 
 above. 
 
 We have also referred above (p. 380) to the fact that 
 some colds or influenzas appear to be infectious. Attempts 
 have been made to detect the microbes of the "grippe" 
 
494 
 
 THE HUMAN MECHANISM 
 
 and other influenzas, and figures are often given of germs 
 found associated with disorders of this class. Figure 130 
 is an example of this kind, although it is not safe to say 
 positively that the microbes chiefly concerned have really 
 been as yet identified. 
 
 But while we patiently wait for more light, we have 
 good reason, because of their general character, to believe 
 that these also are microbic diseases, caused likewise by 
 microparasites transmitted either directly or indirectly from 
 
 oO- 
 
 . i 
 
 ■PCf ' 
 
 
 
 
 '' 
 
 
 ■--,/A ^ ',' , " ^ 'i 
 
 Fig. 130. A so-called " influenza bacillus " 
 
 On the left, as found in the sputum in some " colds" ; on the right, after 
 cultivation in the bacteriological laboratory 
 
 victim to victim. Experience has shown that the same 
 kind of effort which tends to prevent diseases undoubtedly 
 microbic tends to prevent these also ; so that at present, 
 and for all practical purposes, we may consider that we 
 know enough about their causation and spreading to 
 enable us to deal with them intelligently, fearlessly, and 
 hopefully. 
 
 17. Scarlet Fever, Measles, and Chicken Pox belong to 
 the group of eruptive diseases^ a term derived from the fact 
 that persons having any of them are usually, sooner or 
 
MICEOBIC DISEASES. PREVENTION 495 
 
 later, " broken out," or more or less covered with an erup- 
 tion, or rash. When this eruption heals, scales of the skin 
 are shed off, and the wide dissemination of these scales 
 during the process of " peeling " is believed to be one rea- 
 son why eruptive diseases are more contagious than some 
 microhic diseases (like tuberculosis and diphtheria) which 
 are not eruptive. Persons suffering from an eruptive dis- 
 ease should not be allowed to go about among other people 
 until they have ceased peeling. 
 
 As for the prevention of measles, chicken pox, scar- 
 let fever, and whooping cough, the only means at hand 
 at present seem to be isolation and non-intercourse. To 
 maintain or increase the resistance of the bod}' no better 
 means are known than good feeding, temperate living, 
 and, in general, a wise and wholesome conduct of life ; 
 yet even so, immunity is often purchased only at the cost 
 of one or more attacks, and prevention by isolation is 
 frequently difiicult under the conditions of modern life, 
 especially in tenements and crowded districts. 
 
 18. Summer Complaint in Children is a severe and often 
 dangerous summer diarrhea, believed to be caused Ij)^ 
 microbes and apparently due in part to wrong feeding. 
 In cities it appears to be closely connected with the use 
 of stale milk, and is often prevented or overcome by using 
 very fresh milk, or even by pasteiirUini/ ordinary milk 
 (p. 475). This is readily done by immersing bottles of 
 milk in water and then heating the water to a tempera- 
 ture of about 160° F. for five minutes. If no thermom- 
 eter is available, it will suffice to bring the milk nearly 
 (but not quite) to the boiling point and keep it at that 
 temperature for a few minutes. 
 
 19. Smallpox, a disease once so common that "scarce 
 one in a thousand " escaped it, is now happily rare in 
 most highly civilized countries. It is an eruptive fever, 
 small pustules or pocks giving it its name, and while not 
 
496 THE HUMAN MECHANISM 
 
 as fatal as some of the diseases shortly to be consid- 
 ered, it is peculiarly loathsome and very apt to leave after 
 it lifelong disfigurement, or pitting. It is hard to realize 
 to-day the dread and fear with which our ancestors rightly 
 regarded smallpox, and for this reason some people make 
 much more of the slight discomfort and insignificant dan- 
 ger of vaccination than of the loathsome smallpox itself; 
 but if communities ever cease to take the simple but in- 
 dispensable steps required to prevent smallpox, outbreaks 
 of the disease will undoubtedly remind them in alarming 
 fashion of what they had previously escaped. 
 
 It is not yet absolutely proved, although it is now gener- 
 ally believed, that smallpox is caused by microbic activity ; 
 but it is certain that it is extremely contagious, probably 
 through the scales cast off from the skin of those suffer- 
 ing from it, with which scales the specific microbes are 
 blown or handed about. 
 
 Experience has shown that great gain results from " iso- 
 lating " or " quarantining " smallpox patients in a deten- 
 tion hospital (to which the name of " pesthouse " was 
 formerly given). Smallpox patients are thus removed and 
 isolated, while those suffering from typhoid fever or con- 
 sumption are not, partly because of the much greater con- 
 tagiousness of smallpox, and partly because of its graver 
 and more loathsome character. 
 
 In smallpox, as in other diseases, wholesome living 
 diminishes the danger of infection ; but it is a matter of 
 history that in the days when the disease was prevalent 
 and the question was put to the severest test, general 
 good health proved an unreliable defense. Fortunately, 
 however, the human race, which was once so frightfully 
 scourged by this disease, has discovered a more certain 
 means of protection, which consists not in the warding 
 off or destruction of microbes but in an enhancement of 
 the powers of resistance of the organism, so remarkable as 
 
MICEOBIC DISEASES. PREVENTION 497 
 
 to constitute for extended periods virtual exemption, or 
 immunity, from smallpox. The methods by which this 
 extraordinary result is reached are known as inoculation 
 and vaccination. 
 
 20. Immunity. — The best of all defenses against any 
 disease would be complete insusceptibility, or immunity, 
 to it ; for no matter how ingenious, elaborate, or com- 
 plete the devices may be for preventing disease germs 
 from finding access to the body, accidents may always 
 happen which will allow them to enter. Immunity, or 
 insusceptibility, to disease is therefore one of the princi- 
 pal aims of hygiene, one of the goals of sanitary science. 
 Unfortunately, natural immunity is not common, and arti- 
 ficial immunity is not easily conferred or acquired, except 
 in the case of one or two diseases ; but there is good reason 
 to hope that the future may have in store for the human 
 race great gains in this direction. 
 
 Natural immimity means a natural insusceptibility to 
 disease. It is usually constitutional and inherited. The 
 lower animals, for example, are not susceptible to typhoid 
 fever, and birds are immune to anthrax, although mammals 
 take it readily. Diseases common to many species of ani- 
 mals appear to be the exception. In general, each species 
 is immune to many, if not most, of the diseases of other 
 species. 
 
 By artificial immimity is meant a similar exemption 
 from disease, not constitutional or inherited, but acquired 
 in one way or another. The most familiar method of 
 becoming immune to any disease is to have the disease in 
 question. For example, long before inoculation and vaccina- 
 tion were known, it was well recognized that persons who 
 had once had smallpox were not likely to have it a second 
 time, and such persons were, and still are, in demand as 
 nurses for cases of that disease. Again, children who have 
 had scarlet fever or measles or whooping cough are believed 
 
498 THE HUMAN MECHANISM 
 
 (and rightly) for that reason to be less likely to have the 
 same disease a second time. There can be no question that 
 protection is generally secured in this way, although cases 
 are not rare in which such protection, even if once secured, 
 is ultimately lost ; for people sometimes have measles, 
 typhoid fever, and diphtheria twice, or even oftener. 
 
 21. Inoculation for Smallpox. — The first great step 
 towards the prevention of infectious diseases by produ- 
 cing artificial immunity from them was that of inoculation 
 for smallpox. 
 
 For a century after the first English settlements in this 
 country smallpox ravaged Europe and America. But in 
 1721 a novel and ingenious method of producing immu- 
 nity from the disease was introduced into England from 
 Constantinople, and quickly reached the United States. 
 This method, known as inoculation, consisted in inoculat- 
 ing persons while in good health with the virus of true 
 smallpox (not vaccine), for the purpose of causing them 
 to undergo a mild attack of the disease while well and in 
 good condition, so that they might avoid having a severe 
 attack when unwell and in poor condition. Inoculation for 
 smallpox was an effective preventive and met with wide 
 acceptance and approval both in England and in the United 
 States. It was extensively practiced for nearly a century, 
 but was finally supplanted by the much safer process of 
 vaccination, in which the inoculation was with vaccine 
 (the mild and comparatively harmless virus of cozvpor), 
 instead of with the always dangerous smallpox virus. 
 
 22. Vaccination, one of the greatest blessings ever con- 
 ferred upon mankind, was first invented in England in 
 1796 by Edward Jenner, a young physician of Gloucester- 
 shire. It consists in the inoculation of the human beinsf 
 with virus derived from a cow having cowpox. A spot, 
 usually upon the upper arm, is scraped by a lancet, so that 
 the outer layers of the epidermis are removed ; the spot 
 
MICEOBIC DISEASES. PREVENTION 499 
 
 is then rubbed with an ivory point, quill, or tube, carry- 
 ing the virus. A slight and usually unimportant illness 
 or indisposition follows, and the arm is sore for a time, 
 a characteristic scar remaining. In some cases the illness 
 is more serious ; but deaths plainly due to mere vaccina- 
 tion very rarely, if ever, occur. 
 
 The immunity from smallpox produced by vaccination is 
 remarkable, and has been proved over and over again, not 
 onl}' by the experience of armies and nations, but also by 
 actual experiment. It was f ormeiiy thought that " once vac- 
 cinated " was "always protected"; but to-day it is recog- 
 nized that occasional revaccination is essential to complete 
 immunity, the length of the period of protection usually 
 lixed nowadays being not more than ten years. Indeed, so 
 variable is the duration of the immunity in different indi- 
 viduals, and in the same individual at different times, that 
 the only safe course is to revaccinate whenever there is an 
 appearance of smallpox in the community. It should also 
 be remembered that the vaccination may fail to " take " 
 merely because the virus has been rubbed off by the 
 clothing, or because it was not effective to begin with. 
 Consequently when any vaccination fails to " take," it is 
 safest to try again a second or even a third time, espe- 
 cially if we are unusually exposed to the contagion of the 
 disease. 
 
 23. Diphtheria Antitoxin, and Other Antitoxic Serums. — 
 As has been said above, inoculation against smallpox was 
 begun in western Europe and America about 1721, and 
 vaccination for the same disease at about the beginning of 
 the nineteenth century (1796). No further progress was 
 made in the art of immunizing until, in 1879, Pasteur suc- 
 ceeded in extending vaccination to some species of the 
 lower animals, upon which he conferred immunity from 
 certain diseases bj' using a modified, or as he called it, atten- 
 uated, virus of the disease itself. 
 
500 THE HUMAN MECHANISM 
 
 A much more important discovery than this of Pasteur 
 was made in 1892 when von Behring, a German bacteriolo- 
 gist, found that the serum of the blood of an animal im- 
 mune to diphtheria differs from that of one not immune, 
 in that it is capable of neutralizing, both in a test tube and 
 in the body of another animal, the poison (toxin) produced 
 by diphtheria germs. This great discovery naturally led 
 to the use of such neutralizing, antidotal, or antitoxic serum 
 (antitoxin) in cases of diphtheria in man, and such use of 
 it has now become general. 
 
 In order to obtain the antitoxin, horses are inoculated 
 hypodermically with virus, or toxin,^ of diphtheria (from 
 which all germs have been removed), at first in small doses 
 but gradually with larger amounts, until they have become 
 immune to heavy doses. The serum of the horse's blood 
 under this treatment gradually becomes changed, so that 
 it possesses antitoxic or antidotal properties. This serum 
 (or antitoxin) is then carefully collected, bottled, and after- 
 wards used to cure, and sometimes to prevent, cases of 
 diphtheria in human beings. 
 
 Von Behring's discovery is probably one of the most 
 beneficent ever made, because it has pointed out the way 
 for the prevention or cure of other infectious diseases, by 
 showing that when the disease is due to a toxin it may 
 be possible in any case to produce an antidote (antitoxin) 
 which shall neutralize and overcome it. Good progress 
 
 1 If the animal were inoculated with the germ of diphtheria, instead 
 of its toxin, we should have no control of its growth and the severity of 
 the disease produced. By inoculating with carefully measured doses of 
 the toxin, however, — which does not increase in amount, — we may pro- 
 duce the symptoms of diphtheria in very mild form, and always have the 
 course of the disease under control. Immunity is gradually acquired 
 with but trifling discomfort to the animal, and the antitoxic serum is 
 absolutely free from the germs of the disease. The statement sometimes 
 made, that the use of antitoxin is liable to produce diphtheria because the 
 animal was inoculated with the germs of that disease, is untrue, because 
 such germs are never present in antitoxin properly made. 
 
MICKOBIC DISEASES. PEEVENTION 
 
 501 
 
 has, indeed, been made already in tliis direction for a 
 few other diseases than diphtheria, especially for tetanus 
 (lockjaw). 
 
 24. Tetanus, or Lockjaw, is a comparatively rare disease, 
 although in America about the Fourth of July it is quite 
 common among boys as a consequence of accidents attend- 
 ing the celebration of that anniversary. The disease is a 
 
 
 6 
 
 Fig. 131. Microbes of lockjaw (tetanus). After Kolle and Wassermann 
 
 On the left, in the ordinary rodlike stage of active growth; on the right, 
 after having formed resting " spores " in the resting stage 
 
 peculiar one, and prolonged muscular contractions or spasms 
 are a characteristic symptom of its advanced stages. These 
 spasms may cause the lower jaw to become more or less 
 set or fixed ; hence the popular name, " lockjaw." 
 
 The microbe of tetanus is well known, and is common in 
 the intestine of herbivorous animals and in dirt in many 
 places. It grows best in the absence of oxygen, and deep 
 or lacerated wounds, such as are made by toy pistols, etc., 
 appear specially to favor its development. 
 
 25. Asiatic Cholera is a microbic fever formerly greatly 
 dreaded all over the civilized world, and still very de- 
 structive of human life in the East, — for example, in the 
 
502 
 
 THE HUMAK MECHANISM 
 
 Philiiopine Islands. The germ of the disease was discov- 
 ered by Koch in 1883, in the bowel discharges of cholera 
 patients in Egypt. Cholera appeared in Hamburg, Ger- 
 many, in epidemic form as late as 1892, causing great 
 alarm and about eight thousand deaths. It is easily pre- 
 vented by the same means used to prevent typhoid fever 
 
 
 ,'^ ^ ■ 
 
 
 4'''/ *■- 1 Vii 
 
 
 '.'" 
 
 
 
 
 'i^ ' 
 
 'j ' 
 
 
 
 
 'i, 
 
 [I 
 
 K--' 
 
 
 1 
 
 1 
 h 
 
 t 
 
 
 I, 
 
 
 
 Fig. 132. Microbes of Asiatic cholera 
 
 On the left, as they occur in the feces of victims of the disease; on the right, 
 after cultivation in the bacteriological laboratory 
 
 (p. 485), and need no longer be greatly feared in any clean 
 and well-ordered community supplied with pure food and 
 pure water. 
 
 26. The Plague (Bubonic Plague) is the most famous of 
 all the great epidemic diseases of history. It has repeatedly 
 ravaged Europe, and is still very common in some parts 
 of Asia, such as India and China. The Black Death, which 
 was probably a severe form of the bubonic plague, was a 
 severe epidemic disease of the fourteenth century, when 
 it is said to have killed off twenty-five millions of people, 
 or from one fourth to one half of the entire population 
 of Europe. 
 
MICEOBIC DISEASES. PEEVENTION 
 
 503 
 
 The bubonic plague is believed to be transmitted through 
 the agency of rats and fleas, — rats, like human beings, 
 being susceptible to the disease. Rats having the plague 
 are supposed to be bitten by fleas, which thus become in- 
 fected and, later, feeding upon human beings, inoculate 
 them with the germs of the disease. In order to prevent 
 
 
 
 e% / % <= a \eo " 
 
 s 
 
 
 ■ ■^■',ff:: „ ■, •■ '^.i- • 
 
 
 
 
 
 f: f\^ 
 
 ■ ^. O. 
 
 
 r'si ■ , ■"■ \''i ': 
 
 ■'■;>=*■■ 
 
 mm. 
 
