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ELEIilENTS 
 
 OF 
 
 HUMA^ PHYSIOLOGY, 
 
 BY 
 
 HEXEY POWEE, 
 
 M.S. Lo^-B., F.E.C.S., 
 
 OPHTHALMIC SURGEON TO ST. BARTHOLOMEW'S HOSPITAL, LONDON, 
 
 EXAUINEB IN THE BOAHD OF ANAT03IT AND PHYSIOLOGY, 
 
 KOYAL COLLEGE OF SUEGEONS. 
 
 ILLUSTEATEB WITH 47 ENGRAVINGS. 
 
 PHILADELFSIA : 
 HENKY 0. LEA'S SON &i CO. 
 
 188'.t. 
 

 
IBsUitatctr to 
 
 W. S. SAYORY, Esq., F.B.S., 
 
 IX PLEASANT MEMORY 
 
 OF 
 
 NEARLY FORTY YEARS' SINCERE FRIENDSHIP. 
 
Digitized by tine Internet Arciiive 
 
 in 2010 witii funding from 
 Columbia University Libraries 
 
 http://www.archive.org/details/elementsofhumanpOOpowe 
 
P E E F A C E . 
 
 The following pages have been written with tho 
 object of giving to the student a general outline or 
 the Physiology of Man. The simultaneous appear- 
 ance of complementary volumes has caused many 
 subjects to be omitted that would otherwise have 
 found a place, and are generally included in text- 
 books of physiology. Thus it will be seen that all 
 details of structure are passed over in silence, since 
 they are fully given in Klein's Elements of 
 Histology. The volume on Clixical Chemistry, 
 by Dr. Ealfe, has rendered it unnecessary to mention 
 many organic substances and many tests for organic 
 substances that are usually given. All descrij)tions 
 of instrunxients and methods of procedure in prac- 
 tical physiology have been omitted, since th^y vrill 
 be found in Dr. M'Gregor-Eobertson's work on 
 Physical Physiology ; and lastly, the appearance 
 of Prof. Bell's treatise on Comparative Physiology 
 
viii Human Physiology. 
 
 AND Anatomy has led to tlie exclusion of many 
 references to Animal Physiology. 
 
 It need hardly be stated, then, that the five 
 volumes should be read together ; and if the student 
 has mastered them thoroughly, he will, it is hoped, 
 have acquired a sound basis for the future practice of 
 his profession. 
 
CONTENTS. 
 
 -•C-- 
 
 CHAPTER I. PAGE 
 
 Introductory— General Composition op the Body. . 1 
 
 CHAPTER n. 
 The Blood , , . . 8 
 
 CHAPTER lU. 
 
 Heart, and Movement of the Blood 23 
 
 CHAPTER IV. 
 Blood-Vessels 34 
 
 CHAPTER V. 
 Respiration 56 
 
 CHAPTER VI. 
 Food 78 
 
 CHAPTER Vn. 
 Digestion of Food 99 
 
 CHAPTER VIII. 
 Chyle and Lymph 130 
 
 CHAPTER IX. 
 Glycogenic Function of the Liver 133 
 
X Human Physiology. 
 
 CHAPTER X. PAGE 
 
 The Functions of the Skin ....... 139 
 
 CHAPTER XL 
 Animal Heat .......«.•• 1^3 
 
 CHAPTER Xn. 
 The Urine » » , . 155 
 
 CHAPTER Xin. 
 Muscular Movement , , , 178 
 
 CHAPTER XIV. 
 The Nervous System ....-..». 212 
 
 CHAPTER XV. 
 
 The Senses .... ........ 272 
 
 CHAPTER XVI. 
 Generation and Development . . . * . » .322 
 
 Appendix ,...••»*»-••'• ^'^^ 
 Index .,.»«»»»•*»•• ^^ 
 
Human Physiology. 
 
 CHAPTER I. 
 
 INTRODUCTORY — GENERAL COMPOSITION OF THE BODY. 
 
 Human physiology is a branch of biology, and 
 treats of the purposes and functions of the body, and 
 of its several parts. In the lower forms of life the 
 structure is simple, and as there is no distinction of 
 organs every part is capable of discharging a number 
 of functions, each of which in the higher animal is 
 limited to a special organ. The simplest forms of 
 living beings with which we are acquainted are found 
 in the class to which the term Protista is applied. 
 In these the body is composed of a mass of proto- 
 plasm, through which granules are usually scattered, 
 and the whole, or any part of which, can absorb 
 nutriment from without, digest it, apply it to the 
 nutrition of its own structure, cast out the indigestible 
 matter, move from place to place, respond by move- 
 ment to stimulation of various kinds, and undergo 
 multiplication by division. In the higher animals 
 each of these functions is limited to a definite orsran. 
 An alimentary canal is destined to digest the food, 
 and prepare it for absorption into the circulating fluids. 
 Definite vessels are formed, one part of which is 
 developed into a pulsating organ or heart, by which 
 the fluids elaborated into blood are driven to all 
 parts of the system, whilst special organs, as the 
 bones, cartilages, muscles, and nerves, are subservient 
 
 B 
 
2 Human Physiology. [Chap. i. 
 
 to movement, and to the reception, conduction, and 
 the perception of impulses from without, whilst yet 
 others are subservient to reproduction. 
 
 Oeneral composition of tlie body. — The 
 
 body is composed of certain ultimate elements united 
 in part to form inorganic compounds, and in part 
 more complex organic compounds, chiefly made up of 
 carbon and hydrogen either with oxygen alone, or with 
 oxygen and nitrogen, named proximate principles. 
 Some of these are in process of assimilation to the 
 tissues and organs of the body, some form part of the 
 organs themselves, and some are undergoing disinte- 
 gration or decay, represent waste products, and are on 
 their way to be discharged from the body. 
 
 The ultimate elements are carbon, hydrogen, oxy- 
 gen, nitrogen, sulphur, phosphorus, chlorine, fluorine, 
 potassium, sodium, calcium, magnesium, manganese, 
 iron, and silicon. The inorganic compounds are 
 water and free hydrochloric acid ; the carbonates, 
 chlorides, fluorides, sulphates, and phosphates of the 
 alkalies, and alkaline earths. The amount of loater is 
 58 '5 per cent, of the body weight, but different tissues 
 contain very difierent proportions. Thus, the kidneys 
 contain 82-7 per cent., the bones 22, the teeth 10, and 
 the enamel only 0'2 per cent. The gases are oxygen, 
 and perha})S ozone, hydrogen, nitrogen, carbon dioxide, 
 marsh gas CH4, ammonia NH3, and hydrogen sul 
 phide HgS. 
 
 The proximate organic compounds may be divided 
 into two groups ; those containing nitrogen, and those 
 which are destitute of nitrogen. 
 
 The nitrogenous or azotised compounds are : 
 Proteids, including the albumins, fibrin, casein, 
 globulin, and peptones ; and the Albuminoids, mucin, 
 chondrin, glutin, keratin, elastin, and ferments ; the 
 Biliary acids ; cerebrin, lecithin, and many others. 
 
 The no^i-azoiised compounds are the Sugars : grape 
 
Chap. I.] Characters of Proteids. 3 
 
 sugar, milk sugar, inosite, glycogen, and cellulose ; the 
 Fats, stearin, palmitin, and olein ; and the Organic Acids, 
 formic, butyric, capronic, lactic, and sarcolactic acids. 
 The following synopsis of the chief proteid bodies 
 is taken from Gamgee's "Physiological Chemistry," 
 and should be carefully studied. 
 
 ( Class I. — Albumilts. — Proteid bodies, wMcli are soluble 
 in water, and which, are not precipitated by alkaline 
 carbonates, by sodium chloride, or b_y very dilute 
 acids. If dried at a temperature below 40° C. they 
 become yellow and transparent, break with yitreous 
 fracture, and are soluble in water ; coagulation occurs 
 between 65'^ C. and 73° C. 
 
 1. Serum albumin. — Specific rotation (a) d = 
 
 — 56°, not precipitated from its solutions on 
 the addition of ether. 
 
 2. Egg albumin. — Specific rotation [a) d = 
 
 — 3o-5*^, precipitated from its solution on 
 agitation with ether. 
 
 Class II. — Peptones. — Proteid bodies, exceedingly 
 j£ soluble in water ; solutions not coagulated by heat ; 
 
 nor precipitated by sodium chloride, nor by acids or 
 alkalies. Precipitated by a large excess of absolute 
 alcohol, and by tannic acid. In the presence of much 
 caustic potash, or soda, a trace of solution of copper 
 j^ sulphate produces a beautiful rose colour. 
 
 (33 /^ Class III. — €rlol>uliilS. — Proteid substances, which 
 are insoluble in pure water, but soluble in dilute 
 solutions of NaCl. These solutions are coagulated by 
 heat ; they are soluble in very dilute hydrochloric 
 acid, being converted into acid-albumin. They are also 
 readily converted by alkalies into alkali-albumins. 
 1. Vitellin. — Xot precipitated from its solution 
 when these are saturated with common salt. 
 \ 2. Myosin. — Precipitated from its solution in 
 
 weak common salt when these are saturated 
 with sodium chloride. Solutions coagulate at 
 55" to 60° C. Solutions in common salt not 
 coagulated by solution of fibrin ferment. 
 
 3. Fibrinogen. — Soluble in weak solutions of XaCl. 
 Precipitated from them completely on the 
 addition of NaCl, when this amounts to 12 
 
 \ or 16 per cent. Solutions coagulate on the 
 
 'o "« 
 m pi 
 
 +3 O 
 
 4 
 
 P o 
 
 <D O 
 ^ Z 
 
 p! o 
 
 I* 
 
 c .3 
 
Human Physiology. [Chap. i. 
 
 '.S addition of fibrin ferment. Temperature of 
 
 ^ . coagulation 56^ C. 
 
 '%% 4. FaraglohuUn. — Soluble in -weak solution of 
 
 'o ^ sodium chloride. From weak alkaline solu- 
 
 ^ o tions paragiobulin is precipitated by the 
 
 ^ I addition of a very small quantity of NaCl. A 
 
 ^ o further addition of this body effects re- 
 
 ^ _" solution of the precipitate, which is thrown 
 
 ■{ ^ "o down again, though still not completely, 
 
 G § when the amoimt of I^aCl in solution exceeds 
 
 ^ .2 20 per cent. Paragiobulin is completely pre- 
 
 ^^ cipitated when its solutions are saturated 
 
 •'^ § with ammonium sulphate. Its solutions are 
 
 -2 j-i not precipitated by addition of fibrin ferment. 
 
 ^ I Temperature of coagulation varies, ac- 
 
 g ^ cording to the amount of salts present, and 
 
 {^B mode of heating, between 68'' C. and 80° C. 
 
 Class IV. — Derived albmiiiiis. — Proteid bodies, in- 
 soluble in pure water, and in solutions of XaCl, but readily 
 soluble in dilute HCl, and in dilute alkaline solutions. 
 Solutions not coagulated by heat. 
 
 1. Acid albumins. — Obtained by the action of dilute 
 
 acids, especially HCl, upon solutions of proteids, 
 by action of strong acids upon solid proteids, and 
 as first products in the action of gastric juice upon 
 proteids. On neutralising solutions of acid 
 albumins they are precipitated even in the 
 presence of alkaline phosphates. NaCl added to 
 saturation also precipitates them. 
 
 2. (a) Alkali albumins. — Obtained by the action of 
 
 dilute alkalies upon the proteids. Possess the 
 properties of sub-class 1, with the exception that 
 in the presence of alkaline phosphate, the 
 solutions are not precipitated by neutralisation ; 
 when heated with strong solution of caustic 
 potash, potassium sulphide is not fonaied. 
 (;3) Casein. — The chief proteid constituent of milk. 
 Same properties as a, but when treated with strong 
 solution of caustic potash, potassium sulphide is 
 formed. In milk is coagulated by rennet. 
 Class Y. — Fitorill. — Insoluble in water, and in weak solution 
 of NaCl. White, elastic, solid, usually exhibiting fibrilla- 
 tion under the microscope ; swells up in cold hydrochloric 
 acid of -1 per cent., but does not dissolve. When thus 
 swollen, dissolves witli ease when a solution of pepsin is 
 poured over it. When heated for a great many hours at 
 
Chap. I.] Characters of Proteids. 5 
 
 40° C. in dilute HCl it dissolves, and the solution contains 
 acid-albumin. 
 
 Class VI. — CoagXllated proteids.— Insoluble in water, 
 dilute acids, and alkalies. Gives Millon's reaction. Are 
 dissolved when digested at 35° C. to 40° C. in artificial 
 gastric or pancreatic juice, giving rise to peptones. 
 
 Class VII. — Liardaceia or amyloid substance. — 
 Insoluble in water, in dilute acids, in alkaline carbonates ; 
 not dissolved by gastric juice at the temperature of the 
 body, coloured brownish-red or violet by iodine. 
 
 Oeneral characters of the proteids. — All the com- 
 pounds included in this group of substances contain nitro- 
 gen and sulphur, and their chemical composition is ex- 
 pressed by a fomiula more or less closely resembling the 
 following : Cg4ll7]Sr;^g022S4. They are amorphous, with 
 variable solubility in water and acids ; usually soluble in 
 alkalies ; almost insoluble in alcohol and ether. The 
 aqueous solutions are neutral. They are not volatile ; they 
 burn with an odour of burnt feathers, giving off ammo- 
 niacal fumes, and leaving a residue of ash which chiefly 
 consists of lime phosphate. Exposed to the air they easily 
 undergo decomposition. Calcined with potash, or boiled 
 with sulphuric acid, they yield leucin and tj-rosin. Hot 
 concentrated nitric acid converts them into a yellow body, 
 xanthoproteic acid. Treated with acids or with alkalies, 
 or when allowed to undergo putrefactive decomposition, 
 they give, amongst others, the following products of de- 
 composition : Volatile fatty acids ; oxalic, acetic, formic 
 valerianic, fumaric, and asparagic acids; leucin, tyrosin, 
 and ammonia. "When treated with oxydising agents, they 
 yield formic, acetic, propionic, butyric, valerianic, capric, 
 and benzoic acids ; the aldehydes of these acids and volatile 
 organic bases, aceto-nitril, valero-nitril, and propio-nitril. 
 They rotate polarised light to the left. They are pre- 
 cipitated from their solutions by an excess of the strong 
 mineral acids, by acetic or hydrochloric acid, and po- 
 tassium ferrocyanide, the basic acetate of lead, mercury 
 bichloride, tannin, and by potash carbonate in powder. 
 
 Tests for the proteids. 
 
 1. Kitric acid test: — Heat the liquid, and add nitric acid 
 till the reaction is strongly acid : a precipitate falls which 
 undergoes no change on the addition of acid, 
 
 2. Sodium sulphate test. — Add acetic acid till the reaction 
 is strongly acid. Mix with an equal volume of concen- 
 trated solution of sodium sulphate, and boil ; the proteids 
 are precipitated. 
 
6 ITuMAN Physiology. [Chap. i. 
 
 3. PietroicsTci' s test. — AYarm the liquid containing albumin 
 "with a moderate quantity of solution of potash, or soda, and 
 then add one or two drops of copper sulphate ; the 
 liquid assumes a violet colour. 
 
 4. XantJioproteic reaction. — Heat with concentrated nitric 
 acid; the liquid, if it contain a proteid, will assume a yellow 
 tint, which becomes reddish- orange by the action of alkalies. 
 
 5. Millons test. — Millon's reagent is made by dissolving in 
 the cold one part of mercury in its weight of concentrated 
 nitric acid ; the solution is completed by applying gentle 
 warmth. Two volumes of distilled water are then added, 
 and the fluid decanted. This test gives a red colour with 
 liquids containing proteids, which is more marked when 
 they are heated to 60'' C. or 70° C. 
 
 6. Adamkieic lex's test. — Ever^^^ proteid, when dissolved in an 
 excess of glacial acetic acid, gives, on the addition of con- 
 centrated sulphuric acid, a beautiful violet colour and a 
 slight fluorescence (Beaunis). 
 
 Carbohydi'ates, These include : 
 
 1. Starch CgHioOj, formed in and produced by plants 
 growing under the influence of light. It is often stored 
 up as aliment in tubers and fi^uit, and consists of granules 
 presenting concentric markings. It is insoluble in 
 cold, but swells and becomes gelatinous in hot water, 
 which dissolves the granulose, but leaves undissolved the 
 cellulose of the starch granules. Gives a blue colour with 
 free iodine. Exposed to heat the blue tint vanishes, but 
 will return if the liquid be suddenly cooled. Starch, on 
 being heated to 210° C, is converted into dextrin, which 
 also appears in germinating seeds. When starch paste is 
 boiled with dilute acid it is converted through some inter- 
 mediate stages into dextrin and then into sugar, and the 
 same conversion is effected by the saliva, pancreatic and 
 intestinal juices. 
 
 2. Sugars. — These are substances having a more or less sweet 
 taste, usually soluble in water, and destroyed by strong sul- 
 phuric acid, which abstracts the water of these compounds 
 and leaves only the carbon. The most important are glycose 
 CgHioOg, lactose C12H24O12, saccharose Ci2H220n, and glyco- 
 gen CgHio^s- ^^ fermentation they yield CO.2 and alcohol. 
 The tests for glycose are : 
 
 1. Trammer'' s test, which depends on the circum- 
 stance that sugar in. an alkaline solution acts as a 
 reducing agent. To the saccharine fluid about 
 one-fourth of its bulk of soda or potash lye is 
 added, and a dilute solution of copper sulphate. 
 
Chap. I.] Carbohydrates AND Fats. 7 
 
 A slight clouding occurs from precipitatioa of 
 hydrate of copper oxide, which dissolves on 
 shaking. On heating the fluid, if sugar he 
 present, a yellowish precipitate of copper oxide 
 takes place. 
 
 2. Bottger''s test. — An alkaline solution of hismutk 
 
 is reduced hy sugar to hismuth suboxide, the 
 oxide heing precipitated as an olive green and 
 ultimately black powder. 
 
 3. Moore's test. — The fluid is boiled with caustic alkali, 
 
 and becomes, if sugar be present, fir^t yellow, 
 then brown, and then blackish. On the addition 
 of nitric acid a smell of caramel is perceptible. 
 
 4. Mulder atid Neubauer'' s test consists in the addition 
 
 of a sufficient quantity of a solution of indigo 
 carmine to give the saccharine solution a faint 
 blue tint, and applying sodium carbonate ; on 
 applying heat the colour changes to green, purple, 
 and red. On agitation with air the fluid recovers 
 its blue tint. 
 6. Runge's test. — Saccharine solutions, when evaporated 
 to dryness with sulphuric or hydi'ochloric acid 
 in a water bath in a porcelain capsule, leave 
 behind a black shining residue. 
 6. Silver test. — Diluted solution of grape sugar, on 
 being boiled with silver nitrate and ammonia, 
 leaves a metallic deposit on the surface oi the 
 vessel. Alcoholic solution of grape sugar, 
 mingled with alcoholic solution of caustic alkali, 
 causes a precipitation of a compound of alkali and 
 sugar in the form of white flocculi. 
 Fa.tS« — The fats are ethers derived from the triatomic alcohol 
 glycerin C^B.^{OW]^. They are widely distributed both 
 in plants and in animals. They contain very little 
 oxygen. They are soluble in ether, benzol, chloroform, 
 and in boiling alcohol. Dropped on paper they give a 
 characteristic grease spot. Shaken up with colloid sub- 
 stances they give an emulsion. Heated with water above 
 steam heat, or exposed to the action of certain animal 
 ferments, as that of the pancreas, they take up water and 
 split into glycerin and free fatty acids. When these 
 essential fats are boiled with solutions of the alkaline 
 hydrates or carbonates, they undergo the process of 
 saponification, decomposing as before into glycerin, and 
 the fatty acids, but the latter immediately combine with 
 the alkaline metal to form a soap, which is, in fact, a 
 
8 Human Physiology. [Chap. ii. 
 
 soluble salt. The three principal fats of the human body 
 are tristearin, tripalmitin, and triolein. The following 
 formulae represent, according to Gamgee, the composition 
 of these bodies : 
 
 rPalmitin, C,H/OCi6H3iO)3 
 Olycerin C3H5(OH)3 \ Stearin, CoH^fOCjgHsaOJa 
 (Olein, C3H.(OCi8H330)3 
 Palmitic acid, CigHsiOjOH 
 Stenric acid, C^gHsoOjOH 
 Oleic acid, Ci8H330,OH. 
 Stearin melts at about 60'^ C, palmitin at about 45° C, 
 olein at about 30° C. Stellate crystals of stearin and 
 palmitin often occur in fat cells. 
 
 CHAPTER II. 
 
 THE BLOOD. 
 
 Blood may be regarded as a tissue in which the 
 matrix is fluid and in which cell-like elements float, 
 the whole being in constant motion. 
 
 Physical properties of tlie blood. — Blood is 
 of a red colour, varying in difierent parts of the circu- 
 lating system from a carmine or almost purple tint to 
 \ ermillion. It is usually brighter in the arteries than 
 in the veins, owing to the arterial blood being charged 
 with oxygen, whilst that in the veins contains deoxy- 
 dised haemoglobin. Blood is exceedingly opaque, a 
 comparatively thin layer entirely intercepting the 
 passage of light. It presents an alkaline reaction 
 when freshly drawn, which, however, soon becomes 
 less marked, owing to the formation of acids. The 
 reaction may be shown by impregnating neutral 
 porous plates with red tincture of litmus. On allow- 
 ing a drop of blood to fall upon the part, the cor- 
 puscles rest on the surface, whilst the fluid parts of 
 the blood are absorbed, and eti'ect a change of colour 
 in the plate a little below the surface. The smell of 
 blood resembles that of the animal from which it 
 
Chap. II.] 
 
 Spectrum of the Blood. 
 
 is drawn, and may 
 be rendered more 
 perceptible by the 
 addition of a little 
 strono; sulphuric 
 acid, which expels 
 the volatile fatty 
 acids to which it 
 is due. The taste 
 is saline and maw- 
 kish. Its sp. gr. is 
 1056 with extremes 
 of 1045 and 1077. 
 
 The spectriiiii 
 of tlie blood.— 
 When a thin layer 
 of blood is examined 
 with the spectro- 
 scope, its spectrum 
 is found to present 
 two absorption 
 bands, the position ^^ 
 of which is indicat- w 
 ed in the adjoining 
 woodcut (Fig. 1, i). 
 
 The iDands oc- 
 cupy a position be- 
 tween the lines d 
 and E of Frauen- 
 hofer's lines in the 
 orange and yellow 
 part of the solar 
 spectrum. If, by 
 means of reducinof 
 agents, the oxy- 
 gen be withdrawn 
 from the blood, the 
 
 cq 
 
 w 
 
lo Human Physiology. [Chap. ii. 
 
 oxy-hsemoglobin ordinarily present is reduced or con- 
 verted into hsemoglobin, the double absorption band 
 disappears and is replaced by a single band (Fig. 1, 2), 
 wbicli occuj)ies a position nearly intermediate to the 
 two bands of oxy-hsemoglobin. It is remarkable that 
 in blood which has been exposed to the action of car- 
 bonic oxide gas, the spectrum presents two absorption 
 bands which are almost, though not quite, identical with 
 those of oxy-hsemoglobin (Fig. 1, 3). 
 
 Quantity of tlie l>lood. — The quantity of the 
 blood in man is estimated to be from one-twelfth to 
 one-fourteenth of the weight of the body, so that a 
 man weighing twelve stone has about fifteen pounds 
 of blood in his body. It may be determined by 
 Welcker's method, which consists in collecting all the 
 blood that can be obtained by opening the larger 
 vessels. This gives approximatively the total quantity 
 contained in the body, but approximatively only, for a 
 considerable quantity still remains in the capillaries 
 of the muscles, and of various isolated organs, as the 
 brain, spleen, and liver. To estimate this extra 
 quantity the body is finely minced and thrown into 
 a large quantity of water. The colour of the infusion 
 is then compared with that of a series of test liquids 
 in which water is mingled with known quantities of 
 blood, and a tolerably accurate conclusion may thus 
 be drawn of the quantity present in the body after 
 the general bleeding, A modification of this method 
 has been employed by Gescheidlen, in which the tint 
 of the blood is rendered uniform, and decomposition 
 retarded by treating the blood with carbon monoxide. 
 Lehmann, with Weber, obtained a higher number 
 than that above given. These observers determined 
 the weight of two criminals before and after decapita- 
 tion, and having washed out the vessels with water, 
 the quantity of blood remaining in the body was 
 calculated by instituting a comparison between the 
 
Chap. II.] Corpuscles of the Blood. 1 1 
 
 solid residue of the pale red aqueous fluid and that of 
 the blood which first escaped. By waj of illustration 
 the following gives the results of one of the experi- 
 ments with which the other was in close accordance. 
 The living body of one of the criminals weighed 
 60140 grammes, and after decapitation the same body 
 weighed 54600 grammes, consequently 5540 grammes 
 of blood had escaped; 28 '5 60 grammes of the blood 
 yielded 5*36 grammes of solid residue; 60'5 grammes 
 of sanguineous water collected after the injection 
 contained 3*724 grammes of solids; 6050 grammes of 
 the sanguineous water that returned from the veins 
 were collected, and these contained 37 '24 grammes of 
 solids, which corresponds to 1980 grammes of blood; 
 consequently the body contained 7520 grammes of 
 blood (5540 escaping in the act of decapitation and 
 1980 remainiug in the body), hence the weight of the 
 whole blood was to that of the body nearly in the 
 ratio of one to eight. The quantity of blood is greatly 
 increased after a meal, especially when liquid has 
 been taken. It does not diminish but rather increases 
 relatively to the weight of the body in inanition. In 
 the new-born child it is only about one-nineteenth of 
 the total weight of the body. An increase takes place 
 during pregnancy, especially in the latter half. In 
 plethora the quantity of the blood is augmented, in 
 ansemia diminished. After severe haemorrhages the 
 loss of blood is soon restored, though the corpuscles 
 are manifestly fewer in number. 
 
 Morpliological or formed elements of the 
 blood. — Blood contains coloured and colourless 
 corpuscles, the physical characteristics of which are 
 fully detailed in the companion volume, "The Elements 
 of Histology." The coloured corpuscles are elastic and 
 have a specific gravity of about 1088. Their elas- 
 ticity is shown by the temporary alteration in form 
 they undergo in bending round an angle of division of 
 
12 Human Physiology. [Chap. ii. 
 
 a blood-vessel. The essential purpose they fulfil is that 
 of oxygen-carriers ; for the hsemogiobin they contain 
 combines with oxygen during their transit through the 
 lungs, and surrenders it to the tissues as it traverses 
 them. The number of the red corpuscles is enormous; 
 a cubic millimetre contains between 4,000,000 and 
 5,000,000, and it may be estimated by diluting a defi- 
 nite quantity of blood with many times its bulk of 
 water, distributing the mixture evenly over a stage 
 ruled into squares of known size, and then counting 
 the number of corpuscles in each square. Estimates 
 which are, of course, only approximate show that the 
 red corpuscles of an adult present an aggregate sur- 
 face of about 3,000 square yards, whilst the surface 
 they present for the absorption of oxygen in the lungs 
 each second is about 80 square yards. 
 
 The white corpuscles are small nucleated masses of 
 protoplasm, capable of executing amoeboid movements 
 at the rate of progression of 13 millimeters per minute, 
 and much fewer in number than the red, the number 
 in 1 cubic millimetre of blood varying from 4 — 7,000. 
 
 The protoplasm of the colourless corpuscles is a 
 proteid which undergoes partial decomposition, or at 
 least coagulation at 40° C. It swells and becomes 
 transparent when treated with acetic acid. It dissolves 
 in a 10 per cent, solution of NaCl, the nuclei of 
 which consist of nuclein remaining undissolved. The 
 white corj^uscles contain glycogen, recognisable by the 
 reddish colour it gives with potassium iodide and iodine. 
 They sometimes contain a few minute fat granules. 
 
 The use of the colourless corpuscles is less cer- 
 tainly known, but amongst other purposes they 
 probably subserve the repair of tissues and aid in the 
 production of the coloured corpuscles. They contain 
 myosin, fat corpuscles, cholesterin and protagon, 
 nuclein and glycogen. 
 
 Other colourless corpuscles have been described by 
 
Chap. II.] Coagulation of the Blood. 13 
 
 iNTorris, which have been shown bj Mrs. Hart to be 
 red corpuscles from which the colouring matter has 
 been dissolved out. 
 
 Coagpiilatiou of tlie blood. — Blood, on being 
 withdi-awn from a vessel, is perfectly fluid, but, with- 
 in a few minutes, it first becomes viscous, and then 
 sets or coagulates, becoming converted into a solid 
 gelatinous mass, and giving off at the same time a 
 peculiar odour termed the halitus. Small transparent 
 beads soon appear on the surface, and, running together, 
 form a layer of yellowish fluid. The fluid is the 
 serum ; the red solid mass from which the serum 
 exudes is the clot. The time that elapses between 
 the escape of blood from a vessel and the occurrence 
 of coagulation varies. The minute drops that spirt 
 from a divided artery, and fall on a. rough surface, 
 like baize, during an operation, often coagulate in a 
 minute, or, at most, within two minutes ; but, when 
 drawn in mass into a bowl, coagulation does not com- 
 mence for ten minutes, though the blood gradually be- 
 comes thicker. In the horse under ordinary con- 
 ditions, and in other animals, if the blood be rapidly 
 cooled, a triple separation of its constituents may be ob- 
 served. The red blood-corpuscles_, being the heaviest, 
 .sink to the bottom of the vessel ; lying upon them is 
 a thin layer of white corpuscles, whilst the upper part 
 of the fluid column consists of nearly pure liquor 
 sanguinis. Notwithstanding this separation of parts, 
 the column coagulates with nearly equal firmness 
 throughout, and the exudation of serum takes place 
 in the customary manner. It is possible to filter 
 blood, and the corpuscles are found to remain on the 
 filter, whilst the liquor sanguinis passes through it. 
 In this case the liquor sanguinis will set as usual, 
 showing that the presence of the corpuscles is not 
 essential to the act. Under the microscope, it may 
 be seen that, just before coagulation sets in, the red 
 
14 Human Physiology. [Chap. ii. 
 
 corpuscles have a tendency to adhere by their flat 
 surfaces, so as to resemble small piles of coin. 
 
 Cii'dimstances affecting the rapidity of 
 coagulation. — Cold approximating the freezing 
 point of water retards coagulation, and, if the cooling 
 be rapidly effected, the blood may even be frozen 
 before it has had time to coagulate. In this condition 
 it may be kept for an indefinite period, but, if thawed, 
 it quickly sets. Coagulation may be prevented by 
 the addition of large quantities of various neutral 
 salts, such, for example, as sodium, potassium or mag- 
 nesium suljDhate or chloride, the alkaline carbonates 
 nitrate of potash, syrup, pepsin, and white of egg, and 
 by acetic acid added in sufficient quantity to give it an 
 acid reaction. Venous blood coagulates more slowly 
 than arterial, apparently in consequence of the larger 
 proportion of carbonic acid gas it contains. Hence, 
 too, coagulation is slow in the blood of those who 
 have been asphyxiated. Coagulation of normal blood 
 never takes place as long as it is moving within the 
 vessels ; and, even if it be confined to a limited portion 
 of an artery or vein by pressure applied at two points 
 without injury to the vascular walls, it still exhibits 
 little or no tendency to coagulate. If, however, the 
 wall of the vessel is injured, as usually occurs when 
 ligatures are cast around it and tied, the contained 
 blood sets readily enough. Blood rapidly drawn from 
 the body into a smooth vessel, such as glass or china, 
 sets slowly. Under all these circumstances, the cor- 
 puscles have time to settle towards the bottom of the 
 vessel, leaving the plasma at the upper part clear. 
 The coagulation of this clear layer of plasma forms 
 the buff'y coat seen in the slowly coagulating blood 
 of the horse, and in inflammatory affections in man. 
 
 Coagulation is hastened by free exposure of the 
 blood to air, and it occurs quickly in blood flowing 
 from a small vein or from a small orifice in a vein, 
 
Chap. II.] Coagulation of the Blood. 15 
 
 especially when it falls on a rough surface or is 
 received in a metallic vessel which presents many 
 points at which the process may commence. It is 
 hastened by a moderately high temperature. If ex- 
 posed to a temperature approacliing 150° Tahr., 
 coagulation of the serum albumin takes place. 
 
 During coagulation the blood becomes less alka- 
 line, the amount of oxygen it contains diminishes, 
 that of carbonic anhydride increases, and a slight 
 rise of temperature occurs. 
 
 Cause of the coagxilation of tlie blood. — 
 The coagulation of the blood is due to the solidification 
 of fibrin, an albuminous substance which is believed 
 to be formed by the union, under the influence of 
 a ferment and in presence of small quantities of 
 neutral salts and alkalies, of two compounds origi- 
 nally existing in solution named fibrinogen and para- 
 globulin. The ferment does not pre-exist in the 
 blood, but it develops after the blood is withdrawn 
 from the vessels, and appears to be derived from the 
 breaking down of the colourless corpuscles; and these 
 corpuscles also yield a part, and perhaps a large part, 
 of the paragiobulin. Fibrin can be obtained in a 
 tolerably pure state by whipping blood with a bundle 
 of twicjs before coacrulation has commenced. It then 
 appears as a buff'-coloured, stringy substance, which 
 has considerable elasticity. For its chemical charac- 
 ters see page 4. 
 
 It is remarkable that the serous fluids poured 
 forth in inflammatory affections of the pleura, peri- 
 cardium, peritoneum, tunica vaginalis, and other 
 serous membranes, exhibit little or no tendency to 
 coagulate ; but, if a little blood from which the fibrin 
 has been removed by whipping be added to either of 
 them, coagulation at once occurs. The whipped, or 
 defibrinated, blood appears to contain something 
 capable of inducing coagulation when brought into 
 
1 6 Human Physiology. [Chap. ii. 
 
 contact with anotlier substance contained in the 
 transudate ; and Schmidt believes he has been able to 
 show that whilst the ferment, and a large part of the 
 paraglobulin, proceed from the white corpuscles, the 
 fibrinogenous substance is contained in solution in the 
 liquid, and that he has isolated these substances. 
 
 The reasons for believing that the ferment is 
 derived from the white corpuscles are : 
 
 (1) That there is evidence that the ferment does 
 not proceed from the red corpuscles. 
 
 (2) That blood freed from its white corpuscles by- 
 filtration at 0° C. coagulates late and slowly ; yet, if 
 temporarily raised to 10° — 20° C, and re-cooled and 
 filtered, the filtrate, though destitute of Avhite cor- 
 puscles, is rich in ferment, the corpuscles having had 
 the opportunity of parting with it. 
 
 (3) That the quantity of ferment in filtered 
 plasma, which removes the white corpuscles, is not 
 greater at the end of coagulation than at the begin- 
 ning, whilst, in non-filtered plasma, the ferment 
 undergoes gxeat increase during the act of coagula- 
 tion. 
 
 The reasons for believing that the paraglobulin 
 is partly deri^•ed from the white corpuscles are : 
 
 (1) That filtered plasma gives nearly 30 per cent, 
 less fibrin than unfiltered plasma. 
 
 (2) That the number of white corpuscles in the 
 blood is greater before than after coagulation. 
 
 Bizzozero so far differs from Schmidt's views that 
 he believes certain granular masses and plates found 
 in the blood are the active agents, instead of the 
 white corpuscles. 
 
 COMPOSITIOX OF THE BLOOD. 
 
 The quantitative composition of the blood varies 
 considerably in different persons, and even in different 
 
Chap. II.] Composition of the Blood. 
 
 17 
 
 parts of tlie same person ; the following table repre- 
 sents that of the horse, but may be taken as similar to 
 that of man. 
 
 [ Water 200 
 
 rCeUs, 328 ) /Hemoglobin .... 116 
 ' ' ' Other organic com- 
 pounds 10 
 
 Salts ....... 2 
 
 <^ S J 
 
 O O 
 O O 
 
 Solids, 128 
 
 -i-rQ 
 
 ^Plasma, 672 
 
 {■ 
 
 fWater , . 604 
 
 Fibrin ...... 7 
 
 Albumin 52 
 
 Fat ...... . 1 
 
 Other organic com- 
 
 Solids, 68 <^ pounds 3 
 
 Potassium and sodium 
 
 salts ... . . 4 
 Calcium and magnesium 
 
 salts 1 
 
 The Proteids of the blood. — The coloured 
 corpuscles contain from 5 to 12 per cent, of proteids. 
 The most important of these is haemoglobin. 
 
 Hsemog-lobiii.- — This substance stands almost 
 alone in the complexity of its constitution, the formula 
 for each molecule being CgooHggijjSTj^^FeSgOi-g. It has a 
 strong affinity for oxygen, 1 gramme of hsemoglobin 
 being capable of taking up 1*27 cubic centimetres of 
 oxygen at 0° C, and 1 metre pressure. It is then 
 termed oxy-h(emoglohin. The oxygen can be displaced 
 by carbonic oxide gas, and can be abstracted from it 
 by the tissues of the body, which have a stronger 
 affinity for oxygen than hsemoglobin. It is then 
 termed reduced hcemoglohin. Hsemoglobin therefore 
 acts as an oxygen carrier, absorbing and combining 
 with oxygen as it traverses the lungs, parting with 
 it as it courses through the body. Oxy-hsemoglobin 
 crystallises in the forms shown in Klein's " Histolog}^" 
 p. 13. The crystals are doubly refracting and pleo- 
 chromatic. They may be obtained by various methods, 
 c 
 
1 8 Human Physiology. [Chap. ii. 
 
 the principle in all of which is to effect the solution 
 of the haemoglobin, either in serum or water, and then 
 by the addition of alcohol or by the agency of cold, 
 or of both conjointly, to cause the haemoglobin to 
 crystallise out (Gamgee). Moist haemoglobin is a pasty 
 red mass, evincing when pure and Avhen kept in vacuo 
 little or no tendency to decomposition, readily dissolv- 
 ing in w^eak solutions of the caustic, and of the carbo- 
 nated alkalies, and easily decomposed by strong alkaline 
 solutions, as well as by acids and acid §alts with for- 
 mation of haematin. 100 grammes of blood contain 
 1 2 to 1 5 grammes of hsemogiobin. 
 
 Products of tlie decoiiipositioii of lisemo- 
 g:lol>m. — When h£emogiobin undergoes decomposition 
 without access of oxygen it yields a proteid and a 
 body named hcemoclwomogen, but when it is exposed 
 to air for some time haemoglobin loses its blood-red 
 colour, assumes a brownish tint, presents an acid 
 reaction, is precipitated by solutions of basic lead 
 acetate, and gives a spectrum in which the two bands 
 of oxy-hsemoglobin are faint, whilst a new band 
 appears in the red near c. On adding some reducing 
 agent, as sulphide of ammonium, the fluid gives the 
 spectrum of reduced haemoglobin, whilst on shaking 
 the solution containing the latter with air, oxy-haemo- 
 globin is again formed. These characters appear to 
 be possessed by haemoglobin whilst in process of trans- 
 lation into haematin and a proteid, and to this 
 intermediate stage the term rtiethcemoglohin has been 
 applied. 
 
 Haematm. — This is one of the products of the 
 decomposition of haemoglobin by the action of acids 
 and acid salts in presence of oxygen. When blood is 
 treated with acetic acid it assumes a brown tint, and 
 the acid haematin formed can be dissolved out with 
 ether. Pure haematin is of blue-black colour with 
 metallic lustre and does not crystallise. Its formula is 
 
Chap. II.] Plasma of the Blood. 19 
 
 Ce8H;oN8^e20io. It contains 12-6 per cent, of oxide of 
 iron. It is insoluble. The etherial solution presents 
 four absorption bands, two between c and d, one 
 between d and e, and another between h and f. 
 
 Meoniiii. — When a drop of blood is boiled with 
 acetic acid and a little common salt, it immediately 
 becomes brown^ and on evaporation reddish -brown. 
 Prismatic crystals may be obtained, which are those of 
 hsemin. They are insoluble in water, scarcely soluble 
 in hot alcohol and ether, soluble in liquor potassa;. 
 They are of much importance in medico-legal enquiries. 
 
 Il£eiiia,toiditi. — This name is given to the yellow 
 microscopical crystals found in old apoplectic clots and 
 other extravasations of blood. They appear to be 
 identical in form with those of bilirubin, and when 
 treated with fuming nitric acid give the same colour 
 reaction as the chief colouring matter of human bile 
 (Gamgee). 
 
 PLASMA OF THE BLOOD. 
 
 The fluid in which the corpuscles float is the 
 plasma. It is viscous, of yellowish colour when in 
 mass, with specific gravity of 1026 to 1029. Its re- 
 action is alkaline. It is capable of coagulation in the 
 presence of white corpuscles, or of the products of 
 their disintegration. It contains certain proteids, 
 the most important being 
 
 Parag^lobiiliii. — This is also known under the 
 names of serum globulin, and of Schmidt's fibrino- 
 plastic substance. It is obtained by diluting plasma 
 with 10 to 15 times its volume of ice-cold water, and 
 transmitting through the fluid a stream of carbon 
 dioxide. It may also be obtained by adding 4 drops 
 of a 25 per cent, solution of acetic acid to 10 cubic 
 centimetres of serum, diluted with 150 cubic centi- 
 metres of II2O, and, still better, by adding magnesium 
 sulphate to complete saturation, when the whole of the 
 
20 Human Physiology. [Chap. ii. 
 
 paraglobulin falls. It is probably in part contained in 
 solution in the plasma, and in part in the colourless 
 corpuscles. It coagulates at about 75° C. 
 
 Fibriiiog'eii. — After the paraglobulin has been 
 removed from plasma, if the fluid be still further 
 diluted and COj passed, fibrinogen is precipitated. 
 This substance is insoluble in pure water, but soluble 
 in water which holds oxygen in solution. Like para- 
 globulin, it is soluble in a solution of sodium chloride, 
 containing 5 to 8 per cent of the salt. When the 
 quantity of salt attains 12 to 16 per cent., fibrinogen 
 is precipitated, whilst paraglobulin remains in solution. 
 Solutions of fibrinoo-en coagulate at b'o^ 0. 100 
 grammes of the plasma of a horse have been found in 
 one experiment to contain 0*4299 gramme of fibrino- 
 gen, and to yield 0*375 gramme of fibrin. 
 
 The Serum of blood. — The serum is the fluid 
 that is gradually expressed from the clot by the con- 
 traction of the fibrin, and which accumulates upon 
 the clot. It may be regarded as the plasma minus 
 fibrin, or one of the elements of fibrin. It is clear, 
 transparent, and yellowish in a fasting animal, but 
 opalescent after a full meal, owing to the quantity of 
 chyle, containing minute fat drops, that has been 
 poured into it. Its specific gravity is about 1027. A 
 thousand grammes of blood yield between 440 and 
 525 grammes of serum. It contains about 10 per 
 cent, of solids, of which the most abundant are the 
 proteids, and especially serum albumin, but it also 
 contains fats, sugar, salts, and gases. 
 
 Salts of the blood. — There is a remarkable 
 difference in the distribution of the salts of the blood 
 between the corpuscles and the plasma. Thus, 1,000 
 parts of moist corpuscles yield (exclusive of iron) 
 8 parts of mineral matter, of which potassium forms 
 3-3 parts, phosphorus pentoxide 1 '1 parts, sodium 1 
 part, and chlorine 1 "7 parts, calcium, magnesium and 
 
Chap. II.] Gases of the Blood. 21 
 
 sulphuric anhydride making up the rest ; whilst 1,000 
 parts of plasma yield 8*5 parts of mineral matters, of 
 which chlorine forms 3 "6 parts, sodium 3-3, potassium 
 only 0*3 part, with the rest made up as before. 
 
 The Oases of the blood. — When blood is ex- 
 posed to the vacuum of an air-pump, a considerable 
 quantity of gas is given off, amounting to about 
 half its volume, and to about y^^o'o^^ ^^ ^^^ weight. 
 Thus, if 100 parts of blood drawn from the carotid 
 artery of a dog be exhausted, the gases obtained from 
 it show that at 0° C. and 1 m. pressure of mercury, 
 it contains 17 parts of oxygen, 29 parts of carbonic 
 anhydride, and 1*4 parts of nitrogen. 
 
 The quantity of oxygen contained in venous blood 
 varies within wide limits ; thus, the blood returning 
 from quiescent muscles contains only 6 per cent., whilst 
 in the blood of asphyxiated animals it may be almost 
 entirely absent. A small part of the oxygen is simply 
 absorbed, just as oxygen is absorbed by water; but by 
 far the larger part is taken up, not in accordance with 
 the law of Dalton, but as a result of the affinity of the 
 haemoglobin of the red corpuscles for it, and quite in- 
 dependently of pressure. The former portion is given 
 off exactly in proportion as the pressure is diminished, 
 but the second portion is retained when the tempera- 
 ture is low, until the pressure is diminished to about 
 20 mm. of mercury, when it is suddenly released by 
 the haemoglobin. Exposure of the blood to a pressure 
 of 6 atmospheres causes but little increase in the 
 quantity of oxygen absorbed. That portion of the 
 oxygen which is associated with the haemoglobin, is so 
 loosely combined with it that it is given off on boiling, 
 or by the transmission through the blood of other 
 gases, or by the addition of various reducing agents, 
 such as ammonium sulphide, hydrogen sulphide, and 
 iron filings. The oxygen absorbed into the blood 
 seems to be rendered active, or to be converted into 
 
2 2 ■ Human Physiology. ichap. ii. 
 
 ozone, and the haemoglobin is an ozone carrier. This 
 may he demonstrated by adding to oil of turpentine 
 which has been exposed to the air, and which, there- 
 fore contains ozone, some tincture of guaiacum. No 
 change is visible, but if a little blood be added, the 
 mixture immediately becomes blue, owing to the 
 haemoglobin abstracting ozone from the oil of turpen- 
 tine, and parting with it to the tincture of guaiacum. 
 The carbonic anhydride contained in the blood is 
 in a state of chemical combination, since all fluids that 
 simply absorb this gas have an acid reaction, whilst 
 the blood is alkaline, and it appears to be combined 
 with sodium partly as a carbonate and partly as a 
 bicarbonate. The latter portion can be extracted by the 
 air-pump alone, the former requires the addition of an 
 acid. A small quantity is combined with the sodium 
 })hosphate, two equivalents of which take up one of CO2, 
 becoming changed into acid sodium phosphate and 
 neutral sodium carbonate. The proportion of COg in 
 the blood may vary from 30 vol, per cent, in arte- 
 rial blood, at 0° C. and 1 m. pressure, to 35 vol. per 
 cent, in the venous blood of inactive muscle, and it 
 may even rise as high as 52*6 vol. per cent, in asphyxia. 
 Since 100 parts of blood may contain 30*50 vol. CO^, 
 and 100 parts of serum of the same blood only contain 
 31*95 vol. CO2, it is obvious that the corpuscles must 
 also absorb and combine with some carbonic anhydride, 
 for if the carbonic anhydride were combined exclu- 
 sively with the salts of the serum, we should have to 
 admit that in the above example the blood was com- 
 posed of 95 per cent, serum, and only 5 per cent, of 
 corpuscles, which is known to be incorrect. 
 
23 
 
 CHAPTER III. 
 
 MOVEMENT OF THE BLOOD. 
 
 The movement of the blood is requisite that new 
 material and oxygen may be brought to all parts of 
 the system, and that waste material may be carried 
 away. It is effected by the heart, which is a muscular 
 organ divided into four cavities, between which valves 
 are so arranged that the blood can pass in one di- 
 rection only. There are two auricles and two ven- 
 tricles. The right auricle and ventricle receive 
 venous blood, and transmit it to the lungs, where it 
 parts with carbonic acid gas, and takes up oxygen, 
 and then returns to the left auricle and ventricle. 
 This is termed the lesser or J^;^tZ7?^o?^^c circulation. 
 The left auricle and ventricle receive the arterialised 
 or oxygenated blood, and transmit it to the system 
 at large, from whence it is returned to the right 
 auricle and ventricle. This is termed the greater 
 or systetnic circulation. The heart, therefore, although 
 to outward appearance a single organ, is in reality 
 double, the two halves being united for economy of 
 space, and beating simultaneously. 
 
 Action of tlie lieart, — The heart beats about 
 70 times in a minute, and the phenomena that may be 
 observed are the following : In the first place con- 
 traction and relaxation succeed each other in regular 
 order. The period of contraction is termed the systole 
 of the heart, the period of relaxation diastole. The 
 two auricles contract simultaneously, then the two 
 ventricles contract simultaneously, and then there is 
 a pause. This constitutes one entire revolution of the 
 heart's action. If the heart be carefullv examined in 
 
24 Human Physiology. [Chap. in. 
 
 situ^ in the chest of an animal from which the cliest 
 walls have been in part removed, it Avill be seen that 
 the systole commences \sdth a contraction of the great 
 veins at the base of the heart, which rapidly extends 
 over the auricle and ventricle. These become hard and 
 prominent, the apex of the heart is tilted forward, and 
 the whole heart turns a little to the right, so that 
 more of the left ventricle is visible than before. The 
 upper part, or base of the heart, descends to a small 
 extent, but the apex remains nearly at the same level, 
 because its descent, occasioned by the recoil of the 
 great arteries, is compensated by the shortening of 
 the whole heart. The cessation of the contraction is 
 sudden, and the heart becomes flaccid. The impulse 
 of the heart against the chest walls is coincident with 
 the contraction of the ventricles. The duration of 
 the se^xral phases of the heart's action is for the con- 
 traction of the auricles, a little more than one-tenth of 
 a second ; for the contraction of the ventricles, about 
 four-tenths ; and for the pause, a little less than live- 
 tenths. 
 
 The accompanying diagram shows the cardiographic 
 tracings obtained by means of small elastic bags intro- 
 duced into the cavities of the heart, and of a tambour 
 applied to the chest wall. The upper line gives the 
 variations of pressure in the right auricle, the middle 
 line those of the right ventricle, and the lower one 
 the impulse of the heart against the chest. The rise 
 of the line shows an increase of pressure, and is in- 
 dicative of contraction ; its fall shows the period of 
 relaxation and diminution of pressure. 
 
 From a study of these tracings, it may be seen 
 that the auricle contracts before the ventricle, and 
 that its contraction is not synchronous with the im- 
 pulse of the heart, but precedes it. The contraction 
 of the ventricle, on the other hand, coincides precisely 
 in point of time with the impulse of the heart. It 
 
Chap. III.] 
 
 Action of the Heart. 
 
 is seen, also, that the systole of tlie auricle is sharp and 
 short, the line rising suddenly, and as suddenly falling ; 
 
 the line remains low, with some slight undulations, for 
 some time, then slowly rises as the veins continue to 
 deliver their blood into the auricle. It is seen, further, 
 
26 Human Physiology. [Chap. in. 
 
 that the auricular contraction makes itself perceptible 
 by a slight rise in the ventricular tracing ; that the 
 ventricular systole immediately follows, and is long 
 sustained. By proper arrangements it may be demon- 
 strated that the contraction of the two ventricles is 
 precisely synchronous. 
 
 Course of tlie tolood tiu'oiigli tlie heart, 
 and action of the valves. — The blood, during 
 the period of diastole of the heart, swiftly fills the 
 right and left auricles, and a certain quantity passing 
 through the tricuspid and mitral valves^ enters the 
 right and left ventricles. The auricles then contract, 
 and, owing to the peristaltic character of the contrac- 
 tion, which proceeds from the great veins towards the 
 auricles, and then extends over the auricles themselves, 
 drive the blood into the ventricles and distend them 
 fully. The tricuspid and mitral valves float up, and 
 are instantly rendered tense by the contraction of the 
 ventricles, preventing any regurgitation of blood into 
 the auricles. The effect of the contraction of the ven- 
 tricles is to drive the blood they contain into the 
 pulmonary artery and aorta, bursting open the previ- 
 ously closed semilunar valves at the orifices of these 
 vessels, and as the resistance is great^ the contraction 
 is long and vigorous. The diastole follows, and the 
 recoil of the walls of the over-distended arteries effects 
 the closure of the semilunar valves, and the whole 
 cycle recommences. 
 
 Sounds of the heart. — When the ear is ap- 
 plied to the chest, over the region of the heart, two 
 sounds are heard ; one dull and prolonged, which is 
 the first sound ; the second short and abrupt, which 
 is the second sound. The causes of the first sound 
 are essentially the vibration of the auriculo-ventricular 
 valves when made tense by the contraction of the 
 ventricles, with thS^ommencement of which it coin- 
 cides ; and secondly, the sound produced by the 
 
Chap. III.] Impulse of the Heart. 27 
 
 muscular contraction of the walls of the ventricles. 
 The composite nature of the causes producing the 
 first sound has been demonstrated by Wintrich, with 
 the aid of appropriate resonators ; and although some 
 objection has been raised as to the acceptance of the 
 muscular sound as a cause of the first sound, on the 
 ground that the contraction of the heart muscle has 
 been shown to be a single shock, which is not sufficient 
 to produce a series of vibrations recognisable as a 
 sound, yet the very prolonged character of the con- 
 traction must be admitted to modify the conditions. 
 The cause of the second sound is the ^dbration caused 
 by the sudden tension of the semilunar valves, at the 
 orifices of the aorta and pulmonary artery, the former 
 of which closes from g^gth to ^th of a second earlier 
 than the latter. If these are hooked back by a bent 
 needle introduced through the wall of the vessels, 
 the sound ceases to be audible. The first sound is 
 most distinctly heard at the junction of the fifth 
 rib with the sternum, and a little above and to the 
 inner side of that point ; the ventricular wall is 
 here near the chest wall, and the moving column of 
 blood in the ventricle conducts the sound towards 
 it. The aortic element of the second sound is best 
 differentiated at the attachment of the first rib to the 
 sternum on the right side. The pulmonary element 
 is heard most distinctly over the second intercostal 
 space of the left side, just external to the border of 
 the sternum. 
 
 The impulse of the heart. — This is the sen- 
 sation which can be perceived with the eye and felt 
 by the hand when it is placed against the wall of the 
 chest on the left side, and which is particularly evi- 
 dent when the fingers are pressed into the fifth inter- 
 costal space, in a circumscribed region about midway 
 between the left edge of the sternum and a vertical 
 line drawn through the left nipple. It is caused by 
 
28 Human Physiology. [Chap, hi. 
 
 the sudden pressure, due to the tliickeniiig and hard- 
 ening of the heart in contraction against the wall of 
 the chest. It is most perceptible when the body is in 
 the prone position, and in complete expiration. On 
 the contrary, it becomes faint or imperceptible when 
 the body is lying on the back, and a full respii-ation is 
 made, because the heart is then separated from the 
 chest wall by the inflated lung. 
 
 The accompanying diagram is a tracing of a normal 
 impulse. The part included from a — 6, represents the 
 contraction of the auricles. The rapid ascent of the 
 
 Fig-. 3. — Tracing of tlie Cardiac Impulse. 
 
 line from h — c is produced by the contraction of the 
 ventricles, and is coincident with the first sound of the 
 heart. The line then suddenly descends, but shows at 
 d and e slight elevations, which are synchronous with 
 the second sound ; d corresponding to the closure of 
 the aortic, e with the closure of the pulmonary valves. 
 From e—f corresponds with the period of diastole. 
 
 Frequency of cardiac pulsation. — Yaria- 
 tions in the frequency of the beats of the heart are 
 essentially due to shortening or prolongation of the 
 pause, the period of contraction remaining tolerably 
 constant. The frequency of the heart's beats bears a 
 
Chap. III.] Cardiac Beats. 29 
 
 certain relation to the resistance offered to the exit of 
 the blood from its chambers ; with slightly increased 
 resistance the beat is more rapid, but beyond a certain 
 point becomes slower ; with diminished resistance it is 
 accelerated, hence it becomes very rapid when much 
 blood is lost. At birth, the number of beats is about 
 140 ; at the expiration of the first year it is 120, at 
 the end of the second year, 110 ; during middle life it 
 varies from 70 to 80, and in old age it again becomes 
 slightly accelerated. It is slower in man than in the 
 woman. Position of the body distinctly affects it, the 
 beats being about five per minute more numerous in 
 the sitting than in the recumbent position, and ten per 
 minute more numerous in the standing than in the 
 sitting posture. This is probably the result of a 
 greater number of muscles being called into play in 
 the sitting and standing than in the recumbent position, 
 for it is found that all active muscular exertion in- 
 creases the frequency of the beats of the heart, 
 probably by driving more blood to the heart, and thus 
 stimulating it to more active work. The same expla- 
 nation may be afforded of the increased frequency of 
 the heart's beats that occurs when active digestion is 
 in progress, especially when alcoholic beverages are 
 used in moderation, and the society and conversation 
 are bright and cheerful. Increased temperature of the 
 blood increases the rapidity of the heart's action, 
 hence in part the rapidity of the pulse in febrile 
 conditions of the system. 
 
 Force exerted toy the heart. — The entire 
 force exerted by the heart is very great. That of the 
 ventricles has been estimated at about 124 foot-tons 
 daily. The left heart having to drive the blood 
 through the whole of the systemic capillaries, as well 
 as through the double circulation of the liver, is much 
 more powerful than the right heart, which has only to 
 propel the blood through the lungs. Hence the walls 
 
30 Human Physiology. [Chap. iii. 
 
 of the left heart are much thicker than those of the 
 right, and, as might be expected, the left heart becomes 
 thicker and stronger during pregnancy, when it has, in 
 addition, to supply with blood the enlarged utei-us and 
 the placenta. 
 
 The nervous mecliaiiisBii of tlie hesurt. — The 
 heart has nerve ganglia in its substance, and receives 
 nerve fibres which are in communication with these 
 ganglia, from the sympathetic and from the vagus nerves. 
 
 \a) Intrinsic innervation of the heart. — If the 
 heart be removed from the body, and its nutrition 
 maintained by supplying it with defibrinated blood, it 
 will continue to contract rhythmically for some time. 
 It is evident, then, that it has a nervous system in its 
 walls, which is capable of acting automatically, that is, 
 of responding to excitations generated in the heart 
 itself ; and this is supported by the results of micro- 
 scopical examination, which shows that scattered ganglia 
 are to be found in the substance of the base of the 
 heart, and in the frog especially in the auriculo-ventri- 
 cular furrow, and near the point of opening of the vena 
 cava into the auricle. The apex of the heart contains 
 no ganglia, and Ludwig noticed that if this be severed 
 from the heart, its movements cease ; whilst the rest 
 of the heart continues to beat rhythmically. He hence 
 concluded that the above-mentioned ganglia acted as 
 centres. Stannius performed another experiment on 
 the frog. He applied a ligature to the venous sinus 
 leading to the right auricle, at the level of the ganglia 
 that are found at this spot, and which are known as 
 the ganglia of Remak, and the result was that the 
 movements of the heart were at once arrested; but if 
 a second ligature were applied to • the heart at the 
 auriculo-ventricular furrow, where a second set of 
 ganglia, known as the ganglion of Bidder, are found, the 
 heart recommenced to beat. Stannius concluded that 
 there were two kinds of ganglia at these points, the 
 
Chap. III.] Mechanism of the Heart. 31 
 
 former, when stimulated, which he believed to be the 
 effect of the ligation, presiding over the arrest of the 
 heart's action, the latter when stimulated causing it to 
 contract rhythmically. Others, however, admitting 
 the facts given by Stannius, have given a different ex- 
 planation ; holding that the ligature does not act as a 
 stimulus, but destroys the ganglia in question. On 
 this view the s^anoiia of Remak are the motor o-anoiia 
 of the heart, whilst the ganglia of Bidder are the ar- 
 resting, or inhibiting ganglia. Hence, when the 
 ganglia of Remak are destroyed by the ligature, the 
 inhibiting ganglia continue to act, and the heart 
 stops ; whilst, when the action of these ganglia is 
 abolished by the second ligature, the heart recommen- 
 ces to beat, in response to impulses emanating from 
 scattered ganglia in the heart's substance. Gaskell 
 and others have shown that even the apex may, when 
 physiologically severed from the rest of the heart, be 
 made to contract rhythmically. 
 
 ih) Extrinsic innervation of the heart. — The heart 
 receives nerves from two sources : from the vagus, and 
 from the sympathetic. 
 
 Inhibitory nerves of the heart. — The vagus has a 
 special action on the heart, and this influence is of 
 iuterest because it was the ffrst example known in 
 which a stimulus applied to a nerve supplying a 
 muscle causes not contraction, as in most other 
 instances, but relaxation, or rather seems to be able to 
 prevent its contraction, thus exerting what is named 
 an arrestino^ or inhibiting: influence on muscle. This 
 action of the vaofus on the heart was recognised in 
 1843 by the brothers Weber. They observed that 
 when the vagus was stimulated the heart ceased to 
 contract, and remained flaccid or in a state of diastole, 
 and this has been found to be the type of many 
 other cases. The relaxing influence is not exerted 
 persistently, for after the application of the electrodes 
 
32 Human Physiology. [Chap. in. 
 
 of a galvanic battery for a few seconds, the heart 
 begins again to contract rhythmically. The inhibitory 
 action is exerted both directly and reflectorially, as may 
 be shown by direct pressure on the vagus in the necli, 
 but more easily and distinctly by dividing the vagus on 
 one side, and stimulating first the distal and then the 
 proximal stump of the nerve; arrest of the heart's action 
 occurs in both instances, in the former case owing to the 
 impulse travelling peripherally to the heart, and in 
 the latter instance to its passing upwards to the 
 medulla oblongata, and then down the opposite nerve. 
 One or two contractions of the heart, representing 
 a latent period, intervene between the application of 
 the stimulus to the vagus, and the arrest of the cardiac 
 beats. Division of the vagus on one side does not 
 materially alter the heart's action, but division of both 
 vagi causes acceleration. The inhihitory centre is 
 excited (1) by a deficiency of blood, and therefore of 
 oxygen in the medulla oblongata, as occurs in sudden 
 and great haemorrhage ; (2) by increase of blood 
 ))ressure in the cavity of the skull, as may easily be 
 shown by compressing the abdominal aorta of a rabbit, 
 or by arresting the flow of blood in the great veins of 
 the neck, and this influence of variation in blood pres- 
 sure and quality of blood on the inhibitory centre is 
 of importance, since it preserves the brain from exces- 
 sive blood supply, and prevents also the heart from 
 exhausting itself by too frequent contractions. The 
 inhibitory centre may also be excited by increased 
 venosity of the blood ; hence the slight diminution of 
 the frequency of the heart's action during the inspira- 
 tory phase of the respiration ; (3) by mental emotions, 
 as fear, joy, and surprise, all of which may lead to 
 slowing or even to arrest of the heart's action. 
 
 The inhibitory centre may also be excited reflec- 
 torially : (1) By stimulation of certain visceral nerves ; 
 thus, if a frog be smartly slapped on the belly, the 
 
Chap. III.] Mechanism of the Heart. 33 
 
 heart stops, though if the vagi be previously divided 
 no such stoppage occurs ; (2) by stimulation of almost 
 any sensory nerve, that is, by pain ; (3) by stimulation 
 of the proximal stump of one vagus ; (4) by sudden 
 inflation of the lungs with atmospheric air, which 
 lowers blood pressure. 
 
 Certain poisons, as curare, atropin, and nicotin, 
 suppress the action of the vagus on the heart, and 
 anaesthetics generally diminish its inhibitory influence, 
 which is fortunate, since it prevents the occurrence of 
 arrest of the heart's action in operations accompanied 
 by much pain, which is well known to depress the 
 action of the heart. On the other hand, muscarin 
 and disritalis excite the vagus, and lead to arrest of 
 the heart's movement. 
 
 Iiiliibitory ceotre. — The inhibitory fibres seem 
 to arise from a centre situated in the lower part of the 
 medulla oblongata, in common with those of the spinal 
 accessory nerve, and to pass along the anastomosing 
 branch between the two nerves into the vagus ; since, 
 if the spinal accessory be divided at its origin, the 
 inhibitory power of the vagus in the course of a few 
 days can no longer be excited. It has been suggested 
 by SchifF that the vagus is an ordinary motor nerve, 
 which is, however, very easily exhausted^ but the 
 renewed rhythmical contraction during the continued 
 application of the stimulus contradicts this view. 
 
 Depressor nerve of the beart. — In the rabbit, 
 horse, dog, and cat, and probably also in man, a small 
 filament springs by two roots, one from the vagus, and 
 the other from the superior laryngeal nerve, and applies 
 itself to the sympathetic nerve. This nerve is sensory; 
 if it be cut, no effect is observed to follow stimulation 
 of its peripheral extremity, but if the proximal extre- 
 mity is stimulated the animal gives signs of pain, the 
 heart's action is accelerated, and the pressure of the 
 blood in the vessels undergoes remarkable diminution. 
 
34 Human Physiology. [Chap. iv. 
 
 Accelerating- nerves of the heart. — If the 
 
 medulla oblongata, or the lower segment of the spinal 
 cord, after its division in the neck, be stimulated, the 
 heart's action is accelerated; an accelerating centre is 
 therefore surmised to exist in this region. The accele- 
 ration takes place even after the division of the 
 splanchnics, showing that it is not due to diminislied 
 tone of the vascular system generally, and there is 
 evidence to show that fibres arising from the medulla 
 oblongata and upper part of the spinal cord, emerge 
 from the cord by the lower cervical and upper dorsal 
 nerves, and enter the first thoracic ganglion of the 
 sympathetic, from whence they pass directly or 
 indirectly into the cardiac plexus. A few fibres 
 appear also to run in the trunk of the vagus, as may 
 be shown by stimulating the distal stump of the 
 divided vagus, after the inhibitory fibres have been 
 paralysed by curare. 
 
 The quantity of blood expelled at each contraction 
 of the heart has been variously estimated at from three 
 to five ounces. 
 
 CHAPTER lY. 
 
 BLOOD-VESSELS, AXD THE DIRECTrON OF THE MOVE- 
 MENT OF BLOOD IN THEM. 
 
 The blood driven out of the heart at each con- 
 traction of the ventricles, enters a system of tubes that 
 is everywhere closed, except at the point where the great 
 lymphatic ducts open into it ; the tubes vary in struc- 
 ture, in accordance with the statements made in the 
 companion volume to this, on " Histology," and are 
 named respectively arteries, capillaries, and veins. The 
 arteries dividing, but without . intercommunication, 
 
Chap. IV.] The Arteries. 35 
 
 carry the blood to every part of the body, and gradually 
 break up into capillaries, or fine, hair-like vessels, that, 
 freely anastomosing in the substance of each tissue and 
 organ, supply it with blood, and then unite to form 
 the veins, the function of which is to conduct the 
 blood to the auricles ; those commencing in the lungs 
 ending in the left auricle, and those of the body at 
 large in the right auricle, from which the blood passes 
 into the corresponding ventriclfes to begin its circu- 
 lation anew. The right auricle, right ventricle, 
 pulmonary artery, pulmonary capillaries, and pulmo- 
 nary veins form the pulmonary, or lesser circulation. 
 The left auricle, left ventricle, aorta, systemic arteries, 
 capillaries, and veins terminating in the two venae 
 cavse, form the greater or systemic circulation. In 
 the exceptional case of the portal circulation, the 
 great vein named the portal vein, formed by the 
 coalescence of the veins arising in the intestines and 
 other viscera of the abdomen, breaks up like an artery 
 into smaller vessels, which again unite to form the 
 hepatic veins, and conduct their blood into the inferior 
 vena cava. 
 
 The Arteries. — The arteries are highly elastic 
 and contractile, properties which they owe to their 
 structure. Their elasticity permits them to yield, 
 without danger of bursting, to the sudden increase of 
 the strain upon their walls which occurs at each stroke 
 of the heart, whilst it enables them to accommodate 
 themselves easily to the various movements of the 
 body. It has the further effect of converting the 
 intermittent flow of blood in the laro-e arteries, con- 
 sequent on the cardiac beats, into a uniform and con- 
 stant current in the capillaries. Their contractility 
 confers upon them the power of adapting themselves 
 to the variable quantities of blood the vessels contain, 
 and at the same time, as it is under the influence of 
 the nervous system, it enables that system to control 
 
36 Human Physiology. [Chap. iv. 
 
 the amount of blood supplied to each tissue in accord- 
 ance with its requirements. 
 
 Ag'ents aidjiig^ the movemeiits of the blood. 
 
 — The movement of the blood is promoted by several 
 circumstances, first by the aspiration of blood into the 
 thorax owing to the pressure in the chest being nega- 
 tive or below that of the atmosphere, even in expira- 
 tion, and still less, therefore, in inspiration. That such 
 negative pressure exists may be shown by opening the 
 cavity of the pleura, when the lungs are seen to con- 
 tract, showing that under ordinary circumstances they 
 are permanently dilated beyond their natural state, 
 and therefore must tend to draw the blood into the 
 chest both by the vense cavse and by the aorta ; but 
 the efiect is much more marked on the thin-walled 
 veins than upon the thick and strong-walled aorta, 
 and more blood would therefore enter by the veins even 
 if there were no valves to the aorta. Secondly, by the 
 sudden action of the heart in diastole ; thirdly, by 
 the compression of the veins possessing valves during 
 muscular exertion. The influence of the valves is 
 easily intelligiVjle, for if a vein destitute of valves 
 crossed a muscle, the muscle becoming thicker on con- 
 traction would compress it against adjoining parts, 
 and the blood would flow in both directions, but if 
 valves are present the backward flow is prevented, and 
 the blood moves in the forward direction only. 
 
 £lood pressure. — The pressure of the blood in 
 the vessels is the result of the force with which the 
 blood is driven into them by the heart, and of the 
 resistance that is ofiered to its onward movement. 
 An old experiment, dating back to the time of 
 Bernouilli, shows the gradual decrease of pressure in 
 tubes with open mouths, and that there is an inclined 
 plane or fall, the steepness of which regulates the velo- 
 city of the current in each part of its course. If A in 
 Fig. 4 be a cylindrical reservoir, filled to h with fluid, 
 
Chap. IV.] 
 
 Blood Pressure. 
 
 37 
 
 and a 6 a horizontal tube to which several vertical tubes, 
 1, 2, and 3, are attached, it will be seen that as the 
 point of outflow F is reached the lateral pressure of the 
 fluid against the walls of a, as measured by the height to 
 which the fluid rises, di D2 D3, progressively falls. If 
 F were closed the fluid would immediately rise in each 
 of the vertical tubes to the level of the fluid in the cylin- 
 der A, and would remain stationary ; but the moment F 
 
 Fig. 4. — Diagram to explain the flow of Fluids througli rigid Tubes, 
 and tlie variations of pressure at different parts. 
 
 is opened the fluid begins to move, and its level in the 
 
 several tubes falls in a regular order from the level A in 
 the cylinder a to the level of the mouth of f ; the fluid 
 would only again become stationary, if the discharge 
 from F continued, when it had fallen to the same level 
 as f. If, instead of the tube h f, the tube a h pre- 
 sented an open mouth at h, the fluid would of course 
 continue to flow, and the pressure of the fluid in the 
 vertical tubes would fall to zero. If we consider each 
 of the vertical tubes separately, it is seen that a 
 resistance in fi^ont counteracts part of the pressure 
 exerted by the column of water in A, and that this 
 resistance decreases towards the outlet. The resistance 
 
38 Human Physiology. [ciiap. iv 
 
 is attributable to the friction of the fluid against the 
 walls of the tube, and this is necessarily greater at i 
 than at 11, and greater at 11 than at iii. Hence the 
 diflerences in the level of the fluid in the diflerent 
 tubes. In the above diagram A represents the heart, 
 and I, II, III diflerent parts of. the arterial system. 
 If the arteries were a system of rigid tubes, and had 
 no outlet on the side of the capillaries, it would be 
 impossible for tlie heart to drive any blood into them, 
 for fluids are incompressible, and no movement of the 
 blood would occur ; but the pressure of the blood 
 ao-ainst the walls would be increased durino- the heart's 
 beat in exact proportion to the force of that muscle. 
 If, however, instead of being rigid tubes the arteries 
 were elastic tubes, and had as before no outlet on the 
 side of the ca])illaries, the heart would be capable of 
 discharging its blood into them for a few beats, be- 
 cause the elasticity of the walls would permit them to 
 yield and enlarge, but the pressure of blood would 
 continue to rise at each stroke of the heart until the 
 limit of their resistance was overcome and they gave 
 way, or until the resistance ofiered to the entrance of 
 more fluid was equivalent to the force which the heart 
 could exert. But if, as is actually the case, the 
 arteries are highly elastic tubes which terminate in a 
 plexus of capillaries, it is clear that whilst at each 
 stroke of the heart the pressure of the blood is raised 
 throughout the arterial system it must necessarily fall 
 during the inter \al between two beats because the 
 blood is escaping by the capillaries. Pressure exists 
 against the walls of the vessels, because at any moment 
 and in any part, except in the veins close to the heart, 
 the quantity of blood contained in the vessels is greater 
 than they can contain if their elasticity is not called 
 into play, and the difference of pressure in different 
 parts is practically the cause of the motion of the blood. 
 The pressure in the aorta, termed the hlood 'pressure^ 
 
Chap. IV.] Blood Pressure. 39 
 
 is greatly exalted at each stroke of the heart, but 
 rapidly falls during the diastole as the blood escapes 
 through its branches into the smaller arteries, and 
 from thence into the capillaries. The mean blood 
 pressure is the pressure which a manometer attached 
 to the aorta would indicate when a line is drawn mid- 
 way between the highest and the lowest pressure. 
 The pressure necessarily falls in passing from the 
 larger to the smaller vessels, because the resistance is 
 lessened, a portion of the current being directed 
 through lateral channels, and hence the arterial j^ires- 
 sure or tension, which is the pressure exerted by the 
 blood in any particular vessel, is determined by the 
 force with which the heart drives blood into the arte- 
 rial system, and the resistaiice which opposes the exit 
 of blood from that system. 
 
 The influence of the loss of blood on arterial pres- 
 sure is remarkable ; if only moderate in amount the 
 effect is scarcely observable, for although the blood 
 escapes readily from the arteries, and the result might 
 therefore be expected to resemble that of dilatation of 
 the capillaries, and to be a fall of arterial pressure, 
 yet this does not occur to a marked extent because 
 deficiency of oxygenated blood in the nerve centres 
 stimulates them to action, and the smaller arteries 
 everywhere contract, and thus the arterial pressure is 
 maintained nearly at its normal amount. If, how- 
 ever, the loss of blood much exceeds 2 to 3 per cent, 
 of the total weight of the body the power of the heart 
 fails, and the blood pressure suddenly falls. 
 
 The blood pressure can be materially augmented 
 by the transfusion of the defibrinated blood of ano- 
 ther animal, which acts by filling the vessels and 
 increasing the resistance offered to the introduction 
 of more blood into them at each systole of the heart. 
 
 The great size and distensibility of the abdominal 
 veins enable thsm to act as important regulators of 
 
40 Human Physiology. [Chap. iv. 
 
 the arterial pressure. When much fluid is added to 
 
 the blood either by transfusion or absorption, it accu- 
 mulates in these veins. When the general arterial 
 system contains a deficient supply of blood they con- 
 tract and maintain the normal pressure by propelling 
 the blood they contain into the general circulation. 
 A remarkable experiment shows that this really 
 occurs, for if the portal vein be ligatured, the blood 
 continues to enter the abdominal veins but is unable to 
 escape, and so much may thus accumulate that the 
 blood pressure may sink to zero, and the animal dies, 
 having bled itself to death in its own visceral veins. 
 If the ligature be relieved before this result occurs, 
 the normal pressure is soon recovered. 
 
 The pressure of the blood steadily decreases in 
 passing from the larger to the smaller arteries ; thus 
 if in the carotid artery it is 150 mm., in the meta- 
 tarsal artery it will not much exceed 100 mm. T]ie 
 reason that it falls to a certain extent is because 
 the total sectional area of the small vessels is greatly 
 in excess of the primary trunk from which they are 
 derived, and hence the blood is moving in a wider 
 channel, and the only reason that it does not fall still 
 lower is because the friction of the blood against the 
 walls of the vessels is so much greater in the small 
 vessels than in the larger ones. 
 
 Whatever increases the resistance to the flow of 
 the blood through the vessels increases the pressure of 
 the blood ; thus, ligature of one or more large vessels 
 by diminishing the outflow in that direction increases 
 the pressure in the rest of the system. So again, con- 
 striction of the smaller arteries, as by the action of 
 cold, or by the irritation of vasomotor nerves or centres, 
 raises the pressure. 
 
 On the contrary, whatever facilitates the flow of 
 blood from the arteries into the capillaries lowers the 
 pressure of the blood in the arteries. Thus, warmth 
 
Chap. IV.] The Pulse. 41 
 
 applied to the surface, by relaxing the small arteries 
 permits the large arteries to discharge their contents, 
 and the pressure falls. The same result is seen after 
 division of the vaso-constrictor nerves as, for example, 
 of the splanchnics. 
 
 The Piilse. — When the finger is placed upon any 
 artery and light pressure made, a beat or shock is felt, 
 which is caused by the systole of the heart. With each 
 systole from three to five ounces of blood are forced 
 into a system of elastic already distended tubes, and a 
 wave, which is quite different from the translation of 
 the blood injected, is consequently propagated from one 
 end of the body to the other. The whole arterial 
 system becomes suddenly tightened and yields a 
 little to make room for the additional quantity of 
 blood forced into it. The pulse, as it is felt at the 
 wrist, or on the temple, or on the dorsum of the foot, 
 is the effort of the artery to recover its cylindrical 
 form, when it has been compressed against the hard 
 subjacent tissues, and temporarily flattened. The 
 lateral expansion of the vessels is so small that it is 
 inappreciable to the finger when they are surrounded 
 on all sides by soft tissues. Hence, in operations, when 
 it becomes necessary to tie a vessel its pulsation cannot 
 be felt unless it can be compressed against some hard 
 tissue as bone. The wave excited by the systole of the 
 heart, which must not be confounded with the 
 rapidity of movement of the blood itself, takes 
 time to propagate itself to the more distant parts 
 of the arterial system, as may be shown by 
 the application of two sphygmographs to the limb, 
 at parts as remote as possible from each other, and 
 has been ascertained to vary, according to the con- 
 dition of the arterial walls, from 6 — 12 meters per 
 second, the velocity being somewhat greater in the 
 vessels of the lower limb than in the upper, apparently 
 on account of the more rigid nature of the walls of the 
 
42 Human Physiology. [Chap. iv. 
 
 vessels in tlie lower limb. For the same reason it is 
 rather more rapid in old people than in youth. As 
 the wave is propagated through the arterial system it 
 becomes less and less marked. Strong, and of great 
 amplitude in the aorta, it is gradually extinguished in 
 the smaller vessels, and is ultimately lost in the 
 capillaries. 
 
 The rapidity with which the j)ulse wave is propa- 
 gated may be modified by two conditions. Every 
 increase of resistance to the flow of blood by tightening 
 the vessels or raising the arterial tension accelerates 
 the passage of the wave ; but, on the other hand, this 
 very increase of the arterial tension renders it more 
 difficult for the heart to discharge its contents quickly. 
 It propels less blood into the arteries at each pulsation, 
 and the waves are of less volume, and are formed 
 more slowly. On the other hand, when the arterial 
 tension is low, that is, when the arteries are less tightly 
 filled, this cause of the slowing of the wave is com- 
 pensated by the more easy and, consequently, more 
 abundant and brisker penetration of the ventricular 
 blood into the arterial system, and the waves travel 
 correspondingly faster (Marey). And these two con- 
 ditions usually compensate each other in health. 
 
 The form of the pulse wave, as it is exhibited in a 
 sphygmographic tracing, presents certain general fea- 
 tures, whilst there are others which belong to the par- 
 ticular artery examined, to the character of the cardiac 
 contractions, the quantity and quality of the blood, 
 and to conditions of the system at large. The features 
 common to all sphygmographic tracings of the pulse 
 are, that there is a more or less sudden rise of the line 
 indicating the commencement of the wave, a more or 
 less pointed summit, indicating the period of greatest 
 tension of the arterial wave, and an obliquely descend- 
 ing line indicating the gradual reduction of the arterial 
 tension as the blood escapes from the arteries into the 
 
Chap. IV.] Tracing of the Pulse. 43 
 
 capillaries. The descending line usually sliows a dis- 
 tinct secondary or dicrotic wave, and occasionally a 
 third or tricotic wave. The accompanying woodcut 
 shows the form of the radial pulse of a healthy young 
 person at rest, and is of a tyj)ical character. The systolic 
 phase presents two inflections ; the first, 6, forming the 
 summit of the pulsation, is at the end of the vertical 
 line ; it does not constitute a sharp point as in those 
 cases where the arterioles and their capillaries allow 
 
 Pig. 6. — Type of Normal Piuse. 
 
 the blood to pass only with difficulty, and when the ar- 
 terial system suddenly reaches its maximum of tension, 
 but is rounded. Tliis summit is followed by a wave c, 
 which precedes the dicrotic wave d. The wave c has 
 attracted much attention, and some authors hold their 
 opinion in reserve, but jMarey regards it as the end of 
 the systolic plane, which in b is shown by a dotted 
 line. The descending portion of the line corresponds 
 to the diastolic phase of the pulse ; it expresses, by its 
 more or less rapid fall, the greater or less rapidity with 
 which the blood flows from or out of the arteries. The 
 commencement of this is marked by the dicrotic 
 rebound, which is never absent when the sigmoid 
 valves are perfect, though its amplitude varies 
 greatly. The number of subsequent waves increases 
 with a rare pulse and high arterial tension, but they 
 
44 Human Physiology. [Chap. iv. 
 
 are suppressed with the entrance of a fresh wave 
 from the heart. Fig. 5 exhibits three sphygmo- 
 graphic tracings of the pulse. When the heart dis- 
 charges its blood into the arteries briskly the line of 
 ascent is nearly vertical, for the recording cylinder of 
 the sphygmograph has had but little time to move 
 before the artery has attained its greatest tension. 
 When the blood enters more slowly the line of ascent 
 is oblique, for the cylinder has had time to move before 
 the period of greatest tension has arrived. The line may 
 still be oblique if, by reason of the facility with which 
 
 F g. 6.— Three Sphygmograijliic Tracings of different form. In each 
 the period of Ventricular Systole is marked by vertical lines. 
 
 the blood escapes from the arteries into the capillaries, 
 some time elapses before the arteries become quite dis- 
 tended. The ascending line may present an undula- 
 tion, as in cases of insufficiency of the aortic valves, 
 and the atheromatous arteries of advancing age, in 
 which the elasticity of the wall is diminished so that 
 it reaches its limit before the systole is completed, and 
 the heart drives out the last portion of its contained 
 blood with a protracted effort. 
 
 The summit of the curve indicates the period of 
 greatest tension of the arterial wall, and if flattened or 
 rounded shows that a condition of equilibrium has 
 been established between the afflux and outflow of 
 blood from the arteries. 
 
 The descending line indicates the diminution of 
 arterial tension ; the period of systole or diminution 
 of calibre of the arteries, and the escape of the blood 
 
Chap. IV.] Tracing of the Pulse. 45 
 
 from these vessels into the capillaries. It is marked 
 by a secondary rise, the dicrotic wave. This wave is 
 caused by the sudden closure of the sigmoid valves. 
 As the time which the wave occupies in traversing its 
 own length is equal to the duration of the exciting 
 cause, which is the contraction of the systole of the 
 ventricle, and which is known to occupy about ^ of a 
 second, the length of the wave, supposing it to 
 move at the rate of 6 meters per second, would be 
 6 -f 3 = 2 meters. But the distance from the heart 
 to the most distant capillaries is never equal to 2 
 meters, the whole of one wave, therefore (from one 
 crest, that is to say, to the next), is never in 
 the arteries at any one moment. When the 
 systole of the heart is completed, and the ventricle 
 has propelled the whole of its blood into the aorta, 
 the primary pulse-wave is propagated throughout 
 the system; but instantly succeeding this, the elasticity 
 of the highly elastic aorta, over-distended with the blood 
 just driven into it, begins to act, and tends to force the 
 blood both forwards and backwards, but the backward 
 movement is promptly stopped by the closure of the 
 sigmoid valves, and a rebound of the blood takes 
 place from them, analogous to the shock which is felt 
 when a tap through which water is running is suddenly 
 turned. This shock is showTi by the dicrotic rise in 
 the descending line of the sphygmographic curve. If 
 the sigmoid valves are held back it vanishes. It is 
 rendered more prominent by the sudden entrance into 
 the aorta of the ventricular wave, by its small volume, 
 and by feeble arterial tension. The conditions under 
 which several secondary waves may be observed are, 
 when the pulse is slow and the arterial tension is high. 
 
 The volume and strength of the pulse depend on the 
 force of the cardiac beats, the quantity of the blood, 
 the degree of arterial tension, and the greater or lesser 
 elasticity of the arterial walls. C ceteris paribus, if the 
 
46 Human Physiology. [Chap. iv- 
 
 heart beats strongly, the pulse is large and full, as in 
 a healthy man after moderate exercise. In certain 
 inflammatory affections, on the other hand, as in peri- 
 tonitis, the action of the heart is depressed, and the 
 pulse becomes small and weak. In old people the 
 arteries become rigid, their elasticity quickly reaches 
 its maximum, and the heart is feebler than in youth. 
 Hence, the pulse is sharp and comparatively weak ', 
 yet, because firm walls are not easily compressed it is 
 hard. A cold bath, by driving much blood from the 
 capillaries of the skin into the rest of the system, 
 and thus increasing arterial tension, renders the pulse 
 small and hard ; whilst a warm bath, by facilitating 
 the passage of blood from the arteries into the capillaries 
 lowers the arterial tension and makes the pulse full 
 and soft. Mere alteration in the position of a limb 
 will even cause a difference in the character of the 
 pulse in it. Thus, if the arm be raised, the pulse 
 becomes more distinct, and the amplitude of the 
 sphygmographic tracing is greater, because arterial 
 tension is diminished, whilst, if the arm be lowered, 
 the pulse tracing becomes almost a continuous straight 
 line, with but trifling elevations and depressions, because 
 the tension is increased. Lastly, haemorrhage, before 
 the heart is exhausted, greatly increases the amplitude 
 of the sphygmographic tracing by lowering arterial 
 tension. 
 
 The velocity of the blood current. — The 
 velocity with which the blood moves in the vessels has 
 been estimated in various ways. In the first place (1) 
 there is the method of Blake and Hering, in which 
 the time occupied in the passage of a particular salt, 
 as potassium ferrocyanide, from one jugular to the 
 other is determined ; in the horse, it traverses the 
 double circulation in thirty seconds ; in the rabbit, 
 in seven seconds. But this only gives the duration 
 of the circulation as a whole, and affords no indication 
 
Chap. IV.] Speed of the Blood Current. 47 
 
 of the time occupied by tlie blood in passing through 
 any particular vessel, whether artery, capillary, or 
 vein. For this purpose special instruments have been 
 devised. In the arteries and veins the rapidity of 
 the blood current may be approximately determined : 
 (2) By the hcemodromometer of Volkmann, in which 
 by a simple arrangement the current of blood coursing 
 through a tube of glass inserted between the extre- 
 mities of a divided artery can be suddenly diverted 
 into a long bent tube, also made of glass, and the 
 rapidity of its passage noted. (3) By Lud wig's 
 stromuhr^ in which the time occupied in filling a tube 
 of known capacity is measured. (4) By Vierordt's 
 hcematochometer, in which a small cell, from the 
 upper wall of which a light ball is suspended, is intro- 
 duced between the cut ends of an artery; the deviation 
 of the ball from the perpendicular shows the strength 
 of the current. (5) By Chauveau's hasmodromometer, 
 in which a short tube of glass occupies the interval 
 between the cut extremities of the artery, and has in 
 its interior a small circular disk to which is attached 
 an indicator that passes through the wall of the tube 
 and registers its movement on a scale ; the wall is the 
 fulcrum, the current of blood acting on the disk moves 
 it in one direction and the indicator moves in the 
 other. (6) By Chauveau's haemodromograph. {See com- 
 panion volume on " Physical Physiology.") By one 
 or other of these methods, but especially by the last, it 
 is seen that the flow of blood in the arteries is con- 
 tinuous, but undergoes great periodical accelerations. 
 It is swifter in the large arteries than in the small, be- 
 cause the sectional area of the small arteries is much 
 greater than that of the larger ones ; the stream, taking 
 them collectively, is wider, but as there are more chan- 
 nels, the friction is greater. Any obstacle, such as pres- 
 sure, presented to the passage of the blood in an artery 
 retards the current of blood in that artery, whilst it 
 
48 
 
 Human Physiology. 
 
 [Chap. IV. 
 
 accelerates it in the other vessels. The following 
 diagrams from Marey show this clearly. The firet 
 (Fig. 7) shows that whilst there is a great increase, 
 indicated by the sudden ascent of the line, in the 
 
 Fig. 7. — Speed of the Blood Curreut : its siidden increase] witli each, 
 systole of the ventricle, and its arrest when the artery is compressed. 
 
 swiftness of the blood current at each systole of the 
 heart, yet that it still continues to flow, as indicated 
 by the elevation above the zero line of the line con- 
 necting two waves, during the intervals of the pulsation, 
 whilst it is stopped altogether w^hen the artery is com- 
 pressed, the line falling to zero. The next (Fig. 8) 
 shows the speed of the blood current in one carotid when 
 
 Fig. 8.— Tracing of the Speed of the Blood Current in the Eight Carotid 
 of a Horse when the Left Carotid is free. 
 
 the other is free, and the third (Fig. 9) shows the sudden 
 and great augmentation, as indicated by the height 
 the tracing as a whole has risen from the basal 
 line, when the opposite carotid is compressed. The 
 rapidity with which the blood moves in the caro- 
 tid artery of the horse has been estimated at about 
 
Chap. IV.] Circulation IN THE Capillaries. 49 
 
 300 mm., or one foot, per second ; in the maxillary 
 of the horse, 165 mm., or between 6 and 7 inches, 
 per second ; and in the metatarsal artery of the 
 same animal about 56 mm., or 2 inches, per second. 
 These estimates are probably all too low. Speak- 
 ing generally, everything which increases the force 
 
 Fig. 9.— Tracing of the Speed of tlie Blood Current in the Eight Carotid 
 of a Horse when the Left Carotid is compressed. 
 
 propelling the blood towards the periphery increases 
 the speed of the blood current and the arterial 
 tension, and vice versa. On the other hand, every- 
 thing which increases the resistance that the blood 
 experiences in escaping from the arteries diminishes 
 the speed of the blood current but increases the 
 tension. 
 
 Movement of the blood in the capillaries. 
 — The circulation in the capillaries may be seen in the 
 web of the frog's foot, in the tongue of the frog, and in 
 the mesentery of various small, warm-blooded animals 
 placed with suitable precautions against desiccation 
 
 E 
 
5© Human Physiology. [Chap. iv. 
 
 under the microscope. In tlie smallest capillaries the 
 blood corpuscles may be seen moving in single file, 
 separated from the wall of the vessel by only a very 
 thin layer of fluid. Some pursue a straight course, 
 others enter some of the numerous lateral and anasto- 
 mosing channels that exist between the capillaries. 
 At the point of bifurcation of the vessel the corpuscles 
 sometimes hang poised, as it were, for a moment, and 
 then press on to right or left. Certain capillaries 
 become smaller as they are watched, others increase 
 in size in accordance with variations in the size of the 
 small arteries leading to them. The constriction and 
 diminution of the lumen is produced by the cells com- 
 posing the wall of the vessel becoming 'thicker, and 
 thus encroaching upon the cavity. In the larger 
 capillaries, where two or three files of corpuscles have 
 room to move abreast, the comparatively heavy red 
 corpuscles "vntII be seen to occupy the middle of the 
 stream, whilst the relatively light white corpuscles roll 
 more slowly along in the lateral and slower current of 
 the liquor sanguinis. It has been found by experiment, 
 that the difference in their specific gravity leads to this 
 difference in their position. The motion of the blood 
 in the capillaries is uniform, and the flow continuous, 
 no trace of the pulsation in the larger arteries, except 
 under very peculiar circumstances, being perceptible. 
 The pulsation has, in fact, been extinguished by the 
 elasticity of the arteries and by the wider area formed 
 by capillaries as compared with the arteries. The 
 rapidity of the movement of blood in the capillaries is 
 about 0-5 to 1 mm., or from -x^^^ ^^ -io^ of an inch 
 in a second. The pressure of the blood in the capil- 
 laries, determined by the indirect method of ascertain- 
 ing what pressure will empty the capillaries beneath 
 the finger nail, has been found to vary, according to 
 the elevation of the arm, from one-fifth to one half 
 Qf the ordinary arterial pressure. 
 
Chap. IV.] Circulation IN THE Veins. 51 
 
 Movement of tlie blood in the veins. — The 
 
 cuiTent of the blood in the veins is formed by the union 
 of the currents from thousands of capillaries, which, 
 now running in narrower channels, move more 
 rapidly because the friction is reduced. The current 
 is uniform, and free from the pulsation observed in 
 arteries. The only exceptions to this are the pulsations 
 in the larger veins of the neck, due to regurgitation of 
 blood into them during the systole of the right auricle 
 and the pulsations observed w^hen the capillaries are 
 greatly dilated and allow of an extraordinarily fi^ee 
 current of blood through them. The heart is still the 
 main driving force, but it is assisted 
 
 (1) By the aspiration to the chest occasioned by 
 the negative pressure or tendency to a vacuum which 
 is there present, and which is clearly and sometimes 
 fatally shown by the rush of air towards the heart 
 when one of the veins is opened. 
 
 (2) By the suction power exerted by the heart 
 during diastole. 
 
 (3) By movements of the voluntary muscles, which, 
 pressing on veins provided with valves, favour the 
 onward movement of the blood, its regress being pre- 
 vented by the valves. 
 
 (4) By position of the limbs, which acts in a similar 
 manner. Thus Braum saw the veins beneath Pou- 
 part's ligament collapse and the pressure become nega- 
 tive when the thigh was rolled outwards; but fill, and 
 the pressure become positive when the limb was placed 
 in its former position. 
 
 (5) By gravity, which acts, not primarily on the 
 movement of the blood, but upon its distribution, and 
 therefore secondarily upon its movement. 
 
 The pressure of the blood against the walls of the 
 veins is everywhere low, and gradually diminishes 
 from the smaller to the larger veins. In the brachial 
 vein it will support a column of mercury 9 ram. high. 
 
52 Human Physiology. [Chap. iv. 
 
 but in the larger veins at the root of the neck the pres- 
 sure is negative. It varies from — 2 to - 3 mm. syn- 
 chronously with the heart's action, being of course in- 
 creased during the systole of the heart when its flow 
 into the auricle is temporarily arrested, and from — 5 
 to — 8 mm. with the respiratory acts, being increased 
 during expiration. The rate 'of the movement of the 
 blood in the large veins is probably slower than in the 
 arteries in proportion to their relatively larger sectional 
 area. It has been estimated at 100 mm., or 4 inches 
 per second, in the jugular vein of the horse. 
 
 I>i!§tril>utioii of tlie blood. — In the living 
 rabbit when at rest the experiments of Kanke seem 
 to show that the whole mass of blood may be divided 
 into four parts, of which one part is contained in the 
 muscles, one in the liver, one in the heart and great 
 vessels, and one in the remaining organs. Functional 
 activity, however, causes an immediate increase in the 
 quantity of blood circulating through a part, the aug- 
 mentation sometimes amounting to as much as 47 per 
 cent,, which may be estimated by the plethysmograph. 
 Such blood is, of course, taken away from other parts. 
 Hence during physical exercise the muscles are largely 
 supplied with blood, whilst the brain and digestive 
 organs receive less, which teaches that neither severe 
 bodily nor mental exertion should be made imme- 
 diately after a meal. 
 
 I>iaped[esis. — Under certain circumstances, both 
 white and red corpuscles may escape from the vessels, 
 and pass or wander into the adjoining lymphatics. 
 The escape of the white corpuscles appears to occur 
 normally, whilst the escape of the red only occurs 
 when the pressure of the blood against the walls of the 
 capillaries is much increased, or when there is retarda- 
 tion of the blood current, as in inflammation. In the 
 case of the white corpuscles, the attraction between the 
 corpuscle and the capillary wall seems to .be increased, 
 
Chap. IV.] Circulation IN Certain Organs. 53 
 
 the corpuscle begins to bore its way through the wall, 
 assumes an hour-glass form, part being within and 
 part without the lumen of the vessel, and it finally 
 escapes altogether into the adjoining tissues. 
 
 Peculiarities of tbe circulation in different 
 regrious. — (1) The lungs. — The pulmonary circula- 
 tion is peculiar in having its capillaries, unlike those 
 of the system generally, always under negative 
 2)ressure, which varies according to the different phases 
 of the respiratory acts. In mspiration the mean blood 
 pressure diminishes in all the parts contained in the 
 thorax, heart, and large vessels. This diminution of 
 pressure favours the current of blood in the vense cavae, 
 right auricle, and right ventricle, whilst it is opposed 
 to the discharge of the arterial blood from the left 
 ventricle through the aorta. The extensibility of the 
 vense cavse is, however, much greater than that of 
 the aorta, which is comparatively a rigid tube, hence 
 the venous current is favoured, whilst the arterial 
 current is but little interfered with. Expiration 
 exerts an opposite influence. Pressure augments in 
 the veins and in the arteries. The capacity of these 
 vessels, and especially of the large intrathoracic vessels, 
 diminishes ; the arterial circulation is favoured whilst 
 the venous circulation is rendered slower, and the 
 heart receives less blood. 
 
 (2) The hrain. — The four large arteries supplying 
 the brain with blood, namely, the two internal 
 carotids and the two vertebrals, anastomose with 
 extraordinary freedom, so that a ligature applied to 
 one of them does not materially interfere with the 
 nutrition of that portion of the brain which ordina- 
 rily receives its supply of blood from it. The capillaries 
 are very minute, and in the pia mater have a special 
 sheath. The skull, being a closed cavity, retains a con- 
 siderable quantity of blood, even when, from hsemorr- 
 hage, other parts of the body are ex-sanguine. 
 
54 Human Physiology. [Chap. iv. 
 
 (3) The liver. — Tlie peculiarity of the circulation 
 in the liver is that its main supply of blood is obtained, 
 not from an artery, but from a vein, the vena portse, 
 which receives the blood returning from the alimen- 
 tary canal and its appendages, and forms a minute 
 plexus of capillaries in the lobules, where it is joined by 
 the capillaries of the hepatic artery. The hepatic veins 
 arise from the plexus into which the portal vein has 
 divided, and conduct the blood which has circulated 
 through the liver, to the vena cava inferior. The 
 blood of the mesenteric, splenic, gastric, and other 
 visceral arteries, passes therefore through two series 
 of capillaries, before reaching the hepatic veins, one in 
 the walls of the intestines, and in the substance of the 
 spleen, and one in the liver. The circulation in the 
 liver is necessarily slow. The blood traversing its ves- 
 sels certainly contains much new material, directly 
 absorbed from the alimentary canal, and it is probable 
 that this undergoes important assimilative changes in 
 its course through the liver. 
 
 (4) The erectile tissues. — These tissues are remark- 
 able for the great variations in bulk they present, 
 owing to changes in the supply of blood to them, and 
 to the retention of the blood in their substance, 
 which is effected by the relaxation of the muscular 
 tissue of these arteries, and the distension of large 
 sinuses in the substance of the cavernous tissue of 
 which they are chiefly composed. 
 
 The lyiiipli and lympliatics. — A system of 
 tubes is widely distributed through the body, which, 
 commencing sometimes in intercommunicating stellate 
 spaces, sometimes by free blind extremities, and some- 
 times by channels surrounding blood-vessels, at length, 
 after traversing one or more glands, terminates by two 
 large vessels that open at the points of junction of the 
 jugular and subclavian veins. These tubes contain 
 lymph, which is a transparent, slightly yellow, fluid, of 
 
Chap, IV.] Circulation of Lymph. 55 
 
 sp. gr. 1027. It closely resembles the liquor sanguinis 
 of the blood, containing about 5 per cent, of albumin 
 and 1 per cent, of salts, with a minute proportion of 
 fat and of hbrin. It appears to be derived from the 
 blood itself, and to be the surplus material that has 
 traversed the walls of the blood-vessels to supply the 
 tissues, which, in some parts, as the cornea, altogether 
 obtain their nourishment from it. It thus acts as a 
 drainage system. But it probably, also, contains pro- 
 ducts of disintegration, which, moreover, are not so 
 far destroyed as to be incapable of further utility in 
 the system, and hence, after being worked up in the 
 glands and entering the blood, can again be made 
 serviceable in the economy. The lymphatics that com- 
 mence in the villi of the intestine, and form the system 
 of chyle vessels or lacteals, play an important part in 
 absorption, and the fluid they contain, here named 
 chyle, presents a whitish aspect from the molecules of 
 fat that it contains after ordinary food, and is of great 
 importance in nutrition. The lymph traversing the 
 rootlets of the system, has only a feeble power of coagu- 
 lation, and presents few corpuscles ; but after passing 
 through the glands, and especially after it has gained 
 entrance into the thoracic duct, it acquires the power 
 of coagulating with tolerable firmness into a gelatinous 
 clot, and it contains numerous corpuscles, some of which 
 appear to be in process of development into coloured 
 blood corpuscles. {See " Histology," pp. 84 — 93.) 
 
 Movement of tlie lyiiipti. — In some of the 
 lower animals, as the eel and frog, contractile cavities 
 (lymph hearts) which beat rhythmically, eflfect the 
 movement of the lymph and drive it through the 
 vessels ; but none such are known to exist in man, 
 though the muscular coat of the lymphatics may exert 
 a feeble propulsive action. The movement of the fluid 
 in the lymph spaces and tubes is effected primarily by 
 the pressure under w^hich the blood is driven by the 
 
56 Human Physiology. [Chap, v, 
 
 heart, but partly, also, by the pressure exerted by the 
 muscles Avhen in contraction, aided by the valves dis- 
 tributed through, the sj^stem, which are especially 
 numerous in the superficial vessels. Hence exercise, 
 by quickening the flow of fluid, prevents the accumula- 
 tion of waste products in the tissues, and promotes their 
 healthy nutrition. There is, also, a suction influence 
 exerted at the point where the thoracic duct opens into 
 the junction of the subclavian and jugular veins, 
 which is occasioned by the swift flow of venous blood 
 over the oriflce of the duct. A slight additional force 
 may be derived from the act of inspiration. The 
 rapidity of the flow of the lymph is about 4 mm. per 
 second, and the pressure of the lymph against the 
 walls of the larger vessels is about 1 1 mm. Hg. ; but 
 it probably undergoes great variations, M. Colin obtain- 
 ing from a lymphatic in the neck of a horse, 2 mm. 
 in diameter, sixty grammes of lymph per hour when 
 the animal was in repose, and from the same lymphatic 
 when mastication and movements of the neck were 
 being performed, 100 or even 110 grammes per hour. 
 The lympliatic system has close relations with 
 the cavities of the serous membranes, with which it 
 communicates by means of minute orifices or stomata. 
 Milk or other bland fluid injected into the serous cavi- 
 ties is quickly absorbed into the lymphatic system. 
 
 CHAPTER Y. 
 
 RESPIRATION. 
 
 Object and nature of respiration. — The 
 
 object of respiration is the introduction of oxygen 
 into the system, which is accomplished by the expo- 
 sure of the blood in an extremely thm layer in 
 
Chap, v.] Objects of Respiration. 57 
 
 the lungs to a current of atmospheric air. Coincident- 
 ally, and as a secondary process, carbonic acid gas, 
 with which the blood coming to the lungs is charged, 
 is eliminated. The surface presented by the capillaries 
 of the lungs has been estimated to be about 150 square 
 meters, and the blood at any one moment present in 
 the lungs to be about two litres, whilst in the course 
 of twenty-four hours about 20,000 litres traverse the 
 capillaries, the blood corpuscles passing in single file, 
 and being exposed to air on both surfaces. The 
 absorption of oxygen is due to two circumstances. 
 First, to the absorption of oxygen under the partial 
 pressure of that gas at which the blood stands ; and 
 secondly, to the affinity of liEemoglobin for oxygen. The 
 red corpuscles, which are chiefly composed of haemo- 
 globin, are the principal oxygen carriers. The elimi- 
 nation of carbonic acid, on the other hand, is due to 
 the circumstance that the alkaline salts of the blood, 
 charged with this gas, are placed under conditions 
 favourable to its diffusion, and cease to retain it. 
 The carbonic acid gas is generated in the tissues and 
 enters the blood under pressure, but in the capillaries 
 of the lungs the partial pressure of the gas is greatly 
 reduced, and it accordingly escapes. That process by 
 which the oxygen carried by the blood corpuscles 
 through the systemic vessels to the tissues, leaves the 
 blood to unite with the constituents of those tissues, 
 and by which the carbonic acid gas with which the 
 tissues are surcharged leaves them and enters the 
 blood, is termed internal respiratio'n. That process 
 by which the blood traversing the lungs absorbs oxygen 
 from the air and yields up carbonic acid gas to it is 
 termed external respiration. The skin plays a subor- 
 dinate part in external respiration. 
 
 The respii'atory moveineots, — The ingress 
 and egress of air is efi'ected rhythmically by the 
 alternate enlargement and contraction of the cavity of 
 
58 Human Physiology. [Chap. v. 
 
 the chest, which takes place in the adult from 16 to 24 
 times per minute, each complete act occupying there- 
 fore about three seconds. At birth the number of 
 respirations is about 40 per minute, and it becomes 
 slower year by year. The proportion of respiratory 
 acts to the cardiac beats is about two to nine or ten. The 
 act of enlargement of the chest cavity is termed insjn- 
 ration; that by which it is diminished expiration. 
 
 The act of inspiration. — In inspiration the 
 thoracic walls are drawn apart in all directions, and 
 the cavity of the chest is enlarged by muscular action. 
 A tendency to a vacuum is produced ; air immediately 
 rushes through the trachea into the lungs, which 
 passively expand to fill the enlarging space, and to 
 equalise the pressure within and without the chest. 
 If the mouth and nostrils be closed no effort will 
 expand the chest, because the pressure of the atmo- 
 sphere on the outside of the chest is greater than the 
 muscles can overcome ; but if the muscles were of 
 such strength that they could overcome the atmospheric 
 pressure, then the ribs, with the parietal layer of the 
 pleura, would separate from the lungs covered with 
 the visceral layer of the pleura, and a vacuum would 
 be formed between the two layers of the pleura, which, 
 however, would soon be filled by the distension of the 
 lungs with blood, for under normal conditions both 
 blood and air are drawn to the lungs in inspiration ; 
 but air, being much lighter and more mobile, naturally 
 enters in larger quantities. The lungs are always 
 under negative pressure, since they collapse when 
 an opening is made into the pleura. The muscles 
 engaged in effecting inspiration, and especially forced 
 inspiration, are numerous, but one stands out pre- 
 eminent amongst the rest, and probably almost acts 
 alone in tranquil inspiration — tJie diaphragm. This 
 muscle when at rest forms an arch, on the upper 
 convex surface of which the heart and lungs rest. In 
 
Chap, v.] Respiratory Movements. 59 
 
 contraction the lateral portions of tlie arch become 
 flattened, the cavity of the chest is correspondingly- 
 enlarged and the lungs descend. The central tendon, 
 on which the heart rests, and through which the 
 inferior vena cava passes, remains almost stationary. 
 The importance of this muscle is shown by the fact that 
 if both phrenics which are its motor nerves, be cut, 
 death results, the other muscles collectively being 
 unable to maintain respiration eSectively. The next 
 most important group of muscles acting in tranquil 
 inspiration are the scaleni, which fix the first rib and 
 the external intercostals, the fibres of which run 
 downwards and forwards, and which, taking the first 
 rib as a fixed point, act successively on the ribs below, 
 pulling them outwards and upwards, and thus enlarge 
 the intercostal spaces. The levatores costaruTn are also 
 believed to act in tranquil inspiration. 
 
 The act of expiration. — Tranquil expiration is 
 hardly efiected by muscular action, but results from 
 the elasticity of the lungs and walls of the chest. 
 
 Forced respiration. — When from violent muscular 
 effort the right heart and lungs become surcharged 
 with blood, or when the respiratory passages are con- 
 stricted, or when the respiratory centres are supplied 
 with blood containing a deficiency of oxygen and an 
 excess of carbonic acid gas, difficulty of respiration or 
 dyspnoea is induced, and forced efforts of inspiration 
 and expiration are made, in which many muscles take 
 part. The following table, taken from Landois, shows 
 the muscles engaged in tranquil and in forced respira- 
 tion, and gives also their nerve supply. 
 
 A. INSPTEATION. 
 
 I. Ix TrAXQTJIL IxSPIEATION the MrSCLES ACTIS-G ARE : 
 
 1. The diaphragm {Nervus phrenicus). 
 
 2. The three scaleni {Rami muscul. plex. cervicalis et hrachialis). 
 
 3. The levatores costarum [Earn, poster, nerv. dorsalium). 
 
 4. The external intercostal muscles {Nerv. intercostales) . 
 
Go Human Physiology. [Chap. v. 
 
 II. In Forced Ixsfiiiation the Actiye Muscles are : 
 a. Muscles of the Trunk. 
 
 1. The sterno -mastoid {Ramus extermis nervi accessorii). 
 
 2. The trapezius {Ram. ext. ncrv. accessorii et ram. muscularcs 
 
 plez. cervicaUs). 
 
 3. The pectoralis minor [Xn. thoracici anteriores). 
 
 4. The serratus posticus superior (iV. dorsalis scapuUe), 
 
 0. The rhomhoidei {N. dorsalis scapulce). 
 
 6. The extensors of the vertebral column {Ram. post, nervorum 
 
 dorsaliiim). 
 
 7. [The serratus anticus major {N. thoracicus longusy] ? 
 
 h. Muscles of the Larynx. 
 
 1. The sterno-hyoid {Ram. descendens hypoglossi). 
 
 2. The sterno-th^Toid {Ram. descendens hypoglossi). 
 
 3. The crico-ary'tsenoideus posticus {Nerv. laryngeus inferiyt 
 
 i-agi). 
 
 4. The thyro-arytaenoid {Nerv. laryngexis inferior vagi) 
 
 c. Muscles of the Face. 
 
 1. The dilatatores narium, anterior and posterior {N. facialis). 
 
 2. The levator alffi nasi {jS\ facialis). 
 
 3. The expanders of the mouth and nares {JSf. facialis). 
 
 d. Muscles of the Pharynx. 
 
 1. The levator veh palatini (iV./<atci«^is). 
 
 2. The azygos uvulse {N. facialis). 
 
 B. EXPIRATION. 
 
 1. Tranqltil Expiration is effected essentially 
 
 By the elasticity of the lungs, costal cartilages, and abdominal 
 muscles, and by the weight of the chest walls. 
 
 2. In Forced Expiration the Muscles acting are 
 
 1. The abdominal muscles {JSfn. abdominis interni s. anteriores 
 
 e nervis inter costalihus) . 
 
 2. The triangularis sterni [Nn. intercostales). 
 
 3. The serratus posticus inferior {Ram. ext. nerv. dorsalium). 
 
 4. The quadratus lumborum {Ram. mj(sculares e plex. lumhale). 
 0. The intercostales interni so far as they lie between the bony 
 
 ribs, and the infracostales {2\li. intercostales). 
 
Chap, v.] Mechanism OF Respiration. 6i 
 
 The ribs are fixed at their extremities, whilst their 
 central curved part plays comparatively freely in the 
 acts of respiration, like the handle of a bucket. Those 
 muscles which raise the ribs also separate them from 
 each other, and act therefore as muscles of inspiration. 
 Those that depress the ribs approximate them to each 
 other, lessen the cavity of the chest, and act as muscles 
 of expiration. In dyspnoea, as in asthma, the body 
 is bent forwards, the hands grasp a chaii' back, or rest 
 on some solid object, in order to fix the shoulders and 
 to enable the pectorales, scaleni, sterno-mastoid, and 
 other muscles, to act with most advantage in expanding 
 the chest. Much discussion has been held in regard 
 to the action of the intercostals. The points well 
 settled are, that the external intercostals, which run 
 downwards and forwards, and, therefore, from the less 
 movable part of the rib above to the more movable 
 pai-t of the rib below, must act as elevators of the ribs, 
 and as muscles of inspiration, providing the first rib is 
 fixed. And in like manner, that portion of the internal 
 intercostals which is between the costal cartilao-es, 
 since the fibres run downwards and backwards, and, 
 therefore, again from the less to the more movable part 
 of the ribs, for the ribs are fix:ed at the sternum, as well 
 as at the spine, must raise the ribs, and separate them 
 from each other, providing the first rib is fixed. But it 
 is conceivable that if the intercostal muscles were actino- 
 simultaneously, 'per se, and not in concert with other 
 muscles, they would bring the ribs together, contract the 
 chest, and act as muscles of expiration. The absolute 
 enlargement of the chest in the antero-posterior and 
 transverse direction, in tranquil respiration, does not 
 exceed half a centimetre or five mm. In forced respi- 
 ration it is about three centimetres at the level of the 
 seventh or eighth rib. The amount of the vertical 
 enlargement of the chest, caused by the descent of the 
 diaphragm, is not known in man, but in the horse it 
 
62 Human Physiology. [Chap. v. 
 
 has been estimated to be three times greater than the 
 increase of the transverse diameter. 
 
 Duration of the acts of respiration. — Tracings of 
 the respiratory acts show that the act of inspiration is 
 shorter than that of expiration in the proportion of 
 6 : 7 or 6 ; 8. Inspiration commences with moderate 
 rapidity, then becomes more rapid, and towards its 
 close somewhat slower. Expiration begins with 
 moderate rapidity, then becomes quicker, and towards 
 the close of the act very slow. There is no pause 
 between expiration and inspiration. 
 
 Types of respiration. — 1. Abdominal type. — In 
 the adult male, and in infants during the first three 
 years of life, the diaphragm is usually the chief muscle 
 employed in tranquil inspiration, and its descent causes 
 the viscera to press on the abdominal walls, and the 
 belly rises and falls with each act of respiration. This 
 type of respiration may be observed in the horse, cat, 
 and rabbit. 
 
 2. Gosto-inferior type. — This type occurs in many 
 adult males, and especially in boys after the third 
 year. The diaphragm acts, but the abdominal wallb' 
 move but slightly, and the expansion of the chest is 
 chiefly effected by the raising and separation of the 
 ribs from the seventh upwards. It is seen in the dog. 
 
 3. Costo-superior type. — In this type the clavicles, 
 upper part of the sternum, and the upper part of the 
 ribs, play freely, whilst the lower part of the chest and 
 the abdominal walls are almost motionless. It is 
 characteristic of the female, enabling her to respire 
 freely when the movements of the diaphragm and 
 lower ribs are interfered with by the great enlargement 
 of the uterus in pregnancy. 
 
 In infancy, when the child sucks, respiration is 
 carried on exclusively through the nose, and a simple 
 catarrh, by causing tumefaction of the pituitary 
 mucous membrane, may cause death by asphyxia or 
 
Chap, v.] Capacity of the Chest. 63 
 
 by inanition ; bj the child being unable to breathe on 
 the one hand, or take nourishment on the other. 
 
 Capacity of the chest. — The volume of air 
 contained in the lungs of a healthy adult male, after 
 the fullest possible inspiration, is about 330 cubic 
 inches, and it has been found convenient to divide 
 this into four parts. 
 
 (1) Tidal or hreathing air, which is that volume 
 inspired and expired with each act of respiration. 
 
 (2) Complemented air; or that volume of aii' which 
 can be inspired after an ordinary inspiration. 
 
 (3) Reserve, or supplemental air, or that volume of 
 air which can be expired after an ordinary expiration. 
 
 (4) Residual air. — That volume which remains in 
 the lungs after the fullest possible expiration. 
 
 These several volumes may be expressed in cubic 
 inches as follows : 
 
 1. 
 
 Tidal air 
 
 20 cubic inches. 
 
 2. 
 
 Complemental air 
 
 . 110 „ „ 
 
 3. 
 
 Eeserve air 
 
 100 „ 
 
 4. 
 
 Residual air 
 
 100 „ 
 
 The first three of these volumes can be taken into, 
 or expelled from, the chest at will, and have been col- 
 lectively termed the vital capacity. The vital capa- 
 city of a man of 5 feet 8 inches is 230 cubic inches. 
 
 The vital capacity is modified (1) by height, (2) by 
 position, (3) by weight, (4) by age, and (5) by disease. 
 
 (1) In regard to height, every inch of stature from 
 five to six feet, enables the subject to expire eight 
 additional cubic inches by a forced expiration after a 
 full inspiration. 
 
 (2) In regard to position, it is found that more air 
 can be inspired in the erect than in the recumbent 
 position, because the movement of the chest walls is 
 more free in the erect posture. 
 
 (3) Speaking generally, it may be said that with 
 
64 Human Physiology. [Chap. v. 
 
 increased weight, within moderate limits, there is in- 
 creased vital capacity. 
 
 (4) In regard to age, the vital capacity increases 
 from 15 to 35 years, and from 35 to 65 it decreases. 
 
 (5) All abdominal and thoracic diseases, as tu- 
 mours, abscesses, phthisis, diminish the vital capacity. 
 
 The air in the lungs appears to be completely re- 
 newed in the course of from 6 to 10 respiratory acts. 
 
 The respiratory centres. — The centres which 
 preside over the acts of respiration occupy a symme- 
 trical position on either side of the median line in the 
 lower part of the medulla oblongata, a little above the 
 origin of the vagi, and opposite the interspace between 
 the occipital bone and atlas. It is believed that there 
 are two on each side, one reo-ulatinsf the movements of 
 inspiration, the other of expiration. The evidence 
 demonstrating that the centres occupy this position is 
 that the spinal cord may be divided from below, and the 
 brain may be removed by successive slices from above, 
 without arresting respiration, until this spot is injured, 
 w^hen respiration at once ceases ; whilst, if this region 
 is damaged alone, though the rest of the nervous system 
 is intact, the respiratory acts are at once and perma- 
 nently arrested, and death promptly follows. Hence this 
 region of the medulla has been named the nceud vital. 
 
 The respiratory centres can be excited to act both 
 by dii'ect and by reflex stimulation. When all 
 sensory nerves by which these might be stimulated are 
 divided, respiratory movements, though irregular in 
 rhythm and depth, still continue, and it seems probable 
 that these mov^ements are then due to the circulation 
 through the medulla of blood deficient in oxygen and 
 containing an excess of COg. It has been shown by 
 experiment, that after section of the vagi, which pre- 
 vents the passage of stimuli from the lungs, deficiency 
 of oxygen acts as an exciter of the inspiratory centre, 
 whilst excess of COg stimulates the expiratory centre. 
 
Chap. v.] Mechanism of Respiration. 65 
 
 In tranquil respiration, or eupncea, it is probable 
 that the defective supply of oxygen, and excess of CO2 
 in the blood circulating through the capillaries of the 
 lungs, stimulate the peripheric terminations of the 
 vagus. The stimulus is conveyed by this nerve to 
 the respiratory centres, whicli at once respond, and 
 liberate impulses, which, travelling down the spinal 
 cord, pass along the motor nerves supplying the 
 diaphragm and intercostals, and inspiration follows. 
 The entrance and absorption by the blood, of oxygen, 
 then temporarily leads to quiescence of the centres 
 and relaxation of the muscles, and a fresh inspiration 
 is only made when the deficiency of oxygen rises to a 
 certain amount. 
 
 If by rapid and deep breathing a large proportion 
 of oxygen is introduced into the system, the need for 
 the acts of respiration is less felt ; no motor impulses 
 emanate from the respiratory centres, and the condi- 
 tion termed apnoea is established. On the contrary, 
 if the supply of oxygen fail, and CO2 accumulates in 
 the blood, violent muscular efforts, both of expiration 
 and of inspiration, are made, in which almost every 
 muscle in the body takes a part, and the subject is said 
 to suffer from dyspnoaa. 
 
 Other nerves beside the vagi may conduct centri- 
 petal impulses or stimuli to the respiratory centres. 
 The most important of these is undoubtedly the fifth, 
 and its influence is familiar in the deep inspiration 
 that occurs when the face is suddenly exposed to cold, 
 as by being flipped with a cold wet cloth, or drenched 
 with cold water ; and the same effect is produced by 
 the application of cold to other sensory nerves. 
 
 In all violent efforts, a deep inspiration is in- 
 stinctively taken, and the glottis closed by the arytae- 
 noid and lateral crico-arytsenoid muscles, the object of 
 which is to fix the chest, and to afford a point d'appui 
 to the upper limbs. 
 
 F 
 
66 Human Physiology. [Chap. v. 
 
 The sounds of respiratioo. — If the ear be 
 
 applied to the uncovered chest of a person breathing 
 tranquilly, a soft blowing murmur is heard during 
 inspiration, over the whole of those regions beneath 
 which the lungs are situated. This murmur, named 
 the vesicular sound, is not audible during expiration. 
 It is caused by the distension and separation of the 
 walls of the air vesicles and smaller channels leading to 
 them, and to the friction of the air over the numerous 
 points of division of the air-tubes. If the ear be placed 
 over the trachea in front, or the larger bronchi at the 
 back, on either side of the vertebral column, a louder and 
 rougher sound is heard, termed the bronchial murmur. 
 It is audible both in inspiration and in expiration. 
 
 Percussion sounds of the chest. — When 
 the chest is lightly tapped with the finger, or with a 
 percussion hammer, the regions beneath which the 
 lung is situated are resonant, or sound hollow, whilst 
 that part which is over the heart is dull. The apices 
 of the lungs reach an inch or more above the clavicle 
 in front, and nearly up to the level of the spinous pro- 
 cess of the seventh cervical vertebra behind. The 
 lower border of the lung, in moderate expiration, com- 
 mences on the right side, at the border of the sternum 
 opposite the attachment of the sixth rib, then runs 
 nearly parallel with the upper border of the sixth rib, 
 descends a little in the axillary region to the border of 
 the seventh rib, and posterior is level with the tenth rib. 
 The relations are the same on the left side, except in 
 the region near the sternum corresponding to the fourth, 
 fifth, and sixth ribs, where the right ventricle of the 
 heart is in immediate relation with the ribs. In full 
 inspiration, the superior borders of the lungs descend 
 below the seventh rib in front and laterally, and as low 
 as the eleventh rib posteriorly, and on the left side the 
 lung advances considerably in front of the heart, leaving 
 only a small area uncovered. In full expiration, on the 
 
Chap, v.] Physics of Respiration. 67 
 
 other hand, the lower border of the lungs rises nearly 
 the breadth of an intercostal space, and on the left 
 side a large part of the ventricle is uncovered. 
 
 Tariations of pressiu'e in tlie air-passag:©*. 
 
 — If a manometer be connected with an opening in the 
 trachea, whilst the respiratory acts are otherwise 
 normally performed, it is found that, with each inspi- 
 ration, a negative pressure equal to 3 mm. of mercury 
 is exerted, whilst during expiration there is a corre- 
 sponding positive pressure. If the manometer tube 
 be inserted into one nostril, and tranquil respiration 
 carried on through the other, with closed mouth, the 
 inspiratory negative pressure is 1 mm. of mercury, the 
 positive expiratory pressure is from 2 to 3 mm. When 
 all the inspiratory or expiratory muscles are brought 
 into play, the pressures, both negative and positive, are 
 greatly increased. Thus the inspiratory muscles of a 
 medium-sized adult male, can exert a negative pressure 
 capable of raising a column of mercury 75 mm. high, 
 whilst the expiratory muscles, aided by the elasticity of 
 the chest walls and lungs_, can exert positive pressure ca- 
 pable of raising a column of mercury 100 mm. in height. 
 Absorptioia of gases by fluids. — Gases are ab- 
 sorbed to a greater or less extent by water and other 
 fluids, the amount absorbed being dependent (1) on 
 pressure, (2) on temperature, and (3) on affinity ; and the 
 weight that is taken up of any gas by the same liquid at 
 a given temperature is proportional to the pressure, or, 
 which is the same thing, the volume of the gas dissolved 
 is at all pressures the same. Hence it is found that 
 when the pressure is doubled the weight of dissolved 
 gas is also doubled. Temperature is of great importance, 
 since the quantity of gas absorbed decreases with the 
 temperature. At 0° C. and 1 m. pressure, water will ab- 
 sorb 1'79 vol. of CO2, but at 15° C. it will only absorb 
 1 vol. The quantity of gas which a liquid can dissolve 
 Is independent of the nature and of the quantity of 
 
68 Human Physiology. [Chap. v. 
 
 other gases which it may ah^eady hold in solution. If 
 a fluid is exposed to a mixture of several gases, the 
 pressure on its surface is equal to the sum of the pres- 
 sures of the several gases, but each gas only exerts the 
 same pressure that it would do if it occupied the whole 
 space above the fluid, and in a mixture this is called 
 its " partial pressure." Thus oxygen forms one-fifth 
 of any quantity of air, and water exposed to air absorbs 
 only such a quantity of oxygen as it would do if the 
 atmosphere were entirely formed of this gas under a 
 pressure equal to one-tifth that of the atmosy»here. 
 Thus, since the pressure of the atmosphere is 760 mm. 
 at the level of the sea, 
 
 , , 760 X 20-8 
 
 the pressure oi oxvgen = = loo mm,: 
 
 ^ ■'^ 100 
 
 ... 760 X 79-2 -^^ 
 
 the pressure of the nitrosren = = 601 mm.: 
 
 ^ ^100 ' 
 
 ^ , ^^ 760 X 0-00025 
 the pressure of the COg^ = O'Ooz mm. 
 
 The numbers 1»58, 601, 0-052 represent the partial 
 pressure of the several gases mentioned. The partial 
 pressure of oxygen in the air vesicles during calm expi- 
 ration sinks to 1 17 mm., and in a deep inspiration is 130 
 mm,, whilst the partial pressure of COg rises in tranqiiil 
 respiration to 31 '5 mm., and in deep expiration to 66 -4 
 mm. The partial pressure of any gas may be calculated 
 
 TT _r_ Q 
 
 with the aid of the following simple formula : P = — — — 
 
 where P = the partial pressure, H the pressure of the 
 expired or inspired air, and Q the quantity of the gas 
 requii'ed to be determined in 100 volumes. 
 
 Chemistry of respiration. — As the chemical 
 changes in respiration take place between the gases 
 contained in the air and the blood, it is requisite in 
 the first place to ascertain the composition of the gases 
 which can be extracted without chemical decomposition 
 from these two fluids respectively. 
 
Chap, v.] Chemistry of Respiration. 69 
 
 The composition of pure dry air is well known. It 
 contains in every 100 volumes 21 volumes of oxygen 
 gas and 79 volumes of nitrogen gas, or if estimated 
 by weight, in any 100 grains there are 23 grains of 
 oxygen and 77 grains of nitrogen. It further contains 
 four parts of carbon dioxide in every 10,000 volumes. 
 Its composition is constant up to a height of 14,000 feet. 
 In crowded theatres the quantity of oxygen may fall 
 about 1 per cent.^ and the amount of carbon dioxide 
 may rise to 70 parts in 10,000, and in either case the 
 air becomes distinctly prejudicial to health. Ordinary 
 air contains in addition a variable quantity of watery 
 vapom\ If saturated, a cubic metre of air takes up 
 at 10° C. 9'362 grains of aqueous vapour. The higher 
 the temperature the more water can it contain, the 
 lower the temperature, the less. Thus, 1 cub. met. of 
 air at 35° C is saturated with 39'25 grains of aqueous 
 vapour, whereas at 0° C. it can hold only 4*87 grains. 
 Air is rarely saturated with aqueous vapour, and the 
 quantity rarely falls below ^^th of the saturating quan- 
 tity. Lastly, the air contains a trace of ammonia. 
 
 The g"ases of tlie blood. — The most reliable ob- 
 servations that have' been made on this point are those 
 of Pfliiger, who found that nearly 60 volumes of gases 
 coiild be extracted from 100 volumes of arterial blood 
 immediately after it had been withdrawn from the 
 body. These gases consisted of oxygen 22'2 vol., 
 carbon dioxide 34*3 vol, and IS.^ 1"8 volumes. Venous 
 blood, in becomino- chano-ed into arterial, srains from 
 4 to 20 per cent, of oxygen by volume, and loses from 
 3 to 13 per cent, of carbon dioxide. The tension of 
 » in the pulmonary capillaries is 44 mm., of CO2., 
 82 mm. 
 
 Changes in the air that has been once 
 breathed. — (1) The proportion of carbonic dioxide 
 CO2 is increased to 4'3 per cent., so that after a 
 single act of tranquil respiration, air, which previously 
 
70 Human Physiology. [Chap. v. 
 
 only contained four parts in 10,000, now contains on 
 an average 430 parts in 10,000 volumes. 
 
 (2) The proportion of oxygen gas is diminished on 
 an average about 4*8 per cent., so that whilst ordinary 
 air contains 21 per cent, of oxygen, air that has been 
 once breathed contains only 1 6 '2 per cent. The excess 
 of oxygen taken up by the blood over the oxygen dis- 
 charged as carbonic dioxide is probably applied to the 
 oxydation of other constituents of the body, as sulphur 
 and phosphorus. 
 
 (3) A small but variable quantity of nitrogen is 
 added to it. 
 
 (4) It is saturated with watery vapour. One cubic 
 metre of expired air at 37° C. containing about 42 
 grammes of watery vapour. 
 
 (5) Its temperature is increased, usually, in tem- 
 perate climates, rising to 36-3° C, but several degrees 
 lower if the air inspired be very cold, and a degree or 
 two higher if hot. 
 
 (6) Its volume is increased, the slight decrease due 
 to excess of absorbed over COg eliminated* being 
 more than compensated by expansion from heat and 
 addition of watery vapour, 
 
 (7) There is an addition to it of small quantities 
 of ammonia, hydrogen, and CH. 
 
 The absolute quantity of oxygen gas absorbed into 
 the system per diem by the lungs is about 750 grammes, 
 of carbonic dioxide eliminated about 900 grammes, 
 of water eliminated about 450 grammes. 
 
 Many circumstances modify the quantity of car- 
 bonic acid eliminated from the system. Everything, 
 which tends to increase the quantity of oxygen enter- 
 ing the system, or the quantity of readily oxy disable 
 materials in the body, or which renders the oxydising 
 process more active, will increase the elimination of 
 
 * Equal volumes of O and of CO2 contain equal quantities of 
 oxygen gas. 
 
Chap, v.] Chemistry of Respiration. 71 
 
 carbonic dioxide. Thus it is greatly increased by ex- 
 ercise, by food, and by exposure to cold within mode- 
 rate limits. It is relatively larger in children than in 
 adults, in men than in women, in persons of a vigorous 
 constitution than in the languid and phlegmatic. Ifc 
 is also modified by certain physiological changes, as by 
 the frequency and depth of the inspiratory acts, which, 
 when either deeper, or more frequent with the same 
 depth, cause increased elimination of carbon dioxide. 
 
 Muscular exertion is without doubt the condition 
 that exercises the greatest influence on the production 
 and discharge of carbon dioxide, and it has been esti- 
 mated that if a person breathing tranquilly discharges 
 seven or eight ounces of carbon per diem in the form 
 of carbon, the quantity would be more than doubled 
 with severe exertion. 
 
 The mode of comtoinatioo of cartoon 
 dioxide in tlie blood. — It can be easily shown that 
 part of the carbon dioxide contained in the blood is in 
 combination with the coloured corpuscles, and another 
 part with the plasma, and the combination with the 
 plasma is firmer than with the corpuscles, but it has 
 not been quite certainly determined in what state of 
 combination it really exists. It is probably in the 
 form of sodium bicarbonate, which is chiefly con- 
 tained in the plasma, 
 
 ESTects of vai'iatioii in tlie presstu^e of tSie 
 air on respiration. — Man .lives at the bottom of a 
 great aerial ocean, the depth of which is about fifty 
 miles, and which, though so light, exercises an enormous 
 pressure on the surface of his body, a pressure that, at 
 fifteen pounds to the square inch, has been calculated to 
 amount to thirty or forty thousand pounds, according 
 to the extent of surface presented by the skin ; but 
 in ascending lofty mountains, or in balloon ascents, 
 when a height of nearly thirty thousand feet has been 
 attained, the pressure of the air is greatly diminished, 
 
72 Human Physiology. [Chap. v. 
 
 and with it the partial pressure of the oxygen. If 
 the change is effected slowly the system accommodates 
 itself to the altered condition, and no ill effects are 
 experienced ; but if the change from air of ordinary 
 density to air that is greatly rarefied occurs suddenly, 
 remarkable effects are observed, which, in the case of 
 mountain climbing, are known as the mal de montagne, 
 and which are equally noticeable in balloon ascents. In 
 the former case they are intensified by the muscular 
 exertion which is put forth in the ascent, whilst in the 
 latter case the suddenness of the change seems to 
 induce the same symptoms. 
 
 These are : 1. Congestion of capillaries of the skin 
 and free surfaces of the mucous membranes, owing to 
 diminished pressure, which may culminate in haemorr- 
 hage, and certainly leads to sweating and free mucous 
 secretion. 2. The resistance to the passage of the 
 blood through the capillaries being diminished, the 
 cardiac beats become more frequent and dyspnoea is 
 experienced, and the respirations become deeper and 
 irregular as well as more frequent. 3. The insufficient 
 su})ply of oxygen and imperfect elimination of CO. 
 induce extraordinary muscular weakness, the slightest 
 exertion causing exhaustion. Hence, again, in part, the 
 dyspncea and sense of constriction about the chest. 4. 
 The vagal centres are stimulated by the insufficiency 
 of O, and vomiting may occur ; and 5. The blood being 
 drawn from the internal organs to the surface, the 
 brain is imperfectly supplied with oxygen, and faint- 
 ness results, as well as diminished secretion of urine. 
 
 As by ascending the pressure of the air can be 
 decreased, so by descending below the sea level it can 
 be increased, and the amount of such increase in deep 
 mines and in works conducted under water, as in the 
 laying of the foundations of bridges, is sometimes con- 
 siderable, amounting to as much as sixty or seventy 
 pounds avoirdupois on the square inch. In such cases 
 
Chap, v.] Chemistry of Respiration. 73 
 
 the skin has been observed to become pale, and the 
 cutaneous perspiration to diminish. The respirations 
 are reduced in frequency by two to four in the minute. 
 Inspiration is accomplished mth greater facility, 
 expiration is prolonged, and a distinct pause occurs 
 between expiration and inspiration. The capacity of 
 the lungs augments. The urinary secretion is in- 
 creased, and muscular efforts are made with more 
 activity and eneigy. The heart, meeting with more 
 resistance on account of the contraction of the cutan- 
 eous capillaries, beats more slowly, and the pulse curve 
 is lower. There is a subjective sensation of warmth. 
 Great care should be taken that those Avho are sub- 
 jected to such high pressure be not suddenly exposed 
 to air at the normal pressure, for the effect is equiva- 
 lent to the application of a gigantic cupping glass to 
 the whole body. The blood is suddenly drawn to the 
 surface, haemorrhage from the nose, ears, and mouth 
 is likely to occur, and paralysis may result from the 
 sudden abstraction of blood from the nerve centres. 
 Mere exposure to an atmosphere of oxygen produces 
 no ill effects ; but if the animal be exposed to oxygen 
 under pressure, so that the blood is made to take up 
 about thirty-five per cent, of its volume of 0, death 
 occurs in convulsions. 
 
 Effects of breatliiiii: in a confined space. — 
 The effects of breathing in a limited space ditier to 
 some extent in accordance with the size of the space. 
 If the space be small, or only of moderate size in com- 
 parison with the animal, the whole of the oxygen is 
 used up, and a nearly corresponding volume of CO, is 
 eliminated. The fact that the oxygen entirely dis- 
 appears is a clear proof that its absorption by the 
 blood is due to chemical affinity and is not the result of 
 the operation of the usual laws of absorption of gases 
 by fluids. Death results from asphyxia. 
 
 In large but confined spaces, though death results 
 
74 Human Physiology. [Chap. v 
 
 from asphyxia, it is ratlier due to accumulation of 
 CO2 than to a deficiency of oxygen; at least, the animal 
 dies before the oxygen is altogether exhausted. If 
 arrangements are made by which the COg is removed 
 as fast as it is formed, the animal will live much longer 
 than if it be allowed to breathe the same air more or 
 less charged with COg over and over again. In fact, if 
 animals be made to breathe a mixture of half oxygen 
 and half COg death results, not because there is a de- 
 ficiency of oxygen, but because there is an excess of 
 CO2, which is absorbed, and then acts as a poison. 
 
 In pure oxygen animals breathe quite normally, and 
 the substitution of hydrogen gas for nitrogen may be 
 made without in any way interfering with respiration. 
 
 Necessity for ventilation. — The necessity for 
 free ventilation of the air in dwelling-houses, follows 
 from what has just been said. Wherever animals are 
 crowded together, the oxygen of the air by which they 
 are surrounded soon becomes exhausted, and COg takes 
 its place. But this is not all ; the lungs and the skin 
 alike give off" other waste products, which, though 
 minute in quantity, still make themselves perceptible 
 in the odour of the breath and of the skin, which are 
 characteristic of each individual, and which give the 
 peculiarly penetrating and unpleasant odour to the 
 dwellings of the poor, to the out-patient rooms of hos- 
 pitals, and wherever the 'free access of air and water 
 are neglected. Fortunately, the free interchange that 
 takes place between gases, and the porous nature of 
 the materials of which our garments and houses are 
 constructed, allows a natural ventilation to take place, 
 which requires the perverse ingenuity of man to inter- 
 rupt. It may be accepted as a fact, that as soon as 
 air becomes in the slightest degree tainted, or has a 
 distinct odour perceptible to one who enters it from 
 fresh air, it has become unwholesome. In ordinary 
 air, the proportion of CO2 is four parts in 10,000, but 
 
Chap, v.] Internal Respiration. 75 
 
 it has been found to rise as high as tAventy or thirty 
 parts in 10,000, whilst in some schoolrooms in Ger- 
 many it has been noticed by PettenkofFer to rise as 
 high as 72 parts in 10,000. It is generally held that 
 to maintain the air of a room in a sufficient state of 
 purity, 1,000 cubic feet for each individual is not too 
 much, and no doubt, with the ordinary ventilation that 
 is continually taking place through the chimney, and 
 the chinks of windows and of doors, and even through 
 the very walls themselves, this is sufficient ; but it 
 must not be forgotten that such ventilation is abso- 
 lutely necessary, and that in rooms that are unpro- 
 vided with a chimney, and in which, as is sometimes 
 done, the chinks are closed by pasting paper over them, 
 the air soon becomes loaded with poisonous exhala- 
 tions. In order that air should remain sufficiently 
 pure for breathing purposes, at least 200 times the 
 volume of the air expired by each individual should 
 in any given time be added to it. In large factories, 
 the unwholesome results of insufficient ventilation are 
 greatly intensified by the presence of floating particles 
 of the fabrics manufactured in them ; cotton, silk, and 
 steel filings being especially injurious. The preserva- 
 tion of the walls of the dwelling-houses from moisture 
 is of great importance, for not only does moisture fill 
 the pores of wood, brick, or other constructive material, 
 and thus arrest ventilation in this direction, but the 
 absorptive power of moisture for heat exercises a 
 prejudicial influence on the inmates. 
 
 Internal respii'ation. — By internal respiration 
 is meant the exchange of gases which takes place 
 between the tissues, including the blood itself, and the 
 gases contained in the blood. iSTo sooner does the 
 blood become charged with oxygen as it traverses the 
 capillaries of the lungs, than oxydation of the more 
 unstable compounds contained in the blood commences ; 
 not only, however, is the quantity of these substances 
 
76 Human Physiology. [Chap. v. 
 
 small, but the time during which this action can take 
 place is extremely brief, only lasting till the blood 
 reaches the systemic capillaries, hence the oxydising 
 processes which take place within the vessels are com- 
 paratively slight ; but no sooner has the blood reached 
 the capillaries, than it is surrounded by tissues poor in 
 oxygen and rich in CO2, and an active interchange of 
 gases immediately takes place. The of the hsemogio- 
 bin of the coloured corpuscles rapidly escapes, whilst the 
 CO2, with which the tissues are charged, as rapidly enters 
 the blood. That the tension of the COg in the tissues is 
 high, is clearly shown by the tension of this gas in the 
 fluids of the body ; thus, whilst in arterial blood it is 
 equivalent to a column of mercury 21 mm. in height, 
 and in the venous blood of the pulmonary capillaries 
 to one of 41 mm., in the bile it is as much as 50 mm., 
 and in acid urine to 68 mm., though in the latter cases 
 it can only be derived from the tissues. In the lymph 
 it is about 35 mm. of mercury, and the smaller tension 
 of the CO2 in the lymph than in venous blood may be 
 explained in part by supposing that the processes of 
 oxydation continue to take place in the blood, in part 
 by the slower current of the lymph, which, perhaps, 
 allows of a more equal distribution of the CO^, partly 
 to the circumstance that in the muscles, in which the 
 production of CO2 chiefly takes place, the lymphatics 
 are few, and partly that the whole process of inter- 
 change is affected by chemical affinity, and that there 
 are compounds in the blood which have a stronger 
 affinity for COj than in the lymph. 
 
 Respii^atioii toy tlie skin. — The conditions 
 under which the blood travels in the skin are not 
 materially different from those which are present in 
 the lungs. In both cases there is a rich plexus of 
 capillaries separated from the air by epithelium. In 
 the lungs, the epithelium forms a simple layer, whilst 
 in the skin there are many layers, so that the contact 
 
Chap, v.] Various Respiratory Acts. 77 
 
 of the air with the blood is less immediate. The 
 gaseous interchange in accordance with this difference 
 is much less active. The quantity of COj discharged 
 bj the skin as compared with the lungs is estimated 
 to be as 1 : 38. 
 
 The amount of insensible transpiration of vapour 
 of water is estimated to be double that given off bj 
 the lungs, or about 2 lbs. per diem, though, of course, 
 the amount of this vapour given off will depend on 
 the size of the body, the temperature, and the hu- 
 midity of the air. 
 
 In sighing, a deep inspiration is taken, and is 
 followed by an audible expiration. It appears to be 
 a compensatory effort to make up for several previously 
 imperfectly performed respiratory acts. 
 
 In yawning, air enters through the mouth, which 
 is usually widely opened, the nares are closed, and a 
 deep inspiration and expiration follow. It is an indi- 
 cation of fatigue and inadequate working of the respi- 
 ratory muscles. 
 
 In hiccujo, an inspiration is made by contraction 
 of the diaphragm, which is suddenly arrested by closure 
 of the lips of the glottis. 
 
 In snoring, air entering through the nose and 
 mouth causes a relaxed uvula and soft palate to 
 vibrate strongly. 
 
 In laughter, a succession of interrupted explications 
 occurs. 
 
 In coughing, the glottis is at first closed, but is 
 suddenly burst open with a resonant sound, by the 
 contraction of the expiratory muscles, the object being 
 to expel masses of inspissated mucus from the trachea 
 and larynx. 
 
 In sneezing, an expiration of almost convulsive 
 violence succeeds a long inspiration, the blast of air 
 being directed, in some instances, chiefly through the 
 nose, and in others chiefly through the mouth. 
 
78 
 
 CHAPTER YI. 
 
 FOOD. 
 
 Food. — Food is required by all animals to supply 
 the waste of the body, and in the higher animals to 
 maintain the temperature. Now the body of an 
 axlult consists of 58*5 per cent, of water, and 41-5 per 
 cent, of solids, and the analysis of the body of a 
 healthy man, and of a healthy woman, has been found 
 to give the following results : — 
 
 Weight of Body in Grammes. 
 
 Man. Woman. 
 
 69,688 65,400 
 
 Fercentage of the Total Weight of the Body. 
 
 
 Man. 
 
 Woman 
 
 Bones 
 
 15-9 
 
 15-1 
 
 Muscles 
 
 41-8 
 
 35-8 
 
 Thoracic viscera 
 
 1-7 
 
 2-4 
 
 Abdorainal viscera 
 
 7-2 
 
 8-2 
 
 Fat . 
 
 18-2 
 
 28-2 
 
 Skin . 
 
 6-9 
 
 5-7 
 
 Brain 
 
 1-9 - 
 
 2-1 
 
 Each and all of these parts undergo continual 
 waste ; some more, some less rapidly, but all of them 
 to so great an extent that if food be completely with- 
 held remarkable changes in volume and weight are 
 manifest in the course of a few days. This loss is 
 supplied by food, and as a rule the quantity daily 
 ingested corresponds with that which is lost by the 
 skin, lungs, urine, and f feces in the same space of 
 time. The nature of the food is diverse, and it is 
 
Chap. VI.] Uses of Food. 79 
 
 not consumed in a form that at first sight resem- 
 bles tlie tissues it is about to nourish ; it is the 
 purpose of the digestive system to induce such altera- 
 tion in it as may fit it to become a part of the body, 
 or, in other words, to prepare it for assimilation. 
 
 If it be asked, what are the tissues themselves 
 composed of? the reply must be given, that 
 though presenting considerable chemical and physical 
 difierences, they all spring from and contain an 
 organic basis which is composed of C, H, N, and O, 
 with small quantities of P and S, and the chemical 
 characters of which show that it belongs to the group 
 of albumins. These characters are indeed often 
 obscured by the changes it has undergone in the 
 process of development, and the materials it has 
 deposited in its own substance ; but in some instances, 
 as in muscle and nerve and gland, protoplasm 
 still exists as such, and is the active agent in 
 the functional activity of these tissues. In other- 
 instances, as in bone, calcareous salts have been 
 deposited, which more than double its bulk, whilst 
 in the enamel of the teeth the salts are so abun- 
 dant as almost entirely to conceal the protoplasm 
 originally present. In all instances the tissues are 
 built up of small masses of protoplasm, named cells, 
 each of which has its own definite destiny and 
 purjDose ; some becoming converted into blood cor- 
 puscles, some into the walls of blood-vessels, and 
 others into the structure of the different organs. 
 In many instances, as in the cells which form the 
 epidermis and the epithelium of mucous membrane, 
 and the cells of the glands and of the brain, the 
 original cell-form is preserved to so great an extent 
 that there is no difficulty in recognising them as cells ; 
 whilst in others they are so modified by growth that it 
 is necessary to follow them through the whole course 
 of their development to furnish satisfactory proof that 
 
So Human Physiology. [Chap. vi. 
 
 the structure or tissue under examination is in reality 
 formed of them. It is to minister to the wants of 
 these variously modified cells in the discharge of 
 their several functions that food is required. 
 
 Classification of food. — The food of man 
 varies in different regions of the earth in a remarkable 
 manner ; partly in accordance with the productions of 
 the earth, air, or sea by which he is surrounded ; but 
 partly also in accordance with the requirements of 
 his body as determined by the external temperature 
 to which he is exposed, his age, and the particular 
 form of mental, muscular, or other kind of work 
 he performs. The Arab can support life on a few 
 dates ; the Indian on some rice and ghee, with 
 d hurra, or on the fruit of the banana ; whilst the well- 
 fed European requires a great variety, as well as 
 abundance of food ; and the natives of the Arctic 
 regions are compulsorily restricted to a diet rich in 
 oil. But it is found that however different in appear- 
 ance these several dietaries may be, they can be shown 
 to contain certain proximate principles which are to 
 a greater or less extent present in all. These are : — 
 
 1. Proteids, such as the animal albumins and 
 vegetable glutins, which form the greater part of the 
 solids of meat, fish, eggs, milk, and bread-stuffs. 
 
 2. Farinaceous and saccharine compounds, such as 
 starch, gum, sugar, and glycogen and inosite, the first 
 three of which are chiefiy formed by plants and the 
 last two by animals, 
 
 3. Oleaginous compounds, such as the animal and 
 vegetable fats and oils. 
 
 4. Inorganic compounds, such as salts and water. 
 It may be laid down as a rule, without exception, 
 
 that none of these substances, neither albumin, nor 
 sugar, nor starch, nor gum, nor oil, is capable of 
 supporting life if taken alone. Whenever an animal 
 is fed on pure albumin, oil, or sugar, it dies. There 
 
ch^p. VI.] Classification of Food. 8i 
 
 must be a clue admixture of the several proximate 
 principles, though extraordinary variations in the 
 relative amounts occur. In those forms of food which 
 experience has shown to be capable of nourishing 
 every tissue and organ of the body, the types of which 
 are seen in the egg, and in milk, there is an admixture, 
 though in very diffei'ent proportions, of all the above- 
 mentioned groups of proximate principles. 
 
 It was formerly considered that foods were 
 divisible into respiratory and flesh-forming. The 
 former were held to be merely taken to maintain, by 
 their combustion, the heat of the body, and were 
 hence also termed heat-producers ; this class consisted 
 of sugars, starches, and oils. The latter were con- 
 sidered to minister to the nutrition of the tissues, and 
 to be essentially applied to the formation of muscle 
 and nerve ; starches and oils contain no nitrogen, 
 and it was hence thought they could not be applied 
 to the formation of tissue. The general result of 
 modern research is to throw doubt upon this classih- 
 cation of food. We shall see that even in muscle 
 there are non-azotised constituents which are essential 
 components of its tissue, and without which it would 
 not be muscle, and it therefore seems probable that 
 even the non-azotised substances may be applied to 
 the nutrition of muscle, though their chief office is 
 probably to supply the force developed during its con- 
 traction ; on the other hand, it is certain that proteids 
 contain the nitrogen which the muscles and nerves 
 require, and that they are applied to their nutrition ; 
 but there is good reason to believe that they also 
 m.inister to the production of heat, and, in fact, 
 break up into two nearly equal portions, an azo- 
 tised and an unazotised portion ; the former being 
 applied to the nutrition of the azotised tissue, whilst the 
 latter, like the starches and sugars, is applied to the 
 development of force and to the generation of heat. 
 
 G 
 
82 Human Physiology. [Chap. vi. 
 
 We may proceed to consider briefly the composition 
 of some of the principal articles of ordinary diet. 
 
 Meat. — The flesh of animals as consumed at table 
 is composed of about seventy- eight parts of water and 
 twenty- two parts of solids, and the solids consist (1) of 
 myosin-albumin, which is the chief constituent of the 
 contractile muscle substance; (2) serum-albumin, de- 
 rived from tlie lymph and blood coursing through the 
 tissue ; (3) gelatin, derived from the connective tissue ; 
 (4) elastin, from the elastic tissue ; (5) colouring 
 material, or hsemoglobin ; (6) keratin, from the endo- 
 thelium of the vessels ; (7) products of disintegration 
 of proteids, as kreatin, kreatinin, inosinic, and 
 sarcolactic acids, taurin, sarkin, xanthin, and uric 
 acid ; (8) fats, with lecithin and cholesterin ; (9) 
 carbohydrates, as inosite, dextrin, grape-sugar, and 
 glycogen : and lastly salts, amongst which the salts of 
 potassium and those of pliosphoric acid, with magne- 
 sium and calcium, predominate. In 100: parts of the 
 salts, potassium exists in the proportion of 39*4, 
 aodium 4"86, magnesium 3*88, and phosphoric acid 
 46 "74. The remainder is made up of chlorine, 
 calcium, and iron oxide. 
 
 The quantity of fat varies with the state of 
 fattening of the animal. In man, after the visible 
 fat has been dissected off, there still remains in 
 100 parts of flesh from seven to fifteen parts of fat ; 
 in beef, from eleven to twenty -two parts ; in mutton, 
 about four parts ; in fowls, about three parts. The 
 object of cooking meat is to render it more digestible. 
 In roasting and in boiling meat, the outside should 
 be quickly exposed to a high temperature, in order to 
 coagulate the albumin of the surface and prevent the 
 juices escaping from within. The joint may then be 
 more slowly cooked. Broths should, on the other 
 hand, be made by soaking the meat for some time in 
 cold water, and then slowly warming. There is but 
 
Chap. VI.] Values of Different Foods. 83 
 
 little nutriment in broth, as after careful preparation 
 only three parts per cent, are dissolved, and about 
 three parts remain suspended in the form of coagulated 
 albumin. The dissolved constituents are the potash 
 salts, kreatin and kreatinin ; the salts of lactic and 
 inosinic acids, and some extractives ; lastlv, gelatin. 
 A teaspoonful of Liebig's extract represents a pound 
 of meat, but it must not be supposed that it is in 
 any way equivalent to that amount of beefsteak for 
 a healthy person. It is concentrated beef-tea, and 
 nothing more. 
 
 Eggs. — The egg is clearly a perfect form of food, 
 since, with the exception of oxygen, it contains in 
 itself all that is required to form the complete bird or 
 reptile. The ovum of man, however, is extremely 
 small, and, though containing a sufficient store of 
 nutriment for the earliest stages of development, it 
 soon throws out from its surface processes which, 
 dipping into the large blood-vessels of the uterus, 
 draw from thence the material required for further 
 growth. The Qgg of the fowl is composed of the yolk 
 or vitellus, and the white or albuminous substance, 
 which is again enclosed in the shelL The yolk consists 
 of vitellin and other proteids — fat, cholesterin, and 
 lecithin, glycerin, phosphoric acid, grape sugar, 
 pigment, and salts. Yitellin is a globulin-proteid, 
 the solution of which is not precipitated by common 
 salt, in which respect it differs from myosin. It is 
 easily soluble in water containing 1 per cent, of hydro- 
 chloric acid, and is then converted into syntonin. It 
 dissolves easily in weak solution of soda. It is pre- 
 cipitated by alcohol. The average weight of a hen'"s 
 egg is about 1*75 oz., of which the shell forms one- 
 tenth, the wdiite about six-tenths, and the yolk 
 three-tenths. It is estimated that 1 lb. of hard-boiled 
 eggs, if completely oxydised, can set free a force equal 
 to raising 1,415 tons 1 foot high. Eighteen eggs 
 
84 Human Physiology. [Cha?. vi. 
 
 contain an am omit of flesli-forming substance and 
 other pabulum sufficient to maintain the life of an 
 adult man for one day. 
 
 Vegetable foods. — The most important of the 
 foods derived from the vegetable kingdom are supplied 
 by the cereals, and by leguminous plants, which con- 
 tain both proteids and starch, and by the potato and 
 other plants ha\T.ng in their fruit or seed, in root, in 
 tuber, in leaves, or in pith, a large store of starch. Thus 
 100 parts of dry wheat-flour contain 16 '5 parts of pro- 
 teids, and 56 "25 of starch. Barley contains a little more 
 of the proteids and much less starch (13 '38). Hye, less 
 proteids and more starch. Rice has only 7 or 8 parts 
 of proteids, but 78 parts of starch. Other constituents 
 of the flour of the cereals are cellulose and dextrin 
 and salts. In making bread the flour is mixed with 
 water ; salt and yeast are added, and the whole kept 
 at a moderately warm temperature. The proteids 
 soon begin to decompose under the influence of the 
 ferment and themselves act as a ferment to the 
 starch, which becomes converted into dextrin and 
 partially into sugar, and this decomposing yields 
 CO^ and alcohol ; the evolution of the CO2 gas 
 renders bread vesicular and the alcohol is driven 
 off in the baking. A method has been recently sug- 
 gested by which the bread is rendered vesicular 
 without fermenting it. This is accomplished by 
 making the dough with water charged with CO2 
 under pressure ; on removing the pressure the 
 bubbles of CO2 enlarge and the dough swells up. In 
 another method the bread is kneaded with sodium 
 carbonate, and then mingled with hydrochloric acid 
 under pressure. On removing the pressure the gas 
 escapes, rendering the bread light, and the sodium 
 chloride remains. 
 
 Peas and beans contain about 28 per cent, of 
 vegetable casein, or legumin, 38 per cent, of starch. 
 
Chap. VI.] Different Kinds of Drinks. 85 
 
 with lecithin and cholesterin, and from 9 to 19 per 
 cent, of water. As they contain no gluten they 
 cannot be made into dongh. The large quantity of 
 proteids they contain, and their cheapness, render them 
 important articles of diet in an economical point of 
 view. 
 
 Potatoes contain from 70 to 81 per cent, of water, 
 16 to 23 per cent, of starch, and a small quantity, 2 '5 
 per cent., of albumin. In 100 parts of the ash there 
 are about 47 parts of potash, 8 parts of potassium 
 chloride, 2-5 parts of sodium chloride, 13 parts of 
 magnesia, 3*3 parts of lime, and 12 parts of phosphoric 
 acid. 
 
 Fruits contain sugar, salts, organic acids, and a 
 gelatinising substance named pectin Qi^^^^^. 
 
 Oreen food is rich in salts resembling those 
 of the blood, but starch, cellulose, dextrin, sugar, 
 and albumin are all present, though only in small 
 quantity. 
 
 Orinks. — Water is the most necessary and the 
 most wholesome of drinks. It forms a large propor- 
 tion, 58*5 per cent., of the body weight, and if the 
 chief organs be taken separately it is found to consti- 
 tute 30 per cent, of adipose tissue, 69 per cent, of the 
 liver, 72 per cent, of the skin, 75 per cent, of the 
 brain, 75 "7 per cent, of the muscles, and 83 per cent, 
 of the blood. It is constantly being discharged from 
 the body by the lungs and skin, and by the kidneys 
 and faeces. It is essential to the processes of diges- 
 tion, absorption, circulation, and secretion, and is the 
 best solvent of the products of the disintegration of 
 the tissues. Rain water is pure, containing no salts. 
 Spring water is charged with various salts, especially 
 with those of calcium, sodium, magnesium, and iron. 
 It contains usually but little oxygen, whilst it is rich 
 in COv. Hiver water, like the foregoing, is potable, 
 but is apt to become contaminated in populous 
 
86 Human Physiology. [Chap. vi. 
 
 districts with excrementitioiis products and the 
 refuse of factories. It requires in such cases to be 
 purified. 
 
 Drinking water should be tasteless, colourless, 
 and destitute of smell. The lime salts should not 
 exceed 20 parts in 100,000 parts, and these are precipi- 
 tated on boiling, which reduces its "hardness." The 
 presence of organic matter may be ascertained (1) by 
 evaporating a large quantity to dryness and heating 
 the residue, when a well-marked discoloration will be 
 perceived, accompanied with the smell of ammonia ; 
 (2) by the addition of chloride of gold and potassium, 
 which gives a blackish precipitate ; (3) by the addition 
 of potassium permanganate, which become deoxydised 
 and decolorised by organic matter. The presence of 
 more than \\ gr. of common salt in a gallon of 
 water indicates, though it does not absolutely prove, 
 sewage contamination. This quantity scarcely gives 
 more than a mere cloudiness with nitrate of silver in 
 water acidulated with nitric acid, and a specimen of 
 water may be tested by filling a tumbler with it, 
 adding twenty drops of nitric acid and five of a 
 solution of silver nitrate. If there is more than a 
 cloudiness common salt is in excess, and this is 
 probably derived from urine. Drinking impure water 
 is apt to produce typhoid fever, and to favour the 
 spread of cholera, dysentery, and other pestilent 
 jliseases. From one to three pints per diem is sufia- 
 cient for the wants of the ceconomy. 
 
 Beer is obtained from an infusion of malt, which 
 has undergone fermentation, and to which hops, or 
 some other bitter, has been added. The sp. gr. of 
 English beer is from 1010 to 1014. The percentage 
 of dextrin, cellulose, and sugar derived from tlie malt, 
 is from 4 to 15 per cent. The hop extract is in much 
 smaller quantity. The alcohol varies from 1 to 10 
 per cent, in volume. There are some free acids, 
 
Chap. VI.] Different Kinds of Drinks. 87 
 
 chiefly lactic, acetic, gallic, and malic. Free COo, 
 amounting to nearly two cubic inches per ounce, is 
 usually present. Excess in its consumption leads to 
 gouty and bilious disorders due to the impaired elimi- 
 nation of the products of degeneration. 
 
 Wines contain from 6 to 26 per cent, of alcohol 
 by volume; champagnes contain from 6 to 13 per 
 cent., the Rhine wines generally about 10 per cent., 
 port and sherry from 16 to 25 per cent. Many wines 
 are fortified by the addition of brandy for the English. 
 market. Besides alcohol, wine contains ethers which 
 are mainly instrumental in conferring aroma and 
 flavour, albuminous and colouring matters, sugar, free 
 acids, and salts. The total solids vary from 3 to 14 
 per cent. 
 
 Spii'its. — The most common of these are gin, 
 rum, brandy, and whiskey, which should contain 
 from 50 to 60 per cent, by volume of pure alcohol, 
 but which, when sold by retail, are often much 
 diluted. 
 
 Alcohol in one form or other is used by most of 
 the civilised nations of the world as a stimulant. 
 In small quantities and in a diluted condition it pro- 
 bably does little or no harm, but taken in excess it 
 causes exhaustion, and leads to poverty, and vice, and 
 misery. It is believed that under ordinary conditions 
 of exercise and respiratory activity about \\ ounces 
 oi pure alcohol can be consumed or burnt ofl* in the 
 (uconomy per diem, though in fresb air and with great 
 muscular exertion a much larger quantity may be 
 consumed without harm. When an excess is ingested 
 it is only partially oxydised in the body, and the 
 products are partly retained in the system, becoming 
 a fertile source of disease, and are partly eliminated 
 by the lungs and skin, communicating a peculiarly 
 unpleasant odour to the breath. Alcohol in any form 
 is a luxury, and is not necessary, even if it be not 
 
88 Human Physiology. [Chap. vi. 
 
 injurious, whenever severe mental or bodily exertion is 
 required to be made. Its place may be advantageously 
 supplied by tea or coiFee. 
 
 Tea. — Tea leaves contain 1-8 per cent, of thein, 
 2-6 of albumin, 9. "7 of dextrin, 22 of cellulose, 15 of 
 tannin, 20 of extractives, 5 "4 asli. The thein is 
 combined with tannic acid. The drink named " tea " 
 is obtained by pouring boiling water, free, if possible, 
 from lime and iron, upon tea-leaves, which yield six- 
 sevenths of the soluble matters to the first infusion. 
 Tea is stimulant and restorative. 
 
 CoiTee. — Coffee berries contain 1*7 per cent, of 
 caffein, 34 per cent, of cellulose, 10 to 13 of fat. 
 Sugar, dextrin, and a vegetable acid 15-5, legumin 10 
 per cent., Avith an aromatic oil and salts. Coffee 
 infusion contains most of the thein. Coffee is a 
 stimulant to the nervous system, and in excess 
 augments reflex action. 
 
 Milli. — Milk is the secretion of the mammary 
 glands, the activity of which commences at the period 
 of delivery, continues for a period of about nine 
 months, and, if encouraged, may persist for a much 
 longer period. The fluid secreted is intended for the 
 nourishment of the infant, and contains all the mate- 
 rials requisite for its nutriment, growth, and develop- 
 ment. The quantity discharged per diem varies from 
 500 to 1,500 cubic centimetres. Its secretion is ordi- 
 narily a reflex act, the nervous circle being sensory 
 branches of the intercostal nerves, a nerve centre in the 
 spinal cord, and motor fibres emanating from this centre 
 to the gland ; in addition, the sympathetic supplies 
 vaso-motor fibres. It is certain that the mind exerts 
 considerable influence upon the secretion, both in 
 regard to quantity and quality. The sight or the cry 
 of the infant causes a flow of blood (termed the 
 " draught ") to the breast ; and violent emotions may 
 render it unwholesome. 
 
Chap. VI.] Selection of Food. 89 
 
 The sp. gr, of milk varies, but is normally about 
 1030. It contains about 90 parts per cent, of water 
 and 10 parts of solids, of which proteids, fat and sugar 
 each constitute about 3 parts, salts \ j^er cent. On 
 standing, the fat gradually rises, as cream, to the top ; 
 and, if churned, the albuminous envelopes of the 
 fat globules break down, and the oil-drops, running 
 together, form butter. The fats of the milk globules 
 are the triglycerides of stearic, palmitic, myristic, 
 oleic, butyric, and other fatty acids. The 'pro- 
 teids are completely precipitated by tannin. They 
 consist chiefly of casein, with a small proportion of 
 serum- or acid-albumin. Casein is a form of albumin 
 which is not precipitated at 100° C, but is thrown 
 down in flocculi on the addition of dilute acetic or 
 liydrochloric acid. It is also precipitated by the 
 casein-ferment of the stomach, one gramme of this 
 ferment being capable of coagulating 30 litres of 
 .nilk. When thus coagulated, separated from the 
 whey, and pressed, it forms cheese. The plasma of 
 milk contains, besides proteids, traces of urea, kreatin, 
 lactic acid, and milk sugar. 
 
 Milk sugar, which may be obtained by evapora- 
 ting filtered whey, forms a crystalline mass having the 
 composition CellisOg. 
 
 The chief salts of milk are sodium and potassium 
 chloride, calcium and magnesium phosphate. The 
 potassium salts are more abundant than the sodium 
 salts. 
 
 Colostrum is the first milk secreted after delivery. 
 It contains little casein, but much serum, albumin, 
 and fat. It also contains some of the secreting 
 cells. It has a slight purgative action. 
 
 Selection of food. — The principles by which 
 we should be guided in the selection of food are that 
 it should be digestible and wholesome, varied, mode- 
 rately abundant, and ceconomical. The power which 
 
90 
 
 Hum A N Ph ysiology. 
 
 [Chap. VI. 
 
 the gastric juice possesses of arresting putrefactive 
 processes enables " liigli " game and decomposing 
 cheese, and other substances that, but for this power, 
 would be highly deleterious, to be taken as food ; but 
 it is doubtful whether, if taken in excess, they would 
 not prove harmful, and there is much evidence to 
 prove that the constant consumption of decomposing 
 fish in hot climates is associated with, if not pro- 
 ductive of, leprosy. 
 
 Food should be varied, since the persistent use of 
 any particular kind breeds disgust, which is only 
 overcome by absolute hunger, and would probably in 
 no case, though it might enable life to be sustained, 
 supply the necessary materials for the best mental or 
 bodily work. 
 
 In regard to quantity, it would seem to be the 
 practice in all nations where food can be readily 
 obtained that much more material is consumed than is 
 absolutely required on theoretical grounds. 
 
 In regard to oecomomy much may be said. Mole- 
 schott found that a strong and active adult soldier 
 required per diem : 
 
 Albumin . 
 
 Fat . . 
 Starch, sugar, etc. 
 Salts 
 Water 
 
 Total 
 The total nitrogen is here 
 „ carbon 
 
 130 grammes. 
 
 84 „ 
 404 „ 
 
 30 „ 
 
 00 
 
 3,448 
 
 20 "2 grammes. 
 320 
 
 The proportion of N to C is therefore as 1:15. 
 That diet will accordingly be most oeconomical 
 in which this proportion is observed, providing 
 only that it is equally digestible. Moleschott 
 has placed side by side the relative quantities 
 of the chief substances which will afford to the 
 
Chap. VI.] 
 
 Diet and Dietaries. 
 
 91 
 
 consumer tlie 130 grammes of albumin he requires, 
 as follows : 
 
 Cheese 
 
 388 
 
 gramnies 
 
 Lentils 
 
 491 
 
 >) 
 
 Peas . . . 
 
 582 
 
 }> 
 
 Beef .... 
 
 614 
 
 >j 
 
 Eggs 
 
 968 
 
 5> 
 
 Wheaten bread . 
 
 1,444 
 
 )5 
 
 Eice 
 
 2,o62 
 
 >> 
 
 Rye-bread. 
 
 2,875 
 
 >5 
 
 Potatoes . . . . 
 
 10,000 
 
 >l 
 
 It is obvious from this, that, if cheese be compared 
 with potatoes, a much smaller quantity (twenty times 
 less) is required to supply him wdth the necessary 130 
 grammes of albumin with cheese than ^'ith potatoes, 
 and that consequently he must in the latter case con- 
 sume an immense amount of superfluous nourishment. 
 
 In like manner, to obtain the 404 grammes of 
 carbohydrates he requires he must consume 
 
 Rice . 
 
 Wheat-bread 
 Lentils 
 Peas . 
 Eggs . 
 Rye-bread 
 Cheese 
 Potatoes . 
 Meat 
 
 572 
 
 grammes 
 
 625 
 
 
 806 
 819 
 902 
 930 
 
 
 2,011 
 
 
 2,039 
 
 
 2,261 
 
 
 That is, he need only take a moderate amount 
 of rice, but must take a very large quantity of 
 meat, to obtain it. Hence, as common experience 
 shows, the most oeconomical diet is when he consumes 
 bread with meat or cheese, or takes some highly albu- 
 minous compound with potatoes or rice. 
 
 Diet and dietaries. — The diet of different 
 classes in the community varies with the amount and 
 kind of work they have to perform : and it is often a 
 
92 Human Physiology. [Chap. vi. 
 
 matter of importance, not only to determine wliat is 
 suflScient, but also what is least expensive. The 
 general statement may be made that the healthy 
 adult man performing ordinary work requires more 
 proteids and more carbon than one who is at rest. 
 If, as in poor-houses and in prisons, men do little 
 work, they require less proteids, though the carbon 
 cannot be materially reduced ; for, as we have seen, 
 about eight or nine ounces are given off by the lungs, 
 m the form of carbonic acid, by the healthy adult, 
 and this is chiefly derived from the carbohydrates 
 and hydrocarbons, or farinaceous and oily sub- 
 stances he consumes. Boys about ten years of age 
 and women require about three-fourths the quantity 
 of carbon and about one-half the quantity of flesh- 
 formers consumed by healthy men. The following may 
 be regarded as average dietaries for different ages : 
 
 (1) In infancy. — The infant should, if possible, 
 be fed with the milk secreted by the mother. It 
 ]-equires from two to three pints in the twenty-four 
 Lours, which should be given at intervals of three 
 hours. If fed at night at about ten o'clock, and be 
 then kept warm, it will often sleep through the night. 
 Where the mother is unable to feed the infant, the 
 milk of the cow^ ass, ewe, or goat may be substituted. 
 If cow's or ewe's milk be used, since both are richer 
 than human milk, i.e.., contain more casein and butter, 
 but are less sweet, one-third water and a little 
 sugar may be added to each. If artificial feeding be 
 adopted, great care should be taken that the milk is 
 fresh, the vessels sweet and clean, and the temperature 
 at which it is given uniform and about 100° F. When 
 milk cannot be obtained, good beef-tea may be given, 
 and the fat should not all be removed. After from six 
 to ten months' feeding at the breast, the child requires 
 some additional food, which may be bread, or arrowroot 
 and milk, or puddings made with milk, eggs, flour. 
 
Chap. VI.] Diet and Dietaries. 93 
 
 arrowroot, sago, tapioca, semolina, or cornflour. The in- 
 tervals between the meals should be about three hours. 
 In the course of the second year brose or porridge may- 
 be given at breakfast, and a little finely cut-up fresh 
 meat and bread, with gravy, may be given at dinner. 
 
 (2) In youth. — Breakfast should consist of milk or 
 tea or coffee, with bread and eggs, or bacon, or fresh 
 fish, and, if liked, porridge. Dinner should not be post- 
 poned to a later hour than 1.30, and shoidd consist of 
 plain boiled and roast meat (occasionally exchanged 
 for fish), potatoes, or other thoroughly-cooked vegetable, 
 and some farinaceous pudding, with water for drink. 
 The evening meal may consist of bread-and-butter and 
 tea. Children under ten should be in bed by 9 p.m. 
 
 The following represents a sufficient diet for a 
 healthy man performing a moderate amount of work : 
 
 Bbeakjast. — Three-quarters of a pint of milk; quarter of a 
 pint of water, with, coffee or tea ; hread, four ounces to 
 six ounces ; butter, three-quarters of an ounce ; sugar, 
 three-quarters of an ounce ; bacon, three ounces ; or eggs, 
 four ounces ; or cooked meat, three ounces. 
 
 Dinner. — Cooked meat, four ounces to six ounces ; potatoes, 
 eight ounces ; bread, three ounces to four ounces ; pudding, 
 eight ounces; cheese, half an ounce; soup, six ounces; 
 water or beer, haK a pint. 
 
 Tea. — ^Water with tea, three-quarters of a pint ; sugar, three- 
 quarters of an ounce; milk or cream, two ounces; bread, 
 three ounces ; butter, half an ounce to three-quarters of au 
 ounce. 
 
 Supper. — IMilk, three-quarters of a pint ; oatmeal, one ounce ; 
 and bread, three ounces to four ounces ; or eggs, four 
 ounces ; or cooked meat, three ounces ; and bread, three 
 ounces ; butter or cheese, half an ounce ; water or beer, 
 half a pint. 
 