 
 
 Fig. 13.3. Microbes of plague 
 
 On the left, as they occur in the swollen lymph f^land.^; on the right, after 
 citltivation in the bacteriological laboratory 
 
 the plague it is important, therefore, to do away as far as 
 possible with fleas and rats ; and in plague-stricken districts 
 preventive measures are directed chiefly to the destruc- 
 tion of rats. In the Philippines, for example, rat catching 
 is an important branch of sanitary work in combating the 
 plague. 
 
 27. The Care of Wounds. — Proper care is usually taken 
 of a severe wound by the physician or surgeon who is sum- 
 moned to attend the case. Slight cuts, on the other hand, 
 are frequently neglected as trivial affairs ; and without 
 question these cuts usually heal with no bad after-effects, 
 either because no pathogenic organisms are introduced, or 
 because, if introduced, they are killed by one or another 
 
504 THE HUMAN MECHANISM 
 
 of the means of defense possessed by the body against 
 microbic invasion. Infections from such wounds are, how- 
 ever, by no means of rare occurrence, and it is safer to 
 care for them whenever possible. The wound should be 
 washed thoroughly with some antiseptic, — such as a solu- 
 tion of corrosive sublimate (1 to 1000) or carbolic acid 
 (1 to 40), — and then protected by absorbent cotton until 
 healed. Since the bacillus of tetanus grows only in the 
 absence of oxygen, it is generally safer not to close the 
 wound with anything like collodion, which entirely pre- 
 vents access of air. The bleeding from an ordinary cut 
 presents no danger of undue loss of blood from the body, 
 and, by washing out the wound before clotting takes place, 
 is an important safeguard against infection. 
 
CHAPTER XXXII 
 
 PUBLIC SUPPLIES OF FOOD, WATER, ANB GAS. 
 PUBLIC SEWERAGE 
 
 1. Communities and Public Supplies. — Communities 
 (Latin, communis, common) may be of neigtiborhoods, vil- 
 lages, towns, or cities, and are so called because they 
 possess many things, such as roads, bridges, post offices, 
 churches, and general situation, in common. Some sup- 
 plies, such as air, sunlight, and rainfall, are naturally and 
 necessarily public and shared in common. Others, such as 
 water, ice, gas, fuel, milk and other foods, may or may 
 not be so shared. In villages some families keep their 
 own cows, while some buy their milk from their neigh- 
 bors, or from a milkman supplying several families ; but in 
 cities few people keep cows, Avhile most people buy milk. 
 In villages private wells abound, but in large communi- 
 ties most people give up private wells or springs and use 
 a public water supply. 
 
 Public supplies and public services are designed to 
 meet those wants or demands which many families have 
 in common, and the chief supplies are those of water, 
 ice, gas, and milk. Fuel, transportation facilities, libraries, 
 parks, playgrounds, baths, laundries, bakeries, and the 
 like are examples of other public supplies or services. 
 Formerly in rural life each family was comparatively inde- 
 pendent, but the modern town or city family, and to a 
 great extent the rural family, depend upon some public 
 supply for nearly everything that it has or uses ; for 
 books, upon public libraries or bookshops ; for clothing, 
 
 605 
 
506 THE HUMAN MECHANISM 
 
 furniture, household utensils, etc., upon shops or stores; 
 for food, upon markets ; and sometimes even for housing, 
 upon hotels or other public houses. 
 
 2. Public Supplies as Public Conveniences and Safe- 
 guards. — When such public supplies or services are well 
 regulated, cheap, and abundant they may often be superior 
 in safety, comfort, and convenience to private arrangements 
 for the same purpose. In a city it is easier and cheaper 
 to buy milk than it is to keep a cow. It is also better to 
 do so, because cows in cities must be under unnatural, if 
 not unwholesome, conditions, and the milk may suffer in 
 quality. It is more convenient to draw water from a tap 
 than from a well, and city wells are generally objectionable 
 because usually subject to contamination. It is more con- 
 venient, more cleanly, and safer in a city to connect a 
 house with a good sewer than to supply it with a privy 
 and a sink drain. Public supplies may thus serve not only 
 as conveniences but also as sanitary safeguards. 
 
 3. Public Supplies as Public Dangers. — On the other 
 hand, public supplies must be well arranged and well 
 regulated, or they may become sources of public danger. 
 If the water supply, for example, is allowed to become 
 polluted, a whole community may be stricken with typhoid 
 fever or some other infectious disease. Hundreds of cases 
 of typhoid fever have been known to occur among the 
 customers of a single milkman whose milk supply had 
 become infected. Sewage-polluted raw oysters have been 
 known to cause the illness of dozens of persons at a single 
 public banquet. 
 
 It is easy to see that the very convenience and wide- 
 spread use of public supplies which are not pure, makes 
 them doubly dangerous. If a private supjjly becomes 
 polluted, ordinarily only a single family suffers ; but if a 
 public supply is impure, hundreds or even thousands of 
 persons may perish. The moral is plain : tlie purity of 
 
PUBLIC SUPPLIES 507 
 
 public supplies should be iliorouglily established at the oxitset 
 ayid carefully maintained. If, as is often the case, public 
 supplies are owned or controlled by the municipality, then 
 no persons should be put in charge of them who are mere 
 politicians, or in any other way unfit to act as guardians 
 of the public welfare. Expert scientific supervision of pub- 
 lic sup>plies is indispensable for efficiency, for economy., and 
 for public safety. 
 
 4. Food Supplies, Public and Private. — The supply of 
 foods to families or individuals may be largely from private 
 sources, as in the case of a farm upon which many foods 
 may be produced. But with the growth of cities and large 
 towns food supplies are more and more shared in. com- 
 mon by many persons or families ; while certain necessa- 
 ries or luxuries of life, such as hsh, sugar, salt, tea, coffee, 
 spices, are, with rare exceptions, always obtained from pub- 
 lic supplies. 
 
 Furthermore, even on the farm, specialization often 
 leads to the raising of only one thing or a few tilings, 
 such as cotton, corn, or wheat, and so to dependence on 
 public supplies for other things which might be raised if 
 it were worth while. For example, most farmers in New 
 England might cultivate sugar maples and make from the 
 sap of their own trees a year's supply of maple sugar, the 
 purity of which they could control; but most of them pre- 
 fer to raise other things which they can sell or exchange 
 for ordinary cane sugar, of the purity of which they have 
 no knowledge. Flour nowadays is generally bought in 
 barrels or bags taken at random from the output of dis- 
 tant mills over which the buyer has no control. Meat of 
 various kinds is often purchased from a public cart, shop, 
 or market; fish and shellfish, yeast, butter, eggs, cream, 
 spices, canned and dried foods, are likewise obtained from 
 special dealers, whose stores are drawn upon by many 
 families and are therefore public supplies. Obviously 
 
508 THE HUMAN MECHANISM 
 
 impurity or adulteration in any of these public supplies 
 may injure, or at least defraud, an entire community. 
 
 5. The Impurity of Foods may be of many kinds and 
 many degrees, some of them of little or no hygienic signiii- 
 cance. An excellent spring water, for instance, may not 
 be "chemically pure" (i.e. containing nothing but water), 
 and yet may be hygienically wholesome ; and milk might 
 conceivably be somewhat adulterated with distilled water 
 without perceptible damage to the health of the com- 
 munity. 
 
 We may, for convenience, distinguish three principal 
 kinds of impurity in foods : the first kind, caused by the 
 addition of some cheaper substance, either already present 
 in the food (as of water to milk) or altogether foreign to 
 it (as of sawdust to ground spices). Such impurity is pro- 
 duced artificially, intentionally, and fraudulently, and is 
 known as adulteration. It may or may not be prejudicial 
 to health, but it is always a cheat. 
 
 The second kind of impurity of foods, known as their 
 infection, consists in the occurrence in them of parasites 
 or microparasites, such impurity being as a usual thing 
 entirely accidental and unintentional, though often due to 
 ignorance, negligence, carelessness, or uncleanliness. It is 
 always prejudicial to the public health, but is not often 
 due to a desire to cheat. 
 
 A third kind of impurity is that due to the use of unfit 
 or unclean or diseased raw materials, disgusting to the taste 
 and destructive to the appetite. This again arises either 
 from negligence or the desire to cheat. 
 
 6. The Adulteration of Foods The commonest and 
 
 most familiar adulteration of food is that of milk by water. 
 Water is so abundant and cheap, and mixes so readily 
 with milk, that it offers a constant temptation to dis- 
 honest milkmen who profit by the sale of such milk. It is 
 often difficult for the consumer to detect this adulteration, 
 
PUBLIC SUPPLIES 509 
 
 even if he suspects the cheat ; but it is easy for the chemist, 
 and large cities should keep milk inspectors and analysts 
 constantly on the watch, in the interests of the public wel- 
 fare. In some cities the fines imposed upon dishonest milk- 
 men more than repay the cost of the service. 
 
 But while milk is the food whose adulteration is most 
 familiar, it is by no means the only adulterated food. 
 Coffee, spices, beverages, sirups, honey, vinegar, and many 
 other foods are subject to serious adulteration ; and most 
 states and countries are obliged to maintain laboratories 
 devoted to the protection of the public against the adultera- 
 tion of foods and drugs. Massachusetts has such a labora- 
 tory in the Statehouse in Boston, conducted by the State 
 Board of Health. Some of its revelations are surprising 
 and instructive. Milk, for example, is treated not infre- 
 quently with artificial coloring materials, and preservatives 
 such as formic aldehyde, sodium carbonate, and boracic 
 acid. Of about one thousand samples of suspected milk 
 examined during July and August, 181>8, nearly three per 
 cent contained preservatives. Chocolate and cocoa likewise 
 have been found to be frequ!hitly adulterated with wheat or 
 sugar ; coffee with roasted peas, wheat, pea hulls, chicory, 
 and sometimes bark, wood, and charcoal ; honey with 
 cane sugar or glucose ; lard with cotton-seed oil ; maple 
 sugar with other sugars ; maple sirup and molasses with 
 glucose; pepper with rice and buckwheat; cloves with 
 bran, sawdust, and charcoal ; mustard, one of the most 
 commonly adulterated of all spices, with rice, cornstarch, 
 etc. ; cider with salicylic acid. Worse yet, some so-called 
 "patent medicines" which profess to effect cures contain 
 the very substances — such as alcohol and morphine — 
 the effects of which they are supposed to overcome. 
 
 7. The Infection of Foods by Parasites and Micropara- 
 sites. — Another, and from our standpoint much more im- 
 portant kind of impurity sometimes occurring in foods 
 
510 THE HUMAN MECHANISM 
 
 consists in their infection by disease-producing organisms, 
 such as parasitic worms or microbes, — e.g. the germs of 
 typhoid fever, scarlet fever, diphtheria, etc. Here again 
 milk has the unenviable distinction of serving as a familiar 
 example, for some of the worst epidemics of typhoid fever 
 that have ever occurred have been traced conclusively to 
 the infection of some milk supply by persons suffering 
 with that disease. 
 
 But milk is by no means the only food that may be 
 rendered impure and dangerous by infection. Raw hams, 
 bacon, and sausages may contain a kind of almost micro- 
 scopic worm called Trichina, which sometimes occurs in 
 the muscles of hogs, and which, if not killed by cook- 
 ing, is capable of developing in the alimentary canal of 
 man, boring into the tissues and encysting in the muscles 
 (especially the diaphragm), thereby causing severe dis- 
 ease and even death. Extensive epidemics of this disease 
 (trichinosis) have occurred in Germany, in America, and 
 elsewhere, due to ham and other pork products (sausages) 
 either raw or imperfectly cooked. Such foods are fortu- 
 nately seldom eaten underdone or " rare," in America, but 
 even here one should be careful to eat them only when 
 tho7-oughly cooked, or well done. It is required by law 
 that hogs exported to foreign countries shall be carefully 
 examined for Trichina, and numbers of experts are con- 
 stantly employed by the United States Bureau of Animal 
 Industry in making the necessary microscopic tests. 
 
 Other foods subject to infection and capable of con- 
 veying disease are those which are either occasionally or 
 regularly eaten uncooked ; for example, shellfish, such as 
 oysters and clams ; vegetables, such as celery, parsley, water 
 cress, lettuce, tomatoes, cabbage ; and fruits, berries, and 
 the like. The danger lies in the fact that they may have 
 been handled by persons themselves dirty and suffering 
 from infectious diseases ; or they may have been grown on 
 
PUBLIC SUPPLIES 511 
 
 fields manured with sewage or other fecal matters con- 
 taining germs of disease. For all these dangers there is but 
 one sure remedy, namely, stej-ilization ly cooking at a high 
 temperature. But this in the nature of the case is impos- 
 sible for many of the foods cited above. 
 
 Some fruits (oranges, bananas, melons, etc.) are natu- 
 rally protected by their skins, and are consequently espe- 
 cially wholesome. Others (such as cherries, plums, apples, 
 pears) should be washed thoroughly or rubbed with a damp 
 clean cloth before being eaten. Still others (grapes, rasp- 
 berries, strawberries) may be immersed in water and imper- 
 fectly washed, though they are seldom really cleaned by this 
 process. Moreover, such " washing " is apt to injure the 
 texture or flavor of delicate fruits, and is sometimes avoided 
 on that account. 
 
 After all has been said and done, preventive measures may 
 fail and some risks must be taken. The final defense must 
 often come from that vital resistance, that good general 
 health which it is the special object of hygiene to secure and 
 promote. Life is valuable and health is jirecious, but either 
 or both may be safeguarded at too great cost. L'ndue anx- 
 iety about foods, or even about life and death, is unworthy 
 of those who have at most but a few short j-ears to live, and 
 who in those few years have many better things to do than 
 merely to keep alive. " 'T is not the whole of life to live." 
 
 8. Food Preserving and Preservatives. — Processes such 
 as canning, and cold storage in wells, cellars, refrigerators, 
 etc., are of immense value to the human race as conveniences 
 and for the saving of surplus foods. The packing of pork 
 in brine, the salting, smoking, and drying of fish, the 
 " corning " of beef, and the pickling of vegetables are 
 familiar examples of other kinds of food preserving. In 
 these latter cases the foods are saved from sjDoiling by sub- 
 stances (brine or vinegar) which inhibit the growth of putre- 
 factive microbes and are therefore called antiseptics. There 
 
512 THE HUMAN MECHANISM 
 
 are many other antiseptics besides brine and vinegar, and 
 chemistry is constantly adding to the number. Some of 
 the more important are boracic acid, formaldehyde (for- 
 malin or formol), and salicylic acid. 
 
 A difficult and delicate question arises when we come to 
 ask. Does the introduction of chemical antiseptics into foods 
 make them impure or dangerous ? It is obvious that the use 
 of salt to preserve fish, of brine for packing pork or corning 
 beef, of smoke for preserving fish, hams, and dried beef, and 
 of vinegar for pickling have been approved and sanctioned 
 by generations. On the other hand, the use of boracic acid or 
 formalin in milk is an undesirable practice, and at present 
 the employment of any chemical antiseptic in food preserv- 
 ing must be regarded as of doubtful justification. 
 
 Some food substances contain acids which may attack 
 the tins in which they are put up for the market. Blue- 
 berries, for example, readily corrode tin cans, forming salts 
 of tin which in large amounts are harmful. The use of 
 glass is therefore preferable for preserved foods whenever 
 practicable ; but the long-continued and very extensive use 
 of tin cans for tomatoes, peas, beans, pears, etc., without 
 known harm, indicates that for many foods tin cans may 
 be used without much, if any, danger. 
 