 When training for athletics, the object is to 
 diminish superfluous fat and water, and to effect the 
 full nutrition of the nervo-muscular apparatus ; and 
 these results are accomplished partly by judicious 
 feeding, partly by exercise. King when in training 
 
94 
 
 Human Physiology. 
 
 [Chap. VI. 
 
 ate for breakfast two chops, with dry toast or stale 
 bread, and one cup of tea, without butter or sugar 
 (the two last were probably unnecessary restrictions) ; 
 jfor dinner one pound to one pound and a quarter of 
 fresh beef or mutton, toast or stale bread, a little 
 potato or greens, and half a pint of dry old ale ; for 
 tea one cup of tea, an ^g'g^ and dry toast ; and for 
 supper, gruel or half a pint of old ale. The exercise 
 consisted in gentle and fast walking to the extent of 
 at least twenty miles per day, and special exercise in 
 rowing or boxing, to develop certain sets of muscle. 
 
 In old age less food is required than in adult life ; 
 the work done is less, and the nutrition of the tissues 
 is much less active. The diet should be plain, and 
 the staple must be milk and eggs, meat, and bread, 
 whilst wine may in most instances be taken with ad- 
 vantage to the extent of from four to eight ounces daily. 
 
 Balance of the cecoiioiiiy. — Man requires, as 
 a rule, a diet in which the nitrogenous is to the non- 
 nitrogenous substances as 1 : 4. It may be remembered 
 further that the nutritive value of fat as compared 
 with the carbohydrates (starch and sugar) is as 10 : 17. 
 
 Vierordt, who was of light weight, has given a table 
 in which the proportions derived from experiments on 
 himself are slightly different from that stated above, 
 but which shows well the balance of the oeconomy, or 
 the relation of the income of the body to the expendi- 
 ture. It is as follows : — 
 
 An Adult with Moderate Work Coxsumes 
 
 
 
 C. 
 
 H. 
 
 N. 
 18-88 
 
 0. 
 
 1 20 grammes of albumin containing 
 90 „ fat 
 330 „ starch ,, 
 
 64-18 
 
 70-20 
 
 146-82 
 
 8-60 
 10-26 
 20-33 
 
 28-34 
 
 9-54 
 
 162-85 
 
 
 281-20i 39-19 
 
 18-88 
 
 200-73 
 
 To this must be added 744-11 grammes of O absorbed 
 
Chap. VI.] Balance of the (Economy 
 
 95 
 
 from the air in respiration ; 2818 grammes of water 
 consumed as drink, and 32 grammes of inorganic com- 
 pounds (salts) ; the whole weighing about 3|- kilos, 
 or about one-twentieth part of body weight. 
 
 An Adixt with ^Moderate Work gites off 
 
 
 Water. 
 
 C- 
 
 H. 
 
 N. 
 
 0- 
 
 By respiration * ... 
 By transpiration 
 By urine 
 By fteces 
 
 Grammes. 
 
 330 
 
 660 
 
 1,700 
 
 128 
 
 248-8 
 
 2-6 
 
 9-8 
 
 20-0 
 
 8-3 
 3-0 
 
 lo-8 
 3-0 
 
 18-8 
 
 '651-15 
 ' 7-2 
 i IM 
 1 12-0 
 
 
 2,818 
 
 2SI-2 
 
 6-3 
 
 681-45 
 
 To this must be added 296 grammes of water, which is 
 formed by the union of oxygen with hydrogen of the 
 food in the bodv. This would contain 32-89 o-rammes 
 of H, and 2 6 3 '4 1 of O. Twenty-six grammes of salts are 
 also eliminated with the urine, and six in the fseces. 
 
 The balance of the oeconomy requires further to be 
 considered under the heads : (1) Of inanition; (2) of 
 insufficient supply ; (3) of excessive supply. 
 
 (1) In inanition, although no food is taken, the 
 animal continues to take oxygen into the system by 
 the lungs, and to expire carbonic acid gas by the 
 same channel, and by the skin. It still gives ofi' 
 urea by the kidneys. It maintains in temperate 
 climates a temperature much above that of the sur- 
 rounding air. These phenomena indicate that processes 
 of oxydation continue, and as no supplies are introduced 
 from without, it is clear that the animal must live upon 
 its own tissues, and that even though it may be 
 naturally a vegetable feeder it is now a carnivore. 
 As might be expected, those tissues which most readily 
 combine with oxygen are the first to waste. The fat, 
 for example, quickly disappears, and, cceteris paribus, 
 the fatter the animal, the longer is it able to sustain 
 complete deprivation of food. Gradually, Iiowever, 
 
g6 Human Physiologw [f.:hap. vi. 
 
 the albuminous constituents are more and more drawn 
 upon, and the muscular tissue, the solid organs of the 
 body, as the liver, spleen and kidneys, the skin, and 
 nervous system, contribute their portion of nutritious 
 matter to preserve life. "With the reduction of the 
 mass of tlie body all the processes of life are conducted 
 more feebly, and the loss of weight is much slower as 
 the term of life is approached. It is estimated that 
 for each kilo of body weight the loss of M^eight in man 
 on the second day of fasting amounts to 0*13 gramme 
 of nitrogen, and 2*59 grammes of carbon, or upon the 
 whole he loses about 50 grammes of albumin per diem. 
 Death usually ensues in man at the close of the third 
 week, when the body has lost nearly half its weight. 
 Examination of the body shows that the fat has 
 almost wholly disappeared ; the liver and spleen have 
 lost more than half their weight ; the muscles one- 
 third ; the kidneys one-fourth. The brain and heart, 
 however, continue to be nourished at the expense of 
 the rest of the oeconomy, the brain only losing one- 
 tenth of its normal weight, and the heart only one- 
 fortieth part. Access to water enables life to be 
 considerably prolonged, probably by enabling the 
 blood to absorb the waste products of the tissues, and 
 by promoting their combustion. The appearances 
 presented in cases of inanition correspond with the 
 physiological conditions. The body becomes lean and 
 angular ; the eyes sunken ; the cheeks hollow from the 
 loss of fat ; the nerves and muscles lose their powers ; 
 the mental faculties become clouded, and delirium 
 may occur ; the gait is tottering ; the secretions fail ; 
 the mouth is dry ; sordes collect on the lips ; the urine 
 is scanty and turbid ; the stomach usually contains a 
 little acid fluid, and the gall bladder is full owing to 
 the accumulation of the bile, which, slowly secreted, is 
 no longer required for digestion ; the temperature 
 slightly falls, especially towards the close of life. 
 
Chap. VI.] Balance of the CEconomy. 
 
 97 
 
 (2) Akin to inanition is an insufficient diet; 
 and this may be of two kinds, one in. whick the 
 quantity of appropriate food is inadequate in quan- 
 tity to sustain the body, and another in which the 
 diet is limited to a particular aliment, as fat, 
 starch, sugar, gum, or albumin. It is clear that 
 the first four substances cannot supply the wants 
 of the ceconomy; for they contaia no nitrogen, 
 which is an essential element in the composition of 
 the nervous, muscular, glandular, and other tissues, 
 and the animal body is incapable of assimilating the 
 nitrogen of the atmosphere. The utmost, therefore, 
 that an exclusive diet of fat or of farinaceous or 
 saccharine substances can do, is to spare the con- 
 sumption of the fat and proteids of the animal's 
 own body, and to permit these substances to be other- 
 wise applied, and by combining with oxygen to main- 
 tain the temperature. The want of some nitrogenous 
 food is soon perceived, and, after a brief period, the 
 fat or starch is refused. An exclusive diet of albumin 
 would seem at fii^st sight to be possible, since albumin 
 contains nitrogen, and, there is reason to believe, is 
 capable of breaking up in the body into nitrogenous 
 and non-nitrogenous compounds, the former of which 
 supply nourishment to the tissues, whilst the re- 
 mainder, amounting to 50 per cent, of the weight of 
 the albumin, may be regarded as fat, and can be applied 
 to the maintenance of heat. But, as with starch or 
 fat, the use of a single article of diet soon produces 
 an unconquerable aversion to.it, and the subject of 
 the experiment, whether animal or man, turns away 
 from it with dissrust. 
 
 (3) When food is taken in excess of the require- 
 ments of the oeconomy, the body increases in weight 
 up to a certain point, and fat is deposited ; and this 
 tendency may be intensified by hereditary predisposi- 
 tion, by diminished activity of mind and body, and by 
 
 H 
 
98 Human Physiology. [Chap. vi. 
 
 cessation of the reproductive functions. A tendency 
 to corpulency may best be met by reducing the quantity 
 of food, and especially of the saccharine and oily sub- 
 stances. The formation of fat, and its deposition in 
 the body, may result from an excess of either fats, fari- 
 naceous substances, or proteids. The deposit of fat in 
 the body when the diet is rich in oily substances is not 
 surprising, that portion which is not oxydised being 
 simply retained in the body. In carnivora, the 
 quantity of fat taken in the food is sufficient to main- 
 tain the fat of the ceconomy, which is usually small in 
 quantity at its normal amount. There is some evi- 
 dence that the carbohydrates are a source of fat, for 
 the negro fattens during the sugar-cane harvest, when 
 he consumes much sugar; carnivora fatten when starchy 
 substances are added to their food, and bees produce 
 wax when fed almost exclusively on honey. From a 
 chemical point of view, however, the transformation of 
 starch or sugar into fat is difficult to explain ; and it 
 has been observed that, if the proteids are entirely 
 absent from their food, bees cease to form wax. 
 
 It is now very generally admitted that the 
 proteids can produce fat by breaking up in the body 
 into an azotised portion, which is applied to the 
 nutrition of the tissues, and a non-azotised portion, 
 which can be applied to the development of force and 
 the maintenance of heat, or which may be stored up 
 in the body as fat. In support of this, the conversion 
 of muscular tissue into adipocere after death is 
 adduced, as also the formation of fat at the expense 
 of casein in Roquefort cheese, and the well-known 
 fact that in lactation a freer secretion of milk is 
 induced by a more abundant supply of albuminous 
 food. Again, the larvse of flies, placed on coagu 
 ■ lated blood, which contains but little fat, contain 
 in the course of a few days ten times more fat than 
 before. 
 
99 
 
 CHAPTER YTI. 
 
 DIGESTION OF FOOD. 
 
 The first acts to whicli solid food is subjected when 
 introduced into the mouth are those of mastication 
 and insalivation. 
 
 Mastication. — Mastication is effected by the 
 movements of the movable lower jaw against the fixed 
 upper jaw, in both of which teeth are implanted, aided 
 by the tongue and the secretion of the salivary glands. 
 
 The teeth are divisible into groups, each having i bs 
 own function. There are in each jaw four incisors, 
 having cutting edges for the purpose of prehension 
 and detachment of morsels of food of appropriate 
 size ; two sharp-pointed canines or laniary teeth, for 
 piercing hard objects and tearing the food ; and four 
 bicuspids and six molars, having broad and irre- 
 gular surfaces, for grinding the food and reducing it 
 to a pulp by mingling it with the saliva. 
 
 The jaws are ordinarily kept in apposition by the 
 pressure of the air when the mouth is closed ; for 
 this cavity is then perfectly free from air, and the 
 pressure exerted is equal to a column of mercury of 
 about 3 mm. in height. 
 
 The descent of the lower jaw is efiected by its 
 own weight, aided by the platysma and by the ante- 
 rior bellies of the digastrics, the mylo-hyoid and 
 genio-hyoid muscles, which are enabled to act by the 
 fixation of the hyoid bone and larynx, by the sterno- 
 thyroid and thyro-hyoid, and by the sterno- and omo- 
 hyoids. The elevation of the lower jaw is accom- 
 plished by the combined action of the temporal, 
 masseter, and internal pterygoid muscles. The for- 
 ward movement of the lower jaw is effected by the 
 
loo Human Physiology. [Chap. vii. 
 
 external pterygoid, the backward movement by the 
 internal pterygoid. Lateral movements are eftected 
 by the alternate action of the pterygoids of opposite 
 sides. The accumulation of food between the teeth 
 and the cheeks is prevented by the contraction of the 
 buccinator muscle and the orbicularis oris. 
 The motor nervous supply is as follows : 
 
 1. The temporal "^ 
 
 2. The masseter 
 
 3. The pterygoids | are supplied by motor branches 
 
 4. The buccinator \ of the third di\rLsioii of the 
 
 5. The mylo-hyoid [ fifth nerve. 
 
 6. The anterior belly of 
 
 the digastric 
 1. The genio-hyoid "^ 
 
 I ?h: r™.^oid I ^^l^f^^^ ^y *^ 'hypoglossal 
 
 4. The stemo-thyroid [ 
 
 5. The thyro-hyoid J 
 
 1. The posterior belly of "^ 
 
 the digastric | 
 
 2. The stylo-hyoid V are supplied by the facial. 
 
 3. The muscles of the | 
 
 lips ; 
 
 The sensor]/ nerves are the fifth, ninth, and tenth 
 cerebral nerves. The nerve-centre presiding over the 
 movements of mastication is situated in the medulla 
 oblongata. 
 
 The teeth appear in a certain order which it is im- 
 portant to remember, since the order in which the teeth 
 are cut affords one of the best means of determining 
 the age of a child. There are two sets of teeth^ a first, 
 deciduous, or milk set, and a second, or permanent set. 
 
 The deciduous teeth are twenty in number, and 
 appear in the following order : 
 
 Four central incisors ) (lower jaw earlier 7th month after birth. 
 „ lateral incisors \ than upper) . 8 — 10th „ „ 
 „ anterior molars .... 12th ,, „ 
 
 „ canines ..... 14 — 20th „ „ 
 „ posterior molars .... 18 — 36th „ „ 
 
Chap. VII.] 
 
 Digestion of Food. 
 
 lOI 
 
 The permanent set number tliirty-six, and the 
 
 teeth are cut in the following order : 
 
 First true molars 
 
 Central incisors 
 
 Lateral incisors 
 
 First bicuspids 
 
 Second bicuspids 
 
 Canines 
 
 Second molars 
 
 Third molars (wisdom teeth) 
 
 7tli year. 
 8th year. 
 9th. year. 
 10th year. 
 11th year. 
 12— 13th year. 
 12— 14th year. 
 18th year, or later. 
 
 Insalivation. — During the act of mastication, 
 and principally to facilitate this act and that of 
 deglutition, the food is incorporated ^vith saliva, 
 which is secreted by the salivary glands, and is 
 abundantly discharged into the mouth, from whence 
 it may be obtained by allowing it to fall into a vessel ; 
 but, to obtain the secretion of particular glands, a can- 
 nula must be introduced into the duct of the particular 
 gland the secretion of which it is desired to investigate. 
 
 The salivary glands are divisible into two sets, the 
 most important of which, including the submaxillary, 
 sublingual, and the parotid glands, lies external to 
 the oral cavity, whilst the other, including the labial, 
 buccal, palatine, and lingual glands, is embedded in 
 the walls of the mouth. 
 
 Characters of tlie saliva. — The secretion of 
 the farotid gland is thin, has an alkaline reaction and 
 a specific gravity of 1003 or 1004 It contains 98-5 
 per cent, of water and about 1*5 of solids, the most 
 important of which is a ferment named ptyalin and 
 salts, amongst which sulphocyanide of potassium is 
 interesting. This salt gives a red colour with iron 
 chloride and sets free iodine from iodic acid, which 
 may be recognised by a blue colour appearing on 
 the addition of starch. 
 
 Submaxillary saliva is strongly alkaline ; it 
 contains mucin precipitable by acetic acid, the ferment 
 
I02 Human Physiology. iChap. vii. 
 
 ptyalin, and salts, amongst which potassium sulpho- 
 cyanide is again present. 
 
 8iibliiignual saliva is alkaline, tenacious from 
 containing much mucin, has numerous salivary cor- 
 puscles, and a trace of potassium sulphocyanide. 
 
 Mixed saliva is alkaline, has a specific gravity 
 of 1005 — 1009. It is cloudy from the presence of 
 epithelial scales and salivary corpuscles, which per- 
 form amoeboid movements, fragments of food, fila- 
 ments of an alga, named leptothrix buccalis, and 
 some other mineral organisms. It, of course, contains 
 ptj^alin, mucin, and salts. 
 
 The quantity secreted per diem varies from 200 
 to 1,500 grammes. 
 
 Uses of tlae saliva. — In connection with the 
 digestive act the saliva (1) moistens the food and pro- 
 motes mastication and deglutition, for, as a rule, the 
 drier the food the more saliva is secreted ; (2) it dis- 
 solves saccharine, saline, and various sapid substances j 
 and (3) it exercises a saccharifying or diastatic power on 
 starch. This action is effected through the agency of 
 the ptyalin or hydrolytic ferment it contains, and seems 
 to be of a complex nature, starch paste becoming in 
 part converted first into soluble starch or amylo-dextrin 
 and erythro-dextrin, which last reddens with iodine, 
 and then into (1) achroo-dextrin, which does not give 
 any colour with iodine, (2) maltose, (3) grape sugar, 
 whilst part i^esists the action of the ptyalin beyond 
 the stage of achroo-dextrin. Statin^ the chano-es 
 shortly and excluding the intermediate changes, the 
 action of saliva is to cause dextrin to take up an 
 equivalent of water, converting it into glycose. Thus, 
 
 Starch. Dextrin. Dextrin. Glycose. 
 
 Apart from its use in digestion, saliva (1) 
 
Chap. VII.] Digestion OF Food. 103 
 
 moistens the moutli and enables the functions of 
 taste to be duly performed ; (2) it facilitates the 
 movements of the tongue in speech ; (3) it contains 
 potassium sulphocyanicle, which prevents the decay 
 of frao-ments of food lodo-ed between the teeth, 
 and the development of leptothrix and other fungi ; 
 (4) being constantly secreted, it occasions frequent 
 acts of deglutition, which open the Eustachian tube 
 and equalise the pressure of the air within and 
 without the tympanum. 
 
 Ptyalin. — The active ferment of the saliva, ptyalin, 
 may be obtained by acidulating saliva with phosphoric 
 acid and adding lime-water. The precipitate of lime 
 phosphate which falls, carries do^^^l with it ptyalin 
 and proteids, and from this it may be obtained by 
 washing the precipitate with water, and precipitation 
 from the watery solution by means of alcohol. The 
 temperature at which ptyalin acts best as an amylo- 
 lytic agent, that is, at which it is capable of converting 
 starch into sugar most energetically, is between 
 38° and 41° C. It is arrested at 60^ to 70° C. A 
 definite quantity of starch paste only can be converted 
 into glycose by a limited amount of ptyalin, and a large 
 excess of sugar interferes with its action. Ptyalin 
 acts much more quickly on boiled starch than on the 
 raw material. With boiled starch the presence of 
 suojar can be demonstrated in a minute or less after 
 admixture with saliva, whilst with raw starch from 
 haif-an-hour or two or three hours is required. An 
 estimate of the quantity of sugar present may be 
 obtained by allowing the fluid to ferment and collect- 
 ing the CO2 given off. 100 parts of COg by weight 
 correspond to 204*54 of sugar. 
 
 For the tests for grape sugar see page 6. 
 
 The saccharifying action of ptyalin on starch takes 
 place best in neutral or in slightly alkaline solutions. 
 It is impeded or arrested by so small a degree of 
 
I04 Human Physiology. [Chap. vii. 
 
 acidity as is produced by 0'02 per cent, of hydro- 
 chloric acid. Hence it is stopped when the contents 
 of the stomach are strongly acid, though it may re- 
 commence when the acids present are neutralised. 
 Ptyalin is only contained in the saliva of the parotid 
 gland of the new-born child, but at a later period in 
 that of all the glands. 
 
 IiinervatiOM of tHe salivary glands. — ^The 
 secretion of saliva is a reflex act. The spinal nerve- 
 centre presiding over the process is situated either at 
 the level of the facial nucleus or a little above it, and 
 the psychic centre is probably near the sulcus 
 cruciatus ; but the submaxillary sympathetic ganglion 
 acts also as a centre. 
 
 The sensory nerves are, (1) the Fifth, and (2) the 
 glosso-pharyngeal. 
 
 The motor nerves are (1) the chorda tympani of 
 the facial, and (2) the sympathetic. 
 
 The glands may also be excited to action through 
 the olfactory nerve and by the gastric terminations of 
 the vagus. 
 
 The sensory branches of the fifth nerve and of the 
 glosso-pharyngeal nerve conduct gustatory impressions 
 from the tongue or palate, according to their distri- 
 bution, to the medulla oblongata, and excite the 
 gustatory centres situated in that region to originate 
 impulses that travel to the glands through the facial 
 and sympathetic nerves, and stimulate them to 
 secrete saliva. The facial and sympathetic nerves are 
 therefore said to contain secreto-motory fibres. The 
 action of these two nerves upon the submaxillary 
 gland of the dog, where it has been chiefly investigated, 
 is remarkable. Both nerves contain fibres that act 
 upon the blood-vessels of the gland. When the chorda 
 tympani is stimulated the vessels enlarge, and more 
 blood passes through them, and this nerve must either 
 contain fibres that cause the muscular coat to dilate 
 
Chap. VII.] Digestion of Food. 105 
 
 actively, or fibres which inhibit the action of the 
 sympathetic and permit their coats to dilate passively. 
 Under any circumstances, the action of the chorda 
 when stimulated is that of a vaso-dilator. When the 
 sympathetic is stimulated the vessels contract in 
 diameter and less blood passes through them. But 
 stimulation of these two nerves has another effect. 
 It causes an alteration in the quality of the secretion. 
 When the chorda tympani is stimulated the secretion 
 is limpid and abundant. When the sympathetic is 
 stimulated the secretion is scanty and more viscid. 
 Both nerves, therefore, in addition to fibres influ- 
 encing the size of the vessels, contain fibres that 
 stimulate the salivary cells to secrete. But it may 
 be said the effects on the secretion are just those 
 which might have been expected from the influ- 
 ence of the nerves on the circulation. The facial 
 nerve, or the chorda tympani (which is the same 
 thing, since the fibres given off" by the facial nerve 
 to the gland run in the chorda tympani), when stimu- 
 lated causes dilatation of the vessels, and a more 
 rapid circulation through the gland. Hence, the 
 secretion is- thin and copious ; the sympathetic fibres 
 contract the vessels and diminish the fiow of blood 
 through the gland, and therefore when they are 
 stimulated the secretion is slower, and it is more 
 tenacious and of higher specific gravity. What 
 need, then, is there to admit the existence of secreto- 
 motory nerves ] The reply is that the activity of the 
 circulation and the activity of the secretory process 
 bear no direct or necessary relation to each other, for 
 (1) on stimulating the chorda tympani secretion takes 
 place even though the vessels supplying the gland 
 have been ligatured ; (2) if atropia be subcutaneously 
 injected into an animal and the chorda tympani be 
 stimulated, it will be found that the vessels dilate and 
 a greatly augmented quantity of blood passes through 
 
io6 Human Physiology. [Chap. vii. 
 
 the gland, but that there is no increase in the quan- 
 tity of saliva, from which it would appear that the 
 alkaloid has had no effect upon the vaso-dilator fibres, 
 but that it has paralysed the secreto-motor fibres ; (3) 
 it has been ascertained by direct measurement with 
 the manometer that the saliva is secreted under a 
 pressure greater than that of the blood in the blood- 
 vessels, in fact, in some instances, amounting to 
 nearly double this pressure. From all these circum- 
 stances it is concluded that the facial nerve contains 
 fibres which directly stimulate the gland cells to 
 secrete, and Pfliiger believes that he has actually 
 followed nerve fibres into the gland cells. It is con- 
 cluded from analogy that the sympathetic also con- 
 tains two sets of fibres. When all the nerves distri- 
 buted to the gland are divided a thin saliva is con- 
 tinuously secreted, named paralytic saliva, which only 
 ceases when the nerves have undergone complete 
 degeneration. 
 
 The nerves of the parotid gland are less certainly 
 known, but it receives sensory fibres from the fifth, 
 and can be made to secrete by stimulation of the 
 nervus petrosus superficialis minor, which is a branch 
 of the facial, and is joined in the tympanum by a 
 branch from the glosso-j)haryngeal. 
 
 Plieiioineiie, acconipauying' stimulation 
 of tlie secreto-motor nerves. — The gland be- 
 comes more vascular. The arteries dilate, the veins 
 convey scarlet blood, and may, owing to the enlarge- 
 ment of the capillaries permitting freer passage of 
 blood, pulsate. The temperature rises a centigrade 
 degree or more. 
 
 Movements of tlie tongne. — These are of 
 importance in subjecting the food equally to the 
 crushing action of the teeth, in separating it into 
 morsels fit for deglutition, and in pressing each morsel 
 backwards to the fauces, where the act of deglutition 
 
Chap. VII.] Digestion of Food. 107 
 
 commences. The complexity of the muscular fasciculi 
 of the tongue enables it to be moved in all directions. 
 It is protruded by the genio-hyoglossus, aided by the 
 transversus ; retracted by the hyoglossus, styloglossus, 
 and palatoglossus, aided by the longitudinal fasciculi ; 
 depressed by the hyoglossus ; elevated towards the 
 tip by the anterior part of the sujDerior longitudinal 
 fibres ; towards the middle by the action of the 
 mylo-hyoid, which elevates the hyoid ; towards the 
 base by the styloglossus and palatoglossus, and by the 
 stylo-hyoid ; turned to either side by the contraction 
 of the longitudinal fasciculi of the side to which it is 
 turned. Hollowing of the dorsum is effected by the 
 contraction of the geniogiossi, aided by the transverse, 
 and vertical fasciculi. Most of the muscles of the 
 tongue are supplied by the hypoglossal nerve ; the 
 mylo-hyoid receives branches from the fifth ; the stylo- 
 hyoid from the facial. 
 
 Deg-futitioii. — The food taken into the mouth 
 having beein reduced to a pulp by the acts of masti- 
 cation and insalivation, which should be thoroughly 
 performed, is divided into boluses of appropriate 
 size by the tongue, and pressed back by it to 
 the anterior pillars of the fauces, where deglutition, 
 which is an involuntary act, commences. The 
 mass is first seized by the palato-giossal and palato- 
 pharyngeal muscles ; the dorsum of the tongue is 
 rendered prominent and arched by the styloglossi, 
 which press it backwards ; then, secondly, the soft 
 palate is raised and stretched by the levator and 
 tensor palati of each side, which shut off" the opening 
 of the nares, and render the dorsum an inclined plane 
 by which the bolus is guided into the grasp of the con- 
 strictors of the pharynx ; by their peristaltic action 
 from above downwards it is finally propelled into 
 the oesophagus, through which it is driven into the 
 stomach. The bolus in this course has to travel over 
 
io8 Human Physiology. [Chap. vii. 
 
 the upper opening of the glottis, which is a critical 
 moment, and the entrance of food into the trachea is 
 very carefully guarded against. To prevent it the 
 whole larynx rises by the action of the genio-hyoid 
 and by the anterior belly of the digastric and mylo- 
 hyoid muscles, whilst the epiglottis is drawn down- 
 w^ards, which effectually occludes the orifice, and, as an 
 additional protection, the rima glottidis is closed. 
 
 The nerve centre for the acts of deglutition is 
 situated in the medulla oblongata ; the sensory fibres 
 belong to the fifth, which supplies the velum palati ; 
 to the glosso-pharyngeus distributed to the tongue and 
 pharynx, and to the superior laryngeal branch of the 
 vagus supplying the epiglottis and rima glottidis. 
 The motor nerves are derived from the hypoglossus 
 supplying the tongue ; the glosso-pharyngeus supplying 
 the muscles of the pharynx, the facial supplying the 
 levator palati (peristaphylinus internus) ; the fifth sup- 
 plying the tensor palati, the suprahyoid muscles and 
 the muscles of mastication ; and the vagus supplying 
 the muscles of the larynx, and of the oesophagus. 
 
 The gastric juice. — The glands of the stomach 
 secrete a fluid that possesses strong digestive powers. 
 It may be obtained by introducing a clean sponge 
 into the empty stomach of a dog, and giving the 
 animal some fragments of cartilage to digest for a short 
 time, and after a few minutes withdrawing the sponge ; 
 or by making a gastric fistula. It is a nearly clear, 
 slightly yellowish, acid fluid, which does not cloud on 
 boiling. Its specific gravity is 1002*5, and it contains 
 one half part per cent, of solids. The quantity secreted 
 varies from a few to many ounces, according to the 
 nature and quantity of the food. It contains (1) free 
 hydrochloric acid in the proportion of two parts in 
 1000; (2) pepsin, a special hydrolytic ferment, in the 
 proportion of three parts in 1000 ; (3) salts, of 
 which sodium and potassium chloride are the chief, 
 
Chap. VII.] Digestion of Food. 109 
 
 two parts in 1000; (4) mucus; after the use of 
 particular articles of food, as milk and butter, lactic 
 and butyric acids may occur. The acids seem to be 
 secreted by the cells near the orifice of the ducts of 
 the gastric glands, since the deeper part of the glands 
 is alkaline. . Moreover, if potassium ferrocyanide be 
 injected into the veins of dogs, and then iron lactate, 
 a blue discoloration, owing to the decomposition of 
 the ferrocyanide, occurs on the surface only. The 
 pepsin is secreted probably by the chief cells, which 
 line the glands of the stomach, and they contain most 
 pepsin when they are large and clear. The chief cells 
 in a fasting animal are pale and finely granular ; their 
 lines of demarcation, or boundary lines, are not very 
 strongly defi.ned, and they do not become strongly 
 tinged with carmine. During digestion they become 
 cloudy and stain with carmine, and are more granular, 
 contrasting in this respect with the cells of the pan- 
 creas, which become more transparent and lose their 
 granular , character during action. 
 
 The origin of the acid has been attempted to be 
 explained on chemical grounds by the fact that there 
 are fluids of alkaline reaction which may contain two 
 acid and mutually inoflfensive salts, but still have an 
 alkaline reaction, because the acid reaction is obscured ; 
 for instance, a solution of neutral sodium phosphate 
 KajHPO^ and acid sodium phosphate jSraH2P04 
 is alkaline. Such a solution placed in a dialyser 
 after a short time gives up its acid salt to the sur- 
 rounding distilled water, and thus there remains in 
 the dialyser an alkaline fluid, whilst outside of it is 
 an acid fluid. Further, if neutral phosphate of soda 
 be mixed with calcium chloride CaClg, we get calcium 
 triphosphate, sodium chloride, and free hydrochloric 
 acid, as in the following equation : 
 
 2Na2HP04 + sCaCls == CagPO, + 4l^aCl + 2PICI. 
 
 (Ewald.) 
 
no Human Physiology. [Chap. vii. 
 
 But hydrochloric acid possesses a high diffusive power ; 
 it passes three times as quickly through the dialyser 
 as common salt, and hence, once formed in the blood, 
 it may easily pass into the gastric juice. Such 
 chemical theories are, however, not very satisfactory, 
 and the production of the acid, like that of the 
 ferment, must be regarded as a special property of 
 the protoplasm lining the cells of the gastric glands. 
 It is certain that the elimination of a large quantity 
 of acid by these cells leaves so much alkali in the 
 blood that when the stomach is in full digestion the 
 urine, usually acid, becomes neutral, or even faintly 
 alkaline, and often cloudy from precipitation of some 
 of its salts. 
 
 Pepsin. — The active principle of the stomach is 
 a soluble hydrolytic ferment named pepsin. It may be 
 obtained, in a more or less pure state, in several ways. 
 (1) By making a glycerin extract of the gastric 
 mucous membrane of a recently-killed pig or other 
 mammal, and treating it with alcohol. A white 
 precipitate falls, which is soluble in water, and acts 
 like pepsin. (2) By extracting the mucous membrane 
 with a 3 per 1000 solution of hydrochloric acid. 
 (3) By rubbing down the membrane with 5 per cent, 
 of phosphoric acid, and allowing the digestive process 
 to take place upon it for a short time. On the 
 addition of lime-water a voluminous precipitate falls, 
 which carries with it the pepsin. The mass is washed 
 with water, and treated with weak hydrochloric acid. 
 To this fluid, containing the pepsin, a solution of 
 cholesterin in four parts of alcohol and one of ether 
 is added, when a second voluminous precipitate falls 
 which again contains the pepsin. The precipitate is 
 washed with weak acetic acid, and the cholesterin is 
 finally dissolved away by ether. 
 
 When the stomach is empty it is quiescent, pre- 
 sents a pale rosy tint, and contains little or no 
 
Chap, vii.j Digestion of Food. hi 
 
 gastric juice, but the surface is covered with a thin 
 layer of mucus. No sooner is food introduced than, 
 apparently by a reflex action taking place through the 
 sympathetic centres and the fibres of Meissner's plexns, 
 the circulation quickens, the vessels enlarge, the 
 colour of the mucous membrane becomes deeper, the 
 venous blood returning from it is of a brighter hue, and 
 gastric juice is poured forth from the glands. 
 
 Action of tlie gastric juice on proteids. — 
 The essential action of the gastric juice on the food 
 entering the stomach is to seize upon the proteids, 
 and convert them from colloidal and insoluble 
 substances into soluble or crystalloid compounds, so 
 that they easily enter the blood. All varieties of 
 albumin are converted into peptones, and it may be 
 demonstrated that nearly twelve times more peptone 
 will pass through an animal septum than albumin. 
 The action of the gastric juice on albumin may be 
 observed outside of the body by placing small cubes 
 of white of Qg^ in the pure juice obtained from a gastric 
 fistula, or in an artificial juice made by adding a 
 3 per 1000 watery solution of hydrochloric acid to 
 pepsin in the proportion of 2 per 1000, and main- 
 taining the mixture at a temperature of 37° or 38° C 
 (97° F.). In the coiu-se of an hour or two the edges of 
 the cube will be found to be swollen and hyaline, and 
 ultimately a large portion undergoes solution in the 
 fluid, especially if the process be assisted by gentle 
 agitation. The solution thus obtained contains 
 peptones, and there seem to be several intermediate 
 steps in the process ; substances termed A- b- and 
 c-peptones, differing in minor particulars from each 
 other, being formed, whilst slight differences exist be- 
 tween the peptones of albumin, casein, glutin, and other 
 proteids. The general characters of peptones by 
 which they are distinguished from albumins are : 
 (1) That they are soluble in water. 
 
112 Human Physiology. [Chap. vii. 
 
 (2) That they do not coagulate when heated. 
 
 (3) That on the addition of an alkaline solution, 
 and a drop or two of solution of copper sulphate, a 
 purplish colour, differing from the pure violet of 
 soluble albumin, is produced. 
 
 (4) When much diluted, solutions of peptones are 
 not precipitated by nitric or acetic acids, or by neutral 
 salts of soda and magnesia. 
 
 (5) With strong nitric acid they give the xantho- 
 proteic reaction. 
 
 (6) They are not precipitated when treated with 
 strong acetic acid and ferrocyanide of potassium. 
 
 (7) They are precipitated from neutral or weakly 
 acid solutions by solution of corrosive sublimate, nitrate 
 of mercury, nitrate of silver, and of tannic, phos- 
 pho-molybdic and phosjjho-tungstic acids. 
 
 Lastly, when injected into the blood they do not 
 reappear in the urine in the form of albumin. 
 
 If the gastric juice be allowed to act for a long 
 time on peptones, leucin, tyrosin, and other products 
 of the disintegration of albumin appear. 
 
 The conversion of albumins into peptones only 
 takes place in the presence of an acid, and, although 
 it may be imitated outside of the body, the action is 
 much more rapid and perfect in the body. This is 
 due (1) to the circumstance that, in the stomach, 
 each portion of peptone as it is produced is at once 
 removed by absorption, which is of course impossible 
 in artificial digestion ; and (2), which is still more 
 important, fresh acid and fresh pepsin are con- 
 tinuously secreted in gastric digestion, and, it is not 
 improbable, in proportions adapted to the matters that 
 have to be dissolved. In the case of Alexis St. 
 Martin, a young Canadian, who, as the result of a 
 gunshot wound, had a fistulous opening into the 
 stomach, Dr. Beaumont found that meat was digested 
 and absorbed in the course of two hours, that required 
 
Chap, vii.j Classification OF Food. 113 
 
 eight or ten hours to be completely digested when 
 submitted to the action of artificial gastric juice. No 
 absolute period for the digestion of food can be laid 
 down, since it varies with the duration of the pre- 
 vious fast, the amount of exercise taken, the quantity 
 and the quality of the food ingested, and the state of 
 the health. 
 
 Milk is immediately coagulated in the stomach, 
 owing to the precipitation of casein; and, since this takes 
 place when the free acid is neutralised, and does not take 
 place when pure pepsin is added to milk, it is believed 
 that a special ferment, named by Hammarsten lab- 
 ferment^ or rennet-ferment, is present, which possesses 
 the property of quickly developing lactic acid out of 
 lactose or susrar of milk. This ferment seems to be 
 most abundantly produced in infants, and its activity 
 may be estimated when it is stated that 1 part of 
 reiiD.et-/erj7ient can precipitate 800,000 of casein. 
 
 Meat is quickly broken up into detached fibres by 
 the solution of the connective tissue between them ; 
 the transverse striae become very distinct, the clear 
 striae dissolve first, and finally the whole disappears. 
 Beef is more digestible than mutton, whilst veal and 
 pork digest slowly. The flesh of young animals 
 digests more rapidly than that of old ones, and lean 
 meat quicker than fat. Cooking, by softening the 
 tissues, favours digestion. 
 
 Fibrous tissues, as tendons and ligaments, dissolve 
 with a rapidity proportioned to their firmness, and 
 even cartilage is slowly acted on. Gelatin, which is 
 ^obtained by boiling the tendons, skin, and other con- 
 nective tissues, is dissolved in gastric juice, and 
 converted into a kind of peptone. It loses its 
 power of gelatinising. 
 
 Action of the gastric juice on other con- 
 stituents of food. — Gastric juice has no action on 
 the oils and fats ; but it is capable of dissolving the 
 
114 Human Physiology. [Chap. vii. 
 
 connective tissue uniting fat cells into lobules, and of 
 dissohdng also the walls and protoplasm of the fat 
 cells, thus setting the contai]ied oil-globules free. 
 
 Starch is also unacted on by the gastric juice, but 
 it is said to convert the erythro-dextrin and achroo- 
 dextrin resulting from the action of saliva on starch 
 into glycose. Cane sugar is gradually converted into 
 grape sugar. 
 
 Conditions interfering' \i^tti gastric di- 
 gestion. — If a large excess of food be taken into 
 the stomach, imperfect digestion takes place, and the 
 partially-dissolved products are apt to excite irritation 
 throughout the whole length of the alimentary canal. 
 The presence of unwholesome or indigestible material 
 leads to the secretion of an excess of acid and gastric 
 derangement. Raw fruits of close texture, as apples, 
 preserved fruits (as candied lemon and orange-peel), 
 raw turnips and carrots, though they may be eaten 
 with impunity when violent exercise is taken and the 
 supply of more ap]:)ropriate food is insufficient, yet, 
 when given to children, are a fertile source of dis- 
 order. The consumption of alcohol, except in elderly 
 people, is unnecessary, if not harmful. It is useful 
 when the quantity of food is insufficient, and well- 
 matured wine is probably the best form in which to 
 take it. Large quantities impede, or may altogether 
 arrest, digestion. 
 
 It has been found that the ingestion of large 
 quantities of sugar impairs digestion, by causing the 
 secretion of mucus, which acts injuriously in two 
 ways : first, by preventing the gastric juice from 
 becoming incorporated with the food, and secondly, 
 by interfering with the absorption of that which is 
 absorbed. Lastly, it may be remarked that, for proper 
 digestion to be effected, a short period of repose 
 should be permitted after a meal. Exercise, whether 
 of the mind or body, interferes with digestion, by 
 
Chap. VII.] Movements of the Stomach. 115 
 
 withdrawing to the brain or muscles a portion of 
 the blood that should be engaged in supplying the 
 materials for the digestive process. 
 
 The stomach contains more or less gas, partly 
 derived from air swallowed with the food and saliva, 
 partly generated during the process of digestion. In 
 one specimen, obtained from a dog five hours after 
 feeding, 100 volumes contained CO., 25 '2, N 68 "68, 
 O 6-12. 
 
 Moveaiieiits of the stOKiacti. — The movements 
 of the stomach have been chiefly observed in animals, 
 though occasional opportunities have presented them- 
 selves in man, after injuries and in cases of extreme 
 atrophy. They are eflfected by unstriated muscular 
 tissue, so disposed as to form an external layer 
 of longitudinal fibres, an internal layer of circular 
 fibres, which is greatly developed at the cardictc 
 and still more at the pyloric orifices, and of an 
 intermediate layer of oblique fibres, occupying the 
 cardia. When empty, the stomach is probably at 
 rest, but soon after food is ingested movements com- 
 mence. In the earlier periods of digestion, general 
 contraction and shortening of the muscular fibres 
 occur, the walls of the stomach equably compressing 
 the food that has been introduced ; but, after a time, 
 contraction and relaxation succeed each other alter- 
 nately, the contractions sometimes travelling in an 
 undulating manner along the walls, whilst at other 
 times deep constrictions are formed, almost separating 
 the contents into separate portions. This often 
 occurs near the middle of the stomach, or about thi-ee 
 or four inches from the pylorus, the constriction that 
 occurs at this point giving the stomach an hour-glass 
 form. The wave of contraction usually passes from 
 left to right, and is then termed " peristaltic " ; some- 
 times in the opposite direction, and is then termed 
 "anti- peristaltic." The movements are not very 
 
it6 Human Physiology. rchap. vii. 
 
 energetic or rapid until towards tlie close of chymifica- 
 tion^ when tlie contents of the stomach have partially- 
 entered the duodenum, and they are best marked 
 along the greater curvature. A peristaltic wave 
 occupies about a minute in travelling from one end of 
 the stomach to the other. The intervals between 
 successive waves vary, but are shorter towards the 
 close of digestion. Movements have been observed 
 to take place after death. By some observers, as 
 Brinton, the anti-peristaltic movement is denied, and 
 the recurrent movement of the contents of the 
 stomach, which undoubtedly occurs, is explained by 
 considering that the peristaltic action of the gastric 
 walls forces towards the pylorus the food in contact 
 with them, but, the pylorus being closed, a reversed 
 axial current is produced, the presence of which has 
 led to the admission of an anti-peristaltic movement. 
 
 Influence of tlie laervoiis system on the 
 stoma-cli. — The stomach is supplied by the vagus 
 and by the sympathetic nerves. The vagTis contains 
 fibres derived from the spinal accessory, which is a 
 motor nerve. The evidence in regard to the effects 
 both of section and of stimulation of the vagus on 
 the movements of the stomach is unsatisfactory, and 
 even contradictory, perhaps owing to different periods 
 of digestion having been selected, and in part also to 
 different animals having been chosen for exjDeriment. 
 Many observers have noticed paralysis of the stomach 
 after division of the vagus ; but others have seen 
 some movements continue, or even that they have been 
 as vigorous as before. Stimulation of the vagus by 
 means of electricity causes contractions of the stomach 
 to occur in the course of five or six seconds, especially 
 during full digestion ; but, when the stomach is 
 empty, the contractions induced are feeble, or they 
 may be altogether absent. Movements of the stomach 
 have also been noted after stimulation of the two 
 
Chap. VII.] Self-Digestion OF Stomach. 117 
 
 sympathetics, the first thoracic ganglion and the solar 
 plexus. Stimulation of the corpora quadrigemina and 
 optic thalami in some instances excites gastric move- 
 ments. Destruction or removal of the brain and 
 spinal cord renders the stomach more irritable. The 
 splanchnic nerve is thought to be the inhibitory nerve 
 of the stomach. Direct irritation of the stomach, as 
 by scratching, pinching, or the application of irritants, 
 only causes local and limited contractions. Opium 
 inhibits the movements of the stomach. When dis- 
 tended, the stomach is subject to considerable passive 
 movements, through the action of the diaphragm, and 
 even of the heart. 
 
 Self-digestion of tlie stomach. — After death, 
 if this has taken place during the digestion of food, 
 it is not uncommon to find a large ragged orifice with 
 them, and softened edges in some part of the wall of 
 the stomach. In some instances the contents of the 
 stomach have been found in the peritoneal cavity. 
 Perforations of this \ind are due to the action of the 
 gastric juice secreted during life upon the dead walls 
 of the alimentary canal. During life no action of 
 this kind takes place, because the blood is rendered 
 more alkaline in the vessels where the acid juice is 
 secreted, and also brings with it the materials for 
 nutrition. The mere circumstance of the stomach 
 being part of the living body will not alone account 
 for its resisting the action of the juice in ordinary 
 digestion, for Bernard showed in a remarkable ex- 
 periment that gastric juice is capable of digesting 
 living as well as dead tissue, for he inserted the leg 
 of a living frog through a gastric fistula into the 
 stomach of a dog, and found that it underwent 
 ordinary digestion. It may perhaps be stated 
 generally that as long as living blood circulates in 
 the tissues at normal pressure, the gastric juice is 
 incapable of acting on them, but when normal 
 
iiS Human Physiology, [Chap. vii. 
 
 nutrition ceases, either in consequence of the for- 
 mation of embola, or of ligature of vessels, and 
 necrosis of tissue occurs, the gastric juice will exert 
 its full action upon it. Ewald found that division of 
 the cervical or upper dorsal region of the spinal cord, 
 an operation that materially reduces blood pressure, 
 caused, in the course of thirty-six hours, the formation 
 of numerous circular clearly-defined lenticular ulcers 
 "without any trace of inflammatory processes. 
 
 Chyme. — The product of gastric digestion is 
 termed ch37"me, and on ordinary diet is a whitish or 
 creamy acid fluid, which passes through the pylorus as 
 it is formed, and enters the small intestine. It contains : 
 
 1. The products of digestion up to the pylorus : 
 peptone, dextrose, Isevulose, peptonised gelatin. 
 
 2. All matters in a state of minute subdivision, 
 but which are only partially acted upon by the saliva 
 and gastric juice, such as raw starch and gum, the 
 denser connective tissues, gelatin that is merely 
 dissolved, some forms of albumin, and isolated and 
 partially-digested muscular fasciculi. 
 
 3. Substances quite unchanged by saliva and gas- 
 tric juice, such as cellulose, fats, and the fatty acids. 
 
 4 . The fluid solutions and salts not as yet absorbed 
 in the stomach, such as those of sugar, the vegetable 
 acids, and the gastric juice itself. 
 
 The really insoluble and indigestible residue of 
 the food remains for some time in the stomach, but is 
 ultimately propelled through the relaxed pylorus by 
 the peristaltic action of the stomach. 
 
 INTESTINAL DIGESTION. 
 
 As soon as the chyme enters the duodenum it is 
 subjected to the action of three fluids, the bile, the 
 pancreatic juice, and the secretion of the glands of 
 the mucous membrane of the intestine. It has been 
 
Chap. VII.] Characters of the Bile. 119 
 
 ascertained by means of experiments outside of the 
 body that the admixture of the acid chyme with the al- 
 kaline bile causes precipitation of the peptones, the 
 biliary acids being set free by the superior affinity of 
 the gastric juice for the soda with which thej' are 
 combined ; but as soon as the alkaline pancreatic juice 
 is added to the fluid, resolution of the peptones occurs, 
 and the pancreatic ferments are enabled to operate. 
 Some observers, however, deny that any precipitation 
 of the peptones occurs within the body. 
 
 ForniatJon of the bile. — The bile is formed 
 continuously by the cells of the liver, and is in part 
 conducted to the intestine by the ductus communis 
 choledochus, and in part accumulates in the gall 
 bladder, from which it is discharged in considerable 
 quantity soon after food is taken. 
 
 Cliaracters of tlie bile. — The bile, whether ex- 
 tracted immediately after death from man, or obtained 
 by means of a biliary fistula in animals, is a yellow, 
 browii, or greenish fluid, presenting dichroism and 
 some degree of fluorescence, with sp, gr. 1020, of ropy 
 consistence and bitter taste. Its reaction is alkaline. 
 It contains, as its principal ingredients the tauro- 
 cholate and giycocholate of soda, which are present in 
 the 2)roportion of from 2 to 10 per cent. Besides 
 these peculiar biliary salts the bile contains 5 per cent. 
 of ordinary salts, mucus, cholesterin, and lecithin,- 
 collectively. Water varies from 91 to 85 per cent. 
 Finally there is a minute quantity of sugar, and some- 
 times of a diastatic ferment. The colour is due to 
 tlie two colouring matters known as bilirubin and 
 biliverdin. The colouring matter of the bile is probably 
 a derivation of the colouring matter of the blood, hsemo- 
 chromogen, and is itself in process of being decomposed 
 into the colouring matter of the urine or urobilin. Bile 
 gives a green colour when a few drops of w^atery 
 solution of iodine are added to it. 
 
120 Human Physiology. [Chap, vii. 
 
 Gmelin^s test. — Bile is rendered vivid green with 
 nitroso-nitric acid, a play of colours, from green to 
 blue, violet, red, and yellow, being observed, due to 
 the oxydation of the bilirubin. 
 
 Pettenkoffer^s test. — This consists in adding a few 
 grains of sugar to bile or to a solution of the biliary salts, 
 and allowing a drop of sulphuric acid to fall upon it : a 
 beautiful crimson colour gradually makes its appearance. 
 
 Quantity ©f the toifle. — Dalton found, from 
 examination of the fluids obtained from a duodenal 
 fistula, that the largest quantity of bile in a given time 
 was discharged into the intestine immediately after 
 food has been taken. The average quantity secreted 
 per diem may be estimated at from 1 to 2 gi-ammes 
 (15 to 30 grains) per hour for every kilogramme 
 of body weight, or about 1,500 grammes per diem 
 (about 3 lbs. 3 oz. ) for a man of average weight. In 
 the dog the quantity is larger, amounting to about 20 
 grammes of bile containing 1 gramme of solids for each 
 kilogramme of body weight per diem. It is increased 
 with abundant animal food, and is diminished on fat 
 and starch diet. The quantity of bile is greatly 
 increased by certain drugs, and especially by podo- 
 phyllin, corrosive sublimate, and sodium salicylate. 
 The evidence that the bile is really formed in the 
 liver rests on the facts (1) that ligature of the vena 
 portse at once arrests the flow of bile, though neither 
 the blood of the vena portse nor of the hepatic artery 
 contains any of the biliary salts or colouring matters ; 
 (2) that after ablation of the liver the biliary salts do 
 not accumulate in the blood, though they immediately 
 begin to do so after ligature of the common bile duct. 
 
 Uses of tlie toiie. — The uniformity with which 
 the bile is discharged into the alimentary canal just 
 below the stomach throughout the whole of the 
 mammalia seems to indicate that it fulfils some im- 
 portant purposes in digestion, yet it has but little 
 
Chap. VII.] Uses of the Bile. 12 t 
 
 chemical action on either proteicls or on starch paste 
 raw or boiled, or on fats, though it aids in their 
 emulsification or mechanical division. It contains a 
 diastatic ferment, converting starch into sugar, but 
 the action is feeble. If a biliary fistula be established 
 and the bile allowed to flow from the body without 
 entering the intestine, it is found that on ordinary 
 diet serious disturbance of the functions of digestion 
 and absorption ensue, the animal becomes thin, its 
 hair falls off, and it dies apparently from debility. 
 Life can, however, be preserved for years if the food 
 supplied is in large quantity. It is not, therefore, 
 absolutely necessary. The chief uses that have been 
 demonstrated are : — 1. That bile moistens animal mem- 
 branes and permits oil or emulsions of oil to pass 
 through them with facility ; this property is not due to 
 the alkalinity of the bile, for it is possessed by the 
 pure biliary salts. The importance of this property 
 in the process of absorption is great. 2. Bile, more- 
 over, acts as a stimulant alike to the neuro-muscular 
 and glandular apparatus of the walls of the intes- 
 tines, increasing the peristaltic action and the secre- 
 tion of intestinal fluids. Lastly, bile is an antiseptic. 
 Hence, when bile is formed in insufficient quantity 
 hunger is experienced, the bowels are costive, much gas 
 is developed, and the fseces are pale and ill-smelling. 
 In passing along the alimentary canal the biliary 
 salts are decomposed, and the free taurin and glycine 
 probably undergo reabsorption, for little or none 
 of tauro- and glyco-cbolates are discharged with 
 the faeces, though in the dog the fseces contain 
 some cholalic acid, which may undergo further de- 
 composition into dyslysin ; and it is, perhaps, to the 
 loss of sulphur by the oeconoaiy that the ill efiects of 
 biliary flstulse are partly due, the hair in particular 
 being deprived of one of its most important consti- 
 tuents. The bilirubin is decomposed into biliprasin, 
 
122 Human Physiology. [Chap. vii. 
 
 which no longer gives Gmelin's reaction, and urobilin, 
 which is the colouring matter of urine. The choles- 
 terin is discharged by the feces, but the lecithin is 
 either decomposed or absorbed, for none of it appears 
 in them. 
 
 The pancreatic jsiice. — The pancreatic juice is 
 secreted intermittingiy, and enters the alimentary canal 
 by the same orifice as tlie bile. It is a thick trans- 
 parent fluid, secreted sparingij^, the quantity obtained 
 from a fistula in a large dog during one act of digestion 
 being only about one gramme or one gramme and a half. 
 It is colourless and tasteless, and has a strong alka- 
 line reaction. It contains so much albumin that on 
 boiling it coagulates, and its albumin is precipitated 
 by all the mineral acids. The composition of the 
 panci-eatic juice is : Water, 90 per cent. ; organic sub- 
 stances (albumin a.nd ferments), 9 ; and mineral con- 
 stituents, of which sodium chloride forms by far the 
 greatest part, 1 part. The gland is most active two 
 hours after food, then secretes more slowly, and again 
 becomes more active about 5 to 7 hours after the meal. 
 
 Uses of tlie pancreatic juice. — Though 
 secreted in such small quantity, the pancreatic juice 
 is one of the most important of the digestive fluids, 
 for it contains three hydrolytic ferments : a peptone- 
 forming ferment, trypsin; a fat-splitting ferment, 
 steapsin ; and a diastatic ferment, ar)iylo'psin. 
 
 1. Action on albumins. — Trypsin converts proteids 
 first into globulin, and then into true peptones. This 
 action of the trypsin of the pancreatic juice on proteids 
 only takes place in an alkaline fluid, and is of a coito- 
 sive nature. Cubes of coagulated albumin subjected 
 to its action do not swell up, as in gastric juice, but 
 are slowly eaten away. If the action of the trypsin 
 be continued on the peptones, about one half becomes 
 antipeptone , which is not capable of undergoing any 
 further d'gestive changes, whilst the remainder is 
 
Chap, VII.] Uses of the Pancreatic Juice. 
 
 123 
 
 converted into the amido-acids, leucin and tjrosin, and 
 when the peptones are derived from fibrin and gluten, 
 asparaginic acid appears. Still more protracted action 
 leads to the formation of the foetid substances named 
 indol and phenol. Put into a tabular form, the action 
 of trypsin may be thus represented : 
 
 Albumin + trypsin -\- soda solution of 1 per cent, forms 
 at the body temperatui-e, first globulin, insoluble in water, 
 and tben — 
 
 Hemipeptone. 
 
 {Leucin ... 
 Tyrosia .... 
 Hypoxantbin . . 
 Asparaginic Acid. 
 GlycocoU . . . 
 
 Indol. 
 Phenol. 
 Fatty acid. 
 Ammonia. 
 Sulphuretted 
 hydrogen. 
 Hydrogen. 
 Carbonic acid. 
 
 Antipeptone. 
 
 Putrefactive 
 products. 
 
 The trypsin seems to be formed at the expense 
 of a mother substance, named by Heidenhain zymogen 
 substance, the development of which has been care- 
 fully studied. The acini of the glands are Kned by 
 cells, and in each cell, just before digestion, three 
 regions or zones are recognisable, a basal or iperi- 
 pheric zone near the attached extremity, which is 
 homogeneous, free from granules, and which stains 
 with carmine ; a median zone, occupied chiefly by the 
 nucleus ; and an internal zone at the free extremity, 
 which is ojranular and does not stain with carmine. 
 Soon after digestion commences, when secretion is 
 most active, the internal zone, at first large, becomes 
 smaller and more granular, whilst the peripheric zone 
 augments, and ultimately almost fills the cell, the 
 median zone, with the nucleus, remaining unaltered. 
 As the secretion becomes less active, the internal granu- 
 lar zone reforms at the expense of the peripheric zone. 
 
124 Human Physiology. [Chap. vii. 
 
 Thus_, during secretion, the materia] which has been 
 gradually forming in each cell, and which has accumu- 
 lated near its free extremity, is discharged, whilst new 
 matter is taken up from the blood by the protoplasm 
 at the base of the cell, to be elaborated in the interval 
 of digestion. If the 'pancreas taken from the living 
 animal be acted on by glycerin, the glycerin extract 
 does not act on proteids, and only contains traces of 
 trypsin, but it does contain the zymogen substance, 
 which is believed to be a combination of trypsin with 
 a proteid, and the trypsin is set free by the mere 
 presence of water at a moderately high temperature, of 
 weak acids, or of oxygen, whilst its separation is re- 
 tarded by sodium chloride, and by the alkaline carbo- 
 nates. If, instead of the pancreas, the pancreatic 
 juice is extracted with glycerin, no zymogen substance 
 is obtained, but the trypsin itself. Hence at the 
 moment of secretion zymogen ferment is converted into 
 trypsin. The largest quantity of zymogen is found 
 fourteen hours after food. 
 
 2. Action on fats. — Pancreatic juice exerts a double 
 action on fats ; it first converts them into an emul- 
 sion, and then decomposes them with the absorption of 
 water into glycerin and the fatty acids. Two grammes 
 of pancreatic juice are required to emulsionise one 
 gramme of fat. After complete decomposition by the 
 agency of the steapsin, the fatty acids form soaps with 
 the alkali of the juice and of the intestinal fluid. 
 
 3. Action on starch. — The action of the pancreatic 
 juice on starch resembles that of saliva, though it 
 is much more energetic, the amylopsin it contains 
 acting, not only on boiled but on raw starch, convert- 
 ing each into dextrin and grape sugar. 
 
 Iiiiiervation of tSie pancreas. — There ap- 
 pears to be a centre controlling the secretion in the 
 medulla oblongata ; the nerves proceed from the 
 sjplenic, hepatic, and superior mesenteric plexuses, but 
 
Chap. VII.] Intestinal Digestion. 125 
 
 the nervous circle is not accurately known. Induc- 
 tion currents applied to the gland induce secretion. 
 The secretion of the gland is arrested by atropine. 
 It is discharged under a pressure of about 17 mm. 
 
 Hg. 
 
 Movements of the small intestine. — The 
 
 arrangement of the longitudinal and circular layers of 
 muscular fibre in the walls of the intestine enable it to 
 execute peristaltic movements, the effect of which is 
 to propel its fluid or pulpy contents from above down- 
 wards with more or less rapidity. These waves of 
 contraction are excited by the presence of food, which 
 stimulates the sympathetic fibres, and then acts 
 through reflex centres situated in the sympathetic 
 gangha and in the spinal cord, but the precise nervous 
 circle has not been accurately ascertained. A power- 
 ful inhibitory influence on the intestinal movements is 
 exerted through the splanchnic nerves. Stimulation of 
 these nerves causes vascular contraction and anaemia 
 with inhibition of the movements, whilst section of 
 the nerve causes congestion of the intestinal vessels and 
 increased peristaltic action. Under certain circum- 
 stances, as when there is much food in the intestine, 
 stimulation of the vagus excites peristaltic action. 
 
 Function of the §^lands of the small intes- 
 tine in dig^estion. — The walls of the small intestine 
 contain two sets of glands : the glands of Briinuer, 
 chiefly or exclusively found in the duodenum, and the 
 glands of Lieberkiihn, which are closely arranged in the 
 mucous membrane in its whole extent. The secretion 
 of the tubular and convoluted glands of Brilnner 
 appears to resemble that of the pancreas in contain- 
 ing a diastatic ferment capable of converting starch into 
 sugar; and a glycerin extract of the upper part of the 
 duodenum, where these glands are most abundant, 
 yields a ferment which dissolves fibrin easily. The 
 secretion oi Lieberkuhn^ s follicles can best be obtained 
 
126 Human Physiology. [Chap. vii. 
 
 by a Tliiry's fistula, wliicli is thus made : an excised 
 piece of intestine, still coiniected with mesentery, is 
 ligatured at one end, while the other is united with the 
 abdominal wound. The continuity of the intestine is 
 repaired by carefully sewing the ends together. An 
 alkaline opalescent fluid is obtained, which, according 
 to some authors, acts, like the pancreatic juice, upon 
 all the constituents of the food, proteids, fats, and 
 carbohydrates. The use of the valvulce conniventes is 
 not only to present a larger surface for absorption, but 
 to delay the progress of the food, and thus enable the 
 digestive process to be conducted more slowly and 
 perfectly. 
 
 Changes of tiie ciiyme in tlie small intes- 
 tine. — The chyme (described at page 118) becomes 
 alkaline or neutral in the jejunum, owing to admixture 
 with the biliary, pancreatic, and intestinal secretions, 
 but in the ileum it again becomes acid from the for- 
 mation of acids consequent on the putrefaction of pro- 
 teids and upon fermentation processes. Thus : — 
 
 Pepsin and trypsin acting on albumin + n(H20) 
 yield peptone, leucin, tyrosin, xanthin, aspara- 
 ginic acid. 
 
 Steapsin acting on fats converts 
 
 tristearin into glycerin and stearic acid; 
 
 C,,H,,oO, + 3(H,0) = C;H303 -f 3(Ci8H,aO,) ; 
 
 Lactic ferment acting on milk sugar converts 
 
 C12H22O11 + H2O 
 
 it into grape sugar, and this into lactic acid ; 
 
 Butyric ferment acting on lactic acid yields 
 
 2(C3HA) +2(H20) = 
 butyric acid + carbonic acid and hydrogen ; 
 C.HgO^ + 2(C0,H2) + H,. 
 
Chap. VII.] Intestinal Digestion. 127 
 
 Unknown ferments convert 
 
 taurocholic acid into taiirin and 
 
 cholic acid ; cellulose into 
 
 C^H^O^ ^ n(CeH,oO,) + n(H,0) = 
 
 carbonic acid and marsh gas ; 
 3n(C0,) + 3n(CH,); 
 albumin + n(H20) into globulin, peptone, 
 and leucin, tyrosin, xantliin, indol, pbenol, 
 skatol, fatty and carbonic acids, ammonia, 
 sulphuretted . hydrogen ; glycerin CoH-(0H)3 
 into H + CO2 + succinic acid + fatty acids ; 
 malic, tartaric acids into butyric and acetic acids 
 with evolutions of CO^ 
 
 Amongst the final products of the putrefactive decom- 
 position of non-azotised substances after the oxygen in 
 the intestines is consumed are CH^, or marsh gas, and 
 COg. The contents of the small intestine begin to 
 assume the appearance and consistence, and to give the 
 odour of, faeces, in the lower part of the ileum. 
 
 Dig^estiou in ttie large intestine. — The 
 contents of the large intestine are usually acid in the 
 upper part fi'om lactic acid and other fermentations, 
 and by far the larger portion of the nutritive materials 
 of the food have been absorbed in the lower part; they 
 gain in consistence, and are more or less periodically dis- 
 charged as fasces. Theyieces consist (1) of elastic tissue, 
 woody fibre, the husks of grain, and fragments of most 
 of the constituents which have been hurried on before 
 being thoroughly incorporated with and digested by 
 the gastric and other fluids. (2) Of the products of 
 disintegration of the biliary colouring matters. (3) 
 Of unaltered mucin and nuclein. (4) Of combina- 
 tions of fatty acids with lime, especially after milk 
 diet. (5) Of salts, especially those which diffuse with 
 difficulty, as the ammoniaco-magnesian phosphate and 
 
12 8 Hum A n Physiol og \ \ [Chap, vi i . 
 
 phosphate of lime. The quantity of the fieces dis- 
 charged per diem is on the average 170 grammes 
 (about 3,000 grains). It is greater on vegetable than 
 on animal diet. They contain 75 per cent, of water. 
 The foetid odour is due to indol and skatol. 
 
 Oases of the alimeiitary canal. — ^These partly 
 consist, in the stomach, of nitrogen of air that has been 
 swallowed with the food, and partly of carbonic acid 
 gas evolved during processes of fermentation. The 
 oxygen of the air that is swallowed is soon absorbed. 
 In the lower part of the alimentary canal nitrogen 
 again forms the principal constituent of the gases 
 found in this region, but there are also considerable 
 quantities of hydrogen and carbonic acid gas. In 
 some cases marsh gas has been found in the proportion 
 of 50 per cent, or more of the total quantity of gas. 
 
 Defsecatioii. — The lower outlet of the alimen- 
 tary canal is guarded by a sphincter, which preserves 
 a state of persistent contraction, sometimes named its 
 tone, which is evidently due to the constant influence 
 of the spinal cord; since, if this be destroyed, relaxation 
 of the muscular tissue occurs, and the contents of the 
 bowel are discharged. The sphincter is partly formed 
 of striated muscular tissue, and is to some extent 
 under the influence of the will, but is chiefly composed 
 of unstriated muscle, which is altogether withdrawn 
 from voluntary control. The nervous mechanism is 
 complicated. The stimulus is the presence of fseces 
 acting on sensory fibres distributed to the mucous 
 membranes and muscular coat of the rectum, which 
 conduct impulses to the anospinal centre, situated 
 in the lumbar region of the cord ; from this, motor 
 fibres conducting motor impulses proceed to the 
 unstriated muscles of the intestine, and increased 
 peristalsis results. But this is not all. The expulsion 
 of the contents of the rectum must be preceded by the 
 relaxation or inhibition of the sphincter, and there 
 
Chap. VII.] Absorption OF Food. 129 
 
 must therefore be an inhibitory centre. The exact 
 position of this is unknown, but it is clearly, to some 
 extent, under the influence of the will. Under 
 ordinary circumstances the abdominal muscles act but 
 slightly, and then by an exertion of the will ; but if 
 the stimulus be violent, strong motor impulses may 
 radiate to the abdominal muscles, which then act 
 spasmodically and involuntarily, just as in parturition. 
 In diarrhoea, with involuntary discharge of fseces, the 
 voluntary control over the external sphincter is lost, 
 and the stimulus excites the anospinal centre too 
 powerfully for the internal sphincter to resist, whilst 
 the abdominal muscles often contract with great 
 violence. The discharge of the faeces usually takes place 
 once or twice in 24 hours. Irregularity in the per- 
 formance of this function leads to many troubles. 
 
 Absorption of tlie food. — The object of the 
 digestive process is to render the substances taken as 
 aliment soluble, and the conditions that exist in the 
 alimentary canal are well calculated to favour the 
 dift'usion of these substances into the blood and 
 lymph. In the intestine is a slowly-moving fluid 
 charged with diffusible or crystalloid materials, sugars 
 and peptones, whilst in the walls of the intestine are 
 two systems of vessels, the blood-vessels and the 
 lymphatics, both of which contain albumin and 
 colloidal substances moving with considerable rapidity. 
 A current is accordingly established from the intestine 
 towards the blood-vessels, which, it is known, take up 
 and carry ofi*, both from the stomach and the intestinal 
 canal, a large proportion of the products of digestion. 
 The absorption of the fats, however minutely we may 
 conceive them to be divided, is more difficult to 
 explain. Recent investigations seem to show that it 
 is due to amceboid cells which lie at the bases or 
 attached extremities of the columnar epithelial cells 
 of the villi and intestinal mucous m.embrane, and 
 J 
 
130 Human Physiology. [Chap viii. 
 
 send up long processes between the cells to tlie 
 surface, where they seize upon and ingest the passing 
 particles of fat, and conduct them to the commence- 
 ment of the lymphatics, here called lacteals. 
 
 CHAPTER YIII. 
 
 CHYLE AXD LYMPH. 
 
 The ctayle and lympli. — These fluids may be 
 considered together, since they are contained in the 
 same system of tubes, and only differ from each 
 other at certain periods. During fasting they are 
 alike, but when di^-estion is in progress the lympha- 
 tics distributed to the intestines and abdominal viscera 
 become charged with the products of digestion, and 
 their contents are materially modified both in aspect 
 and in chemical composition. 
 
 Properties of t!ie lympBi. —Lymph is clear 
 and transparent, contains albumin, and, after passing 
 through a gland, or perhaps even before, lymph cor- 
 puscles. It is capable of feebly coagulating, and of 
 setting into plasma and clot. 
 
 derivation of Ij^iipli. — Lymjoh plasma is be- 
 lieved to be the supertiuous fluid part of the blood 
 which has escaped from the blood-vessels, and which 
 has irrigated the tissues and ministered to their 
 nutrition. The salts it contains correspond nearly 
 with those of the blood j)lasma. It is, however, a 
 more watery fluid, the albumin bemg reduced to 
 about one-haif and the fibrin generators to two- 
 thiiYls of the quantity in which they exist in the 
 blood. It is i)robable that it contains some of the 
 products of the waste of the tissues. 
 
Chap. VIII.] Chyle and Lymph. 131 
 
 The Synipli coi'piiscSes. — The corpuscles, or 
 morphological elements of the lymph, resemble the 
 white corpuscles of the bloody and they are derived 
 (1) from the corpuscles which are so abundant in the 
 tissue of the lymphatic glands ; (2) from the adenoid 
 tissue of other parts of the body, as the mucous and 
 submucous tissue of the intestinal tract, the spleen, 
 and the marrow of bones ; (3) some may also be true 
 white corpuscles of the blood which have wandered 
 into the lymphatic system, especially when that 
 system invests the blood-vessels as with a sheath ; 
 (4) and lastly, by fission or division of previously- 
 formed lymph corpuscles. 
 
 Properties of cliyle. — The fluid absorbed by 
 the lymphatics of the intestine during digestion is 
 termed the "chyle," or "lacteal fluid," because it 
 presents, especially when much fat has been contained 
 in the food, a milky aspect. It is an alkaline fluid, 
 with a sp. gr. of 1012 — 1022. It contains sugar, and 
 albumin in the form of peptone and salts in solution, 
 and fat molecules in suspension or in the state of 
 emulsion. In addition, even in the villi, there are a 
 few lymphoid cells, and these increase in number 
 after the chyle has passed through one or more of the 
 lymphatic glands of the mesentery. In these glands 
 a process of elaboration or assimilation takes place, as 
 a result of which the proteids absorbed are converted 
 into fibrin generators. Hence the chyle taken from 
 one of the more centrally situated lacteals, or from 
 the thoracic duct itself, where it is mingled with the 
 lymph coming from other parts of the body, is capable 
 on standing of coagulation, separating into a soft and 
 easily-broken clot and a plasma. In the upper part of 
 the thoracic duct, corpuscles, presenting the character 
 of red blood-corpuscles, are found, which, however, 
 have probably escaped from the vascular system and 
 entered the lymphatic system of the spleen, or other 
 
132 Human Physiology. [Chap.viii. 
 
 abdominal organ, under the greatly increased venous 
 pressure attendant upon digestion. 
 
 Qiiaiitity of tlie cliyle. — Tlie chyle enters the 
 circulation through the thoracic duct intermittently. 
 The quantity is large ^T.th ordinary diet, an hour or 
 two after digestion has commenced, and sooner if 
 such an emulsion as milk be taken \ and it is esti- 
 mated that the constant and more equable supply of 
 lymph from the body generally, and the variable 
 supply of chyle from the digestive organs, each con- 
 stitute about one-half of the fluid traversing the 
 thoracic duct. The absolute quantity is very large. 
 In one case, six kilos or about 13 lbs. of lymph 
 were obtained in twenty-four hours from a fistula 
 in the thigh which opened into a large lymphatic. 
 
 Movement of tlie cliyle and lympli. — The 
 onward current of the chyle is maintained : 
 
 (1) By the contraction of the layer of unstriafced 
 muscular tissue in the villi, which, aided by the valves 
 in the larger lacteals, act like so many little force- 
 pumps, driving the fluid absorbed into the central 
 lacteal onwards to the subjacent tubes. 
 
 (2) The current thus established will be increased 
 in those lymphatics which form sheaths around blood- 
 vessels by any dilatation of these blood-vessels, the 
 backward current being prevented by valves, whilst 
 
 ^ there will be under these circumstances an increased 
 
 quantity of interstitial fluid. 
 / (3) In a similar manner, increased blood-pressure 
 
 in all j)arts of the body will, by causing increased 
 
 filtration through the blood-vascular walls, lead to 
 
 more rapid flow of lymph. 
 
 (4) The act of inspiration is a vis a fronte^ which 
 draws the lymph contained in vessels outside the 
 chest into the vessels within the chest, the opposite 
 efiect of expiration being neutralised by the valves. 
 
 (5) A powerful agent is the rapid current of 
 
Chap. IX.] FUNCTIOISf OF THE LiVER, 
 
 03 
 
 venous blood over the mouth of the opening of the 
 thoracic duct into the angle of junction of the jugular 
 and subclavian veins. 
 
 (6) The last cause to be mentioned which accele- 
 rates the onward current of lymph is the contraction 
 of the lymphatics themselves, aided again by the 
 valves. These contractions are not much accentuated 
 in man and the higher animals ; but in the amphibia, 
 and some fishes, the muscular tissue by which it is 
 effected is collected into definite regions, forming 
 true lymphatic-hearts, which contract rhythmically 
 and propel the lymph with considerable vigour. The 
 rate of movement in the chief cervical duct of the 
 horse is estimated at about one foot per minute. The 
 lateral pressure in the thoracic duct of the horse is 
 about 1-2 mm. Hg. 
 
 The nervous system acts on the lymph current 
 through its distribution to the smooth muscular fibres 
 of the lymphatics. 
 
 CHAPTEK IX. 
 
 GLYCOGE^S'IC FUJ^CTIOX OF THE LIVEE. 
 
 Crlycogen. — This substance, named also hepa- 
 tine, and bernardine, and zo-amyline, is widely distri- 
 buted. It is found in all developing animal cells and 
 nerves, in embryonal tissues, and in the villi of the 
 chorion ; in colourless blood-corpuscles, in the muscles, 
 and especially, where it was first recognised by Bernard, 
 in the cells of the liver, where it exists in an amor- 
 phous condition. Bernard believed that this sub- 
 stance, derived from the food and elaborated in the 
 liver, was converted in that organ into sugar by the 
 aid of a ferment. The sugar thus formed, he thought, 
 
134 Human Physiology. [Chap. ix. 
 
 immediately entered the circulation, and underwent 
 oxydation. Pavy has, however, shown that no sugar is 
 contained in the blood returning in the hepatic veins 
 from the liver to the heart during life, and that it 
 only makes its appearance in this blood after (though, 
 it must be admitted, very quickly after) death. 
 
 Mode of o]l>S:aiiiiiig' glycogen. — Bernard's 
 method was as follows : — Cut up a portion of liver 
 recently removed from an animal into small fragments, 
 and throw them into boiling water ; pound in a 
 mortar, and boil for a quarter of an hour in a little 
 water ; pass the fluid through a linen cloth, adding to 
 it a little animal charcoal To the opaline fluid thus 
 obtained, add four or five times its volume of alcohol 
 at about 40° C (100° F.), w^hen glycogen is precipi- 
 tated. This must be well washed with alcohol. It 
 may be further purified by boiling with caustic potash, 
 precipitating with alcohol, and removing any adherent 
 potash with acetic acid. 
 
 Briicke's method is to throw the liver into boiling 
 water ; then, when hardened, to pound it in a mortar, 
 and boil the mass for half an hour in Avater. The 
 milky liquid is decanted, and fresh water added as 
 long as it acquires an opaline tint. These liquids are 
 put together, refrigerated and filtered, and then there 
 are added alternately hydrochloric acid and mercuro- 
 potassic iodide as long as a precipitate, consisting of 
 albuminous compounds, falls, and the fluid is filtered. 
 The filtered liquid is treated with alcohol, which 
 precipitates the glycogen. This may then be collected 
 and purified as usual. 
 
 Clieinlca.! composition and properties of 
 glycogen. — The composition of glycogen is expressed 
 by the formula CgHioOj, and it is therefore isomeric 
 with starch and dextrin. It is a white powder 
 without smell or taste, insoluble in alcohol and ether, 
 but dissolving in boiling water, and forming an 
 
Chap. IX.] GlYCOGENY. 
 
 OD 
 
 opalescent fluid, which rotates the plane of polarised 
 light strongly to the right. On the addition of an 
 alkali the opalescence disappears; with ioduretted potas- 
 sium iodide it gives, not a blue colour like starch, but 
 a red colour, which disappears with heat, and reappears 
 on cooling. It does not reduce the oxide of copper 
 in an alkaline solution of copper sulphate, which 
 distinguishes it from giycose. It is precipitated by 
 lead acetate, which distinguishes it from dextrine ; 
 when boiled with acids it is converted into achroo-dex- 
 trin and ptyalose, and it undergoes a similar change 
 with the animal ferments, ptyalin and amylopsin. 
 
 Quantity of glycogen contained in tlie 
 liver. — In health the liver always contains glycogen, 
 the quantity varying from about \\ to 1\ per cent, 
 of the weight of the liver in man. The proj)ortion 
 differs in other animals. In the case of the fowl it 
 has been known to rise to 12 per cent. It seems 
 to be chiefly stored up around the nucleus in the cells 
 in relation with the hepatic vein. 
 
 Influence of food on glycogen. — The first 
 and most important factor is the food. If a dog be 
 supplied, in addition to its proper food of proteids and 
 fat, with a considerable quantity of starch, sugar of 
 almost any kind, such as cane, grape, milk, fruit 
 sugar, or glycerin, a large accumulation of glycogen 
 takes place. JMannite and inosite, however, and gum, 
 do not increase it. The addition of fats and soaps to 
 the ordinary food causes no increase. The effect of 
 the supply of proteids, even if starch and its congeners 
 be witlidrawn from the food, is to maintain a mode- 
 rate formation of glycogen in the liver. Gelatin 
 does not conduce to the production of glycogen. 
 
 Prolonged fasting leads to its total disappearance. 
 
 Influence of the nervous system on gly- 
 cogeny. — It has been found by experiment that 
 lesion of the floor of the fourth ventricle near its 
 
J 
 
 6 Human Physiology. [Chap. ix. 
 
 lower pait produces diabetes or sacclaarine urine. 
 But this part of the medulla oblongata is the centre 
 of the hepatic vaso-motor nerves, and lesion of these 
 nerves in any part of their course from the medulla 
 oblongata to the liver, whether in the spinal cord or 
 in the sympathetic cord, also leads to diabetes. The 
 explanation that is offered is, that such lesions as 
 those just referred to seriously interfere with the 
 circulation through the liver, the vessels dilate, the 
 current of blood moves at a slower rate, and the liver 
 becomes charo-ed with susjar because the blood ferment 
 has now time to act on the glycogen and effect its 
 conversion into that substance. But the sugar 
 diffuses easily into the blood, and is immediately 
 filtered off by the kidnej^s, rendering the urine 
 secreted saccharine. Section of the splanchnics 
 prevents the occurrence of diabetes after puncture 
 of the floor of the fourth ventricle, and even if 
 diabetos has been induced by this means it arrests its 
 production. The reason of this is that section of 
 the splanchnics causes such an immense accumulation 
 of the blood in the portal vessels and abdominal 
 viscera, that the liver is rendered anaemic, and hence 
 no sugar is formed in it. 
 
 Iiiflueiice of dTBigs oai g-5ycog:eiiy. — A variety 
 of drugs, which are capable of paralysing the vaso- 
 motor nerves of the liver, act like puncture of the 
 floor of the fourth ventricle, and by retarding the 
 current of blood through its vessels lead to an 
 accumulation of sugar in its cells, and secondarily to 
 diabetes. Such are curare, when artificial respiration 
 is not maintained; chloroform, ether, chloral, and 
 amylnitrite. 
 
 The ferment to which reference has been made is 
 considered to be present in the blood. 
 
 MotUer substance of glycogeia. — There seems 
 to be strong reasons for believing that glycogen may 
 
frrrt'ff rise 
 Chap. IX.] Functions of the Spleen. 137 
 
 be derived directly from tlie carbohydrates of tlie food 
 to which it is, in its chemical composition, so closely 
 allied; it may, however, also be derived from taurin 
 and giycin ; the latter substances splitting into 
 glycogen and urea. Its increase on abundant flesh 
 diet renders it probable that the albumins can be 
 broken up into a non-nitrogenous portion — glycogen 
 and a nitrogenous portion. 
 
 I>e§tiBiatio£i of g-Iycogen. — Pavy and others 
 believe that during life no conversion of glycogen into 
 sugar takes place, or at least that there is no evidence 
 of such conversion, but most experimenters are of 
 opinion that sugar is being constantly formed from 
 glycogen in small quantities, and is taken up by the 
 hepatic venous blood, which is known to be richer in 
 sugar than ordinary blood, and that it is applied 
 either to the production of heat, or to the development 
 of muscular force by oxydation. 
 
 Functions of tlie spleen. — The functions of the 
 spleen are obscure. It is believed to be a place where 
 red blood-corpuscles are broken down, and also one 
 where white corpuscles are formed. It is evidently an 
 important organ, for it receiA^es a large supply of 
 blood ; it undergoes great changes in volume ; it 
 executes slow rhythmical movements of contraction 
 and expansion. The relations between the small 
 arteries and veins are peculiar, the intermediate 
 channels being of the nature of lacunar spaces rather 
 than of capillary vessels. On the other hand, the 
 spleen may be extirpated, both in animals and in man, 
 without serious result, the chief eiiect which has 
 been observed being hypertrophy of the lymphatic 
 glands and red marrow of bones ; and it is reasonable 
 therefore to conclude that the hypertrophied tissues 
 discharge a vicarious function, and this is probably 
 the production of lymphoid and white corpuscles. 
 Little information in regard to its functions can be 
 
r,r.,;viir /inM"-^ 
 
 138 Human Physiology. [Chap. ix. 
 
 obtained from examination of the blood going to and 
 returning from it, but such as it is, it seems to 
 show that white corpuscles have been added to it. In 
 diseases, again, in which the spleen is much enlarged, 
 as in ague, the blood is found to contain an excess 
 of white corpuscles, a condition that is named 
 leucaemia. The splenic arterial blood in one case 
 contained one white to 2,200 red corpuscles, whilst 
 in the blood of the splenic vein the proportion was 
 one to sixty. Chemical examination of the splenic 
 pulp aflbrds support to the view that red corpuscles 
 are broken down in the spleen, for it contains many 
 of the products of their regressive metamorphosis, 
 as pigment, leucin, xanthin, hypoxanthin, and albu- 
 minous compounds rich in iron; inosite ; cholesterin; 
 and lactic, acetic, formic, butyric, uric, and succinic 
 acids. Moreover, under the microscope large masses 
 of protoplasm are found containing red corpuscles, 
 more or less discoloured, to the number of ten or more. 
 
 The rhythmical contractions of the spleen, as 
 shown in tracings taken with plethysmograph, occur 
 in. the cat and dog at intervals of about one minute. 
 
 Functioais of tlie tliysiiiis and tlayroid. 
 bodies. — These organs, sometimes included with 
 the suprarenal bodies under the general title of tJie 
 ductless glands^ seem to be remains of organs which 
 may once have been of importance in the oeconomy, but 
 which, in the process of evolution, have come only to 
 play a subsidiary part. This view is suggested by the 
 temporary activity of the thymus^ which attains its 
 largest size about the end of the second year of life, 
 and then gradually atrophies. It is thought that both 
 the thymus and thyroid bodies may minister to the 
 formation of the white corpuscles of the blood. The 
 chemical composition of their expressed juice resembles 
 that of the spleen. The suprarenal capsules receive a 
 remarkable nervous supply, but their use is unknown. 
 
139 
 
 CHAPTEE X. 
 
 THE FUXCTIOXS OF THE SKIN. 
 
 The skin, by its elasticity, its density, and 
 toughness, protects the subjacent parts from injury ; 
 the desquamation or scaling of the surface, which is 
 constantly taking place, preserves to a certain extent 
 its cleanliness, and enables the body to throw off 
 adherent particles of a harmful nature, such as the 
 spores of fungi, thorns, and the like, which may have 
 accidentally implanted themselves in or on its surface. 
 Secondly, the rich supply of nerves in its tissue 
 renders it a highly efficient sensory organ, the im- 
 pressions it receives being termed, from their wide 
 distribution, those of common sensation, whilst the 
 long outrunners that it possesses in the form of hairs, 
 and the exquisite sensibility of their bulbs, convey to 
 the mind important information of the approach of a 
 foreign body. Thirdly, the immense capillary network 
 of blood-vessels that ramifies in its substance renders 
 it an important agent in the regulation of the heat of 
 the body ; for when the capillaries are contracted the 
 bad conducting power of the dermis and epidermis 
 for heat preserves the internal temperature at a high 
 standard, whilst, when the capillaries are dilated, the 
 blood they contain is drawn from the deeper-seated 
 and warmer parts of the body, and loses much heat 
 by conduction, radiation and evaporation. Fourthly, 
 the skin is an efficient organ of secretion, giving off 
 water charged with various soluble substances, and 
 oily material. Lastly, it absorbs O and eliminates 
 CO2, and plays therefore a subsidiary part in the 
 function of respiration. 
 
140 Human Physiology. [Chap. x. 
 
 The cutaneous respiration. — In man ana 
 
 the higher animals the part played by the skin in 
 respiration is quite subsidiary to that of the lungs, 
 but in the frog it is so important that life can be 
 preserved even after the lungs have been removed. 
 The quantity of oxygen absorbed by the skin as com- 
 pared with that by the lungs is as 1 : 127. The 
 quantity of carbonic acid that is eliminated by the 
 skin is estimated at about 10 gramimes in 24 hours, 
 the results obtained by different observers varying, 
 however, from 2-23 grammes to 32 grammes. The 
 quantity increases with temperature and with 
 muscular exertion. The quantity of watery vapour 
 eliminated by insensible perspiration varies also with 
 the temperature and moisture of the air, the amount 
 of clotliing and of exercise, and generally with all 
 conditions that modify the flow of blood through the 
 capillaries. It is difficult or impossible to separate and 
 estimate it from sweat, but it is interesting to notice 
 that it bears a certain inverse relation to the 
 elimination of water from the system by the kidneys. 
 When the temperature of the air is low, and when it 
 is nearly saturated with watery vapour, the capillaries 
 of the skin are contracted, little fluid is lost by 
 cutaneous evaporation, and the kidneys secrete freely ; 
 while, on the other hand, if the air be warm and dry, 
 the cutaneous capillaries are charged with blood, the 
 kidneys secrete but a small quantity of water, and 
 the insensible perspiration is greatly augmented. 
 
 The cutaneous secretions. — Regarded as an 
 organ of secretion the skin presents a surface of 
 about 15,000 square centimeters, or one and a half 
 square meters, and secretes a fluid named sweat, 
 and a peculiar sebaceous or oily material. Sweat 
 is a colourless, slightly cloudy fluid, with a salt 
 taste and characteristic odour ; its specific gravity is 
 1004; its reaction is primarily alkaline, but from 
 
Chap. X.] Functions of the Skin. 141 
 
 its admixture with sebaceous matter whicli contains 
 fatty acids, it, as ordinarily examined, gives an acid 
 reaction. Its alkalinity can be shown in such regions 
 as that of the palm of the hand, where there are no 
 sebaceous glands. The quantity secreted per diem is 
 about 1,000 grammes, or about 2 lbs. ; but great 
 variations result from differences of both internal 
 and external conditions. Large quantities of fluid 
 ingested, violent muscular exertion, high external 
 temperature with dryness of the air, all tend to 
 increase the amount of secretion, and if acting 
 together may cause the elimination of many pounds of 
 fluid per diem. Chemical analysis of the sweat shows 
 that it contains about 1 per cent, of solids, of which 
 urea, a proteid resembling casein, the neutral fats, 
 palmitin and stearin, cholesterin, and the volatile fatty 
 acids, are the most important organic constituents ; 
 whilst the inorganic include sodium and potassium 
 chlorides, sulphates and phosphates. 
 
 Iiifiiieiice of tlie nerves on tlie secretion of 
 sweat. — The nerves seem here, as in so many other 
 cases, to influence the secretion in two ways ; first, by 
 influencinoj the size of the blood-vessels, and the 
 activity of the circulation through the glands ; and 
 secondly, by a direct action on the gland-cells them- 
 selves. Paralysis of the constricting vaso-motor 
 nerves, and stimulation of the dilating vaso-motor 
 nerves, causes the blood to flow with increased rapidity 
 through the blood-vessels of the glands, and the 
 excretion of sweat is increased. This effect is well 
 shown in cases where the sympathetic nerve in the 
 neck has been divided, the skin of the face and neck 
 on the same side as the lesion becoming moist with 
 perspiration, whilst electrical stimulation of the upper 
 cut extremity arrests the secretion. That a direct 
 action can be exerted on the glands through the ner- 
 vous system apart from any change in the circulation 
 
142 Human Physiology. [Cimp.x. 
 
 is sllo^vIl, first, by the fact that stimulation of the 
 sciatic nerve will produce increased secretion of sweat, 
 even in an amputated limb ; and secondly, by the 
 circumstance tliat if the sciatic nerve of one leg be 
 divided^ and the animal be placed in a hot tempera- 
 ture, and even if, in addition, the vein of the same 
 leg be tied so that the conditions most favourable to 
 congestion of the cutaneous capillaries are established, 
 no secretion of sweat takes place from the damaged 
 limb, though the others, Avith the rest of the body, 
 perspire freely. The nerve centre for the secretion of 
 sweat lies in the anterior part of the grey substance of 
 the spinal cord, and the motor fibres partly issue with 
 the motor roots of the nerves forming the sciatic 
 nerve, and partly enter (in the cat) the sympathetic 
 cord. The sweat centre can be stimulated directly (1) 
 by venous blood ; (2) by blood, the temperature of 
 which has been artificially raised considerably above 
 the normal ; (3) by certain poisons, as by nicotin ; and 
 reflectorially by stimulation of the sensory nerves of 
 the part. There must also be some fibres of commu- 
 nication between the brain and sweat centres, since 
 the influence of mental conditions on the production 
 of sweat is very marked. 
 
 The sebaceous matter secreted by the skin is 
 the product of the cells of the sebaceous glands, 
 situated near the roots of the hair, and opening into 
 the hair-follicles. It is at first fluid, but in process of 
 excretion forms a consistent mass that, as in the alse 
 of the nose in some persons, may be squeezed out in 
 the form of a white wormlike body. Examined under 
 the microscope fat granules are found with the debris of 
 cells, crystals of cholesterin, and, in many instances, a 
 small acaroid animal, the Demodex foUiculorum. Its 
 chemical composition is olein with some soaps and 
 cholesterin, the insoluble phosphates of lime and mag- 
 nesia, and extractives. Its purpose is to lubricate the 
 
Chap. XL] A^'rI\fAL Heat. 143 
 
 skin, diminish excessive evaporation, and render the 
 hairs bright and glossy. The skin of the new-born 
 child is sometimes covered with a layer of this seba- 
 ceous matter, which is then named the vernix caseosa. 
 The functions of the skin as an orojan of sensation 
 will be considered under the head of special senses. 
 
 CHAPTER XI. 
 
 AXIMAL HEAT. 
 
 The chemical processes which are constantly taking 
 place in the living body consist essentially in the 
 decomposition of complex organic compounds under 
 the influence of oxygen, by which various forms of 
 force are set free, and amongst others, heat. Every 
 living body generates heat, but in some the pro- 
 cesses of oxydation are so slow and feeble that the 
 temperature of the body does not materially differ 
 from that of the surrounding medium, whilst in others 
 they are so active that a tolerably constant tempera- 
 ture is maintained. The former class, represented by 
 such animals as the fish and the reptile, are sometimes 
 incorrectly named " cold-blooded," but more properly 
 " poikilothermal ; " and the latter, represented by tlie 
 bird and mammal, improperly " warm-blooded," but 
 more appropriately " homoiothermal." 
 
 Mode of detersniiiing the temperatiire. — 
 The instrument employed for this purpose is the 
 thermometer, and, as very minute differences have to 
 be measured, it is expedient that the bulb should be 
 large and the bore of the stem very fine. For con- 
 venience' sake a small indicator is inserted into the 
 bore in the form of a bubble of air separating a small 
 
144 TIcMAN Physiology. [Chap. xi. 
 
 segment of mercury from the main body, which shows 
 the highest point reached, and obviates the erroneous 
 readinsf that would otherwise occur from the fall of 
 the mercury during the interval between its contact 
 with the body and the examination of its height on 
 the scale by the observer in a good light. In England 
 Fahrenheit's thermometer is in common use, in which 
 zero is the point at which the mercury stands when 
 ice and salt are mixed ; and the space between whicb 
 and that where the mercury stands when placed in 
 boiling water is divided into 212 parts, the freezing 
 point in water being at 32. On the continent the 
 thermometer of Celsius, or the centigrade thermo- 
 meter, is used, in which the freezing point of water, or, 
 rather, the melting point of ice is taken as zero, and 
 the boiling point of water as the upper limit, the space 
 between these two being divided into 100. One degree 
 centigrade is equal to nine-fifths of a degree of Fahren- 
 heit's thermometer. To convert centigrade degrees, 
 or the degrees of Celsius' thermometer, into those of 
 Fahrenheit, multiply by 9, divide by 5, a,nd add 32'^. 
 To convert those of Fahrenheit's scale into centigrade 
 degrees, subtract 32, divide by 9, and multiply by 5. 
 Thus, to convert 40°C. into Fahr., 40 x 9 = 360; 360 -f 5 
 = 72 ; 72 + 32 = 104°. Therefore, 40° C. = 104° Fahr. 
 In order to determine the temperature of the body, or 
 of any part of it, time must be given for the full 
 expansion of the mercury, and this commonly requires 
 two or three minutes. In man, the temperature of 
 the body is usually ascertained by placing the bulb of 
 the instrument in the axilla and covering the stem 
 with the bedclothes. In animals it is often taken by 
 inserting the bulb into the rectum or vagina, 
 
 Temperatiu'e of iiia.ii. — It is remarkable that, 
 notwithstanding the roughness of the instruments at 
 his disposal, the estimate made by John Hunter (99° 
 Fahr., or 37 '2° C.) coincides with the most careful 
 
Chap. XI.] Temperature of the Body. 145 
 
 recent researches. Perhaps 100° Fahr. might be 
 accepted, for the general temperature of the interior of 
 the body. 
 
 Cii'ciinistaiices modif^Tiig the tempera- 
 ture of the body. — Influence of age. — The tempe- 
 rature of the child before birth is a little higher than 
 that of the vagina of the mother, owing to independent 
 production of heat and protection from loss. Shortly 
 after birth the child cools rapidly. During childhood, 
 and up to puberty, the temperature gradually falls 
 about two-tenths of a desjree centigrade. From 
 puberty to the age of fifty it falls about two-tenths 
 more. Charcot remarks that the temperature of the 
 axilla in old people may be as much as 3° C. below 
 that of the rectum. The temperature in old people is, 
 however, nearly as high as in the new-born child, 
 probably owing to anaemia of the skin, and consequent 
 smaller loss by radiation ; but, like infants, they have 
 little power of resisting the depressing influence of 
 cold. Sex has very little influence on the tempe- 
 rature. 
 
 Influence of the 'period of the day . — The results of 
 many experiments show that the temperature of the 
 body rises quickly from 6 a.m. to 10 or 11 a.m., and 
 more slowly up to about 6 p.m., when it begins to fall, 
 reaching its minimum between 4 and 6 a.m. The 
 diiference between the maximum and the minimum is 
 about 1° C. 
 
 Influence of food. — As the oxydation of the ma- 
 terials which compose the food constitutes the source of 
 heat, it is natural that the temperature should be modi- 
 fied both by their quantity and quality, and by the tem- 
 perature at which they are introduced into the body, 
 but the eflects are slighter than might be expected. 
 The temperature rises after food, but only a few 
 tenths of a degree centigrade, and it is said even to 
 decline if alcohol be taken. Hot fluids raise the 
 
 K 
 
146 Human Physiology. [Chap. xi. 
 
 temperature one or two-tenths of a degree, whilst 
 after the ingestion of ice, or of a glass or two of iced 
 water, the temperature may fall one or two degrees. 
 The fall of temperature after the use of alcohol is to 
 be explained by its effect in dilating the capillaries of 
 the skin, thus allowing freer transpiration of watery 
 vapour and radiation of heat, and this supplies a 
 strong , argument against the consumption of alcohol 
 by those who are likely to be exposed to a very low 
 atmospheric temperature. In inanition the oxydising 
 processes continue in the body, which is, so to speak, 
 living upon or burning itself, and the temperature 
 remains unchanged until shortly before death, when it 
 suddenly falls. When the decline amounts to about 
 23° or 24^ C. death invariably occurs. 
 
 Lnft'oence of muscular exertion. — Muscles in con- 
 tracting liberate heat, which is carried off by the 
 blood circulating through them, and warms the body 
 generally. TJie effect, however, soon passes away, 
 owing to the existence of compensatory arrangements, 
 such as more rapid respiration and more rapid circu- 
 lation through the skin. A quick march of the 
 duration of an hour and a half has been found to raise 
 the temperature 1'2° C, whilst in tetanus, in which 
 many muscles are thrown into a state of spasmodic 
 contraction, the temperature has been seen to rise to 
 44*75° C. The same effects have been observed in 
 dogs tetanised by the application of an electiic cur- 
 rent to the spinal cord ; but it is to be noted that 
 there is not only increased liberation of heat in such 
 cases owing to muscular contraction, but that the 
 vaso-motor nerves may be stimulated, causing con- 
 traction of the cutaneous capillaries, and therefore 
 diminished loss. Marcet found that when at rest 
 during the ascent of Mont Blanc the temperature of 
 the body did not vary much at different heights. Re- 
 peated vigorous contraction of a single muscle, like 
 
Chap. XL] Animal Temperature. 147 
 
 the biceps, causes its temperature to rise half a degree 
 C, and the temperature of the body rises nearly half 
 a degree C during labour pains. 
 
 Injiuence of 'mental exertion. — A slight rise of 
 temperature, varying from 0*5° to 1° C, occurs in the 
 whole system as the result of intense mental ejQTort. 
 
 Influence of suri'ounding tentperaticres. — Dr. Davy, 
 who made many experiments in different climates on 
 the temperature of man, found that climate had very 
 little influence on the temperature of the adult white 
 man. It scarcely differs more than 1° in the native of 
 India and in the Icelander, though the absolute 
 amount of heat generated in the two must differ to 
 a very great extent. The larger quantity of heat 
 which the man develops who lives in a temperature 
 where the thermometer falls many degrees below zero, 
 as compared with that generated by those who dwell 
 in torrid zones, where the temperature does not, 
 for a considerable part of the day, differ much from 
 that of their own bodies, is derived, as we have seen, 
 from the nature of the food, which is not only greatly 
 increased in absolute quantity, but consists of com- 
 pounds, like the fats, which in burning give off much 
 Jieat. The degree of moisture in the air is of great 
 importance, the influence of moist hot air being much 
 more marked than of dry hot air, in consequence of 
 the interference in the former case with the reo-ulatinsr 
 action of evaporation from the skin. Many workmen 
 are exposed without inconvenience to temperatures in 
 dry air, which would be quite unbearable were it even 
 partially saturated with moisture, the ill effects of 
 such exposure being prevented and the temperature of 
 the body maintained at the normal standard by drink- 
 ing freely of some watery fluid and by copious perspira- 
 tion. Exposure to very high degrees of heat, espe- 
 cially if accompanied with exercise, is apt to cause 
 death, either by apoplexy, as in the " sun stroke," 
 
148 Human Physiology. [Chap. xi. 
 
 that occurs in summer even in temperate climates, or 
 by " heat tetanus," in which the heart and respiratory- 
 muscles suddenly become rigid. A frog stiffens and 
 dies if plunged into water at a temperature at, or a very 
 little above, that of the blood of a mammal. The 
 greatest cold which has been noted was by Capt. Back, 
 who observed a temperature of— 70° E., but, with suffi- 
 cient food, little inconvenience is felt even by so low a 
 temperature as this, providing the air is still. When 
 the air is in motion, however, it is impossible to face 
 it, and those parts of the body which are exposed, or 
 remote from the heart, as the nose, ears, fingers, and 
 toes, are certain to be frost-bitten or killed. The prac- 
 tice of rubbing such parts with snow to restore anima- 
 tion in them is right, as it prevents the vessels from 
 yielding too suddenly to the influx of blood and becom- 
 ing over-distended. Exposure to cold under unfavour- 
 able circumstances, as under the influence of alcohol, 
 may cause great reduction of the temperature of the 
 body. Thus, in one case recorded, a drunken woman, 
 after falling into a ditch covered with ice, remained in 
 it all night. On admission into the hospital the 
 temperature was found to be reduced to 26° 0. In 
 the course of six hours it had risen to 36 "3° C, and 
 she eventually recovered. 
 
 Caflorisnef I'y* — By calorimetry is understood the 
 measurement of the total amount of heat produced in 
 a given time by the body, and this is ascertained by 
 placing the animal in a cage surrounded by ice or by 
 water for a definite period, and collecting the water 
 which proceeds from the melting ice or ascertaining the 
 elevation of temperature which the water has under- 
 gone. In either case the amount of heat given off 
 can by calculation be exactly determined. Instead 
 of determining the amount of heat generated in this 
 direct fashion, it may also be estimated by indirect 
 methods, one of which is to calculate the quantity 
 
Chap. XL] SoUIiCES OF HeAT. 1 49 
 
 of carbon and hydrogen contained in a carefully 
 analysed diet, and to compare it with, that eliminated 
 by the urine and fseces ; the difference gives the 
 quantity of these elements oxydised in the system. 
 A second method is to ascertain the quantity of 
 oxygen taken into the ceconomy, and the amount of 
 CO2 eliminated by the skin and lungs. The excess 
 of oxygen not employed in oxydising carbon is sup- 
 posed to be applied to the oxydation of hydrogen. In 
 order that a standard of reference may be obtained to 
 which variations in the quantity of heat produced may 
 be referred, it is found convenient to take as a unit of 
 heat the quantity of heat required to raise one kilo- 
 gramme of water at 0° C. one degree. This is called a 
 calory^ and the theory of the correlation of forces, sup- 
 ported by the experiments of Joule and others, has 
 shown that there is a direct relation between heat and 
 mechanical work. The unit of work which has been 
 taken is named a kilogrammeter, and is that amount 
 of force which is required to raise one kilogramme one 
 meter high. It has been found by experiment that 
 the expenditure of one calory is capable of raising 
 430 kilogrammes one meter high. That is to say, the 
 mechanical equivalent of one calory is 430 kilogram- 
 meters ; and, vice versa, friction or a blow caused 
 by the vertical descent of 430 kilogrammeter s through 
 one meter is equal to one calory. The mechanical work 
 of the muscles estimated in kilogrammeters may be 
 reckoned also in calories, since it is sufficient to convert 
 calories into kilogrammeters to multiply them by the 
 number 430, whilst to convert kilogrammeters into 
 calories they must be di^dded by 430. 
 
 Sources of Iieat in the body. — The gene- 
 ration of heat in the body proceeds chiefly from 
 chemical, but in part from mechanical, processes. 
 The chemical actions are the oxydation of the various 
 substances ingested as food, which descend to lower 
 
150 Human Physiology. [Chap. xi. 
 
 and still lower planes of chemical composition, till 
 at length they are eliminated from the body in the 
 forms, already noticed, of water and carbonic acid, and 
 of substances to be hereafter described, such as urea 
 and uric acid, which are discharged by the kidneys j 
 and it is important to remember that the quantity 
 of heat generated by the combustion of these bodies 
 is the same whether they are burnt directly with 
 the free access of oxygen, or whether they pass 
 through a succession of grades of more and more 
 complete oxydation. One gramme of hydrogen in 
 combining with oxygen generates 34'462 calories, 
 and one gr. of carbon 8 "080 calories. One gr. of 
 starch gives 3 '2 calories, one gr. of albumin 4'998 
 calories, and one of fat 9-069 calories. The high 
 heat-generating power of fat is due to the fact that 
 it is entirely burnt up, whilst it contains originally 
 very little oxygen. Starches and sugars have less 
 power of generating heat, because the hydrogen they 
 contain is already partly oxydised, whence their appel- 
 lation of carbohydrates. And albumins, though they 
 contain much carbon and hydrogen, are yet less effective 
 than fats, because they are not wholly consumed in 
 the body, but are eliminated in the form of urea, one 
 gramme of which is still capable, when burnt, of yield- 
 ing more than two calories. Besides the processes of 
 oxydation, the combinations of acids with bases, the ab- 
 sorption of water, as in the decomposition of fats, and 
 the like, act in a subsidiary manner in the production 
 of heat. 
 
 The Hiecliamcal actions generating heat are 
 chiefly those of friction : such as the friction of the 
 blood against the walls of the vessels, and of tendons 
 in their sheaths; but inasmuch as the friction is the 
 result of muscular contraction, which is attended with 
 oxydation, even this may be referred to chemical 
 action. 
 
Chap. XL] HeA T GeNERA TED IN THE BoDY. I 5 1 
 
 The SocaJily of tlie g^eMcratioai of heat. — 
 Lavoisier maintained that as the lungs were the 
 organs bj which oxygen was absorbed, oxydising pro- 
 cesses also took place in them, and that they might 
 therefore be re2:arded as the foci or centres of heat in 
 the oeconomy; but were this the case, the lungs should, 
 be the hottest parts of the body, which experiment 
 shows they are not. The chief centres of heat pro- 
 duction are really the muscles, in which, even when 
 at rest, but especially wdien contracting, active pro- 
 cesses of oxydation occur. A considei'able quantity of 
 heat is also disengaged in all glandular organs v/hen 
 discharging their functions, and the liver, the largest 
 gland in the body, has been shown to develop so 
 much heat, that the blood returning from it (that, 
 namely, which is contained in the hepatic veins) 
 possesses a higher temperature than that of any other 
 part of the body. After the muscles and glands, the 
 brain and nervous system are the principal organs 
 which participate in the production of heat. 
 
 Total quantity of lieat g^eiierated io the 
 body. — If, with Eanke, the diet of a healthy man be 
 reckoned on the following scale, it will be seen to 
 what the total quantity of heat generated per diem 
 ?anounts : — 
 
 100 grains of alViumin give 426"300 calories. 
 100 „ of fat „ 906-900 „ 
 
 240 „ of starch „ 938-880 
 
 Total . . . 2272080 
 
 In round numbers, a man generates per diem 2272 
 calories, which is equivalent to about one million 
 kilogrammeters, i.e., would be capable, if converted 
 into mechanical work, of raising one million kilo- 
 grammes one metre high. The force generated by 
 the oxydation of the food is not altogether, though it 
 
152 Human Physiology. [Chap. xi. 
 
 is in part, expended in tlie performance of external 
 meclianical work. Part is expended in warming the 
 air and food introduced into the body, part is lost 
 by radiation and conduction, and part is used up in 
 the performance of internal work, as in the move- 
 ment of the blood. The body may, therefore, be 
 regarded in the light of an engine, in which the 
 force liberated by combustion becomes apparent 
 partly in the form of heat and partly in the form of 
 work done. 
 
 ISegTila^tion of tlie temperature of tlie 
 body. — As the temperature of the body remains 
 tolerably constant, while both the processes of oxyda- 
 tion and the external conditions which lead to loss of 
 heat undergo considerable variation, it is evident that 
 some regulatory influences must be in operation. The 
 variations in the production of heat depend essentially 
 on the amount of muscular exertion that is made, 
 whilst the variations in the loss of heat depend partly 
 on the state of the body and partly on the temperature 
 of the surrounding medium, and the nervous system 
 constitutes the necessary bond of union by which 
 the conditions leading to the generation and the 
 loss of heat are associated. The chief causes of 
 loss of heat are : (1) The heat required to warm the 
 air inspired. The average daily amount of air 
 inspired, thirteen kilogrammes, which enters the body 
 at a mean temperature of 12° C, and leaves it after 
 being warmed up to 37° C, that is, after it has been 
 heated 25 degrees, is estimated to require 84 calories. 
 (2) The heat lost in heating the food and drink, 
 amounting to 1900 grms., which is estimated at 47 
 calories. (3) The heat lost in cutaneous transpiration, 
 which amounts to 660 grms., and requires 384 
 calories. (4) The heat lost by pulmonary evaporation 
 amounting to 330 grms., and requires 182 calories. 
 These collectively amount to 677 calories lost. The 
 
Cha p. XI . ] Anima l Hea T. 1 5 3 
 
 remaining 1595 calories, required to account for tlie 
 2200, are lost by radiation from the skin. 
 
 The compensatory arrangements, by which the 
 temperature is maintained at a uniform standard, are : 
 (1) In cases of increased generation of heat by pro- 
 moting its loss. (2) In cases of diminished generation 
 of heat, by preventing its loss. For example : If the 
 external temperature to which the body is exposed be 
 high, the arterioles of the skin relax, a freer current of 
 blood passes through the capillaries, and radiation 
 from the surface is increased, whilst at the same time 
 evaporation of fluid, both by sensible and insensible 
 transpiration, takes place, which powerfully contributes 
 to lower the temperature. At high temperatures, 
 moreover, the appetite for food is diminished, and 
 there is consequently diminished production as well 
 as increased loss of heat, and the temperature of the 
 body remains stationary. 
 
 If, on the other hand, the temperature of the 
 surrounding medium is lowered, the arterioles and 
 capillaries of the skin contract, and loss of heat by 
 radiation is greatly reduced ; food is taken in larger 
 quantities, and of a nature like fat to yield more heat 
 on oxydation, and the temperature is again maintained 
 at an equable standard. 
 
 Influence of tlie nervous system on the 
 generation of tieat. — The influence of the nervous 
 system on the production of heat is rendered evident 
 by the division of the sympathetic nerve in the neck. 
 The effect of this is to relax the arterioles of the side 
 of the head corresponding to the lesion, those of the 
 ear, for example, becoming more conspicuous, and the 
 temperature of that organ considerably increased, the 
 difference between the two ears augmenting in pro- 
 portion as the external temperature is low, Various 
 circumstances tend to show that there is a generation 
 of heat in those tissues which thus receive a fuller 
 
154 Human Physiology. [Chap. xi, 
 
 and more abundant current of blood, and especially 
 the fact that, if the head of the animal be enveloped 
 in wool, the venous blood returning from the ear 
 after section of the sympathetic may be actually 
 warmer than the arterial blood passing to it, indicating 
 that a local generation of heat takes place. The 
 function of the sympathetic would therefore appear to 
 be, when in the normal and uninjured state, to bridle 
 the chemical changes, and consequently the develop- 
 ment of heat in the tissues ; it is not only, there- 
 fore, a vaso-constrictor nerve, but also a " frigorific 
 nerve." 
 
 Stimulation of a sensory nerve, such for example 
 as the auricularis, causes diminution of temperature 
 in the ear when the sympathetic is uninjured, but 
 raises the tem])erature of the ear when the sympathetic 
 is divided. In the first case a slight rise precedes the 
 fall, which depends on reflex paralysis of the vaso- 
 dilator nerves, and is not observed if the vaso- 
 dilator nerves are stimulated by cura,re, because the 
 action can then be exerted upon the vaso-constrictor 
 nerves. 
 
 Division of the spinal cord is followed by a 
 gradual fall of temperature till death occurs, the fall 
 being more rapid in proportion as the cord has been 
 divided at a higher point. If the section is made 
 higher than the sixth cervical nerve, the respiratory 
 nerves are divided, and artificial respiration must be 
 maintained. 
 
 The existence of a heat-regulating centre in the 
 spinal cord is still debated. 
 
^5S 
 
 CHAPTER XII. 
 
 THE URINE. 
 
 CI!iaracter§ of tSie itriiie. — The urine is a fluid 
 secreted continuously by the kidney, and is the chief 
 means by which the nitrogenised waste products are 
 discharged from the body. It is a clear amber- 
 coloured fluid, with slight fluorescence, depositing a 
 light cloud of mucus on standing. Its reaction is 
 acid. Its mean specific gravity, 1020. Its odour is 
 aromatic. It should contain no morphological elements 
 except a few epithelial cells. 
 
 Qwaaitity of the iii'me. — The average quantity 
 of urine discharged by a healthy man per diem is 
 about fifty ounces, or two pints and a half; but it 
 varies greatly with the amount of fluid ingested, and 
 with the evaporation of fluid by the skin and lungs. It 
 increases with increase of blood pressure. It is 
 greatest in the morning, less in the evening, and least 
 at night during sleep. The first day after birth it is 
 scarcely more than an ounce, but at the end of the 
 first month it rises to half a pint or more, and 
 from three to five years of age it is about a pint and 
 a half ; it is less in women than in men. It is 
 increased by certain drugs, as potash nitrate and 
 acetate, which are termed diuretics. 
 
 Specific gravity of tfiie urine. — Though the 
 average specific gravity is 1020 it exhibits great 
 variations, the extremes being 1002, which occurs 
 after drinking much water, and 1040 after abstinence 
 from fluid, and copious perspiration. The specific 
 gravity is essentially dependent on the quantity of 
 solids relatively to that of the water. It is estimated 
 
i^b Human Physiology. [Chap. xii. 
 
 bj an instrument termed an Ar'aometer, or urino- 
 meter which consists of a float weighted with mercury, 
 and with a long graduated neck. The graduation 
 begins above at 1000 because the heavier the urine the 
 less deeply will the instrument sink, and the further 
 the neck will protrude from the surface. If very 
 little urine is accessible, the specific gravity may be 
 ascertained by adding two, or three, or four times its 
 volume of water, then taking the specific gra^dty 
 with the urinometer, and multiplying the number 
 obtained by two, three, or four, according to the 
 number of volumes of distilled water that has been 
 added. Attempts have been made to discover an easy 
 method of estimating the quantity of solids in a given 
 quantity of urine, and an approximation may be 
 obtained by multiplying the two last numbers of the 
 specific gravity by 2*2 or by 2 '3. Thus, if urine be of 
 specific gravity 1015; 15 multiplied by 2'2 gives 33. 
 The number 33 represents the number of solid parts 
 in 1000 of such urine. 
 
 Colour of tlie urine. — As a rule it may be 
 said that the larger the quantity of urine, the paler it 
 is ; whilst the smaller the quantity, and the more 
 concentrated it is, the higher the colour. The urine 
 passed on rising in the morning is usually the most 
 deeply coloured of the day. It is called urina 
 sanguinis. That passed after copious draughts is 
 pale, urina 'potus. The urina cibi, or urine passed 
 soon after a meal, is intermediate in colour, and often 
 cloudy. Some drugs, as senna, rhubarb, and especially 
 the prickly pear, confer a deep colour on the urine. 
 The colour of normal urine is due to urobilin, or to 
 some modification of this suljstance, which is again 
 derived from haemoglobin, and is probably a product 
 of its disintegi^ation. 
 
 The reaction of tlie urine. — The acid reaction 
 of urine is due to the presence of acid phosphate of 
 
Chap. XII.] Characters of Urine. 157 
 
 sodium, and not to free acid. The acid sodium 
 phosphate occurs in the urine in consequence of uric 
 acid, hippuric acid, and COg each seizing a portion of 
 the sodium of the basic phosphate. The acid reaction 
 is increased by fasting, muscular exercise, and the 
 ingestion of acids, whilst it is diminished, and may be- 
 come neutral or even alkaline, after food, apparently 
 as the result of the separation of acid from the blood 
 in the gastric juice leaving an excess of alkali. The 
 ingestion of caustic alkalies and their carbonates, and 
 of the salts of the vegetable acids, such as the tar- 
 trates, malates, and citrates, which become converted 
 into carbonates in passing through the body, renders 
 the urine less acid, neutral, or even alkaline. 
 
 The reaction of urine is ascertained by dipping 
 into it a strip of violet litmus paper, which, when 
 placed in acid urine, becomes red, and when in 
 alkaline urine, blue. The degree of acidity is 
 determined by finding how much solution of soda 
 is requisite to make a definite quantity of urine 
 exactly neutral. 
 
 Cliemical reactions of tlie mine. — The 
 addition of hydrochloric acid to urine renders it 
 darker and causes the precipitation of uric acid 
 crystals in the course of twenty-four hours. With 
 ^reat excess of hydrochloric acid urine becomes 
 reddish-brown, violet, or blue. The addition of 
 sulphuric or nitric acid deepens the colour of urine, 
 and if the nitric acid be cautiously added, the surface 
 of contact of the two fluids presents a reddish hue. 
 The addition of "picric acid causes crystals of uric acid 
 to be precipitated. If acidulated with nitric acid, 
 and immediately treated with 'phospho-r)iolyhdic acid, 
 and made to boil, it assumes an indigo-blue tint. The 
 addition of alkaline solutions makes urine cloudy from 
 the precipitation of the phosphates of lime and mag- 
 nesia. Urine causes the blue colour of iodine and 
 
158 Human Physiology. [Chap. xii. 
 
 starch to disappear. A precipitate forms on tlie ad- 
 dition of barium chloride as well as with nitrate of 
 silver, acetate of lead, and oxalate of ammonia ; dilute 
 solution of mercury nitrate renders it hazy, the cloud 
 disappearing on agitation. Lastly, when heated with 
 an ammoniacal solution of copper oxide it destroys its 
 blue colour (Beaunis). 
 
 Cliemical composition of tlie urine. — The 
 urine contains on an average about 60 grammes 
 (about 900 grains) of solids in twenty-four hours, 
 of which 40 grammes (600 grains) are organic, and 
 20 grammes (300 grains) are inorganic. 
 
 The organic substances are partly azotised, partly 
 non-azotised. The former includes urea ; uric and 
 hippuric acids ; creatinin, xanthin, oxaluric acid ; 
 and sometimes aliantoin. The latter includes oxalic 
 and lactic acids, and glycose. Then there are certain 
 compounds of sulphuric acid named phenol-sulphuric 
 and cresol- sulphuric acid, and sulpho-pyrocatechuic 
 acid, and certain colouring matters, as urobilin. 
 
 Lastly there are the inorganic substances, which 
 include sodium and potassium chloride, acid sodium 
 phosphate, phosphate of lime and magnesia, alkaline 
 sulphates, and traces of ammonia and iron. 
 
 Many substances appear exceptionally in the urine 
 either after the consumption of certain kinds of food, 
 or in particular states of the constitution. Thus 
 albumin may appear if a large quantity of white 
 of egg be consumed. So also peptones are sometimes 
 eliminated with the urine, and a diastatic ferment. 
 In other instances mucin, inosite^ hypoxanthin, 
 leucin, tyrosin, and cystin are discharged. The 
 appearance of albumin in large quantities, as shown by 
 the formation of a dense precipitate on boiling, and 
 the addition of nitric acid, is a well-known sign of a 
 serious disease of the kidney, Bright's disease or 
 albuminuria. The urine in this affection is uniformly 
 
Chap. XII.] Composition of the Urine. 159 
 
 of low specific gravity, usually below 1010. The 
 presence of sugar in the urine in considerable quantity 
 is the characteristic feature of the disease known as 
 diabetes, and the specific gravity of the urine in this 
 affection is often high, sometimes remaining for a 
 lengthened period at 1040. 
 
 The organic constitiieots of tlie iirine. — 
 (1) Vrea CO(NH2)2. The biamide of CO2, or car- 
 bamide. This is the most important constituent of 
 urine, for iu man it is the substance which con- 
 tains by far the largest proportion of the waste 
 nitrogen of the body ; in fact, when the diet is such 
 that the weight of the body is preserved the same from 
 day to day, it may be accepted that almost the whole 
 of the nitrogen which enters the body in the food 
 is discharged from it in the form of urea. The healthy 
 adult Englishman performing moderate work excretes 
 about 500 grains of urea per diem, or about 1 oz. av., 
 of which nearly one half by weight is nitrogen. The 
 quantity is not materially different in the French 
 and Germans, in whom it is given as varying from 
 thirty to forty grammes, with an average of about 
 thirty-four grammes (which is 524 grains), though 
 their food differs somewhat in containing less meat. 
 Women excrete less urea than men. Children, by 
 reason of their activity, relatively more. In old age 
 the quantity falls considerably. The quantity excreted 
 may be expressed differently by stating that it is in 
 the proportion of 0*5 grammes (7*o grains) for every 
 kilogramme (2 lb. 2 oz.) of body weight. Blood 
 contains 0'025 parts per cent, of urea. 
 
 Physical and chemicai characters of isi'ea. 
 — Urea is a crystalline substance, vefy soluble in 
 water and in alcohol, but almost insoluble in ether. 
 It dialyses with great rapidity. When rapidly crystal- 
 lised, the crystals are acicular, but, when slowly 
 crystallised, they form four-sided prisms, with oblique 
 
i6o Human Physiology. [Chap. xii. 
 
 extremities, belonging to the rhombic system. They 
 are without smell, but have a cool taste like that of 
 nitre. Considerable interest is attached to urea, 
 from the circumstance that it is capable of being 
 formed artificially. Thus, it can be obtained from a 
 solution of ammonium cyanate, with which it is 
 isomeric, by evaporation. When heated to 120° C, it 
 is decomposed into ammonia, which is volatile, and a 
 residue of biuret and cyanuric acid. In putrefying 
 urine, or when treated with strong mineral acids, or 
 when boiled with the hydrates of the alkalies, urea 
 takes up two equivalents of water, and becomes 
 converted into ammonium carbonate, the reaction 
 being represented by the formula CO(NH2)2 + 2H2O 
 = CO(0]SrH4)2. When acted upon by nitric acid, it 
 breaks up into water, CO2, and N. 
 
 The separation of iirea in a pnre state. — 
 (1) Evaporate the urine of a dog, which has been well 
 fed with meat, to a syrupy consistence ; add alcohol ; 
 filter and evaporate the alcoholic extract; set aside 
 to allow crystals to form. 
 
 (2) Evaporate human urine to one-sixth of its 
 bulk j cool to freezing point ; add nitric acid. Nitrate 
 of urea falls, with colouring matter of urine. Sepa- 
 rate the precipitate by filtration, dissolve in boiliiig 
 water, and pass through animal charcoal. On cooling, 
 crystals of urea nitrate form. Dissolve these in hot 
 water, and add barium carbonate till efiervescence 
 ceases. The fluid now contains barium nitrate and 
 pure urea. Evaporate it ; exhaust with alcohol, and 
 set aside to crystallise by slow evaporation. 
 
 Tests for nrea in urine. — (1) Evaporate urine 
 to half its bulk, and add strong HNO3. Impure 
 urea nitrate separates out. 
 
 (2) Russell and West^s test. — This test consists 
 essentially in decomposing urea into water, carbonic 
 dioxide, and nitrogen gas. The quantity of the 
 
Chap. XII.] Tests for Urea. i6i 
 
 latter produced is a measure of the quantity of urea 
 originally present. The method adopted to effect the 
 decomposition is to dilute the urine to be examined, 
 and to mingle it suddenly, by a special arrangement, 
 with a solution of sodium hypobromite and caustic 
 soda in a test-tube inverted over water. Decomposi- 
 tion immediately takes place according to the follow- 
 ing formula : CO(NHo), + 3]S^aBrO + 2KaH0 = 
 sNaBr ^ 3H2O + NaoCOs + IST^. That is, Urea + 
 Sodium hypobromite + Caustic soda = Sodium 
 bromide -h Water -1- Sodium bicarbonate + Nitro- 
 gen. The nitrogen thus produced is given off as gas, 
 and displaces the water in the graduated tube which 
 is held over it. The gas is at first evolved briskly, but 
 afterwards more slowly ; to facilitate its evolution, 
 the bulb of the tube may be slightly warmed with a 
 spirit-lamp. After ten minutes, the amount of water 
 displaced by the gas should be read off on the tube, 
 which is divided into tenths. Each number on the 
 tube represents one gramme of urea in 100 centi- 
 metres of urine. Normal urine should yield roughly 
 1 "50 parts of nitrogen by this test. If urine contain 
 albumin, it should be first heated with two or three 
 drops of acetic acid and then filtered (Harris and 
 Power). 
 
 (3) Biuret test. — Heat urea crystals cautiously in 
 a dry test-tube till the smell of ammonia ceases to be 
 perceptible ; then add a few drops of solution of 
 caustic potash and of copper sulphate, and a violet- 
 red colour appears. 
 
 (4) Liebig's test — Forty cubic centimetres of urine 
 are collected in a glass. The sulphuric and phosphoric 
 acids are removed by adding 20 cc. of solution of 
 baryta. The liquid is filtered, and 15 cc, containing 
 10 cc. of urine, placed in a glass. To this is added, 
 drop by dro]D, a test solution of silver nitrate (of 
 which 1 cc. unites with 10 mm. of urea) until no 
 
 L 
 
1 62 Human Physiology. [Chap. xii- 
 
 further precipitation takes place. Soda solution is 
 added to neutralisation. If now one drop of the 
 mixture and one drop of a pap of sodium bicarbonate, 
 placed upon a watch-glass, give a yellow colour, all 
 the urea may be considered to be precipitated. The 
 quantity of the test liquid used is read off, and, as 
 each centimetre represents 10 milligrams of urea, the 
 quantity of urea in 10 cc. is easily determined by 
 multiplying by 10. The solution of baryta employed 
 in this test is made of 1 vol. cold saturated solution of 
 barium nitrate and 2 vol. cold saturated solution of 
 caustic baryta. 
 
 CirciMiistaiaces Buodifyiiag" tlie excretioBi of 
 urea. — (1) The nature and quantity of the diet. — If 
 the food contain much albumin, casein, glutin, or 
 other proteid, the amount of urea is increased. If, 
 on the other hand, the food contain but little nitrogen, 
 the quantity of urea diminishes. A diet rich in pro- 
 teids will cause the urea eliminated per diem to rise 
 from about 35 grms. to 80 and even to 100 grms. in 
 twenty-four hours, as is seen in some diabetic patients 
 who eat an enormous quantity of food ; whilst a 
 farinaceous and vegetable diet makes it fall to 20 grms. 
 Though in greatly diminished quantity, it still con- 
 tinues to be eliminated when food is wholly with- 
 drawn ; and, under these circumstances, there is a 
 diurnal increase about mid-day and a diminution in 
 the early hours of the morning. The consumption of 
 food at each meal is followed by a rise in the quantity 
 of urea eliminated. 
 
 (2) The influence of muscula,r exercise. — The 
 experiments that have been made both on animals 
 and man show that muscular exertion causes a slight 
 increase in the amount of urea excreted. The ex- 
 periments that are most relied on to establish this are 
 those of Yoit on the dog, those made by Flint and 
 Parkes on Mr. Weston the pedestrian, and those by 
 
Chap. XII.] Excretion OF Urea. 163 
 
 Parkes on soldiers. In Yoit's experiments, a large 
 dog, weighing about seventy pounds, was selected, 
 and carefully trained, first, to perform certain regular 
 work, as the turning of a tread-mill, the force required 
 to accomplish which had been calculated with great 
 nicety ; and, secondly, to evacuate the contents of the 
 bowels and bladder at stated intervals. The excreta 
 of the dog, when under ordinary conditions and with 
 his food and body weight in equilibrium, were then 
 examined chemically, both in the fasting state and 
 when consuming a minimum diet. It was then made 
 to work, by turning the tread-mill for ten minutes at 
 a time six times during the day, (1) when fasting 
 except from water, and (2) when supplied with just 
 sufiicient food to cover loss when no work was done. 
 The results showed that there was an increase in the 
 quantity of urea when work was done, but that the 
 amount of increase was very small, viz., an increase 
 of from 0*1 to 0*3 gTamme in the fasting experi- 
 ments when the total in repose was about one 
 gramme per diem; and an increase of from 0'3 to 
 0'7 gramme in the experiments with food when the 
 excretion of the animal at rest was about 7 grammes 
 per diem. Voit drew the conclusion that, during 
 work, the substance of the muscle cannot undergo 
 any large amount of disintegration, as was generally 
 supposed before his experiments were made, and that 
 the force exerted must be derived from the oxydation 
 of other materials, of which the fat of the body or of 
 the food was the most probable. 
 
 The experiments of Fick and Wislicenus afforded 
 important confirmation of the general truth of Voit's 
 statement. These observers climbed an Alpine peak, 
 the Faulhorn, the height of which is 1,956 metres, on 
 food from which nitrogen was excluded. The work 
 done in the case of Fick, who weighed 66 kilogram- 
 mes, was 129,096 kilogrammeters, and in the case of 
 
164 Human Physiology. [Ciiap. kii. 
 
 Wis! icenus, whose weiglit was 76 kilogrammes, 148,656 
 kilooframmeters. In addition, other muscular work 
 was done within the body of each man, as the action 
 of the heart and of the muscles of respiration, tlie 
 exertion required to maintain the erect position and 
 the movements of the arms, which would all add 
 their quota to the products of muscular exertion. 
 They took no albuminous food for seventeen hours 
 previous to making the ascent, none during the 
 ascent, which occupied eight hours, and none for six 
 hours after ; but they did take a moderate amount of 
 non-azotised food, consisting of cakes made with rice, 
 fat, and sugar, with beer, tea, and wine, as solid food. 
 The urine of different periods of the experiment was 
 carefully tested to determine the amount of urea. 
 The periods selected were, that passed on the night 
 before the ascent, that passed on gaining the summit, 
 that passed after the descent, and, lastly, that passed 
 after a full meal had been taken. The comparison of 
 these specimens of urine showed that in both observers 
 there was a slight decrease in the amount of urea in 
 the urine during the ascent and after the descent, 
 as compared with the period before the ascent, the 
 quantity discharged by Tick in grammes per hour 
 during the four periods indicated being 0-63, 0*4:1, 
 0-40, and 0*45, whilst, in Wislicenus, it was 0'61, 
 0*39, 0*40, and 0-51, the unanimity of the observa- 
 tions on the two men being remarkable. It would, 
 therefore, appear that in man, as in the dog, muscular 
 exertion causes no perceptible disintegration or waste 
 of the proper muscular tissue. 
 
 The chief element of possible error in this conclu- 
 sion lies in the fact that it is assumed that muscular 
 tissue, if wasted at all, would cause an immediate 
 increase in the quantity of urea in the urine, but it 
 is clearly possible that the decomposition of muscle 
 into urea may not be immediate, and that other 
 
Chap. XII.] Excretion OF Urea. 165 
 
 nitrogenous compounds may be formed, whicli may 
 remain for some time in the body, and be only slowly 
 eliminated from tbe body, and perhaps in some other 
 form than urea. Gamgee, however, considers that 
 Fick and Wislicenus' experiments show beyond all 
 doubt that during and after muscular contraction no 
 quantity of effete nitrogenous material passes out of 
 the body which is at all adequate to effect the 
 mechanical work done in contraction. What they 
 do not show is, whether or not any nitrogenous 
 waste occurs in muscle during activity. If the 
 urea be taken as a measure of the decomposition 
 of albumin or proteids in the body, it may be 
 calculated that the quantity of proteids decomposed 
 by Fick was 37 "17 gTammes, and by Wislicenus 
 37 grammes. These numbers afford data for deter- 
 mining the exact amount of force generated in 
 the oxydation of this quantity of proteids, and from 
 Frankland's researches it is certain that the total 
 burning of 37 grammes of albumin would only yield 
 about 80,000 kilogrammeters of force ; but we have 
 already seen that Fick required upwards of 129,000, 
 and Wislicenus of 148,000, kilogra,mmeters for the 
 mere ascent, without reckoning the amount of force 
 required for the operations carried on within the 
 body. Experiments, similar to those of Fick and 
 Wislicenus, were undertaken by Professor Haughton, 
 who found that with a daily walk of five miles for five 
 consecutive days the quantity of urea eliminated was 
 501*28 grains, whilst with a daily walk of 20 miles, 
 other conditions remaining unchanged, the amount of 
 urea discharged was 501-16 grains, or a trifle less than 
 before. Prof. Parkes made a still more interestins: 
 series of researches on two soldiers at ISTetley. In one 
 series the effects of work and of rest were contrasted 
 in regard to the elimination of urea on a diet which 
 was abundant in quantity but contained no nitrogen, 
 
1 66 Human Physiology. [Chap. xii. 
 
 and in a second series the effects of work and rest were 
 contrasted on a normal diet containing nitrogen. In 
 both sets of experiments it was found that there was 
 a slight total increase of nitrogen eliminated during 
 muscular exertion, though there was, as in Haughton's 
 and in Tick's experiments, at first, and for about 36 
 hours, a slight decrease in the amount of nitrogen 
 eliminated when work was performed. The decrease 
 was subsequently over-compensated by an increased 
 discharge of urea. In the experiments on. Weston, 
 who, in one instance, walked 100 miles in 21 hours 
 39 minutes, and subsequently attempted, but failed, 
 to walk 400 miles in five consecutive days, were im- 
 portant, because Dr. Flint determined on the last 
 occasion the elimination of nitrogen by the urine for 
 five days before the walk and for five days after it. 
 Mr. Weston was 31, weighed 126 lbs., smoked mode- 
 rately, and was an almost total abstainer from alcohol. 
 A sample of every kind of food consumed was care- 
 fally analysed. During the five days of the walk 
 Weston consumed in all 1173-8 grains of nitrogen in 
 ])is food, and he eliminated 1807 grains of nitrogen in 
 the urine and faeces. This leaves 633-8 grains of 
 nitrogen over and above the nitrogen of the food, 
 which it seems probable must be attributed to the 
 waste of the tissues, and probably almost exclusively 
 to the waste of muscular tissue. In this experiment 
 Mr. Weston broke down on the morning of the fourth 
 day clearly from utter exhaustion of the nervo- 
 niuscular apparatus, and ifc is probable that this was 
 the result of an insufficient supply of food. It is 
 enough to add that in Pavy's experiments on Weston, 
 though he lost weight, the nitrogenous waste during 
 the walking period was incompetent to account for 
 the mechanical v/ork done. 
 
 The results of all these experiments, then, seem to 
 demonstrate that there is a slight increase in the total 
 
Chap. XII.] Origin of Urea. 167 
 
 excretion of nitrogen after exercise, and this probably 
 in part proceeds from the disintegration of muscular 
 tissue. But the relation of waste of the proper tissue 
 of muscle to the amount of work done may not 
 inaptly be compared with the loss undergone by the 
 iron framework of a locomotive whilst running. 
 During action a part of the framework is disin- 
 tegrated and cast off, but the wear and tear of 
 the machine is comparatively trifling ; the real 
 source of the power is the fuel in the one in- 
 stance and the carbohydrates and hydj'ocarbons of 
 the food in the other. 
 
 Origin of iirea. — Urea, with carbonic acid and 
 water, must be regarded as the final stages of the 
 regressive changes through which the albuminous 
 compounds pass in their transit through the body. 
 Some, but not all, of the steps by which the proteids 
 are converted into urea are known. In the alimentary 
 canal, for example, they are first converted by the 
 action of the gastric juice into peptones, and these 
 again, by the action of the pancreatic ferment, yield 
 leucin, glycin, tyrosin, and asparaginic acid, whilst, 
 by putrefaction, the proteids yield various salts of 
 ammonia. Similar changes take place in the pro- 
 teids in the organs and tissues of the body after 
 their absorption in the blood, and a number of 
 compounds have been isolated, such as allantoin, 
 alloxan, xanthin, hypoxanthin, guanin, and uric 
 acid, which are so closely allied to urea that some, 
 especially the last two, replace it in the urine of 
 various animals, or appear instead of urea as the 
 result of some disturbance of the ceconomy, whilst 
 the relationship is further shown by the circumstance 
 that if administered with the food they greatly 
 augment the quantity of urea excreted. The best 
 physiological chemists admit, however, that it is not 
 at present possible to arrange a table in which the 
 
1 68 Human Physiology. [Chap. xii. 
 
 albuminous compounds shall appear at the head of the 
 list and urea at the bottom. An approximation may 
 be made, no doubt ; but there must be many gaps, and 
 it seems probable that urea proceeds either (1) from 
 a combination of carbonic acid and ammonia with 
 subtraction of water ; or (2) from a conversion of 
 carbaminate of ammonia j or, lastly, from cyanate of 
 ammonia. 
 
 If the antecedents of urea in the body are still 
 somewhat uncertain, the same obscurity hangs over 
 its place of origin. It is natural to suppose that, as 
 the liver secretes bile, or the parotid glands saliva, 
 the kidneys form urea ; and it is to be observed that 
 they receive an unusually large supply of oxygenated 
 blood, so that it is not improbable that oxydising 
 processes take place with activity in their cells ; but, on 
 the other hand, it is found that urea is not only 
 constantly present in the blood and lymph, but that it 
 accumulates in the system after extirpation of the 
 kidneys. And there is other evidence, such, for 
 example, as the great decrease in the quantity of 
 urea excreted in fatty degeneration and in other 
 diseases of the liver, which indicates that this 
 organ, and perhaps the lymj^hatic glands, are places 
 where urea is always being generated. One or two 
 observers have, however, noticed that the quantity of 
 urea in the blood after ligature of the ureters is much 
 greater than after ablation of the kidneys, which 
 supports the view that the kidneys themselves aid 
 in its production ; and if this be admitted it is 
 probable that it proceeds from the metamorphosis 
 of kreatin, the quantity of which is always in- 
 creased in the muscles after the ureters have been 
 tied. 
 
 Uric acid CgH^lST^Os. — This compound is closely 
 allied to urea, and is the chief mode in which nitro- 
 gen is eliminated from the body by birds, reptiles, 
 
Chap. XII.] Tests for Uric Acm 169 
 
 and insects. The ordinary quantity that is discharged 
 in a healthy adult per diem is about 0*75 gramme, 
 or 10 grains, the proportion to urea being about 
 1 : 50; but if the diet be highly nitrogenous it may 
 rise to 2 grammes or more. Uric acid is colourless, 
 without taste or smell, crystallises in various forms, 
 the type of which is the rhombic tablet. It does 
 not exist in the free state in the urine, but forms 
 an acid urate of sodium and potassium. It is very 
 insoluble in water, one part only being dissolved 
 in 18,000 of cold and in 15,000 of hot water. Hence 
 when formed in excess it is apt to apjoear as a red 
 sediment in the urine, to accumulate in the pelvis 
 of the kidneys, forming a singularly painful form of 
 stone, and to be deposited, in combination with bases, 
 in the joints. Uric acid is a less perfectly oxydised 
 compound of nitrogen than urea, and therefore makes 
 its appearance when with abundant food there is 
 insufficient respiratory activity. 
 
 Tests for uric a.eid. — 1. The addition of a 
 few drops of hydrochloric acid to urine causes uric 
 acid to separate in thQ course of a few hours in the 
 form of crystals, the form of which may be recogTiised 
 under the microscope. 
 
 2. TJte murexide test. — Uric acid or urates when 
 gently heated in a saucer with nitric acid become 
 yellow and decompose into N and CO2, which are 
 volatile, and urea and alloxan, which remain. If these 
 are slowly evaporated to dryness, and a drop of 
 ammonia liquor being added murexide or ammonium 
 purpurate forms with the production of a purple-red 
 tint, which is very characteristic. 
 
 3. Silver test. — Drop a drop of the fluid containing 
 a urate or uric acid dissolved in an alkaline carbonate 
 upon a piece of blotting paper saturated with solution 
 of silver nitrate, and a black spot appears, owing to 
 reduced silver. 
 
170 Human Physiology. [Chap. xii. 
 
 Quantitative d.eter5iimatl©Bi of Mric acid 
 [SalkowskVs metlwcT). — Take 200 centimetres of urine, 
 and make strongly alkaline with sodium, carbonate. 
 In tlie course of an hour add 20 centimetres of con- 
 centrated solution of ammonium chloride, which 
 causes a precipitate of acid ammonium-urate. The 
 mixture is set aside for 48 hours in a cool place, passed 
 through a weighed filter, and washed. Sufficient 
 dilute hydrochloric acid is poured on the filter to 
 dissolve all the ammonium urate, the filtrate being 
 received in a clean glass. After six hours' standing, 
 the whole of the uric acid separates, and is collected 
 and placed on the same filter. The filter is twice 
 washed with water and with alcohol till the acid 
 reaction disappears. It is then dried at 110° C. and 
 weighed. To the weight must be added, in addition 
 to the weight of the original filter, 0-030 gramme. 
 
 The place ©I" origin of nric acid. — Experi- 
 ments to determine this point have been made on 
 birds and on snakes, and it has been found that a few 
 hours after the ligature of the ureters in birds, the 
 canaliculi of the kidneys are filled with urates (which, 
 however, are not found in the Malpighian capsules), 
 and that at a subsequent period a deposit of urates 
 takes place on the surface of all the serous mem- 
 branes, the lymphatics of which are completely 
 occluded by amorphous precipitate of these salts, 
 on the joint ends of bones, in the parenchyma of 
 the lungs, and elsewhere, whilst the blood, which 
 under normal conditions is free from uric acid, 
 contains a quantity large in proportion to the time 
 that has elapsed after the performance of the opera- 
 tion. Parallel experiments in snakes are followed 
 by the same effects ; but if in these reptiles the 
 kidneys are extirpated, very little deposit of the 
 urates occurs, and none is found in the muscles, 
 lungs, or liver. The conclusion is therefore obvious, 
 
Chap. XII.] Other Constituents of Urine. iji 
 
 that uric acid is chiefly formed at the kidneys, and 
 by the action of the cells of those organs. 
 
 Kreatimn C4H7^30. — This substance proceeds 
 from the kreatin contained in muscle, from which it 
 may be obtained by heating it in water, when HgO 
 is given off. From 0*5 to 1"5 gi*ammes are eliminated 
 per diem by an adult. It does not appear to be 
 contained in the urine of the infant at the breast. It 
 is a strong base, crystallising in colourless oblique 
 rhombic prisms. It is increased on an albuminous 
 diet, and diminishes materially when no food is 
 ingested. When boiled with baryta water It breaks 
 up into urea and sarkosin. 
 
 Hippuric acid. C9H9XO3 is found in small quan- 
 tity in the urine, about one gramme being eliminated 
 per diem, especially after the use of certain articles of 
 diet, as after asparagus, greengages, and the ingestion 
 of benzoic, kinic, and cinnamic acids. It is largely 
 contained in the urine of herbivora, and in them pro- 
 ceeds from the cuticular tissue of plants, which is 
 nearly allied to it in composition. 
 
 Xantliin C5H4N4O2 is an amorphous yellowish- 
 white powder, which dissolves with tolerable facility 
 in boiling water. Traces of it are found in the nervo- 
 muscular system, and in some glands. It exists in 
 urine in the proportion of about one gramme in. 300 
 kilos of urine. It is intermediate in composition 
 between sarkin and urea. Heated with a drop of 
 nitric acid it yields a yellow stain, which becomes 
 yellowish-red with potash, and on further heating 
 violet-red, 
 
 HjTJOxaiitlim or Sarl^iii C5H4^40. — This 
 substance has only been found in the urine in 
 leucsemia, but has been obtained from muscles and 
 various glands. It is of interest on account of its 
 near relationship to urea and to xanthin. From the 
 former it can be obtained by the r.ction of hydrogen 
 
172 Human Physiology. [Chap. xii. 
 
 upon it in the nascent state, and it can be converted 
 into tlie latter. 
 
 Oxaliu^ic acid C3H4N2O4. — Tliis substance is 
 found onlj in small quantity in the urine in the form 
 of ammonium oxalurate. 
 
 Aliantoiu C4Hg]Sr403. — Allantoin is found in 
 the urine of the new-bom child for the first few days 
 after birth. It is especially interesting from its re- 
 lation to uric acid. For it can be shown that uric 
 acid under the influence of oxydising agents yields 
 alloxan and urea. Alloxan by oxydation yields COg- 
 and parabanic acid. Parabanic acid -h HjO gives rise 
 to oxaluric acid, and oxaluric acid when dissolved 
 in water and heated yields oxalic acid and urea. 
 
 II. NONAZOTISED CONSTITUENTS OP THE XJrINE. 
 
 Oxalic acid C2H2O4. — Oxalic acid, which has 
 just been shown proceeds from the decomposition 
 of oxaluric acid, exists in small quantity in the urine 
 in the form of oxalate of lime ; it is augmented by the 
 use of all foods containing oxalates, as tomatoes and 
 rhubarb, and by fruits containing citric acid. 
 
 I^actic acid CgHeOg. — This acid is found in the 
 urine after violent muscular exertion, and is probably 
 always present in minute proportion. 
 
 Sitgar. — The quantity of sugar in normal urine 
 is, if any, very small, though in conditions of disease 
 it increases to a remarkable extent. It is sometimes, 
 in abnormal conditions, replaced by inosite. 
 
 Succinic acid C4H6O4 is found after meat 
 diet, and especially after eating asparagus and after 
 the ingestion of alcohol. 
 
 III. Conjugated Sulphur Acids of the Urine. 
 
 Indican. — ^This substance, sometimes named iru- 
 digogen, results from the combination of SO.H with 
 
Chap. XII.] Colouring Matters of Urine. 173 
 
 indol. It is a brownish-yellow, bitter, disagreeably- 
 tasting, syrupy substance, which is a stronger acid 
 than acetic or hippuric acid. It exists in small pro- 
 portions only in the urine. Its quantity is increased 
 by any circumstance that delays the passage of nitro- 
 genous aliment through the intestine, as by ligature of 
 the intestine. It is augmented after the ingestion of 
 indol. Its presence in the urine may be demonstrated 
 by adding to a drachm of urine some strong hydro- 
 chloric acid with a drop or two of nitric acid. On heat- 
 ing, a violet-red colour appears, with the separation of 
 rhombic crystals of indigo-blue and indigo-red. The 
 same change is induced by putrefactive changes, and 
 a coloured pellicle consisting of microscopic crystals of 
 indigo-blue may not unfrequently be seen on the 
 surface of decomposing urine. 
 
 Plieiiol CeHgO, or carbolic acid, exists in the 
 urine in combination with sulphuric acid, as phenol- 
 sulphuric acid, CgHjO + SO3H, which forms salts with 
 the alkalies. The pheno-sulphates form about one- 
 tenth of the sulphates eliminated by the kidneys. The 
 quantity of phenol is increased by the ingestion of 
 phenol, tyrosin, benzol, and indol. The acid named 
 creso-sulphuric, and the sulpho-pyrocatechuic acids, are 
 occasionally present. 
 
 lY. Colouring Matters of the Urine. 
 
 XJrotoiliii. — This substance is a product of the 
 metamorphosis of haematin, and is associated with 
 the colouring matter of the bile. It confers upon 
 the urine its red or reddish-yellow colour. It is 
 especially abundant in the urine of febrile patients. 
 Another substance, termed urochrome, has also been 
 recognised, which oxydises when exposed to the air, 
 and yields uroerythrine, which colours the precipitates 
 of sodium urate. 
 
174 Human Physiology. [Chap. xii. 
 
 V. Salts of the Urine. 
 
 Tlie more important salts contained in the urine 
 are: — 
 
 1. Sodiiisn cliloride, of which about 11 or 12 
 grms, are excreted per diem, or 0'176 grm. per kilo 
 of body weight. Women excrete less, and children still 
 less. There are two periods of the day at which 
 maximum quantities are eliminated, one in the 
 morning and the other in the afternoon. It diminishes 
 to two or three grams in inanition. It is increased 
 by the ingestion of food, and by muscular and nervous 
 work. 
 
 2. Pliosplia,tes. — The quantity of phosphoric 
 acid daily discharged is, on the average, 2*8 grms., 
 or 044 ju per kilo of body weight. One-third 
 of this is combined with lime and magnesia, the 
 other third with the alkalies sodium and potassium. 
 The quantity of the phosphates eliminated is 
 augmented by abundant food, by muscular work, 
 and by the ingestion of phosphates. They are in- 
 creased by mental work. The elimination of phos- 
 phates is, for obvious reasons, less free during preg- 
 nancy, and during the early years of growth and de- 
 velopment. 
 
 3. SulpSiates. — A man eliminates about two 
 grammes of sulphuric acid per diem, or 0'032 gramme 
 per kilo of body weight. The quantity is increased 
 by food and by muscular exertion. 
 
 4. Ammoiiia. — The quantity of ammonia elimi- 
 nated by the urine in twenty-four hours is about 0*7 
 or 0*8 gramme. It is increased by some articles of 
 diet, as asparagus. 
 
 Spontaneous chang^cs in urine on stand- 
 ing. — Urine, on standing in a cool place, first becomes 
 more acid, owing to the development of an organic 
 ferment (fungus), which acts on the minute quantity 
 
Chap. XII.1 AfoDE OF Secretion OF Urine. 175 
 
 of sugar contained in the urine, and leads to the 
 development of lactic and acetic acids, the presence 
 of which causes the precipitation of uric acid, 
 acid sodium urate, and lime oxalate, rendering the 
 urine cloudy. After the lapse of some time the urine 
 passes into alkaline or ammoniacal fermentation, a 
 bacilliform microccus urince appearing, which de- 
 composes urea with addition of water into ammo- 
 nium carbonate. Urea CO(NH2)2 + 2H0O = am- 
 monium carbonate C03(]S^H4)2. 
 
 Mode of secretion of the iirioe. — The con- 
 ditions under which the urine is secreted differ from 
 those of all other glands in the circumstance that the 
 capillary vessels constituting the tutts of Malpighi 
 enter, without the intervention of any lymph spaces, 
 directly into relation with the gland tissue, separated 
 from it only by a delicate layer of epithelium and the 
 capillary wall. In other parts of the gland, however, 
 the usual arrangements prevail. Mr. Bowman pointed 
 out long ago that the disposition of the blood-vessels 
 in the Malpighian capsule was eminently favourable 
 to the transudation of the watery parts of the urine, 
 and that it was probable the salts were also excreted 
 at this point, whilst the essential constituents of the 
 urine, as the urea, were eliminated by the cells lining 
 the convoluted portion of the tube, and were washed 
 away by the fluid coming down from the capsule. By 
 Ludwig, on the other hand, and his school, the urine 
 is believed to be secreted in a dilute condition, with 
 all its constituents, in the Malpighian tufts, with a 
 rapidity varying with the blood pressure. As it de- 
 scends the tubes it becomes more or less concentrated, 
 in accordance with the laws of diSusion and the 
 density of the fluids moving in the spaces surrounding 
 the convoluted portion of the tubes. The result of 
 recent enquiries, and especially those made by Hei- 
 denhain, on the whole favour the view taken by 
 
176 Hum A N Physiology. [Chap . x i i . 
 
 Bowman, for lie has shown that certain substances are 
 excreted by the cells of the convoluted portion of the 
 tubes, and not by those of the glomeruli, and hence 
 that the functions of these two sets of cells are 
 different. Thus, if a few minims of a solution of a pure 
 sulpho-indigotate of soda be injected into the blood of a 
 rabbit, after section of the cord, which is done to reduce 
 the blood pressure, and to allow a slower current of 
 blood to traverse the kidneys, it will be found that if 
 the animal be killed, the cells of the convoluted por- 
 tion of the kidney will, in the course of a few minutes, 
 be stained of a blue colour, whilst those lining the 
 capsules are quite uncolouredo If, however, an hour 
 be allowed to elapse, neither set of cells presents any 
 coloration, but the granules of colouring matter lie 
 free in the lumen of the tube, none being contained in 
 the capsules, clearly showing that the cells lining tho 
 convoluted tubes have seized on the sulpho-indigotate 
 of soda, and have excreted it into that part of the 
 tube to which they are attached. 
 
 IiLflueiice of blood pressure on tlie secre- 
 tion of urine. — The kidneys receive a very largo 
 proportion of blood, and in dogs it has been found by 
 experiment that the arteries can be compressed till 
 they are not more than about half a millimetre in 
 diameter before the flow of blood through the veins is 
 sensibly diminished, thus showing that the quantity 
 of blood reaching the veins is, within wide limits, 
 independent of the calibre of the arteries, but depends 
 essentially upon the blood pressure, and the resistance 
 it meets with in the organ itself. When the blood 
 pressure in the aorta falls below 40 — 50 mm. the 
 excretion of water by the kidneys is entirely arrested ; 
 but above this pressure the amount of watery excre- 
 tion is directly dependent on the variations of the 
 blood pressure. The quantity of urine discharged 
 diminishes, therefore, on stimulation of the vagus, 
 
Chap. XII.] Physiology OF Micturition. 177 
 
 after hsemorrliage, and after section of the cord. On 
 the other hand, it increases if many large arteries are 
 tied, which raises the pressure in those that still re- 
 main patent. Section of the renal nerves leads to 
 increase of the urinary secretion. Stimulation of 
 them causes a diminution of the flow. 
 
 Pressure under AViiicli tlie urine is dis- 
 charged..— If a mercurial manometer be inserted 
 into the ureter of an animal, it is found that the pres- 
 sure under which the urine is secreted will rise until 
 it supports a column of mercury 60 mm. in height, 
 when secretion stops. The pressure of the blood in 
 the aorta in the same animal was observed to be 
 from 100 to 105 mm. of mercury. 
 
 Micturition. — The urine as it is secreted collects 
 in the bladder and is discharged at irregular intervals, 
 but usually four or five times a day. The bladder 
 holds about one and a half pints, and the pressure 
 of urine in it in the healthy man produces no 
 effect till it has accumulated to a moderate amount, 
 when some discomfort is felt, and, by a slight effort 
 of the will, the bladder contracts and completely 
 empties itself. If this amount be much exceeded the 
 desire to evacuate it becomes imperious, and if it 
 cannot be gratified pain is experienced, and violent 
 straining: efforts are made. The nervous mechanism 
 appears to be that there is a " micturition centre ' in 
 the lumbar region of the spinal cord, to which sensory 
 fibres convey impressions from the mucous membrane 
 of the bladder, and from which motor impulses are 
 transmitted to the bladder. This centre is under the 
 control of the will. Under ordinary eii'cumstances the 
 stimulus of the bladder, when moderately full, is con- 
 ducted to the centre, and the requisite motor impulse 
 is transmitted to the detrusor urinee, and the urine 
 would be discharged were it not that the sphincter 
 vesicae is contracted, and requires a relaxing impulse 
 II 
 
178 Human Physiology. [Chap. xiii. 
 
 to be liberated. This impulse may proceed from the 
 cord, but is probably ordinarily effected by the brain, 
 in order that a convenient time and place may be 
 selected. As soon as the sphincter is relaxed the 
 detrusor acts and the urine is discharged. That the 
 act is in part voluntary is within the experience of 
 every one ; but that the Avhole nervous mechanism, in- 
 cluding reflex contraction of the detrusor muscles, and 
 the relaxation of the sphincter, is under the control 
 of the centre of the lumbar region of the cord, is shown 
 by an experiment of Goltz, in which, the cord of a dog 
 being divided, the application of a sponge dipped in 
 cold water to the perinseal region at stated intervals, led 
 ultimately to the discharge of the urine at these periods, 
 though under ordinary circumstances division of the 
 spinal cord leads to retention and then to incontinence, 
 the urine dribbling away as fast as it is formed, owing 
 to loss of tone of the sphincter muscle. 
 
 CHAPTER XIIL 
 
 MUSCULAR MOVEMEXT. 
 
 The movements that are observed in the body are 
 referable to one of three kinds, amoeboid movement, 
 ciliary movement, muscular movement. 
 
 Ainceboid movement. — The amoeba is an 
 organism presenting great simplicity of structure, if 
 structure it can be called, when it ap[)ears only as a 
 minute speck of animal jelly or protoplasm. When 
 unexcited by mechanical or other stimulus, it forms a 
 disk with irregular outline, in which slow movements 
 can, by careful observation, be seen. These consist in 
 protrusions of one part of the mass or another, which 
 
Chap. XIII.] Ciliary Movement. 179 
 
 often extend to a considerable distance from the main 
 body, and, after acting as feelers, are either again re- 
 tracted, or draw after them the rest of the body. The 
 movements of the amoeba are most active at and about 
 a temperature of 36° C, but they cease or become im- 
 perceptible towards the freezing point, and when the 
 temperature is raised as high as forty-five degrees. The 
 irritability or contractility of protoplasm is rendered 
 evident by the application of a stimulus whilst it is 
 performing these slow movements. The mass then 
 draws itself together and assumes a spheroidal form. 
 The stimulus may be mechanical, as by the contact of 
 a needle ; chemical, as by the addition of some salt ; 
 thermic, or electric, or even the simple change from 
 liofht to darkness or from darkness to light. In all 
 cases it must be of a certain degree of intensity, and 
 must be sudden in its application. 
 
 Ciliary movefineiit. — The cells of many parts of 
 the body present processes of their protoplasm, which, 
 during life, and for some time after death, execute 
 rapid vibratory or lashing movements, the effect of 
 which is to drive the fluid in which they move, ^dth 
 any particles that may be suspended in it, towards the 
 outlet of the body. The movement does not appear 
 to be under the influence of the nervous system, but 
 it is affected by external agents, moderate heat accele- 
 rating, cold retarding it. The extent of motion of the 
 individual hairs is through an arc of fifty or sixty 
 degrees, or occasionally as much as ninety degrees, 
 and they are usually set in an inclined position in 
 regard to the cell, bending forwards. The forward 
 stroke of the hair is more rapid than the return stroke, 
 and, when movinofslowlv, the movement runs in the form 
 of a wave along the hair like the undulation of the lash 
 of a whip. In some instances the motion resembles 
 that of the arm in circumduction. The rapidity 
 with which the strokes succeed each other is veiy great, 
 
i8o Human Physiology. [Chap. xiii. 
 
 for even when considerably retarded by chemical agents 
 or loss of power, tliey still may be as many as six or 
 eight in the second. Attempts have been made to 
 estimate the force of ciliary movement, and it appears 
 to be much greater than that of striated muscle. 
 The weight that ean be distinctly moved, when cover- 
 ing a surface of one centimetre, by ciliary motion is 
 accepted as its absolute force, and it has been found 
 that the lowest value for the pharyngeal mucous mem- 
 brane of the frog was 3*36 grammes. Ciliary move- 
 ment may be observed to occur between 0° C and 
 45° C, and the most rapid motion is observed when the 
 cells are exposed to temperatures near the higher limit. 
 If heated beyond this the motion begins to fail, and 
 ultimately leaves the cilia all inclining forwards. 
 Kestoration of motion may occur on cooling, but at 
 temperature of about 48° C. the motion ceases altogether 
 and permanently. Short exposure to the temperature 
 of melting ice temporarily suspends movement, which 
 may, however, recommence when the temperature rises, 
 provided the cilia have not been quite frozen. The 
 presence of water is requisite for ciliary movement, 
 and also of oxygen, the motion soon ceasing when 
 oxygen is wholly withdrawn. Exposure to oxygen 
 under a pressure of eight atmospheres or more arrests 
 the motion. Ozone always acts as a poison. Alkalies 
 and acids alike prove fatal to the movement of cilia, 
 even in small doses. Small doses of ether, alcohol, 
 amyl nitrite, and carbon bisulphide first accelerate, 
 and then in somewhat large doses stop the motion. 
 Chloroform arrests it without a primary stage of ac- 
 celeration. The ordinary vegetable poisons, veratria, 
 strychnia, atropin, eserin, curare, quinine, morphia, and 
 hydrocyanic acid, do not appear to be more injurious 
 than various indifferent substances, according to the 
 degi^ee of concentration. 
 
 Moderately strong currents of electricity act as 
 
Chap, xiii.i Muscular Tissue. i8i 
 
 excitants, strong currents or shocks kill the cells and 
 stop the movement. On theoretical grounds it has 
 been supposed that there must be in the substance, or 
 forming the substance of the cilia, serially arranged 
 particles, which, when at rest, are elongated, and when 
 in action are contracted, and the movement of which 
 takes place in response to external stimuli, which here 
 act directly and not through the intervention of a 
 nervous system. 
 
 The object of the motion in many of the lower 
 animals is to sweep aliment into the mouth, and to 
 maintain respiration ; but in man it is limited, in 
 general, to the propulsion of mucus and any particles 
 of matter, as dust, the debris of cells, and in some 
 instances the products of secretion, towards the ex- 
 ternal orifice of the cavity or tube Kned by the 
 ciliated cells. 
 
 MiBSCMlar tissoe. — The muscles, which constitute 
 forty-five per cent, of the weight of the body, are the 
 agents by which the movements of the body and its 
 members are efiected. There are two varieties, t]:ie 
 striated and the unstriated, the former of which are 
 usually, though not always, under the control of the 
 will, the heart being a notable exception. (For the 
 details of structure, see companion volume, Klein's 
 " Histology.") The striated muscles consist of 
 fasciculi^ bound together into a mass by peri- 
 raysium. The fasciculi can be split into fibres 
 having: a lencrth of about one and a half inches 
 m the longest specimens, though only a fraction of 
 this in many of the minuter muscles, and a 
 tolerably uniform diameter of about ^o"*^^ °^ ^^ inch. 
 The fibres, between which are the blood-vessels 
 and nerves, are composed of a transparent nucleated 
 sheath or sarcolemma enclosing fibrillse. The fibres 
 may be split either transversely or longitudinally ; in 
 the former case each fibre seems to be composed of a 
 
i82 Human Physiology. [Chap. xiii. 
 
 series of disks, in the latter of a series of fibrils. If 
 a fihril be examined it is seen to present alternately 
 disposed dark and light portions, and these being ar- 
 ranged on the same plane in adjoining fibrils, confer 
 upon the fibre which they form its striated aspect. 
 The dark particles are anisotropous, or doubly refract- 
 ing. The isotropous, or bright and clear bands, are 
 singly refracting. The substance of the fibrils is 
 divided at regular intervals by septa named Krause's 
 disks or membranes. The dark particles, with a small 
 portion of clear substance at either end, are Bowman's 
 sarcous elements, and the same dark particles are held 
 by Briicke to be made up of still smaller uniraxial 
 crystals named disdiaclasts. 
 
 Unstriated amisclee — Sometimes called smooth 
 muscle, forms an important part of the walls of the 
 blood-vessels, alimentary canal, and genito-urinary 
 apparatus. It consists of bands which, by appropriate 
 means, can be separated into long fusiform nucleated 
 cells. The colour of unstriated muscles is paler than 
 that of striated, but they are fairly well supplied with 
 blood-vessels, and abundantly with lymphatics, both of 
 which present oblong meshes. 
 
 Cliemical cliaracters of Mittscle. — The re- 
 action of muscle is alkaline. The sarcolemma 
 resembles elastic tissue, being unacted on by acetic 
 acid, and resisting long boiling in water. It differs 
 from elastin in being slowly dissolved when heated in 
 dilute solutions of acids and alkalies. It is slowly 
 acted on by the gastric and pancreatic ferments. 
 
 The anisotropoics, or dark substance, is solid, and, 
 though allied to the proteids, differs from them in not 
 being affected by alcohol or by salicylic acid, both of 
 which precipitate the proteids. 
 
 Muscle plasnAa. — The isotropous, or light sub- 
 stance of muscle, can be obtained by pressure at 0° C. 
 from the perfectly fresh muscles of frogs, thoroughly 
 
Chap. XIII.] HEMOGLOBIN OF Muscle. 183 
 
 freed from blood by injection. It is a fluid of syrupy 
 consistence, with faint alkaline reaction, which coagu- 
 lates like blood plasma. The coagulating substance is 
 named myosin. It forms and coagulates slowly at 
 0° C, but instantaneously at 40° C. Pure myosin is 
 obtained by dropping muscle plasma into distilled 
 water, when it coagulates in the form of little balls. 
 It is a neutral substance, insoluble in distilled water, 
 but soluble in water containing between five and 
 ten per cent, of NaCl. It decomposes peroxide of 
 hydrogen. 
 
 Muscle seiTim is the liquid which remains after 
 the separation of the spontaneously coagulating sub- 
 stance from muscle plasma. It is alkaline, and contains 
 three proteids in solution, viz., potassium albummate or 
 casein, serum albumin, and an albumin coagulating at 
 45° C. 
 
 Il£eiuo§'lobm of muscle. — The red colour of 
 muscle is due to the presence of haemoglobin, which is 
 in combination with the plasma and not with the 
 dark substance. Its presence in the muscle may 
 be shown by holding a thin portion before the slit 
 of the spectroscope after all blood has been removed 
 by washing out the vessels with weak solution of 
 salt. 
 
 When muscle is treated with cold water the whole 
 of the constituents of the muscle serum are dissolved, 
 and perhaps some of the anisotropous substance. The 
 fluid is found to contain the nitrogenous compounds, 
 kreatin, kreatinin, carnin, xanthin, hypoxanthin_, and 
 urea. 
 
 Noil-nitrogenous constituents of muscle. — 
 These are fats, glycogen, and inosite, with the volatile 
 fatty acids and ]3aralactic acid. The quantity of gly- 
 cogen is about 0"5 per cent. Inosite CgllioOe-}- 2H2O 
 is a non-fermentable sugar crystallising in large colour- 
 less monoclinic tables. It does not reduce Fehling's 
 
184 Human Physiology. tchap. xiii. 
 
 solution, but colours it green. Muscle contains a trace 
 of pepsin, and probably also of a diastatic ferment which 
 has not been isolated. The proportion of water in 
 muscle is about seventy-five per cent. The organic 
 matters, chiefly albuminous, are about twenty per cent., 
 and the salts make up the remainder, the phosphates 
 and potassium salts being particularly abundant. 
 There are about six times more salts of potassium by 
 weight than of sodium. 
 
 Healthy muscle during life, even when at perfect 
 rest, abstracts oxygen from the blood passing through 
 it, in addition to the materials required for its nutrition, 
 and gives off carbonic acid, though in less quantity 
 than the oxygen absorbed. Even after death, or in 
 excised muscle, is taken up and COg eliminated. 
 The exchange of gases is greatly augmented during 
 contraction. 
 
 Properties of miiscislaT tissue. — The most 
 important properties of muscular tissue are extensi- 
 bility, elasticity, and contractility. The extensibility 
 of muscle is the elongation it undergoes when it is 
 stretched by a weight. The elasticity is the power of 
 recoil which muscle possesses when it is either elon- 
 gated or compressed. The contractility of muscle is 
 the power it possesses of shortening when stimulated 
 either directly or through the nerves. 
 
 ExtensifolBity of musele. — This is difficult to 
 determine, but Donders has endeavoured to estimate it 
 by supporting the fore-arm at the elbow, when at right 
 angles to the upper arm, and then suspending a weight 
 from the wrist, and, after allowing it to act for ten 
 seconds, suddenly snipping the thread that attached 
 it. The result obtained was that the elongation was 
 precisely proportioned to the weight. The extensi- 
 bility of muscle is essential to the movements of the 
 bones, for since the bones are surrounded by muscles on 
 all sides, if these were rigid no movement could occur ; 
 
Chap. XI II. J Contractility OF Muscle. 185 
 
 but as it is, when one set of muscles contracts and 
 draws the bone with them, their antagonists yield to 
 a corresponding extent. 
 
 Elasticity ol muscle. — The ela,sticity of 
 muscles is small, but perfect. That is to say, a very 
 small weight will extend a muscle, but when that 
 weight is removed it returns exactly to its original 
 dimensions. The elongation of a muscle is not in 
 exact proportion to the weight extending it ; but, 
 according to Wertheim, the elongation diminishes, at 
 first quickly, and then more slowly, in proportion as 
 the weight increases, and the curve of muscular 
 elasticity, instead of being nearly straight, resembles 
 a hyperbole. The limit of elasticity of a muscle is 
 soon reached, so that the gastrocnemius of a frog 
 extended by a weight of 100 grms. will no longer 
 return to its original length. Yet the cohesion of the 
 same muscle is sufficient to resist a weight of about 
 250 grms. even after death, whilst during life the 
 breaking strain is about a kilogramme, or 1,000 grms. 
 The importance of the elasticity of muscle is consider- 
 able, for it is due to this alone that, notwithstanding 
 the distance between its points of attachment is con- 
 siderable, it is sufficiently tense to prevent time being 
 lost before contraction occurs. Moreover, it permits 
 the muscle, when brought into sudden action, to act 
 smoothly and uniformly without danger of tearing. 
 That the muscles of the body are always in a state of 
 slight extension is shown by the retraction of the ends 
 of a divided tendon. 
 
 Contractility of muscle. — This is the property 
 which muscular tissue possesses of shortening when 
 stimulated. The question was formerly much discussed 
 whether the contraction is due to a vis msifa, as Haller 
 termed it, or property of the muscle itself, or whether 
 it is due to the action of the nervous system. But 
 such a discussion has long since been recognised as 
 
1 86 Human Physiology. [Chap. xiii. 
 
 unprofitable. The nervous and muscular systems are 
 in reality fused together, and in some of the lower 
 animals one part of a cell may be nervous and another 
 muscular. And although the contraction of muscle 
 ordinarily takes place in consequence of an impulse 
 propagated from the nervous system, there can be 
 little doubt that if the nerve tissue could be entirely 
 removed from a muscle, it would still contract on the 
 direct application of a stimulus to it. There are 
 indeed instances of muscular tissue, such, for example, 
 as the foetal heart and the allantois, which contract 
 rhythmically, notwithstanding that there is a complete 
 absence of any differentiated nervous system ; whilst 
 in curare a drug is known which is capable of para- 
 lysing the extremities of the motor nerves, whilst the 
 muscles still remain capable of responding to direct 
 stimulation. 
 
 The irritability or contractility of both striated and 
 unstriated muscle is rendered much feebler by exposure 
 to cold, but is at the same time much longer preserved. 
 Thus, the gastiocnemius of a frog in winter, when 
 detacLed from the body of the animal, and exposed, 
 with some precautions against drying, to a temperature 
 a little above the melting point of ice, will preserve its 
 contractility and alkaline reaction for as long a period 
 as ten days, whilst in summer the muscle may cease to 
 respond to a stimulus within twenty-four hours. The 
 same holds good of the heart of a frog, which will some- 
 times, in winter, continue to contract rhythmically after 
 removal from the body for nearly a week, whilst in 
 summer it ceases to beat in a few hours. This effect of 
 cold is also observed in the muscles of the higher ani- 
 mals, though the time is relatively much shorter. In 
 man the muscles lose their irritability after sudden 
 death, as by hanging, very quickly, no trace being ob- 
 servable after the lapse of from three to seven hours. In 
 some cases of disease it lasts longer ; and a case is on 
 
Chap. XIII.] Rigor Mortis. 187 
 
 record in whicli^ the cause of death being aneurism of 
 the heart, the muscles retained slight irritability for 
 twenty-seven hours after life was extinct. The muscles 
 of the new-born child lose their irritability sooner than 
 those of the adult, providing the two are maintained 
 at an equal temperature ; but the body of the infant, 
 being of smaller size, loses its heat, if no precautions 
 are taken to prevent it, more rapidly than that of the 
 adult, and the muscles may hence appear, deceptively, 
 to retain their irritability longer. 
 
 The order in which the several muscles of man 
 lose their contractility is : first, the left ventricle, which 
 ceases to respond to any stimulus about three-quarters 
 of an hour after death ; then the large intestine, then 
 the small intestine ; and, after a few minutes, the 
 stomach, the urinary bladder ; and, about an hour 
 after death, the right ventricle ; about one hour and 
 a half after death the oesophagus, the iris ten minutes 
 later, and then the muscles of animal life, those of 
 the trunk losing it before the members, and those of 
 the legs before the arms. The right auricle, which 
 Haller rightly designated the " primum movens " 
 and " ultimum moriens," retains its irritability the 
 longest. 
 
 Rigor mortis. — Immediately after death the 
 irritability of muscular tissue increases, but it soon 
 begins to diminish, and as soon as all traces of irrita- 
 bility have died out rigor mortis commences. In this 
 state the muscles become hard and stiff, presenting 
 many of the characters of contraction. It appears to 
 be of invariable occurrence. The instances in which 
 it has been considered to be absent, as in death by 
 lightning, in hunted animals, and in asphyxia, have 
 probably been cases where it has occurred very early 
 or late, and has been overlooked. In some cases of 
 gunshot wound it has succeeded spasm of all the 
 muscles so quickly, that the body has preserved the 
 
i88 
 
 Human Physiology. 
 