 Sometimes food products, such as peas, clams, etc., are 
 chemically treated in order to make them more attractive. 
 French peas (canned) have frequently been found to con- 
 tain copper, and canned clams are sometimes bleached to 
 make them whiter. It is needless to say that such treat- 
 ment is almost always objectionable, even if not positively 
 dangerous. 
 
 The best preservative, hygienically speaking, is heat, 
 and this, carefully applied, may be made wonderfully 
 effective. The processes of canning and preserving are 
 too familiar to need description, but it is not always under- 
 stood that if the temperature employed is high enough and 
 
PUBLIC SUPPLIES 513 
 
 sufficiently long-continued it is of great hygienic value, 
 because it tends to destroy any disease germs which may 
 be present. It is, in brief, a kind of cookery, and cook- 
 ing tends not only to preserve but also to purify foods. 
 
 9. The Purity of Public Water Supplies. — Public water 
 supplies should be derived from the purest possible sources. 
 Villages and small cities are often supplied from driven 
 wells or open basins located near a lake or a river, and 
 thus receive gromid ivater (see p. 451). Large cities and 
 many small ones often secure their supplies from lakes, 
 ponds, or rivers, or from smaller streams, the water of which 
 is stored in reservoirs. Supplies of this sort are called 
 surface-water rather than ground-water supplies, and the 
 water from them is naturally softer (see p. 4.51). 
 
 Ground-water supplies are apt to be of good quality but 
 limited in quantity. Surface-water sup2:)lies are generally 
 ample in quantity but more easily subject to pollution. For 
 this reason they should not, as a rule, be drawn from thickly 
 inhabited districts or from rivers, lakes, or smaller streams 
 into which sewage or other polluting matters may find their 
 way ; and they should never be drawn from such sources 
 unless they have first been purified in some manner. 
 
 Some cities, like Brooklyn (New York) and Lowell 
 (Massachusetts), rely for their public water supply in part 
 or wholly upon driven wells ; some, like Boston, New 
 York, and Liverpool, upon water collected in large reser- 
 voirs from streams upon comparatively uninhabited water- 
 sheds ; and some, like Philadelphia, Paris, St. Louis, Lon- 
 don, Hamburg, Lawrence, Albany, upon impure river water 
 which is purified by filtration, or otherwise treated, before 
 it is distributed to the citizens.^ 
 
 1 The student, unless already informed, should familiarize himself -with 
 the sources and the possibility of pollution of the public water supply, if 
 any, of his own village, town, or city, and should satisfy himself, if pos- 
 sible, as to its purity. 
 
514 THE HUMAN MECHANISM 
 
 It was formerly thought that running water sufficiently 
 purified itself, although as early as 1874 a Royal Commis- 
 sion of experts on water supply reported in England that 
 " there is no river in the United Kingdom long enough to 
 purify itself from any sewage introduced into it even at its 
 source," and the river Thames is more than two hundred 
 miles long. It is true that sewage or other filth in streams 
 often disappears, and that great improvement in polluted 
 streams frequently takes place; but such "self-purifica- 
 tion" is too often partial, incomplete, and untrustworthy. 
 
 In many cases the disappearance of obvious pollution is 
 due to a mere dilution of the filth with purer water, and 
 such dilution may greatly improve or even " purify " it. A 
 drop of ink in a quart of water makes a mixture far less 
 inky than the original drop. On the other hand, dilution 
 does not necessarily mean destruction. A flock of birds 
 may be lost sight of, but not destroyed, by scattering, and 
 the purification of sewage filth should mean its destruction 
 as such and its conversion into harmless substances. Much 
 true purification does take place in a flowing stream, but 
 tliis is not usually adequate, and towns and cities nowa- 
 days are generally turning toward filtration, or other arti- 
 ficial treatment on a large scale, of waters which for any 
 reason are suspected of possible contamination. Some of 
 these municipal purification works are elaborate and costly, 
 as, for example, those in Albany, Philadelphia, St. Louis, 
 Ithaca (New York), Lawrence (Massachusetts), Washington. 
 
 10. Public Gas Supplies and their Dangers There is 
 
 no more danger from the products of combustion of illu- 
 minating gas than from those of oil or other illuminating 
 materials. The air of rooms naturally becomes heated and 
 more or less vitiated by these products, just as it does by 
 human I>reath or any other waste product of oxidation; 
 but illuminating gas properly burned is no more danger- 
 ous to life than is kerosene oil or any similar illuminant. 
 
PUBLIC SUPPLIES 515 
 
 Unburned gas, on the other hand, escaping from pipes or 
 fixtures is often extremely dangerous, both because it is 
 poisonous and because in certain proportions it forms with 
 air an explosive mixture. 
 
 Illuminating gas is generally either "natural " gas drawn 
 ready-made from the earth, or gas made from gasoline, oil, 
 wood, coal, or coal and water, and hence known as " oil 
 gas," " wood gas," " coal gas," or " water gas," as the case 
 may be. 
 
 11. Natural Gas consists chiefly of marsh gas, or methane 
 (CH^), this making from ninety to ninety-seven per cent 
 of the whole. It never contains more than one half of 
 one per cent of carbonic oxide (CO), — a quantity too small 
 to do serious damage. Though irrespirable, — that is, not 
 fitted to support life, — natural gas is not poisonous. It 
 may even leak into an apartment in considerable quanti- 
 ties without endangering life or seriously damaging health. 
 
 12. Coal Gas is made by distilling " soft " or bituminous 
 coal, and consists chiefly of hydrogen and marsh gas, with 
 smaller amounts of carbonic oxide and other compounds of 
 carbon. It contains from six to ten per cent of carbonic 
 oxide, a highly poisonous gas, and cannot be admitted into 
 living or sleeping rooms in any great quantity without ex- 
 treme danger to life. It also readily forms explosive mix- 
 tures with air. 
 
 13. Water Gas is made by passing steam over red-hot coal 
 or coke (carbon), which decomposes the water vapor, pro- 
 ducing, among other gases, an abundance of carbon monox- 
 ide. As it leaves the generator, water gas burns with a 
 pale blue flame only. For lighting purjDoses it is therefore 
 enriched by the addition of naphtha or other vapors which 
 give it good illuminating qualities. But even after this 
 treatment water gas generally contains from twenty-five 
 to thirty per cent of carbonic oxide and is therefore ex- 
 tremely poisonous. 
 
516 THE HUMAN MECHANISM 
 
 In cities supplied with water gas, cases of asphyxiation 
 and death from gas poisoning are common. These come 
 chiefly from ignorance (in blowing out the gas instead of 
 shutting it off) or carelessness (in turning the gas on again 
 after extinguishing the light), or from suicidal intent, or 
 drunkenness, or from leaky fixtures, or from change of 
 pressure, — a light turned low being extinguished by a 
 decrease of pressure in the pipes and the gas escaping into 
 the room afterwards when the pressure is renewed. 
 
 The most remarkable (and often the most extensive) 
 cases of poisoning by illuminating gas are those in which 
 the inliabitants of houses or apartments not piped for gas 
 have been poisoned by gas which has escaped from a 
 broken or leaking main in an adjoining street. In these 
 cases the gas makes its way underground to the base- 
 ment of the house in question, and then, perhaps partly 
 robbed of its warning odors by passage through the earth, 
 rises through the house to sicken or kill those within. 
 Whole families, and even groups of families, have occa- 
 sionally been poisoned in this way, even in houses or tene- 
 ments not piped for gas at all. The fact is that heated 
 houses act like cliimneys in producing a strong up-draft ; 
 and in winter, when windows and doors are shut tight, this 
 draft sucks in air from the surrounding ground. If the 
 ground air happens to be charged with gas from a leaky 
 main, both air and gas may enter the house and sicken 
 or even kill the inmates, although the house itself is not 
 supposed to receive any gas. It has been estimated that 
 " fourteen per cent of the total product of gas plants leaks 
 into the streets and houses of the cities supplied." 
 
 Headaches and malaise (a convenient term for " feeling 
 poorly ") may be caused by small and imperceptible leaks 
 of illuminating gas, and great care should be taken to 
 have all gas fitting well done, and all leaky joints or fix- 
 tures made perfectly tight, especially if the gas used is 
 
PUBLIC SUPPLIES 517 
 
 water gas, now very generally supplied to the public in 
 American cities. 
 
 One of the great advantages of lighting houses by elec- 
 tricity is that it does away with all possibility of gas 
 poisoning except that from leaky mains in public streets, 
 already referred to. 
 
 The use of gas for heating and cooking requires especial 
 caution, owing to the large quantities used and the tem- 
 porary connections often employed (pp. 438 and 440). 
 
 14. The Purity of Public Milk Supplies. — Milk is one 
 of the most universal and most important of foods. It is 
 also one of the most peculiar, in that it is a secretion drawn 
 directly from the bodies of living animals. This remark- 
 able animal secretion, when fresh, is very sweet, smooth, 
 and bland to the taste, but on exposure to the air generally 
 spoils quickly and sours. It is obviously not the air alone 
 which causes it to sour, for milk is easily kept sweet a long 
 time if kept in a cold place, or if scalded when it threatens 
 to turn sour. 
 
 The spoiling and souring of milk are caused by certain 
 bacterial microbes which, having got into the milk as it 
 was drawn, or later from dust, air, dirt, or unclean pails or 
 strainers, live and multiply enormously at the expense of 
 the sugar and other food stuffs which milk contains. The 
 so-called lactic-acid bacteria, in particular, thrive in milk, 
 especially if it is kept warm, and spoil it by converting 
 the milk sugar (lactose) into milk acid (lactic acid). 
 
 Milk that is pure sliould be free from dirt, and sweet 
 rather than sour, but such milk is unfortunately not always 
 easy to obtain, especially in cities. A black sediment in 
 milk indicates dirt (usually cow dung), and so does a 
 "cowy" taste. Milk may also be adulterated with water, 
 with antiseptics, or with other substances, as has been 
 shown above (p. 509) ; but the most serious impurity in 
 public milk supplies is the occurrence oi (/enns of contagious 
 
518 THE HUMAN MECHANISM 
 
 or infectious diseases. Many epidemics of typhoid fever 
 and diphtheria have been conclusively traced to a public 
 milk supply which served as the unsuspected vehicle of 
 the disease. In all of these cases uncleanness of some sort 
 — on the farm, in the dairy, among the milkmen, or else- 
 where — is believed to have been always at the bottom of 
 the trouble. 
 
 Persons supplying milk to the public should take pains 
 to keep their cows healthy and their cow stables clean; to 
 milk only after careful washing of the hands, pails, cans, 
 strainers, etc., and also only after washing the udders of 
 the cow ; and it should always be remembered that milk is 
 a rich animal secretion which readily sujDports bacterial 
 life and therefore should be scrupulously guarded against 
 any invasion of dirt or disease. To secure rich, pure, clean, 
 and fresh milk in cities, a higher price must probably be 
 paid than has been the custom hitherto. The demand is 
 for better, purer, cleaner milk ; and for this it is reasonable 
 to expect that more must be charged. 
 
 It should also be remembered that the number of bacteria 
 in milk, unlike that in water, does not depend simply on 
 the number that get in, since germs multiply very rapidly 
 in this rich food supply. Hence milk as soon as drawn 
 should be chilled as far as possible before delivery. The 
 mere souring of milk lessens its digestibility, especially 
 in the case of infants, so that it is a matter of hygienic 
 imjjortance, particularly in warm weather, to lessen the 
 growth of bacteria in it by immediate cooling as soon 
 as drawn from the cow, and keeping as cold as possible 
 afterwards. 
 
 It must also be borne in mind that the milk-producing 
 industry, while one of the oldest known to man, is still 
 largely in a primitive condition. What is needed is a more 
 scientific knowledge of the subject, more intelligence, skill, 
 and cleanliness among those engaged in it, and, finally, 
 
PUBLIC SUPPLIES 519 
 
 expert supervision both on the part of the producer and of 
 the sanitary authorities of cities. 
 
 15. Public Sewerage and the Disposal of Sewage. — One 
 of tlie most beneficent procedures in any community is 
 the establishment of a system of public drains which shall 
 quickly and effectually remove all liquid and many solid 
 wastes, especially the excreta of human beings and other 
 animals. Well-built sewers not only do this but also carry 
 off much " ground " water, making the soils of cities drier 
 and therefore more wholesome. The term "sewerage" is 
 applied both to the act of draining and to the system of 
 sewers; the word "sewage," to the contents of sewers. 
 
 The disposal of the sewage of cities and towns is often 
 a very serious, difticult, and costly problem. Sometimes the 
 sewage can be safely emptied into a river, a lake, or the 
 sea, but more often it is necessary to purifi/ it, either upon 
 land (where it may be made useful, though rarely profit- 
 able, for agricultural purposes), or by chemical treatment, 
 or by microbic (bacterial) action during cesspool or filtra- 
 tion processes. The proltlem of the final disposal of sew- 
 age is not yet fully solved, and at the present time is 
 engaging the anxious attention of the world's ablest sani- 
 tary engineers. 
 
CHAPTER XXXIII 
 
 THE HYGIENE AND SANITATION OF TRAVELING, PUBLIC 
 CONVEYANCES, PUBLIC HOUSES, ETC. 
 
 1. Migration, Past and Present. — One of the most strik- 
 ing characteristics of the present as compared with the 
 past is the increased and increasing movement of masses 
 of people not only permanently out of one country (emigra- 
 tion) and into another (immigration), but also temporarily 
 from place to place, and back and forth (traveling). Such 
 migration inevitably removes the traveler, temporarily at 
 least, from one environment, and subjects him to another 
 and often very different one ; so that from the hygienic 
 point of view a change of this sort is of great impor- 
 tance and interest. It also often affects the environments 
 of others besides the migrant himself, by introducing into 
 those environments new elements of disease. 
 
 2. Traveling and Change of Scene. — Even before start- 
 ing upon a journey conditions for the prospective traveler 
 have often begun to change. The bustle and the thought 
 of the necessary preparations constitute a kind of excite- 
 ment, sometimes pleasurable, sometimes wearisome, accom- 
 panied, it may be, by temporary loss of appetite or even 
 sleeplessness (especially in children), or by other abnormal 
 conditions sometimes described by the phrase "journey 
 proud." With the start come leave takings, farewells, and 
 partings more or less unusual and exciting, and then begins 
 a series of tolerably rapid changes of environment or scene. 
 The body is moving and possibly shaken about or jarred ; 
 unusual and shifting scenes fall upon the retina and come 
 
 520 
 
HYGIENE OE TRAVELING 521 
 
 before the mind, calling for attention and arousing new sen- 
 sations ; strange sounds are heard, strange odors detected; 
 the air (if in an open vehicle) beats against the face and the 
 ordinary atmospheric " blanket " is diminished or otherwise 
 interfered with. 
 
 Arrived at a stopping place or the journey's end, streets, 
 houses, and hotels are new or strange ; strange faces meet 
 the traveler ; there are strange rooms and walls, strange 
 furnishings, strange sounds and odors, — in short, a strange 
 or unusual environment. 
 
 All this may or may not be wholesome, according to 
 circumstances on the one hand and the individual on the 
 other; but it is certainly stimulating and physiologically 
 exciting, as may be readily proved by observing its effects 
 upon children and the aged. The change of scene is only 
 one element in the hygiene of travel, and its value must be 
 determined by weighing it together with other equally influ- 
 ential factors, namely, the change of occupation, the change 
 of air, and the change of food, and finally by applying all 
 of these considerations to particular cases or individuals. 
 