 [Chap. XIII. 
 
 kneeling or sitting position it occupied at the moment 
 of death. Rigor mortis commences in the muscles of 
 the jaws, then in those of the neck and trunk, and, 
 generally, in those of the lower limbs before those of 
 the upper, but occasionally the latter stiffen first. 
 From observations made on a considerable number of 
 subjects, Niderkorn has constructed the following 
 table, which shows the time when the rigor mortis 
 was complete : — 
 
 Eleventh hour 
 
 after 
 
 death 
 
 1 
 
 Seventh hour after death 
 
 11 
 
 Second „ 
 
 
 
 2 
 
 Third 
 
 )j 
 
 }) 
 
 14 
 
 Thirteenth,, 
 
 
 
 2 
 
 Fifth 
 
 9> 
 
 ; ) 
 
 14 
 
 Ninth „ 
 
 
 
 4 
 
 Sixth „ 
 
 » 
 
 J> 
 
 20 
 
 Eighth „ 
 
 
 
 7 
 
 Fourth ,, 
 
 >> 
 
 J> 
 
 31 
 
 Tenth „ 
 
 
 
 7 
 
 Total 
 
 
 
 113 
 
 So that as a rule the rigidity of the body is complete 
 from four to six hours after death. It usually sets in 
 more rapidly when the body preserves more heat than 
 usual, hence it is early in its appearance in those who 
 meet with sudden death. And Brown Sequard records 
 a case in which rigor mortis was established in the 
 jaws several minutes before the heart ceased to beat. 
 On the contrary, cold retards its appearance. 
 
 Section of the sciatic nerve causes the muscles of 
 the lower limbs to stiffen more slowly than when the 
 nerve is uninjured. Paralysed limbs are earlier 
 affected than the opposite sound ones, perhaps because 
 the healthy nerves maintain some degree of tonicity 
 and chemical activity. The injection of defibrinated 
 arterial blood has been found to render the muscles 
 once more supple after they have passed into the con- 
 dition of rigor mortis, but venous blood is inoperative. 
 
 The cause of rigor mortis is believed to be the 
 coagulation of the myosin, and it is remarkable that 
 the injection into the vessels of a rigid limb of a ten 
 per cent, solution of common salt, which is known to 
 
Chap. XIII.] Rigor Mortis. .189 
 
 dissolve myosin, restores the limb to its original con- 
 dition of suppleness. Muscle in the condition of 
 rigor mortis is shorter, thicker, and of firmer consis- 
 tence, resisting pressure and any attempt to elongate 
 it. It ruptures with less facility. The muscular 
 current of electricity disappears, or is reversed. Its 
 reaction changes from the alkaline reaction of healthy 
 living muscle to acid, which is attributed to the 
 presence either of sarcolactic acid, which is an isomeric 
 modification of lactic acid, or to that of glycerin- 
 phosphoric acid, or of both. 
 
 Stenson's experioiefiit. — The muscles can be 
 thrown into a state resembling or identical with rigor 
 mortis by arrest of the supply of blood, a fact origi- 
 nally noticed by Stenson. The first efiect of the liga- 
 ture of a vessel passing directly to a muscle is, that 
 its irritability is increased ; it then rapidly lessens, and 
 finally the muscle passes into a condition of stiifness. 
 In the earlier period of the action the muscle is 
 capable of completely recovering itself after removal 
 of the ligature from the vessel, but in the later stage 
 the muscular rigidity is permanent. 
 
 Meat rig'or. — The muscles of mammals pass into 
 a state of rigor at a temperature of 48° — 50° C, those 
 of frogs at 40° C, and those of birds about the tempera- 
 ture of 48°— 50° C. 
 
 Water rigor. — The exposure of muscle to the 
 action of distilled water induces a condition of rigor, 
 and even ordinary sea-water acts in the same way, as 
 is seen in the crimping of cod. 
 
 Acid rigor. — The injection of one part of lactic 
 or hydrochloric acid into the muscles of frogs produces 
 rapid stiffening. 
 
 Idiomiisctilar contraction. — When a muscle 
 is nearly exhausted, and a direct stimulus is applied to 
 it, a swelling or local contraction is often observed, 
 which is slowly propagated in the form of a wave from 
 
1 9© Human Physiology. [Chap.xiii. 
 
 the point stimulated to both extremities. It may be 
 observed even during life, the most favourable situation 
 being the pectoralis major of patients affected with 
 phthisis or other wasting disease, in whom there is 
 little subcutaneous fat. In such patients a smart tap 
 raises a swelling which may be seen to extend over 
 the Avhole length of the muscle. Akin to idiomuscuiar 
 contraction is the " fibrillar contraction," or quivering 
 of various facial muscles, which is often experienced 
 in enfeebled states of health, and may sometimes be 
 seen in the gastrocnemii if the leg be supported and 
 at rest. The same occurs in the muscles of the tongue 
 after section of the hypoglossal nerve. 
 
 TosiBclty of muscle. — By tonicity is meant that 
 property of muscles by which they preserve a certain 
 degree of firmness and slight contraction, which is 
 best seen in the sphincters. It appears to be under 
 the influence of the nervous system, since it is lost as 
 soon as the nerve distributed to a muscle is divided, 
 the muscle immediately becoming flaccid and relaxed. 
 It fulfils the important purposes of aiding the elasticity 
 in preventing any loss of time in the execution of 
 movements when muscle is called into play, and of 
 rendering smooth and uniform the movements of the 
 limbs by antagonising the contraction of the opposing 
 muscles. It also serves to maintain the surfaces of 
 joints in a state of coagulation. It passes, by insensible 
 degrees, into those pathological conditions that are 
 seen in contractures of the muscles in paralysis, and 
 which are usually associated with organic changes 
 in the muscle itself. 
 
 Muscle stimiili. — (1) Nervous imjjulse. — The 
 normal stimulus of muscle may be propagated to 
 it through a motor nerve from one of the higher 
 intellectual centres, when it is termed automatic or 
 voluntary ; or from some other centre, when it is 
 usually of a reflex nature. It differs from most other 
 
Chap. XIII.] Muscle Stimuli. 191 
 
 stimuli in bringing all the fibres of the muscle into 
 action simultaneously. 
 
 (2) Chemical. — Such as the mineral acids, which 
 act promptly" upon muscle even in the dilute state, 
 though they require to be rather concentrated to affect 
 nerve. Lactic acid and glycerin are said to act on 
 muscle only in the diluted state, and on nerve only 
 when concentrated. Neutral alkaline salts act equally 
 on nerve and muscle, and upon both with much energy, 
 whilst alcohol and ether act upon both comparatively 
 feebly. 
 
 (3) Thermic stimuli. — If the muscles of a frog be 
 exposed suddenly to a temperature of 28° C. they 
 gradually shorten, the contraction becoming suddenly 
 strongly marked at 30° C, and attaining its maximum 
 at 45° C, at which temperature it easily passes into 
 heat rigor. There is a difference between the smooth 
 muscles of mammals and the striated muscles in their 
 relation to warmth, for heat causes contraction of the 
 latter and relaxation of the former. Striated muscle 
 of the frog, cooled down to the melting point of ice, 
 is rendered very excitable, and if it be then sub- 
 jected to lower grades of cold still, it is stimulated to 
 contract. The muscles of animals that have been 
 artificially cooled preserve their excitability many 
 hours after death. 
 
 (4) Jlechanical stimuli. — A sudden blow or prick 
 of a muscle excites a contraction, and if the shocks 
 be sufficiently frequently repeated, tetanus is induced. 
 
 (5) Electric stimuli. — These will be considered 
 under the head of nerve ; but it may be remarked 
 that an electric current passed transversely across 
 a Diuscle is a much less powerful stimulus than 
 when made to transmit it in the direction of its 
 length. 
 
 Plienomeiia of contraction. — (1) When ex- 
 amined with the naked eye m.uscle is seen, in the act 
 
T92 Human Physiology. [Chap. xiii. 
 
 of contraction, to become shorter and correspondingly 
 thicker. It is not difficult to show that there is but 
 little chano;e in volume, and that the loss in lens^th is 
 almost entirely compensated for by a gain in thickness 3 
 for if a portion of the body of an eel be placed in a 
 vessel, the stopper of which is drawn out so as to form 
 a narrow neck, and which is filled with water, if the 
 muscles be made to contract by some stimulus, and 
 there was either increase or diminution of absolute size, 
 the water would rise or fall in the capillary neck, 
 but the most exact researches show that the variation 
 in the level of the vessel is almost inappreciable, and 
 that if there is any change during contraction it is 
 slight diminution, but this diminution of volume 
 certainly does not exceed -^ ^^ ^th of the total volume 
 of the muscle. 
 
 (2) Under the microscope the distinction between 
 the isotropous and the anisotropous substance becomes 
 in the first instance obscured, so that the whole con- 
 tents of one of the compartments of Krause presents 
 a uniformly dim aspect ; but when the contraction is 
 more advanced or complete, the central transverse dark 
 band becomes lighter, and the terminal portions of the 
 compartment darker, so that there is either a shifting 
 or interchange of position between the isotropous and 
 the anisotropous disks, or these parts undergo a change 
 in their physical properties. 
 
 (3) Certain chemical changes take place in muscle 
 during contraction, the most important of which are 
 that four or five times more oxygen is used up, and 
 more CO2 is produced, the venous blood returning 
 from muscle containing more CO2, and more CO2 being 
 eliminated by the lungs. The muscles become more 
 watery. The components soluble in cold water 
 diminish, those in alcohol increase. The amount of 
 glycogen diminishes. Acids are developed, especially 
 a form of lactic acid which has been named the 
 
Chap. XIII.] Muscular Contracttqn. 193 
 
 sarcolr.ctic acid, glycerin-pliosphoric acid, and car- 
 bonic acid, and the reaction of muscle becomes acid. 
 
 (4) A sound is emitted during the contraction of 
 muscle which may be heard when a muscle is made to 
 contract, either by the influence of the will or by any 
 external stimulus. It is audible when the masseter 
 muscles are strongly excited, and also when the 
 muscles of the thumb are contracted and the little 
 finger is inserted into the ear. The number of vibra- 
 tions is 19"5 per sec, but it is not this deep note that 
 is heard, but the first octave, overtone, or harmonic 
 above it, having 39 vibrations per sec. When a 
 muscle is thrown into continuous spasmodic con- 
 traction by the action of an induced current, the 
 vibrations, and consequently the pitch of the sound, 
 agree with the frequency of the shocks of the in- 
 duction apparatus. 
 
 (5) The temperature rises. In the muscles of a 
 frog tetanised for a few minutes a rise of temperature 
 amounting to about 0*16° C has been observed. In 
 a single contraction its rise was from 0*001° C to 
 0-005^ C. 
 
 More heat is disengaged when the muscles per- 
 form no work than when work is done by their 
 contraction. 
 
 ( 6 ) D uring contraction the norm al electrical current 
 of muscle diminishes in strength, and even becomes 
 reversed, constituting the negative variation of Du Bois 
 Reymond. 
 
 (7) In contraction the elasticity of muscle dimi- 
 nishes, that is, it yields to a greater extent with the 
 same weight in the contracted as compared with the 
 un contracted state. This circumstance affords an 
 explanation of the " Paradox of Weber," viz., that a 
 muscle in repose, and heavily weighted, will, when 
 strongly stimulated, become longer than in the 
 quiescent state, instead of shortening. 
 
 N 
 
194 Human Physiology. [Chap. xiii. 
 
 "Wave of muscular contraction. — When a 
 
 stimulus is applied to the nerve supplying a muscle, of 
 sufficient streno;th to excite contraction, the fusion of 
 the two tissues is so complete that the whole of the 
 muscle is made to contract, strongly or weakly, as the 
 case may be, hut still simultaneously, and the same 
 occurs if a current of electricity be passed from one 
 extremity of the muscle to the other, so as travel 
 through the whole length of the muscle ; but if the 
 current be applied to one extremity only it can 
 readily be shown, by the graphic method and the 
 attachment of a succession of levers or tambours to i 
 in the direction of its length, that the contraction 
 travels as a wave with considerable speed from end 
 to end. In the gastrocnemius of a frog the rapidity 
 with which the wave travels has been determined 
 to be from 4 to 5 metres per second, whilst in the 
 arm muscles of man it is from 10 to 13 metres per 
 second. 
 
 The most rapid movements that can be executed 
 by muscles under the influence of the will, as in 
 Avriting, elocution, or in musical performances, do not 
 exceed ten or twelve contractions per second. 
 
 A single muscular contraction. — There is no 
 short term in English to designate the sudden, short, 
 and transient shortening which is termed by the 
 French a secousse, or shock, and by the Germans a 
 zuckung^ or convulsion. It is the response of the 
 muscle to a single stimulus, and much interest is 
 attached to a tracing taken of it with a myograph ion.* 
 Such a tracing is shown in the adjoining cut (Fig. 10). 
 
 It will be seen that there are four lines. The 
 lowermost at i exhibits an abrupt descent which 
 indicates the moment at which the shock was applied. 
 The next undulating line represents the vibrations of 
 a tuning fork, each double vibration of which occupied 
 * See companion volume on " Physical Physiology." 
 
Chap. XIII.] Muscular Contraction. 
 
 195 
 
 Y^th of a second, and consequently constitutes a re- 
 liable and perfectly uniform measure of time. The 
 straight line L is the basal line, and is made by the style 
 or brush attached to the muscle on the cylinder when 
 this is revolving and the muscle is at rest. The line 
 
 Fig. 10.— Tracing of a Single Muscular Contraction or Shock of tlie 
 Muscle of a Frog. 
 
 s, Line showing at r, the po'nt where it descends, the instant at which the stim- 
 ulus of the electric shock was applied to the muscle ; d. Tracing of the 
 vihrations of a tuning fork vibrating iCO times pf^r second; l, Basalline of 
 muscle at rest ; a'a. Latent (period of contraction ; ah. Period of contrac- 
 tion; HB, Period of relaxation. 
 
 AA'HB is the tracing of the muscle curve. It is di- 
 visible into three parts: (1) The latent period, a'a; 
 (2) the period of ascent, a — h; (3) the period of 
 descent, h — b. 
 
 The latent period. — Careful examination and 
 comparison of time by means of the undulating line 
 D renders it evident that the point A, at which 
 the muscle begins to contract, and the curve 
 to spring from the basal line, does not corre- 
 spond with the point a' or i^ which represents the 
 moment at which the shock was applied, but that 
 a short interval a'a intervened. This is the latent 
 period. It lasts less than half a complete vibration 
 of the tuning fork vibrating one hundred times in a 
 second, and is therefore about o^T7''^h of a second in 
 duration. During this period the muscle seems to be 
 
196 Human Physiology. [Chap.xin. 
 
 gathering itself togetlier for the effort it is about to 
 make, and we may suppose that some chemical change 
 is taking place by which the liberation of muscular 
 force is effected ; and it is found that whatever tends to 
 promote or facilitate chemical change tends to shorten 
 the latent period, whilst all that acts prejudicially to 
 chemical action, and impedes it, tends to lengthen it. 
 Thus, the latent period is shorter in muscles that 
 are slightly extended by a weight than in those that 
 are absolutely quiescent, for in the latter case the 
 chemical changes are at their lowest and have to be 
 started, whilst in muscles doing a little work the 
 changes are already in progress and are easily ren- 
 dered more active. Again, the latent period is shorter 
 at a moderately high than at a low temperature, 
 and hence it is shorter in summer than in winter frogs. 
 Again, it is shorter in mammals than in frogs ; shorter 
 when a powerful electric shock is applied than when 
 it is feeble ; shorter when the muscle is fresh than 
 when it is exhausted by exercise, and, above all, shorter 
 when it still forms part of the body of the animal 
 than after its removal and manipulation. An addi- 
 tional argument is found in the circumstance that if a 
 very feeble shock be applied to a muscle, a shock so 
 feeble as to be insufficient to cause it to contract, the 
 application of a second shock instantly induces con- 
 traction, the latent period almost disappearing. It 
 seems probable in such case that the muscle is pre- 
 pared for the effort by the shock first applied. The 
 latent period is shortened by certain drugs, such as 
 strychnia and veratria. In mammals it is very short, 
 and is believed to oscillate about 0*008 sec, a little 
 more or less. A very heavy weight appended to a 
 muscle prolongs the latent period, and it is prolonged 
 by curare. 
 
 Period of ascent. — The ascending portion of 
 the line representing the muscle curve corresponds 
 
Chap. XIII.] Muscular Contraction. 197 
 
 with tlie contraction or shortening of the muscle. The 
 rise is observed to be steep, and it can be shown by it 
 that muscle contracts more rapidly at the commence- 
 ment of its contraction than when it has nearly 
 attained the maximum of its shortening, unless the 
 muscle is heavily weighted, when the opposite condi- 
 tions obtain. 
 
 Period of descent. — This is always slower t)r 
 more prolonged than the period of ascent. It corre- 
 sponds with the period of relaxation of the muscle, 
 and, as in contraction, is more rajDid at first than 
 when the muscle has nearly resumed its ordinary 
 conditions. It is succeeded by one or two secondary 
 vibrations due perhaps to elasticity. When a muscle 
 is lightly weighted, the descending portion of the 
 curve does not reach the basal line, but when heavily 
 weighted it may not only reach it, but descend below 
 it, and then rise above it again. In the ordinary con- 
 traction of the gastrocnemius of the frog the relation 
 in point of time of the period of ascent to that of 
 descent is such that if the former occupies one-tenth 
 of a second the latter occupies three-tenths. 
 
 Exhausted muscles have a longer latent period, 
 contract more slowly and to a less extent, and have a 
 longer period of relaxation. The curve is therefore 
 lower and longer with the same stimulus. 
 
 ISuimiiiatioii of stioiuli. — As a general rule, 
 when a stimulus of moderate strength is applied to 
 a muscle it contracts suddenly and completely, that is, 
 to its fullest extent ; but a stimulus may be applied to 
 a muscle so slight that no contraction follows ; if this 
 feeble stimulus be quickly reapplied two or more 
 times contraction ensues, or if the first feeble stimulus 
 produces slight contraction, the second stimulus, 
 though it may not be any stronger, excites a vigo- 
 rous contraction. This gradually increasing action 
 is termed summation by the Germans-, and latent 
 
198 
 
 Human Physiology. 
 
 [Chap. XIII. 
 
 addition by the French. The interval of time between 
 two successive stimuli may be as brief as yQ—gth of a 
 second and as long as ten seconds. Two stimuli, 
 though removed from each other by an interval as 
 short as the former of these periods or as long as tho 
 latter, are consequently clearly perceived by the 
 muscle to be two, and the muscle curve produced by 
 them differs in its characters from that produced by a 
 
 Fig. 11. — Summatiou of Two Stimuli. 
 
 single stimulus. If the two stimuli both occur within 
 the latent period, or soon after the commencement of 
 shortening, only one contraction will result, but it 
 will be more vigorous than one alone. Thus, in Fig. 1 1 
 the first shock is applied at 0, and — 1 := latent 
 period, and ba8 represents the curve that would 
 be produced if that stimulus alone acted ; but if 
 now, when the muscle is just beginning to contract, 
 a second stimulus is applied at B, the second contrac- 
 tion superimposes itself upon the first, and the stronger 
 contraction b c 11 follows. The excitability of a 
 muscle seems to increase when a succession of stimuli 
 are applied, for it is possible to apply to a muscle a 
 shock of such feeble intensity that no contraction 
 
Chap, XIII.] Summation of Stimuli. 
 
 199 
 
 follows, but if several such shocks are applied consecu- 
 tively, a vigorous contraction may be induced. Thus 
 it is seen in Fig. 12 that the application of the first 
 
 Fig. 12,— Muscle of Lobster. 
 Indications of seven electric shocks of equal intensity. 
 
 two shocks was not followed by any contraction ; the 
 third, however, produced a slight rise in the myo- 
 graphic tracing, the fourth a distinct but slight mus- 
 cular contraction, whilst the three following shocks 
 caused the muscle to contract energetically. Hence it 
 
200 
 
 Human Physiology. 
 
 [Chap. XIII. 
 
 appears that when successive and equal stimuli are 
 applied they add themselves together and produce 
 a much more powerful effect than either of them 
 singly. 
 
 Secondary \%^ave. — If a muscle to which a 
 light weight is attached be thrown into tetanus and 
 then left, it suddenly elongates, but soon begins again 
 to contract without tlie further application of a 
 stimulus. This contraction is termed the " secondary 
 wave," and it is necessary, in order that it should 
 appear, that the muscle should be very fresh and the 
 weight light. 
 
 Tetanus. — Tetanus is the fusion of a series of 
 successive contractions into one continuous con- 
 traction. It occurs when stimuli are applied so 
 rapidly that the muscle has not time to relax between 
 the times of application of two consecutive stimuli. 
 The number of stimuli required to throw a muscle 
 into tetanus varies greatly in different instances. 
 
 Thus, in the case 
 of the pedal 
 muscle of the 
 snail, tetanus re- 
 sults even when 
 the interval be- 
 tween two shocks 
 is as much as 
 ten seconds ; two 
 sh ocks per second 
 are sufficient to 
 throw the striat- 
 ed muscle of the 
 tortoise into te- 
 tanus ; the rapidly vibrating muscles of the insect 
 require 300, of the bird 100 ; the gastrocnemius of 
 a frog requires twenty-seven, and the pale muscles 
 
 Fi?. 13. — Incomplete Tetanus ; the undulation 
 of the successive shocks being still perceptible. 
 
 of the rabbit a very much 
 
 greater 
 
 number ; the 
 
Chap. XIII.] 
 
 Tetanus. 
 
 20I 
 
 striated muscles of man require 
 forty, the unstriated two. In the 
 graphic tracing of tetanus the con- 
 tractions caused by successive 
 stimuli are seen to be super- 
 imjiosed, till at length a maxi- 
 mum height is observed, which 
 is preserved as long as the stimu- 
 lation is continued till exhaustion 
 occurs, when the line suddenly 
 falls. So long as the line, though 
 high, presents undulations, the 
 tetanus is said to be incomplete 
 (Fig. 13); but when the tracing 
 of the muscle forms a uniform 
 Ime (Fig. 14) then the tetanus is 
 complete. Before tetanus is es- 
 tablished, when the number of 
 excitations is considerable, there 
 is generally one contraction, fol- 
 lowed by relaxation. This is 
 named the initial contraction ; 
 but instruments have been de- 
 vised by means of which 22,000 
 stimuli can be applied in a 
 second, and it is then said by 
 some that no initial contraction 
 occurs, the muscle passing at once 
 into tetanic spasm. 
 
 Action of curare on 
 muscle. — Curare, or woorara, is 
 the arrow poison of the South 
 American Indians, and is remark- 
 able for its power of rendering 
 even large and 2:)owerful animals 
 motionless, when introduced into 
 their blood. In small doses it 
 
 o 
 
 
202 Human Physiology. [Chap. xiil. 
 
 increases the irritability of both nerve and muscle, but 
 in large doses it primarily aflfects the intramuscular 
 termiaations of the motor nerves without impairing 
 the functions of the sensory nerves. Its fatal effect is 
 the result of its paralysing action on the respiratory 
 muscles, and where, as in the frog, the skin plays an 
 important part in the act of respiration, life may be 
 preserved for a long time, though the animal remains 
 motionless. Though the intramuscular extremities 
 of the nerves are paralysed, the muscles themselves afe 
 capable of responding to direct stimuli In large 
 doses the inhibitory fibres of the vagus are paralysed, 
 and the heart consequently beats more frequently. 
 Curare produces little effect when taken into the 
 stomach, because as fast as it is absorbed it is excreted 
 by the kidneys, but if the ureters are tied, the poison 
 accumulates in the blood, and the usual effects afe 
 observed. 
 
 Action of some ottier poisons. — Yeratria re* 
 sembles curare in its effects ; chloroform, ether, and 
 chloral retard the whole process of contraction ; oxide 
 of carbon, which at ordinary temperature has no action 
 on muscular irritability, abolishes it at a pressure of 
 five atmospheres. 
 
 Necessity for fi'ee supply of blood. — The 
 capability of muscle to respond to nerve stimulation, 
 whether direct or indirect, is diminished by fatigue, 
 and abolished by exhaustion, and the same effect is 
 produced by any means preventing the supply of blood 
 to it, whether by pressure or ligature of the a-rtery 
 of the limb, by the injection into the vessels of any 
 fine powder which obstructs the capillaries, or by the 
 application of an Esm arch's bandage. On re-admission 
 of the blood, if the supply have not been too long cut 
 off, it is soon recovered. The muscle does not lose its 
 irritability in anaemia, for it may still respond to 
 direct stimuli, but nervous impulses can no longer be 
 
Chap. XIII.] Effects of Nerve Lesions. 203 
 
 communicated to it. The heart, however, seems to 
 be diflferentlj affected, since it stops instantly after 
 ligature of the coronary arteries. In frogs the whole 
 of the blood may be replaced by solutions of common 
 salt in the proportion of six grains to the litre. Yet 
 voluntary and reflex movement, and the excitability 
 of the muscles, may be preserved for many hours. 
 When all signs of irritability have ceased this property 
 of muscle may be restored by the injection of warm 
 defibrinated blood into the vessels. In the dog 
 restoration may take place six hours after the last 
 signs of irritability have vanished, and in pigeons 
 after one. Speaking generally, muscle can live and 
 contract independently of the rest of the body, but it 
 requires oxygen for the manifestation of its activity, 
 and it is necessary that the products of its waste 
 should be removed ; both of these conditions are 
 fulfilled by the blood current. The circulation of 
 blood through muscle excited to contract is rendered 
 more active. The intramuscular arteries dilate, and 
 the flow of blood through them is more active. Small 
 dilatations have been observed in the capillaries of 
 muscle in which blood may accumulate, and a supply 
 of nutriment and of oxygen is thus afforded which can 
 be drawn upon during prolonged effort. 
 
 Eflfects of nerve lesions on muscle. — In the 
 course of three or four days after lesion of a motor 
 nerve, the muscle supplied by it reacts less powerfully 
 to both direct and indirect stimulation of the nerve. 
 This period is succeeded by one in which constant 
 currents act more powerfully whilst induced currents 
 are almost inoperative. There is also increased 
 excitability for direct mechanical stimuli. This occurs 
 about the seventh week, and from this time it 
 gradually sinks up to the sixth or seventh month, 
 when it disappears altogether. Under the microscope 
 fatty degeneration a])pears about the second week, 
 
204 Human Physiology. [Chap. xui. 
 
 and gradually progresses until complete atrophy results 
 (Landois), 
 
 Work done by muscle. — The work done by a 
 muscle is estimated by multiplying the weight raised 
 by the height to which it is raised. If there is no 
 weight to be lifted no work is done, and, on the 
 other hand, if the weight is so heavy that it cannot 
 raise it at all, no work is done. If the weight be 
 gradually increased from a minimum it is found that 
 the work done steadily increases up to a certain point ; 
 when this is passed the height to which the weight can 
 be raised diminishes, and the work done diminishes. 
 Thus, in one experiment Weber found that with 
 
 Weight attached to SS^^^ ^^t^^^ Work done, in gram- 
 B.uscle,ingramnxes. ^St^XSet'' xnilLxuetrer 
 
 5 27-06 138 
 
 15 25-01 376 
 
 25 11-45 286 
 
 30 6-03 220 
 
 From which it appears that most work was done in 
 this case with a we'ght of 15 grammes. 
 
 The " absolute muscle force " is the weight which 
 a muscle stimulated to the utmost is just unable to 
 raise, so that it retains its natural length before being 
 weighted, though at the same time it does not elon- 
 gate at the moment of stimulation. As a means of 
 comparison of the absolute muscle force of different 
 muscles, it is estimated on 1 square centimetre of the 
 mean transverse section, and the mean transverse section 
 is obtained by dividing the volume of a muscle by its 
 length, and the volume equals the absolute weight of 
 the muscle in question, divided by the specific gravity 
 of muscle, which is equal to 1'058. The absolute force 
 of 1 square centimetre of frog's muscle is thus esti- 
 mated at 2-8 — 3 kilogrammeters ; for the same area 
 of human muscles it is about 8 or 9 kilogrammeters. 
 
Chap. xiiL] Electric Currents in Muscle. 205 
 
 The greatest exertion of a man, reckoned at eight 
 hours per diem, is estimated to be about 10 kilo- 
 grammeters per second, or 300,000 per diem. One 
 horse-power equals seven times this amount. 
 
 The electi'ic current in muscle. — This mav 
 best be shown by excising a muscle such as the gastro- 
 cnemius of a frog, and dividing it transversely imme- 
 diately after removal from the body. If one end of 
 a galvanometer wire be now applied to the cut surface, 
 and the other to the outer or longitudinal surface 
 of the muscle, the movement of the needle indicates 
 that a current is passing from the uninjured surface 
 towards the cut surface, and hence it may be con- 
 cluded that a corresponding current is passing inside 
 the muscle from the cut surface towards the outer 
 surface. In other words, imtJdn the muscle the cut 
 surface represents the positive pole, the free surface 
 the negative pole, and outside the muscle the longi- 
 tudinal surface represents the positive, and the cut sur- 
 face the negative pole. This subject will be more fully 
 discussed under the head of electrical currents in nerve. 
 
 Unstriated muscular tissue. — The physio- 
 logical investigation of unstriated muscular tissue 
 is rendered dithcidt, on account of the difficulty of 
 obtaining a mass of it in an isolated state, its in- 
 accessibility, and the circumstance that it is often 
 arranged in alternating layers, which have a different, 
 if not, as in the case of the intestinal wall and iris, a 
 precisely opposite action. In experimenting on the 
 retractor penis of the horse, dog, and some other 
 mammals, where the fibres form a mass of considerable 
 size, almost pure, and easily accessible, Sertoli has 
 found that this example of smooth muscle retains its 
 excitability for an extraordinary time (with proper pre- 
 cautions to prevent loss of heat and desiccation), con- 
 tracting, in response to stimuli, five, six, or even seven 
 days after removal from the body. At a temperature 
 
2o6 Human Physiology. [Chap. xiii. 
 
 of 104' F. it soon became fatigued, and lost its sensi- 
 bility. Its most remarkable power was found to be 
 that of executing spontaneous movements even when 
 removed from the body, the contractions lasting from 
 two to six minutes, and the degree of contraction being 
 about one-fifth of the total length of the muscle. The 
 intermissions were short. The movements cease in 
 deep narcotism, in anaemia, and with deficient supply 
 of oxygen. Direct stimulation with a constant current 
 causes elongation with complete cessation of spon- 
 taneous movements when the current is passing. The 
 application of an induced current causes contraction. 
 The graphic tracing resembles that of striated muscle, 
 but exhibits a latent period of 0*8 second, which is 
 nearly 100 times longer than that of striated muscle, 
 and a period of contraction lasting from 90 to 120 
 seconds. The contraction is moderately rapid, the 
 relaxation at first slow, then more rapid, and finally 
 very slow. Tetanus supervenes when the shocks do 
 not succeed each other more rapidly than at intervals 
 of one in five seconds, or twelve per minute. 
 
 Arraiig:emeiit and application of muscles 
 in tlie body. — The muscles constitute nearly one half 
 of the total weight of the body, and they are divided 
 into several groups according to their mode of action. 
 Some have no definite origin and insertion, but 
 surround cavities, and form the walls of tubes, as in 
 the case of the muscular walls of the alimentary 
 canal, of the bladder, and uterus ; the coats of blood- 
 vessels, glands, ducts, and lymphatics, all of which have 
 for their purpose the contraction of the spaces they 
 bound, and the onward movement of their contents. 
 Others surround the orifices of the various apertures 
 of the body, and are termed sphincters. Other groups 
 of muscles have one attachment to some fixed point, and 
 the other in a soft tissue, the movement of which they 
 effect j such, for example, is the azygos uvulae. And 
 
Chap, xiii.j Joints, 207 
 
 others again, which are bj far the most numerous, 
 have two points of attachment into bone, and with the 
 bones act as levers. Prof. Haughton has pointed out 
 that in most kinds of labour which are effective and 
 usually employed, almost all the muscles of the upper 
 and lower extremity are exerted, together with the 
 muscles of the lumbar and dorsal vertebrae. He takes 
 the work of the Oxford or Cambridge eight-oar boat 
 as an example of extreme muscular exertion, and 
 shows that the work done by each man is nearly 
 4 foot-tons per minute. The average daily work of a 
 labourer is about 400 foot-tons, accomplished in 10 
 hours. The oarsman, therefore, performs in one 
 minute the hundredth part of a fair day's labour, and if 
 he could continue to work at the same rate he would 
 finish his task in one hour forty minutes, instead of 
 the customary ten hours. The work done, therefore, 
 in rowing one knot in seven minutes is, while it lasts, 
 performed at a rate equal to six times that of a hard 
 worked labourer. The most effective mode of em- 
 ploying human labour is to make a man lift his own 
 weight through a height for many hours. 
 
 Joints and joint movements. — Joints result 
 in some instances from the necessity of allowing for the 
 enlargement of the contents of cavities, as in the case 
 of the cranium. In such joints, termed synarthroses, 
 there is no movement, but in other cases they exist to 
 enable parts to move more or less freely upon one 
 another. The movement is sometimes very slight, as 
 in the articulation of the pelvic bones with each other 
 or with the sacrum, and the attachment of the bones 
 of the vertebrae to one another. These are termed 
 amphi arthroses, and they combine great strength with 
 some degree of movement. The other joints are those 
 which are intended to permit free movement, and 
 which exist between the extremities of the bones of 
 the limbs. In those which are termed diarthroses the 
 
2c8 Human Physiology. [Chap. xui. 
 
 opposed surfaces of the bones are covered with cartilage, 
 over which, as well as over the inner surface of the 
 ligaments binding them together, is originally a thin 
 membrane resembling the serous membranes, but 
 differing from them in the character of its secretion, 
 which is glairy, and adapted to lubricate the surfaces, 
 named synovia. The membrane has hence been called 
 the synovial membrane. As soon as the joint is used, 
 the cells covering the cartilage are worn away and are 
 not replaced ; and, in the adult, the membrane, which 
 is composed of a matrix of delicate connective tissue 
 with elastic fibres, and lining membrane of flattened 
 epithelial cells, appears to terminate at the margin of 
 the cartilage by a zone of polygonal endothelial cells. 
 Processes are often given oflf from the inner surface of 
 the membrane, which, projecting into the joint, contain 
 fat and vessels. The synovia is a colourless viscid 
 fluid, containing mucus, albumin, and traces of fat and 
 salts. It is diminished in quantity after active exer- 
 tion, when it also becomes thicker in consistence 
 owing to increase of the mucus. 
 
 The movable joints are divided into the ginglymus 
 or hinge joint, like that of the elbow or knee ; the 
 arthrodia, where two plane surfaces are in apposition, 
 like the os calcis and cuboid ; the enarthrodia, or ball- 
 and-socket joint, as the hip and shoulder joints; and 
 the rotatoria, as in the radio-ulnar articulation and the 
 articulation of the axis and atlas. 
 
 Positions and Movements of the Body. 
 
 The bones constitute a system of levers or rigid 
 bars, which are acted on by the muscles and eflfect the 
 movements of the body. The different kinds of levers, 
 in which the fulcrum, the weight, and the power are 
 respectively in the middle, are all represented in 
 the body. An example of the first order of lever is 
 found in the muscles moving the head forwards and 
 
Chap, xiii.l Levers in the Body. 209 
 
 backwards on the vertebral column. Thus, in the case 
 of the backward movement, the i-ectus capitis posticus 
 minor, amongst other muscles, represents the power, 
 the occipito-atlantal articulation is the fulcrum, and 
 the fore part of the head is the weight to be raised. 
 Other examples of this form of lever are found in the 
 movement of the trunk on the pelvis, of the foot on 
 the leg, and of extension of the fore-arm. An example 
 of the second form of lever, in which, as in the first 
 form, great power can be exerted by lengthening one 
 arm of the lever, is found in the gastrocnemii when 
 acting on the foot in such a manner as to raise the 
 body. The force is here applied to the os calcis, the 
 fulcrum is the point of contact of the ball of the great 
 toe with the ground, and the body is the weight to be 
 raised. The third form of lever, in which the power is 
 in the middle, is by far the most frequent, because, 
 with some sacrifice of power, there is great gain in 
 rapidity of action ; and, as a rule, rapidity is required 
 rather than strength. It is to be seen in the biceps 
 of the arm and leg, the deltoid and brachialis anticus. 
 In every position of the body, except lying, muscular 
 effort is required, though the number of muscles 
 brought into play in the sitting position is much less 
 than in standing. 
 
 Standing:. — In this position of the body it is 
 necessary that a line dropped from the centre of gravity, 
 which is situated about the second sacral vertebra, 
 should fall on the area covered by the feet, or the space 
 between them, and that the body, as a whole, in spite 
 of its numerous articulations, should be kept rigid. 
 The slightest inclination to right or left, forwards on 
 backwards, tending to cause the centre of gravf-feyf 
 to fall outside the surface covered by the fe^jis 
 counterbalanced by the action of opposing m^urii^fefi^ 
 which serve to restore the equilibrium, arid Ht^at 
 these really act is shown by the impossibiyiy^iof 
 o 
 
2IO Human Physiology. [Chap.xiii. 
 
 setting a dead man, or one who has only fainted, on 
 his feet. 
 
 Sitting:. — In this position the body rests on the 
 tubera ischii, on which a rolling movement, forwards 
 and backwards, can be made. The body and head are 
 maintained in the vertical position. If the forward 
 movement be carried so far as to allow the centre of 
 gravity, which is in front of the tenth dorsal vertebra, 
 to fall in front of the two tubera ischii, the body must 
 be supported by resting the arms on some solid body, 
 or the thighs must be supported. If the body be 
 inclined far backwards, the coccyx may help in sus- 
 taining it, whilst the psoas-iliac muscle, pectineus, 
 and other muscles contract, and fix it upon the thigh, 
 and the quadriceps extensor femoris again fixes the 
 thigh upon the leg ; the leg is then either raised 
 and forms the long end of a lever, of which the fulcrum 
 is at the tubera, and the weight is the trunk ; or 
 the flexors of the leg act and press it against the 
 support. 
 
 Walking:. — Walking is the horizontal forward 
 movement of the body, by the alternate use of the two 
 lower limbs and with the minimum of exertion. The 
 leg resting on the ground is the "active" leg; the other, 
 which is swinging through the air, is the " passive " 
 leg. The following are the two phases of the act, as 
 given by Landois. In the first, the active leg stands 
 vertically, the knee joint slightly bent and immediately 
 beneath the centre of gravity ; the passive leg is com- 
 pletely extended, and rests only with the point of the 
 great toe on the ground. In this position a rectangular 
 triangle is formed, of which the active leg forms the 
 hypothenuse. In the second phase, the active leg 
 inclines forwards, the body is maintained at the same 
 height, and, in order to accomplish this, the active 
 leg is elongated by fully straightening the knee, and by 
 raising the heel from the ground so that the foot rests 
 
Chap. XIII.] Act of Walking. 211 
 
 on the ball, and ultimately on the tip of the great toe. 
 During the period that the straightening and forward 
 inclination of the active leg is completed, the passive 
 leg leaves the ground ^Yith the tips of the toes. 
 The knee is now somewhat bent and executes a pen- 
 dulum-like swinging movement forwards, and comes 
 to occupy a position as far before the active leg as it 
 was previously behind it ; the sole of the foot is placed 
 flat on the ground, and the position of the centre of 
 gravity is now so altered by the lateral movement of 
 the body as to act through this, which is now become 
 the active leg, and is slightly bent at the knee, and 
 the first of the two phases recommences. The lateral 
 movement of the body, made with the object of shifting 
 the centre of gravity to the active leg, is accomplished 
 by the glutsei and tensor fasciae femoris, and in 
 persons of heavy build, and in women who have 
 naturally broad pelves, gives rise to more or less of a 
 waddling gait. In moderate walking the body is in- 
 clined slightly, in fast walking strongly forwards. 
 During the swinging movement of the leg, the body 
 executes a slight rotatory movement on the head of 
 the active femur; but this is to some extent com- 
 pensated by the arm of the same side swinging in the 
 opposite direction, whilst that of the opposite side moves 
 with the swinging leg. As the rapidity with which 
 the swing of the leg is accomplished varies vsdth its 
 length, it is apparent that each person must have a 
 certain natural rapidity of walk, but the duration of 
 each step depends, in addition, on the time that both 
 feet are simultaneously in contact with the earth, 
 which is dependent on the will. In very fast walking 
 this period is nil, as the moment that the active leg 
 touches the ground the passive one is raised. The 
 length of the step is about two and a half feet 
 and is longer in proportion as the length of the hypo- 
 thenuse of the passive leg surpasses the opposite side 
 
212 Human Physiology. [Chap. xiv. 
 
 of the triangle formed by the active leg. Hence, in 
 striding, the active leg is shortened by bending at the 
 knee, and the body is carried lower. 
 
 Riuiniiig' differs from walking in the circumstance 
 that for a short interval both feet are off the ground 
 together, and the body is in the air. The active leg is 
 more bent at first, and is then straightened with a 
 force that corresponds with rapidity of movement. 
 
 Leaping:. — In leaping, the hip, knee, and ankle 
 joints, previously bent in opposite directions, are sud- 
 denly extended, both leaving the ground at the same 
 time. The muscular effort made is more than is 
 sufficient to straighten the limbs, and an impulse is 
 therefore imparted to the centre of gravity of the 
 body, which is thus propelled in the manner of a pro- 
 jectile in the mean direction of the joints which are 
 thus extended. 
 
 CHAPTER XIY. 
 
 THE NERVOUS SYSTEM. 
 
 Nervous tissue is composed of nerve cells and 
 nerve fibres, which are processes given off from the 
 cells. In the central parts of the nervous system, 
 such as the brain and spinal cord, the cells are accu- 
 mulated in large numbers, and are imbedded in a 
 substance variously termed neuroglia, reticulum, 
 supporting tissue, and polio-synectic tissue. 
 
 The cells and the fibres are organically continuous 
 with one another, and impressions affecting the one 
 are propagated to the other ; but these functions are 
 so far different that the nerve cells act as receivers 
 of impressions and as originators of impulses, whilst 
 the nerve fibres are the conductors which transnrit 
 
Chap. XIV.] The Nervous System. 213 
 
 impressions from the periphery towards the cells, or 
 from the cells to the periphery. The parts of the 
 nervous system are the spinal cord and its continua- 
 tion upwards, the medulla oblongata, the pons, a chain 
 of gansrlionic masses situated at the base of the 
 brain, the cerebrum, the cerebellum, and the various 
 nerves that originate in or are indirectly connected 
 with these several parts. Before describing the 
 functions of the centres of the nervous system it is 
 necessary to make some general observations on the 
 characters and properties of nerve tissue in general, 
 and of the mode in which it can be excited to action. 
 Chemistry of the nervous system. — The 
 nerves are essentially composed of protoplasm, that is 
 to say, of a proteid substance, and of fat compounds. 
 The grey matter which forms the outer part of the 
 brain and the inner part of the spinal cord, contains a 
 larger proportion of water than the white substance, 
 the proportion being 81-6 per cent, in the grey sub- 
 stance, and 68 "4 percent, in the white. When dried 
 at a gentle heat the grey nervous matter yields 55 '4 
 per cent, of albumin and giutin, 17 '2 of lecithin, 18 "7 
 of cholesterin and fats, 6-7 of extract insoluble in 
 ether, and 1 *5 parts of salts. The dry substance of 
 the white matter of the brain and spinal cord yields 
 only 24*7 of albumin and giutin, 9*9 of lecithin, 51*9 
 of cholesterin and fats, 3*3 of substances insoluble in 
 ether, and 0*6 of salts. Of the salts, those of potassium 
 and phosphoric acid greatly preponderate. The albu- 
 minous or proteid compounds of nervous tissue in part 
 resemble myosin, in part casein, and a form of glo- 
 bulin. The albuminoids are nuclein and neuro-keratin. 
 The principal mass of the brain is considered by 
 Liebreich to be composed of protagon, which is a 
 proteid containing both nitrogen and phosphorus. 
 The chief fatty substance is cerebrin, which is soluble 
 in hot alcohol and ether. The extractives include, 
 
214 Human Physiology. [Chap. xiv. 
 
 amongst other substances, xantliin and hypoxanthin, 
 kreatin, inosite, leucin, and lactic acid. The reaction 
 of nervous tissue at rest is either neutral or feebly 
 alkaline, but when it has been excited to action and 
 after death it becomes acid, apparently, as in the case 
 of muscular tissue, owing to the development of lactic 
 acid. 
 
 The cohesion of nerve fibres is very small, but 
 when bound totjether with connective tissue to form 
 nerve cords it presents considerable resistance to 
 rupture. The nerves possess little or no elasticity, so 
 that when cleanly divided the ends remain in appo- 
 sition. 
 
 Nerve stimiili. — Nervous tissue may be regarded 
 as matter in a very unstable state of chemical compo- 
 sition, so that the application of various external 
 agents, named stimuli, causes it to readily undergo de- 
 composition. These, as in the case of muscle, may be 
 mechanical, chemical, thermic, electric, or they may 
 be waves or impulses transmitted from other cells. 
 The effect of the stimulus is to alter the composition of 
 the cell substance and to liberate force. In the case 
 of all stimuli it has been found that a certain sudden- 
 ness of application is requisite to excite the nerve, and 
 that if the stimulus be applied with extremely gradual 
 increments of intensity, beginning from 0, it may 
 be augmented till the nerve is destroyed without pro- 
 ducing any noticeable effect. The mechanical stimuli 
 acting upon nerves may be of various kinds, such as 
 a blow, or succession of blows, pressure, traction, 
 puncture, and division. If these act on a sensory 
 nerve they give rise to impulses, which, travelling 
 centripebally along different nerves, produce sensations 
 of characteristic nature, which experience enables us 
 to refer to their cause, and to the particular nerve 
 excited. If they act on a motor nerve the impulses 
 travelling centrifugally excite contraction in the 
 
Chap. xnM Nerve Stimuli. 215 
 
 muscle they supply, or secretion in the gland to 
 which they are distributed. The smallest mechanical 
 stimulus which can be perceived is that produced by 
 the fall of 900 milligrammes through a height of one 
 millimeter. Slight pressure or extension of nerve 
 fibres increases their excitability, whilst strong stimuli 
 exhaust them. The increased excitability in the 
 former case soon disappears, and in the latter 
 case, though the part struck or injured loses its 
 excitability, the whole nerve is not exhausted, since 
 the application of the stimulus at a lower point 
 produces nearly the same effect as before. When 
 persistent pressure is made upon a motor and a 
 sensory nerve simultaneously, the excitability of the 
 motor nerve is first lost. 
 
 Thermic stimuli. — A sudden elevation of 
 temperature or a sudden depression acts as a stimulus 
 to nerves. • "Within moderate limits heat augments and 
 cold depresses their excitability. Exposure of the 
 nerves of a frog to a temperature of 50° C. (122° F.), 
 after first exalting, quickly abolishes the irritability of 
 nerve. Nerves which have been gradually frozen and 
 slowly thawed preserve their irritability for a long 
 time. 
 
 Ctiemical stimuli. — These act upon the nerves 
 when of sufiicient strength to efiect a rapid alteration 
 in their composition. The motor nerves are more 
 readily acted on than the sensory, in which respect 
 chemical differ from thermic stimuli, which more 
 readily act on the motor nerves. Rapid withdrawal 
 of water from a nerve, as bv foldino- it in blotting 
 paper or exposure to a very dry air, acts as a stimulus 
 to it. Strong solutions of the neutral salts of 
 the alkaline metals constitute stimuli ; common salt, 
 however, acts only on the motor nerves. Free acids, 
 with the exception of phosphoric acid, the alkalies, 
 many organic acids, and most of the salts of the heavy 
 
2i6 Human Physiology. [Chap. xiv. 
 
 metals, act vigorously. Much stronger solutions of the 
 acids are required than of the alkalies ; the latter are 
 still effective when in strength not exceeding 0*5 per 
 cent. Such dilute solutions first augment the excit- 
 ability and then depress it. Dilute alcohol, ether, 
 chloroform, bile and the biliary salts, and sugar, all 
 act as stimuli. They usually first induce convulsions 
 and then cause death of the nerves. Lime-water, 
 carbon bisulphide, and some others kill the nerve 
 without exciting convulsions. Carbolic acid, if 
 applied to the spinal cord, produces spasms, but if to 
 the nerves kills them without producing convulsions. 
 The last -mentioned substances act as excitants to 
 muscle when directly applied to it. Tannic acid acts 
 neither on nerve nor muscle. 
 
 Pliysiolog-ical stitiittli. — The nature of these 
 is unknown, but they may act on the centripetal 
 nerves, as in the sensations, or on the cenirifugal 
 nerves, producing either motion or the inhibition 
 of motion or secretion. 
 
 Electrical stimuli. — Electrical currents may be 
 applied to nerves either in the form of the constant or 
 of the induced current, and the effects differ in some 
 particulars. As in the case of other stimuli, no effect 
 is produced if either form of current be very gradually 
 increased from zero. Sensations or muscular contrac- 
 tions only occur when the current is suddenly applied, 
 stopped, or greatly increased or diminished in intensity. 
 In the case of the constant current applied to motor 
 nerves^ contraction only takes place at the moment 
 when the current, of which the nerve forms a part, is 
 either made or broken. During its steady passage no 
 contraction occurs when weak, but with a certain 
 moderate strength of current the muscle may be thrown 
 into permanent tetanus, a condition that is sometimes 
 termed " galvanotonus." In the case of secretory and 
 vasomotor nerves no effect is produced during the 
 
Chap. XIV.J EXCITA TION OF NeR VE. 2 1 7 
 
 passage of the current of uniform strength. Yery 
 little effect or none is produced when the current is 
 made to pass transversely across a nerve. It must, to 
 produce contraction, pass through it in a longitu- 
 dinal direction, and the longer the portion the weaker 
 is the current required to produce a given effect. 
 A certain duration of the current is necessary ; if 
 it is briefer than y^^^Q-ths of a second no effect 
 follows. In the case of constant current applied to 
 sensory nerves, the stimulating effect is also most 
 marked at the moment of closing and opening the cur- 
 rent, the sensation being comparatively slight during 
 the passage of a current of uniform density. A greater 
 effect is produced by an electrical current of a given 
 strength when it is applied near the nerve centres than 
 when it is applied to some part near the periphery, 
 and it has been supposed that the wave of excitation 
 as it travels downwards gathers strength like an 
 avalanche, but this effect is only observed with 
 <dectrical and not with chemical stimuli. Nerve 
 tissue responds more readily to feeble currents of 
 electricity than muscle, and some nerves, as for 
 example those supplying the flexors of the leg in the 
 frog, are more easily excited than others, as the 
 extensors. The contact of a single pole of a battery, 
 by disturbing the electric relation of the nerve, will 
 produce contraction, which is then called "unipolar 
 induced action." 
 
 Action of induced ciu^rents. — Induced or in- 
 terrupted currents of electricity act very powerfully 
 upon the nerves. The induced current which is 
 established on breaking a current, has the same direc- 
 tion as this current, and is of very high tension. The 
 induced current which is established on making con- 
 tact is of feeble tension, is produced more slowly than 
 the foregoing, and runs in the opposite direction. 
 With equal strength of the inducing current, the 
 
2i8 Human Physiology. [Chap. xiv. 
 
 induced cuiTent wliich is established on breaking 
 contact constitutes a mucli more powerful stimulus 
 tlian t]iat which, is established on making contact. 
 The maximum stimulus is always produced at the 
 cathode, or negative pole. If induced currents are 
 applied to a motor nerve, beginning with those of 
 very feeble intensity, contraction first occurs with the 
 induced current established on breaking contact. As 
 the current i]i creases in force, contraction occurs also 
 on closing, so that with each interruption of the cur- 
 rent two shocks are experienced, and by degrees, as 
 the current is still increased in intensity, the two 
 shocks tend to become equal in force. 
 
 Necessity for coutmuity of tissue. — In 
 order that a nerve should act as a conductor of 
 nerve impulses it must be in direct continuity with 
 its centre. If a nerve be divided the centres can 
 no longer act upon muscles through it, nor can any 
 sensation be conducted by it from the periphery to the 
 nerve centre v^dthin a week after division. " Trau- 
 matic degeneration " begins in the peripheric 
 extremity, the nerve becoming softer, its axis cylinder 
 and medullary sheath breaking up into a fatty sub- 
 stance that ajDpears in the form of drops, and an 
 albuminous portion, the former of which, after a time, 
 undergoes absorption. In the regeneration of mixed 
 nerves the feeling first returns and then the power of 
 motion. At the point of section the ii-ritability of the 
 nerve is at first increased, and this condition of 
 increased irritability then gradually travels down- 
 wards from the centre towards the periphery, and is 
 succeeded by loss of irritability. 
 
 Electric currents in nerve and muscle. — 
 In perfectly fresh muscle and nerve as they exist in 
 the living body it is doubtful whether any electrical 
 current exists, but a few minutes after removal from 
 the body, if one end of a wire be placed on the surface 
 
Chap. XIV.] Electric Currents IN Nerved. 219 
 
 or natural longitudinal section of either a nerve or a 
 muscle, whilst the other is applied to the cut surface 
 or artificial transverse section of the same, and a 
 galvanometer be introduced into the circuit, it can be 
 shown that a strong current sets from the longitu- 
 dinal to the transverse surface ; the former must 
 therefore be positive, as compared with the latter, 
 which is negative; as a consequence there must be 
 within the muscle a current in the opposite direction, 
 setting, that is, from the transverse section to the 
 longitudinal section; and it has been suggested that a 
 reason for this may be that the cut surface, being 
 exposed to the air, oxydises more rapidly than the 
 longitudinal surface, which is protected by the sheath 
 of the muscle, and being the seat of more active 
 chemical changes, is therefore positive to the longi- 
 tudinal section. Two points that are equally distant 
 from the equator of the longitudinal section of the 
 nerve, or from the centre of the transverse section, 
 give no current, but if at unequal distances, then the 
 most remote is positive as regards the other. The 
 electro-motor power of a large nerve is equal to 0*02 of a 
 Daniell's cell, of a laroe and stronoj muscle 0-05 to 0-08. 
 It increases on moderate elevation of temperature. 
 
 Physiological rheoscope. — This is a pre- 
 paration of a frog, in which the longitudinal and 
 transverse sections of the gastrocnemius are connected 
 by any moist conductor ; as soon as the sciatic nerve of 
 a frog, which is isolated, but still in connection with 
 the lower leg, is placed on this, a contraction of its 
 gastrocnemius follows, and the same occurs when the 
 sciatic is raised. 
 
 Negative variation of tfee natural electric 
 current of muscle. — If a muscle be thrown into 
 tetanus, and the electric current be tested, it will be 
 found that the natural current rapidly diminishes, 
 and sinks to zero, or that it may be actually reversed, 
 
2 20 Human Physiology. [Chap. xiv. 
 
 especially if the primary muscle current have been 
 strong, and if the muscle contract energetically. 
 
 Ritter-VaBli's law. — If a nerve be divided, or 
 if the centre is destroyed, the nerve, beginning at the 
 central end, is rendered more excitable throughout ; 
 its excitability then diminishes till it becomes totally 
 extinct. In cases of divided nerves the loss of excita- 
 bility takes place much more quickly in the part con- 
 nected with the centre than in the peripheral portion. 
 
 Electrotoniis. — If a constant current be made 
 to traverse a portion of the length of either a motor or 
 sensory nerve, not only the part between the poles or 
 " intrapolar '' area, but the rest of the nerve, both 
 above and below, undergoes a change. The current is 
 called a polarising current ; the change is that the 
 nerve is thrown into a condition termed electrotonus. 
 The excitability of that portion of the nerve which is 
 situated near the positive pole is lowered, whilst the 
 excitability of that portion which is near the negative 
 pole is exalted. This change may be shown by the 
 application of any stimulus to the parts in question. 
 Thus, if an electric current be used, a weaker current 
 is required to produce the same effect if applied in 
 the neighbourhood of the negative as compared with 
 that required near the positive pole. The condition 
 of the nerve on both sides of the positive pole is 
 termed anelectrotonus, and on both sides of the 
 negative pole, cathelectrotonus. There is, with 
 moderately strong constant currents, a neutral region 
 in the intrapolar area at which the excitability of the 
 nerve is neither increased nor diminished ; with weak 
 currents this indifferent point is nearer the anode or 
 positive pole, with strong nearer the cathode or 
 negative pole of the constant current. The nerve 
 experiences a stimulus at the moment when a current 
 is closed and at the moment when it is opened, that 
 is, at the moment when electrotonus is established ; 
 
Chap. XIV.] ElECTROTONUS. 221 
 
 and when it disa^jpears on closure of the current, this 
 stimuhis occurs at the cathode alone, or at the moment 
 when cathelectrotonus is established ; on opening the 
 current the stimulus occurs at the anode alone, or at 
 the moment when anelectrotonus is established. Of 
 these two stimuli, that caused bj the production of 
 cathelectrotonus acts more powerfully than that caused 
 by the disappearance of anelectrotonus. The con- 
 traction of muscle resulting from closure and opening 
 of the current transmitted through a nerve differ 
 with the direction and the strength of the current. 
 (1) Very weak currents, whether directed upwards or 
 downwards, only produce contraction on closure of the 
 current. The disappearance of electrotonus is so 
 feeble a stimulus that the nerve scarcely reacts. "With 
 currents of middle strength contraction occurs both 
 with the ascending and with the descending current 
 on opening and on closure. Yery strong descending 
 currents cause contraction only with closure of the 
 current. The opening current fails because nearly 
 the whole intrapolar area is rendered incapable of 
 conduction. Yery strong ascending currents present 
 only contraction on opening, on the same ground. 
 
 Functions of nerves. — The different nerves 
 may be divided into deffnite groups, which have 
 certain features in common, whilst they present minor 
 differences. The chief groups are : 
 
 (1) The centrifugally conducting nerves, which 
 include : 
 
 a. Motor nerves for the striated muscles, 
 
 usually voluntary. 
 h. Motor nerves for the smooth muscles, 
 usually involuntary. 
 
 c. Secretory ^ 
 
 d. Trophic 
 
 nerves. 
 f. Yaso dilator 
 
 e. Inhibitory ( 
 
222 Human Physiology, [cuap. xiv. 
 
 (2) The centripetally conducting nerves, which 
 include, 
 
 a. Nerves of common sensation. 
 h. is'erves of special sense. 
 c. Nerves ministering to reflex action, or 
 excito-motor nerves. 
 
 (3) Intercentral fibres. 
 
 The functions of these nerves are determined by 
 the terminal apparatus and special structures with 
 which they are connected at their extremities. The 
 effects observed, for example, when a motor nerve is 
 stimulated, being due to its distribution to muscle 
 and connection with a motor nerve cell ; and of a 
 nerve of common sensation, to its distribution to the 
 skin at one end, and to a sensory nerve cell at the 
 other. The intimate relation between nerve and 
 muscle is shown by the fact that cells have been 
 found, one part of which is sensory while the other is 
 contractile. 
 
 Mode of determiiiiiig: t2ie function of a 
 nerve. — If a hitherto undescribed nerve were dis- 
 covered in the body, its function would be determined 
 by a consideration both of its anatomical origin and 
 distribution and of its physiological characters. In 
 regard to its anatomical disposition, it would be 
 noticed whether it sprang from a tract giving origin 
 to other known motor nerves, and from large ganglion 
 cells, and whether it was distributed to muscle, gland, 
 skin, or membrane, or whether it simply connected 
 different parts of the nervous system together. In 
 regard to its physiological characters, it would be sub- 
 jected to experiment ; the effects of stimulation at 
 some part of its course would be observed ; it 
 would be divided, and the effects of division, both 
 immediate and remote, would be noticed ; the peri- 
 pheric stump would be stimulated with electrical 
 currents, or other stimuli, and the results of such 
 
Chap. XIV.] Functions of Nerves. 223 
 
 irritation carefully noted, whether muscle contracted, 
 or gland secreted, or some change occurred in the 
 structure or function of the organs to which it 
 had been ascertained, by anatomical investigation, 
 to be distributed ; and, lastly, the proximal stump 
 of the divided nerve would be similarly stimu- 
 lated and the effects observed. By the adoption 
 of these measures, it has been ascertained that the 
 purposes fulfilled by nerves, or their functions.^ differ 
 remarkably. 
 
 Sensory nerves. — These are divided into nerves 
 of common sensation and nerves of special sense. The 
 nerves of touch, or common sensation, issue from the 
 spinal cord by the posterior roots of the spinal nerves, 
 which, after a short course, have a ganglion upon 
 them. Immediately beyond the ganglion the sensory 
 roots join with the anterior or motor roots, and the 
 ordinary mixed nerves (that is, nerves containing both 
 motor and sensory fibres) are formed. If ti-aced into 
 the spinal cord, many of the sensory fibres are seen to 
 be connected with small ganglion cells in the posterior 
 horns of the grey matter. When traced in a peri- 
 pheral direction, the nerves of common sensation are 
 found to terminate chiefly in the skin. The evidence 
 on which the function of a nerve is determined to be 
 sensory is that, if it be divided, stimulation of the 
 distal extremity is witliout effect ; stimulation of the 
 proximal end produces pain and reflex movements. 
 The nerves of special sense, including those minis- 
 tering to vision^ hearing, taste, and touch, will be 
 subsequently considered. 
 
 Motor nerves. — Motor nerves are those which 
 are distributed to muscle, and incite it to contract. 
 The motor nerves of the body generally spring from 
 the spinal cord or its continuation upwards in the 
 medulla oblongata and brain. They issue from the 
 spinal cord by the anterior roots, Vvdiich are connected 
 
224 Human Physiology. [cuap. xiv. 
 
 witli large cells in the cord, and fceiminate by piercing 
 the sarcolemma of the muscle fibres, and becoming 
 continuous with the proper muscular substance. On 
 section of any motor nerve, the muscles supplied by 
 it cease to be under the influence of the will, or 
 become paralysed. Stimulation of the proximal end 
 of the divided nerve is without effect ; stimulation of 
 thejii|istal end causes contraction of the muscle until 
 degeneration sets in. 
 
 Reciu'rent sensibility. — It has been said that, 
 after division of a motor nerve, stimulation of the 
 proximal portion is without effect \ that is to say, 
 that it produces neither motion nor pain. But this is 
 not strictly correct ; for, if the motor root of one of 
 the spinal nerves be divided, and a stimulus applied 
 to the cut extremity of the portion still attached to 
 the cord, some pain is felt. It is believed that this is 
 due to recurrent fibres of the sensory root, which, 
 after having joined the motor root^ instead of going as 
 usual to the periphery, revert to the spinal cord, and 
 enter it by the motor root. It has even been shown 
 that the peripheral mixed nerves have some degree of 
 recurrent sensibility. Thus, if the median nerve be 
 divided, it is found that, after a few hours, the distal 
 stump possesses sensibility. The sensory fibres, there- 
 fore, which run in the median nerve cannob all ter- 
 minate in the periphery, but some of them must turn 
 back and run in a recurrent direction in the ulnar or 
 musculo-spiral nerves. 
 
 Secretory nerves. — This term is applied to 
 those nerves which, when stimulated, excite secretion. 
 The best-known examples are those of the chorda 
 tympani, which, when stimulated, causes a flow of saliva 
 from the submaxillary and sublingual glands ; the 
 nervus petrosus superficialis minor, which is a branch 
 of the glosso-pharyngeal, and, after entering the otic 
 ganglion, joins the auriculo-temporal nerve, and is 
 
Chap. XIV.] Trophic Nerves. 225 
 
 distributed to the parotid gland ; when stimulated, it 
 excites the parotid to secrete saliva ; the lachrymal 
 and subcutaneus malse, which, when stimulated, 
 cause a flow of tears ] the nerves exciting the sweat- 
 glands to secrete ; and those distributed to the 
 mammary gland. 
 
 Trophic nerves. — The belief in the existence of 
 trophic nerves, or nerves ministering to the nutrition 
 of a part, has been chiefly derived from observation of 
 the efiects of lesion of nerves. Thus, if the fifth 
 nerve be divided in the skull, it is found that, after 
 six or eight days, inflammation of the cornea sets in, 
 and the whole eye, becoming gradually involved, is 
 lost; and it is supposed that, the influence of the 
 nerves having been removed by the section, the nutri- 
 tion of the tissues is interfered with, and they die. 
 But it has been pointed out that another explanation 
 may be given of the phenomena, and that whereas 
 in healthy conditions the surface of the eye is 
 acutely sensitive, and the contact of foreign bodies is 
 prevented by movements of the head, closure of the 
 lids, and flow of tears ; after section of the fifth nerve 
 the sensibility is entirely lost, and particles of dust, or- 
 ganic ferments, and germs of various kinds, may easily 
 adhere to the surface, whilst, no sensation of dryness 
 being conducted to the reflex centres of the seventh 
 nerve, the lids are not duly closed when required, and 
 the cornea desiccates. In proof of this, it is asserted, 
 first, that similar changes, though of a less serious 
 character because the vaso-motor nerves are not 
 implicated, may be observed when, from paralysis of 
 the seventh nerve, the lids are kept permanently 
 open ; and secondly, that, if the cornea be pre- 
 served from injury by luting a watch-glass over 
 the eye, the inflammatory symptoms may be pre- 
 vented from occurring, and cured if they have com- 
 menced. 
 
2 26 Human Physiology. [Chap. xiv. 
 
 Inhibitory nerves. — This term is applied to 
 nerves which arrest or prevent action. The brain 
 exerts an inhibitory power over reflex actions in gene- 
 ral. Thus, it is natural to start, or move the body or 
 limbs, from any sudden stimulus, as a cut or burn ; 
 but the will can overpower this movement, and main- 
 tain the limb in position notwithstanding the pain. 
 The best examples of inhibition are the action of the 
 vagus nerve on the heart and the action of the 
 splanchnics on the movements of the intestine. When 
 the vagus is stimulated, the heart stops in diastole. 
 The mode in which inhibitory impulses act is not 
 clearly understood ; but it may be conceived that 
 impulses coming from one centre counteract or sup- 
 press those which are about to be liberated from 
 another, just as two beams of polarised light of equal 
 intensity, when meeting under a certain angle, occa- 
 sion darkness. 
 
 Taso-dilator nerves. — The best examples of 
 these are seen in the nerves distributed to glands and 
 to the erectile organs. In the case of the salivary 
 glands, the blood, when the gland is at rest, flows 
 from an opened vein slowly and with its usual dark 
 colour ; but when the nerves containing the vaso- 
 dilating fibres are stimulated, the arteries distributed 
 to the gland enlarge, and the resistance to the passage 
 of blood through the gland falls so remarkably that 
 the speed of the venous current is greatly augmented, 
 and presents arterial pulsation, whilst the colour of 
 the blood is as bright as that contained in the arteries. 
 This change in the activity of the circulation is not 
 necessarily accompanied by increased secretion. Simi- 
 lar phenomena may be seen in the erectile organs, but 
 it is not known how the dilatation is efltected. 
 
 Vaso-motor fibres. — These fibres might with 
 propriety be termed vaso-constrictor fibres. They 
 have for their function the contraction of the smaller 
 
Chap. XIV.] VaSO-MOTOR FiBRES. 227 
 
 arteries. The vaso-motor fibres are chiefly contained 
 in the sympathetic nerves, and, like the vaso-dilator 
 nerves, have been particularly studied in the salivary 
 glands and in the vessels of the ear. It has been 
 found that when the sympathetic fibres distributed to 
 the ear or ofland are stimulated the arteries contract, 
 the supply of blood is greatly diminished, the veins 
 return but little blood of a dark colour, and the 
 temperature of the part falls ; and in the case of the 
 gland the secretion is checked and altered in quality. 
 There is little doubt that each gland has its vaso- 
 constrictor and its vaso-dilator fibres, but it is not 
 practicable in most instances to isolate them. 
 
 Rapidity of conduction in nerves. — The 
 rapidity with which nerve impulses are propagated 
 through motor nerves is 21 \ meters, or a little more 
 than 30 yards per second in the frog. In man it is 
 estimated to be 33-9 meters, or about 38 yards per 
 second. The mode in which this has been determined 
 consists, in the case of the frog, in isolating as long a 
 portion as possible of the sciatic nerve, and applying 
 a stimulus first to the end of the nerve most remote 
 from the gastrocnemius, and then to the part which is 
 nearest to the muscle, and obtaining simultaneous 
 tracings with a rapidly-revolving cylinder, or with 
 the pendulum myograph, of the moment when the 
 current is applied, and of the contraction of the 
 muscle. The rapidity with which impressions are 
 conducted through sensory nerves has been given very 
 difi*erently by difierent observers, and even by the 
 same observer experimenting on difierent persons. 
 Thus Kohlrausch, from numerous experiments made 
 on four persons, found the rapidity of conduction of 
 sensory impressions to be 56, 82, 129, and 225 meters 
 per second. The mean of all his experiments gave 94 
 meters. De Jaager believed it to be 26 meters. The 
 rapidity of conduction is retarded by cold, and by the 
 
228 Human Fhvsiologv. [Chap. xiv. 
 
 condition of anelectrotonus, and in motor nerves by 
 curare. It is augmented by warmth, by an increase 
 in the strength of the stimulus, and by the establish- 
 ment of cathelectrotonus. In man the contraction of 
 the muscles of the thumb has been used for experi- 
 mental purposes, the nerve-stimulus having been 
 applied at one time to the median nerve in the 
 axilla, and at another to the same nerve at the vi^rist. 
 When a stimulus is transmitted through a nerve, it is 
 found that the part of the nerve through which it is 
 made to pass exercises considerable influence on the 
 rapidity of conduction. This was first shown by 
 Munk, who stimulated the motor nerve of a frog in 
 three places, and found that with equal length of 
 nerve the portion between the middle and lower points 
 of stimulation conducted twice as rapidly as that' be 
 tween the upper and middle point. 
 
 Conduction in both directions. — It may be 
 taken as certain that when either a nerve of common 
 sensation or a motor-nerve is stimulated, the stimulus 
 is propagated in both directions ; so that a wave 
 travels not only downwards in a motor and upwards 
 in a sensory nerve, but upwards in a motor and 
 downwards in a sensory nerve. The nerves are, in 
 fact, only conductors, and the effect produced when they 
 are stimulated depends upon the nature of the organ 
 with which they are connected at their termination. 
 Thus, if the hypoglossal and lingualis nerves, where 
 they are in close contiguity to each other, be divided, 
 and the peripheric extremity of the hypoglossus be 
 united by a suture Avith the central end of the 
 lingualis, when union has taken place, contractions of 
 the muscles of the tongue can be produced by 
 stimulation of the lingualis. The stimulus must here 
 be propagated in the opposite direction to that which 
 is natural in the sensory lingual nerve. Similar 
 evidence has been obtained by Bert, who implanted 
 
Chap. XIV.] Automatic Movements. 229 
 
 the end of tlie tail of a rat into a cut in the skin of the 
 back, and when it had become thoroughly adherent, 
 divided the tail at its root. After a time it was 
 found that sensory impressions were propagated along 
 the tail in the reverse direction to that which they 
 pursued previously to the section. The phenomeDa 
 of electrotonus also show that the nerve is altered 
 throughout its whole length by the application of a 
 stimulus to any part of it. 
 
 Automatic movements. — This term is appKed 
 to movements which take place after removal of the 
 whole of the nervous system, with the exception of 
 that part from which the motor nerve springs. The 
 best examples of it are the following : — (1) Those of 
 the continuance of the respiratory movements after 
 the brain has been gradually removed from above and 
 the spinal cord from below, until only a small portion 
 of the medulla oblongata is left, giving origin to the 
 phrenic and intercostal nerves. It is supposed in 
 this case that the absence of oxygen, and the pres- 
 sure of carbonic acid gas in excess in the blood 
 circulating through the medulla oblongata, stimu- 
 late the cells forming the respiratory centre, and 
 that these automatically originate the movements 
 observed. 
 
 (2) Those of the heart, in which rhythmical con- 
 tractions occur after the organ has been entirely re- 
 moved from the body, and therefore when no action 
 of the central nervous system can take place. It is 
 supposed in this case to be due to the rhythmical 
 liberation of impulses from the ganglia situated in the 
 heart itself. 
 
 (3) Those of the stomach and intestines, where, 
 after isolation of a segment of the canal, peristaltic 
 movements may still be observed, apparently without 
 the application of any stimulus. 
 
 (4) Those of the bladder and ureters, which move 
 
230 Human Physiology. [Chap. xiv. 
 
 after the whole of the nerves distributed to them have 
 been divided. 
 
 (5) Those of the iris, which has been seen to con- 
 tract and dilate in some animals even after the 
 anterior half of the eye has been removed from the 
 body. 
 
 (6) Those of the arteries and of the lymphatic 
 hearts, which have been observed in some animals to 
 present contractile movements after destruction of the 
 brain and spinal cord. 
 
 (7) Certain secretions, as those of the saliva and 
 bile, which continue to be formed when all the nerves 
 supplying the glands have been divided. 
 
 Reflex acts. — This term is applied to movements 
 or secretory processes, which take place in response 
 to a stimulus applied to some part of the nervous 
 system. The conditions which are requisite are : — 
 (1) A stimulus, (2) afferent or centripetal nerves, 
 (3) a nerve centre, (4) efferent or centrifugal nerves, 
 (5) muscles or glands, 
 
 (1) The stimulus may be one of the various kinds 
 already mentioned, mechanical, chemical, thermic, or 
 electric ; or it may be of a special kind, as in the case 
 of odorous or luminous stimuli. It may be applied 
 either to the peripheric extremity of the nerves, which 
 are in almost every instance connected with a special 
 terminal apparatus, or to the nerves in some part of 
 their course ; the reflex movements are, however, 
 much more orderly and purposive, as well as swifter, 
 in the former than in the latter case. The stimulus 
 may also be applied to one of the special nerves, as 
 in the vomiting produced by a disagreeable sight or 
 smell. 
 
 The distinction between a purely voluntary and a 
 reflex act is in general sufficiently evident. The act of 
 writing is a voluntary one ; the formation of each 
 letter is slowly learned, and even in those most 
 
Chap. XIV.] Laws of Reflex Action. 231 
 
 accustomed to write the process requires a distinct 
 mental effort ; but the act by which food is propelled 
 down the oesophagus is a reflex act, and is performed 
 quite independently of the will. In many instances 
 reflex acts are performed with consciousness, in many 
 without. The movements of the intestinal canal, of 
 the gall-bladder, of the arteries, of the ducts of many 
 glands are unattended by consciousness, whilst such 
 reflex acts as cougliing, sneezing, and winking of the 
 eyelids are clearly perceived. In some instances, as 
 in contraction of the pupil, the stimulus may be 
 clearly perceived ; but the contraction of the muscular 
 tissue of the iris is wholly without consciousness. 
 
 The vivacity and energy of reflex acts bears no 
 relation to the amount of pain produced. Thus, if 
 an eel be decapitated, and the surface of the body be 
 lightly touched, strong reflex movements are pro- 
 duced ; but if a thermocautery be applied to the skin, 
 a severe bum may be inflicted with very slight 
 muscular contractions. In man, in the same way, 
 tickling will produce more vigorous movements than 
 a cut. A stimulus which is so feeble as not to excite 
 a reflex act may by repetition induce it. 
 
 liSiTV's of reflex a.ctioo. — Keflex actions are not 
 irregular, but respond in a very definite manner to 
 particular stimuli, and certain laws have been dis- 
 covered to exist^ the more important of which are the 
 followinoj : — 
 
 (1) Law of unilateral action. — If the skin or 
 other sensory surface be stimulated, the muscles of 
 the same side or of the immediate vicinity are stimu- 
 lated to contract. If the skin of one leo^ be stimu- 
 lated, that leg will be drawn up by its muscles. 
 If one conjunctiva be touched, the lids of that side 
 close. 
 
 (2) Law of irradiation. — If the stimulus be 
 stronger than that required to produce unilateral action, 
 
232 Human Physiology. [Chap. xiv. 
 
 the next effect is that the corresponding muscles of the 
 opposite side are made to contract. . This is sometimes 
 called the law of reflex symmetry. If the stimulus 
 be still stronger, it affects not only the opposite side, 
 but those muscles which are supplied by nerves 
 arising higher np from the spinal cord. 
 
 (3) Law of co-ordination. — This, which might also 
 be termed the law of purposive adaptation, indicates 
 that the movements made in response to a definite 
 .stimulus are not irregular, but are performed with a 
 distinct object, and are co-ordinated to that end. 
 Thus, if a frog be decapitated, and the body be 
 suspended by the fore-limbs, if the side of the 
 abdomen be touched with a feather dipped in acetic 
 acid, the leg of that side is raised, and the foot is so 
 moved as to brush away the stimulus ; and if this leg 
 be held down, the opposite foot will be raised, and 
 after being made to cross the back will attempt to 
 perform the same movement. 
 
 (4) Law of jyrolonged irritation. — This law shows 
 that a powerful stimulus produces a persistent action 
 on the spinal cord. Thus, if the head of a frog be 
 violently struck against a hard body, the animal is 
 thrown into a tetanic state, and this tetanic condition 
 is maintained even after decapitation. 
 
 Rapidity of reflex acts. — The rapidity with 
 which a reflex act is performed has been determined 
 to vary, according to the intensity of the stimulus, 
 between 0"025 and 0"05 second. The time occupied 
 between contact with the cornea and contraction of 
 the orbicularis palpebrarum is 0-05 second. Cold 
 lowers the rapidity of reflex acts. Chloroform retards 
 them. The administration of strychnia accelerates 
 thenL Moderately high tempei-ature accelerates 
 them. 
 
 Some reflex acts are of so complex and purposive 
 a character as almost to sn2;2:est a certain decjree 
 
Chap. XIV.] Cerebral Nerves. 233 
 
 of consciousness in the spinal cord, since tliey take 
 place after the brain has been removed ; such are the 
 crawlinof movements of a froo^ when the board on 
 which it lies is tilted, and the fluttering of the wings 
 of birds. A remarkable experiment is known as 
 " Goltz' Quarrversuch," in w^hich a frog, from which 
 the brain has been removed, is made to croak by 
 stroking the skin of the back. 
 
 Functions of tlie cerebral nerves. — The 
 cerebral nerves are twelve in number, and their func- 
 tions may here be briefly given : — 
 
 (1) The olfactory nerve. — The olfactory nerve 
 arises from the olfactory bulb, which is really a pro- 
 cess of the brain itself, and whilst small and degenerate 
 in man, is large and highly developed in many animals, 
 with extensive intracerebral relations. It is distri- 
 buted to a definite region of the nose, the epithelium 
 of which presents special characters. {See Klein's 
 " Histology.") Stimulation of this nerve by its proper 
 stimuli excites odorous sensations. It reacts but 
 slightly, if at all, to mechanical or other stimuli. Its 
 division or non -development is attended with absence 
 of the sense of smell. For the sense of smell to act 
 it is necessary that the membrane should be moist, 
 and that the stimulus should be in the gaseous state, 
 or at least in a state of such fine division as to be 
 suspended in the air. In consequence of the distribu- 
 tion of the fifth nerve to part of the Schneiderian 
 mucous membrane, pain or tingling may be ex- 
 perienced with certain odorous substances, such as 
 ammonia or carbonic acid gas. 
 
 (2) Optic nerve. — The optic nerve and tract con- 
 duct visual impressions from the retina to the brain. 
 The optic tract arises chiefly from the grey matter of 
 the optic thalamus, and from the corpora quadrigemina, 
 but other fibres may be traced into the medulla oblon- 
 gata and spinal cord, and others again radiate towards 
 
234 Human Physiology. [Chap. xiv. 
 
 the apex of tlie occipital lobe of tlie cerebrum, where, 
 according to some authors, the psychic centre of the 
 visual sense is located. At the chiasma, a more or 
 less complete decussation of fibres takes place. In 
 some instances it is complete, the tracts crossing with- 
 out any interchange of fibres, but in the great majority 
 of cases there is a partial decussation, the fibres of the 
 rio-ht tract being distributed to the left halves of the 
 two retinse, and those of the left tract to the two right 
 halves. The terminal apparatus of the optic nerve in 
 the eye is the retina. The normal stimulus of the 
 optic nerve is light, but luminous sensations can be 
 excited by other stimuli, as by pressure, concussion, 
 and electricity. Various reflex acts can be induced 
 through the optic nerve ; thus light falling on the 
 retina causes contraction of the iris by a reflex influ- 
 ence acting through the third nerve upon the sphincter 
 pupillse, whilst the absence of light acts through the 
 sympathetic nerve in inducing dilatation of the pupil. 
 Exposure to bright lights, again, causes closure of the 
 lids by reflex impulses conveyed through the facial 
 nerve ; and, lastly, a flow of tears may be induced by 
 reflex impulses conducted through the fifth nerve. 
 Section or destruction of the nerve causes complete 
 and permanent blindness. 
 
 Third or oculo-motor nerve. — The third nerve arises 
 in common with the fourth from a nucleus situated 
 just below the Aquseductus Sylvii, which is a continua- 
 tion of the anterior horn of the grey substance of the 
 spinal cord. It is in relation with the nates, and with 
 the lenticular nucleus. It is connected in the cavern- 
 ous sinus with the first branch of the fifth nerve, 
 and thus obtains fibres ministering to muscular 
 sensibility, and with the sympathetic nerve which 
 supplies it wdth vasomotor branches. It is dis- 
 tributed to the muscles rotating the globe of the 
 eye, named the rectus superior, rectus intemus, 
 
ciiap. XIV.] Cerebral Nerves. 235 
 
 rectus inferior, and the obliqiuis inferior, and also 
 to the levator palpebrse siiperioris. It also supplies 
 two muscles Avithin the eye, the sphincter pupillse and 
 the ciliary muscle or tensor choroideee, by means of 
 which the accommodation of the eye for near objects 
 is effected. The three centres which usually act toge- 
 ther, viz., for accommodation, for contraction of the 
 pupil, and for convergence of the eyes, accomplished by 
 the recti interni, are situated in that order from before 
 backward in the posterior part of the floor of the third 
 ventricle. The centre for the reflex contraction of the 
 pupil from light falling on the retina is situated in 
 the medulla oblongata. Atropin, daturin, duboisin, 
 and other drugs paralyse the intraocular fibres of the 
 third. Calabar bean, opium, and others act as stimuli 
 to them. Complete paralysis of the third nerve is 
 followed by (1) drooping of the upper lid, termed ptosis, 
 (2) immobility of the eye in all directions, (3) eversion 
 of the eye or external squint owing to the unant agonised 
 action of the external rectus, (4) slightly increased 
 prominence of the eye owing to the action of the 
 obliquus superior, (5) dilatation of the pupil, (6) im- 
 mobility of the pupil on exposure of the eye to light, 
 and (7) loss of the power of accommodation. 
 
 Fourth nerve^ or trochlear nerve. — This is the 
 smallest of the cerebral nerves, and supplies one 
 muscle only, the superior oblique muscle of the eye. 
 The fibres of origin spring by an anterior root from 
 the trochlear nucleus, which is continuous with the 
 anterior horn of the spinal cord, and is situate near 
 the valve of Yieussens, and by a posterior root con- 
 nected with the ganglion of the fifth. Stimulation 
 of the nerve causes contraction of the superior oblique 
 muscle, and the eye rolls downwards and outwards. 
 
 Fifth nerve, or nervus trigeminus. — This nerve is 
 the nerve of common sensation for the side of the head 
 and face, and supplies the muscles of mastication with 
 
236 Human Physiology. [Chap. xiv. 
 
 motor fibres. It is named the trigeminal from its 
 dividing into three large branches just beyond the 
 Gasserian ganglion. It arises by two roots, which 
 appear at the side of the pons ; the anterior is the 
 smaller, and is a motor nerve ; the posterior is larger, 
 and is sensory. The motor root springs from a grey 
 mass near the middle line of the floor of the fourth 
 ventricle. The sensory root springs from a grey 
 nucleus situated in the pons, and from the grey sub- 
 stance of the posterior cornu of the cervical portion 
 of the spinal cord, as far down as the third or fourth 
 cervical vertebra. The trophic root springs from a 
 grey mass at the side of the Aquseductus Sylvii. Other 
 fibres spring from the substantia ferruginosa, from the 
 cerebral peduncle, and from the cerebellum. If the 
 distribution of the nerve be followed in the order of 
 the three divisions, the functions of the branches are 
 as follow : — 
 
 First or ophthalmic branch, 
 
 (1) Recurrent branch supplies sensory branches 
 
 to the tentorium cerebelli. 
 
 (2) Lachrymal nerve supplies secreto - motory 
 
 branches to the lachrymal gland, sensory 
 branches to the conjunctiva, upper lid, and 
 temple. 
 
 (3) Frontal nerve supplies sensory branches to 
 
 the brow and upper lid. 
 
 (4) Nasal nerve supplies sensory branches to 
 
 the conjunctiva, caruncle and lachrymal sac, 
 to the tip of the nose and part of the septum, 
 and gives off the long or sensory root to the 
 ciliary ganglion. 
 
 TJie second or siqjerior maxillary nerve. 
 
 (1) Recurrent nerve supplies sensory branches to 
 the dura mater. 
 
Chap. XIV.] Ganglia of the Fifth Nerve. 237 
 
 (2) Subcutaneous malar supplies sensory branches 
 
 to the cheek and temple, and some secreto- 
 motor fibres to the lachrymal gland. 
 
 (3) Superior, posterior, and middle alveolar 
 
 nerves supply the teeth and ujoper jaw with 
 sensory fibres. 
 
 (4) Infraorbital nerve supplies the lower lid, 
 
 cheek, ala of the nose, and upper lip with 
 sensory branches. 
 
 The third or inferior maxillary nerve. 
 
 (1) Recurrent nerve supplies sensory branches to 
 
 the dura mater. 
 
 (2) Pterygoid, temporal, masseteric nerves supply 
 
 motor branches to the muscles of mastication. 
 
 (3) Buccinator nerve supplies sensory fibres to the 
 
 buccal mucous membrane. 
 
 (4) Lingualis nerve supplies nerves of special 
 
 sense to the tongue, anterior arches of the 
 palate, tonsils, and fioor of the mouth. It 
 contams vasomotor and vasodilator nerves 
 for the vessels of the tongue. 
 
 (5) Alveolar nerve supplies sensory fibres to the 
 
 gums and skin of the chin and lower lip, 
 and motor fibres to the mylo-hyoid, anterior 
 belly of the digastric, triangularis menti and 
 platysma muscles. 
 
 Four ganglia are connected with the branches of 
 the fifth pair of nerves, the ciliary, sphenopalatine, 
 submaxillary, and otic. 
 
 (1) The ciliary or lactaryma] gfaoglion. — 
 
 This is connected with the ophthalmic division of the 
 fifth, lies in the orbit, and has three roots : a short 
 motor root from the third, a long sensory root from 
 the nasal nerve, and a sympathetic root from the 
 carotid plexus. 
 
238 Human Physiology. [Chap. xiv. 
 
 It gives off sensory fibres to tlie cornea and con- 
 junctiva, tile iris, choroid and sclera ; vasomotor fibres 
 for the vessels of the iris, choroid, and retina ; inotor 
 fibres for the dilatator of the pupil ; trophic nerves for 
 the eye. 
 
 (2) The splieuo-palatine g'aaig'lioii is situ- 
 ated just below the second division of the fifth, 
 where it traverses the spheno-maxillary fossa. It 
 receives sensory fibres from the second division of the 
 fifth ; motor fibres from the facial, through the nervus 
 petrosus superficialis major; sympathetic fibres from 
 the carotid plexus. 
 
 It gives ofi" sensory fibres to the interior of the 
 nose, hard and soft palate, and tonsils ; motor fibres to 
 the levator palati, and azygos uvulae ; vasomotor fibres 
 to the mucous membrane of the nose ; and probably 
 secretory fibres to the glands of the Schneiderian 
 mucous membrane. 
 
 (3) The otic gang-lion is situated just below 
 the foramen ovale on the inner side of the inferior 
 maxillary nerve, close to the origin of the internal 
 pterygoid branch. It receives motor fibres from 
 the third division of the fifth ; vasomotor fibres from 
 the sympathetic plexus surrounding the arteria 
 meningea media ; and sensory fibres from the glosso- 
 pharyngeal through the nervus superficialis minor. 
 
 It gives off motor fibres to the tensor tympani and 
 to the tensor palati muscles ; and secretomotory fibres 
 to the parotid gland, which join the auriculo-temporal 
 nerve. 
 
 (4) The submaxillary ganglion. — This is 
 situated above the deep portion of the submaxillary 
 gland. It receives motor fibres from the chorda 
 tympani, and therefore from the facial nerve ; sensory 
 fibres from the third division of the fifth ; vasomotor 
 fibres from the sympathetic plexus surrounding the 
 facial artery. It gives off vasodilator fibres obtained 
 
Chap. XIV.] Submaxillary Ganglion. 239 
 
 from the chorda tympani, causing dilatation of the 
 vessels of the gland ; secretomotoi- fibres through the 
 same channel, stimulation of which excites the secre- 
 tion of the submaxillary gland, and trophic fibres to 
 the same gland. 
 
 The sixth nerve, or abducens oculi. — This 
 nerve springs from a nucleus continuous with and 
 above that of the facial ner^-e, and continuous also with 
 the anterior horn of the grey substance of the spinal 
 cord. It is situated at the upper part of the floor of 
 the fourth ventricle. It appears at the posterior 
 border of the pons. It is a motor nerve and supplies 
 one muscle only, the external rectus. It contains 
 some vasomotor fibres, and some fibres conferring 
 upon it muscular sensibility, which are derived from 
 the sympathetic and the fifth, from both of which it 
 receives branches in the cavernous sinus. 
 
 The seventh nerve, or portio du7'a. — The portio dura 
 is a motor nerve distributed to the muscles of expres- 
 sion and to the salivary glands. It arises from the facial 
 nucleus, situated near the upper part of the floor of 
 the fourth ventricle, and from the lenticular nucleus 
 of the opposite side. It appears at the upper pai-t of 
 the medulla oblongata in a triangular space, bounded 
 in front by the pons, in front by the olivary and 
 behind by the restiform bodies, and is here divided 
 into two parts, the smaller of Avhich is named the 
 portio intermedia. This intermediate portion is a 
 remnant of a condition present in the lower animals, 
 in which the facial and glosso-pharyngeal nerve issue 
 together as a mixed nerve. The fibres of the portio 
 intermedia may be traced into the glosso-pharyngeal 
 nucleus. It is supposed that they may contribute 
 gustatory fibres and vascular nerves. The portio 
 dura is distributed : (1) By the nervus petrosus super- 
 ficialis major to the levator palati and azygos uvulae, 
 having previously entered the sphenopalatine ganglion. 
 
240 Human Physiology. [Chap. xiv. 
 
 (2) By branches passing through the otic ganglion to 
 the tensor palati and tensor tympani. (3) It sup- 
 plies the stapedius. (4) It gives off secreto-motor 
 fibres and vaso-dilator fibres to the sublingual and sub- 
 maxillary glands. (5) It contains some gustatory fibres, 
 probably derived from the glosso-pharyngeal nerve at 
 its origin. (6) It contains the sweat-gland fibres of 
 the face. Lastly, it supplies all the muscles of the 
 face, the stylo-hyoid, posterior belly of the digastric, and 
 the muscles of the external ear. Section or lesion of 
 this nerve causes facial paralysis. The lids cannot be 
 closed, the face is without expression on the side of 
 the injury, the mouth is drawn up to the opposite 
 side, speech and smell are rendered imperfect, and the 
 secretion of saliva is diminished. 
 
 The eighth or auditory nerve, or portio mollis 
 of the seventh. — This nerve arises from two 
 nuclei, situated in the floor of the fourth ventricle; 
 some fibres spring from the grey substance of the 
 si^inal cord, and some extend into the cerebellum, and 
 even into the cerebral peduncle and cerebrum. The 
 auditory makes its appearance in close proximity to 
 the facial, from which it is separated by the portio 
 intermedia. The nerve fulfils two functions ; it is the 
 nerve of hearing, or that by v,^hich sound undulations 
 are conducted from the labyrinth ; and secondly, by the 
 distribution of some of its fibres to the semicircular 
 canals and amjDuUse, it aids in the preservation of the 
 upright and normal position of the body, or affords the 
 information by which the equipoise of the body is 
 maintained. Division of the nerve causes permanent 
 deafness ; stimulation of it, auditory sensations. 
 Section of the semicircular canals leads to vertio-o and 
 pendulum-like movement of the head. 
 
 Ninth or glosso-pharyngeal nerve. — The glosso- 
 pharyngeal nerve is partly motor, partly sensory, 
 and partly a nerve of special sense. It arises from a 
 
Chap. XIV,] GlOSSO-PHARYNGEAL NeRVE, . 2^1 
 
 nucleus situated in the lower half of the floor of the 
 fourth ventricle, consisting partly of small cells, from 
 which the sensory and special sense fibres arise, and 
 partly of large cells ; some fibres also arise from 
 the spinal cord. The motor fibres are distributed to 
 the stylo-pharyngeus, the middle constrictor of the 
 pharynx, and perhaps also to the palato-glossus, 
 levator palati, and azygos uvulae, but these branches 
 may be derived from the facial by the communicating 
 branch between this nerve and the petrosal ganglion 
 of the glosso-pharyngeal. 
 
 The sensory fibres supply the posterior part of the 
 tongue, the anterior surface of the epiglottis, the 
 tonsils, the anterior pillars of the fauces, the soft 
 palate, and part of the pharynx. 
 
 The special sense fibres which minister to the 
 sense of taste are distributed to the posterior third of 
 the tongue, the sides of the soft palate, and the pillars 
 of the fauces. 
 
 Tenth nerve, vagus or jjneumo gastric nerve. — 
 This nerve has the widest distribution of any nerve in 
 the body. It supplies the larynx and pharynx, the 
 heart and lungs, the stomach, intestines, and liver, 
 spleen, pancreas, kidneys, and bladder. It arises from 
 a nucleus in the lower part of the floor of the fourth 
 ventricle, and appears at the side of the medulla 
 oblongata, in front of the restiform body and just 
 below the glosso-pharyngeal nerve. The names, dis- 
 tribution, and functions of the chief branches .which it 
 gives off are : 
 
 (1) The meningeal branch, distributed to the me- 
 ningeal artery, and to the occipital and transverse 
 sinuses. The fibres are sensory. 
 
 (2) The auricular branch, distributed to the meatus 
 auditorium, sensory in function. 
 
 (3) Connecting branches, the function of which 
 is unknown, between the ganglion petrosum of the 
 
242 Human Physiology. tchnp.:xtv. 
 
 ojlosso- pharyngeal and the ganglion jngiilare of tlie 
 vagus. A large communicating branch enters the 
 vagus from the spinal accessory nerve, just below its 
 lower or plexiform ganglion This contains the 
 fibres, which, after coursing in the trunk of the nerve 
 for some distance, leave it, to be distributed as motor 
 nerves to the muscles of the larynx and oesophagus, 
 and as inhibitory nerves to the heart. 
 
 (4) Tlie superior laryngeal nerve gives off sensory 
 fibres to the mucous membrane of the larynx, the 
 stimulation of which induces coughing by reflex 
 action through a centre situated on each side of 
 the raphe, near the ala cinerea, and in addition motor 
 fibres to the crico-thyroid . 
 
 The inferior laryngeal nerve supplies motor fibres 
 to the trachea and oesophagus, and to the various 
 muscles of the larynx. Section or lesion of the superior 
 laryngeal nerve is apt to cause death by the entrance 
 of food into the air passages, which are no longer pro- 
 tected by the natural exquisite sensibility of the laryn- 
 geal mucous membrane. Section or lesion of the 
 inferior laryngeal nerve alters the voice, and is apt to 
 cause death on slight exertion, owing to the falling 
 too'ether of the vocal cords. 
 
 (5) The depressor nerve, given off from the superior 
 laryngeal nerve and the trunk of the vagus in the 
 rabbit, and passing into the cardiac plexus. It con- 
 ducts impressions centripetally, and lowers the energy 
 of the vaso-motor centre^ causing the blood pressure 
 to sink, and the heart to beat less frequently and 
 less vigorously. 
 
 (6) The vagus gives off cardiac branches, some of 
 which contain sensory fibres, others inhibitory fibres, 
 and some accelerating fibres. 
 
 (7) The pulmonary branches of the vagus are 
 partly motor, supplying the muscular fibres of the 
 bronchi ; partly vaso-motor, the fibres being derived 
 
Chap. XIV.] Distribution of the Vagus. 243 
 
 from the sympathetic ; and partly sensory, some of the 
 latter conduct impulses from the mucous membrane, 
 which excite the cough-centre, whilst others excite the 
 respiratory centre. 
 
 (8) The branches distributed to the oesophagus, 
 stomach, and intestines are partly motor and partly 
 sensory. 
 
 Landois observes that the trunk and branches 
 of the vagus contain various centripetal fibres, which 
 act on certain nervous mechanisms. These are : 
 
 (1) Fibres acting on the vaso-motor centre, some 
 of which are pressor fibres, chiefly contained in the 
 laryngeal nerves, which act in a reflex manner, and 
 cause the arteries to contract, the blood pressure 
 consequently to rise, and the heart to beat more 
 frequently. Others are dep>ressor fibres, which have 
 an opposite effect. 
 
 (2) Fibres acting on the respiratory centre^ some 
 of which excite the centres and accelerate the respira- 
 tory acts, whilst others, situated in the two laryngeal 
 nerves, depress and inhibit them. 
 
 (3) Fibres acting on the cardiac inhibitory centre, 
 which run to this centre, and excite it to action. The 
 existence of these is shown by stimulation of the proxi- 
 mal end of the divided vagus, which arrests the heart 
 in diastole. Blows or sudden distension of the stomach 
 produce the same effect. 
 
 (4) Fibres acting on the vomiting centre, which 
 can be excited by stimulation of the proximal end of 
 the divided vagus. 
 
 (5) Fibres which, when stimulated, induce arrest 
 of the pancreatic secretion, and run centripetally, since 
 this also may be induced by stimulation of the proxi- 
 mal cut extremity. 
 
 (6) Lastly, fibres may be shown by the same 
 means to exist, which act in a reflex manner in 
 inducing the formation of sugar in the li^>Tr. 
 
244 Human Physiology. ichap. xiv. 
 
 Eleventh or spinal accessor!/ nerve. — This nerve 
 springs by one fasciculus from the accessory nucleus 
 of the medulla oblongata, which is in near relation 
 with the nucleus of the vagus, and by another fasciculus 
 from a nucleus which extends down the spinal cord, 
 between the anterior and posterior roots of the spinal 
 nerves, as far as to the fifth or sixth cervical vertebra. 
 This portion, which confers its specific name on the 
 nerve, joins the vagus, and contributes to that 
 nerve the inhibitory nerve-fibres it gives off to the 
 heart, as well as its motor fibres to the larynx. The 
 accessory nerve terminates in the sterno-mastoid and 
 trapezius muscles, and it also contains some fibres of 
 muscular sensibility, derived from the posterior roots 
 of the first and second cervical nerves. The evidence 
 demonstrating that the spinal accessory nerves con- 
 tain cardiac inhibitory fibres is, that if the lov/er 
 roots be divided, the fibres of the vagus going to the 
 heart cease to exert an inhibitory action upon that 
 organ ; and, further, that these fibres undergo fatty 
 degeneration. 
 
 Twelfth or hypoglossal nerve. — This nerve arises 
 from two nuclei with large cells, and one nucleus with 
 small cells, situated near the point of the calamus 
 scriptorius. Some fibres also proceed from the brain, 
 and from the olivary body. The apparent origin is 
 from the side of the medulla oblongata, on a line with 
 the anterior roots of the spinal nerves. The hypo- 
 glossal nerve is the motor nerve of the tongue, and 
 supplies all the muscles by which it is moved, including 
 the genio-hyoid and thyro-hyoid. It contains vaso- 
 motor nerves for the vessels of the tong-ue, which it 
 receives from its connection with the superior cervical 
 ganglion. It further receives branches from the 
 ganglion of the trunk of the vagus, and a small branch 
 from the lingual of the fifth, by which it acquires 
 the power of conducting impressions of muscular 
 
Chap. XIV.] Sympathetic Nerves. 245 
 
 sensibility. From its loops of communication with the 
 upper cervical nerves branches are given off to the 
 sterno-hyoid,? sterno-thyroid, and omo-hyoid. Section 
 of the hypoglossal nerves in man abolishes the power 
 of movement of the tongue, and therefore interferes 
 with mastication, deglutition, and speech. 
 
 The Sympateetic System of Nerves. 
 
 This consists of a series of ganglia arranged on 
 either side of the vertebral column, connected by 
 cords with each other, and also with the spinal cord, 
 and of numerous scattered ganglia, from which 
 branches are given off, chiefly to the organs of 
 vegetative life, as to the alimentary canal and its 
 appendages, the blood-vessels, and generative system. 
 The nerves respond to the same stimuli as those 
 belonging to the cerebro-spinal system. 
 
 Cervical portion of tlie sysaapatSietic. — This 
 consists of three well-known ganglia, and their con- 
 necting cords, and the influence of this part of the 
 sympathetic has been chiefly ascertained by observing 
 the effects of section and of stimulation. The effects of 
 section are for the most part attributable to dilatation 
 of the smaller arteries, increased blood-pressure, and 
 the passage of a freer current of blood through them. 
 Such hypersemia, for example, affecting the retina 
 renders it more sensitive to light; the pupil, there- 
 fore, contracts ; the eye in the rabbit is drawn 
 inwards ; the lids are partially closed ; the membrana 
 nictitans is drawn over the eye ; the secretion of tears 
 is augmented; the sensibility becomes more acute. The 
 temperature is notably heightened ; the secretions of 
 cerumen and of sweat are increased ; a galvanic 
 current too weak to act on the other side may 
 here produce contractions. When the animal is 
 killed the reflex faculty lasts longer there than on the 
 
246 Human Physiology. [Chap. xiv. 
 
 other side j cadaveric rigidity comes later and lasts 
 longer ; putrefaction supervenes later ; lastly, certain 
 trophic disturbances occur chiefly in the region of the 
 eye. On the other hand, on stimulation of the nerve 
 the opposite effects are observed, such as dilatation 
 of the pupil, diminished temperature, and contrac- 
 tion of the blood - vessels. There appear then to 
 be in this part of the sympathetic pupil- dilating 
 fibres, motor fibres for Miiller's smooth muscle of the 
 orbit, vaso-motor branches, secretory und trophic fibres. 
 Besides these, the cervical symjDathetic seems to con- 
 tain branches which join the automatic ganglia. 
 
 Tti®i'acic p®rti®ii ©ftlae sympatlaetic. — The 
 chief nerves emanating from these ganglia, which are 
 about twelve in number, jom the solar plexus and 
 semilunar ganglia, and contain accelerating fibres for 
 the heart, which, when stimulated, act centripetally on 
 the inhibitory cardiac centre in the medulla oblongata ; 
 also fibres which excite the vascular nerve centres 
 in the medulla oblongata ; and lastly, the splanchnics, 
 which act as inhibitory nerves on the movements 
 of the intestine so long as a normal current of 
 oxydised blood traverses the vessels, but if the 
 blood has become venous the movements are in- 
 tensified. The splanchnics are also the chief vaso- 
 motor nerves of the vessels supplying the abdominal 
 viscera. When the splanchnics are divided an im- 
 mense accumulation of blood takes place in the 
 abdominal vessels. The thoracic portion also contains 
 inhibitory fibres for the renal secretion ; fibres, stimu- 
 lation of which causes sugar to appear in the urine. 
 
 Abdominal asid lo^^^er parts ®f tSae syisi- 
 pattieiic. — The nerves which are given off from 
 the abdominal and lumbar ganglia chiefly enter into 
 the plexuses which surround the vessels passing to 
 the genito-urinary apparatus, and, so far as is kno^vn, 
 are chiefly vaso-motor in functiori.. 
 
Chap. XIV.] The Spinal Cord. 247 
 
 FUXCTIOXS OF THE SpIXAL CoRD. 
 
 The cord is a centre for many reflex actions. It 
 is a conductor between tlie brain and the various 
 tissues and organs of tlie body. In man it is incapable 
 of originating impulses, and is destitute of con- 
 sciousness, but in many of the lower animals there is 
 reason to believe that it participates with the brain in 
 these functions. (For details of structure see Klein's 
 "Histology," p. 127, et seq.) 
 
 The spinal cord gives origin to thirty-one pairs of 
 nerves, which are divided into groups in accordance 
 with the part of the cord from which they spring, 
 and are named respectively cervical, dorsal, lumbar, 
 sacral, and coccygeal. Each nerve springs by two 
 roots, an anterior and a posterior. Experiments 
 devised by Sir Charles Bell and by IMagendie have 
 demonstrated that the jjosterior roots are sensory 
 in function, the anterior motor; and the general 
 characters of these nerves have been already de- 
 scribed (page 223). The evidence on which the 
 function of these roots has been determined is that 
 when the p)osterior root is touched or cut acute pain is 
 experienced, and simultaneous reflex muscular contrac- 
 tions occur; the parts supplied by the nerve beyond the 
 point of section are deprived of sensation. Stimulation 
 of the distal stump when the nerve has been divided 
 has no effect ; stimulation of the proximal stump pro- 
 duces pain and reflex actions. Fatty degeneration takes 
 place throughout the whole of the peripheric portion. 
 
 When the anterior root is divided contraction oc- 
 curs in the muscles supplied by the nerve owing to the 
 mechanical irritation of the knife, pain is experienced 
 owing to recurrent sensibility. The muscles supplied 
 by it are paralysed. Stimulation of the peripheric 
 stump causes muscular contractions in the muscles 
 supplied by the nerve. Stimulation of the proximal 
 
248 
 
 Human Fhysiology 
 
 [Chap. XIV. 
 
 extremity is without effect. The peripheric portion 
 undergoes fatty degeneration quickly, the proximal 
 more slowly. The common sensation of the parts 
 supplied by the nerve is not interfered ^dth. 
 
 Modern research has shown that, besides sensory 
 and motor nerves, the spinal nerves contain fibres 
 which are distributed to the smooth muscular tissue 
 of various organs, as the uterus and urinary 
 bladder, and that there are vaso-motor, vaso-dilator, 
 
 secreto-motor, and trophic 
 fibres. 
 
 The course of these 
 fibres after their entrance 
 into the cord is not very 
 accurately known ; but 
 there seems to be good 
 reason for believing that 
 the cord, besides its appa- 
 rent division into grey 
 and white substances, 
 may be mapped out into 
 certain columns formed 
 by ascending or descend- 
 ing groups of fibres, 
 which are represented 
 in the accompanying 
 woodcut (Fig. 15) made transversely across the spinal 
 cord at the level of the third dorsal vertebra. 
 
 The gi'ey §iil>staiaice of tlie c©rd. — This is 
 essentially composed of cells and their processes, and 
 although it may be regarded as a series of centres 
 which respond readily to appropriate stimuli, it is 
 certain that it is quite unexcitable to direct irritation, 
 such as cuts, punctures, or electric stimuli. The cells 
 are roughly divisible into two groups : those situated 
 in the anterior comu, which are large, and appear to 
 be capable of liberating motor impulses ; and smaller 
 
 Fig. 15. 
 
 The black pai't in the centre of the figure 
 is the grey substance ; m, motor aute- 
 rior root ; s, posterior sensory root ; 
 a and g, pyramidal columns ; 6, fun- 
 damental fasciculus of the anterior 
 column; c, Goll's column; d, Biir- 
 dach's column ; e and /, mixed la- 
 teral columns ; h, cerebellar columns. 
 
Chap. XIV.] Course of Fibres in Cord. 249 
 
 cells, situated in tlie posterior, which receive and 
 transmit sensorj impressions. 
 
 The u^Iiite sitfestance of tlie cord. — This is 
 composed of fibres, and surrounds the grey substance. 
 It is divisible into several columns or groups of fibres, 
 which have received special names. [See explanation 
 of Fig. 15.) 
 
 Coiu'se of tlae fibres of the anterior s^iid 
 posterior roots of tSie spiMsaS nerves. — If the 
 anterior roots of the spinal nerves be followed after 
 their entrance into the cord, they appear for the most 
 part to join the large ganglion cells of the anterior 
 cornu of the grey substance, of which they form 
 the axis-cylinder processes. From the grey fibrous 
 plexus which these ganglion cells give off, broader 
 fibres take origin. Some of these, constituting the 
 median fasciculus, pass through the anterior white 
 commissure to the opposite side^ and ascend in the 
 anterior column of the latter. Others, consti- 
 tuting the lateral fasciculus, enter the lateral column 
 of the same side, and ascend until they decussate at 
 the lower part of the medulla oblongata, forming the 
 decussation of the anterior pyramids. 
 
 If the posterior roots of the spinal nerves be 
 followed, they are found to penetrate the posterior 
 cornu of the grey substance, and to break up into 
 delicate fibrillse, which join and lose themselves in 
 the grey fibrous plexus of the cornu, by mea,DS of 
 which they are brought into relation with the small 
 ganglion cells of the posterior cornu. 
 
 The gTey fibrous plexus which links together the 
 ganglion cells of the anterior and posterior cornu 
 gives off fibres which pass to the opposite side of the 
 cord before and behind the central canal in the grey 
 commissure, and from this they pass backwards, part 
 ascending in the posterior cornu, and part in the pos- 
 terior columns of the cord. 
 
250 Human Physiology. [Chap. xiv. 
 
 Of the several groups of fibres differentiated in 
 Fig. 15, the columns marked a and g contain all 
 fibres emanating from the central convolutions of the 
 cerebral cortex and conducting voluntary impulses, or 
 impulses to muscles under the control of the will. 
 The decussation of the pyramids at the lower part of 
 the medulla oblongata is formed by the decussation of 
 these fibres, and, before they emerge from the cord by 
 the anterior roots, they enter the grey substance of 
 the cord. 
 
 The column h is composed of fibres connecting the 
 cerebellum with the grey substance of the spinal cord. 
 6, e, and f are the columns connecting the reflex 
 centres situated in the grey substance of the cord and 
 medulla oblongata, aud they also contain those fibres 
 which are continuous with the anterior roots of the 
 spinal nerves, and which subsequ.ently enter the grey 
 substance, c is a column composed of fibres con- 
 necting the posterior roots with the grey nuclei of the 
 funiculi graciles of the medulla oblongata ; d is com- 
 posed of fibres connecting the posterior roots and 
 the grey substance of the cord and medulla oblongata 
 (Landois). 
 
 Coisi^se puiFsiied fey sensory iMapffes§i®ias 
 aiidoiotor liMpialses tlirowgli tlie c®i*d, — Tactile 
 sensations, such as those of temperature, pressure, and 
 muscular sensibility, travel through the posterior roots 
 of the spinal nerves into the posterior cornu, and up- 
 wards along the lateral . column of the same side ; 
 painful impressions enter through the posterior roots, 
 and travel upwards through the grey substance gene- 
 rally. Motor impulses travel in the upper part of the 
 cord along the anterior and lateral columns, in the 
 lower part along the lateral columns alone. E/oflex 
 centres are probably connected by fibres running in the 
 white anterior and posterior columns. Inhibitory fibres 
 controlling reflex acts travel in the anterior columns. 
 
Chap. XIV.] CeATRES IN THE SpIjVAL CoRD. 25 1 
 
 The vaso-motor nerves run in the lateral columns, and 
 issue with the anterior roots of the spinal nerve. 
 
 The centres existiiag isi the spinal cord. — 
 
 Certain acts require for their due performance the 
 co-ordination of many muscles. If these do not con- 
 tract in a regular and orderly manner, the act is 
 imperfectly performed. The segment of the nervous 
 system which governs and regulates the movements is 
 termed a reflex centre. Proceeding from below up- 
 wards, the following centres have been demonstrated 
 in the cord : 
 
 1. Tlie ano-spinal centre, or centre controlling the act of 
 
 defaseation. 
 
 2. The vesico-spinal centre, regiilating micturitioiL 
 
 3. The erection centre, ) -i • i x 
 
 4. The ejaculation centre, | or gemto-spinal centre. 
 
 5. The partuiition centre. 
 
 6. The vaso-motor centre. 
 
 7. The vaso-dilator centre. 
 
 8. The sweat centre, 
 
 9. The ciho- spinal centre. 
 
 The first five centres are situated in the lumbar 
 region ; the sixth, seventh, and eighth extend through 
 a large portion of the cord ; and the last occupies the 
 lower part of the cervical and upper part of the dorsal 
 region. 
 
 The functions of tlie metliilla oblongata. 
 — The medulla oblongata may be regarded as a collec- 
 tion of reflex centres, traversed by fasciculi of white 
 fibres in all directions. (6'ee Klein's " Histology," p, 
 142.) It is inferior to the brain proper in possessing 
 no power of initiating movement, apart from the cardiac 
 and respiratory reflex movements, the mechanism of 
 which is contained in this part of the nervous 
 system. Even in the frog, if time be given for the 
 prostration caused by the removal of the brain to pass 
 away, no movements are observed, except as the result 
 
2^2 
 
 Human Physiology. 
 
 [Chap. XIV. 
 
 of the application of some external stimulus. The 
 body assumes the position it takes up naturally when 
 the animal is at rest, and if carefully protected from 
 external stimuli remains immovable ; but if stimuli 
 be applied, the reflex movements induced are ex- 
 traordinarily complicated, and closely simulate those 
 of the will. If laid upon its back it endeavours, 
 though generally unsuccessfully, to turn on its belly. 
 If placed in water, it swims, and comes to the surface 
 to breathe. If the water be gradually warmed, it 
 makes efforts to escape. In the higher animals, 
 sections made through the pons, or junction of the 
 pons with the medulla oblongata, are attended with 
 so much prostration from pain and bleeding that the 
 results are not very trustworthy ; but movements 
 of various kinds not involving change of place, and 
 cries in response to strong stimulation of sensory 
 nerves, have been observed. The animals are of 
 course rendered blind, and deprived of the power of 
 smell, but hearing is to some extent retained. 
 
 Centres in tSie imecliilia ©Moogata. — These 
 are both numerous and important. 
 
 2. 
 
 3. 
 
 ( Sensory fibres. 
 Centre for sue- J Motor fibres, 
 tion. I 
 
 Pp^x__ f„_ -^„. ( Sensory fibres, 
 centre tor mab ) ^^^^^ ^^^^^^ 
 
 tication. I 
 
 Centre for in- f Sensory fibres. 
 
 salivation. ( Motor fibres. 
 
 ( Sensory fibres. 
 Centre for de- j 
 
 glutition. j Motor fibres. 
 
 Centre forvomi- Sensory fibres, 
 tinor. 
 
 Centre for clos- f Sensory fibres, 
 ure of eyelid. ( Motor fibres. 
 
 Fifth and glosso-pbaryngeal. 
 
 Facial, hypoglossal, and mo- 
 tor fibres of third divi- 
 sion of fifth. 
 
 Fifth and glosso-pharyngeal. 
 
 Facial, hypoglossal, thu"d 
 division of fifth. 
 
 Glosso-pharyngeal and fifth. 
 
 Facial, symj)athetic. 
 
 Fifth, glosso-pharyngeal, va- 
 gus. 
 
 Spinal accessory, vagus, hypo- 
 glossal, sympathetic. 
 
 Fifth, glosso-pharyngeal, va- 
 gus, and afferent fibres 
 from many organs — 
 stomach, intestines, and 
 kidney. 
 
 First division of fifth. 
 
 Seventh or facial nerve. 
 
Cnap.xiv.] The Medulla Oblongata. 253 
 
 7. Centre for dila-^ 
 
 tationof i>upil, | Sensory fibres. Optic, fiftli. 
 
 and for smooth f>Motor fibres. From the spinal cord which 
 muscle of or- first enter the sympa- 
 
 bit. J thetic nerve. 
 
 8. Eespu-atory f Sensory fibres. VagT.;s, sympathetic, 
 centre. 1, ]Motor fibres. Phrenics, spinal nerves. 
 
 9. Centre for inliibition of cardiac movements. (This has 
 
 already been considered, page 226). 
 
 10. Centre for accelerating cardiac movements. 
 
 11. Vaso-motor centre. 
 
 12. Vaso-dilator centre. 
 
 The ftniiictioiBS ©f iMe. feasal gaiig-lia.— 
 
 Several o-ancrlionic masses are situated at the base of 
 the l3rain, which, both in their mode of development 
 and in their anatomical and physiological characters, 
 may be regarded as a continuation of the spinal cord 
 and medulla oblongata. These are the grey substance 
 of the pons ; the corpora quadrigemina ; the gTey sub- 
 stance of the cerebral peduncles, the optic thalami; 
 and the corpora striata. The functions of these parts 
 are very imperfectly knovni, for, lying deeply, great 
 damage to other parts and prostration occurs in 
 animals subject to direct experiment upon them, 
 rendering the observations made upon them very 
 untrustworthy : vvdiilst as they occupy an intermediate 
 position between the cerebral hemispheres and the 
 spinal cord, and many of the fibres connecting these 
 parts pass through them, it is difficult to distinguish, 
 in experiments what effects are attributable to the in- 
 jury inflicted on the special centre under examination, 
 and what to the lesion of the connecting fibre. Some 
 points may, however, be regarded as determined, 
 partly from the results of experiments, partly from a 
 consideration of their histological characters and 
 anatomical connection, and partly on pathological 
 gro\i]ids, that is, from observation of the symptoms 
 of disease limited to these structures. Still more 
 would probably be gained if we possessed accurate 
 
2 54 Hum A n Physiol ogy. [Cimp. x i v . 
 
 knowledge of their ontological development, and of 
 their comparative anatomy. 
 
 FiiBictioBis of the p©iis. — The structure of the 
 pons shows that it is composed of masses of grey sub- 
 stance traversed both longitudinally and transversely by 
 white fibres. The superficial joortion is chiefly composed 
 of transverse fibres acting as a commissure between the 
 hemispheres of the cerebellum, whilst the deeper parts 
 present longitudinal fibres that are partly continuous 
 with the motor, partly with the sensory fibres of the 
 spinal cord. Stimulation of the pons produces, there- 
 fore, pain and convulsions, whilst section occasions 
 paralysis of sensory motor and vaso-motor nerves, as 
 well as compulsory m.ovements. Complete section, of 
 course, abolishes all influence of the higher centres on 
 the body. There is good reason, therefore, for be- 
 lievinof that it is a centre for the co-ordination of 
 movement. 
 
 FitM€ti®Eis ®f ttie co>i'p®a'a qitadi'tgesuBKia, 
 — The larger portion of the fibres of the optic tract 
 penetrate the corpora quadrigemina of tlieir own side ; 
 and experiment shows that if these bodies are de- 
 stroyed on either side, blindness of the opposite eye 
 results. If both are destroyed, the blindness is com- 
 plete. The co-ordination of movements, which under 
 all ordinary conditions is so closely connected with 
 visual sensations, is interfered with, and animals find 
 a difl&culty in preserving their balance. Electrical 
 stimulation causes dilatation of the pupil, but the eye 
 afifected is not constant. Stimulation of the rioiit 
 anterior corpus quadrigemina causes both eyes to turn 
 to the left, and vice versa ; and if the stimulation be 
 continued, the head also turns in the same direction. 
 Other phenomena that have been observed are increase 
 of blood pressure, retardation of the pulse, and deeper 
 inspiration and expiration. 
 
 Functions of the cerebral peduncles.— 
 
Chap. XIV.] The Basal Ganglion: 255 
 
 The cerebral peduncleS; like the pons, contain gray 
 substance and longitudinally-aTranged fibres, Avhich are 
 divisible into two groups, an anterior or inferior one. 
 the crusta; and a superior one, the tegmentuvi. Little 
 more can be said of them with certainty than that 
 they are co-ordinating centres. 
 
 FiHietiOBis of tBie corpora striata. — Tliese 
 bodies contain two masses of grey substance : one, 
 the nucleus caudatus, intraventricular nucleus, or 
 corpus striatum proper, is the larger ; the other, or 
 lenticular nucleus, is the smaller of the two. The 
 functions, though certainly distinct, have not been 
 differentiated. Electrical stimulation of the caudate 
 nucleus produces, according to Ferrier, pleurostho- 
 tonos or latei'al flexion of the body : whilst ISTothnagel 
 finds that destruction of the inner part near the 
 ventricle, by means of a needle, or by the injection of 
 a drop of chromic acid through a fine syringe on 
 one side in rabbits, causes them, after a short in- 
 terval, to execute violent running or leaping move- 
 ments, or the " mouvements de manege," that is to say, 
 "circus movements," the animal running round and 
 round continuously in one direction, till it falls ex- 
 hausted. He has hence termed it the " nodus cur- 
 soriiis." If the whole nucleus caudatus is destroyed, 
 however, these effects are not observed. Similar 
 experiments made upon the lenticular nucleus were 
 always followed by motor paralysis. 
 
 Functions of tlie optic tliaiasni. — These 
 seem to be centres towards which many sensory 
 fibres converge, for not only have they large con- 
 nections with the optic tracts, but the greater part of 
 the tegmentum of the cerebral peduncles, which 
 appears to be in great measure a continuation of the 
 sensory columns of the cord, enter them. Direct 
 stimulation of them in animals by means of electricity 
 has not been found to produce any movements. 
 
256 Human Physiology. [Chap.xiv. 
 
 Lesion of tliem, which is not unftequentiy observed 
 in man as the result of haemorrhage, produces anses- 
 thesia, or loss of sensation, of the opposite side 
 of the body ; and if it be localised in the posterior 
 third of the thalamus, visual disturbances have 
 been, as might be expected, observed. It is par- 
 ticularly worthy of notice that various actions 
 ordinarily regarded as voluntary may be performed 
 by an animal from vdiich the cerebral hemispheres 
 have been removed. Thus, Longet found, that in a 
 pigeon in which this operation had been performed the 
 animal gave many indications of consciousness of 
 light; for not only did the pupil contract, but the 
 lids closed when a strong light was suddenly made 
 to fall upon the eye, the animal having been pre- 
 viously kept in darkness ; and when a lighted candle 
 was made to move in a circle before it, the animal 
 executed a corresponding movement with its head. 
 
 The general result of the observations made on 
 the nuclei of the corpora striata and optic thalami is 
 to favour the supposition that, like the anterior and 
 posterior cornua of the grey substance of the spinal 
 cord, they are respectively connected with the motor 
 and sensory operations ; and that whilst on the one 
 hand they may act as independent centres of reflex 
 action, and in this relation may be the centres for 
 various automatic and instinctive movements which 
 are commonly prompted and guided by sensations, 
 they may also be subservient to the operation of the 
 hemispheres, with the increase and development of 
 which they, j^ari 2')assu, augment. 
 
 CoisipuSsory siaoveisieMts. — The two halves of 
 the brain, basal ganglia and medulla oblongata, act 
 together, under ordinary circumstances, in preserving 
 a certain equipoise and harmony between the opposite 
 sides of the body, which is rendered evident by the 
 effects of lesions involving one half of these parts. 
 
Ch:v'p.%iY.] Functions of the Cerebellum, 257 
 
 Unilateral sections cause in many instances move- 
 ments which are not under the control of the will of 
 the animal, and seem to be the consequence of an 
 irresistible imjDulse. In some cases the animal is 
 compelled to run in a circle, in others to whirl round 
 and round, in others to roll over and over. These move- 
 ments sometimes take place towards, sometimes away 
 from, the side on which the injury has been inflicted. 
 FiiactioBis of ttae cerebelliiMi. — The size and 
 complexity of the cerebellum increase with the 
 number and variety of the movements that each 
 animal is capable of executing, attaining its maximum 
 size in man, by whom the most elaborately complex 
 actions are capable of being performed. If the cere- 
 bellum be cut away by successive slices, the animal 
 soon becomes sullen and its movements irregular, and 
 by the time the whole of the organ is removed, all 
 power of springing, flying, walking, standing, or pre- 
 serving the balance (that is, of performing any com- 
 bined muscular movements which are not of a simply 
 reflex character) is lost. On the other hand, neither 
 the will nor the consciousness seems to be affected. 
 Many pathological cases of cerebellar disease have 
 been recorded in man, the distinguishing characters 
 of which have been staggering gait, convulsions, and 
 tendency to fall, or loss of balance. From these 
 various circumstances the conclusion is drawn that 
 the cerebellum is the esseoitial organ for the co-ordina- 
 tion of muscular tnovements. Anatomical evidence is 
 in favour of this view, for the cerebellum is directly 
 connected with all the columns of the spinal cord, as 
 well as with the basal ganglia and the hemispheres of 
 the cerebrum. The cerebellum is insensitive to direct 
 irritation. Some of the most remarkable examples of 
 compulsory movements are observable on unilateral 
 section of the cerebellar peduncles. In some cases the 
 animal has been seen to roll over for many days 
 R 
 
2 5-5 Human PiiYsioLOGW [Chap. xiv. 
 
 together at the rate of sixty times in a second. An 
 obscure relation exists between the cerebellum and 
 the sexual functions. 
 
 It is reasonable to suppose that in the performance 
 of various actions an impulse is propagated from a 
 few, or even from one ganglion cell of the cortex of 
 the cerebral hemispheres, which communicates directly 
 with a centre composed of many cells capable of 
 bringing into simultaneous action numerous muscles. 
 
 FuiictioBis ofttoe ceretorum. — The hemispheres 
 of the cerebrum are the seat of all psychical processes, 
 that is to say, they contain the higher centres which 
 minister to thought and volition. They are the 
 centres of memory. In them are performed all 
 operations of reason and judgment, and they initiate 
 the voluntary impulses which consist in the intelligent 
 adaptation of means to ends. Usually acting coinci- 
 dentally, they are probably capable by practice of 
 being brought into separate action. Their removal 
 appears to plunge the animal into a deep sleep, from 
 v»^hich no irritation ever seems able to rouse them into 
 full activity, although they give manifestations of 
 co-isciousness. It is worthy of observation, however, 
 that large portions of the brain may be destroyed by 
 disease, or removed in consequence of accidents, with- 
 out very material impairment of function, so that it is 
 possible that a process of substitution may take place. 
 
 Two opinions are entertained in regard to the 
 action of the brain. Both agree in considering that 
 the brain is the instrument of all the higher intellec- 
 tual or psychical functions. From the one point of 
 view, however, it is maintained that the nerve cells 
 are accumulated in certain regions to form centres, the 
 definition of which is, indeed, obscured by the com- 
 plexity of the communications intervening between 
 themselves and between the centres situated on lower 
 planes, as those of the basal cerebral ganglia and the 
 
Chap. XIV.] Functions OF THE Cerebrum, 259 
 
 ganglia of the medulla oblongata and spinal cord, but 
 which act like other centres in a reflex manner to 
 stimuli that affect them from without or from within. 
 From the other point of view it is considered that 
 although the brain is physically a part of the nervous 
 system, it is to be regarded as xhe organ through which 
 the mind of man operates, as having altogether 
 superior functions to any other part of the nervous 
 system, operating as a whole, directing, controlling, 
 and otherwise influencing the lower centres. It is 
 regarded as the organ of the will, which acts in a mode 
 that is incomprehensible indeed, but which is evident 
 enough, through the cerebral cells. 
 
 The difference of opinion in regard to the mode of 
 action of the brain between physiologists is analo- 
 gous to that which exists between materialists and 
 spiritualists. 
 
 To illustrate the views by an example. A man 
 voluntarily thrusts forth his arm, or he determines to 
 think of some object, as a horse ; and says, See ; I 
 believe in the independence of the will. I choose to 
 thrust forth my arm, and I do so ; I choose to direct 
 my thoughts to a horse, and I do so. The spiritualist 
 contends that herein is the evidence of a something 
 separate from the body, the mind, which has called into 
 play the centres which are capable of exciting the con- 
 traction of the extensor muscles of the arm, or those 
 which, when collectively excited, make up our idea of 
 a horse. He maintains there need be no antecedent 
 stimulus, but that the mind has originated within itseK 
 the necessary conditions for the acts in question. 
 
 The materialist, on the other hand, sees in such 
 operations nothing more than reflex acts, though of a 
 somewhat higher or complex nature than those 
 ordinarily performed by the spinal cord or medulla 
 oblongata. The man, he maintains, does not thrust 
 forth his arm, or think of a given animal, until other 
 
26o Human Physiology. [Chap. xiv. 
 
 groups of cells have been excited or aroused, either by 
 the mere stimulus of discussion or by the sequence of 
 a train of ideas, each of which is immediately related 
 to that which has preceded it. From this point of 
 view there is no such thing as free will. The acts 
 and thoughts of a man are the results partly of original 
 and hereditary contirmation, partly of the direction of 
 his own occupation and pursuits ; and were it possible 
 for us to know the whole antecedents of the individual, 
 it would be possible to predict what his conduct 
 would be on any given crisis. 
 
 The consideration of these questions is not, how- 
 ever, adapted for an elementary book, and must be 
 looked for in larger treatises. 
 
 It was long considered that it was impossible to 
 excite the brain to the manifestation of its powers by 
 direct stimulation of any kind. But the experiments 
 of Hitzig and Ferrier have shown that this conclusion 
 was the result of imperfect observation, and that the 
 application of electric currents to diflferent parts of the 
 cerebral surface is followed by movements of very 
 various kinds, and by evidences of sensation, whilst 
 the destruction of these regions occasion loss of 
 sensation in the one case, and loss of voluntary control 
 over the muscles that respond to the stimulation in 
 the other. In Fenier's experiments the electric 
 stimulus employed was that derived from a single 
 Daniell's cell, which is strong enough to be just borne 
 when applied to the tongue. The application of the 
 electrodes produces very decided movements ; and if 
 they are shifted over a sulcus to a different convolution, 
 a different set of muscles is immediately called into 
 action. The following regions have been mapped out 
 by Ferrier in the brain of a monkey : (Figs. 1 6 and 1 7) 
 Stimulation of i produces movement of the hind limb 
 as in walking ; 2, complex movements of the hind leg, 
 especially adduction of the foot to the middle line ; 
 
Chap. XIV.] Functions OF THE Cerebrum. 261 
 
 ^\ 
 
 
 Fig. 16. — Side View of the Brain of a' Moiikey, showing Localised Areas. 
 
 Pig. 17. — Diagram of tTie Brain of a Monkey, seen from above, with 
 L' calise^i and Niimhprefl Are^s. The stimulation of each of them is 
 fo l^wed by tLe results stated in the text under the corresponding 
 numbors. 
 
262 Human Physiology. [chap. xiv. 
 
 3, movements of liind foot and tail ; 4, action of latis- 
 simus dorsi ; 5, extension forward of tlie arm ; a, 6, c, d, 
 successive movements of the hand and wrist, termi- 
 nating in closure of fist ; 6, action of the biceps ; supi- 
 nation and flexion of the fore-arm ; 7, action of the 
 zygomatici ; 8, conjoint action of the elevators of the 
 upper^ and depressors of the lower, lip ; 9, opening of 
 the mouth and protrusion of the tongue ; 10^ retraction 
 of tongue; 11, action of platysma ; 12, elevation of 
 the eyebrows and eyelids, dilatation of the pupils, and 
 rotation of the head to the opposite side; 13, eyes 
 directed to the opposite side, and either upwards or 
 dowuAvards, with, usually, contraction of the pupils ; 
 14, retraction of the opposite ear, head turning to the 
 opposite side, the eyes widely opened, and the pupils 
 dilated ; 1 5 (not shown in figures, but corresponding 
 to tip of uncinate gyrus), torsion of the lip and nostril 
 of the same side. The antero-frontal and occipital 
 regions give no result on stimulation, nor does the 
 island of Eeil react. Upon the whole, it would appear 
 that the centres of motion for the anterior and pos- 
 terior limbs are situated in the convolution immediately 
 surrounding the frontal fissure, which runs transversely 
 to the longitudinal fissure. 
 
 Terrier believes he has been able to localise certain 
 regions of special sense ; viz., 1. T\iq psycho-optic centre. 
 Blindness results from destruction of the angular gyrus 
 (Ferrier), or the occipital lobes (Munk) ; but this is only 
 temporary if the other angular gyrus is uninjured ; if 
 both are destroyed, the blindness is complete and 
 permanent. 2. The psycho-acoustic centre. Deafness 
 results from destruction of the superior temporo- 
 sphenoidal convolutions. 3. Loss of tactile sensibility 
 results from destruction of the hippocampus major and 
 hippocampal convolutions. 4. Zoss o/smeZ^ results from 
 destruction of the subiculum cornu ammonis, or tip of 
 the uncinate convolution. 5. Loss of taste results from 
 
Chap. 5civ.] Psychical Processes. 26 
 
 J 
 
 destruction of tlie lower part of the temporo-splienoidal 
 lobe. 
 
 The speecla centre. — One of the best defined 
 regions, and that one which has been longest known, 
 is the region through which speech is effected. It is 
 the third left frontal convolution, which is in part 
 continuous with the island of Reil. In rare instances 
 lesions have been observed in the right third frontal 
 convolution, which have led to loss of speech. Most 
 men, therefore, are left-brained speakers, just as most 
 men are right-handed, which is also associated with 
 special develo23ment of the left brain. In the cases 
 where lesion of the right third frontal convolution has 
 caused loss of speech the individuals have also been 
 left-handed. 
 
 The diu^atioii of psychical processes. — It 
 has been ascertained that a visual impression induced 
 by a constant stimulus is not constant in intensity, 
 but increases from zero till it reaches a maximum, and 
 then gradually diminishes. The same holds in regard 
 to the ears, except that it is n'^t known whether after a 
 certain time the sensation diminishes in intensity j and 
 it is observed also in the perception of gustatory and 
 olfactory sensations. In the latter cases the process 
 is so slow that it becomes highly probable that it is to 
 be referred to conditions of the terminal apparatus, 
 rather than to peculiarities of the cerebral nervous 
 organs. It is known that for a visual stimulus to give 
 a continuous impression there must be more than 
 twenty-four impulses in the second ; yet if the optic 
 nerve be directly stimulated by means of sixty shocks of 
 electricity per second, the impression is by no means 
 continuous, though the latter stimulus may be less in- 
 tense than the first. Hence, again, it may be deduced 
 that the tardy course of visual sensations is due to the 
 slow process of excitation of the retina, and not to 
 slowness of perception on the part of the brain. It 
 
264 Human Physiology. [Chap. 3iiV. 
 
 is to the different rapidity with which the terminal 
 organs respond to impressions, though partly also to 
 the different speed of conduction in the nerves, that 
 the difference in fixing the exact time of the occurrence 
 of any phenomenon is due. That such difference 
 exists has long been known to astronomers, by whom 
 it is recognised, and allowed for under the name of 
 jjersonal equafAon. In astronomical observations it is 
 of great importance to fix the precise moment when, 
 for exam])le, the transit of a star takes place across 
 the hair line of a telescope, and this is done by com- 
 paring Avhat is seen with what is heard, as the click 
 of a clock ; or by pressing the knob of an electric 
 apparatus at the moment of observation of the 
 transit ; but if one observer fixes the moment of transit 
 as occurring at seven o'clock, another observer may 
 make it one half second or even a whole second later. In 
 this case the time as given by the last observer must have 
 a certain deduction made, when compared with the 
 first, in order that correct results should be obtained. 
 
 Kapidity ®f perception. Tlae siiialiest 
 dMereaice. — How quickly can two sensory impres- 
 sions follow each other, and the succession in point of 
 time be correctly stated 1 The answer to this question 
 gives the smallest difference. In the case of the 
 visual sense, where the same part of the retina is 
 stimulated, this has been determined by rotating a 
 disk with black and white sectors before the eye, and 
 it has been found, that with ordinary daylight the 
 retina must be stimulated about twenty-four times in 
 the second, before it assumes a uniform grey aspect, 
 that is, for the smallest difference to be surpassed. In 
 this way a smallest difference of ^^g-th second is obtained. 
 It is not necessary, however, that the white sectors 
 should be of the same size as the black, in order that 
 with twenty-four stimuli in the second a uniform grey 
 should appear ; which shows that it is the alternation 
 
Chap, XIV.] Stimuli by the Senses. 565 
 
 and not tlie duration of each impression which is 
 of importance. When different parts of the retina 
 are affected, as when a point is watched where two 
 electric sparks can be made to appear quickly, one after 
 the other, the images of which upon the retina are not 
 separated bj a greater interval than O'Oll mm., and 
 which, therefore, both fall on the yellow spot, it can be 
 determined which spark first appears, when the inter- 
 vening period is not more than 0'044 sec. In the case 
 of the ear, when the sound attended to is that caused 
 by two electrical sparks quickly succeeding each other, 
 it can be perceived that there are two, and that one 
 is earlier than the other, when it precedes it by no 
 more than 0*002 sec. It is remarkable that the 
 minimum difference for one ear when both sounds 
 affect it, is smaller than when one sound affects one 
 ear, and the other the other ear ; as is shown by the 
 fact that a slight inequality of beat can be distinctly 
 perceived when two watches are held before the §ame 
 ear, which is imperceptible when one watch is held 
 before one ear, and the other before the opposite one. 
 In regard to tactile impressions, great differences 
 appear to exist in regard to the shortness of the 
 interval between a series of tactile impressions, in 
 order that they should no longer be perceived to 
 be discontinuous but fused into one. Preyer stands 
 alone in stating, that from twenty-eight to thirty- 
 eight beats per second give a continuous sensation. 
 Valentin gives the limits as varying from 480 to 640, 
 and Y. Wittich places it at about 1000. 
 
 Kecog^iiitioii of simtiltaiieoMS impii'essaoiis 
 on eye and ear, — If an electric spark be made to 
 appear, and simultaneously a bell be struck, the 
 coincidence of the two in point of time is not easily 
 or certainly recognised ; and if it is unknown whether 
 they are precisely simultaneous or not, the subject of 
 the experiment will often rightly state that they are 
 
266 Human Physiology. [Chap. xiv. 
 
 simultaneous, but often also that the sound precedes 
 the spark ; he will rarely state that the spark pre- 
 cedes the sound. In a large series of experiments, 
 in which a judgment has to be formed between the 
 precedence of a sight or sound, it has been found that 
 the perception of sound is a trifle earlier than that of 
 light. The ear is quicker than the eye. 
 
 Keactaoii time. — By this is meant the time 
 between the application of a stimulus to some sensory 
 nerve, and the time when a sional is made to indicate 
 that it has been perceived. This experiment may be 
 conducted in a variety of ways. By Helmholtz and 
 others it has been done by applying a stimulus to 
 one hand, and signalling with the other ; and the 
 results of observations made by different experi- 
 menters have given very different results. One ob- 
 server, for example, found it to be 0-1087 sec, and 
 another 0'1911, or nearly twice as long. But the 
 experiment may also be conducted by stimulating 
 other sensory nerves, as the eye and ear, whilst still 
 making the hand signal. The sudden appearance of a 
 light required a period of 0-1139 sec. to be signalled, 
 and the occurrence of a sudden sound 0-1360 sec, by 
 the same observer. 
 
 Instead of the hand, the lower jaw and foot have 
 been made to signal. The reaction time from the eye 
 stimulated by an electric spark to the lower jaw 
 is 0-1377 sec, and from eye to foot, 0*1840 sec 
 
 Personality has much to do with the rapidity 
 of propagation of nervous impulses. Thus, an intel- 
 ligent young man, much interested in this question, 
 was compared with an old man of seventy-seven 
 from the Union, and it was found that, whilst the 
 reaction time from hand to hand for the youth 
 was 0-3311 sec, for the old man it was 0'9952 
 sec. But practice can be shown to be of importance, 
 for experiments continued for more than half a year 
 
Chap- 5CIV.] Sleep. 267 
 
 with the same old man showed extraordinary im- 
 provement, since in June his reaction-time was 
 as above, but with ten days' practice it had fallen 
 to 0*3576 sec, and in January of the following 
 year it was only 0"1866 sec. Exhaustion of the 
 attention prolongs the reaction time. Intensity of 
 stimulus abbreviates it. The processes that are in- 
 cluded in the reaction time are numerous. They are : 
 1. The conduction of the stimulus or sensory impulse 
 through the nerve and the spinal cord to the central 
 organ ; 2, Conversion of the sensory impression into 
 a motor impulse; 3. Centrifugal conduction of the 
 motor impulse through the spinal cord and motor 
 nerve ; 4. Liberation of the muscular movement. If, 
 in addition to the mere reaction- time, a judgment 
 has to be formed (that is to say, that one of two 
 colours is presented to the eye, or two sounds to the 
 ear, and a signal corresponding to each has to be 
 touched), it is found that a period of 0*036 sec. has 
 to be added to the reaction period. 
 
 Sleep. — Sleep consists in the more or less com- 
 plete suspension of the psychical operations, whilst 
 the purely vegetative processes are continued, though 
 usually with diminished energy. The heart, for 
 example, beats less frequently, the respirations are 
 slower, and the digestive operations are less active. 
 The cause of sleep is unknown. It is probably 
 primarily associated with the great cosmical alterna- 
 tion of day and night, and with the diminished mole- 
 cular activity that takes place in the cerebral cells 
 during tbe hours of darkness. It may also, as Dr. 
 Capper believes, be in part attributable to altered 
 conditions of the cerebral circulation, and perhaps also 
 to diurnal variations in the amount of oxygen absorbed. 
 Its duration varies greatly with age. The infant and 
 old person sleep half their time away, whilst in mid 
 age, especially when mental strain is experienced and 
 
2 68 Human Physiology. [Chap. XiV. 
 
 work has to be done, five or six hours are all that 
 is needed. The conditions most favourable for sleep 
 are moderate fatigue both of mind and body, recent 
 but moderate supply of food, quietude or monotonous 
 sound, absence of anxiety, and habit. It has been 
 observed in some pathological cases in which several 
 of the senses have been abolished (as those of 
 touch, sight, smell, and taste, whilst hearing has 
 been retained) that the suppression of this remaining 
 sense rapidly induces sleep. In many persons sleep 
 is very profound, and some time elapses on being 
 awakened before they can gather their senses together; 
 whilst others sleep with their faculties about them, or 
 at Irast within ready call, so that the slightest noise or 
 touch awakes them. Such persons are often capable of 
 fixing very accurately the time when they shall awake. 
 Oiiidiiig seMsati€>si§. — In order that the will 
 should call a definite group of muscles into play, it 
 must be cognisant of the position of the parts about 
 to be moved, and of the state of the muscles it is 
 about to cause to contract. This knowledge is 
 afforded by guiding sensations, the existence of which 
 in health has been deduced rather from the observa- 
 tion of the phenomena of disease than by direct 
 evidence under normal conditions, for it is found that 
 if the sensibility of any part be lost, or if the guiding 
 sensations derived from the visual and aural senses 
 be suddenly removed, great difiiculty is experienced 
 in the performance of movements that seem, under 
 ordinary conditions, to be executed quite uncon- 
 sciously. Many examples of this might be cited, but 
 amongst others may be mentioned that in certain 
 conditions of ataxia, though there may be no loss of 
 power in the muscles of the limb, the patient walks 
 with difiiculty, unsteadily, and with tottering gait, 
 because, in consequence of the loss of tactile sensibility 
 in the feet, he does not exactly recognise their 
 
Chap. XIV.] Guiding Sensations. 269 
 
 position, and is therefore unable to plant them 
 properly. In like manner, a woman is unable for 
 any length of time to hold her child in her arms, but 
 the sense of position of the brachial muscles being 
 lost, they unconsciously yield to gravity, and fall. 
 The guiding sensations derived from the eye are those 
 upon which we chiefly depend in our ordinary life ; 
 and every one must have experienced the sense of 
 insecurity, and the difficulty of pursuing a straight 
 course, when the eyes are blindfolded. It is inter- 
 esting to observe, moreover, that guiding sensations 
 derived from one sense may supply the loss of those 
 from another, for in such cases of ataxia as those 
 above mentioned, the deficiency of the muscular sense 
 may be made good by the visual, standing and walking, 
 or the preservation of the child, being accomj)lished, 
 provided the patient looks at the limbs. A sense of 
 vertigo or insecurity may, in a similar manner, be 
 produced by the supervention of sudden deafness. 
 The precise adjustment of the muscles of the larynx 
 which is required for the emission of vocal sounds is 
 effected through the auditory sense, for when this is 
 absent, vocal intonation is learned with difficulty 
 through the eye, and the voice is never pleasing. It 
 would be impossible for a person born deaf to become 
 a brilliant or even moderately good singer. Many 
 habitual actions are, when once commenced by the 
 will, maintained by the existence of guiding sensa- 
 tions. Thus, in mastication the act may be con- 
 tinued, whilst the mind is wholly absorbed in a train 
 of thought, and directs no attention to the process, 
 though there is a disposition in such cases to continue 
 to turn the same morsel over and over in the mouth 
 rather than to proceed to the next act, of deglutition. 
 Guiding sensations may occasionally be antagonistic, 
 and the movements may then become uncertain. In 
 walking across a plank at a great height, although the 
 
27© Human Physiology, [Chap. xiv. 
 
 feet are in contact with solid material, yet the sense 
 of insecurity" occasioned by the unusual condition of not 
 being able to see or fix any object with the eye between 
 the near plank and the remote ground, renders the 
 movements of the limbs and the maintenance of the 
 equilibrium exceedingly difficult, especially if there be 
 any swaying of the plank itself ; yet no difficulty is 
 experienced in walking along a narrow plank in the 
 flooring of a room, when all accessory guiding sensa- 
 tions are brought into action. 
 
 Ides&s. — Ideas are to be regarded as a mental 
 state or representation, which assumes the character 
 of an independent intellectual reality (Carpenter). 
 In forming them, the mind is determined by the 
 nature and intensity of the various affections of its 
 consciousness which have been excited by the object; 
 and as these depend in part upon the original consti- 
 tution of the cerebrum, and in part upon the mode in 
 which its activity has been habitually exercised, it 
 follows that the ideas of the same object or occurrence 
 which are formed by different individuals may be widely 
 discrepant. There are some ideas which are so con- 
 stantly present in every soundly constituted mind 
 that they have been named primary beliefs^ or funda- 
 mental axioms. The chief of these are : Belief in our 
 present and in our past existence ; belief in the 
 external and independent existence of the causes of 
 our feelings, leading us to distinguish the ego from the 
 non ego ; the belief in the existence of an efficient 
 cause for the changes we witness around us ; the belief 
 in the uniformity of the order of nature; and the 
 belief in our own free will. 
 
 Eisiota®ii§. — These may be regarded as being 
 constituted by certain classes of ideas to which feelings 
 of pleasure or of pain are attached. They include the 
 mental states of joy, hope, surprise, fear, and the like. 
 They are apt to lead to the liberation of movements 
 
Chap. XIV.] Association of Ideas, 271 
 
 which are peculiarly characteristic of the emotion 
 experienced. 
 
 JLa^vs of mental association of ideas. — The 
 
 most important of the laws which have been laid 
 down by psychologists in regard to the succession of 
 our mental states, are those which relate to the asso- 
 ciation of ideas. The first of these is the Icno of 
 contiguity, which is to the effect that two or more 
 -states of consciousness habitually existing together or 
 in immediate succession, tend to cohere, so that the 
 future occurrence of any one of them restores or 
 revives the other. Good examples of this may be 
 drawn from the memory of any common object, such 
 as an orange, the idea called up by the sound of the 
 word blending together many attributes of the fruit, 
 as colour, surface, form, consistence, and even internal 
 structure, which, by having been frequently presented 
 coincidentally to our senses, have gro^vn together in 
 the mind, and recur together when one or other of 
 them is thought of. Of this law of contiguity, we 
 have. Dr. Carpenter remarks, a most important ex- 
 ample in the association which the mind early learns 
 to form between successive events, so that when 
 the first has been followed by the second a sufficient 
 number of times to form the association^ the occur- 
 rence of the first suggests the idea of the second ; if 
 that idea be verified by its occurrence, a definite 
 exi^ectation is formed ; and if that expectation be 
 unfailingly realised, the idea acquires the strength of 
 a belief. And thus it is that we come to acquii*e that 
 part of the notion of "' cause and effect," which rests 
 upon the " invariability of sequence," and to form our 
 fundamental conception of the uniformity of nature. 
 
 L.aw of sisniiarity. — This law is to the effect 
 that present sensations and thoughts have a tendency 
 to revive preceding states of consciousness which 
 resemble them. Having once seen and eaten a bunch 
 
2^2 Human Physiology. [cuap. xv. 
 
 of grapes, we recognise a second bunch, though, it may 
 be ranch larger or smaller, and even though the colour 
 may be different, the form of the individual berries, 
 as well as their arrangement, jDredominating over the 
 colour, and rendering us certain, when the colour 
 might engender a doubt. Some minds, however, take 
 note of differences with greater quickness than of 
 similarity. Minds that are capable of perceiving 
 minute points of resemblance or of difference, like that 
 of Linnasus, are well adapted to classify and arrange 
 natural objects; and the recognition of similarity and 
 dissimilarity betv/een mental images and ideas is a 
 characteristic of the highest order of minds, such as 
 those of Shakespeare and Bacon. 
 
 There is yet another law, termed the la^^v of coii- 
 strwctive association, which is the foundation of 
 the imagination, and examples of which are found in 
 those cases in which, from an outline or a sketch, we 
 build up a complete form, or in which we combine 
 two or more dissimilar ideas into a concordant whole. 
 
 CHAPTER XV. 
 
 THE SENSES. 
 
 The eye. — The eye is a camera obscura, and 
 the images of external objects are depicted on the 
 retina, which is a concave screen lining its posterior 
 surface. The images here formed are inverted, and 
 greatly reduced in size. The similarity of the eye to 
 a photographic apparatus is rendered very striking by 
 the circumstance that the rhodopsin, visual purple, or 
 colouring matter of the rods and cones, is bleached 
 by exposure to light, so that, by the application of 
 appropriate chemical agents, the image of a brightly 
 
Chap. XV.] Field of Vision: , 273 
 
 illuminated object can be fixed, and rendered apparent 
 for some time. 
 
 The optic axis. — This is a line which passes 
 through the nodal point and the centre of the cornea j 
 if prolonged backwards, it falls upon the retina on 
 the inner side of the yellow spot. 
 
 The T'isiial line. — The visual line joins the 
 macula lutea with the point on which the eye is fixed. 
 It passes through the cornea a little to the inner side 
 of its centre, and therefore forms an angle with the 
 optic axis, which is termed the angle a. This angle 
 does not exceed in the normal eye 4° or 5°. In look- 
 ing at a distant object, as a star, the visual lines are 
 parallel, and the optic axes are directed outward ; but 
 since we judge of the position of the eyes of another 
 person by the position of the centres of the cornea, 
 the eyes appear to diverge slightly, or in other words 
 it may be said that there is slight apparent strabismus. 
 That there is no real squint is shown by covering, 
 when a distant object is regarded, first one eye and 
 then the other, with the hand, when it will be found 
 that neither eye alters its position. 
 
 FieM of vision.— This is the area which can be 
 seen by each eye, when the head and body, being 
 maintained in a fixed position, the eye is rotated to 
 the utmost in the different meridians. The field of 
 vision is most extensive below and on the outer side, 
 being limited above by the brow, below by the cheek, 
 and to the inner side by the nose. The limits are, for 
 each eye : Outwards, 45 ° ; downwards and out- 
 wards, 47° — 50°; inwards, 45°; upwards and in- 
 wards, 45°: downwards, 50° — bh° ; upwards and 
 inwards, 38° — 40° ; upwards, 43°; upwards and out- 
 wards, 47°— 50°. 
 
 Tisual angle.— This is the angle included between 
 straight lines drawn from the extremities of the object 
 to the nodal point (0, Fig. 18) of the rays refracted by 
 
 S 
 
274 
 
 Human Physiology. 
 
 [Chap. XV. 
 
 tlie media of the eye, which is situated a little behind 
 the centre of the lens. Our estimate of the size of 
 any object is mainly dependent upon the visual angle 
 under Avhich it is seen. 
 
 The size of the visual angle 
 
 Pig. 18. — The line a b lias a visual angle a', and a retinal angle i/, both, 
 of which are identical with the visual angles of any lines drawn 
 parallel to a b between the lines a o, b o. 
 
 depends, first, on the size of the object, and, secondly, 
 on its distance from the eye. The distance remaining 
 unaltered, the size of the visual angle varies with the 
 size of the object ; and on the other hand, the size of 
 the object remaining the same, the size of the visual 
 angle diminishes with the distance of the object. 
 
 In Fig. 18, A B or the angle o is the visual 
 angle of the line A b. The retinal angle 6 o a is 
 equal to the visual angle, and is formed by the same 
 lines continued beyond the nodal point, o, and it is 
 limited by the extremities of the image formed on the 
 retina. It is seen from the figure that objects of 
 different magnitude, e, d^ c, placed at different dis- 
 tances, may be seen under the same angle, and in the 
 absence of other means of correcting the impression, 
 may be all considered to be of the same size. 
 
Chap. XV.] Sharpness of Vision. 275 
 
 SSiarpness of vision. — This corresponds with 
 the defining power of the eye, the power which the 
 eye possesses of distinguishing two points as separate. 
 It is measured by the smallest angle under which two 
 objects of definite and constant size can be dis- 
 tinguished from each other when separated by an 
 interval of corresponding size to the diameter of one 
 of the objects, or by the determination of the smallest 
 retinal image, the form of which can be perceived by 
 the eye, providing it is not a line. The smallest 
 object that can be seen at the distance of a foot is 
 ■—-^th. of an inch, which subtends an angle of about 
 1 minute. The retinal image corresponding to this- 
 visual angle of one minute has been calculated to be 
 about 0*004 mm., or g-Q^Q-th of an inch, which is the 
 diameter of a cone, but brilliantly illuminated objects 
 of much smaller size may be seen. It follows from 
 what has been stated above, that the smallest object 
 that can be seen at two feet is -g-fg-ths of an inch ; 
 at five feet, 2~fo^^^^ ^^ ^^^ inch, and so on. In 
 ophthalmic practice, the meter is taken as unity of 
 distance, instead of the foot, and letters have been 
 selected of a certain size, usually subtending 5', 
 and the patient's sharpness of vision is tested 
 by determining at what distance he can name the 
 
 letters correctly. Donder's formula, Y = —, enables the 
 
 sharpness of vision to be immediately estimated, 
 for Y, which stands for sharpness of vision, is 
 measured by a fraction of which the numerator cl is the 
 distance in meters at which a given letter can be read, 
 whilst the denominator n is the number of meters at 
 which it ought to be read by any one possessing 
 average or normal acuity of vision. Thus, a letter 
 which by experiment it has been ascertained ought to 
 be read by a normal eye at 12 meters from the 
 subject, is placed before the person to be tested. If 
 
276 Human Physiology^ [Ghap.xv. 
 
 he reads it at 12 meters, his vision is perfect, 
 V = -i|- = 1 ; but if he can only read it at 6 meters, 
 then his vision is expressed by V = y*V = f ; if only at 
 4 meters, Y = y*^ = |-, and so on. 
 
 The sharpness of vision is modified by various 
 circumstances. It is greater in youth, and becomes 
 gradually less as age advances. It is gi^eater when 
 the object is well illuminated (provided the illumina- 
 tion is not excessive) than when indifferently lighted. 
 The size of the pupil influences the sharpness of 
 vision ; for a large pupil, by flooding the retina with 
 light and increasing the circles of dispersion, impairs 
 vision. Wild races have usually very sharp vision. 
 
 I>irect and indirect vision. — Vision is said to 
 be direct when the image of the object falls on the 
 macula lutea ; it is indirect when it falls on any other 
 part of the retina. It is found by experiment that the 
 sharpness of indirect vision rapidly diminishes with 
 the distance from the macula at which the image of 
 the object is formed. Thus, if the sharpness of vision 
 at the macula be taken at 1, at 5° from the macula, 
 it is reduced to ^ ; at 10°, to -jig ; at 20°, to :^ ; at 25°, 
 to -gJg- ; at 30°, to -J-q- ; and at 40°, to g-g-Q ; and beyond 
 this distance, though a moving object can be readily 
 discerned, its form cannot be recognised with accuracy. 
 
 Marriotte's iBlind spot.— Though not ordinarily 
 recognised, there is a spot in the field of vision which 
 is incapable of percei^dng light. This corresponds to 
 the entrance of the optic nerve or optic papilla. It 
 is situated 15° to the outer side of the point of 
 fixation, 'and about 3*^ below the horizontal meridian. 
 
 Spherical atoerration. — If a pencil of diverging 
 rays of light fall on a lens made of homogeneous 
 material, those that fall near the periphery of the lens 
 are brought to a focus sooner than those that fall upon 
 it near its centre. Hence the definition is imperfect. 
 In optical instruments this defect has to be remedied \ 
 
Chap. XV.] Chromatic Aberration. 277 
 
 and there are two modes in which it can be accom- 
 plished. In one, the curvature of the periphery 
 of the lens is diminished as compared with the 
 central part ; and in the other, the density, and con- 
 sequently the refractive power, of the periphery 
 is diminished. The former is the most practicable 
 method in art ; and calculation has shown what is the 
 precise curvature of the surface which enables a lens 
 to give a perfectly defined image, and that it must not 
 be the segment of a sphere, but of the end of an 
 ellipsoid of revolution about its major axis. In the 
 lens of the eye both of the above means of correction 
 are employed ; for in the first place, the outermost 
 layers of the lens have less refractive power than 
 the central and denser portions ; and secondly, the 
 curvature of the peripheral parts of the lens is less than 
 the central part ; in addition, the iris cuts off' the 
 most external rays, whilst the cornea contributes some- 
 thing to the effect, since its surface is really that of 
 an ellipsoid of revolution around its major axis. 
 
 Chromatic atoeiTatioii.— The different coloured 
 rays of light possess different degrees of refrangibility. 
 Red rays are least refrangible ; violet rays are most 
 refrangible. As white light is made up of rays of 
 all degrees of refrangibility, it is clear that each 
 colour will have its own focus ; the violet rays will 
 be brought to a focus first ; the red, latest. Helm- 
 holtz has calculated, that for a reduced eye, -i.e., an 
 eye reduced to its simplest condition for purposes of 
 calculation, the focal distance of the red rays would 
 be 20 •524mm., whilst for violet rays it would only be 
 20"140mm., so that there is a difference amounting to 
 about half a millimeter, or about one-fiftieth of an 
 inch, and on a line of this length all the rays would 
 be successively focussed. The result of this is that 
 white objects, situated beyond the far point have an 
 edging or border of red, because the red rays have 
 
27S Human Physiology. [Chap. xv. 
 
 not as yet met on tlie retina ; whilst if they are 
 nearer than the near point they have a violet edging, 
 the \iolet rays having met and crossed. This defect 
 in the structure of the eye is, however, so slight that 
 it scarcely attracts notice. In optical instruments it 
 is corrected by combining lenses of different dispersive 
 power ; as, for example, those of crown and flint glass. 
 The bi-convex lens of a telescope made of crown glass 
 possesses both great refractive and great dispersive 
 power ; but if combined with a concave lens of flint 
 glass, the curvature of which is much less, its dis- 
 persive power may be neutralised without greatly 
 diminishing its refractive power. 
 
 Use of the ii*is. — The iris is a thin highly 
 vascular membrane, pierced by a hole in the centre, 
 and continuous with the choroid at the periphery. The 
 hole is the pupil, which is capable of undergoing 
 gTeat variations of size. It is surrounded by circular 
 unstriated muscular tissue, by which it can be contrac- 
 ted to the size of a pin's head ; and by radiating smooth 
 muscular fibres, by which it can be expanded till it 
 almost disappears behind the sclero -corneal junction. 
 The sphincter pupillae is under the influence of the 
 third cerebral nerve; the dilatator pupillee is under 
 the influence of the sympathetic. The iris, in addition 
 to these nerves, receives branches of the first division 
 of the fifth nerve, which confers upon it acute sensi- 
 bility. Light enters the eye through the pupil, and 
 the first purpose fulfilled by the iris is to regTilate the 
 amount of light admitted into the interior of the eye. 
 When exposed to bright light the pupil contracts by re- 
 flex action ; the nervous circle being the retina and optic 
 nerve, which is the sensory apparatus ; a centre situated 
 in the medulla oblongata ; and the third nerve, which is 
 the motor nerve. The brighter the light and the more 
 sensitive the retina, the greater is the contraction of 
 the pupil. Hence, in coming from a dark room into 
 
Chap. XV.] Uses of Iris. 279 
 
 Lright sunlight tlie pupil contracts to the utmost, and 
 pain may result from excessive stimulation of the fibres 
 of the fifth nerve; and the stimulus radiating in various 
 directions causes contraction of the orbicularis, and 
 flow of tears. Light falling on one eye causes the 
 opposite pupil to contract consentaneously. The 
 stimulus for the dilatation of the pupil is the absence 
 of light ; and the nervous circle is the retina and optic 
 nerve for the sensory apparatus, the eilio-spinal ce?z^re; 
 and the sympathetic for the motor nerve. This centre 
 can be excited by stimulation of other sensory nerves, 
 acute pain causing dilatation of the pupil. It is 
 stimulated also by imperfectly aerated blood, as is seen 
 in conditions of dyspnoea. 
 
 A second purpose fulfilled by the iris is to aid in 
 correcting the spherical aberration of the lens. The 
 pujDil contracts when the eyes are rolled inwards, or 
 converged to see a near object. The efifect of this is 
 to cut ofi" the outer rays of the divergent pencil, 
 which, falling on the periphery of the lens, would 
 be brought to a focus sooner than those passing 
 through that body nearer its centre. The contrac- 
 tion of the pupil thus aids in making the images 
 of near objects on the retina more clear and defined. 
 The pupil contracts as an associated movement when- 
 ever the eye is rolled inwards; and as it turns inwards 
 and upwards in sleep, the pupil is then also contracted. 
 
 The large supj)ly of blood-vessels in the iris renders 
 it almost an erectile organ. Hence, any circumstances 
 causing congestion of the head, or repletion of the 
 vessels of the eye, tend to produce contraction of 
 the pupil. The escape of the aqueous humour on 
 paracentesis of the cornea, by reducing the pressure 
 under which the blood is movincj iii the vessels of 
 the iris, leads to a rush of blood into them, and con- 
 traction of the pupil occurs. 
 
 Ue:ftain drugs possess remarkable po\Yers o|' clilating, 
 
2 So Human Physiology. [Chap. xv 
 
 and otliers of contracting, the pupil. The alkaloids of 
 the Solanacese, atropin^ hyoscyamin, daturin, as well as 
 duboisin, cause, by paralysing the third nerve, wide 
 dilatation of the pupil. Nicotin, pilocarpin, on the other 
 hand, and especially eserine, cause, by paralysing the 
 sympathetic nerve, great contraction of the pupil. It 
 is probable that in addition to paralysing one set of 
 nerve fibres, these agents stimulate their antagonists. 
 
 Near point and far point. — The points at 
 which an object can be distinctly seen, when the ac- 
 commodation is exerted to the utmost, is termed 
 the near point, or 'punctum proxiimmi, of the eye. 
 It varies with the form of the eye, the strength of 
 the ciliary muscle, and the elasticity of the lens. 
 A child can bring an object Avithin three inches of 
 the eye, and still see it distinctly. Its power of 
 accommodation is great, the ciliary muscle acts 
 vigorously, and the lens is highly elastic, therefore 
 it can bring strongly diverging rays to a focus on 
 the retina. At fifty years of age an object cannot, as a 
 rule, be distinctly focussed when it has been brought 
 within twenty inches of the eye, for at that age the 
 lens is of firm consistence, and its elasticity is reduced, 
 whilst the ciliary muscle acts less energetically. As 
 age advances, a book is held at a continually increasing 
 distance from the eye, until at length the distance is so 
 great that, although the letters are accurately focussed 
 on the retina, the size of the image is so small that it 
 can no longer be recognised. The loss of power in 
 the ciliary muscle, and the diminishing elasticity of 
 the lens, require to be supplemented by a convex 
 glass, which renders the rays of light convergent. 
 
 The far point, or punctum rennotumy for a normal 
 eye is infinite distance, for the normal eye at rest is 
 adapted to focus parallel rays on the retina, and it is 
 only bodies that are at an infinite distance that emit 
 parallel rays (Fig. 19). 
 
Chap. XV.] ACCOMMODA TION OF THE EyE. 
 
 281 
 
 AccossMModatiosi of tSae eye, — In looking at 
 two objects, one of which is nearer than the other, 
 with- a telescope, the position of the glasses has to be 
 altered to see the nearer object, by drawing out the 
 tube of the telescope. The same result can be ob- 
 tained by thickening the glasses and rendering them 
 
 Fig. 19.— Accommodation of tlie Eye for Distance, Parallel Eays being 
 brovTilit to a Focus on the Eetina. The eye is at rest, and uu 
 muscular effort is made. 
 
 L, Lens ; p, parallel rays; e, retina. 
 
 Fig. 20.— Accommodation of the Eye for Distance, Depiction of an 
 Object on the Eetina. 
 
 A, B, Object ; a, b, retinal image ; 0, nodal point. 
 
 stronger lenses. In the eye the latter plan is adopted. 
 The muscle employed for this purpose is the ciliary 
 onuscle, or tensor choroidece. (lo, ii, 12, Fig. 21.) When 
 the attention is fixed on a distant object the muscle is 
 relaxed ; the suspensory ligament of the lens is in 
 action, and by its compression the lens is kept in a 
 flattened state ; the degree of flattening being just such 
 as will allow parallel rays to be brought to a focus upon 
 the retina. Thus we may consider the object to 
 
282 Human Physiology, [Chap, xv, 
 
 consist of several parts^ as the barb, feathers, and 
 stem, of an arrow. It is obvious from Fig. 20 
 that, as the rays proceeding from each part are 
 parallel, they will be exactly focussed on the retina, 
 and that the image will necessarily be inverted. 
 If, however, the object be situated near to the eye, the 
 rays proceeding from it will not be parallel, but 
 divergent, and divergent rays would not be brought 
 to a focus so soon as parallel rays ; in fact, would 
 only be brought to a focus behind the retina, and 
 not upon it. Either, therefore, the distance between 
 the lens and the retina must be increased^ or some 
 change in the refractive power of the lens must be 
 made; the latter plan is adopted, and it is accom- 
 plished by the action of the ciliary muscle. When this 
 muscle contracts, the choroid membrane is drawn 
 forwards. The effect of this is to relax the suspensory 
 ligament of the lens, which permits the lens to be- 
 come more convex, the increase of convexity affecting 
 the anterior part, and the degree of convexity being 
 dependent in part upon the contractile power of the 
 ciliary muscle, and in part upon the elasticity of the 
 lens. This change in the form of the lens is shown in 
 Fig. 21, where the upper half shows the state of the 
 lens when the eye is adapted for distant vision; 
 and the lower half when it is rendered more convex 
 by the action of the ciliary muscle, and is therefore 
 adapted, adjusted, or accommodated for the distant 
 vision of near objects. 
 
 That there is really such a change in the convexity 
 of the lens may be shown by observing the reflections 
 from the cornea, and from the surfaces of the lens, of a 
 candle held to one side of the person observed, whilst 
 he adjusts his eyes for near and distant objects 
 alternately. It is then found that, while the erect 
 corneal image remains stationary, the erect image from 
 th§ anterior surface of the lens changes its position, 
 
Chap. XV.] Accommodation of the Eye. 
 
 283 
 
 Fig. 21.— Diagram of tlie Front Half of the Eye, to sliow tte means "bj 
 wMch. Accommodation is effected, 
 
284 
 
 Human Physiology. 
 
 [Chap. XV. 
 
 '£» H: B' 
 
 Pig. 22. — Scheiner's 
 Experhneut. 
 
 c, Card with two holes ; s, left ; 
 D, right ; R, puiictum remo- 
 tum ; p, inmctuin proximuiu ; 
 &' , focus of distant object ; p', 
 course of rays through dextral 
 aperture from near poiut ; p", 
 course of rays from near point 
 through sinistral aperture. 
 
 whilst the inverted image from 
 the posterior surface of the lens 
 becomes smaller. 
 
 Scfiieiiier's experSaiaeaftft.— 
 This experiment demonstrates the 
 necessity for accommodation, in 
 order that single images should be 
 formed on the retina. It consists 
 in making two minute holes in a 
 card [see Fig. 22) with a needle, 
 the distance between which is 
 less than the breadth of the 
 pupil. If a single j)in be stuck 
 into a board and fixed (that is to 
 say, looked at intently) through 
 the card, a single image only is 
 seen, because all the rays pro- 
 ceeding from it, though entering 
 by two separate holes, are focussed 
 on the retina ; but if two pins^ p 
 and R, are now stuck into the 
 board, one behind the other, and 
 looked at through the holes, it 
 will be found that when the 
 proximal pin, or the one nearest 
 to the eye, is fixed, the other, or 
 remote one, is double, and vice 
 versa. The reason of this is, that 
 if the remote pin be fixed, and its 
 image be formed distinctly on the 
 retina, the more divergent rays 
 proceeding from the nearer pin 
 are not brought to a focus soon 
 enough, but are focussed behind 
 the retina ; and as it is looked at 
 through two holes, the image is 
 double. On the other hand, if 
 
Chap. XV.] Presbyopia. 285 
 
 the nearer pin be fixed, the lens is rendered a stronger 
 one, and the rays coming from the more distant object 
 are brought to a focus in the vitreous, or in front of 
 the retina, and therefore cross. If the nearer pin is 
 fixed, and the right hole is covered, the left-hand image 
 of the double image of the more remote pin vanishes, 
 because the rays have crossed in the vitreous. If 
 the more distant jDin is fixed, and the right-hand hole 
 is covered, the right-hand image of the two images of 
 the near pin vanishes, because the rays have not yet 
 come to a focus. 
 
 Elnuneti'opic eye. Normal or healthy eye. — 
 The healthy eye is so constructed that parallel rays, 
 or those coming from infinitely distant objects, are 
 brought to a focus upon the retina when the eye is at 
 rest and no exertion of the ciliary muscle is made. 
 When near objects are looked at, the diverging rays 
 which proceed from them are brought to a focus by 
 the exercise of the muscle of accommodation. The 
 most distant object which can be seen distinctly is an 
 infinitely distant one, or one so distant that the rays 
 are approximatively parallel. The nearest object that 
 can be seen distinctly depends on the strength of the 
 ciliary muscle and the elasticity of the lens. 
 
 Presbyopia. — The vision of old age. This is 
 really only failure of the power of accommodation. 
 In age, supposing the eye to have been originally 
 of normal formation, distant objects are seen as well 
 as in youth, except in so far as the eye, in common 
 Avith the rest of the nervous system, reacts less 
 vigorously to impressions, and is less sensitive to 
 them. The presbyope sees near objects with difiiculty. 
 The images of these are confused and blurred, because 
 he cannot exert his power of accommodation sufliciently 
 to cause a distinct image to be formed on the retina ; 
 and this defect maybe attributed in part to loss of power 
 of the ciliary muscle, which is unable to draw the 
 
286 Human Physiology. [chap. xv. 
 
 choroid forwards, and therefore to relax the suspensory 
 ligament, but partly also to defective elasticity of the 
 lens, which, when the suspensory ligament is relaxed, 
 does not become thicker, as occurs with the eye of a 
 young person. The image of any luminous object 
 situated within twenty or thirty inches of the eye 
 (that is, within such distance that the rays proceeding 
 from it are divergent) falls behind the retina, and no 
 exertion that he can make enables him to obtain a 
 picture of it on the retina. He requires convex 
 glasses to enable him to see near objects distinctly ; 
 that is to say, the refractive conditions of his eye re- 
 quire that the entering rays should be rendered parallel 
 before he can obtain a well-defined image. If in old 
 age distant objects are not seen distinctly without a 
 convex glass, the subject must have been hyper-metropic. 
 
 Myopia, — Short-sightedness. In myopia the far 
 point is at a measurable distance. The globe of the 
 eye is elongated. Parallel rays are brought to a 
 focus in front of the retina. Distant objects, there- 
 fore, are not clearly seen. On the contrary, rays 
 more or less strongly diverging can be recognised 
 distinctly. Hence, objects are brought into close 
 proximity to the eye. Hence, also, concave glasses, 
 which render parallel rays diverging, assist the vision 
 of myopes ; but the weakest power should be used 
 compatible with clear vision. 
 