 3. The Change of Occupation. — It is an old saying that 
 " all work and no play makes Jack a dull boy," and expe- 
 rience teaches clearly enough that a change of occupation 
 is wholesome. One of the best features of travel is that 
 it necessitates a change of occupation. A common expres- 
 sion contains the idea of " going away from home to get 
 a change." One of the most valuable characteristics of the 
 home is the repose and restfulness which result from its 
 uniformity of conditions, and one of the best things about 
 travel is the mild stir and excitement involved. Routine 
 and regularity of occupation are on the whole the more 
 natural and normal, and " a steady job," whether it be in 
 shop, mine, or factory ; on farm, plantation, or shipboard ; 
 in bank, school, or professional life, is naturally sought and 
 prized by everybody. 
 
522 THE HUMAN MECHANISM 
 
 And yet most persons profit from time to time by "a 
 day off," or a vacation, or a journey which affords change 
 of occupation with freedom from responsibihty. Once on 
 the way, tlie traveler is not responsible for the train or the 
 steamer, for the cookery or the beds, for the house or hotel, 
 or its furnishings or management ; and this freedom from 
 responsibility is a complete and often refreshing change. 
 
 4. The Change of Air. — It is difficult to say in what 
 way and to what extent a change of air is beneficial in 
 traveling. Much of the benefit, even when attributed to 
 the " change of air," is no doubt really due to other things, 
 such as the change of work and the change of scene ; but 
 after making all allowances, it would still seem to be true 
 that a change of air has a perceptible effect, and often does 
 great good or great harm. Air that is drier or damper, 
 or warmer or cooler than usual, or air in the forest or by 
 the sea, often seems to have decided effects for good or for 
 evil, all other conditions remaining apparently much the 
 same. At times, obscure atmospheric influences at home, 
 unknown to ourselves, may be the source of lessened vital 
 resistance, and so of a lowered tone of general health, and 
 the change of air may be the means of restoring nor- 
 mal conditions by removing the obscure cause of trouble. 
 Moreover, when the change is from the close, "stuffy " air 
 of an office to the open air of country or of sea, with their 
 agreeable odors, there is a " bracing " or stimulating effect 
 which reacts favorably upon the entire constitution, but 
 especially upon the nervous system. The tendency to "fill 
 our lungs " with it is only a sign of the general beneficial 
 influence upon the system as a whole. Many a case of 
 " the blues " has been successfully overcome by this simple 
 expedient. 
 
 On the other hand, air as a vehicle of infection may 
 affect the traveler unfavorably; for he must almost inevi- 
 tably be exposed to air (as well as to other things) which has 
 
HYGIENE OF TEAVELING 523 
 
 recently been in contact with persons having incipient tuber- 
 culosis, diphtheria, measles, typhoid fever, or other infec- 
 tious diseases. To this subject we shall return in the next 
 section but one. 
 
 5. The Change of Food. — It is uncertain how much or 
 how little influence a change of food has upon the organ- 
 ism. It is commonly believed that a change of food is 
 often beneficial, or the reverse, and that much of the good 
 or bad effects of travel is due to the inevitable change of 
 diet and of cookery which goes with it. How far this is 
 true is unknown, but it is easy to see that a simpler diet 
 for some and a more abundant diet for others may in 
 itself alone be helpful. It is doubtful if any special virtue 
 resides in "sea food," or in " country living," or in " camp 
 cookery," apart from that which consists in its palatability 
 or its novelty, — qualities which affect appetites and there- 
 fore nutrition ; but in so far as a change makes food appe- 
 tizing or acceptable, such food is, of course, more valuable 
 to the body. However this may be, there can be no ques- 
 tion about the increased danger of infection from food and 
 drink taken at random from unknown sources. 
 
 6. The Dangers of Infection away from Home. — At home 
 the traveler, in theory at least, has an environment well 
 under his control ; but when he starts upon a journey, — 
 whether afoot, or riding, or driving ; by bicycle, railway, 
 steamship, or other means of conveyance, — he enters into 
 new environments, of whose precise nature lie is ignorant, 
 and which are usually beyond his control. Of the san- 
 itary or unsanitary condition of the water supply, ice 
 supply, milk supply, etc., he is entirely ignorant ; and he 
 may at any time be thrown in contact with persons suf- 
 fering from infectious diseases, especially in a mild or 
 incipient form. The public vehicle (carriage, wagon, car, 
 or omnibus) in which he travels, the hotels, rooms, chairs, 
 and even the beds which he uses may have been recently 
 
524 THE HUMAN MECHANISM 
 
 occupied by diseased persons. His laundry work may be 
 done or delivered by workers suffering from contagious 
 diseases ; uncleanness may attend the preparation and serv- 
 ing of his food. In short, in leaving his own familiar 
 and controllable environment and passing into others unfa- 
 miliar and beyond his control, the traveler clearly takes 
 large risks. 
 
 7. Safeguards of the Traveler. — If it be asked what 
 one can do to protect himself or his family from the dan- 
 gers of travel, it may be pointed out, in the first place, that 
 jt is often better not to travel at all. When one is in poor 
 condition, although a change to some new scene whose 
 hygienic conditions are known to be good is likely to be 
 beneficial, a railroad journey with frequent stops is apt to 
 increase the danger of infection at a time when vital resist- 
 ance is low. When a journey is necessary the traveler 
 should try to avoid marked fatigue, which always dimin- 
 ishes vital resistance and thus predisiDoses to disease ; he 
 should seek to avoid unclean hotels, unclean conveyances, 
 badly aired rooms, and unclean fellow-travelers ; he should 
 avoid the use of public drinking cups, public towels, pub- 
 lic razors, and the like ; he should, if possible, drink only 
 waters of estahlished reputation ; he can, if need be, forego 
 the use of raw milk, raw oysters, and other uncooked 
 foods the antecedents of which he knows nothing about, 
 and he can take other obvious and useful precautions that 
 will suggest themselves as he goes along. 
 
 But, after all, it must be admitted that precautions, 
 even if rigorously observed, will often prove insufficient, 
 and also that too much thought about them, or about the 
 dangers of travel, would rob it of most of its advantages. 
 People always have traveled and probably always will 
 travel without much consideration of the dangers involved 
 in ti'aveling. Some risks must always be taken, even at 
 home, and most travelers cheerfully accept the necessary 
 
HYGIENE OF TRAVELING 525 
 
 risks for the sake of the gains to be derived. With the 
 increase in the amount of traveling, many of the risks 
 are gradually decreasing, and in highly civilized countries 
 adults in robust health who know how to take care of 
 themselves may now go upon a journey without very much 
 more risk of infection than they would undergo if they 
 stayed at home. This is probably less true of children, for 
 children and old people are not only more easily excited 
 and more easily fatigued, but they also suffer more severely 
 from exposure, and children are especially apt to contract 
 infectious diseases when away from home. 
 
 8. Public Drinking Cups should be avoided by travel- 
 ers, theater goers, and all persons in parks or other public 
 places. Few sights are more distressing to a sanitarian 
 than to see (on a hot day in a crowded railway car) jnen, 
 women, and children, of all ages, sorts, and conditions, 
 clean and unclean, sick and well, one after another in 
 rapid succession applying their mouths to the one public 
 drinking cup. If the student will once carefully observe 
 for himself the use to which this cup is put during even a 
 short journey under such conditions, he will realize that 
 every traveler had better carry his own drinking cup, or, 
 in default of this, go thirsty. In some theaters, between 
 the acts, trays containing glasses of water are passed to 
 patrons in their seats. Here also the lips of many persons 
 touch successively the same glasses, and one wlio is wise 
 will avoid the obvious danger involved in using one of 
 these glasses, which may have become infected. Sani- 
 tary drinking fountains in which, by a simple device, the 
 obnoxious common drinking cup is made unnecessary, are 
 now being gradually introduced in parks, schools, and 
 other public places. 
 
 9. The Influence of Travelers upon the Environment. — 
 We have thus far considered chiefly the effects of strange 
 environments upon the traveler, but before leavmg the 
 
526 THE HUMAN MECHANISM 
 
 subject we must not fail to point out some of the reactions 
 of travelers upon the environments in which they journey 
 or linger. Many epidemics of infectious diseases have 
 sprung from germs left by travelers, and most of the 
 great plagues and pestilences of history have followed the 
 routes of pilgrims, caravans, crusaders, conquerors, traders, 
 or travelers. " Walking cases " of typhoid fever, diphthe- 
 ria, etc., are perhaps most dangerous to the public health, 
 and tramps, peddlers, and other roving characters do much 
 to spread disease. Persons coming down with an infec- 
 tious disease, such as typhoid fever, are very apt to leave 
 off work and go a-fishing, sometimes upon or along the 
 shores of a public water supply, which tliey may unwit- 
 tingly contaminate. Life away from home has its dangers 
 for the traveler; it is no less true tliat life at home has 
 its dangers, these often arising from travelers themselves. 
 
 10. Public Conveyances, because they are used promis- 
 cuously by the well and the ailing alike, are subject to 
 infection, and for this reason carriages, cars, and steamboats 
 should be kept clean and occasionally should be thoroughly 
 disinfected. Steamboats and steamships are essentially 
 floating hotels, and should be treated as such. Sleeping 
 cars bear less resemblance to public houses, and may be 
 cleaned partly by washing, partly by blasts of compressed 
 air, and partly by disinfectants, and when properly cared for 
 are less likely to endanger health than are many hotels. 
 Their lavatories should be kept scrupulously clean and 
 should be frequently disinfected. In modern times vast 
 improvements have been made in all kinds of public con- 
 veyances in the direction of greater steadiness, less noise, 
 better heating, better air, and better lighting. The public 
 drinking cup, however, remains as objectionable as ever, 
 and its use ought to be abolished. 
 
 11. Public Houses. — Hotels and other public houses 
 may be either clean, wholesome, and restful, or unclean. 
 
HYGIENE OF TKAVELING 527 
 
 noisy, and unsanitary. Owing to the fact that their popu- 
 lation is constantly changing they are far more exposed to 
 infection than are private houses, and great pains should 
 be taken to keep thenr always in good sanitary condition. 
 The simplest (iron) bedsteads are the best, and in hotels all 
 carpets, draperies, etc., should either be avoided or sub- 
 jected to frequent and thorough cleaning. The kitchen, 
 especially, requires careful supervision to insure cleanliness, 
 and in the laundry the linen should be so treated as to be 
 sterilized during the process of washing. Employees should 
 be instructed in the art of cleanliness, and any suffering 
 from contagious or infectious diseases should be excluded 
 or quarantined. All lavatories should be kept scrupulously 
 clean and should be frequently disinfected. 
 
 12. Public Places, such as streets, j^arks, plaj^grounds, 
 and cemeteries, are dangerous only when infected. Dirty 
 streets are unsightly and disagreeable, but it is very hard to 
 trace the source of much disease directly to them. Never- 
 theless, few things sooner or more agreeably impress a 
 visitor than clean streets, and in the lower portions of the 
 town or city clean streets are particularly important because 
 the streets are the home and the playground of the children 
 of the poor. Pavements in cities should be hard and non- 
 absorbent rather than porous, and should be kept clean 
 and free from rubbish. 
 
 13. Public Parks are desirable for fresh air, recreation, 
 rest, and change of scene, and in these respects are impor- 
 tant hygienic factors in city life. They are of special 
 benefit to those living in tenement houses or under crowded 
 conditions. Public playgrounds minister to the needs of 
 the same class of people. Their importance can hardly be 
 overestimated, since they furnish to city children almost 
 the sole opportunity for normal physical development and 
 some contact witli nature. It has been well said that "the 
 boy without a playground is the father of the man without 
 
528 THE HUMAN MECHANISM 
 
 a job." But here also wise supervision and cleanliness are 
 the conditions of hygienic success. 
 
 Public cemeteries in America are usually well conducted 
 and unobjectionable from the hygienic standpoint. The 
 objections sometimes urged against them as centers of 
 infection and sources of disease are seldom well founded. 
 Cremation, or the burning of the dead, is slowly but stead- 
 ily growing in favor and has much to recommend it from 
 the sanitary standpoint, since it prevents slow decay and 
 destroys completely all germs of disease. Near most of 
 the larger American cities there are now one or more 
 crematories. 
 
CHAPTER XXXIV 
 PUBLIC PROTECTION OF THE PUBLIC HEALTH 
 
 1. The Public Health. — By this term is meant the 
 health of the community ; and of some community every 
 family and every individual is a member. The public 
 health is obviously of vital importance to the individual, 
 and, conversely, the health of the individual is of vital 
 importance to the comnnmity. Personal hygiene, or the 
 hygiene of tlie individual, and public hygiene, or the 
 hygiene of the community, are thirs closely bound together. 
 Not only because it is his duty, but also because from the 
 selfish point of view it is to his advantage, the individual 
 should, therefore, interest himself in and seek to promote 
 the public health. If, for example, small|)Ox appears in 
 his community, he cannot afford, even from a selfish point 
 of view, to fail to do liis best to aid in suppressing it. 
 Conversely, if he himself falls ill of smallpox, his neigh- 
 bors and the whole public naturally feel a similar interest 
 in isolating him and preventing the spread of the disease. 
 
 FoT these and similar reasons, people living in commu- 
 nities, and especially in villages, towns, and cities, by com- 
 mon consent usually elect or appoint a few of their own 
 number as sanitary authorities or officials to attend to 
 matters affecting the public liealth. The citizens thus 
 chosen are endowed with special powers and privileges, 
 and, taken together, are generally designated as the Board 
 of Health, or Health Commissioners. Sometimes, especially 
 in small communities, there is no formally organized board 
 of health, the duties of such a board being performed by 
 
 529 
 
530 THE HUMAN MECHAJSTISM 
 
 some governing body of the community, such as the 
 selectmen, etc. 
 
 2. Boards of Health, their Powers and Duties. — Very 
 much as boards of police are chosen by the people to pre- 
 serve public order and to prevent disturbance and crime, 
 so boards of health are chosen to preserve the public 
 health and prevent disease and death. And as the police 
 officer could not possibly do tlie work assigned to him 
 without unusual powers and privileges, these sometimes 
 involving a considerable interference with personal liberty, 
 so the health officer cannot do the work expected of him 
 without unusual powers and privileges. But it should 
 never be forgotten that in each case both the officers 
 themselves and the powers which they possess exist by 
 the common consent of the community, which desires, and 
 thus provides for, protection at the cost of surrendering 
 some personal rights and privileges. Boards of this sort 
 derive their powers solely from the consent of the majoritj' 
 of the community which they serve, and those members of 
 the community who disapprove of their existence, powers, 
 and acts must either persuade the majority to do other- 
 wise, or must submit, or must go elsewhere. 
 
 Among the important pozoers of hoards of health are the 
 right of quarantine, isolation, entrance and search, and 
 vaccination. A board of health may, in many cities of 
 the United States, detain a vessel, perhaps full of pas- 
 sengers impatient of delay and eager to land, at quarantine, 
 even for many days, subjecting the owners, passengers, and 
 others to great inconvenience, expense, and damage. A 
 board of health, finding smallpox in a hotel or boarding 
 house, may quarantine or isolate the building, surround it 
 by police, and forbid all persons to enter or leave it, thus 
 causing great alarm and annoyance to the inmates, great 
 damage to the proprietor, and a heavy expense to the com- 
 munity. A board of liealth may declare general vaccination 
 
PUBLIC PROTECTION OF PUBLIC HEALTH 531 
 
 necessary for the protection of the public health, and may 
 even enforce vaccination upon the careless, reluctant, or 
 resisting. It may forbid a dairyman to sell milk thought 
 to contain typhoid fever or other disease germs, thus caus- 
 ing the dairyman great inconvenience and even financial 
 ruin. In all these cases the board is, as a rule, simply 
 obeying the wishes of a majority of the community, and 
 those who are delayed, constrained, or financially injured 
 have to submit as best they may, unless the general senti- 
 ment of the community undergoes a change in their fa\'or. 
 The duties of hoards of health are manifold. Some of the 
 most obvious and general are usually prescribed by public 
 statute or ordinance. Such, foi' example, are the following 
 in the state of Massachusetts : 
 
 The State Board of Health shall take co,unizai)CP of the interests 
 of health and life ainoug' the citizens of the Commonwealth. It shall 
 make sanitary investigations and inquiries in respect In the causes 
 of disease, and especially of epidemics and the sources of mortality, 
 and the effects of localities, employments, condififins, and circum- 
 stances on the public health; and shall gather such information in 
 respect to those nurtters as it may deem projier, for diffusion among 
 the people. It sliall advise the government in regard to the location 
 and other sanitary conditions of any public institutions. 
 
 Others are less general and more specific, like the fol- 
 lowing : 
 
 The State Board of Health shall have the general supervision of 
 all streams and ponds used by a city or town as sources of "water 
 supply, with reference to their purity, together vs'ith all springs, 
 streams, and water courses trilnitary thereto ; andshall have authority 
 to examine the same from time to time and inquire what pollutions 
 exist and ^Yhat are their causes. 
 
 When the Board of Health of any city or town has had notice of 
 the occurrence of a case of smallpox or of any other disease dan- 
 gerous to the public health in such city or town, such Board of Health 
 shall, within twenty-four hours after the receipt of such notice, notify 
 the State Board of Health of the same. 
 
53:^ THE HUMAN MECHANISM 
 
 A nation may and should have a national sanitary 
 authority charged with the protection and promotion of 
 the public health and provided with large powers ; it 
 should also be supplied with trained experts and money 
 enough to enable these to deal with emergencies, to study 
 large practical problems, and to carry on researches into the 
 causes of disease and the improvement of methods for their 
 prevention. Germany has such an organization in its Impe- 
 rial Board of Health, and the United States, for a short 
 time, had a National Board of Health. At present the 
 United States Public Health and Marine Hospital Service 
 does this same work to some extent. 
 
 The states also, in the United States, have for the most 
 part their own boards of health; but such boards do not, 
 as a rule, have very large powers, these being reserved for 
 the so-called " local boards " of the various cities and towns. 
 It is believed by many experts that a larger grant of powers 
 and resources to state and national boards would be of 
 immense benefit to the public, and would secure for all a 
 much more constant and efficient sanitary protection. 
 
 3. What the Individual may do to protect the Public 
 
 Health The first duty of the individual to the public 
 
 health is to remember that he himself, his family, his house, 
 and all his belongings constitute one important and funda- 
 mental element in the health of the community of which 
 he is a unit. He should therefore seek, first of all, 
 to maintain and promote good health in himself, in his 
 family, and in all his household ; for the prevention of 
 disease and premature death in one household is a distinct 
 and genuine contribution to the public health of all other 
 households. 
 
 In the next place, he should cheerfully conform to all 
 reasonable regulations of the board of health or other sani- 
 tary authority of his community, duly prescribed by them 
 under powers conferred by the community as a whole. 
 
PUBLIC PROTECTION OP PUBLIC HEALTH 533 
 
 Finally, he should inform himself as fully and as accu- 
 rately as possible upon hygienic and sanitary subjects, in 
 order not only to protect and promote his own health and 
 that of his household, but also to enable him to become 
 an intelligent, critical, and yet cooperative member of the 
 community, thus doubly aiding in preserving and promot- 
 ing the public health. 
 
 Having done, or tried to do, these three things, the good 
 citizen has still one further duty of the utmost importance 
 to perform for the maintenance and betterment of the 
 public health, — which, as we have shown above is also 
 of great consequence both to himself and his family, — and 
 that is to aid and assist in all their good works boards of 
 health and all others in sanitary authority. Tliis, one may 
 do by reporting the existence of cases of infectious disease, 
 nuisances, etc. ; by trying to secure the election or appoint- 
 ment of intelligent, upright, and expert oflicials ; by loj'ally 
 upholding such officials in the performance of their dut}' ; 
 by refusing to countenance opposition to necessary public 
 procedures such as vaccination, gas inspection, plumljing 
 inspection, the f)lacarding of houses containing cases of 
 infectious disease, the isolation of patients, etc., and in 
 many other ways which are certain to arise. 
 
 At times this individual responsibility for public health 
 involves personal inconvenience and hardsliip, severely 
 testing the good citizenship even of those most desirous 
 of cooperating with the public health authorities. This is 
 well illustrated in the case of diphtheria. When antitoxin 
 is given in this disease, the toxin produced by the bacteria 
 is neutralized in the blood and tissues and is thus pre- 
 vented from injuring the organism ; but not all the bacteria 
 are immediately killed by this treatment. Consequently it 
 sometimes happens that, long after the clinical symptoms 
 have disappeared and when the patient is apparently per- 
 fectly normal, examination of the throat reveals the presence 
 
534 THE HUMAN MECHANISM 
 
 of the bacillus ; and it has been proved beyond question 
 that germs from this source are often capable of transmit- 
 ting the disease to healthy persons. It is a real hardship 
 to such a patient to be kept in quarantine for days and 
 weeks, until the disappearance of the germ in the throat 
 is established by bacteriological examination, and boards 
 of health are frequently criticised severely for enforcing 
 quarantine under such circumstances; but it is obvious 
 that these measures are demanded in the interests of the 
 community and that resistance to them can arise only from 
 ignorance or selfishness, or both. 
 
 4. What the Public may do to protect and promote the 
 Health of the Individual. — On the other hand, the com- 
 munity, through its paid or unpaid officials, may do much 
 to protect and promote the health of its individual mem- 
 bers. It may see to it that the public water supply, if any, 
 is pure ; it may maintain an eificient system of milk in- 
 spection ; it may provide investigations of food adultera- 
 tion, and prosecutions and penalties for the same ; it may 
 require prompt and efficient scavenging, and the collec- 
 tion and removal of wastes such as sewage, garbage, and 
 other refuse ; it may establish a wholesome system of school 
 hygiene ; it may prevent the concealment of the existence 
 of cases of infectious or contagious disease ; it may pro- 
 vide for vaccination against smallpox, and for the use of 
 antitoxic serum in diphtheria ; and in many other ways it 
 may protect the individual and his family even better than 
 he, unaided, could protect himself. 
 
CHAPTER XXXV 
 THE HEALTH OF NATIONS 
 
 1. The Modern World One Vast Community. — Ever since 
 the inveation of the ruariner's compass, followed as this 
 was by the voyages of discovery of Columbus, Vasco da 
 Gama, and Magellan, the world has become, century b}^ 
 century, more and more one great communit}^ or neigh- 
 borhood. With the introduction of steam transjiortation 
 on land and sea our globe has practically shrunk so that 
 intercourse between the various nations of the earth has 
 become both frequent and easy, and if no other means than 
 those formerly known existed for the prevention of disease, 
 plagues and pestilences would, without question, ravage 
 mankind worse than ever before. The isolation of the 
 ancient world was its sanitary salvation, but to-day there 
 is no isolation. Steamers ply regularly and frequently 
 between Orient and Occident, commingling the people and 
 the products of the whole world. Books, newspapers, let- 
 ters, food materials, fabrics, and many other sorts of mer- 
 chandise pass freely back and forth, and yet plague and 
 pestilence to-day seldom follow in their train. It is possi- 
 ble, and even probable, that some milder diseases such as 
 influenza, or the grippe, may still owe their sudden and 
 wide distribution to the modern ease and extent of com- 
 munication ; but mails and steamers come and go, and 
 bubonic plague, and Asiatic cholera, and smallpox, though 
 occasionally brought by them to Europe or America, do 
 not make great headway there after their arrival. 
 
 2. Ancient Paths of Pestilence and Plague. — Although 
 it is true that modern civilization is indebted to the Orient 
 
 535 
 
536 THE HUMAN MECHANISM 
 
 for its first knowledge of the art of inoculation for the 
 prevention of smallpox, it is no less true that many of its 
 worst epidemic diseases have often come from the same 
 source. 
 
 The plague, a world-famous disease (p. 502), has afflicted 
 mankind for centuries, and has repeatedly appeared in 
 Europe, traveling westward from the Orient and from 
 Africa. The Black Death, which is held to have destroyed 
 one fourth of the population of Europe in the fourteenth 
 century, was probably a virulent form of the oriental 
 plague which entered Europe from the south and east. 
 The Great Plague of London (in 1665) probably came 
 from Holland, in bales of merchandise brought from the 
 Levant. 
 
 The Asiatic cholera, as its name suggests, has repeat- 
 edly come to Europe and America from the East, and is 
 believed to exist almost constantly in Lidia, from which 
 place its germs are readily conveyed to western countries. 
 The germs of the great Hamburg (Gei'many) epidemic of 
 1892 were probably brought there by immigrants from 
 Russia. 
 
 3. The Modern Impotence of Pestilence and Plague. — 
 The modern increase of the means of communication has 
 no doubt tended to spread far and wide all sorts of conta- 
 gious and infectious diseases, but with that increase there 
 has come, especially within the last few years, such a 
 scientific knowledge of these diseases and of the ways of 
 holding them in check that, in spite of vastly greater facil- 
 ities for their distribution, they are actually less dangerous 
 to mankind, and far less dreaded, than formerly. The 
 appearance of the bubonic plague in China or in India, or of 
 Asiatic cholera in Japan or in the Philippines, still causes 
 international anxiety, and vigorous local precautionary or 
 corrective measures are taken to overcome them ; yet little 
 widespread alarm is felt. The closer intimacy between 
 
THE HEALTH OF NATIONS 537 
 
 Cuba and the United States since the Spanish War of 
 1898, while in itself favoring the spread of yellow fever, 
 has had the marvelous and happy consequence — thanks 
 to the brilliant researches and able administration of the 
 medical and sanitary officers of the American army — not 
 of bringing more yellow fever to the United States, as 
 would formerly have been the case, but of virtually extir- 
 pating that disease, for the present at least, in Cuba. 
 
 4. The Use and Abuse of Quarantine. — The word 
 "quarantine" comes from the French word qiiaraiite, 
 meaning forty, because a detention of forty days was for- 
 merly enforced upon travelers crossing frontiers. Quar- 
 antine is of great value in some cases, as, for example, iri 
 ports like Boston or New York, and when thoroughl}' 
 enforced may be an important means of protecting the 
 country against infectious disease. Wlien a vessel wliicli 
 has been long enough at sea to give contagious disease (if 
 present) time to appear, comes into port witli cases of 
 such disease on board, its detention is a wise precaution. 
 On the other hand, indiscriminate quarantine between 
 states or cities, or of vessels that have come from near 
 ports, so that little or no time lias been given for disease, 
 if present, to show itself, is necessarily severe and often 
 useless and unwarrantable. 
 
 Quarantine is also liable to abuse on other grounds, for 
 it is claimed that it has sometimes been unjustifiably 
 employed to keep out of a country foods or other products 
 which came into competition with domestic products, the 
 plea of sanitary danger being raised for commercial reasons. 
 
 5. International Sanitary Congresses. — From time to 
 time there are held nowadays international sanitary con- 
 gresses which undertake to deal with the larger questions 
 affecting the health of nations. There is also held annually 
 an International Congress of Hygiene and Demography ; 
 and the annual meetinc^s of the American Public Health 
 
538 THE HUMAN MECHANISM 
 
 Association, in wliicli the United States, Canada, Mexico, 
 and Cuba are represented, are really international con- 
 gresses for a large part of the western hemisphere. 
 6. Health and Longevity in Various Countries. — It is 
 
 interesting to inquire how different nations compare, one 
 with another, in respect to health and longevity. It might 
 be supposed that somewhere on the earth's surface the 
 climate should be so salubrious, the food so wholesome, 
 the conditions so favorable, and life so normal, that sick- 
 ness would be unknown and death indefinitely postponed. 
 Invalids in large numbers do, in fact, turn to Colorado or 
 California, to Madeira or to the Riviera, seeking in these 
 places more favorable conditions for sustaining or prolong- 
 ing life ; but no place has ever been found altogether free 
 from disease, and no climate, however salubrious, seems 
 capable of causing any great increase in longevity. It 
 was many centuries ago in the Orient, and of a race singu- 
 larly strong and persistent, that the Jewish poet wrote 
 those majestic lines which for every land and every people 
 are no less true to-day : " The days of our years are three- 
 score years and ten ; and if by reason of strength they be 
 fourscore years, yet is their strengtli labor and sorrow; 
 for it is soon cut off and we fly awaj^" 
 
 The general death rate — that is, the number of deaths 
 per year per thousand of the population — is not a com- 
 plete measure either of health or of longevity, but is some- 
 times the only test we have, and the following table for 
 1900 shows how great is the difference in the death rates 
 of some of the larger cities of the world. 
 
 Moscow 30.0 
 
 Rome 16.5 
 
 Madrid 33.3 
 
 Stockholm 17.1 
 
 Boston 20.8 
 
 London .... 
 
 . . 18.7 
 
 New York 
 
 . . 20.6 
 
 Paris 
 
 20 r-) 
 
 Berlin 
 
 . . 18.9 
 
 Vienna 
 
 . . 20.6 
 
 St. Petersburg 
 
 . . 27.0 
 
THE HEALTH OF NATIONS 
 
 539 
 
 The following table (from the United States Census of 
 1900) gives the death rates for the periods specified of 
 some of the principal countries of the civilized world. 
 
 Comparative Death Rates per 1000 Populatiox for 
 Certain Countries 
 
 Austria . 
 
 Belgium 
 
 Denmark 
 
 England and Wales . . . . 
 
 France 
 
 German Empire 
 
 Prussia 
 
 Hungary 
 
 Ireland 
 
 Italy 
 
 Netherlands 
 
 Norv\'ay 
 
 Scotland 
 
 Spain 
 
 Sweden 
 
 Switzerland 
 
 United States (registration area) 
 
 L'4 
 
 IS.; 
 ■-'0. 
 ■Jll. 
 17.: 
 li).' 
 
 TWE>TV-FlYE 
 
 Yi;ak,s 
 1870-1900 
 
 28.6 
 
 20.1 
 
 IS. 13 
 
 l!).l 
 
 21 .<:> 
 
 21.2 
 2:;." 
 
 1S.L> 
 
 2n..i 
 20.:; 
 10. i; 
 
 II). 2 
 
 30.:; 1 
 
 17.0 
 20. 
 
 25.1 
 10.3 
 16.9 
 18.2 
 21.9 
 22.1 
 21.3 
 2(5.9 
 19.0 
 23.8 
 17.8 
 1.3.9 
 18..) 
 28.7 
 10.8 
 19.3 
 17.8 
 
 7. The Sanitation of the World. — Enough has been said 
 in the foregoing paragraphs to show that while the hopes 
 of dreamers seeking after an elixir of life have no founda- 
 tion, and while a wholl}' salubrious environment cannot 
 greatly prolong human life beyond the usual period, much 
 is being done and much still remains to be done for a 
 more complete and perfect sanitation. Infectious diseases 
 
 1 Average for twenty years, 1878-1884, 1888-1900. 
 
540 THE HUMAN MECHANISM 
 
 still sweep over communities, carrying sickness and death 
 among the people, increasing tlie death rate, and diminish- 
 ing longevity. Here and there nations and individuals are 
 devoting themselves with energy, public spirit, and wisdom 
 to investigation of the causes of disease, and to improve- 
 ment of the environment by careful organization of boards 
 of health, by municipal sanitation, by sanitary engineer- 
 ing, by purer water and milk supplies, by proper sewerage 
 and sewage disposal, by food inspection, and the like. All 
 this is wise and encouraging, but it is only a beginning. 
 Far more might and ought to be done both by nations 
 and by individuals. Many of the nations, especially those 
 known as half-civilized or barbarous, have as yet hardly 
 made a beginning in hygiene or sanitation, and as long 
 as this is the case they are, and will continue to be, a men- 
 ace not only to themselves but to the whole world, which, 
 as one vast communit3% is in these respects closely bound 
 together. 
 