 It is often thought that myopic eyes improve with 
 age, and those who are short-sighted congratulate 
 themselves that they will see better as they grow 
 older ; but here also the same changes occur as in 
 presbyopia. It is really a failure in the power of 
 adjustment that gives a semblance of truth to the 
 statement. The myopic person, as he advances in 
 years, becomes unable to accommodate his vision for 
 what was in youth his near point. He can only see 
 objects distinctly when they are placed at his far 
 
Chap. XV.] Myopia. Hypermetropia. 287 
 
 point. Hence he holds objects at a greater distance 
 than formerly, and he appears to have improved to 
 that extent; but it is only at the expense of his 
 near vision. For objects beyond his far point, he 
 must still wear, as he always has done, concave glasses. 
 In the accompanying Fig. the globe of the eye is 
 seen to be elongated. The continuous dark lines, g g. 
 
 rig. 23.— Diagram of the Coiu-se of tlie Eays ia a Myopic Eye. 
 
 represent parallel rays, and these, by reason of the 
 length of the eye, are brought to a focus at 0, and, 
 decussating, form a blurred image on the retina. In 
 order to see an object distinctly, he brings it nearer to 
 the eye, as to f, which we may regard as his far 
 point. The diverging rays which then proceed from 
 it he can, without effort, focus on his retma at c, as 
 indicated by the dotted lines. If he bring it nearer 
 thany^ he can still focus it clearly, pro^dding he exerts 
 his accommodation, till at last he reaches a point (his 
 near point) beyond which he cannot focus the rays by 
 any effort of his accommodation. In high degrees of 
 myopia, either one eye alone is used, or the power of 
 converging the eyes must be very great, for the object 
 has to be approxunated very closely to the eye, and 
 fatigue is soon experienced. 
 
 Hypermetropia. — Long-sightedness. In hyper* 
 metropia the eye is flattened. The far point ia 
 
288 Human Physiology. [Chap. xv. 
 
 beyond infinifce distance, if the term may be allowed. 
 Rays of light emanating from all objects at a mea- 
 surable distance are divergent. Those from objects at 
 an infinite distance are parallel. Neither of these 
 can be focussed on the retina by a hypermetropic eye. 
 At rest, the diverging rays emanating from a near 
 object must be rendered convergent by a convex lens, 
 or by a more or less energetic contraction of the 
 
 Fig. 24. — Diagram of tlie Course of the Eays in a Hj^permetropic Eye. 
 
 ciliary muscle. This muscle soon becomes fatigued, 
 and then exhausted, and near objects can no longer be 
 seen. 
 
 The conditions are shown in Fig. 24. It is 
 here seen that parallel rays_, indicated by the dark 
 continuous lines, are not brought to a focus on the 
 retina, but behind it, at o. Objects that emit parallel 
 rays of light, as, for example, the moon, are not seen 
 distinctly by the hypermetropic person when his eyes 
 are at rest ; but, if he exerts his accommodation, and 
 renders his lens thicker, then he brings up the focus 
 from o to c, and he sees the object distinctly ; but he 
 is in worse case with near objects. Rays proceeding 
 from near objects are divergent, and hence are only 
 brought to a focus, as indicated by the dotted lines, 
 far behind the retina, as at f. By an extraordinary 
 exertion of the ciliary muscle, the suspensory ligament 
 
Chap. XV.] 
 
 Astigmatism, 
 
 289 
 
 may be for a short time so relaxed, and tlie lens 
 rendered thicker, that the focus is brought up from f 
 to ; but the eflbrt cannot 
 be long sustained, the mus- 
 cle relaxes, and all near 
 objects become indistinct. 
 The appropriate means of 
 relief consist in the appli- 
 cation of glasses of such 
 strength, that, as shown by 
 the thin lines, they cause 
 rays of light from distant 
 objects to converge suffi- 
 ciently to unite on the re- 
 tina without any effort on 
 the part of the , subject. 
 The lonof - sifjhted man 
 should wear the strongest 
 glasses with which he can 
 see distant objects well. 
 
 Astigmatism. — This 
 is so common a defect of 
 the eye that it may almost 
 be regarded as the normal 
 condition. When consider- 
 able, it occasions great 
 impairment of vision. It 
 consists in an inequality of 
 the refraction in the dif- 
 ferent meridians of the eye, 
 and is usually caused by 
 the curvature of the cornea 
 being different in the two 
 meridians. The curvature 
 in the vertical meridian is 
 usually sharper than in the 
 horizontal. Its refractive 
 
 power IS 
 
 therefore greater 
 
290 
 
 Human Physiology 
 
 [Chap. XV. 
 
 in the vertical than in the horizontal meridian. 
 Under these circumstances, the eye is unable to bring 
 the image of a point of light to a focus on the retina. 
 If p (Fig. 25) be the point of light, and vv' represents 
 the vertical meridian or sharper curve of the cornea in a 
 case of simple myopic astigmatism, and h h' the hori- 
 zontal meridian, the rays p v, p v' will be brought to a 
 focus at F ; whilst those falling in the horizontal 
 meridian, p h, p h', will not be brought to a focus so 
 soon, but at a farther point, f'. Hence, at f the 
 image of the point will be horizontally elongated, as 
 at K, by reason of the rays v f, v' f having come to a 
 focus, whilst the horizontal rays p h p h' have not yet 
 met. At F, on the contrary, the image will be 
 vertically elongated, as at l, because the vertical rays 
 have come to a focus and crossed. 
 
 "When, in such an eye, a series of lines, radiating 
 from a centre, is placed before it, that line will be 
 
 seen most distinctly which is 
 parallel to the astigmatic meri- 
 dian, Avliich w^ould here be the 
 vei-tical one ; for the image 
 of a vertical line will be 
 formed by the superposition of 
 small vertical lines. These 
 overlap each other in the 
 meridian, and 
 line somewhat 
 
 V 
 
 n 
 
 H 
 
 Fig. 26. — Appearance of a 
 Cross in a case of Simple 
 Myopic Astigmatism, axis 
 vertical. 
 
 astigmatic 
 
 make the 
 
 longer indeed and blurred at 
 
 the ends, which is not noticed, 
 but blacker and more distinct; 
 whilst the line at right-angles to the astigmatic 
 meridian is blurred and confused, being formed by the 
 apposition of a series of vertical lines (Fig. 26). 
 
 The defect is corrected in ophthalmic practice by 
 cylindrical glasses, the curvature being nil in one 
 axis_, and more or less considerable in the opposite 
 
Chap. XV.] EnTOPTIC PHENOMENA, 39 1 
 
 axis, in correspondence with tlie degree of astigmatism 
 that has to be corrected. 
 
 Irradiation. — By this term is understood the 
 tendency of a brightly illuminated surface to encroach 
 upon an adjoining black surface, as a result of im- 
 perfect accommodation. A black letter upon a white 
 ground, therefore, appears smaller than it really is, 
 whilst a white letter on a black ground appears larger. 
 When accommodation is perfect, irradiation is not 
 observed. 
 
 £iitoptic plieuonieua. — This term is applied 
 to subjective visual sensations, sensations that are 
 perceived by the eye itself. The most important 
 of these are Purkinje's figures, phosphenes, and muscse 
 volitantes, of which there are several varieties. Thus, 
 if the clear blue sky be looked at for some time, a 
 number of bright spots will be seen moving to and 
 fro, like the sparks in tinder, or like the movements 
 of the small black water beetle named Gyrinus. These 
 are the blood corpuscles moving in the retinal capil- 
 laries. Another form is that of strings of transparent 
 pearl-like bodies, which are the debris of cells in the 
 vitreous. These are not very persistent, and are 
 of much less importance than the presence of one or 
 more black spots, sometimes with a tail of semi- 
 transparent cell-like bodies, or a chain of globules 
 attached to it or them. These move Avith the eye, 
 and continue to move for some time after the eye has 
 been brought to rest. They are usually considered to 
 be masses of pigment, which have separated from the 
 choroid or ciliary processes, and have entered the space 
 between the lens and the vitreous, or have become 
 entangled in the vitreous. They may also be exuda- 
 tion masses, or small masses of efifased blood. They 
 are of common occurrence in cases of hypermetropia, 
 and appear to be caused by the subject pressing and 
 rubbing the eye after exertion. Muco-lachryraal 
 
292 Human Physiology, [cvap. xv. 
 
 figures may be brought into view by looking through 
 a minute hole in a card at the flame of a lamp. 
 These move with the eyelids. 
 
 Piirkiiije's figwi'es. — If, after the pupil has 
 been widely dilated by remaining in a dark room for 
 some time, a candle be waved before it in various 
 directions, the outlines of the vessels will be seen. 
 These, as is well known, lie in the anterior layer 
 of the retina, and their shadows are consequently 
 thrown on the rods and cones which form the posterior 
 layer of the retina, and are the light - perceiving 
 elements. 
 
 Pliosplieiies. — When the eye is turned strongly 
 inwards, and the finger is lightly pressed on the outer 
 side of the globe as far back as possible, a phosphor- 
 escent-like luminous ring is seen on the side opposite 
 to that on w^hich the pressure is made. It is due 
 to the mechanical irritation of the retina at the point 
 pressed, which is projected outwards. 
 
 Pui'kinje's imag'es. — When a ray of light 
 falls on the eye it is in part reflected from the 
 successive limiting planes of the different media, and 
 before the invention of the ophthalmoscope much 
 use was made of the images thus formed, and specially 
 described by Purkinje, to determine the presence of 
 cataract. If the jDupil be large, or, better still, artifi- 
 cially dilated with atropine, and a candle with a 
 steady flame be placed a little to one side of the 
 subject, whilst the observer stands in front of him, 
 three images of the candle will be seen : A large 
 erect one, reflected from the convex surface of the 
 cornea ; a smaller erect one, from the anterior surface 
 of the lens (both of tbese move with the candle if the 
 position of this be altered) ; lastly, there is a still 
 smaller image, which is inverted, and moves in the op- 
 posite direction to the candle ; this is reflected from 
 the posterior concave surface of the crystalline lens. 
 
Chap. XV.] Cause of Erect Vis 102^. 293 
 
 The presence of an opaque lens is rendered evident 
 by the disappearance of the inverted iioage, vrhich 
 is formed bv light that has passed through the lens 
 to the posterior surface, and has been reflected from 
 it. The non-appearance of the second or smaller 
 erect image demonstrates that the pupil is obstructed 
 bj an exudate, or that there is turbidity of the 
 aqyeous humour, or cloudiness of the cornea, all of 
 which interfere with its production. 
 
 Caiase of erect vision. — The images of 
 external objects formed on the retina are necessarily 
 inverted, and it has been thought that we really see 
 the Avorld upside clown ; and attempts have been made 
 to explain the mode in which "visual sensations are 
 made to harmonise with tactile, auditory, and other 
 impressions. There is, however, no reason for 
 believing that such antagonism between the senses 
 exists. The rays coming from the lower part of the 
 field of vision affect the rods and cones situated in the 
 upper part of the retina, and those from above affect 
 the lower ones ; but it is easy to conceive that at the 
 central extremity of the nerve fibres, the mind takes 
 cognisance of the direction from which the rays have 
 entered the eye, and refers them back to their normal 
 source, or, as it is said, projects the image outwards. 
 A good illustration of the "law of visible direction" 
 is given by Leconte. If a pin-hole be made in a 
 card, and the card be held at a distance of four or 
 five inches before the right eye, with the left eye 
 shut, and a pin-head be now brought very near to the 
 open eye, so that it touches the lashes, and in the line 
 of sight, a perfect inserted image of the pin-head will 
 be seen in the piu-hole. If, instead of one, several 
 pin-holes are made, an inverted image of the pin-head 
 will be seen in each pin-hole. The explanation is as 
 follows : — If the pin were farther away, say six inches 
 or more, then light from the pin would be brought to 
 
294 Human Physiology. [Chap. xV. 
 
 focal points, and produce an image on the retina, and 
 this image being inA^erted would, by projection, be 
 reinverted, and the pin would be seen in its real 
 position. In the above experiment, however, the pin 
 is much too near the retina to form an image. But 
 nearness to the retinal screen, though unfavourable 
 for producing an image, is most favourable for casting 
 a sharp shadow, and whilst retinal images are inverted, 
 retinal shadows are erect. The light streaming 
 through the pin-hole into the eye casts an erect 
 shadow of the pin-head on the retina. This shadow 
 is projected outward into space, and, by the law of 
 direction, is inverted in the act of projection, and 
 therefore seen in this position in the pin-hole. It is 
 further proved to, be the outward projection of a 
 retinal shadow by the fact that, by multiplying the 
 pin-holes or sources of light, the shadows are multi- 
 plied, precisely as shadows of an object in a room are 
 multiplied by multiplying the lights in the room. 
 
 Siog-le "Vision ^witli two eyes. — The two eyes 
 act together, and when directed to any object, that 
 object is seen single, because its image falls upon 
 what are termed "corresponding" or "identical" 
 points of the retina ; if the images fall on any other 
 points, double images will be formed. If the concave 
 right retina were lifted bodily out of the eye, and 
 superimposed upon the left retina, the " corresponding 
 points " of the two retinae would be in contact ; the 
 outer half of the right retina would cover the inner 
 half of the left retina, and the inner half of the right 
 would cover the outer pa.rt of the left retina. The 
 images of all objects falling on the outer side of the 
 riffht retina and the inner side of the left retina, or 
 vice versa, fall therefore on corresponding points, and 
 are seen single. The area of space, rays proceeding 
 from any part of which fall on corresponding 
 points, and in which, therefore, all objects appear 
 
Chap. XV. ] V/S UA L PuRPL E. Op TOGR A MS. 29^ 
 
 single, is termed the horopter. The precise form of 
 the horopter is unknown. The difficulty of determin- 
 ing it lies in the difficulty of recognising double 
 images when they are formed at some distance from 
 the macula lutea. 
 
 Rtiodopsiii or visual piu'ple. — The stnic- 
 ture of the retina is given in detail in Klein's " His- 
 tology," and it is there stated that the outer segments 
 of the rods, but not of the cones, contain in the fresh 
 and living state a peculiar diffused purplish colouring 
 matter, named rhodopsin. It is not present in the 
 rods near the ora serrata. It is non-diffasible, in- 
 soluble in solutions of urea, or in melted paraffin. It 
 is soluble in solutions of the biliary salts. It is easily 
 destroyed by chlorine, nitrous acid, ether, chloroform, 
 aldehyde, and oil of turpentine ; but it resists the 
 action of ozone, permanganate of potash, ammonia, 
 and sodium chloride. When exposed to the light of 
 day, it rapidly bleaches. Exposure to a temperature 
 of 50° C. in the dark causes bleaching to begin, be- 
 coming very rayjid as the temperature rises to 70° C 
 When, after being bleached, the retina is kept in 
 darkness, it is capable of regaining its colour if pre- 
 served in contact with its natural background. 
 
 Opto^^anis. — The action of light upon rho- 
 dopsin was first demonstrated by Kiihne in the fol- 
 loAving experiment : — In the opaque wooden wall of a 
 dark chamber he bored a hole, which he covered with 
 a circular diaphragm 5 mm. in diameter. The hole 
 looked into a second chamber, in which was only one 
 ground-glass pane, on which the bright noon-day 
 light fell. In order to see how this bright pane, 
 which was about 5 "77 metres from the wooden wall 
 just mentioned, came out as an image in the rabbit's 
 eye, he first of all hung over it an intensely coloured 
 chrome-yellow tissue paper, and arranged an eye in 
 the following way : an Albino rabbit, after being 
 
2^6 Human Physiology. [Chap. xV. 
 
 kept fifteen minutes in the dark, was decapitated. 
 One eye was removed from the head under the mono- 
 chromatic light of a sodium light, was somewhat cleared 
 at its posterior surface, and fastened on to the edge 
 of a cork by means of needles run through the re- 
 mains of the conjunctiva. Thus prepared, the eye was 
 placed in position in the dark chamber, with the 
 cornea pressing softly against the diaphragm. The 
 imao;e was visible on the sclerotic on one side of the 
 optic nerve, a portion of which had been left attached 
 to the eye, and so far beneath the point of entrance 
 of the nerve into the bulb that he was sure that it 
 fell on the more deeply coloured division of the 
 retina, and could readily mark its place in the appro- 
 priate quadrant. Thereupon the yellow curtain was 
 removed from the pane, and the eye, after five 
 minutes' exposure, was taken away, divided along 
 the equator, and examined in feeble gaslight. Being 
 unable to recognise any image on it, he brought the 
 preparation out into darkened daylight, and showed 
 it to several witnesses. There was evident on the 
 retina a most distinct brighter difiiised spot, the 
 small dimensions of which corresponded to those of 
 the image he had previously seen, and the position of 
 which made him already sure that it was the optogram. 
 All the witnesses recognised the spot as being in the 
 same place. The eye Avas removed from its support, 
 and for his own satisfaction he tried to find on the 
 sclerotic from behind the previously observed position 
 of the image. In this he was completely successful, 
 owing to the help given by the small remains of the 
 ocular muscles, the position of which, in reference to 
 the position of the image, had previously been ob- 
 served, and he was able to thrust a needle through 
 from behind, which went straight through the pale 
 spot ; still the image was not precise, and it was only 
 in after- experiments, in which he used a 4 per cent. 
 
Chap. XV.j Binocular Vision: 297 
 
 solution of potash-alum to harden the retina and fix 
 the retinal purple, that the details of the image 
 thrown on the retina, as the cross-bars of a window, 
 became clearly visible. 
 
 JSinociilar vision. — The importance of bino- 
 cular vision is shown by the diihculty that is 
 experienced in threading a needle, or in pouring 
 wine into a glass, with one eye closed ; and this is 
 due to the circumstance that pictures on the retina 
 convey to the mind only the notion of a plane surface. 
 The mode in which the mind estimates distance with 
 one eye alone is by variations of light and shade, by 
 degree of exertion of the accommodation to obtain 
 distinct vision, by experience of form and size, by 
 parallactic motion, and by probability ; but when 
 both eyes are used, an impression of solidity is at 
 once obtained ; a stereoscope effect is produced, and 
 the relative position of objects is much more 
 accurately estimated. If a card with a name be 
 held edgewise before the nose, it will be found that 
 the name may be read with one eye, whilst the 
 back of the card will be seen by the other. It is 
 the combination of two dissimilar images that enables 
 us to judge that the card has a certain depth, and that 
 a pillar is round and not flat. The stereoscope itself 
 is only an instrument by which two plane figures drawn 
 or photographed, as seen with the two eyes, are super- 
 imposed upon each other, and which immediately give 
 the effect of solidity ; the objects in the foreground 
 standing out in strons: relief acjainst those in the 
 middle distance and background. 
 
 Decomposition of light. — The white light of 
 the sun is composed of colorific rays of various degrees 
 of refrangibility, which are therefore separated from 
 each other when they are made to traverse a prism. 
 The red rays are least refrangible, and then in succes- 
 sion the orange, yellow, green, blue, indigo, and violet 
 
298 Human Physiology. [dhap. Xv\ 
 
 rays, wliich are the most refrangible. The rapidity of 
 the undulations is very great ; the red ray vibrates 481 
 billions of times in a second; the violet 764 billions. 
 There are other rays in the solar sj^ectram besides those 
 of light. There are rays, which are invisible indeed, 
 but which act strongly on the thermometer [calorific 
 rays) which are less refrangible than the red; and 
 there are rays which are more refrangible than the 
 violet, which are also invisible, but which are re- 
 cognised by their powerful chemical action, and are 
 sometimes called " actinic,'^ or " ultra violet," rays. 
 Attempts have been made to distinguish the several 
 coloured rays of the solar spectrum into pure 
 colours and mixed colours. At one time it was 
 thought that the fundamental colours were red, 
 yellow, and blue ; orange was believed to result 
 from the blending of red and yellow ; green from 
 the mixture of yellow and blue. More recently the 
 fundamental colours have been held to be red, green, 
 and violet ; whilst Hering reverts to the view long 
 ago held by Leonardo da Yinci, that the primary 
 colours are red, yellow, green, and blue. 
 
 CoiiapleiMciitary colours. — Complementary 
 colours are those which, when mingled, theoretically 
 produce white light. Examples of complementary 
 colours are said to be found in red and bluish-green ; 
 orange and light blue, yellow and indigo, greenish- 
 yellow and violet, green and purple, which last is a 
 compound colour ; but none of these compounds really 
 produce the impression of white light on the eye ; 
 they are merely antagonistic colours. 
 
 Perception of colours. — On the theory of 
 Young it is believed that the retina possesses three 
 kinds of elements, the stimulation of which gives 
 respectively the sensation of red, green, and violet 
 rays. White light excites all the elements equally ; 
 but if homogeneous or monochromatic light be received 
 
Chap. XV.] 
 
 Perception oi< Colours. 
 
 299 
 
 upon the retina, then each of the three kinds of fibres 
 is stimulated, with an intensity which varies with 
 the length of the waves. Thus : red light, which has 
 waves of the greatest length, stimulates the red 
 elements strongly, the green more feebly, and the 
 violet elements slightly, and the sensation experienced 
 is red. Green, which has waves of intermediate length, 
 stimulates the red and the violet elements feebly, but 
 the green elements strongly, and the sensation per- 
 ceived is green ; and so with violet, which has waves 
 of the shortest length, and which acts as a powerful 
 stimulus to the violet elements of the retina, but 
 scarcely affects the green and red elements. 
 
 &T* 
 
 Fig. 27.— Showing the Distribution of the Primary Colours in the 
 Sj^ectrum. e, red ; o, orange ; t, yellow ; g, green ; b, blue ; v, violet. 
 
 The three curves culminating at R, or, and v (Fig. 
 27), show that the retinal impression of these colours in 
 greatest intensity are produced by their own special 
 colour with slight admixtures of the other special 
 colours. 
 
 The intermediate colours are, of course, produced 
 by the mixture of sensations ; thus, the sensation of 
 yellow is caused by the nearly equal stimulation of the 
 red and green perceiving elements with slight stimu- 
 lation of the violet, and so on. 
 
 A very different view has been advanced by 
 Hering, who considers that colour is the mental per- 
 ception of the clianges taking place in the visual 
 substance, which under the influence of light is 
 
3o6 Human Fhysiology. ichap. x^^. 
 
 constantly undergoing a double process of disinte- 
 gration and reparation. Perception of white light is 
 coincident with disintegration ; of blackness or dark- 
 ness, with redintegration ; and the degrees of white and 
 black depend on the activity of the processes of disin- 
 tegration or repair. Besides white and black there are 
 two other pairs of antagonistic colours, the perception 
 of which is similarly dependent on processes of 
 construction or destruction. E-ed and yellow result 
 from destructive or disintegrating processes ; green and 
 blue, on constructive or redintegrating processes. The 
 visual substance, rhodopsin, or material, by whate^'er 
 name it is called, which is sensitive to colour, is thus 
 supposed to be composed of three constituents ; the 
 black and white perceiving, the blue -yellow, and the 
 red-green perceiving substance ; all luminous rays 
 produce disintegration of the black-white perceiving 
 substance, though the different rays act with different 
 degrees of energy upon it. There are only certain 
 rays, however, which act as decomposers of the blue- 
 yellow or red-green constituent, whilst other rays act 
 as constructive agents, and others again have little 
 or no action. 
 
 Coloiii' toliiicliiess. — About three or four per 
 cent, of men, and a less proportion of women, fail to 
 distinguish certain colours. The most common defect 
 is the inability to distinguish between red and 
 green. The peculiar character of red rays is not per- 
 ceived ; the subject is unable to recognise, otherwise 
 than by their form, the cherries from the leaves of a 
 cherry tree. This defect (red-blindness, as it may be 
 called) is supposed, on Young's theory, to be due to the 
 absence of the red-perceiving elements of the retina ; 
 and, on the theory of Hering, to the absence of the red- 
 green constituent of the colour perceiving substance. 
 In the violet blind, the yellow-blue constituent is absent. 
 
 There are degrees of colour blindness ; in some, 
 
Chap. XV.] Movements of the Eye. 301 
 
 whilst saturated colours can be readily recognised, 
 the different shades of the same colour fail to be dis- 
 tinguished. This is termed partial colour blindness. 
 In complete colour blindness only shades of black and 
 white can be perceived, and both the yellow-blue and 
 red-green constituents of the visual substance are absent. 
 
 Positive and negative after imag^es. — If 
 the retina be exposed to a very bright light, which is 
 then suddenly extinguished, there remains for a short 
 time an impression of the same colour, as though the 
 retinal elements still continued to vibrate in response 
 to the same stimulus. This is the iiositice after- 
 image. After a little while, however, the positive 
 image is replaced by a negative after-image^ in which 
 the bright parts of the real image and of the positive 
 after-image, become dark, and the dark parts light. 
 The appearances in question are well seen on looking 
 intently at the sky, through cross-barred or cottage 
 windows for a minute or two, and then closing the 
 eyes ; the bars, at first dark on a light ground, soon 
 become light on a dark ground. 
 
 Muscles and movements of tlie eye. — 
 Each eye is moved by six muscles, which are arranged 
 in pairs, each pair rotating the eye in opposite 
 directions round a definite axis. To facilitate the 
 comprehension of their action, it is also useful to 
 remember that a vertical line halvino- the cornea is 
 termed the vertical 'meridian. The names of the 
 muscles and their actions are : 
 
 Internal rectus ; rotates the eye inwards. 
 External rectus ; rotates the eye outwards. 
 
 The axis of rotation is vertical. The vertical 
 meridian is not altered in direction by their action, 
 but remains vertical. 
 
 \ Superior rectus ; rotates the eye upwards. 
 [ Inferior rectus ; rotates the eye down-wards. 
 
302 
 
 Human Physiology, 
 
 [Chap. XV. 
 
 The axis of rotation is horizontal from nose to 
 temple. The superior rectus acting alone, not only 
 turns the eye upwards, hut causes the vertical meridian 
 to incline inwards, and to obtain a true rotation 
 upwards the inferior oblique is also brought into play, 
 which inclines the meridian outwards. 
 
 In the case of the inferior rectus the same is 
 observed. The inferior rectus not only rotates the 
 
 
 R.iii 
 
 R.exf. 
 
 R.inl. 
 
 ''R.sun. 
 R.i4 
 
 Fig. 28. — Diagram showing- relative Attachments of the Ocular 
 
 Muscles. The thin lines show the axes of rotation. 
 
 eye downwards, but inclines the vertical meridian 
 outwards, and this is corrected by the coincident 
 action of the superior oblique, which includes the 
 vertical meridian inwards. 
 
 ^ Superior oblique — rotates tlie eye down-wards and outwards. 
 ( Inferior oblique — ^rotates the eye upwards and outwards. 
 
Chap. XV.] Lachrymal Apparatus. 303 
 
 The axis of rotation of the oblique muscles lies 
 in the plane of the horizontal diameter of the globe, 
 but forms an angle of 60 '^ with the transverse axis. 
 The superior oblique inclines the vertical meridian 
 inwards. The inferior oblique inclines the vertical 
 meridian outwards. {See Fig. 28.) 
 
 Lachryiiial apparatus. — The secretion of the 
 lachrymal gland, named tears, is a protective fluid. 
 It is discharged into the conjunctival sac at its upper 
 and outer part, and serves to wash away any foreign 
 body that may have entered between the lids and 
 become adherent to the conjunctiva. The secre- 
 tion is very thin and watery, containing nearly 99 
 parts of water and one part per cent, of albumin, 
 mucin, and salts, and may be excited through the 
 optic nerve by exposure of the eye to a bright light, 
 through the branches of the fifth nerve distributed to 
 the conjunctiva and nose, and by mental excitement. 
 The centre is probably in the pons or medulla ob- 
 longata. The motor nerves are the lachrymal of the 
 fifth and the small tem]Doro-malar nerve. After having 
 traversed the surface of the globe of the eye, the tears 
 enter the puncta, partly by capillary attraction and 
 partly by the action of Horner's muscle and some 
 fibres of the orbicularis, which on contracting dilate 
 the lachrymal sac, and produce a tendency to a vacuum, 
 which the tears rush in to fill. The continued action 
 of the same muscle compresses the sac and forces 
 the fluid down the nasal duct, regurgitation being- 
 prevented by several but irregularly placed folds. 
 The inspiratory act also promotes the passage of 
 tears through the duct. The surface of the globe is 
 further kept moist by the mucous secretion of the 
 inner surface of the eyelids. The lids are prevented 
 from adhering during sleep by the secretion of the 
 Meibomian glands, which open on the free margin of 
 the lids behind the attachment of the cilia. 
 
304 Human Physiology. tchap. xv. 
 
 THE SENSE OP HEARING. 
 
 This sense is due to the excitation of the eighth 
 or auditory nerve. The organ of hearing is divided 
 into three parts — the external ear, including the 
 auricle and the meatus ; the middle ear, or tympanum ; 
 and the internal ear, or labyrinth. The external ear 
 has for its function the collection and transmission of 
 sounds to the membrana tympani. The importance 
 of the auricle is shown by its constant motion in 
 animals like the horse, cat, and hare. In man its 
 function is subsidiary ; it has become flattened, the 
 muscles are atrophied, and its movements are scarcely 
 perceptible. If the inequalities of one auricle be filled 
 with wax, the perception of sounds is not impaired, as 
 compared with the opposite ear, when the meatus is 
 directed towards the source of the sound ; but in all 
 other positions it is much diminished. 
 
 The membraiia tympani, or di'umi of the 
 ear. — This is a membrane composed of a basis of 
 fibrous tissue, with a prolongation of the external 
 skin on the one side, and of the mucous membrane of 
 the tympanic cavity on the other. It is concave when 
 viewed externally, is not very tightly stretched, and is 
 inelastic". The handle of the malleus is firmly attached 
 to it. The planes of the two tympani converge 
 anteriorly, and if prolonged would meet at an angle 
 of about 130°. The tympanum vibrates to and fro 
 as a w^hole with undulations of the air, and its 
 vibrations are communicated directly to the ossicula 
 auditus, which not only conduct the vibration to the 
 labyrinth, but act as dampers, and prevent after- 
 impressions to a very material extent. The membrana 
 tympani has no fundamental note of its own, but 
 vibrates with nearly equal readiness to notes of very 
 different pitch. 
 
Chap XV.] Function of Semicircular Canals. 305 
 
 The ossicula. — These, consisting of the malleus^ 
 incus, and stapes, conduct the vibrations of the air 
 from the luembrana tympani to the labyrinth, through 
 the foot of the stapes, which closes the foramen ovale. 
 By their means also the muscles of the tympanum are 
 able to modify the tension of the membrana tympani. 
 The extent of movement impressed on the foot of the 
 stapes by vibrations of the membrana tympani i.i very 
 small, and has been estimated at O'OTmm. The 
 tensor tympani muscle, as its name implies, stretches 
 the membrana tympani by pulling the malleus in- 
 wards. This renders it more capable of vibrating 
 with acuter sounds. 
 
 The EiistacSiiaw tiatoe. — The Eustachian tube 
 permits the air in the tympanic cavity to be renewed, 
 and the secretion of its mucous membrane to escape. 
 The aperture of the tube is usually closed, but it is 
 opened at the moment of swallowing, by the action of 
 the fibres of the dilatator tubcf^. 
 
 Ftioctiou of the semicircular canals. — The 
 semicircular canals partly supply information in regard 
 to the direction of sounds, partly aid in maintaining 
 the equipoise of the body. Their division does not 
 appear to interfere materially with the perception of 
 sounds, but induces very peculiar movements of the 
 head alone, or head and body. Thus, section of the 
 horizontal canal in a pigeon causes it to turn its head 
 alternately to right and left for months together. 
 Lesion of the posterior vertical canal occasions nodding 
 movements, so that the animals often either fall 
 backwards or forwards. Lesion of the superior an- 
 terior canal also leads to vertical movements of the 
 head ; and it is believed by some that the position 
 of the head is recognised by the pressure exerted by 
 the endolymph upon one or other of the ampullae. It 
 will be observed that the three semicircular canals 
 correspoiid to the three sides of a cube, and that 
 U 
 
o 
 
 06 Human Physiology. ichap. XVv 
 
 consequently in whichever direction sounds are pro- 
 pagated to the ear, they will affect one of tjie canals 
 more readily than the others. 
 
 FwBictioii of the lab^niliitli. — The peculiar 
 arrangement of the arches of Corti naturally led to the 
 supposition that they acted like the successive wires 
 of a piano, and vibrated in unison with the sounds 
 affecting them from without ; and it is only necessary 
 to imagine that each arch is in connection with a 
 nerve fibre, which is excited by its vibration, in order 
 to understand how sounds of various pitch, intensity, 
 and timbre come to be recognised. But since the 
 arches of Corti are absent in birds, which m.ust un- 
 doubtedly have very clear perceptions of musical 
 sound, some other part must be looked on as fulfilling 
 this function ', and those which seem most likely are 
 either the radial fibres of the membrana basilaris, on 
 which the organ of Corti rests, and which are shortest 
 in the first turn of the cochlea, and become longer 
 towards the cupola, or the hairs of the hair-cells, which 
 are known to be of different length. 
 
 Sounds are divisible into musical and non- 
 ■ musical. Musical sounds result from aerial undula- 
 tions, which reach the ear in a certain order and 
 regularity. Non-musical sounds, or noises, consist of 
 undulations which have no periodic relation to each 
 other, and reach the ear irregularly. The undulations 
 strike the drum of the ear, are conducted chiefly by the 
 chain of bones, but partly by the air contained in the 
 tympanum, to the vestibule, semicircular canals, and 
 cochlea, and are supposed to set the auditory hairs 
 into vibration. Their vibration excites the extremities 
 of the auditory nerve, and the impression being con- 
 ducted to the auditory centre, produces there the 
 sensation of sound. 
 
 The interval of two notes. — This may be 
 expressed by a fraction, representing the proportion 
 
'chap.xv.] Function OF Semicircular Canals 307 
 
 the "vibrations producing the two notes bear to each 
 other. Thus, if one note is caused by 300 vibrations 
 per second and another by 200, the proportion of the 
 ftwo is |-^, or I". Certain intervals are represented 
 "bj comparatively simple ratios ; and these the ear 
 I'eceives most readily, and are the most agreeable 
 to it. They are those which are ordinarily 
 emitted by the human voice. The simplest proportion 
 is that termed the octave^ in which the ratio is \. 
 The higher note is here produced by double the 
 number of vibrations by which the lower note is 
 formed. The following table gives the relations of the 
 chief simple intervals which are less than an octave. 
 
 Intervals. 
 
 Eatio. 
 
 jS'iTinter of Yibra- 
 
 tions of tlie Higher 
 
 ISTote. 
 
 Number of Vibra- 
 tions of the Lower 
 Note. 
 
 Fifth 
 Fourth . 
 Major third 
 Minor third 
 Minor sixth 
 Major sixth 
 
 2 : 3 
 
 3 : 4 
 
 4 : 
 
 5 : 6 
 5 : 8 
 3 : 5 
 
 3 
 
 4 
 5 
 6 
 8 
 5 
 
 2 
 3 
 4 
 5 
 5 
 3 
 
 The gamut is produced by preserving the more 
 simple intervals, as the fifth, fourth, and third, and 
 intercalating in the intervals of an octave a series of 
 notes separated from each other by detinite intervals. 
 The notes of the gamut are seven in number, and their 
 vibrations bear the following ratio to the vibrations of 
 the fundamental note, or tonic do. 
 
 do 
 1 
 
 mi 
 5 
 
 fa 
 
 sol 
 
 la 
 
 do. 
 2 
 
 This is called the major gamut. 
 The minor gamut is also composed of seven notes, 
 but in these the ratios of the vibrations between 
 
3o8 . Human PmsioLOGV. tcKap. XV. 
 
 themselves and to each other differ from those of the 
 major gamut. They are as follows : — 
 
 do re miV fa sol la'^ silj do. 
 
 19643890 
 
 ■I-qH q <5 Si S ^ 
 
 There are other forms of the minor gamut, but this 
 is the earliest and perhaps the most important of them. 
 
 The tonic of the gamut, whether major or minor, 
 may be placed on any note indifferently, but the ratio 
 of the vibrations in the successive notes in the major 
 and minor keys do not correspond. 
 
 I>ii§igoiia,Mce. — Seatis. — As lonsj as sounds have 
 a certain simple relation to each other, so that one is 
 to the other as 1 : 2, 1 : 3, 1 : 4, and the higher note 
 makes two, three, or four vibrations to each vibration 
 of the lower note, harmony results ; but if the relation 
 of the higher note to the lower is not in the ratio of 
 the multiple to the single, interferences must occur, 
 and dissonance result. Thus, if one sound is produced 
 by 33 vibrations per second and another by 34, the 
 waves of the one must advance upon those of the 
 other, till the crest of one undulation is exactly 
 opposed to the depression of another. A distinct 
 beat is then heard, which in this case would occur once 
 in the second, and would recur at regular intervals of 
 a second. Such isolated beats are frequently heard, 
 and are distressing to a musical ear; but if the 
 difference be greater, the beats of course recur with 
 greater frequency, and dissonance is produced of so 
 marked a character as to be perceptible to the most 
 unmusical person. 
 
 Pitcli ©f a soisiid,,— The pitch of a sound de- 
 pends on the number of vibrations in a given time. 
 The greater the number of vibrations, the higher is 
 the pitch. The perception of notes of successively 
 higher pitch by the ear corresponds therefore with the 
 perception of the succession of colours by the retina ; 
 
Chap. XV.] Pitch and Timbre of Notes, 309 
 
 the difference between tliem lies in the rapidity of the 
 vibrations, which in the case of light are counted by- 
 millions of millions, whilst in the case of sound they 
 are at most only a few thousand in the second. The 
 greatest number of sonorous ^dbrations that can be 
 perceived is rather less than 41,000 per second, though 
 few can hear sounds produced by more than 35,000 
 per second ; the lowest that will give the sensation 
 of a musical sound is about 16. Above the former 
 number, no sound is perceived ; below the latter, only 
 a succession of beats, or puffs, when the instrument 
 used is a wind instrument. A whistle has been 
 constructed which, by being rendered shorter, can 
 be made to yield shriller and shriller notes. If such 
 a whistle be sounded in a mixed audience, it will 
 be found that as the note is made sharper, the 
 ears of a certain number of persons become incapable 
 of responding to the vibrations, and they are perfectly 
 deaf to them ; whilst others are still capable of 
 distinctly recognising them. Contraction of the 
 tensor tympani muscle enables sounds of about 4,000 
 vibrations higher than normal to be perceived. In 
 the same way, the cry of the bat, the squeak of the 
 mouse, or the sound made by the cricket, are inaudible 
 to many who have otherwise fairly good ears, fine- 
 ness of hearing, or the possession of a " good ear," 
 signifies that minute differences in the pitch of two 
 notes, produced by a nearly equal number of vibra- 
 tions, can be perceived. 
 
 Tiiiibi'e of a note. — The timbre of a note is the 
 peculiar difference which enables even an ordinary 
 ear to say whether a particular note is produced by a 
 piano, a violin, a flute, or an organ. It depends on 
 the number and nature of the harmonics which ac- 
 company nearly all musical sounds, and which may be 
 rendered evident, either by resonators, which are large 
 hollow vessels that respond to particvilar notes 
 
3IO Human Physiology. [Chap. xv. 
 
 strongly, and less or not at all to all others ; or by 
 examining the vibrations of a series of stretcliecl cords 
 in the neiahbourhood of the note the timbre of which 
 is required to be determined. Thus, if the wires of a 
 piano are carefully examined whilst any particular 
 note is sung by the human voice, or elicited by the 
 bow from a violin, it will be found that not only the 
 wdre that is in unison with the note sung or played 
 vibrates, but that several other wires are also thrown 
 into vibration. These are harmonics or partial tones, 
 thirds and fifths, which respond to corresj)onding 
 vibrations in the vocal cords or in the cords of the vio- 
 lin, and it is the existence of these harmonics, over- 
 tones, or partial tones, that enable the difference between 
 two instruments or their timbre, to be recognised. 
 
 Dtiratioai of tSae auditory seiisations. — It 
 has been ascertained by experiment that the human ear 
 is capable of recognising as distinct beats 133 beats in 
 a second, but beyond this number the successive im- 
 pulses fuse into one, and the sound becomes con- 
 tinuous. Occasionally after-sounds are perceived, but 
 as a rule the persistence of the individual sounds is 
 very short. The recognition of diiierences between two 
 notes varies greatly in different persons. Thus, some 
 are scarcely disturbed by a sound which is half a note 
 flat or sharp, whilst to others the difference produces, 
 a sensation that is akin to pain ; and it is said that 
 practised musicians will distinguish between notes the 
 difference of which is not greater than 1 in 1,000. 
 The A of musicians in Germany has 440 vibrations 
 per second ; in France, 435. 
 
 VOICE AND SPEECH. 
 
 "Voice or vocal sounds are produced by most mam- 
 mals. Speech is peculiar to man. Yoice is produced 
 by the vibrations of the inferior vocal cords. Speech 
 
Chap. XV.] Production OF Voice, 311 
 
 consists of the same vibrations, with modifications 
 induced by varying size of the oral cavity, and vary- 
 ing position of the tongue and lips. 
 
 The glottis is the opening between the vocal cords. 
 During ordinary respiration, and when no sounds are 
 emitted, it is of triangular form, the apex being in 
 front, and the base behind. The margins are formed 
 by the chordce vocales, or thyro-arytenoid ligaments. 
 These are composed of extremely fine and deKcate 
 fibres of pure elastic tissue, connected on their outer 
 surface with the thyro-arytenoid muscle. As soon as 
 a vocal sound is required to be produced the cords 
 are, by muscular action, rendered moderately tense, 
 and their edges are brought into perfect parallelism. 
 The passage of air through them with a certain degi*ee 
 of force, greater than that of ordinary expiration, 
 throws them into vibration, and produces notes of 
 various pitch and intensity, which may even be 
 imitated in the dead subject by appropriate arrange- 
 ments for drivinor air through them. The tighter the 
 cords, the higher is the pitch of the note ; the more slack 
 the cords, the lower is the pitch. The vocal cords are 
 stretched by the depression of the thyroid cartilage on 
 the cricoid, through the agency of the crico-thyroid 
 muscle ; they are relaxed by the thyro-arytenoid 
 muscles. The fissure of the glottis is widened pos- 
 teriorly by the rotation outwards of the arytenoid 
 cartilages caused by the crico-arytasnoidei postici. It 
 is narrowed by the simultaneous action of the crico- 
 arytsenoidei laterales, which rotate the arytenoid 
 cartilages inwards, anteriorly, and by the arytenoid 
 muscles jDOsteriorly, The nerves implicated in the 
 acts of vocalisation are the sujDerior and inferior 
 laryngeals. The former is the sensory, the latter the 
 motor nerve. Lesion of the inferior laryngeal causes 
 loss of voice from inability to bring the vocal cords 
 into parallelism, 
 
IIuMA N Physiol ogy. 
 
 [Chap. XV. 
 
 Tlie oi-jraais of voice auul spcecli. — Potter 
 
 gives the follo^ving classificatioji: { Ordhmnj. 
 
 j Diaphragm. 
 I Intercostals. 
 .IMuscles of re- j Levators of ril 
 I spiration --' S!'^q1oi-.i 
 
 Motor 
 
 Generating sound ■ 
 
 J Thorax, 
 j Lungs. 
 I Bronchi. 
 Trachea. 
 
 ^ 
 
 .Yibratile. I Jt^'^'^'i ^ 
 ( V oeal chcrJs. 
 
 Scaleui. 
 Extra ordina rjf. 
 I Serrati maginni. 
 I Latiss. dorsi. 
 LPectorals. 
 
 Eesonant 
 (vowel forming) 
 
 JModifying sound i 
 
 ^•Vestibule of larynx. 
 ' Ventricles of larnyx. 
 
 Pharynx. 
 
 Oral cavity. 
 
 Nasal cavity. 
 
 Frontal sinus. 
 
 Sphenoidal sinus. 
 
 Epiglottis 
 
 Velum palati. 
 
 Inferior maxilla. 
 
 Articulating 
 (consonant formin; 
 
 f Tongue. 
 i Lips. 
 ^ i Velum palati. 
 °^ ' Teeth. 
 
 , Inferior maxilla. 
 
 Compass of tlie voice.— The ordinaiy com- 
 pass of the voice includes about four octaves, viz., 
 
 from E gE= to C" ^Se f"^ ^^'''^f /^^^e 
 ^^— |- gj — ■ — being produced by 
 
 80 vibrations per second; the highest by 1,024. 
 Individual voices are stated by Potter* to have de- 
 
 scended to F 
 
 while Catalani sansf G" 
 
 -^ 
 
 and Aguiari is said by Mozart to have risen to 
 
 C"" ~ 
 
 * " fSpeech and its Defects." Lea, Prize Thesis, 
 
 1882. 
 
Chap. XV.] 
 
 Range of Voice. 
 
 313 
 
 The compass and classification of ordinary voices 
 is thus given by the same authority : 
 
 Soprano. 
 
 Female Voices. 
 
 E,F,G|A|B,C DE 
 
 ]\Iale Voices. , , 
 
 .^ ; — I — . — 
 
 FG 
 
 Mezzo-Soprano. 
 
 A B 
 
 Contralto. 
 
 =i=lK: 
 
 — _-»-^C 
 
 •^3B- 
 
 C'D' 
 
 I 
 
 ~"=i?^-«- 
 
 Tenor. 
 
 Barytone- 
 
 Bass. 
 
 E'F' 
 
 G'A' 
 
 —■—tr. 
 
 -x:z^ 
 
 B'C"D"E"F"G"A"B"C'" 
 
 Falsetto voice. — This is a different and a higher 
 register than the ordinary or chest voice. The 
 aperture of the glottis is wider, and the superior 
 vocal cords are more widely separated from each other, 
 than in the production of the ordinary voice. It is 
 believed to be produced by the vibration of the edges 
 only of the vocal cords, whilst these are in a state of 
 greater tension than in chest notes. A falsetto note 
 cannot be sustained as long as a chest note of the same 
 pitch. 
 
 Vo-^vell soiancts. — Vowels are continuous voice 
 sounds produced in the larynx, but the overtones of 
 which are modified by the different forms assumed by 
 the oral cavity. If the trachea is opened, voice ceases. 
 Tlie buccal cavity either forms a cavity of equal 
 diameter throughout, or enlarges its anterior segment 
 whilst it contracts the posterior. The former occurs 
 with the vowels A (broad), 0, and u; the latter for a short, 
 E and I. The buccal cavity is of maximum size in 
 pronouncing the vowel A, and is smallest with u. For 
 all the vowels the orifices of the nasal cavities are 
 
.14 
 
 Human Physiology. 
 
 [Chap. XV. 
 
 closed by the elevation o£ the velum palati, without 
 which nasal sounds v/ould be produced (Budge). 
 
 Consonants. — Consonants are produced by the 
 emission of shorts puffs of air, which are thrown into 
 vibrations as they traverse the narrowed air passages. 
 Simple expiration reinforced by the mouth produces 
 A, or when the lips are closed, and the current of air 
 directed through the nose, «z or n. In the case of the 
 majority of consonants the opening for the passage of 
 air is at first closed, then suddenly opened. Thus, the 
 explosive consonants, h and p, are formed by the sudden 
 opening of the lips ; d and t by the sudden separation 
 of the tongue from the palate or teeth ; g and h by 
 the sej)aration of the tongue from the posterior part of 
 the palate. Simple contraction and closure of the lips 
 takes place for f and v ; of the tongue for s and I ; 
 and of the palate for ch, and s pronounced in German, 
 and t. Consonants produced by approximating the 
 lips to each other or to the teeth are termed labials ; 
 those formed by approximating the tip of the tongue 
 to the teeth or to the hard palate are dentals, or 
 palatals ; and those produced by approximating the 
 root of the tongue to the soft palate are gutturals. 
 
 Potter gives the following tabular arrangement of 
 the consonants according to their acoustic relation : 
 
 Explosives 
 A.spirates 
 Resonants 
 A^ibratives 
 
 Labials.! 
 
 Dentals. 
 
 P.B 
 F V 
 
 M 
 
 T. D 
 S,Z,L, Sh J. Th (hard & soft) 
 
 N 
 
 Gutturals. 
 
 KG 
 
 Ch Y (initial) 
 
 E 
 
 'Wliisperiiig' consists in the movements required 
 for articulate speech being efiected whilst the vocal 
 9ords are not thrown into vibration, 
 
Chap. XV. ] Ta C tile SeNSIBILI TV. 3 1 5 
 
 Sense of tOMcli. — The cutaneous nerves terminate 
 in the skin, partly by free extremities, and partly in 
 special organs, termed tactile corpuscles and Pacinian 
 corpuscles. Several different sensations are perceived by 
 the skin. There are sensations of touch, of pain, of space 
 or position, of pressure, of ticklings of temperature, and 
 of muscular effort. Whether each of these has its own 
 special nerve-end apparatus, and its own channel of 
 communication with the central nervous system, is 
 still undetermined. The sensations of touch proper, 
 or tactile sensations, are only perceived by the skin, 
 and the mucous membranes near theii* orifices, i^o 
 tactile sensations are aroused by the passage of ordinary 
 bodies along the intestinal canal, or along any of the 
 ducts connected ^sdth it ; or if any are excited, they are 
 sensations of pain. All tactile sensations are most 
 distinctly felt when they affect the periphery of the 
 nerves ; and in order that any kind of sensation should 
 be felt, it is necessary that the stimulus should be 
 suddenly increased in intensity, and not increased 
 slowly or gradually. There must also be a free circu- 
 lation through the part. The action of cold in 
 numbing a part is well known. 
 
 Sense of space. — The most sensitive part of the 
 body is the tip of the tongue, which is capable of dis- 
 tinguishing that the points of a pair of compasses are 
 separated when there is only an interval of I'l mm., 
 or -oV^li of an inch between them. The tip of the 
 third phalanx of the fingers is nearly as sensitive. The 
 skin of the lips distinguishes the two points when they 
 are only about 4 mm. apart ; the middle of the dorsum 
 linguae and the skin of the metacarpus of the thumb, 
 when they are about 8 mm. apart ; the skin over the 
 malar bone, when they are about 20 mm. apart. The 
 least sensitive parts are the lower part of the back, 
 and the outer part of the thigh, where the points of 
 the compasses may be applied at a distance from each 
 
o 
 
 1 6 Human Physiology, [Chap. xv. 
 
 other of 2|- inches, and yet not be certainly differen- 
 tiated as two. An instrument termed an cesthesioineter 
 has been devised, in which the distance between two 
 points can be accurately noted, and by means of 
 which the relative acuteness of sensibility of different 
 parts of the skin in any case can be readily ascertained. 
 The simultaneous but unusual excitation of two parts 
 of the skin by any object gives the impression of the 
 presence of two bodies, as in the experiment of Aristotle, 
 in which a marble is touched by the outer side of the 
 second and the inner side of the first finger when these 
 fingers are crossed, which gives the impression of two 
 marbles. In experimenting -on the sensibility of the 
 skin for all varieties of sensation, it should be remem- 
 bered that great improvement takes place after short 
 practice, and that experiments should be repeated 
 many times before any trustworthy conclusions can 
 be drawn. Those who are blind, and who have 
 therefore to depend largely on the sense of touch for 
 guidance, acquire extraordinarily delicate and accurate 
 powders of perception with the fingers, differences of 
 form, size, character of surface, consistence, tempera- 
 ture, and other characters, being readily recognised 
 that are quite inappreciable to those who possess good 
 vision, Avithout special education. Tactile sensibility 
 is most acute at temperatures near that of the normal 
 temperature, 38*6° C The greater the common sensi- 
 bility of any part, the more rapidly may a succession of 
 shocks or pulsations succeed each other, and still be 
 distinguished as separate impulses. On the inner 
 side of the thigh 52 shocks per second can be dis- 
 tinguished ; on the back of the hand, 6 1 ; and on the 
 points of the fingers, 70 per second. 
 
 Sense of pressure. — By this special form of 
 tactile sensibility we recognise the degree of resistance 
 presented by different bodies. Its acuteness is ascer- 
 to-ined by placing weights of the same size but of 
 
Chap. XV.] S£NS£ OF TEMPERATURE. 317 
 
 different amounts on the skin of various parts of the 
 body, and endeavouring to estimate them. The parts 
 which perceive the sense of touch most acutely are 
 generally also those which have the keenest sense of 
 pressure, but not always. The smallest weight which 
 can be perceived is 0*002 gramme, which is recognised 
 by the skin of the forehead, the temples, the back of 
 the head, and the fore-arm. The pulp of the fingers 
 can perceive a weight of 0-005 to 0-015 \ the chin, 
 nose, and belly, 0*04 to O-Oo gramme; and the 
 finger-nail, 1 gramme. The points of the fingers 
 can distinguish that one weight is heavier than 
 another when the proportion between the two is 
 as 29 : 30, providing they are not very light or very- 
 heavy. Experiments have been made to determine 
 what additional weight must be added to one gramme 
 in order that it should be perceived, and it has been 
 found that on the third phalanx of the fingers the 
 addition of 0*499 gramme to 1 gramme is perceived. 
 On the lower leg a whole gramme must be added, and 
 on the back no less than 3*8 grammes. The judgment 
 is materially influenced by the length of time that is 
 allowed to elapse between the trials, and no trust- 
 worthy conclusions are drawn when a little more than 
 a minute and a half have elapsed. Considerable 
 pressure may be exerted without its being perceived, 
 if it is uniform. Thus, v/hen the hand is plunged 
 into mercury, the increased pressure is only felt at the 
 line corresponding with the surface of the fluid. 
 
 Sense of temp erat lire* — The sensations of heat 
 and of cold are relative, and dependent upon the 
 temperature of the part of the body exposed. This 
 piay easily be shown by dipping one hand into hot 
 water, the other into cold, and then both together 
 into water of medium temperature. This will feel hot 
 to the one hand, and cold to the other. The fingers 
 are capable of perceiving differences in temperature 
 
o 
 
 1 8 )'iu MAN Physiology. . [Chap. xV. 
 
 of about 0"2°C. It is mucli more easy to perceive 
 slight differences of temperature when a large surface 
 of the skin is exposed to them, than when only a 
 finger or limited surface is acted on. The left hand ■ 
 is more sensitive than the right. Temperatures near 
 those of freezing water and of 50° C usually pro- 
 duce pain, but custom, as. in other cases, enables both 
 low and high temperatures to be borne with impunity 
 that are under ordinary circumstances quite unbear- 
 able. The skin, when deprived of the ej^idermis, 
 appears to be incapable of perceiving variations of 
 temperature, both heat and cold causing the sensation 
 of pain. The sensitiveness of the mucous membranes 
 for variations of temperature is very dull, the in- 
 gestion of a tumbler of cold water into the stomach, or 
 the injection of cold water into the rectum, scarcely 
 giving the sensation of cold, or producing it only by 
 withdrawing heat from the adjoining skin. 
 
 Faiu. — Pain results from excessive stimulation of 
 any sensory nerve, and may be excited by the violent 
 application of all forms of stimuli, whether mechanical, 
 chemical, thermic, electrical, or due to some alteration 
 in the body itself, such, for example, as inflammation. 
 Some nerves are much more acutely sensitive to pain 
 than others. The fifth nerve stands pre-eminent in 
 this respect, and acute neuralgia of the several 
 branches of this nerve almost paralj^ses the sufferer, 
 every movement being inhibited lest it should in- 
 tensify the pang. The splanchnics are also highly 
 sensitive nerves. The organs which, when inflamed, 
 give rise to the most intense pain are those which are 
 well supplied with nerves, and which have a dense 
 unyielding fibrous investment, examples of which are^ 
 found in the testes, in the ovary, and in the eye. 
 Pain can be excited by violent stimulation of a nerve- 
 in any part of its course, but it is often referred to its 
 peripheral extremity. Kemarkable examples of this- 
 
Chap. XV.1 Muscular Sensibility. 319 
 
 are found in cases of ampntation, in whicL. irritation 
 of a nerve at the line of section is referred to the 
 distal part of the limb which has been removed. 
 
 Muscular sensibility. — The muscles, though 
 they are not verj sensitive organs to ordinary stimuli, 
 yet, when contracted spasmodically, occasion severe 
 pain. They ache when fatigued, and pain is felt 
 when they are contused or cut. They also possess a 
 certain sensibility, which enables us to tell not only 
 whether they are in contraction, but to what degree 
 they are contracted, and this has been distinguished as 
 the '' muscular sense " proper. Experiment seems to 
 show that the acuteness of the muscular sense for the 
 amount of exertion made is greater than that of the 
 sense of pressure or resistance possessed by the skin, 
 for, whereas the skin, as we shall see, can only dis- 
 tinguish between weights diiFering from each other in 
 the proportion of 29 : 30, weights can be recognised as 
 different when weisjlied in the hands which do not 
 differ more than 39 : 40. Experiments of this kind 
 can be best made by concealing the weights to be 
 estimated in small bags, and endeavouring to form a 
 correct judgment as to their respective weights. The 
 muscular sense affords indications (1) of the energy of 
 contraction of the muscle, (2) of the extent of contrac- 
 tion, (3) of the rapidity of the movement, (4) of the dura- 
 tion of movement, and, lastly, (o) of the position of the 
 limbs and of the body. By some observers it is main- 
 tained that the occurrence and degree of contraction 
 in a muscle is only known by the effort, or nervous 
 expenditure, put forth to cause the muscle to contract ; 
 whilst others think we only become aware of the 
 contraction of a muscle by the cutaneous or tactile 
 sensations that are coincidently excited. In accord- 
 ance with this, the heart and diaphragm, which only 
 remotely affect the skin, have very feeble muscular 
 sense. It is, however, now very generally admitted 
 
320 Human Physiology. ichap. xv. 
 
 that there are special nerve fibres charged with the 
 office of ministering to the muscular sense. Section 
 of the posterior roots of the nerves supplying the 
 hind limbs in dogs produces much more disturbance in 
 the movements of the animal than section of all the 
 cutaneous nerves supplying the same limb. 
 
 Sense of taste. — The faculty of taste is localised 
 in the tongue, especially at its posterior part, and in 
 the lovv'er part of the velum palati. The papillse cir- 
 cumvallatse possess special organs which are believed to 
 be adapted for this purpose. {See Klein's " Histology," 
 p. 193.) The nerves implicated are the glosso- 
 pharyngeal, which is the proper nerve of taste, and 
 supplies the posterior part and sides of the tongue 
 with the circumvallate papilke ; the pterygo-palatine 
 branches of the fifth pair, which supply the velum 
 palati ; and the lingual of the fifth, which is distributed 
 to the tip of the tongue. The conditions necessary for 
 the exercise of the sense of taste are that the object to 
 be tasted should come into direct contact with the oriran 
 of taste, and consequently sapid substances must be in 
 solution ; secondly, that the nerves of taste should be 
 specifically excited ; lastly, attention and judgment. 
 
 There are four chief varieties of savours : sweets, 
 bitters, acids, and salines. Sapid substances are more 
 distinctly perceived and distinguished in proportion (1) 
 to the extent of surface to which the substance is ap- 
 plied, (2) to the degree of concentration of the sub- 
 stance, (3) to the duration of the time that it is 
 applied, (4) to the degree of practice, and (5) to the 
 approximation of the temperature to the normal tem- 
 perature of the body. Wine and tea tasters take a 
 moderate quantity of the fluid into the mouth, and roll 
 it over the tongue and clieeks so as to increase the 
 surface to which it is applied. Some substances^ as 
 quinine, have a very persistent flavour, and it has been 
 estimated that a solution of common salt must b(5> 
 
Chap. XV.] Se.vse of Smell. 32 1 
 
 twenty times as strong to produce the same vividness 
 of impression as quinine. The rapidity with which 
 the gustatory impressions of different substances are 
 perceived varies. Salt is tasted most quickly, being 
 perceived 0'17 sec. after its application, whilst quinine 
 requires 0"258 sec, and sweets and acids occupy an 
 intermediate period. Many gustatory sensations are 
 materially assisted by the sense of smell. 
 
 Sense of smell. — The organ of smell is situated 
 in the upper part of the nose, a portion of the 
 mucous membrane covering the upper and middle 
 turbinals, and the septum nasi, being specially modified 
 for this purpose. The nature of odorous emanations 
 is not certainly known. They may consist of aerial 
 undulations, or they may be aerial particles of the 
 odorous substance. In either case they are extremely 
 delicate, air containing only a millionth part of hydro- 
 gen sulphide having a distinct odour, whilst a minute 
 portion of musk will continue, without appreciable loss 
 of weight, to render its presence perceptible in a large 
 apartment for years. Odorous emanations are con- 
 ducted to the nostrils by the air ; and water or eau de 
 Cologne, however strongly they may be impregnate<:l 
 with odorous substances, convey no impression of smell 
 when they are made to fill the nostrils. The air must 
 also be in motion, and to effect this a respiratory effort 
 is made, by which a current is directed towards the 
 upper part of the nasal cavities. The olfactory nerve 
 is the nerve of smell, and through it we i^ei'ceive 
 aromatic, nauseating, and other odours. Attempts have 
 been made to classify odours, without success ; but after 
 section of the olfactory nerves in animals, and in cases 
 of disease in man, certain stimulating odours can still 
 be perceived; and the retention of this power must be 
 attributed to the sensibility of the mucous membrane 
 generally, which is supplied by the fifth nerve. In 
 order that distinct perception of smell should exist, it 
 V 
 
322 Human Physiology, [Chap. xvi. 
 
 is necessary that the nasal mucous membrane should 
 be moist. The use of the sense of smell is, to guide 
 the animal in the selection of food, and to the respira- 
 tion of pure air. Foal smells are often, though not 
 always, associated with unwholesome emanations. 
 
 CHAPTER XVI. 
 
 GENERATION AND DEVELOPMENT. 
 
 The fact that any animal or vegetable substance, 
 when l^ept moist and exposed to the air and light, 
 soon begins to teem with life, formerly led to the 
 belief in the occurrence of spontaneous generation, 
 and even animals so highly organised as files or mice 
 were supposed to originate without parentage. The 
 application of more exact methods of observation to 
 physiology has, however, sho^^^l that in proportion to 
 the perfection of the means by which the decaying 
 bodies of animals or plants are protected from the 
 access of living animals or their ova or germs, the 
 appearance of new life becomes more and more rare, 
 until at length, when they are totally excluded, 
 though chemical changes may take place which result 
 in the decomposition of the body, no form of animal 
 or plant life can be discerned. The difficulty of 
 suggesting any mode of preventing the access of 
 germs to an infusion of plant or animal tissue, 
 satisfactory to an opponent, is, however, remarkably 
 great, and attempts have even recently been made to 
 demonstrate the appearance of life without the exis- 
 tence of parents, though the organisms thus believed 
 to be produced belong to the simplest forms of life 
 with which we are acquainted. If an infusion of hay, 
 
Chap. XV I .] Genera tion, 323 
 
 turnip, carrot, muscle, nerve, or skin be enclosed in a 
 tightly-stoppered vessel, and exposed to the warmth and 
 light of the sun, it is found in a day or two to swarm 
 with minute, and indeed microscopic, beings — monads 
 infusoria, microspores ; but it is easy to suggest that 
 the circumambient air contains enormous numbers 
 of spores, which, though invisible even to microscopic 
 investigation, may yet be capable of producing new 
 Kfe. The reply that has been made to this, and which 
 appears at first sight satisfactory, is, that such infusions 
 may be placed in vessels with long necks, and made 
 to boil, which may reasonably be supposed to kill all 
 germs, and that whilst boiling, the neck may be 
 hermetically closed by fusion of the glass with a blow- 
 pipe flame, and that still organisms appear in the 
 infusion. It is here, however, that the controversy 
 rests, and the misfortune is that the positive experi- 
 ments of the one side are the negative of the 
 other. Those who maintain the doctrine of biogenesis, 
 and that life proceeds from life alone, contend that 
 in proportion to the care wdth which the whole pro- 
 ceedings tending to exclude germs are conducted 
 (as, for example, the purity of the surrounding air ; the 
 length of time the ebullition has been continued ; the 
 avoidance of the entrance of a bubble of air in the act 
 of fusion of the neck, or, if this expedient be not 
 adopted, the packing of the neck, made tortuous, and 
 filled with wool to act as a filter) the more com- 
 pletely sterile do the infusions become ; whilst those 
 who hold the doctrine of ahiogenesis can only point to 
 the presence of life on the globe, and ask, apart from 
 creation, how it could have orio;inated, and whv it 
 should be held to be impossible that a certain aggre- 
 gation of chemical elements should possess those 
 properties to w^hich the term '' vital " is applied. 
 Grantino;, then, that so far as we know at present, all 
 life proceeds from some antecedent fonn of life, the 
 
324 Human Physiology, [Chap. xvi. 
 
 modes in which new creatures appear may be reduced 
 to two, the asexual and the sexual. Both forms are 
 met mth in plants and both in animals. 
 
 The chief varieties of the asexual mode are fission, 
 gemmation or proliferation, as in the evolution of 
 young from detached buds, and parthenogenesis. 
 The simplest form of asexual generation is that 
 presented by the Amoeba, in which the body 
 separates into two parts, each of which is as capable 
 as the other of maintaining its own life. This is 
 termed fission. It is probably performed under the 
 influence of abundant nourishment and a certain 
 temperature. A form of fission is common in Algae, 
 in which the protoplasm of the parent cell breaks up 
 into separate masses, which, growing, burst the cell, 
 and begin life on their own account. In gemTnationj 
 an outgrowth takes place, without sexual connection, 
 from some part of the parent, which under favourable 
 circumstances may reproduce the entire plant or 
 animal. In some instances the bud remains connected 
 vnth. the parent stem ; in others it becomes detached. 
 Gemmation may be coincident with the occurrence in 
 the same organism of sexual generation. A third 
 form is conjugation., or coalescence, seen in some of 
 the lowest forms of life, as in the algie and in the 
 Gregarinida, when two neighbouring cells each develop 
 a protrusion, and when these meet, the septa break 
 down, the contents of the cells mingle, and a new 
 individual ap]3ears between the parents. The process 
 resembles in some important respects the sexual mode 
 of reproduction. In "partlienogenesis new individuals 
 ai'e developed from virgin females, by means of ova, 
 without the intervention of a male. The remarkable 
 mode of development termed the alternation of genera- 
 tion constitutes a kind of intermediate form between 
 the sexual and asexual modes of reproduction, for 
 whilst embryos are produced by sexual reproduction. 
 
Ghap. 'K.Vl.i CeNJ^RA fl6N. 3 2 g 
 
 these develop into organisms wliicli are capable of 
 propagating their like in an asexual manner, some- 
 times for several generations, when thej suddenly 
 assume sexual characters, and once more produce 
 their young by sexual generation. One of the ex- 
 amples which has been most carefully and completely 
 followed is the tape-worm, which is an hermajDhrodite 
 animal, living a parasitic life within the intestinal 
 canal, with testes and ovary in the same animal. 
 Segments of the tape-worm containing numerous 
 impregnated ova are discharged with the faeces, and 
 are called loroglotiides. In the interior of each ovum 
 an embryo, provided with six hooks, is developed, and 
 enters the body of some other animal with its food ; 
 the act of digestion frees it from its covering, and it 
 immediately bores its way into the tissues, and be- 
 comes a vesicular worm, or cysticercus. One or more 
 heads or vesicles develop within the vesicle. The 
 cysticercus is now swallowed by another animal, and 
 the head fixes itself in the intestinal canal of the new 
 host, and forms a scolex, which by budding produces 
 a long chain of segments, each of which, when fully 
 developed, is the sexually mature tsenia. 
 
 Sexual generation is the mode in which reproduc- 
 tion is effected in all the higher forms of animals. 
 It is usually and best accomplished in the maturity of 
 life, after full growth and development are completed, 
 and before the physical powers are Aveakened by the 
 advance of age. It is the result of the union of a male 
 element named the sperm with a female element named 
 the ovum. In some instances, as in fishes, the fertili- 
 sation of the ova is effected without the body, but in 
 the majority of cases the impregnation of the ovum 
 takes place within the body. In some instances the 
 ovum is sufficiently large to supply all the material 
 for the growth and development of the young, and is, 
 as in birds, discharged from the body, and kept warm 
 
326 Human Physiology. tChap.:5tVt. 
 
 either bj the heat of the body of the parents or by 
 exiDosure to the sun's rays. In mammals, however, 
 the ovum is very small, and is retained in the body of 
 the Titerus, from the vessels of wliich it indirectly 
 receives its supplies of oxygen and of food. 
 
 Fiiiictioiis of tlie male org^ans. — The male 
 organs are the penis and testes. The testes prepare 
 the fertilising agent, or sj)ermatic secretion. The 
 penis serves as an intromittent organ, by which the 
 sperm is injected into the neighbourhood of the os 
 uteri, or directly into the cavity of that organ. For 
 the structure of the testes and spermatozoa see Klein's 
 " Elements of Histology," p. 249. 
 
 The seHiiiial fluid. — The sperm or seminal fluid 
 is semi-solid or gelatinous, of whitish colour, peculiar 
 odour, which has been likened to that of moist dough, 
 and which is developed only at the period of emission, 
 as it is imperceptible in the fluid contained, in the 
 vesiculse seminales, or in any of the other secretions 
 which are discharged coincidently with the sperm, 
 such as the prostatic secretion, or \h& secretion of 
 Cowper's glands. Its reaction is slightly alkaline. 
 Exposed to the air it dries up, and stiffens linen like 
 starch. When such spots are moistened and examined 
 with the microscope, spermatozoa may be recognised, 
 the addition of carmine solution bringing the heads 
 prominently into view. The spermatic fluid obtained 
 from the vas deferens is pasty, whitish, and is com- 
 posed of nine-tenths of spermatozoa and one-tenth 
 clear fluid ; but when it has reached the ampulla 
 of the vas, it is mingled with a brownish fluid, which 
 alters its colour and renders it thinner. In cases of 
 repeated coitus at short intervals, the fluid contains 
 few spermatozoa, and is chiefly composed of prostatic 
 fluid and the mucus of the vesiculse seminales. The 
 object of the prostatic fluid seems to be to render 
 it more fluid, and to facilitate the movement of the 
 
Chap, xvi.j Genera tioN: 32; 
 
 spermatozoa. The chemical composition of the sper- 
 matic fluid is : 
 
 Water 
 
 . 88-0 
 
 Spermatin ..... 
 
 . 6-0 
 
 Fat 
 
 . 2-0 
 
 MagTiesiuin and calcium phosphate . 
 
 . 3-0 
 
 Sodium phosphate .... 
 
 1-0 
 
 Ammoniaco-magnesian phosphate 
 
 a trace. 
 
 Spermatin is an organic substance, resembling 
 mucin and albumin. It coagulates on the addition of 
 alcohol, and the coagulum dissolves when treated with 
 liquor potassse ; but when the potash is neutralised 
 with nitric acid, it does not, like albumin, undergo 
 precipitation. It also differs from albumin in not 
 being coagulated by heat. Spermatin is chiefly 
 formed in the vesiculee seminales. The spermatozoa 
 are formed in the tubuli seminiferi. They are not, as 
 a rule, formed before the age of sixteen or seventeen. 
 They continue to be produced to an advanced period 
 of life, having been found at the age of ninety and 
 upwards. They move in a spiral manner by a vibratile 
 motion of the long tail. The movements are arrested 
 by cold, the addition of a little snow stopping tixem 
 in less than a minute, by the addition of acids, one 
 part of hydrochloric acid in 7,500 of w^ater rapidly 
 killing them, and by various poisons, such as opium 
 and strychnia. They are killed by a temperature of 
 60° C, and by the passage of an electric current. 
 Their activity is favoured by the addition of 
 slightly alkaline fluids, such as the serum of 
 blood and milk ; and they have been seen in 
 action in the generative passages of the female 
 eight or ten days after coition. It is owing to these 
 movements that the spermatozoa are able to perforate 
 the thick albuminous coating which surrounds the 
 vitellus. The movements are not often perceptible in 
 spermatozoa taken from the tubuli seminiferi, but are 
 
328 tluMAN Physiology. [Chap. xvt. 
 
 apparent in those taken from the vas deferens or 
 vesiculfe seminales. The motility of spermatozoa 
 must not be confounded with their vitality. Their 
 motility is the persistence of their movements ; 
 their vitality is the persistence of their power to 
 fecundate the ova. In the practice of pisciculture 
 the ova of the trout, salmon, or other fish, are 
 obtained in a dry state by gentle pressure of 
 the abdomen of the female over a vase. The 
 spermatic fluid of the male is, by the same means, 
 made to cover them. But the two may long remain 
 in contact without fecundation occurring, the sper- 
 matozoa being motionless. As soon, however, as a 
 little water is poured over them, the spermatozoa 
 begin to move, and almost all the ova are fecundated 
 in the trout in less than half a minute. The sper- 
 matozoa of man can traverse 2*7 mm. in the space of 
 one minute. According to Fort, monorchids, or those 
 in whom one testis has not descended, but remains in 
 the abdomen, produce no spermatozoa on the side 
 of the retained testis, but are capable of procreating 
 children of both sexes with the other. Those in 
 vrhom neither of the testes has descended are in- 
 fertile, though the testes present theii- normal 
 structure. The spermatic secretion is formed con- 
 tinuously, and though in great part re-absorbed, 
 yet gTaclually accumulates in the vesiculss seminales, 
 from whence it is discharged, in the absence of 
 all sexual excitement, in many cases perfectly 
 naturally, about once in three weeks or a month. 
 Self -abuse, too often practised by youths, is strongly 
 to be deprecated, since it produces both mental and 
 physical exhaustion, incapacity for work, and un- 
 vdiolesome trains of thought. 
 
 Erection. — Under ordinary circumstances the 
 penis is soft, flexible, pendulous, and of small volume. 
 During sexual excitement it acquires a larger volume, 
 
Chap. 3cvt. ] Genera tion. 3 i § 
 
 and becomes erect, rigid, hotter, and nmch more 
 sensitive. The object of these changes is to render it 
 efficient as an intromittent organ, and to enable it to 
 conduct and deposit the semen at or near the month 
 of the uterus. The cause of erection has been tlie 
 subject of much controversy. There can be no doubt 
 that the vessels not only contain more blood, but that 
 the circulation through them is more active. The 
 pressure of the blood in the vessels rises to one-sixth 
 that of the pressure in the carotid. The a^rteries 
 undergo active dilatation under the influence of the 
 vaso-dilator fibres contained in the nervi erigentes, 
 which in the dog arise chiefly from the second sacral 
 nerve, and contain ganglion cells in their course. The 
 centre for these nerves is in the lower part of the 
 spinal cord, and it may be excited in various ways, as 
 by stimulation of the sensory nerves of the penis, and 
 by psychical processes. The fulness of the vessels thus 
 induced is aided by the contractions of the erector es 
 penis, which, forming a tendinous expansion over the 
 dorsal vein of the penis, hinder the return of blood ; 
 and also by the contractions of the transversus perinsei 
 profundus, and of the accelerator urince, which con- 
 tribute to render the bulbus urethrae rioid. 
 
 Ejaculation. — The semen is stored up in the 
 vesiculae seminales, which are long sacculated tubes 
 with muscular walls, contractions of which are in- 
 duced in response to stimulation of the nerves of the 
 penis. The fluid they discharge is driven along the 
 urethra by the contraction of the muscular Avails of that 
 tube, of the accelerator urin^e and erector penis muscle. 
 The nervous circle is completed through the sensory 
 nerves of the penis ; the ejaculator, or genito-spinal 
 centre in the lower part of the spinal cord, and the 
 motor fibres which are distributed to the vesicul^e 
 and to the erector muscles. The sphincter vesicas is 
 spasmodically contracted, to prevent the entrance of 
 
oo 
 
 '\6 ifi/MAiV Physiology. [chap. xvt. 
 
 the fluid into the bladder; and this is further pre- 
 vented bv the swellino; of the verumontaniim. The 
 quantity discharged at each emission varies from one 
 to two or three drachms. 
 
 Metisti'iiatioM. — This periodical occurrence is 
 coincident Avith the rupture of a Graafian follicle, and 
 the setting free of an ovum. It is characterised by 
 the discharge through the vagina from the uterus of a 
 variable quantity of blood, which lasts for one, two, 
 or three days, once in four weeks. It is the result of 
 a process termed ovulation^ which produces a flush of 
 blood to the whole of the generative apparatus ; but it 
 is long preceded by a growth of the tunica propria of 
 the Graafian follicle which is accompanied with abun- 
 dant formation of vascular loops, that fill its cavity 
 and constitute the expelling force, resulting in the ex- 
 trusion of an ovum ; whilst the expulsion is facilitated 
 by fatty metamorphosis of the follicular wall, which 
 renders it more friable (Fig. 29). The distension of the 
 follicle probably constitutes a stimulus to the nerves 
 supplying the ovary, and the afflux of blood to the gener- 
 ative organs is a reflex action. The congestion thus 
 induced causes increased transudation into the follicle, 
 diapedesis of numerous white corpuscles, and in- 
 creased thickening of the wall, which at length gives 
 way at its crown or weakest part, which is destitute of 
 blood-vessels and lymphatics, and the ovum escapes. 
 The whole of the generative organs participate in the 
 congestion, the mucous membrane of the uterus be- 
 coming thicker, more spongy, and hypersemic ; forming 
 the decidua menstrualis^ which differs from the decidua 
 of the impregnated uterus in the small size of the 
 spheroidal cells that are developed in the inter- 
 clandular tissue. According to Dr. "Williams, the 
 epithelium and the uterine glands, with the mucous 
 membrane, undergo fatty degeneration, and are in 
 gi-eat part thrown ofl', renewal taking place from the 
 
Chap. XVI. i 
 
 Mens tr ua tiou. 
 
 M 
 
 deeper portions whicli still remain. With the changes 
 in the generatiA-e organs other reflex or consensual 
 effects are observed. The breasts enlarge and are 
 rendered more sensitive. There is some gastric dis- 
 turbance, with general lassitude and indisposition to 
 
 Fig. 29. — Mature Graafian Follicle coutainiug au Ovum, wliisli is aTDOilt 
 to escape throu!?h a rupture of tJie wall of the Follicle, carrying with 
 it some of the cells of the surrounding Discus Frohgerus. 
 
 work. The quantity of blood discharged varies 
 considerably^ but is estimated as amounting on the 
 average to aboat 300 grammes. It presents no 
 peculiarities, except that on account of its admixture 
 with mucus and o^her materials secreted by the 
 vagmal mucous membrane, it has little or no tendency 
 to coagulate. The majority of women menstruate 
 during the first quarter of the moon, few only at the 
 
33^ Human Phy$iologv. tchap. XVI. 
 
 time of tlie new or full moon. The first occurrence 
 of menstruatioD. is nsualiy betAveen the thirteenth and 
 the fifteenth year. Brunettes are said to commence 
 earlier than blondes, but in this respect heredity, 
 social position, and mode of life, together with cli- 
 matic conditions, are powerful factors. Warmth, 
 abundant food, and luxurious habits of life tend to 
 render its appearance earlier. Menstruation is a 
 sign of sexual maturity, and as long as it lasts it is 
 possible that a woman may bear a child. It usually 
 ceases, after first becoming irregular, between the ages 
 of forty-five and fifty. 
 
 Foriiaation of corpus liiteiaiii, — As soon as the 
 ovum has escaped, the cavity of the Graafian follicle in 
 which it was contained becomes filled with blood, which 
 soon coagulates, and with it the cells of the membrana 
 granulosa. The rupture of the ovarian wall cicatrises, 
 the serum of the coagulated blood is re-absorbed, the 
 hfemoglobin becomes converted into hsematoidin, and 
 the vascular wall of the follicle develops villous-like jDro- 
 cesses, which project into its interior, and contain many 
 capillaries and numerous cells. Diapedesis of white 
 corpuscles takes place to a large extent. The cells of 
 the membrana granulosa multiply and form laminae, 
 which are suj)erimposed upon each other, and sub- 
 sequently undergo fatty degeneration with the forma- 
 tion of lutein and fat, the yellow colour of which has 
 led to the application of the term coiyus luteunfii 
 spuriv.m to the now gradually disappearing remains of 
 the Graafian follicle, which is scarcely visible at the 
 expiration of four weeks. If pregnancy take place, 
 the larger sujDply of blood to the whole of the genera- 
 tive apparatus causes the corpus luteum, then called 
 the corpios luteum verum, to increase to so great an 
 extent, that even at the time of delivery it may 
 measure nearly half an inch in length, whilst its 
 colour is much deeper, and some traces of it remain 
 
Cnap. XVI.] Fecundation, 353 
 
 for years. Menstruation is, as a rule, arrested by the 
 occurrence of pregnancy. 
 
 Fecuiidatioo. — This consists in the fusion of 
 the male with the female element, the spermatozoon 
 with the ovum. In order that it should be effected 
 it is necessary that the spermatozoa, after en- 
 trance into the uterus, should be living and active, 
 and that in the female the passage from the 
 ovary to the vagina should be free. The ex- 
 periments of Newport render it probable that the 
 fusion of many, or at least of several spermatozoa, with 
 one ovum are required to thoroughly fertilise it, for 
 he found that in the case of the frog, when one of the 
 ova was touched with only so much of the sperm as 
 was taken up on the point of a fine needle, fertili- 
 sation either did not take place at all or was 
 evidently only partial, the subsequent development 
 being imperfect ; whilst other ova, to which a 
 larger portion of male element was applied, under- 
 went complete development. In some animals, as in 
 most of the carnivora and in ruminants, there is a 
 definite time, known as the rut, or rutting season, at 
 which alone the female will permit coitus to take 
 place, and when fecundation occurs ; but in other 
 instances, as amongst rodents, marsupials, and others, 
 coitus is frequent, and there is no definite season for 
 the fertilisation of the ova. In man, coition and 
 fecundation may apparently occur at any period. 
 Fertilisation of an ovum is not necessarily coincident 
 with coitus ; nor can the females, except in rare cases, 
 state precisely when it occurs. In all instances the 
 spermatozoa must make their way^ by virtue of their 
 vibratile movements, along the cervix uteri into the 
 body of that organ, and the occurrence of ovarian, or of 
 extra uterine, or of tubarian pregnancy, in which the 
 fecundated ovum begins to develop within the 
 ovary, or becomes attached to some part of the 
 
334 Human Physiology, ichap. xvi. 
 
 peritoneal cavity, having missed the upper opening of 
 the Fallopian tube, or develops in the Fallopian tube 
 itself, conclusively proves that the spermatozoa can 
 make their way through the whole length of the 
 uterus and Fallopian tube to the ovary. It is pro- 
 bable, however^ that impregnation generally takes 
 place in the upper part of the Fallopian tube, whilst 
 the ovum is descending partly by the action of the 
 vibratile cilia that line the tube^ and partly by the 
 peristaltic action of the muscular walls ; and up to 
 which point the spermatozoa have made their way by 
 their own movement in opposition to that of the 
 uterine cilia. The unfecundated ovum descends 
 through the uterus, and is dissolved or discharged ; 
 the fertilised ovum, on the contrary, is arrested in its 
 course through the uterus, probably owing to the 
 greater turgidity of its walls when fecundation has 
 taken place, and becoming attached undergoes its full 
 development. According to Landois, twins, or 
 double impregnation, occurs once in 87 deliveries, 
 though more frequently in hot climates ; three at a 
 birth once in 7,600 ; four at a birth once in 330,000, 
 More than six at a birth have not been observed. 
 The average number of impregnations for each woman 
 is 41 
 
 'I'he ©VUBM. — The ovum, or egg, is produced by 
 the female. It is small in man, but very large in the 
 bird or reptile, and its several parts are best studied in 
 these animals. In its young condition in all animals 
 it is a simple cell. 
 
 Ovum of tlae tolrd. — The parts of the ovum of 
 a bird are : 1. An external calcareous coating or shell 
 which is porous, and permits the passage of air and 
 moisture. 2. Two thin membranes, which, closely 
 applied to each other throughout the greater part 
 of their extent, becom.e separated, shortly after the egg 
 has been laid, at the large end of the egg, and 
 
Chap. XVI.] Structure of Oi'um. 335 
 
 contain a bubble of air between them at this point. 
 3. Next comes the albumin^ which is deposited in 
 layers, and can be detatched in a hard-boiled Qgg in 
 flakes which have a spiral direction ; the outer layer 
 of che albumin is less firm and consistent than the 
 inner. 4. At each end of the egg the albumin 
 forms a distinct gelatinous mass, which stretches 
 between the yolk and the shell, and which resists 
 coagulation by heat rather longer than the other parts 
 of the albumin. These masses are twisted spirally, 
 but in opposite directions, and are believed to aid in 
 keeping the yolk in position. They are named the 
 chalazce. 5. If the albumin be entirely removed the 
 yolk is seen to be surrounded by a delicate membrane, 
 termed the vitelline niemhrane. 6. If an egg be laid 
 on its side and the shell be broken by gentle taps, it 
 is easy to pick away the shell and lining membrane 
 over a space about equal to a threepemiy piece with- 
 out damaging the yolk. The part of the yolk exposed 
 presents a small round spot of lighter colour than the 
 rest of the yolk, which is an indication of a separation 
 of the yolk into two parts, which have a marked 
 difference in their use and purpose ; the light yolk is 
 the germ yolk, or blastoderm, which alone under- 
 goes segmentation, and is directly converted into the 
 body of the embryo ; the yellow yolk, though it 
 ministers to the development and growth of the 
 embryo, does not undergo segmentation, and is hence 
 called the " food yolk." 
 
 The circular patch of white yolk, also named the 
 cicatricula or tread, may be seen on section to give 
 off a process that dips into the yellow yolk to near its 
 middle, where it dilates into a kind of vesicle ; the 
 whole somewhat resembling a Florence oil flask, the 
 mouth of which is a little expanded. Whatever part 
 of the side of the shell is chipped away, the blastoderm 
 invariably comes into view, and the yolk being lighter 
 
336 Human Physiology. [Chap.xvi. 
 
 than the albumin floats up to near the surface. 6, 
 The yellow yolk when boiled, or otherwise hardened, 
 presents concentric markings, that are termed 
 halones, and it is then seen to be invested ex- 
 ternally by a thin layer of the white yolk prolonged 
 from the margin of the blastoderm. The yellow yolk 
 is composed of small spheroids, which contain no 
 nucleus, but are filled with minute highly refractile 
 albuminous granules. The white yolk is also com- 
 posed of spheroids, but these differ from the spheroids 
 of the yellow yolk in being smaller and in containing 
 a nucleus. Eggs which, like those of the fowl, consist 
 of two parts, one of which undergoes segmentation, 
 whilst the other does not, are called " meroblastic. " 
 Eggs which, like those of mammalia, undergo total 
 segmentation, are termed " holoblastic." 
 