 The student should never forget that the foundation of 
 municipal, national, and international hygiene and sani- 
 tation, and therefore of the health of nations, ultimately 
 rests upon the liygiene and sanitation of individuals, — that. 
 is, upon personal hygiene and sanitation. If all human 
 beings were healthy and clean, the nations of the world 
 would of necessity be in the same condition. Personal 
 hygiene and personal sanitation thus form the basis of 
 all hygiene and sanitation, whether of home or village, 
 of town or city, or of the world ; and the essentials of 
 personal hygiene and sanitation are simply the proper 
 management and care of the human mechanism and its 
 surroundings. 
 
Fiii. l:-!4. Tlie tlinnicic anil abilomiiial eavitk'S, after the removal of 
 tlie orii'aiis shown in Fii;. 2 
 
 The dia[>lu-agan has he^Mi ih'awii s()ine\^'liat lor\\ar(l 
 
Fi';. l.'-;-'>. Geiieial virw of tlie clii^rstive tract. After Spaltelinlz 
 
 riioiitlM';t\-ity ; />', pliai-ynx ; ' ', (rsi)])liaiins : /'. (Iia))lirai;in : /^, stoniarh ; 
 /•', small intrsliiH' : <,'. ascriiHiim colon : //, ilrscriiiliii^ colon ; /, recti! 111. 
 'Ilic t raii,s\ci-sc colon lias been cut away, it.s position bciiii; iniiicatctl by 
 dotted lines 
 
Fid. 130. The flmmcelike folding of the mesentery, as seen after 
 removing the small intestine. After Spalteholz 
 
Fir,. 1.37. Median dnr.so-ventral section of tlie trunl; in the abdominal 
 region, showing tlie suspension of the stomach and intestine by tlie 
 mesentery. After Spalteholz 
 
 -t, liver; />', stomach; <J, transverse colon; />, mesentery; 7?, rectum; 
 F^ miliary Iiladder 
 
Fig. 108. The permanent teeth in (he jaw-liones, viewed 
 from the right. After Spalteholz 
 
 Fig. 139. The network of capillaries on the lining of the 
 air cells of the kuigs. After Kolliker 
 
 See page lt>7 
 
; E 
 
 Fig. 140. First layer of niu.scles of the breast and shoulder region. 
 After Spalteholz 
 
 .1, hiceps of the arm (p. ."4); B, deltoid; (_\ portion of the trapezius (see 
 Fi,i;s. KIG and 107); />, ehi\irle ; /'.'.sternum or lircastiKtue; /^ pecto- 
 ralis major (.see p. :t'4 and Fig. 1U7) 
 
Fk;. 141, Second layer of muscles of the lireast. exposed by disserting 
 away tlie pectoralis niaior in Fig. 140. After Spaltelmlz 
 
 .i, B. the two " heads " of the hieeps ; l\ cut cud ol the pert.u'ali,-; luaj.iv; 
 /'.deltoid: A', i>rctiiralis aiiiinr; /■', trapezius ; fv,rhn'icle; //, tirst i-il> : 
 /v, stermim. Note tiie direct attachment of the intercostal muscles to 
 the rihs (p. S). (Jouipare Fig. 140 
 

 
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The parts of the nervous system 
 represented a re the cereltrum. 
 eerel)el]uiii,linlb,:uid seg'iiieiit 
 of the spinal euni. Afferent 
 nerves in red, efferent nerves 
 ill bhick. /;/, //(, motor neu- 
 rones to some of the nius<'les 
 tlie lei;-. These may l)e 
 stimulated to eoiir-linate ae- 
 tion l)y neurones (/') from 
 the cerebrum, neurones (cb) 
 from the eerebelluni, <^)r liy 
 tlie afferent neurones (t//i) 
 rom the ten(h:)ns, etc In the 
 Imlb this afferent neurone 
 eonnects with a seeond ni_'U- 
 rone (af^), and this with a 
 third (a/^), thus providing- 
 the path to the cerebrum and exeitin.i; 
 in eonseiousness sensations of position 
 of the h'g (muscular sense). Tliesame 
 ncurrmes connect with the eerel)ellum, 
 as do also neurones from the inner ear. 
 For further explanations see Chapter 
 XV, pp. '2V.)-2H2. 
 
 Fio. 145. Diagram of the nervous mechanism of walking 
 
Fn:. 14(1. Siile vie\Y uf tliL' brains of rabbit, cat, aud ujnnke 
 Sfe pago L'Tl 
 

 > SI- 
 
 -r f_i 
 
INDEX 
 
 Abdominal breathing, 170. 
 
 Abdominal cavity. See Peritoneal 
 cavity. 
 
 Abdominal muscles, action in 
 breathing, 172. 
 
 Absorption from the intestine, 12.3. 
 
 Accommodation, for near objects, 
 218 ; muscle of, 217. 
 
 Adipose tissue, 182, 2.30. 
 
 Adrenal, 68. 
 
 Adulteration of foods, 508. 
 
 Afi'erent impulses, 77 ff. ; reflex 
 and conscious oft'ects of, 281. 
 
 Afferent neurones, 77, 78. 
 
 Air cell, 168. 
 
 Air, good and bad, pure and im- 
 pure, 411 ; change of, 522. See 
 also Fresh air. 
 
 Albuminoids, 01, 218. 
 
 Alcohol, physiological action of, 
 366; as a stinuilant, 367, 360 ; as 
 a food, 372 ; as a defense against 
 cold, 372 ; pathological conditions 
 due to, 373 ; in muscular work, 
 370 ; influence on self-control, 
 375. 
 
 Alcoholic beverages, composition of, 
 303. 
 
 Alimentary canal, structure of, 20 ; 
 as a digestive laboratory, 97. 
 
 Alimentation, 88, 06. 
 
 Alveolus of gland, 31, 32 (fig.), 33 
 
 ■ (fig.); of lungs, 107, 108 (fig.). 
 
 Amceba, amojboid movement, 135. 
 
 Amylopsin, 119. 
 
 Anesthetics, 284. 
 
 Animal foods, 94, 111, 240, 241. 
 
 Ankle, bones of, 19, 403 (fig.), 404 
 
 (%)■ 
 
 Anoplieles, 490, 491 (fig.), 492 (fig.). 
 
 Anterior, 9. 
 
 Antiseptics, 475, 504 ; in food pres- 
 ervation, 512. 
 
 Antitoxin, 488, 499. 
 
 Aorta, 13, 22, 24 (fig.), 2-5, 141. 
 
 Apical lobes of lungs. See Linvjs. 
 
 Appendicular skeleton, 14, 19. 
 
 Appetite, 265; as a guide in feed- 
 ing, 348. 
 
 Aqueduct of Sylvius, 208, 270. 
 
 Acjueous humor, 247. 
 
 Arborization. See Hi/napse. 
 
 Arterial reservoir, 139. 
 
 Arterial tone, 158. 
 
 Arteries, 22, 38, 114 (fig.). 
 
 Asphyxia, 168. 
 
 Astigmatism, 2.53. 
 
 Auditory nerve, 28, 259. 
 
 Augmentor nerves of heart, 157. 
 
 Auricle, 22, 24 (fig.), 137. 
 
 Auriculo-ventricular valves, 138. 
 
 Automatic nervous actions, 82. 
 
 Axial skeleton, 14. 
 
 Axon or axis cylinder, 73, 77. 
 
 Bacilli, 469. 
 
 Bacteria, 469. See also Microbes. 
 
 Balance exercises, 331. 
 
 Balance movements, 276. 
 
 Bathing and baths, 413 ff., 476. 
 
 Beds and bedsteads, 431. 
 
 Bile, 119. 
 
 Bile duct, 107 (flg.). 
 
 Bladder, urinary, 179, 180 (fig.). 
 
 553 
 
554 
 
 THE HUMAN MECHANISM 
 
 Blood, arterial and venous, 23, 163 ; 
 microscopic structure of, 133 ; 
 gases ol, 163 ; as a common car- 
 rier, 132 ; distribution among 
 organs, 144. 
 
 Blood corpuscles, red, 133-135; as 
 carriers of oxygen, 104. 
 
 Blood corpuscles, white, 133, 134 ; 
 behavior during inflammation, 
 382. 
 
 Blood plasma, 134, 135 ; gases of, 
 163. 
 
 Blood vessels, 38. See also Arteries, 
 Capillaries, and Veins. 
 
 Boards of Health. See Health. 
 
 Body cavity, 10. 
 
 Bone, 37. See also Skeleton. 
 
 Brain, 27 ; of frog, 207 ; of mam- 
 mal, 270 ; histological structure 
 of, 271 ; the seat of sensations, 
 245; functions of, 274 ff. 
 
 Brea.stbone, 17. 
 
 Breathing movements, 108, 109 
 effect on circulation, 145, 173 
 effect on flow of lymph, 148, 1 73 
 hygiene of, 172 ; in muscular ac- 
 tivity, 308, 315. 
 
 Breathlessness, 174. 
 
 Bronchiole, 108. 
 
 Bronchitis, 381. 
 
 Bronchus, 12, 21, 167. 
 
 Bulb, 207 (fig.), 208, 270 (fig.); 
 functions of, 275. 
 
 Caffeine, 301. 
 
 Calendered paper, 399. 
 
 Calorie, 212. 
 
 Canal, spinal or vertebral, 18. 
 
 Canning of fnods, 512. 
 
 Capillaries,- 25 (fig.), 27, 32 (fig.), 
 39. 
 
 Capsule of gland, 31. 
 
 Carbohydrates, 01, 218 ; digestion 
 of, 103, 110, 119 ; waste products 
 of, 177 ; as source of power for 
 work, 213 ; fuel value of, 213 ; as 
 
 food in cold climates, 216 ; as 
 source of fat, 231. 
 
 Carbon dioxide (carbonic acid) 
 formed during muscular work, 
 49, 60, 175, 300; in lymph, 162; 
 in blood plasma, 163. 
 
 Cardiac region of stomach, 107. 
 
 Carpets, 432. 
 
 Catalytic actions, 106. 
 
 Catarrh, chronic, 394. 
 
 Catarrhal conditions, care of, 383. 
 
 Cellar, sanitation of, 428. 
 
 Cell walls in plants, 94. 
 
 Cells, 32, 33, 36, 38, 40, 41 (fig.), 
 43 ; as chemical factories, 52 ; 
 as living mechanisms, 03 ; waste 
 and repair of, 220. 
 
 Cellulose, 94. 
 
 Cement of tooth, 99. 
 
 Cemeteries, 528. 
 
 Central canal of spinal cord, 268. 
 
 Cerebellum, 267(fig.), 268, 273 (fig.). 
 
 Cerebrum, 83, 206 (fig.), 207 (fig.), 
 209 (fig.), 270 (fig.); connections 
 with other parts of the nervous 
 system, 279; functions of, 277, 
 283. 
 
 Cervical vertebrie, 14. 
 
 Cesspools, 459. 
 
 Chemical change, relation to work, 
 52 ; as source of heat, 193 ; as 
 influenced by temperature, 187. 
 
 Chicken pox, 493, 494. 
 
 Chloral, 377. 
 
 Chloroform, 284. 
 
 Chocolate, 302. 
 
 Cholera, Asiatic, 501. 
 
 Choroid, 247, 248, 251 (fig.). 
 
 Chyme, 112. 
 
 Cilia, 474. 
 
 Ciliary muscle, 248. 
 
 Ciliary region of eye, 248 (fig.). 
 
 Cinders, removal from eyes, 400. 
 
 Circulation, organs of, 22, 132, 
 140 (fig.); time of, 130; mechan- 
 ics of, 136 ; in warm and cold 
 
INDEX 
 
 555 
 
 weather, 150, 200; during mus- 
 cular activity, 151; during diges- 
 tion, 155, 150; during mental 
 work and sleep, 155 ; nervous fac- 
 tors of , 156 ft.; essential to respi- 
 ration, 174 ; in foot, 409. 
 
 Cisterns, 454. 
 
 Clavicle, 20. 
 
 Cleanliness, sanitary value of, 4.S.3. 
 
 Climate, and mental work, 204 ; and 
 feeding, 236 ; in treatment of tu- 
 berculosis, 482. 
 
 Clothing, hygiene of, 418 ft. 
 
 Coagulation of proteids, 91. 
 
 Coarse foods, importance of, 355. 
 
 Cocaine, 377. 
 
 Cocci, 469, 470 (fig.). 
 
 Coccygeal vertebrte, 14. 
 
 Cochlea, 259. 
 
 Cocoa, .362. 
 
 Coffee, 361. 
 
 Cold, effect on circulation of the 
 blood, 150; effect on body as a 
 whole, 200 ft. 
 
 Cold, sensations of, 261. 
 
 Cold-blooded animals, 189. 
 
 Colds, nature, prevention, and care 
 of, 380 ft. 
 
 Collagen, 91, 218. 
 
 Collar bone. See Clavicle. 
 
 Collaterals, 79. 
 
 Colon, 21. 
 
 Color, sensations of, 256. 
 
 Compensatory adjustments of the 
 circulation, 150. 
 
 Conduction of heat, 194-190. 
 
 Congestion of blood in internal 
 organs, 307 ; during inflamma- 
 tion, 382. 
 
 Connective tissues, structure of, 7, 
 8, 37, 228, 230 ; of glands, 31, 32 ; 
 of muscles, 34, 35 ; of nerves, 72 ; 
 of lungs, 107 ; of skin, 182 ; re- 
 lation to blood vessels, 38 ; to 
 lymphatics, 39 ; digestion of, 109, 
 115 (footnote). 
 
 Consciousness, 244, 275, 277, 283, 
 
 285. 
 Consciousness of correct posture, 
 
 education of, 326. 
 Constant temperature of the body, 
 
 188. 
 Constipation, 130. 
 Consumption. See Tuberculosis. 
 Contagious diseases. See Diseases. 
 Contraction of muscle, 34, 48, 56. 
 Convection of heat, 194, 195. 
 Convolutions of cerebrum, 270 (fig), 
 
 271. 
 Cooking, 243 ; as an aid in nutri- 
 tion, 349 ; sterilization of food 
 
 by, 511. 
 Cooling oft suddenly, 391. 
 Coordination, 70, 80, 275 ; training 
 
 of, 80, 280, 330. 
 Corium, 182. 
 
 Cornea, 247, 248 (fig.), 251 (fig.). 
 Corpuscles. See lUood corpuiicles. 
 Corrective muscular exercises, 321 ; 
 
 hygienic value of, 332. 
 Cortex of cerebrum and cerebellum, 
 
 272. 
 Costal breathing, 170. 
 Cotton underwear, 421. 
 Cough medicines, 387. 
 Course dinners, 242. 
 Cranial nerves, 271. 
 Cranium, 18. 
 Creatinin, 178. 
 Culex, 492 (fig.). 
 Curd of milk,'' 90, 108. 
 Curvatures of vertebral column, 16. 
 Curve of fatigue, 68. 
 Cutaneous sensations, 201. 
 Cutis, 182. 
 Cycling, 317. 
 Cytoplasm, .32, 36, 43, 70. 
 
 Dampness, 380. 
 
 " Dangerous region " of atmos- 
 pheric temperature, 201, 390. 
 Deafness, 401. 
 
656 
 
 THE HUMAN ME(JHANISM 
 
 Dendrites, 76, 77, 272, 273, 274. 
 
 Dentine, 99. 
 
 Derrai.s, 182. 
 
 Dextrines, 92, 105. 
 
 Dextrose, 98. 
 
 Diaphragm, 10 ; action in respira- 
 tion, 170-172. 
 
 Diarrliea, 130, 380, 456. 
 
 Diastase, 304. 
 
 Dia.stole, 137, 139 (fig.). 
 
 Diet, 226. 
 
 Digestibility of food, 226. 
 
 Digestion, organs of, 20 ; nature of, 
 88. 98 ; external and internal, 97 ; 
 in the mouth, 98 ; in the stomach, 
 105 ; in the intestine, 115 ; cooper- 
 ation of processes of, 129 ; and 
 the circulation, 155 ; andtemper- 
 ature regulation, 205 ; as affected 
 by muscular activity, 309 ; dur- 
 ing fatigue, 342. 
 
 Diphtheria, 486, 533. 
 
 Diseases, 293 ; mental cures of, 345; 
 infectious and contagious, 467 ; 
 prevention of microbic, 475. 
 
 Disinfectants, 475. 
 
 Distilled liquors, 365. 
 
 Divisive movements of intestine, 
 121. 
 
 Dorsal, 9. 
 
 Drainage in hou.se sanitation, 428, 
 458. 
 
 Drinking cups, public, 525. 
 
 Driving force for circulation through 
 capillaries, 144. 
 
 Drug habit, 358. 
 
 Drugs, 357 ff. ; in the operation of 
 the nervous system, 344 ; in the 
 catarrhal conditions, 387 ; as a 
 substitute for hygienic living, 
 388 ; in treatment of tubercu- 
 losis, 482. 
 
 Ducts of glands, 20, 20, 32 (fig,), 
 33 (fig.). 
 
 Dust as vehicle of tuberculosis, 479, 
 
 Dyspepsia, 113, 
 
 Ear, .structure of, 258 ; care of, 401. 
 Efferent nerve fibers and impulses, 
 
 76, 80 (fig.). 
 Elasticity of arteries, 142. 
 Elimination of intestinal waste, 
 
 128. 
 Emmetropia, 250. 
 Enamel, 99. 
 End organs of nerves, 28, 76-84, 
 
 245. 
 Engine, analogy of, 62. 
 Enjoyment of food, hygienic value 
 
 of, 114. 
 Enjoyment of muscular activity, 
 
 311. 
 Environment, 299. 
 Enzyme, 45, 48, 52; action of, 
 
 101 ff. ; of saliva, 103 ; of gastric 
 
 juice, 108 ; of small intestine, 
 
 118, 120. 
 Epidemics, 466, See also Plagues. 
 Epidermis, 182. 
 Epiglottis, 20 (fig.). 
 Equilibrium, nervous factors in, 
 
 276 ; in physical training, 330. 
 Esophagus. See (Esophagus. 
 Ether, 284. 
 
 Eustachian tube, 259, 401, 
 Excitation, 285, 367. 
 Excretion, 177 ; in relation to feed- 
 ing, 225 ; essential and incidental 
 
 organs of, 178. 
 Extractive, 135, 218. 
 Eye, structure of, 246 ; care of, 395. 
 
 Face, bones of, 18. 
 Farsightedness, 253, 
 Fasciculus of muscle, 34, 35, 
 Fatigue, 55 ff. ; hygienic value of, 
 66; influence on digestion, 353 ; 
 and taking cold, 389 ; practical 
 considerations concernin.g, 316, 
 Fatigue level, 58, 61, 
 Fats, 92, 218 ; of meat, 115 ; diges- 
 tion of. 119; waste products of. 
 177 ; fuel value of, 213 ; as food 
 
INDEX 
 
 557 
 
 in cold climates, 217 ; storage of, 
 224, 227, 229, 231 ; a hindrance 
 to output of heat, 205. 
 
 Fatty acids, 92, 119. 
 
 Feces, 128. 
 
 Feeding, muscular activity after, 
 1-56; effect on chemical changes 
 in the body, 221 ; for muscular 
 activity, heat, cold, mental -wnrk, 
 and sedentary occujiations, 235- 
 237 ; hygiene of, 347 ; during 
 colds, 380. 
 
 Femur, 19. 
 
 Fermentation, 304. 
 
 Fever, 210. 
 
 Fibula, 19. 
 
 Filters, house, 455. 
 
 Filth and filth diseases, 484. 
 
 Filtration of public water supplies, 
 513 f. 
 
 Fires, open, 190, 4.34. 
 
 Flagella, 474 (fig.). 
 
 Flat foot, 400. 
 
 Flesh of body, 227 ; increase of, 
 228. 
 
 Food accessories, 357 ff. 
 
 Food stuff. See Nutrient. 
 
 Foods, chemical composition of, 89, 
 95 ; animal and vegetable, 94, 
 240 ; as source of energy or power, 
 88, 211; as material f'lr growth 
 and repair,88, 220-222; fuel value 
 of, 211 ; heating, 210; choice of, 
 233 ; adulteration and infection 
 of, 508 ff. ; preservatives and 
 antiseptics in, 511; canning of, 
 512. 
 
 Foot, hygiene of, 403 ff. ; arches 
 of, 404 ; deformation of, 405 ; 
 physical training of, 410. 
 
 Force-pump action of heart, 137, 
 139 (fig.). 
 
 Forebrain, 207 (fig.), 283. 
 
 Fresh air, as a substitute for exer- 
 cise, 320 ; as a cure for colds, 385. 
 
 Fried foods, 352. 
 
 Fuel substances, storage of, in mus- 
 cle, 49. 
 Fuel value of food, 211. 
 
 Gall bladder, 107 (fig.). 
 
 Games, 318. 
 
 Ganglion, 73 ; of the dorsal root, 
 
 77. 
 Garbage, disposal of, 401. 
 Gas, illuminating, 440 ; natural, 
 
 coal, and water, 515; poisoning 
 
 by, 510. 
 Gaseous exchange in capillaries, 
 
 104, 105 ; during muscular activ- 
 ity, 175. 
 Gastric digestinn, 350. 
 Gastric juice, 108 ; secretion of, 113. 
 Gelatin, 91. 
 
 General muscular exercise, 314,332. 
 Germ theory of disease, 407. 
 Germicides, 475. 
 Glands, 20, 29, 31, 32 (fig.), 33 
 
 (fig.); working and resting, 44, 
 
 47 ; blood supply during activity, 
 
 45 ; ductless, 29, 08. 
 Glottis, 20 (fig.). 
 Gluten, 90. 
 Glycerin, 92. 
 Glycogen, 227, 232. 
 Granules, storage of, in gland cells, 
 
 47. 
 Gray matter of spinal cord and 
 
 brain, 75. 
 Grippe, 493. 
 Ground water, 451, 513. 
 Growth, favorable period for the 
 
 acquisition of defdrmities, 325. 
 Gums or dextriues. 92. 
 Gymnasium, 332, 470. 
 
 Habits, physical basis of, 287. 
 Hair and hair follicle, 183, 185 
 
 (%•)■ 
 " Hardening" to cold, 392. 
 Harvey, William, 136. 
 Headaches. 128, 396. 
 
658 
 
 THE HUMAN MECHANISM 
 
 Health, 293 ; public, 529 ; national, 
 state, and local boards of, 530 ; in 
 different countries, 538. See also 
 Public health. 
 
 Hearing, 258. 
 
 Heart, 10, 12, 22; valves of, 138; 
 nerves of, 157 ; effect of muscular 
 activity on, 307. 
 
 Heart beat, 136 ; force-pump action 
 of, 137, 139 (fig.); regulation of, 
 157. 
 
 Heat, effect of, on the circulation 
 of the blood, 150; unit of, 212; 
 transfer of, from internal organs 
 to the skin, 195 ; production and 
 transfer of, 192, 108 ; supply of 
 energy for production of, 216; 
 as a food preservative, 612. 
 
 Heating foods, 216. 
 
 Hemoglobin, 134, 135, 165. 
 
 Hepatic artery, 24 (fig.). 
 
 Hepatic vein, 24 (fig.), 27. 
 
 Hind brain, 267. 
 
 Hip bones, 20. 
 
 Hoarseness, 21. 
 
 Horny layer of skin, 182, 183. 
 
 Hotels, sanitation of, 526. 
 
 House, sanitation, 425 ff. ; construc- 
 tion, 429 ; furnishings, 430 ; 
 floors, 431 ; care of, 432. 
 
 Humerus, 19, 20. 
 
 Humidity, influence on temperature 
 regulation, 202 ; influence on 
 mental work, 204. 
 
 Hunger, 264. 
 
 Hygiene, scope and subdivisions 
 of, 301 ; personal, 304 ; domestic, 
 425 ; public, 463. 
 
 Hypermetropia, 253, 255. 
 
 Hypnotics, 344. 
 
 Ice supply of the house, 455. 
 Illumination for near work, 255. 
 Illusions, optical, 257. 
 Immunity, natural and artificial, 
 497. 
 
 Indigestible material in food, 93. 
 
 Indigestion, 113, 125. 
 
 Infection of foods, 509. 
 
 Infectious diseases. See Diseases. 
 
 Inflammation, 381. 
 
 Influenza, 493. 
 
 Inhibition, 158, 367 ; in the nerv- 
 ous system, 285, 367 ; an active 
 process in muscular relaxation, 
 343. 
 
 Inhibitory nerves of heart, 157. 
 
 Inorganic salts. See Salts. 
 
 Instep, bones of, 19. 
 
 Interdependence of organs, 65. 
 
 Internal secretion, 67. 
 
 Intestinal juice, 117, 120. 
 
 Intestinal waste, elimination of, 
 128. 
 
 Intestine, small, 10, 21, 24 (fig.), 
 26, 116; large, 11, 21, 127; aetion 
 of muscular coat of, 120. 
 
 Iris, 247, 248 (fig.), 251 (fig.). 
 
 Irritability, 46, 50. 
 
 Jugular vein, 24 (fig.). 
 Junket tablets, 108. 
 
 Kidneys, 11, 13, 24 (fig.), 26, 68; 
 
 structure of, 179, 181. 
 Kilogrammeter, 212. 
 Koch, Robert, 473, 478. 
 
 Labyrinth of ear, 260. 
 
 Large intestine. See Intestine. 
 
 Laryngitis, 381. 
 
 Larynx, 21. 
 
 Lateral costal breathing, 171. 
 
 Lateral curvature of spine, 329. 
 
 Lens of eye, 247, 248 (fig), 251 
 
 (fig.); formation of image by, 
 
 249, 250 (fig.). 
 Levulose, 98. 
 Ligaments, 8, 14, 10, 17. 
 Lighting of the house, 439. 
 Linen underwear, 421. 
 Lipase, 110. 
 
INDEX 
 
 559 
 
 Liver, 11, 21, 24 (fig.), 26, 27, 29, 
 68, 107 (fig,), 117. 
 
 Lobes and lobules of glands, 30, 31 
 (fig.); of the lung, 13, 172. 
 
 Lockjaw, 501. 
 
 Locomotion, nervous factors in, 
 270, 277, 280. 
 
 Longevity, 538. 
 
 Lumbar vertebrEe, 14. 
 
 Lungs, 10, 12, 21, 24 (fig.) ; struc- 
 ture of, 107 ; apical lobes of, 
 172, 309. 
 
 Lymph, 39; origin of , 40 ; environ- 
 ment of cells, 40, 135 ; gases of, 
 102. 
 
 Lymph flow, function of, 42 ; cause 
 of, 137 ; influenced by respira- 
 tory movements, 173 ; influenced 
 by muscular activity, 308. 
 
 Lymph spaces, 39. 
 
 Lymphatics, 41, 147. 
 
 Malaise, 128, 396. 
 
 Malaria, 489 ff. 
 
 Malt liquors, 304. 
 
 Massage, 147. 
 
 Master neurones, 84. 
 
 Mastication, hygienic aspects of, 
 349. 
 
 Mattings, 432. 
 
 Measles, 493, 494. 
 
 Meat, a proteid food, 90 ; composi- 
 tion, 220 ; in diet, 242 ; eating 
 of, 350 ; as a vehicle of infection, 
 479. 
 
 Mediastinum, 11, 21. 
 
 Medulla oblongata. See Bulb. 
 
 Mental cures of disease, 345. 
 
 Mental states, influence on the 
 health of the nervous system, 
 344. 
 
 Mental work and the circulation, 
 155; after meals, 156, 353; as 
 influenced by climatic condi- 
 tions, 204 ; feeding for, 237 ; 
 effect on nutrition, 237. 
 
 Mesentery, 12, 26. 
 
 Mesiiwork underwear, 421. 
 
 Microbes, 407, 468 ; as scavengers, 
 471 ; in decomposition and decay, 
 472 ; as disease germs, 473 ; 
 growth, multiplication, and spore 
 formation, 473 (fig, )• 
 
 Microbic action on food, 225. 
 
 Microbic life in the intestine, 125, 
 128. 
 
 MicromiUimeter, or micron, 133. 
 
 Microparasites, 407, 
 
 Microscopic work and the eyes, 
 399. 
 
 Midbrain, 207 (fig,). 
 
 Milk, 108 ; as a vehicle of infection, 
 479, 517, 531. 
 
 Milk supplies, purity of, 517. 
 
 Moisture, influence on temperature 
 regulation, 202. 
 
 Monosaccharides, 92. 
 
 Moral c<")nilLict as a part of nervous 
 
 * hygiene, 344, 
 
 Morphine, 370, 
 
 Mosquito, as transmitter of malaria, 
 491 ; as transmitter of yellow 
 fever, 493, 
 
 Motor nerves, 28. 
 
 Mouth, 183, 
 
 Movements, active or passive, effect 
 on circulation, 14t> ; respiratory. 
 See Breatlihui movoDents. 
 
 Jlucin, 45, 127, 
 
 Mucous coat of stomach, 108 ; of 
 intestine, 110, 127, 
 
 Muscle fibers, 35, 36 ; of stomach 
 and intestine, 108, 116, 127 ; of 
 arteries and veins, 144, 
 
 Muscles, 8. 16 : structure of, 34 ; 
 isolated. 49. 56 ; physiology of, 
 48, 50 ; and temperature regula- 
 tion, 207 ; antagonistic action of, 
 17, 208 ; in faulty carriage, 322 ff. 
 
 Muscular activity, effect on circu- 
 lation, 146, 151, 156; on respira- 
 tion, 175 ; and the regulation of 
 
560 
 
 THE HUMAN MECHANISM 
 
 the temperature of the body, 203, 
 390, 423 ; hygiene of, 304 ; minis- 
 try to body as a whole, 304 ; after 
 meals, 160,353 ; and fatigue, 316 ; 
 for women, 313 ; physiology of, 
 300. 
 
 Muscular exercises, general charac- 
 ter of the most useful, 312 ; for 
 special purposes, 321. 
 
 Muscular sense, relations to loco- 
 motion and maintenance of efjui- 
 librium, 280. 
 
 Mu.scular work, pjower for, 213 ; 
 feeding for, 235 ; of stomach, 
 110 ; of intestine, 120. 
 
 Myopia, 252. 
 
 Narcotics. 344. See also Anes- 
 thetics. 
 
 Nasal cavity, 20. 
 
 Nearsiglitedness, 252. 
 
 Near vision, 24!), 254, 396. 
 
 Neck, carriage of, 32«. 
 
 Nerve cells, 73 ff., 271. 
 
 Nerve fiber.';, stnirture of. 73 ; affer- 
 ent and efferent, 70. 77 ; of .spinal 
 cord and brain, 75, 271. 
 
 Nerve roots, 75. 
 
 Nerves, 28; distribution to organs, 
 42 ; structure of, 72 : cranial, 271. 
 
 Nervous impulse, nature of. 87; in 
 reflexes and sen.sation, 24G, 266. 
 
 Nervous strain, 335 ff. 
 
 Nervous system, general anatomy 
 of, 26, 27, 266 ; structural organi- 
 zation of, 72 ff. ; training by prac- 
 tice, 86, 286 ; physiology of, 70, 
 273 ; and tlie circulation, 157- 
 161 ; and respiration, 173 ; and 
 secretion of persjiiration, 185; 
 and regulation of bndy tempera- 
 ture, 209 ; and carriage of body, 
 324 ; hygiene of, 334. 
 
 Nervousness, 396. 
 
 Neurones, 77; "master," 84; of 
 brain, 272. 
 
 Nitrogenous equilibrium, 223. 
 
 Noise, 401. 
 
 Non-proteids, 218 
 
 Nucleus, 32, 30. 
 
 Nutrients, 89 ; classification of, 2X8 ; 
 sjjecial effects of each on oxida- 
 tions of the body, 221 ; on flesh 
 weight, 222 ; on storage of fat, 
 224 ; choice of, 233, 239. 
 
 Nutrition, 211 ff. 
 
 0-:sophagus, 10, 12, 20 (fig.), 183. 
 
 Opium, 376. 
 
 Optic lobes, 267. 
 
 Optic nerve, 28, 247, 251 (fig.). 
 
 Organs, typical structure of, 41 
 
 (fig.), 42. 
 •Overfeeding, 351. 
 Overheating of houses, 430. 
 Oxidation, 49, 50, 52, 92, 162 ; 
 
 products of, 177. 
 Oxygen, role of, in cell life, 48, 
 
 162 ; in lymph, 162 ; in blood 
 
 plasma, 163 ; absorbed during 
 
 muscular activity, 175. 
 
 Pain, sensations of, 263 ff. ; signs 
 
 of, 284- 
 Palate, 20. 
 Pancreas, anatomical relations, 11, 
 
 21, 26, 29, 107 (fig.), 117 ; the 
 
 source of an internal secretion, 
 
 68. 
 Pancreatic duct, 107. 
 Pancreatic juice, 109, 118. 
 Paper, in pjrinting, 399. 
 Papiillfe of skin, 182. 
 Parasites, 407. 
 Parks, 476, 527. 
 Parotid gland, 30. 
 Pasteur, Louis. 473. 
 Pasteurization, 475. 
 Pelvis, of skeleton, 19 (fig.), 20 
 
 (footnote) ; of ureter, 180, 181 
 
 (fig-)- 
 Pepsin, 108, 118. 
 
INDEX 
 
 561 
 
 Peptones, 98. 
 
 Peristalsis, 122. 
 
 Peritoneal cavity, 10, 11, 12. 
 
 Peritoneum, 10, 11, 1.3. 
 
 Peritonitis, 11. 
 
 Perspective, idea of, 2.56. 
 
 Perspiration, composition of, 179 ; 
 secretion of, 184 ; sensible and 
 insensible, 185 ; and the output 
 of heat, 197 ; in relation to feed- 
 ing, .'352. 
 
 Pharyngitis, .381. 
 
 Pharynx, 20, 21, 183, 259. 
 
 Plague, bubonic, 502. 
 
 Plagues, 460, .5.35. 
 
 Plasma. See lUood plasma. 
 
 Play, 287, 318. 
 
 Playgrounds, 476. 
 
 Pleura, 10, 11. 
 
 Pleural cavity, 10, 11 ; pressure in, 
 169. 
 
 Pleurisy, 11. 
 
 Plumbing of the house, 450. 
 
 Pons Varolii, 270 (fig.). 
 
 Portal vein, 24 (lig.), 27, 125. 
 
 Position, sense of, 262, 28(1, 327. 
 
 Po.sterior, 9. 
 
 Posture, nervous factors in, 276 ; 
 faulty, as a cause of deformity, 
 322. 
 
 Presbyopia, 253. 
 
 Preservatives i)f food, 511. 
 
 Pressure in arteries and veins, 142, 
 143, 152 (fig.); in pleural space, 
 109. 
 
 Processes of nerve cells, 76. 
 
 Proteids, nature of, 90, 115, 218 ; in 
 blood plasma, 135 ; in living cells, 
 220 ; digestion of, in stomach and 
 intestine, 109, 118 ; fuel value of, 
 213 ; quantity in body, 222 ; waste 
 products of, 177 ; influence on 
 secretion of urine, 181 ; disinte- 
 gration of, 221 ; native, 120. 
 
 Proteoses, 109. 
 
 Protozoa, 469, 470. 
 
 Pseudopodium, 134. 
 
 Psychic secretion of gastric juice, 
 
 113, 350. 
 Ptyalin, 103. 
 Public health, 463 ff.; rules and 
 
 regulations, 464 ; authorities, 465; 
 
 problems, 465. 
 Pulmonary arteries, 22, 24 (fig.), 
 
 25, 167. 
 Pulmonary circulation, 23, 140 (fig.). 
 Pulmonary veins, 22, 23, 24 (fig.), 
 
 25, 167. 
 Pulp cavity, of foolli, 99. 
 Pupil of eye, 247. 
 Purposeful character of reflex and 
 
 volitional actidus, 71, 80, 275. 
 Pylorus, 107 (fig.). 
 
 Quarantine, 530, 537 ; in tubercu- 
 losis, 480; in smallpox, 496; in 
 diphtheria, 533. 
 
 Padiation of heat, 195, 196. 
 
 Radius, 19. 
 
 Pailroad trains, reading on, 399. 
 
 Rectum, 130, 18-".. 
 
 Reflexes, 81; of locomotion, etc., 
 
 280 ; disappearance of, during 
 
 anesthesia, 284. 
 Relaxation, nnisriUar, in sleep, 342. 
 Renal arteries. 26. 
 Renniu, 108, 118. 
 Repair of cells, 220. 
 Reservoirs, arterial and venous, 
 
 139. 
 Resistance to the flow of blood, 
 
 143, 152 (fig.). 
 Respiration, organs of, 21 ; of the 
 
 cells,162 ; nervous factors in, 173; 
 
 and mttscular activity, 175, 308, 
 
 315. 
 Respiratory movements. See Breath- 
 ing movements. 
 Rest, in relaxation and sleep, 334, 
 
 338 ; in change of work, 340. 
 Retina, 247. 
 
562 
 
 THE HUMAN MECHANISM 
 
 Rheumatism, 380. 
 
 Rhinitis, 381. 
 
 Ribs, 17 ; action in respiration, 170, 
 
 172. 
 Rugs, 432. 
 
 Sacrum, 14, 20. 
 
 Saliva, chemical composition of, 45 ; 
 secretion of, 44-48 ; action in di- 
 gestion, 101, 10.5. 
 
 Salivary glands, 21, 30. 
 
 Salts, inorganic, 45, 93, 218, 219. 
 
 Sanitation, scope and subdivisions 
 of, 301; domestic, 425; public, 
 303, 463. 
 
 Saprophyte, 467. 
 
 Sarcolactic acid, 49, 60. 
 
 Sarcolemma, 35. 
 
 Sarcostyles, .36, 53. 
 
 Scapula, 20. 
 
 Scarlet fever, 494. 
 
 Sclerotic coat, 247, 248. 
 
 Sebaceous glands, 184. 
 
 Secondary aids to the circulation, 
 14.5. 
 
 Secretion, general physiology of, 
 44-48 ; internal, 67 ; of gastric 
 juice, 113; of pancreatic juice, 
 bile, and intestinal juice, 116-120; 
 of urine, 180; of perspiration, 
 184. 
 
 Sedentary occupations, 237. 
 
 Segmenting movements of intestine, 
 121. 
 
 Semicircular canals, 259, 260 (fig.), 
 263, 280. 
 
 Sensations, 244, 245, 246 ; refer- 
 ence of, 244. 
 
 Sense organs, 78, 244, 245. 
 
 Sense of position, importance of, in 
 physical training, .'!27. 
 
 Septum, of gland, 31 ; of muscle, 
 .34. 
 
 "Setting-up" drill, 326. 
 
 Sewage, disposal of, 459, 519. 
 
 Shivering, 208. 
 
 Shoes, 405, 407 ; for deformed feet, 
 409 ; temperature and moisture 
 within, 410. 
 
 Shoulder blade, 20. 
 
 Shoulder girdle, 20. 
 
 Sinuses of temporal bone, 260. 
 
 Skeleton, 14, 15, 19. 
 
 Skin, 7 ; structure and functions, 
 181 ; regulator of the output of 
 heat, 199 ; as an organ of absorp- 
 tion, 186; care of, 185, 413. 
 
 Skull, 19. 
 
 Sleep, circulation during, 153 ; hy- 
 giene of, 334 ff. 
 
 Smallpox, 495 ; inoculation and vac- 
 cination for, 498. 
 
 Smell, sensations of, 261. 
 
 Soaps, 92, 119. 
 
 Soda water, 363. 
 
 Soil, an element in house sanita- 
 tion, 428. 
 
 Solidity, ideas of, 256. 
 
 Somnambulism, 278. 
 
 Speech, 282. 
 
 Spinal column, 14, 17 ; faults of 
 carriage of, 328. 
 
 Spinal cord, gross anatomy of, 18, 
 27; structure of, 74, 267; func- 
 tions of, 274. 
 
 Spirilla, 469, 471 (fig.). 
 
 Spitting nuisance, 488. 
 
 Spleen, 11, 26. 
 
 Sporozoa, 470. 
 
 Spring.*^, 463. 
 
 Sputum, as vehicle of tuberculosis, 
 479. 
 
 Starch, 91, 94 (fig.); digestion of, 
 102, 110, 119. See also Carbo- 
 hydrates. 
 
 Steapsin. See Lipase. 
 
 Stegomyia, 493. 
 
 Sterilization, 475 ; of food by cook- 
 ing, 511. 
 
 Sternum, See Breastbone. 
 
 Stimulants, 344, 357 tf. 
 
 Stimulation, 46, 52, 80, 81. 
 
INDEX 
 
 663 
 
 Stomach, anatomical relations of, 
 10, 21, 26; structure of, 107 ff. ; 
 digestive work of, 108-115. 
 
 Storage of material in the cell, 52. 
 
 Stoves, oil and gas, 4.37. 
 
 Streets, sanitation of, 527. 
 
 Submaxillary gland, 30. 
 
 Submucous coat of intestine, 116 
 (fig.), 127 (fig.). 
 
 Suction action of breathing move- 
 ments, 148. 
 
 Sugars, 92, 104, 105, 119. See also 
 Carbohydrates. 
 
 Summer complaint in children, 495. 
 
 Sunshine, 428. 
 
 Supplies, public, of food, water, and 
 gas, 505 ff. 
 
 Supporting organs and tissues, 37, 
 220. 
 
 Suprarenal, 68. 
 
 Surface water, 451, 51.3. 
 
 Suspensoiy ligament of eye, 248. 
 
 Sweat glands, structure of, 184. 
 
 Synapse, 79, 273 (fig.). 
 
 Systemic circulation, 23. 
 
 Systole, 137. 
 
 Tannic acid, 361. 
 
 Tartar, 100. 
 
 Taste sensations, 261. 
 
 Tea, 301. 
 
 Teeth, 98. 
 
 Temperature, external, influence 
 on chemical change and vital ac- 
 tivities, 187, 188 ; reactions of 
 body to changes of, 200; "dan- 
 gerous region" of, 201, 390; of 
 living rooms, 390, 439, 447 ; in- 
 fluence on secretion of urine, 
 181. 
 
 Temperature of the body, 188-205. 
 
 Temperature sensations, 192, 195, 
 262. 
 
 Tendon, 8, 34, 35. 
 
 Tetanus, 601. 
 
 Theine, 301. 
 
 Thermal phenomena of the body, 
 
 187. 
 Thirst, 264. 
 
 Thoracic cavity. See Pleural cavity. 
 Thyroid glands, 60. 
 Tibia, 19. 
 Tobacco, 377. 
 Toes, bones of, 19; flexion of, in 
 
 hygiene of foot, 405. 
 Tone, arterial, 158; of skeletal 
 
 muscle, 209. 
 Touch, 201 . 
 Trachea, 10, 11, 21. 
 Traveling, hygiene and sanitation 
 
 of, 520 ff. 
 Trichina, 510. 
 Trunk movements, 315. 
 Trypsin, 118. 
 Tuberculdsis, 477 ff. 
 'Tweenbrain, 267 (fig.). 
 Tympanic membrane, 258, 259 (fig. ). 
 Tympanum, 259. 
 Type, size of, 398. 
 Typhoid fever, 483 ff. 
 
 Ulna, 19. 
 
 Underclotliing, 420. 
 Urea, 177. 
 
 Ureter, 179, 181 (fig.). 
 Uric acid, 178. 
 Urine, secretion of, 180. 
 Use and disuse in the training of 
 the nervous system, 286. 
 
 Vaccination, 498, 530. 
 
 Valves, of heart, 22, 138, 141 ; in 
 
 veins, 146. 
 Vasoconstrictor nerves, 159. 
 Vasodilator nerves, 160. 
 Vasomotor. See Vasoconstrictor 
 
 and Vasodilator. 
 Vegetable foods, 94, 111, 240, 3.50. 
 Vegetarianism, 240. 
 Veins, 7, 23, 26, 38, 39, 144 (fig.) ; 
 
 intermittent compres.sion of, in 
 
 muscular activity, 146. 
 
564 
 
 THE HUMAN MECHANISM 
 
 Veiife cav», 13, 22, 24 (fig.): 26. 
 
 Venous reservoir, 139. 
 
 Ventilation, 442 ff. ; natural, 443; 
 meclianical systems of, 449. 
 
 Ventral, 9. 
 
 Ventricles, of heart, 22, 24, 137 ; of 
 brain, 268, 269. 
 
 Vertebra, 14, 16. 
 
 Vertebral column. See Spinal col- 
 umn. 
 
 Villus of intestine, 116 (fig.), 117, 
 118 (fig.), 123 (fig.). 
 
 Visual judgments, 256. 
 
 Visual sensations, 255. 
 
 Vital resistance, 297, 482. 
 
 Vitreous liumor, 247, 251 (fig.). 
 
 Volitional actions or movements, 
 82, 279, 282. 
 
 Walking as a means of exercise, 
 
 319. See Locomotion. 
 Wandering cells, 134. 
 Warm weather, and feeding, 156 ; 
 
 circulation in, 150. 
 Warm-blooded animals, 189. 
 Warming of the house, 434 ff. ; 
 
 by open fire, 434 ; by stoves, 435; 
 
 by hot-air furnaces, 435 ; by 
 
 steam and hot water, 437. 
 Warmth, sensations of, 261. 
 Waste products, 49, 52, 59, 65, 
 
 177 ; excretion of, 225. 
 
 Water, as a food, 93, 218, 219; 
 
 and the secretion of urine, 181 ; 
 
 use as a drink, 354. 
 Water supply, of the house, 451; 
 
 purity of public, 513, 626. 
 Waters, liard and soft, 454. 
 Wells, 462. 
 White matter of spinal cord and 
 
 brain, 75. 
 Whooping cough, 493. 
 Will, 82. 
 Wind and temperature regulation, 
 
 203. 
 Wines, 365. 
 Winking, muscular and nervous 
 
 mechanism of, 70. 
 Woolen underwear, 421. 
 Work, unit of, 212. 
 Wort, 365. 
 
 AVounds, care of, 603. 
 Wrist, bones of, 19. 
 Writing, 283. 
 
 Xanthin bases, 178. 
 
 Yeast, .364. 
 Yellow fever, 492. 
 
 Zymogen, 48.