 Clieinical composition of tlie fowl's egg. — 
 The shell is composed of 4-15 per cent, of organic sub- 
 stance, chiefly albumen, impregnated with calcium 
 carbonate to the extent of 93*7 per cent., and contain- 
 ing also small quantities of magnesium carbonate (1'39 
 percent), calcium phosphate (0'76), and iron phosphate. 
 The yolk consists of 47-1 per cent, of water, 15-6 per 
 cent, of albumin, 31 '3 per cent, of ether extract, chiefly 
 fats, 4-8 per cent, of alcohol, and 0*96 per cent, of in- 
 orsranic salts. Amongst the substances which have 
 been isolated are : vitellin, which is the characteristic 
 albuminous compound of the yolk ; nuclein, chiefly 
 obtained from the white yolk ; lecithin, glycerin, 
 phosphoric acid, cholesterin, palmitin and olein, 
 a pigment named lutein, and grape sugar. 
 
 Ovaam of siiaui. — Tlie human ovum differs in 
 several particulars from that of the bird. It is 
 extremely small, having a diameter of only about 
 . i^th of an inch. It has an external investment named 
 the zonapellucida (Fig. 30, b), which is of considerable 
 thickness, and capable of resisting tolerably strong 
 
CKap. XVI.] Ovum of Mammal. 337 
 
 pressure, returning to its spheroidal form as soon 
 as the pressure is removed, by virtue of its elasticity. 
 It j)resents a fine radial striation, due to the presence 
 of fine canals. This membrane {zona radiata) cor- 
 responds apparently to the vitelline membrane of the 
 
 ^^fi^^^ 
 
 Fig. 30. — OTum of JMamioal. 
 
 • a. Cells of discus proligerus ; &, zona pellucida ; c, vitellus ; A, germinal 
 vesicle ; e, genuinal spot. 
 
 ^^^g of the bird, for it directly invests the vitellus, 
 which is here gi^anular and contractile. In the midst 
 of the vitellus is a nucleus named \h^ germinal vesicle. 
 or vesicle of Purkinje, containing nucleo-plasma and a 
 nucleolus named the gerTninal spot, or spot of Wagner, 
 both of which may be seen under the microscope. 
 Besides the nucleolus, some small corpuscles have been 
 noticed by v. Beneden, which have been named 
 pseudo-ni'.clear bodies. 
 
 Mode of orig^ of the ovum. — If a trans- 
 verse section be made through the body of the embryo 
 w 
 

 Human Physiology. 
 
 [Chap. xvi. 
 
 of ■ a fowl about tlie close of tlie fourth day, the 
 abdominal cavity is seen to be lined by pavement 
 epithelium, which becomes columnar at the level of the 
 Wolffian body {see Fig. 31), and is here named the 
 germinal epithelium. As the embryo develops, this 
 columnar epithelium becomes limited to the inner and 
 outer parts of the surface of the Wolffian body ; the in- 
 termediate cells being flat. The canal or duct ofJIuUer, 
 which subsequently becomes the oviduct, is formed at 
 the expense of the external germinal epithelium ; the 
 ovary, or sexual gland of the female, is developed 
 
 uo 
 
 Fig .31. — Diagram stowing early Differentiation of Parts in Blastoderm. 
 ch. Chorda dorsalis ; pr, provertebrss ; W, Wolffian body ; ao, aorta. 
 
 from the internal germinal epithelium, for in the 
 midst of these columnar cells other cells make their 
 appearance, which are larger, isolated, and spheroidal ; 
 these are the primordial ovules. These ovules, or 
 female germs, exist at an early period in both sexes. 
 In the male, according to Waldeyer, they disappear, 
 the male elements developing from the Wolffian canal ; 
 but according to Semper, both male and female ele- 
 ments originate from the columnar epithelium. 
 
 I><evel®p5iaeiit of the ovuiii. — The ovum, as it 
 descends the Fallopian tube, usually has numerous 
 cells of the discus proligerus still adherent to it. In 
 some animals, as the rabbit, the ovum receives an 
 albuminous covering of considerable thickness, whilst 
 in the dog it is very thin. Segmentation commences 
 
UiAp. Xvi.] DKVELOPM^tJT OP Embryo, 339 
 
 during its descent. In this process a groove forms, 
 wliicli speedily divides the ovum into two parts, of which 
 one, named the epiblastic sphere, is larger and more 
 transparent than the smaller, or hyjjohlastic, sphere. 
 Each sphere soon undergoes subdivision into 2, 4, 8, 1 6 
 spheres, and so on, but with such change of position 
 that at the close of segmentation (that is, at the close 
 of the third day in the rabbit) the larger granular 
 hypoblastic spheres constitute a central solid mass, 
 almost entirely surrounded by the smaller, clearer, 
 and somewhat cubical epiblastic spheres. The seg- 
 mented ovum now enters the uterus, and undergoes 
 a series of changes, which result in the formation of 
 the blastodermic vesicle. This is produced by the 
 separation of the epiblast from the hypoblast, and by 
 the formation of a cavity in the centre of the ovum 
 filled with fluid. The greater part of the walls of 
 this cavity are formed at first of a single row of 
 flattened epiblastic cells, whilst the hypoblast cells 
 form a small lens-shaped mass attached to the inner 
 side of the epiblast cells ; but very soon the hypo- 
 blast cells spread out on the inner side of the epiblast, 
 the central thickening, however, forming an opaque 
 circular spot on the blastoderm, which is the com- 
 mencement of the emhryonic area. A day or two 
 later this spot presents three layers, a new inter- 
 mediate layer, termed the mesohlast, having made its 
 appearance. The blastodermic vesicle now comes to 
 present three areas : — (1) The embryonic area, formed 
 of three layers, epiblast, mesoblast, and hypoblast ; 
 (2) the ring around the embryonic area, where the 
 walls of the vesicle are formed of epiblast and hypo- 
 blast ; (3) the area beyond this again, where the 
 vesicle is formed of epiblast only. Speaking generally, 
 the epiblast gives rise to the central nervous system, 
 to the epidermis and epidermoid tissues, as the hair and 
 nails, and to the epithelium of the organs of sense ^ 
 
340 
 
 Human Physiologv. 
 
 [Chap. XVI. 
 
 the mesohlast forms the greater part of the tissues of 
 the body, such as the cutis, muscles, bones, and the 
 large glands ; whilst from the hypoblast originate the 
 epithelium of the intestines and the cells lining the ducts 
 of those glands which arise from it by a process of invo- 
 lution. The blood and blood-vessels and connective 
 tissue arise in the fowl from the elements of the white 
 yolk which lie external to the embryonal rudiment, 
 and have hence been called imrahlastic formations in 
 opposition to the arcliihlastic formations which arise 
 from one or other of the three layers of the embryo. 
 
 Priifliitive streak.— Medullary folds.— ^Early 
 on the seventh day in the rabbit the embryonic area, 
 previously oval, becomes pyriform, and at its posterior 
 
 and narrower end a jyrimitive 
 streak makes its appearance, due 
 to a proliferation of rounded 
 cells from the epiblast. On the 
 eighth day two folds arise, 
 bounding a shallow median 
 groove. These meet in front, 
 and diverge behind, enclosing 
 between them the front end of 
 the primitive streak. They are 
 the medullary /olds, and consti- 
 tute the first deiinite traces of the 
 embryo. The thickened axial 
 portion of hypoblast beneath and 
 between the medullary folds becomes separated from 
 the lateral parts and forms the notochord, the fore part 
 first becoming distinct. That part of the blastodermic 
 vesicle which is included in the embryonic area alone 
 takes part in the formation of the embryo ; the re- 
 maining, or non-embryonic part, forms the umbilical 
 vesicle, which is pa.rticularly well seen in fish, where it 
 is large, and long remains visible as a sac appended to 
 the body. The umbilical A^esicle contains the food yolk. 
 
 Fig. 32.— Ovum of Fowl, 
 with. Primitive Streai, 
 siirroiinded by the Area 
 Pellucida, which is boun- 
 ded by a Line, and is 
 Pjnriform, and by the 
 Area Opaca, which is 
 Oval. 
 
 a, Zona pellucida; 6, vitellus. 
 
Chap. XVI.] Development of Embryo. 
 
 341 
 
 Fig. 33, — Diagram showing the 
 gradiial folding oif of the Em- 
 bryo, which comes to he saited 
 on an eminecce, and has a ciirved 
 form. 
 
 a. Zona pellucida ; 6, Titellus ; c, em- 
 bryo ; d, e, folds of amuion. 
 
 which is very small in quantity in mammals, and is 
 gi'adually absorbed, being applied to the nutrition of 
 the growing embryo. The lateral masses of meso- 
 blast, from which the 
 notochord has separated, 
 begin to divide into a 
 vertebral zone adjoining 
 the embryo and a more 
 peripheral lateral zone, 
 and the vertebral zone 
 exhibits indications of 
 two somites, the fore- 
 most of which marks the 
 junction of the cephalic 
 region and trunk. A 
 delicate clear ring now 
 appears around the em- 
 bryonic area, which is 
 believed to represent the 
 peripheral part of the area pellucida of birds, and ex- 
 ternal to this is a well-defined area vasculosa. Even 
 ail this early period, before any 
 folding off of the embryo has 
 occurred, traces of the heart are 
 visible, in the form of two tube- 
 like structures running; lonoji- 
 tudinally in the lateral masses 
 of the mesoblast. In the process 
 of the folding off of the embryo 
 the head fold appears first, and 
 is deepest, and it is soon followed 
 by the tail fold. The medullary 
 groove, still open, now begins to 
 close, from before backwards. 
 Nearly coincidently with the formation of the primary 
 vertebrae, a fissure appears in the lateral masses of the 
 mesQblast, w^hich is tbe commencement of the pleuro- 
 
 Pig. 34. — Formation of 
 
 Head fold. 
 a, Zona pellucida ; b, vitcl- 
 lus ; c, head fold, 
 

 Human Physiology. 
 
 [Chap. XVI. 
 
 jjeritoneal cavity ; tlie lower lamina is the spla7ichno- 
 fletiTe^ "wliich, folding inwards sharply, forms the walls 
 of the intestine, which is lined by the hypoblast ; whilst 
 the outer, or ujDper, lamina is named tiie somatopleure, 
 and also folding inwards, but with a much wider curve, 
 forms the wall of the body. The heart a2:)pears in the 
 space between the two. 
 
 F©a'iiiafi®ii ©f tSae aBiisii®M. — "When the em- 
 bryo (Fig. 35) has become separated ofi' by an inflec- 
 tion of the wall of the blastodermic vesicle from the 
 general yolk cavity, a circular fold of the somato- 
 pleure and epiblast arises (ks, ss), the circumference 
 
 Fig. 35. — Diagram to show the mode of Origin of the AUantois. 
 
 hy, Somatopleure ; ks, head fold of amnion; ss, tail fold of amnion ; ek, epiblast; 
 kk, head cap ; sk, tail cap ; dsli, cavity of the yolk sac ; dovi, ductus omphalo 
 mesentericus or omphalo mesaraicus ; kdh, pharyngeal, or anterior portion 
 of intestinal cavity: vdp, hypoblast lining the cavity of the intestine; lig, 
 rudiment of heart ; hh, part of pleuro-peritoneal cavity. 
 
 of which, gradually approximating towards the middle 
 of the back of the embryo, ultimately meets, and the 
 double septum existing at all points is absorbed. The 
 embryo thus comes to be invested, except below, by a 
 membranous sac, which is continuous, with the skin, 
 and which is formed of the somatopleure, lined by 
 the epiblast. Fluid is secreted both within and 
 without the sac. The fluid between the skin of the 
 embryo and the inner layer of the sac occupies the 
 
Chap. XVI.] Development of Embryo. 
 
 343 
 
 cavity of the amnion, and is termed the liquor amnii, 
 whilst the free membrane separating the two portions 
 of fluid is the true amnion. The outer layer of the 
 sac, consisting of somatopleure internally and ejiiblast 
 externally, which is called the false amnion, ov suhzonal 
 membrane, soon coalesces with the vitelline membrane, 
 
 elv.jir 
 
 Fig. 36.— Diagram to show the complete Formation of the AUantois. 
 
 rr/?i, Amnion ; n/, allantois ; dom, umljilical duct; f7s, umlDilical vesicle; 3, villi ; 
 ch. pr., cliorium primitivuui. 
 
 and the fluid between it and the true amnion under- 
 goes absorption. The amnion may now be regarded as 
 a serous membrane, being a continuous sheet of mem- 
 brane, and having an internal or visceral, and an ex- 
 ternal or parietal layer. The \isceral layer, near the 
 point where it becomes continuous with the skin of the 
 embryo, curves downwards and inwards {sl^, Fig. 35), 
 and comes to invest the vitelline duct (dom), and thus 
 ultimately forms a tubular sheath around the umbilical 
 cord, The amnion possesses some contractile power, 
 
344 Human Physiology. [Chap. xvi. 
 
 owing to the presence of fusiform cells of un'striated 
 muscular tissue, which are derived from the soma- 
 topleure. It executes slow rhythmical pulsations, 
 which rock the embryo to and fro in the ^gg. The 
 serous layer {cli.pr^ Fig. 36) sends forth numerous 
 processes, or villi, which project from the whole 
 periphery of the vitelline membrane, and constitute 
 the irrimiti've chorion. 
 
 liiquor amiitii. — The liquor amnii is a clear 
 yellowish-green fluid, of peculiar odour, in which 
 epidermis, hairs, scales, and particles of fat, cast 
 off from the skin of the embryo, float. The quantity 
 is larger near the middle of pregnancy than towards 
 the close, when it usually amounts to about a pint. 
 Its reaction is alkaline j its specific gravity varies from 
 1*002 to 1'028, and in general it resembles diluted 
 blood serum. It contains albumin and globulin-like 
 substances, a fermentable sugar, lactic acid, kreatinin, 
 urea, and the usual salts of the blood. The urea is ex- 
 creted by the kidneys of the foetus. The liquor amnii is 
 essentially formed by the foetus, part proceeding from 
 the skin, part from the foetal chorionic capillaries, 
 which are closely applied to the amnion and to the vasa 
 propria of the limiting membrane of the placenta, and 
 partly from the kidneys, the evidence of which is the 
 very small quantity of licpior amnii found in cases 
 where the fcetal kidneys are congenitally absent. A 
 portion, however, is probably also supplied by the 
 mother from the numerous vessels of the decidua vera. 
 The uses of the liquor amnii are to fill the uterine cavity 
 evenly ; to protect the foetus from injury through the 
 abdominal walls of the mother ; to give room for, and 
 to prevent the mother from being incommoded by, the 
 foetus ; and to facilitate delivery by forming an elastic 
 pad, equably dilating the external parts. 
 
 The allaiitois. — This organ is far more highly 
 developed in reptiles and birds than in mammals. It 
 
Chap. XVI.] Development of Embryo. 
 
 345 
 
 is the means by which the blood is aerated, a process 
 which in mammals is very early effected by the 
 interchange of gases between a thin layer of the 
 foetal blood and a corresponding layer of the maternal 
 blood in the placenta, but which is provided for in 
 birds and reptiles by the expansion of the vascular 
 
 Fig. 37.— Diagi-am to show the rormation of the Allantois. 
 
 cft.pr, Primary chorion ; cft.s, secondary chorion ; am, amnion ; ds, remains of 
 yolk sac; a\, allantois ; aV, neck of allantois. 
 
 allantois immediately beneath the shell of the (i^g. 
 It arises as two projections, which soon coalesce, from 
 the splanchno-pleure at the posterior extremity of the 
 embryo. It is seen in Fig. 37, where it is marked 
 «Z, aV , and grows first into the pleuro-peritoneal cavity 
 of the embryo and then into the space between the true 
 and the false amniotic cavities. It gradually extends 
 gver the embryo in the direction of the arrows in Fig. 
 
3-1.6 Human Physiology, [Chap. xvi, 
 
 37, and lies over the embryo and true amnion imme- 
 diately beneath the shell in the fowl's %gg^ from which 
 it is separated only by the false amnion and vitelline 
 membrane. It never attains this size in mammals. 
 From its mode of origin it is necessarily at first com- 
 posed of two layers, the entoderm and that portion of 
 the mesoblast which forms the fibro-muscular layer of 
 the intestine j but as it increases, the entodermal layer 
 aV ceases to grow, whilst the mesodermal plate en- 
 larging applies itself to the primitive chorion {ch.'pr, 
 Fig. 37). The allantois is the medium of the second 
 circulation of the embryo, for in it run the 
 embryonic vessels, the nmbilical artery, and umbilical 
 vein, the branches of which reaching the periphery in 
 this way, shoot into certain of the villi of the chorion, 
 converting this from the primary into the secondary 
 chorion. Some of the villi, as shown at cli.idr (Fig. 
 37), remain relatively small, and form the clwrion 
 Iceve, and ultimately disappear ; whilst others, enlarg- 
 ing ch.s, becoming leafy and highly vascular, have 
 received the name of chorion frondosum, or placenta 
 foetalis. The commencement or root of the allantois re- 
 mains as the urinary bladder, its outer part, still patent, 
 forming in the mammal the urachus, which, however, 
 disappears about the second month. The allantois is 
 the organ which receives the urinar}^- constituents, for 
 the excretory ducts of the primary kidneys, or Wolffian 
 ducts, open into it, and the secretion, passing by the 
 allantois through the nmbilicus, enters the peripheral 
 part of the urinary sac. The fluid of the allantois has 
 been found to contain [urate of ammonia and soda, 
 urea, allantoin, grape sugar, and salts. 
 
 The placeBita fcctalis, the origin of Avhich from 
 the amnion and allantois has just been described, 
 applies itself to a corresponding hypertropliied portion 
 of the mucous membrane of the uterus, named the 
 placenta uterina (F'ig. 38, ^l.u). Whilst the yolk 
 
Chap. XVI.] Development of Embryo. 
 
 147 
 
 sac ids) gradually diminislies in size, owing to 
 the absorption of its contents, the amnion enlarges, 
 and everywhere becomes applied to the inner surface 
 
 Fig. 38. — Diagram to sliow the Formation of the Placenta. 
 ?«, L'tenis: dv, dccidna vera; dr, decidua reflexa ; c7(.?, claorion ; a7H, amnioTi ; 
 rt/, allantois; ds, vitelline diu-t and sac: ;j?./, i)lacentafoetalis ; v^.u, placenta 
 uterina ; doin, ductus oiuplialo-mesentericus ; hn, the point of junction of the 
 amnion with the skin ; dn, the cavity of the amnion. 
 
 of the chorion verum, formed by the union of the 
 chorion primitivum and allantois. It is seen to 
 invest the vitelline duct (dom), the stalk of the 
 allantois, and the vessels it supports, which collectively 
 form the umbilical cord connecting the embryo and 
 the Ti terns. The embryo is thus contained within two 
 QOncentrically arranged vesicles, the chorion and the 
 
348 
 
 Human Physiology, 
 
 [Chap. XVI. 
 
 amnion, whicli are both continuous with the body of 
 the embrvo at the umbilicus or navel. But it receives 
 
 additional coverings from the mother, for as soon as 
 the fecundated ovum is arrested in its passage dowi^ 
 
Chap. XVI.'] Development of Embryo. 
 
 349 
 
 the uterus, the uterine mucous membrane begins to 
 grow up around it like the cup of an acorn, and 
 ultimately entirely invests it. The proper wall of the 
 uterus in the rest of its extent also increases in thick- 
 ness and vascularity, and, as the foetus enlarges, that 
 portion which has gi'own over it, and which is known 
 as the decidua reflexa, comes into contact with the true 
 mucous membrane, which is termed the d,ecidua vera. 
 A space exists between the decidua rera and reflexa up 
 to about the third month, but at the fourth month the 
 whole cavity of the uterus is 
 filled with the ovum and the 
 decidua reflexa. At one limited 
 spot {uu, Fig. 38) the ovum 
 lies in direct contact with the 
 decidua vera, but in the rest of 
 its extent it is separated from 
 the decidua vera by the decidua 
 reflexa. This spot is the decidica 
 serotina, which becomes the 
 placenta. At this spot the 
 uterine vessels undergo great 
 enlargement, and form irre- 
 gular cavernous spaces, into 
 which the villi of the chorion 
 dip. The vascular villi of the 
 fcetus are received into the wide 
 blood-vessels of the uterine pla- 
 centa, so that the capilla,ries of 
 the foetal system are bathed in 
 uterine sinuses; and although 
 there is no direct communication between the blood 
 of the mother and of the foetus, a free interchange of 
 gases takes place ; and whilst the uterine blood sup- 
 plies nutritious matter to the fostus, the foetal blood 
 yields up to it again eflete and disintegrated material. 
 The ends of thu villi are formed by the inosculating 
 
 Cb (i 
 
 Fig. 40.— The Extremity of 
 
 a Villus taken from a 
 
 fresh. Placenta. 
 a a a. h. Loops QUcd with Mood; 
 
 o^, empty loop; b, margin of 
 
 pellucid villus. 
 
350 Human Physiology. [Chap. xVt. 
 
 loops of the small arteries and veins of the foetus, 
 which are characterised by the peculiarity of making 
 several turns into difierent loops before terminating 
 in a vein. 
 
 The structure of the placenta is somewhat compli- 
 cated. The maternal portion forms the general frame- 
 work, into depressions of which are received the 
 convoluted vessels distributed to the chorionic villi 
 of the foetus. The maternal placenta presents a 
 cavernous texture, and sections exhibit wide spaces 
 and cavities, which intercommunicate freely, and are 
 separated by trabeculse. If a vertical section be made 
 through the placenta, the following structures are met 
 with in passing from the uterus to the umbilical cord : 
 
 (1) The decidua serotina. 
 
 (2) The intermediate cavernous portion, filled with 
 hypertrophied and frondose foetal villi. 
 
 (3) The subchorionic septum, Avhich bounds the 
 maternal portion of the placenta. 
 
 (4) The placental part of the chorion. 
 
 (5) The gelatinous layer. 
 
 (6) The placental part of the amnion. 
 
 The iBMitoilical cord. — This constitutes the con- 
 nection between the mother and the foetus. Its 
 average length is 50 to 55 ctm., or about 20 inches. It 
 contains the two umbilical arteries, the umbilical A^ein, 
 the remains of the allantois with the urachus, and is 
 invested by the amnion. It presents a spiral testing, 
 which is usually, though not always, from right to left 
 in passing from the foetus towards the placenta, 
 
 Developnieaat of the lieart and vessels. — 
 The heart is formed in the head fold of the embryo, in 
 the general pleuro-peritoneal cavity formed by the 
 separation of the splanchno-pleure from the somato- 
 pleure. In the duck it has assumed a flask-like form 
 as early as the first half of the second day. At its 
 anterior end is a slight swelling indicating the future 
 
O^^.tyii.^ Development of Blood-vessels, 351 
 
 buibus arteriosus, and a bulging behind indicates tlie 
 position of the auricles. It receives behind the two 
 omphalo-mesaraic veins, which pass outwards into the 
 folds of the splanchno-pleure. In front it divides into 
 two primitive aortse, each of which curves upwards 
 and then backwards, pursuing a parallel course 
 towards the tail in the mesoblast on each side of 
 the notochord, immediately beneath the protovertebrae. 
 About the middle of their course each gives off, at 
 right angles to the axis of the embryo, the omphalo- 
 mesaraic artery, which is distributed over the pellucid 
 and vascular areas. The vascular area is bounded by 
 the sinus or vena terminalis. The blood returning 
 from the vascular and pellucid areas runs in vessels 
 which are the omphalo-mesaraic veins, and joins the 
 heart behind. This is the first circulation of the 
 embryo. Both vessels and corpuscles are formed 
 entirely from the cells of the mesoblast, some 
 coalescing to form the walls of the vessels, others 
 remaining free and becoming converted into blood 
 corjDuscles. The rhythmical pulsatile movements of 
 the heart are visible in the course of the second day of 
 incubation in the fowl, and before any differentiation 
 of tissue can be observed. 
 
 The next phase is the increasing complexity of the 
 heart, which doubles upon itself, that portion which is 
 turned to the right, including the curved part, being 
 the ventricular portion, the left and more dorsally 
 situated part being the auricular portion. At the 
 same time a series of four additional aortic arches are 
 formed anteriorly, though they are never all present 
 together, each arch occupying a visceral fold, but succes- 
 sively unite to form the dorsal aorta, which, after a short 
 course, divides into the common iliacs, each of which 
 gives off an omphalo-mesaraic artery to the yolk sac and 
 capillaries to the tail. The blood returning from the 
 capillaries distributed over the yolk sac, enters the 
 
352 
 
 Human Physiology. 
 
 lOiap. xvi. 
 
 omplialo-mesaraic veins, which unite to form a common 
 trunk. The first or most posterior part is called the 
 ductus venosKS, whilst the part near the auricle is the 
 simcs venosus. The object of the first fcetal circulation 
 is the absorption of the yolk, and with the shrivelling 
 
 WT-ad SS 
 
 %.x,scl 
 
 Fig. 41. — Dia^am to show tlie Early Stages of the Second Embryonic 
 Circulation. (Seen from behind.) 
 
 js. Left jugular ; jd, right jugular ; dc, ductus Cuvieri ; ci, cava inferior ; vcd, 
 right cardinal vein ; «c.s, left cardinal vein; csd, superior right vena cava ; 
 c.","!. superior left vena cava; vu, umbilical vein; a, azygos vein; ha, hemi- 
 azygos vein ; ad, right innominate ; as, left innominate ; sd, right sub- 
 clavian vein ; ss, left subclavian vein-; cs, cava superior. 
 
 of the yolk consequent on the absorption of its contents, 
 the first circulation disappears. About this time, how- 
 ever, small arteries beajin to be distributed to the body 
 of the embryo by the aorta, and the capillaries reunite 
 to form two main trunks on each side, the jugular an- 
 terior or cardinal veins and the posterior cardinal 
 veins, which run parallel to the long axis of the body, 
 a little external to the protovertebrse. The two veins 
 of eacl*. side soon unite to form a common trunk at 
 
CKap.xvi.i Development of Embryo. 353 
 
 right angles to themselves, "which, running inwards, 
 oper.s into the sinus venosus. These trunks are the 
 ductus Cuvieri (dc, Fig. 41 a). The jugular or siqoerior 
 cardinal vein receives the blood returning from 
 the head and neck, and is soon joined by the vertebral 
 and subclavian veins, returning the blood from the 
 head and wing. The inferior cardinals return the 
 blood from the Wolffian bodies. 
 
 About the fourth week of foetal life the vena 
 cava inferior {ci) makes its appearance, conveying 
 the blood to the heart from the tissues above the 
 Wolffian bodies ; and as these bodies diminish in size, 
 and the true kidneys increase, the inferior cardinals 
 atrophy, as shown in dotted outline (Fig. 41, b), 
 and the vena cava inferior enlarges ; the anterior 
 ends of the inferior cardinals unite to form, with 
 others on the left side, the small or hemi-azygos vein 
 {ha), and on the right the azygos (a), which are 
 connected by an oblique communicating vein. The 
 ductus Cuvieri become absorbed into the growing 
 sinus venosus, or, according to some, form the 
 superior vense cavse, so that there are now (Fig. 41) 
 three large veins returning blood into the heart, the 
 inferior vena cava, with which the ductus Cuvieri and 
 umbilical vein have coalesced, and the right and left 
 superior venee cavge. A new vein establishes a com- 
 munication between the right and left jugular veins, 
 and is the rudiment of the future vena innominata 
 sinistra. The left vena cava soon disappears (shown 
 in dotted outline. Fig. 41, c), so that the blood 
 coming from the left jugular vein passes into the now 
 single vena cava superior {cs) ; the portion of the 
 right jugular below the point of opening of the right 
 subclavian vein (sd) now becomes the right innomi- 
 nate (ad). 
 
 Circulation of tlie foetus. — It has been shown 
 that in the fowl the first circulation in the embryo was 
 
 X 
 
354 Human Physiology. [c^ap.xvt. 
 
 developed, in order to enable it to absorb the materials 
 for its nutrition, from the vitelline sac; and it was 
 noted, that whilst the extent of this circulation 
 through the omphalo-mesaraic vessels was great in the 
 meroblastic ^gg^ in accordance with the large amount 
 of food yolk it contains, that it was comparatively 
 small and evanescent in the holoblastic %gg^ on account 
 of the early attachment of the ovum to the uterine 
 walls, from whence it can draw its supply of nutri- 
 ment by the formation of the chorionic villi with their 
 contained allantoic vessels. These gradually form the 
 placenta, and it is at the placenta that the blood of 
 the foetus, without intermingling with the blood of the 
 mother, receives by osmosis the pabulum it requires, 
 and gives up the carbonic acid it contains in exchange 
 for oxygen. The necessity that exists for these pro- 
 cesses of nutrition and respiration to take place 
 external to the body of the mature foetus involves some 
 special arrangements of the vascular system, and the 
 course of the blood in the foetus is as follows : — Com- 
 mencing at the placenta, aerated and well-nourished 
 blood runs up the single umbilical vein to the navel ; 
 here it enters the body of the foetus, and after a short 
 course reaches the liver, where the first special 
 arrangement occurs, for it splits into two parts, one 
 supplying the right and left lobes of the large liver, 
 the other passing through the ductus venos2cs, which 
 lies in the longitudinal fissure of the liver, into the 
 vei2C6 cava inferior, which also receives the hepatic 
 veins containing the blood that has circulated through 
 the liver. The blood traversing the inferior vena 
 cava enters the right auricle of the heart ; and here 
 the second special arrangement occurs, for instead of, 
 as in Uie adult, passing into the right ventricle, it 
 is directed by the Eustachian valve along the back 
 of the auricle to the foramen ovale, and immediately 
 enters the left auricle. The left auricle contracting 
 
Chap. XVI.J FCETAL CIRCULATION. 355 
 
 propels it into the left ventricle^ through the mitral 
 Valve, and from them it is driven into the aorta 
 through the sigmoid valves, and is distributed to the 
 head and body as usual. The object of this disposition 
 is apparently to allow aerated blood to reach the head 
 in as 23ure and highly aerated a condition as possible. 
 The blood returning from the head descends through 
 the jugular and innominate veins into the superior 
 xencb cava, and from thence into the right auricle. 
 This current or column of blood is situated in front of 
 that ascending from the inferior vena cava, and 
 enters the right ventricle through the tricuspid valve. 
 When the right ventricle contracts, the blood is driven 
 into the imlmonary artery through the semilim.ar 
 valves ; and here the third special arrangement exists, 
 for the lungs are not yet in action, and instead of 
 being distributed to them, as in the adult, it is directed 
 through a channel given off by the left pulmonary 
 artery, named the ductus arteriosus^ into the aorta, just 
 beyond the point where the left subclavian is given ofl 
 from that vessel. At this point of the aorta, then, 
 there is a junction of two streams, one which has 
 come direct from the placenta, and is therefore fully 
 aerated, and the other which has traversed the head, 
 and is unaerated. The body generally is supplied 
 with this mixed blood, which, passing through the de- 
 scending aorta, the common iliacs, and the umbilical 
 artery, reaches the placenta, where it again under- 
 goes aeration, and recommences its course. 
 
 As soon as the child is born respiration com- 
 mences, the lungs become expanded, the pulmonary 
 vessels permit the blood to traverse them freely, and 
 the ductus arteriosus, no longer required, contracts and 
 shrivels, though its remains are always visible. The 
 detachment of the placenta leads to immediate arrest 
 of the flow of blood through the umbilical arteries ; 
 no return current consequently takes place through 
 
35^ 
 
 Human Physiology. 
 
 tChap. XVI. 
 
 the umbilical vein ; tlie ductus venosus contracts, the 
 currents of the superior and inferior vense cavse mix, 
 and with this the Eustachian valve and foramen ovale 
 become obsolete. 
 
 ©eveaoipsiieMt of iSie toraiii. — The brain is 
 formed from the fore part of the medullary plate, and 
 
 before the closure of the 
 medullary folds. Two ves- 
 icles first appear, and the 
 posterior of these divid- 
 ing intwo to make three, 
 known respectively as the 
 fore-, mid-, and hind-brain. 
 The fore-hrain becomes 
 converted into the cere- 
 bral hemispheres, the 
 thalamen-cephalon, and 
 the primary optic vesicles. 
 The mid-hrain develops 
 into the optic lobes, or 
 corpora quadrigemina and 
 the crura cerebri; and the 
 hind-brain becomes con- 
 verted into the cerebel- 
 lum. The brain early 
 presents a remarkable 
 curvature, named the 
 cranial flexure, the fore 
 part bending down in 
 front of the extremity of 
 the notochord. Soon 
 after the appearance of 
 this, the hind-brain, the 
 cavity of which is the 
 fourth ventricle, becomes divided into two regions : 
 a posterior, the medulla oblongata; and an anterior, 
 the cerebellum. The roof of the medulla becomes 
 
 Fig. 42.— Diagram showing the 
 curved Form of the young Em- 
 bryo : A, natural size, b, magni- 
 fied. The head is seen to be 
 sharply curved downw2,rds. In 
 the fore and lower part of the 
 head is the eye; immediately he- 
 low which are the visceral arches. 
 The vitelline sac is seen reduced 
 in size and attached by a long pe- 
 dicle to the body of the embryo. 
 
Chap, xvi.i Development of the Brain. 357 
 
 attenuated, then breaks down, and the nerves of the 
 two sides, originally in contact at the dorsal summit 
 of the brain, appear to arise at the sides of the brain. 
 The blood-vessels of the pia mater form a plexus 
 over it, which is the choroid plexus of the fourth 
 ventricle. The anterior portion thickens, and becomes 
 the cerebellum, with its central and two lateral 
 lobes. 
 
 The mid-hrain is the single vesicle which has the 
 aquseductus Sjlvii for its cavity, with the pons 
 surrounding it posteriorly, and the corpora quadri- 
 gemina anteriorly. The fore-hrain is primarily a 
 single vesicle, but early buds off the optic vesicles, 
 and almost coincidently with them a prolongation in 
 front, which is soon divided off from the fore-brain 
 by a constriction. The posterior part becomes the 
 thalamen-cephalon ; the anterior forms the rudiment of 
 the cerebral hemisjjheres and olfactory lobes. 
 
 The cavity of the thalamen-cephalon is the third 
 ventricle ; posteriorly it is continuous with the ven- 
 tricle of the mid-brain, and anteriorly it opens into 
 the cerebral rudiment, the aperture becoming the 
 foramen of Monro. 
 
 The floor forms anteriorly the optic nerves and 
 chiasma, and posteriorly the corpora cdbicantia and 
 the infundibidum, which, coming into contact with 
 an involution from the mouth, gives rise to the 
 pituitary body. The sides of the thalamen-cephalon 
 thicken to form the optic thaiami, which become 
 subsequently united by the gi^ey or middle com- 
 missure. The roof forms the pineal gland and the 
 posterior commissure, and a vascular plexus forms 
 above it, which is the tela choroidea of the third 
 ventricle. 
 
 The cerebral liemisplieres. — These develop 
 from the cerebral rudiment of the fore-brain, and 
 gradually attain theii* predominating size. The floor of 
 
358 Human Physiology. LChap.xvi. 
 
 the fore-brain gives rise to the corpora striata, and the 
 roof to the hemispheres, which are separated by a deep 
 groove. The cavity on each side is a lateral ventricle; 
 a septum also forms, from which the falx cerebri is 
 derived. The outer wall of each hemisphere undergoes 
 great thickenino-, whilst the inner wall becomes thinner, 
 and two curved folds form in it, the upper of which 
 is the hippocampus major, or cornu Ammonis, and the 
 lower gives rise to the choroid plexus of the lateral ven- 
 tricle. By the fusion of the imier walls of the hemi- 
 spheres the septum lucidum is formed, which in man 
 is partially separated by a cavity, the fifth ventricle, 
 which has no communication with the true ventricles 
 of the brain. In the anterior part of this septum the 
 fi.bres of the anterior commissicre are formed. 
 
 The cerebral hemispheres in their primitive state 
 are quite smooth, but there arise a series of convolu- 
 tions, separated by sulci, which give it a characteristic 
 aspect. The most important, and the first to 'appear, 
 is the Sylvian fissure, situated at the root of the 
 hemispheres. The part of the brain lying in this 
 fissure is the island of Reil. 
 
 I>eveiopiiteiit of tlae ca'asiiai asicl spiBiafl 
 nea^ves. — The nerves are outgrowths of the central 
 nervous system. The spinal nerves arise from a 
 median and dorsal ridge of cells, named the neural 
 crest, by two lateral outgrowths, which are the rudi- 
 ments of the posterior roots. The posterior roots are 
 connected by a longitudinal commissure running along 
 the v/hole length of the neural crest. They develop a 
 ganglion, a.nd shift their point of attachment from the 
 crest to the lateral region of the cord, becoming for a 
 time altogether detached. The anterior roots grow, 
 somewhat later in point of time, from the anterior 
 part of the cord ; they have no ganglion, are not 
 united by a commissure, and do not shift their point 
 of attachment. The cranial nerves seem to arise, like 
 
Chap. XVI.] Development of the Eye. 
 
 359 
 
 the spinal nerves, from a neural crest, vfliich extends 
 as far as the root of the mid-brain, and is continuous 
 with the crest of the cord. The nerves arisino- from 
 it are the third, fifth, seventh, and auditory as a single 
 root, the giosso-pharyngeal, and the vagus. 
 
 I>eve!os>iiaeMt ®f the eye. — The numerous and 
 complicated structures of which the eye is composed 
 are developed in part from the integument of the 
 embryo, partly from the mesoblast, and in part from 
 the central nervous system. It commences with the 
 appearance of the o-ptia vesicles, a pair of hollow 
 outgrowths from the anterior cerebral vesicle, the 
 
 ■Ol? 
 
 'vv 
 
 A B 
 
 Fig. 3i,— A, Early ; B, Later Stage of Development of the Eye. 
 
 A. e, Epiljlast tiiickeued at t' in front of the optic vesicle, the indented portion 
 of which, ov, forms the retina, whilst ov' hecomes the optic nerve ; p, cavity 
 in the rudimentary lens. B. The letters have the same signification, bat the 
 lens, I, is seen to he completely detached, and to lie in the optic cup in con- 
 tact with the rudiment of the retina. 
 
 cavity of which extends into their interior. The 
 stalk elongates, and becomes the optic nerve. The 
 formation of the lens and of the optic ciij), or secondary 
 optic vasicle, now commences. The lens is formed by 
 a thickening of the epiblast, which indents the ex- 
 tremity of the primary optic vesicle, and pushes it 
 back till the front wall of the vesicle is in contact 
 with the posterior wall, and the cavity of the vesicle 
 becomes almost obliterated. The front wall of the 
 cup becomes the retina • the posterior wall, the 
 tesselated pigment layer of the choroid. By the 
 closure of the mouth the pit of the involuted epiblast 
 
360 Human Physiology. [Chap. xvi. 
 
 becomes a completely closed sac ; and the lens, 
 separating from the epiblast, forms an oval vesicle 
 with a small central cavity, lying in the pit. Soon, 
 however, a space is formed between the lens and the 
 wall of the cup, which comes to be occupied with the 
 vitreous humour. And now it must be explained, that 
 owing to the obliquity of the position of the optic 
 nerve, which slants downwards, inwards, and backwards 
 towards the vesicle, the lenticular thickening presses 
 in the front wall of the vesicle, not in a line with the 
 axis of the stalk, but in a line forming an obtuse angle 
 with it. The margins of the cup contiiiue to grow up 
 around the lens everywhere, except at the lower part, 
 which corresponds with the stalk, or optic nerve, and 
 here a fissure, the clioroidal fissure, long remains, 
 which sometimes, instead of closing, remains patent, 
 and constitutes a coloboma of the choroid. The 
 fissure is in the same line as the optic nerve, and 
 through it the mesoblastic tissue, from which the 
 connective tissues within the eye are formed, gains 
 entrance into the cavity of the optic cup. The 
 anterior wall of the optic cup, which it has been said 
 forms the retina, now presents a distinction of parts : 
 the posterior half develops into the true retina ; the 
 anterior half becomes the seat of the deposition of 
 pigment, is thrown into folds, covers the ciliary 
 ' processes, and forms the uvea at the back of the iris. 
 The ciliary processes themselves, with their vessels, 
 the iris, the ciliary muscle, and the vitreous, are all 
 formed from the mesoblast, which has entered the eye 
 through the choroidal fissure. The cornea, which is 
 composed essentially of three layers, the external 
 epithelium, the cornea proper, and the membrane of 
 Descemet, appears to be derived from two sources. 
 The external epithelium proceeds from the epiblast ; 
 the internal layer of cells, or membrane of Descemet, 
 and the cornea proper, originate in iqaesoblast, 
 
Chap. XVI.] Development of the Ear. 361 
 
 Development of tlie ear. — The auditory vesicle 
 in all vertebrata commences, according to Balfour, 
 with the formation of a thickened patch of epiblast, 
 at the side of the hind brain, on the level of the 
 second visceral cleft. This patch soon becomes in- 
 vaginated in the form of a pit, to the inner side of 
 which the ganglion of the auditory nerve, which is 
 primitively a branch of the seventh nerve, closely 
 applies itself. The pit closes, and the vesicle so 
 formed retreats from the surface, but remains con- 
 nected with it by an elongated duct. The inferior 
 part of the sac is produced into a process, which is the 
 rudiment of the cochlear canal ; the superior part is 
 prolonged into a blind sac, the aqitceductus vestibuli. 
 The main body of the vesicle becomes the utriculus 
 and semicircular canals, whilst the inferior prolonga- 
 tion forms the sacculus hemisphericus and cochlea. 
 The organ of Corti is developed from the epithelium 
 of the cochlear canal. The Eustachian tube and 
 lymphatic cavity are believed to be derived from the 
 inner part of the first visceral, or hyomandibular, 
 cleft. The meatus audit orius externus is formed at the 
 region of a shallow depression, when the closure of 
 the first ^dsceral cleft takes place. The tympanic 
 membrane is derived from the tissue which separates 
 the meatus auditorius externus from tympanic cavity, 
 and presents a hypoblastic epithelium on its inner 
 aspect, an epiblastic epithelium on its outer aspect, 
 and an intermediate mesoblastic layer. 
 
 Olfactory org:aiis. — In all the vertebrata the 
 olfactory organs commence, like the eyes and ears, 
 from a pair of thickened patches of epiblast, which, 
 in the case of the olfactory organs, are situated on the 
 under side of the fore-brain, immediately in front of 
 the mouth, and soon become involuted into the form 
 of a pit^ the lining cells of which become the olfactory, 
 or Schneiderian, membrane ; the surface of the walls 
 
362 Human Physiology. [Chap. xvi. 
 
 of the pit become greatly increased hj foldings. The 
 olfactory nerve attaches itself to the olfactory epi- 
 thelium at a very early period 
 
 ©evelopjfiaeiit ©f tlie aliiHeiitary cstiial* — 
 
 The alimentary canal results from the folding in of 
 the splanchnopleure, and is at first straight and 
 parallel to the vertebral column. It is connected 
 with the omphalo-mesaraic duct at a point which 
 corresponds with the lower segment of the ileum, 
 but the duct atrophies, and usually disappears 
 about the fourth month, though a diverticulum some- 
 times remains persistent. The attachment is at first 
 very broad, and only a thin stratum of mesoblast 
 separates the hypoblast of the canal from the noto- 
 chord and protovertebrte ; but it subsequently atten- 
 uates, and becomes the mesentery. In the fourth 
 week the part connected with the umbilical vesicle 
 loops forward. The part above the umbilical opening 
 becomes the small intestine ; the part below, almost 
 wholly large intestine ; the limit between the two is 
 soon indicated by a projection, the csecum. The 
 intestine separates from the abdominal wall, the 
 remains of the attachment appearing at the third 
 month, and sometimes later, as a thread-like appen- 
 dage to the lower part of the ileum. The convolutions 
 then begin to form, and an enlargement in the 
 region of the liver, which is the stomach. 
 
 Mecoiiiaisia. — Meconium is the feculent matter 
 contained in the rectum and large intestine of the 
 new-born child before any food has been taken. It 
 represents the products of the decay of the body 
 during intrauterine life. The chemical constituents 
 are bilirubin and biliverdin, biliary acids, cholesterin, 
 mucin, traces of formic acid, and other volatile fatty 
 acids, and non-volatile fatty acids. It contains 80 per 
 cent, of water, and about 1 per cent, of ashe^ 
 (Zweifel). 
 
Chap. XVI.] GeNITO-URINARY OrGANS. 
 
 363 
 
 The posterior opening of the intestine is formed by 
 the establishment of a communication between the 
 cloaca, or tube common to the gut and allantois, and 
 a depression on the 
 outside of the body, 
 about the sixth or 
 seventh week. A 
 septum, which is the 
 future perinseum, 
 now grows up, which 
 separates the intes- 
 tine from the organs 
 forming the allantois. 
 
 Various glands 
 arise as outgrowths 
 of the intestinal 
 canal, the mass of the 
 gland being formed 
 of mesoblast, and the 
 lining of the ducts of 
 hypoblast. Amongst 
 these glands are the 
 salivary glands, the 
 lungs, the pancreas, 
 and the liver. The 
 lungs appear as two 
 hollow vesicles, 
 which give off hol- 
 low branches like a 
 gland, and subse- 
 quently have a duct 
 or tube common to 
 both, which is the 
 trachea, and the 
 larynx forms at the upper part of the trachea. 
 The epiglottis and thyroid cartilage proceed from the 
 
 Fig. 44.— Embryo of the Tentli Week. 
 The abdomen has been laid open and 
 the hver and intestines removed. 
 
 a. Palatine fissure ; &, tongue ; c, carotid of ricrbt 
 side ; d, thyroid body ; e, tliymus gland'; f, 
 right ventricle; g, left ventricle; /(, richt 
 auricle ; i, left auricle ; fc, right lung ; I, dia- 
 phragm, still membranous ; m, portion of 
 liver attached to diaphragm ; ?i, n, supra- 
 renal capsules ; 0, kidneys; q, laminaj of 
 mesentery ; p, ureters ; r, rectum, divided ; 
 s, s, "Wolffian ducts and remains of Wolffian 
 bodies ; t, t, ovaries ; %i, sinus uro-genitalis ; 
 V, future Fallopian tubes ; w, future 
 round ligament ; x, clitoris ; y, cleft of 
 clitoris ; z, fold behind anus. 
 
 rudiment of the tongue. 
 
 The large liver commen,ces 
 
364 
 
 Human Physiology. 
 
 [Chap. XVI. 
 
 as a projection formed of two primitive liej)atic 
 ducts, which divide and subdivide. At the peri- 
 phery of the ducts are solid masses of cells, which 
 proceed from the hypoblast. The liver is of large 
 size at the second month, and secretes as early as the 
 third month. The spleen arises in a fold of the meso- 
 gastrium at the second month. The adrenals are at 
 tirst larger than the kidneys. 
 
 GeBiito-iiriaiary apparatus. — The excretory 
 organs of the vertebrata consist of three distinct 
 glandular bodies, and of three ducts : 
 
 (1) The i^ronephTos, or head kidney, a small glan- 
 dular body, usually with one or more ciliated funnels 
 
 somjil 
 
 TVD 
 
 snlan-jil 
 
 Fig. 45. — Diagram showing First Appearance of Genito-Urinary 
 System. 
 
 Som. pi., Somatopleure ; splan. pi., splanchnopleure ; md, Miillerian duct ; 
 WD, Wolffian duct. 
 
 opening into the body cavity, near which is a vascular 
 glomerulus. Its duct, which forms the basis for the 
 generative and urinary ducts, is the segmental duct. 
 
 (2) The Wolffian body, or mesonephros, consisting 
 gf a series of segmental tubes, opening at one 
 
Chap, xvi.j Urii^o- Genital Apparatus. 365 
 
 extremity into the body cavity, and at the other into 
 the segmental duct. This dnct divides into two, the 
 Wolffian duct and the Miillerian duct. 
 
 (3) The kidney proper, or metanephros. Its duct 
 is an outoTOwth from the AYolffian duct. 
 
 In the amniota (reptiles, birds, and mammals) the 
 Wolffian body is a purely embryonic organ and 
 atrophies, whilst the metanephros takes its place and 
 forms the permanent kidney. In the development of 
 each organ the duct is first seen. The first appearance 
 of the Wolffian duct^ which in mammals is really the 
 homologue of the segmental duct, is a solid rod of 
 cells, primarily derived from the somatic mesoblast of 
 the intermediate cell mass (Fig. 45, wd). The' solid 
 rod soon becomes tubular. The Wolffian body then 
 appears in the form of a series of convoluted tubules, 
 closely resembling the future kidney, commencing in 
 Malpighian bodies with vascular glomeruli and opening 
 into the duct, and the duct opens into the lower part 
 of the alimentary canal. The duct of Muller now ap- 
 pears as a furrow, which soon becomes a tube^ on the 
 outer surface of the projection formed by the Wolffian 
 body, and opens below into the cloaca, above the 
 Wolffian duct. The duct of the true kidneys now 
 forms, as the result of a constriction of the enlarged 
 Wolffian duct, the new iireter lying on the dorsal 
 surface of the Wolffian duct, and soon opening 
 separately into the cloaca. From the upper end of 
 the ureter^, diverticula are given off, which are the 
 tubuli uriniferi, and the kidney is formed from the 
 mesoblast surrounding them. The ridge of mesoblast 
 at the base of the somatopleure is covered with 
 epithelium of a columnar character, whilst that 
 covering the adjoining portions of the somatopleure 
 and splanchnopleure is tesselated. It is from the 
 proliferation of the columnar cells, and of the 
 subjacent fusiform cells of the mesoblast, that the 
 
366 
 
 Human Physiology. 
 
 [Chap. 5tVl. 
 
 sexual organs arise, and in both males and females 
 the appearances are identical ; large cells, the 
 primordial ova proceeding from the columnar epi- 
 thelial cells lying near the surface of the genital 
 ridge. In males the cells and ova disappear, but in 
 females they enlarge, sink into the 
 stroma, and carry with them some 
 ordinary cells, which unite to form 
 the Graafian follicles. The la^rge nu- 
 cleus of the primordial ovum becomes 
 the germinal vesicle, while the ovum 
 itself remains as the true ovum (Foster 
 and Balfour). 
 
 In males the testes arise in close 
 proximity to the Wolffian bodies^ by 
 the formation of tubuli and the 
 growth of the mesoblast. The sub- 
 sequent changes in the genito-uri- 
 nary apparatus are, that in birds the 
 Wolffian body is converted, in the 
 cock, into the coni vascidosi and 
 epididymis ; in the hen, into parts 
 of the parovarium. The Wolffian 
 duct remains as the vas deferens 
 in the male, and atrophies in the 
 female. The duct of Midler remains in the female as 
 the oviduct, in the male it atrophies. 
 
 SJkeletoai. — The notochorc!, probably developed 
 from the hypoblast, is the first rudiment of a skeleton, 
 and occupies the middle line of the body just beneath 
 the medullary canal. It is a cjdindrical rod composed 
 of nucleated cells, at first in immediate contact with 
 each other, but subsequently separated by a blastema, 
 or matrix, both cells and matrix being enclosed in a 
 sheath. It reaches at first nearly from one end of the 
 body to the other, but the brain soon begins to extend 
 beyondj and bend over in front of it. In some of the 
 
 Fig. 46,— Magni- 
 fied representa- 
 tion of the Urin- 
 ary and Genital 
 Organs of a Hu- 
 man Embryo 
 eight lines in 
 length. 
 
 a, Right suprarenal 
 capsule, complete- 
 ly covering kid- 
 ney; &, left kidney, 
 exposed liy the re- 
 moval of supra- 
 renal capsule ; c,c, 
 ducts represent- 
 ing vas deferens 
 or Fallopian tubes; 
 rl, ducts of Wolf- 
 flan body ; c, testis 
 or ovary; /, sinus 
 urino-genitalis. 
 
Chap. XVI.] JDeVELO'PMENT OF SkULL. 367 
 
 lower fishes the notochord is persistent, but in higher 
 animals it forms a kind of basis, around which the 
 vertebrae are formed. The vertebrae appear in the 
 form of cubical masses in the vertebral plate on either 
 side of the notochord, and these masses are tsnned 
 protovertebrcE. Ossification commences at an early 
 period in the proto vertebra as early as the twelfth 
 day in the fowl, the first vertebra to ossify being the 
 second or third cervical, but a portion of each proto- 
 vertebra remains as a muscle plate and a nerve 
 ganglion. The ribs are developed from the lateral and 
 inferior part of the protovertebra. In man the ossi- 
 fication of the vertebrae begins at the end of the 
 second or the beginning of the third month, with three 
 centres for each vertebra, one for the body, and one 
 for the arch on each side; but these parts do not 
 coalesce till the second year after birth. Accessory 
 centres of ossification are formed at the tips of the 
 spinous and transverse processes, and on the upper and 
 lower surfaces of the bodies. The first appearance of 
 the skull is in the form of a mass of mesoblastic tissue 
 in front of the protovertebrae, which, unlike the 
 vertebral plates, does not undergo any segmentation. 
 This is named the investing mass, because it surrounds 
 and invests the end of the notochord, and extends 
 forwards, forming the base of the skull. Its anterior 
 part pushes forth two horns, the trabeculse cranii, 
 which separate to enclose the pituitary fossa, and re- 
 unite in front of it to form the naso-frontal jJTocess. 
 From the margins of the basal mass, a membranous 
 investment, which soon becomes partly cartilagi- 
 nous, rises, which covers the brain. The primordial 
 cranial axis, therefore, consists of three parts : a 
 membranous roof ; membranous and partly car- 
 tilaginous lateral walls ; and a cartilaginous base. The 
 several bones are formed by separate points of ossifica- 
 tion appearing in these parts. 
 
368 
 
 Human Physiology. 
 
 tChap. XVI. 
 
 Limbs. — The extremities appear about the fourth 
 week in man in the form of buds from the somato- 
 pleure. The arms are the first to appear, and in the 
 first instance no division into fingers or toes are per- 
 ceptible ; a division into upper-arm and fore-arm, and 
 into thigh and leg, is perceptible about the eighth 
 week, but previously to this the extremity of the limb 
 presents indentations indicative of the future digits. 
 
 Fig. 47. — Diagram of Formation of Limb. 
 
 A, The first appearance of digits formed tjy slight indentations of the extremities ; 
 a. &, layers cf amnion ;"c, umbilical vesicle; d. umbilical cord; e, ear; B, 
 palmar aspect of hand ; c, division of fore from upper arm ; d, more ad- 
 vanced stage. 
 
 The structures which enter into the composition of the 
 limb are gradually differentiated. The bones first 
 appear as masses of blastema, which develop into carti- 
 lage, and in this again points of ossification are formed. 
 niiration of pregnancy. — Parturition, or the 
 delivery of the child, takes place at the expiration of 
 forty weeks (280 days) from the period of concep- 
 tion. At this period the uterus and its contents 
 have attained a large size, and the act of labour 
 is induced partly owing to the reflex excitation of 
 the genito-spinal centre in the lumbar region of the 
 cord, and partly, perhaps, owing to the direct stimula,- 
 tion of the uterine muscular tissue, and the numerous 
 
Chap. XVI.] Physiology OF New-born Child. 369 
 
 ganglia that supply it with motor fibres. The extreme 
 limits of pregnancy compatible with the birth of a 
 child capable of living are difficult to define, but 
 instances have been recorded Avhere a child was born 
 at about the close of the fifth month, and yet lived, 
 though much difficulty is in such cases experienced 
 in maintaining the temperature and in supplying it 
 with food. The longest period allowed for pregnancy 
 by the French law is 300 days. No limits are fixed 
 by the English law, and a disputed case is decided 
 on the evidence. 
 
 Physiology of the nenv-toom child. — The 
 foetus is born with the lungs unexpanded and the chest 
 in the condition of the most complete expiration. It is 
 in a state of apnoea, the blood being fully charged with 
 oxygen. The act of birth and the contractions of the 
 uterus interfere with the due interchano-e of o-ases 
 between the mother and child, and the circulation 
 of imperfectly arterialised blood through the medulla 
 oblongata at once stimulates the respiratory centres 
 tq liberate the motor impulses requisite for inspiration, 
 and the rhythmical sequence of inspiration and expi- 
 ration is at once commenced. The number of respira- 
 tions is 44 per minute, and the number of cardiac 
 beats 130. The temperature in the rectum in 37*8° C, 
 but falls during the first few hours 1° to 1'5° C, to rise 
 again to 37*5° C. The circulation through the liver 
 is reduced in activity, owing to the umbilical venous 
 circulation being arrested. The rectum contains me- 
 conium. The quantity of urine contained in the 
 bladder is from 8 to 10 cubic centimetres ; the quan- 
 tity of urine discharged in twenty-four hours is from 
 50 to 60 grammes. The mammary glands often secrete 
 a little milky fluid. Soon after birth hunger is felt, 
 and the child becomes restless, cries, and greedily 
 seizes and sucks the nipple or finger if introduced into 
 the mouth. The cortical motor centres are inactive, 
 
 Y 
 
370 Human Physiology. [Chap. xvi. 
 
 but a light is followed by the eye, and sounds produce 
 a start. The iiiliibitory action of the vagus is demon- 
 strated with difficulty, and the supervention of death 
 from asphyxia by drowning or suffocation is resisted 
 for a considerable period. 
 
 Infancy. — Infancy extends from birth to the 
 eruption of the first teeth, or to about the eighth 
 month ; all the vegetative functions of the body are 
 in full activity ; the lymphatic and blood-forming 
 organs, as the thyroid, thymus, spleen, and lymphatic 
 glands, are largely developed \ the periods of sleep 
 and of waking are about equal ; the mental faculties 
 gradually develop ; notice is taken of external objects, 
 and the infant can smile and shed tears. The quan- 
 tity of food taken is considerable ; the fseces are 
 yellowish, semi-fluid, without much odour, and contain 
 some unchanged bile, much fat, and coagulated casein. 
 The urine is frequently discharged ; its quantity 
 about the fourth month is 300 to 400 grammes. The 
 height augments 30 centimetres in the first year, and 
 the weight of the body is tripled. Some voluntary 
 movements can be performed. 
 
 Cliildliood extends from the first dentition to 
 the beginning of the second dentition, or to about the 
 age of seven years. The number of cardiac beats and 
 of respirations gradually falls. At five years of age the 
 heart's beats are 105 ; the vital capacity is 900 cubic 
 centimetres ; and the respirations are 26 in the minute. 
 During the second year the child learns to walk and 
 speak. Sleep is protracted to nine or ten hours. 
 
 Healthy children should grow from 2 to 3 inches a 
 a year. They should weigh as nearly as possible to these 
 averages. There is danger if a child falls 7 lbs. below 
 this standard, or grows under 2 or over 3 inches a year. 
 Arrest of growth and loss of weight indicate 
 malnutrition. They are the frequent forerunners of 
 disease, and should always excite suspicio^, 
 
Chap. XVI.] Characters of Different Ages. 371 
 
 The following is a proportionate table of height 
 and weight : 
 
 Height. 
 
 Weigtb. 
 
 HeigM. 
 
 Weiglit. 
 
 Feet in. 
 
 st; Iba. 
 
 Feet in. 
 
 St. lbs. 
 
 2 
 
 1 4 
 
 3 7 
 
 3 8 
 
 2 1 
 
 1 6i 
 
 3 8 
 
 3 10 
 
 2 2 
 
 1 7 
 
 3 9 
 
 3 12 
 
 2 3 
 
 1 Si 
 
 3 10 
 
 4 
 
 2 4 
 
 1 10 
 
 3 11 
 
 4 2 
 
 2 5 
 
 1 Hi 
 
 4 
 
 4 4 
 
 2 6 
 
 1 13" 
 
 4 1 
 
 4 6i 
 
 2 7 
 
 2 Oi 
 
 4 2 
 
 4 9 
 
 2 8 
 
 2 2' 
 
 4 3 
 
 4 Hi 
 
 2 9 
 
 2 3i 
 
 4 4 
 
 5 0^ 
 
 2 10 
 
 2 5" 
 
 4 5 
 
 5 2i 
 
 2 11 
 
 2 Gi 
 
 4 6 
 
 •5 5 
 
 3 
 
 2 8' 
 
 4 7 
 
 5 7J 
 
 3 1 
 
 2 10 
 
 4 8 
 
 5 10 
 
 3 2 
 
 2 12 
 
 4 9 
 
 5 12i 
 
 3 3 
 
 3 
 
 4 10 
 
 6 1"^ 
 
 3 4 
 
 3 2 
 
 4 11 
 
 6 3i 
 
 3 5 
 
 3 4 
 
 5 
 
 6 6" 
 
 3 6 
 
 3 6 
 
 
 
 YoMtli. — This period extends from the seventh 
 to the fifteenth year, or to the occurrence of puberty. 
 The milk teeth are shed and the thymus disappears ; the 
 bones become firm and solid ; the mental faculties a.re 
 often exceedingly acute, and the memory very exact 
 and tenacious. The vital capacity is about 2,000 cubic 
 centimetres at twelve years of age ; the cardiac beats 
 at the same age are about 82. The quantity of urine 
 is about 21 or 22 grammes. The fat of the body is in 
 great part absorbed ; and as the period of puberty is 
 reached, the voice alters and becomes deeper, and the 
 conservation of the individual begins to make way for 
 the conservation of the species (Beaunis). 
 
 ^doilt age. — Puring the earlier period growth 
 
372 Human Physiology. [Chap. xvi. 
 
 continues, but about the age of twenty, or earlier in 
 "^v'omen, growth ceases, and for many years the body 
 remains stationary in point of bulk. The faculty of 
 observation and power of acquiring knowledge are 
 active, but at first judgment is defective, and the 
 actions are largely influenced by the emotions. It is 
 fi'om twenty to forty that tlie most remarkable mental 
 efforts are made, and Avlien genius, if present, usually 
 effects its greatest triumphs. At a later period, 
 though intellectual development may still progress, 
 it is more uniform and sober in its manifestation. In 
 women the period of adult age is interrupted by the 
 occurrence of the menopause. 
 
 Old age; senility. — This may be taken to 
 commence about the sixtieth or sixty-fifth year. It is 
 marked by general decay of the bodily powers ; the skin 
 begins to be wrinkled, owing to the absorption of fat ; 
 the teeth to decay and to be shed ; the hair to be white 
 and fall out \ the virile powder is less active, or alto- 
 gether ceases ; the respirations and the number of 
 cardiac beats are reduced in frequency ; the arteries 
 have a tendency to ossify, the veins to dilate ; the 
 muscular movements lose their force and precision ; 
 the cartilages ossify ; the voice alters to " childish 
 treble ; " the digestion and the vegetative functions 
 generally are less perfectly performed ; the eye is no 
 longer capable of being accommodated to clear vision 
 of near objects, and, with the other senses, loses its 
 acuteness of perception ; the mind, however, may long 
 preserve its fi-eshness, and in some instances even 
 seems to mature with advancino^ ao-e. 
 
 I>eatli. — Death, when perfectly natural, occurs 
 as the result of the cessation of the cardiac or respira- 
 tory movements, and this is probably the result of 
 inadequate nutrition of the nerve centres governing 
 these acts. In by far the greater number of cases, 
 however, one or other of the vital organs is smitten 
 
Chap, xvi.i Death. 373 
 
 with disease, which interferes with the functions o£ 
 the body at large, aiid it was from observations of 
 this fact that Bichat maintained that death began at 
 the brain, the heart, or the lungs. To these the 
 kidneys and the intestines might be added. In some 
 instances it is sudden, and scarcely preceded by any 
 appreciable failure of health ; whilst in other instances 
 it is prolonged and painful. Beaunis gives the fol- 
 lowing as characteristic features of the death agony : — 
 The face is livid and sharp-featured ; the cheek-bones 
 prominent ; the cheeks pendent ; the nose sharpened ; 
 the forehead covered with a cold, clammy moisture ; 
 the eyes dull and unobservant, the lids drooping ; the 
 lips livid and discoloured ; the mouth partly open ; 
 the gums dry, and the teeth covered with sordes ; the 
 body inert and yielding to gravity, save only some 
 involuntary movements of the fingers and hands ; the 
 extremities cold, the coldness extending gradually 
 upwards ; respiration feeble ; mucus accumulating in 
 the trachea gives rise to a rattling sound ; the cardiac 
 beats, at first frequent, become slower and impercep- 
 tible ; sensibility is reduced ; the eye no longer sees 
 the light ; the dying man feels himself to be shrouded 
 in darkness ; the hearing often persists to the last ; 
 voice fails, and he mutters some incomprehensible 
 words ; intelligence may be preserved, but usually 
 fails, and he seems to remember as in a dream some 
 events of his past life ; at length the heart ceases to 
 beat, and the last breath is an expiration. 
 
APPEISTDIX. 
 
 A. list of Home of the more imjwrtant suhsta)ices not described in 
 the text, that have physiolog iced relations. 
 
 CgHfjO. — A colourless molDile neutral fluid , 
 specific gravity 0*782 at IS'' C. Its odour resembles that of 
 acetic ether and peppermint. It vaporises easily, boils at 
 66-3° C, and burns with a sKghtly smoky flame. It dissolves 
 in water, alcohol, and ether, and acts as a solvent on camphor, 
 fat, resins, and gun cotton. It has been found in the urine of 
 patients suffering from diabetes mellitus. It is formed hy the 
 dry distillation of the acetates, or by the dry distillation with 
 lime of citric, tartaric, and lactic acids, sugar, gum, and starch. 
 
 AcSiroo-dextfiai. — An intermediate substance hetween 
 starch and dextrin, formed by the action of saliva on starch. 
 It does not give any colour reaction on the addition of iodine. 
 
 A^piC>cere. — A compound consisting of lime, palmitate, 
 and stearate. It is found in dead bodies exposed to much 
 moisture. 
 
 AI!aiit©iiJ C^HglST^Oa. — A product of the decomposition 
 of u.ric acid. It is found in the allantoic fluid of the calf, and 
 has also been found in the urine of the dog in cases where the 
 respiration has been seriously interfered with for a long time, 
 and in the mine of man after the use of large quantities of 
 tannic acid. It crystallises in thin fascicles, the crystals be- 
 longing to the mono-klino-metric system. It is tasteless, and 
 has no action on litmus. It dissolves at 20° C. in 160 parts, 
 at a boiling heat in 30 parts of water, and more easily in spirit 
 of wine. It is an anhydride of 2 molecules of urea 2(N2C0H4) 
 and glycoxylic acid C2H4O4. Acids and alkalies split "it into 
 urea and allanturic acid. 
 
 A8lfl>xail C4H0N2O4. — A substance having close relations 
 with uric acid, from which it can be obtained by oxydation. 
 It crystallises with 4 equivalents of water of crystallisation in 
 large brilliant rhombic pyramids, which eflioresce when exposed 
 to air. It dissolves easily in alcohol and in water. The watery 
 solution stains the skin of a purplish colour, and communicates 
 
Appendix. 375 
 
 a disagreeable odom- to it. It has an acid reaction, and dis- 
 agreeable taste. With, salts of iron, it gives a dark indigo blue 
 colour. It is a powerfiil oxydising agent. 
 
 Asparaglll 04113X203. — The amide of aspartic acid ; a 
 substance obtained from the turios of asparagus, but also con- 
 tained in Symphytum officinale (comfrey), convallaria majalis 
 (Solomon's seal), and Paris quadi'ifolia (herb Paris), and in the 
 seeds of leguminous plants. It forms colourless and odourless 
 transparent stable crystals, belonging to the rhombic system. 
 It dissolves in 58 parts of water at 13'^ C, and in 4-5 parts of 
 boiling water. It is quite insoluble in alcohol at all tempera- 
 tui'es, in ether, and in oils. 
 
 Aspartic acid C4H7XO4. — It is formed by the decom- 
 position of asparagus under the influence of alkalies ; also by 
 the decomposition of proteids, such as legumin, egg albumin, 
 and casein, by means of sulphuric acid. By fermentation or 
 putrefaction, aspartic acid is converted into succinic acid. 
 
 Camill C-HgiSr403 + H2O. — This substance is contained 
 in Liebig's extract of meat, in which it exists in the proportion 
 of about one per cent. It is allied to sarkin, into which it can 
 be converted with loss of H and COo by the action of bromine 
 or chlorine, with a little nitric acid. 
 
 Ceretorill. — This substance is only found in nerve-tissue 
 and in pus cells. It forms a delicate white tasteless powder, 
 destitute of smell, and composed of rounded granules. "When 
 heated to 80° C, it becomes brown, then, at a higher tempera- 
 ture, forms bladders and decomposes. When in a somewhat 
 impure form, and containing some lecithin and cholesterin, it 
 gives the appearance of myelin drops, or double-bordered 
 intestine - like clumps, which prolong themselves into long 
 threads, and refract light strongly. Solutions of cerebrin do 
 not act on litmus paper. When boiled with acids it yields a 
 kind of sugar, that rotates polarised light to the left, but does 
 not ferment. 
 
 CBieno-taiU'OClioIic acid. — An acid found in the 
 bile of the goose. 
 
 \ CllOlatic acid 004114505. — A product of the decompo- 
 sition of glycocholic acid with absorjDtion of water at a boiling 
 temperature, glycocol being separated at the same time. 
 
 aeH43NO, + H,0 = CoH,N0.3 -f C,4H4o05 
 Glycccliolic acid glycocol cholalic acid. 
 
 Cliolesteriil Cogll440 + HgO. — This substance is best 
 obtained from gall stones, which are in great part composed of 
 it. Anhydrous cholesterin crystallises out of benzol, chloro- 
 form, or pure ether, in fine colourless silky- needles. Hydrated 
 
37^ Human FhysiologV. 
 
 cholesterin crystallises out of boiling- alcohol in microscopic 
 oblique rhombic plates. It is insoluble in water, alkalies, and 
 diluted acids, and with difficult}^ in cold alcohol, but very easily 
 in hot ether or boiling alcohol, chloroform, benzol, volatile 
 oils, and fatty acids. Anhydi-ous cholesterin melts at 140" C, 
 and distils unchanged in vacuo at 360° C. Concentrated sul- 
 phuric acid converts it in the cold into a red mass, which, on 
 the addition of water, becomes gTcen. On account of its form- 
 ing ether-like compounds with one molecule of acid, which are 
 saponifiable by alcoholic solution of potash, it is regarded as a 
 univalent alcohol. 
 
 CllOlill CgHjgXOo- — A product of the decomposition of 
 lecithin ; it does not appear to be a constituent of the body. It 
 is a thick syrup, easily soluble in water and in alcohol ; it turns 
 litmus paper blue. 
 
 Dyslyslll ^.^^^ifiy — This substance is the ultimate pro- 
 duct of the action of boiling hydrochloric acid upon giycocholic 
 acid or upon cholalic acid. 
 
 Glycocliolic acid glycocol djslysin water. 
 
 It is a white amorphous tasteless mass, soluble in ether, with 
 more difficulty in alcohol, and scarcely at all in water, alkalies, 
 acetic or hydrochloric acids. It melts at 140" C, and bums 
 with a smoky flame. 
 
 Formic acid CHoOa- — An acid obtained, as its name 
 implies, from ants, but present also in many plants. It has 
 been found in blood, in urine, milk, sweat, and in the juice of 
 muscle of man. 
 
 Olycerill CgHgOg. — A secondary product obtained by the 
 action of superheated steam upon fats. It has not been shown 
 to be a constituent of the body. It is without colour or odour, 
 but has a burning sweet taste, is of oily consistence, has a 
 neutral reaction and a sp. gr. of 1-26. It boils at 290°, and the 
 vapour ignites, burning with a feeble but not smolcy flame ; at 
 — 40° it becomes a gummy mass, but does not crystallise ; on 
 withdrawal of water by rapid boiling condensed glycerin-mole- 
 cules are formed, such as diglyceride and polyglyceride. Placed 
 in contact with j^east it yields propionic acid C3Hg02. With 
 chalk and putrefying cheese it yields at 40° C. butyric acid 
 C^Hj^Oo and ethji alcohol CoHgO. By long heating of glycerin 
 with excess of fatty acids in hermetically sealed tubes at a tem- 
 perature of 200'^ to 270°, triglycerides are produced, which are 
 identical with the ordinary fats. The oxydation of glycerin by 
 means of nitric acid yields tartaric acid, and when heated with 
 potassium hydroxyd to a temperature of 200° C, potassium 
 
Appendix. 377 
 
 formiate and potassium acetate are prodaced. Glycerin is a 
 trivalent alcohol. It is intimately associated with, acetone, 
 acrolein, and the allyl series generally, and consequently with 
 the propyl series of compounds. 
 
 Olycerisi-pliospaioric acid C3H5(OH)2,0,PO(OH)2. 
 — An acid syrup, which, when slightly warmed, hreaks up into 
 glycerin and phosphoric acid. It is a hibasic ether acid, which 
 combines with bases to form cryst alii sable salts. Easily soluble 
 in water, but soluble with difficulty in alcohol. It is a product 
 of the decomposition of lecithin. 
 
 llypoxailtliiia CgH^X^O. — K. constituent of muscle, in 
 which it is found in the proportion of U-02 per cent. It may 
 be obtained by acting on xanthin or uric acid with sodium 
 amalgam. The relation between these organic substances is 
 immediately apparent on considering the following formulae • 
 
 Sarkm xantlun xa'ic acid. 
 
 IiKtlcan C2r,HgiNOi7. — The quantity of indican excreted 
 in the urine of man is very inconsiderable ; it is most abundant 
 on meat diet. It can be obtained in larger quantities from the 
 urine of the horse. It appears in the form of a bright brown 
 strongly odorous syrup, of bitter and na-useous taste, which has 
 a neutral reaction, and is precipitated by lead acetate. It is 
 not altered by dilute sulphuric acid or by alkalies. Acidified 
 solution of indican with a trace of chlorine gas gives a blue 
 colour from the forination of indigo. The indican contained in 
 urine is not identical with that in the plant named Isatxs tinc- 
 toria. When acted on by concentrated acid, leucin, volatile 
 fatty acids and a purphsh red body named urrhodin are formed. 
 When its aqueous solution is treated with hydrochloric acid in 
 presence of oxj^gen, it ^delds indigo blue. 
 
 Indig'O. — A crystalline blackish-blue powder, consisting of 
 needles or oblique rhombic plates, the surface of which has a 
 copper-red metallic glint. Indigo is insoluble in water, 
 alcohol, ether, alkalies, and dilute acids, but is easily soluble in 
 boiling chloroform, amyl-alcohol, and melting paraffin. "ViHien 
 dissolved in amyl-alcohol its spectrum presents between d and 
 D a dark absoi-jjtion band. 
 
 Indol CsH;.X. — A substance crystallising in large pearly 
 thin tablets of peculiarly ofliensive odour. It melts at 52*' C, 
 and volatilises unchanged m vacuo at 218^ C. It is soluble in 
 water. The dilute solution on the addition of chromic acid 
 gives a dark violet very voluminous precipitate, which is in- 
 soluble in ether, chloroform, and benzol, but is soluble in 
 alcohol with a red tint, and in hydrochloric acid with a violet 
 
378 Human PhysiologV-. 
 
 colour. It is one of the final products of the action of 
 pancreatic juice on proteids, and it may also be obtained by 
 the distillation of proteids with the caustic alkalies. 
 
 Imosinic acid Q^^H.-^^ ^O-^^ — An acid obtained in the 
 proportion of about O'Ol per cent, from the muscles of cats and 
 rabbits, but not certainly from that of other mammals, and in 
 the proportion of about 0-02 per cent, from the muscles of the 
 goose, duck, and pigeon ; it has a taste and an odour resembling 
 that of broth, and appears in the form of a syrup from which, 
 on the addition of bases, crj-stalline salts can be obtained. 
 
 Isiosite CgHjoOg + aHoO. — Muscle sugar or phaseoman- 
 nite, found in muscles and in various fruits of the leguminosse. 
 It forms large colourless monokUnic tabular crj'stals. It is 
 soluble in six parts of water at 19° C It is incapable of 
 undergoing fermentation, and does not reduce alkaline solution 
 of copper sulphate, nor does it exert a rotatory action on light. 
 When moistened with a little nitric acid and evaporated to dry- 
 ness, and then mixed with ammonia and calcium chloride, it 
 gives a rose colour ; when placed in contact with putrefying 
 cheese it yields jjropionic, butyric and paralactic acids. 
 
 M-eratill. — A substance of inconstant composition obtained 
 after the successive action of boiling water, alcohol, ether, and 
 dilute acids on various epidermoid tissues, as upon fish scales, 
 nails, horn, hoofs, feathers, and the epidermis. It is insoluble 
 in alcohol and ether, swells up in hot water, and still more 
 easily in acetic acid ; it does not undergo putrefaction ; melts 
 when heated, and burns with a luminous smoky flame, having 
 a peculiar odour. Boiled with dilute sulphuric acid or with 
 hydrates of alkalies, it yields aspartic, acetic, butyric, propionic, 
 and valerianic acids, with ammonia, leucin, and tyrosin; 
 treated with nitric acid it yields oxalic acid as a terminal 
 product. 
 
 Kreatisi C4II9X3O2+II2O. — A substance found in the 
 muscles of all vertebrata. It crystallises in colourless oblique 
 rhombic prisms, readily soluble in hot water. Soluble in 
 9400 parts of alcohol, and insoluble in ether. It has a bitter 
 taste. Boiled with baryta water it breaks up in urea and 
 sarkosin, taking up one equivalent of water. Kreatin is an inter- 
 mediate product of the disintegration of muscle and nerve 
 tissues. It is not found in gland tissue. Heated with mercuric 
 oxide it yields oxalate of methylguanidin, which establishes a 
 relation between it and guanin. 
 
 KreatiBiin C4H7N3O. — A substance found in the urine. 
 It can be artificially made by boiling kreatin for four or five 
 days with concentrated acids, when it gives up one equivalent 
 of water and becomes converted into kreatinin. Kreatinin 
 
Appendix. 379 
 
 Crystallises in long colourless monoklinic prisms, which are 
 soluble in 100 parts of cold alcohol, and in 11-5 parts of cold 
 water. The solution turns litmus paper hlue. 
 
 Idecitllin C44HgQNP09. — A constituent of the hrain and of 
 yolk of Q^g. It crystallises in fine needles on slow cooling of 
 its alkaline solution. It swells up like starch on the addition 
 of water. It dissolves with difficulty in cold alcohol and in 
 ether, hut easily in chloroform, henzol, boiling alcohol and ether, 
 and in hot acetic acid. Lecithin is in close relation with the 
 triglycerides or fats. 
 
 ILiMteiDl. — This substance forms microscopic red crystals 
 insoluble in water, soluble in alcohol, ether, cliloroform, benzol, 
 and fat oils. On the addition of nitric acid it becomes green, 
 blue, yellow, and then colouiiess. It is thought to be identical 
 with hsematoidin. 
 
 Malic acid 043^0.5. — An acid widely distributed 
 amongst plants, often in association with tartaric and citric 
 acid ; thus it is found in apples, cherries, pine-apple, tamarinds, 
 in aniseseed, in the berries of the mountain ash, and in 
 rhubarb. The crystals melt at 83° C. When ingested into the 
 stomach the malates are decomjDosed, and appear in the urine in 
 the form of carbonates and bicarbonates of the alkalies. 
 
 l^JTociEl. — A substance which may be procured from the 
 salivary glands and from the snail. It forms a white or yellow- 
 ish substance insoluble in water, but capable of swelling up 
 when immersed in it. It does not diffuse through animal mem- 
 branes. It is immediately precipitated on the addition of 
 alcohol, to a mixture of water and mucin. It is soluble in con- 
 centrated acids and alkalies. It does not coagulate on boiling, 
 it is not precipitated by metallic salts excepting basic acetate of 
 lead, or by potassium ferrocyanide, or by tannic acid. It gives a 
 rose-red colour with Millon's reagent, and a yello w coloiu' with 
 nitric acid. 
 
 NMCieln, — A suhstance obtained irotn the nuclei of cells. 
 It may be obtained from pus cells by digesting them in warm 
 alcohol to remove fat and lecithin ; on the addition of gastric 
 juice the albumin of the cells is converted into peptones, and 
 dissolved when the nuclei, which resist the action of the gastric 
 juice, gravitate to the bottom of the vessel ; and they are then 
 further purified from fat and lecithin by the successive appli- 
 cation of ether, cold water, and alcohol. Pus cells dried at 
 110° C. contain 33 per cent, of nuclein. 
 
 Paralactic acid CgllyOg. — An acid that, notwithstand- 
 ing its alkaline reaction, seems to be present in muscle ; 
 during muscular effort it accumulates in the tissue to such an 
 extent as to confer upon it an acid reaction. It is of syrupy 
 
380 Human PhysioLo&Y. 
 
 consistence, and mixes ^vith alcohol, ether, and water. It is 
 raonobasie. It rotates a ray of polarised light to the right. 
 
 PlieilOl CgHgO is found in the urine in the form of a 
 conjugated sulphur acid (phenosulphuric acid) combined with 
 potassium. Phenosulphate of potassium crystallises in brilliant 
 white scales, soluble in water, almost insoluble in alcohol. 
 Acids decompose it into phenol and sulphuric acid. When 
 rapidly heated it melts, dissolves in water, and assumes a red 
 tint on the addition of perchloride of iron. 
 
 PyrocatCCllill CgH40H2 is an occasional constituent 
 of the urine, either pure or in conjugation with sulphuric 
 acid. In this case the urine on standing acquires a deep brown 
 shade in its upper layers, and becomes of a blackish-brown tint 
 on the addition of alkalies. It reduces ammoniacal solutions 
 of silver nitrate, mercury nitrate, and copper sulphate. In 
 watery solution pjTOcatechin gives an emerald gTeen colour 
 with perchloride of iron, which becomes violet on the addition 
 of tartaric acid and ammonia. 
 
 ^ariiiM. — The same as Hypoxaxthix. 
 
 §arl40Sitl CgH-NO.,. — An amido-acid-methyl-glycocol, 
 not certainly ascertained to be a constituent of the body, but 
 interesting as being a derivative of kreatin, for when this sub- 
 stance is boiled on by bar}i;a water, it yields ammonia and 
 barium carbonate, and sarkosin can be obtained by evaporation 
 of the liquid. Caff ein treated in the same way also yields sar- 
 kosin. It crystallises in colourless rhombic prisms, easily 
 soluble in water, less so in alcohol, and not at all in ether. It 
 has a burning sweet taste. 
 
 §MCCBB1EC acid C4llg04. — An acid found in small quan- 
 tities in several animal fluids, but usually obtained artificially. 
 It forms colourless monoklinic prisms, or microscopic six-sided 
 tablets, soluble in three parts of boiling, and 17 parts of cold 
 water; specific gravity 1-552. It begins to sublime at a tem- 
 perature of 126° C, melts at 180°, and boils at 235°, decom- 
 posing into anhydride and water. The vapour tastes acid, and 
 excites cough. It stands in genetic relation with the fatty 
 acids, as the butyric and valerianic, and with the fats with 
 fumaric and maleic acids, with the glycerin- acids, and with 
 some organic acids, as the malic and tartaric. 
 
 T'yi'OSBll CgH^NOg. — An aromatic compound obtained 
 by boiling horn shavings, or any of the proteids, with sulphiiric 
 acid, neutralising with lime-water, and, after filtration, adding 
 lead acetate to the filtrate, when tyrosin-lead is formed. The ty- 
 rosin is set free by the addition of sulphuric acid. It forms white 
 silky needles on crystallising, which are insoluble in ether and 
 alcohol, soluble in 1900 parts of cold water, and easily soluble 
 
Appendix. 381 
 
 in dilute acids and alkalies, with the exception of acetic acid. 
 Solution of tyrosin heated with mercury nitrate turns of a rose 
 colour. Tyrosin may be regarded as one of the products of 
 the oxydation of all proteid bodies, or of their putrefaction or 
 regressive metamorphosis. It is an alanin in which H is sub- 
 stituted by oxy-phenyl CgH40H. 
 
 Xanttliil C5H4N4O0. — ^An amorphous powder of yellowish- 
 white colour, or crj^staUine lamellae, slightly soluble in water, 
 insoluble in alcohol and ether, and soluble in caustic ammonia. 
 Heated with nitric acid, it giyes a yellowish residue, which, ou 
 the addition of soda, assumes a red colonr, and becomes purple 
 when heated. It forms crystallisable salts. 
 
 Zo amy J ill* — The same as Glycogen. 
 
INDEX. 
 
 Abdominal type of respiratiou, G2. 
 Abducens ocuJi nerve, 239. 
 Abei-ration, Chromatic, 277. 
 
 , Spherical, 276. 
 
 Absolute muscle force, 201. 
 Absorption of fat, 129. 
 
 of food, 129. 
 
 of gases byffluids, 67. 
 
 Accelerating nerves of heart, 34. 
 Accelerator centre, 253. 
 Accessory, Spinal, nerve, 244. 
 Accommodation of the eye, 281. 
 Acetic acid test for proteids, 6. 
 Aceton, 374. 
 Achroo-dextrin, 374. 
 Acid albumins, 4. 
 
 rigor, 189. 
 
 Actinic rays, 298. 
 AdamMewicz's test for proteids, 8. 
 Adipocere, 374. 
 Adidt age, 371. 
 .3]sthesiometer, 316. 
 Albumins, 3. 
 
 , Acid, 4. 
 
 , Alkali, 4. 
 
 Alcohol as food, 87 
 
 Allautoin, 172, 374. 
 
 Allantois, 344. 
 
 Alloxan, 374. 
 
 Alternation of generations, 321. 
 
 Amnion, 342. 
 
 Amoeboid movement, 178. 
 
 Amyloid substance, 5. 
 
 Amylopsin, 122, 124. 
 
 Anaemia, Effect of, on muscle, 202. 
 
 Anelectrotoniis, 220. 
 
 Animal heat, 143. 
 
 Anisotropous substance, 182. 
 
 Anospinal centre, 128. 
 
 Anterior jugular veins, 352. 
 
 Antipeptone, 122. 
 
 Antiperistalsis, 115. 
 
 Aortic arches, 351. 
 
 Apnoea, 65. 
 
 Appendix, 374. 
 
 Archibjastic formations, 340, 
 
 Aristotle's experiment, 31J. 
 Arteries, 35. 
 Asexual generation, 324. 
 .Asparagin, 375. 
 Aspartic acid, 375. 
 Astigmatism, 289. 
 Auditory nerve, 240. 
 
 sensations, 310. 
 
 Automatic movements, 229. 
 Avalanche theory of nerve im- 
 pulse, 217. 
 Azotised compounds, 2. 
 
 Balance of the ceconomy, 94. 
 
 Barley as food, 84. 
 
 Basal ganglia, 253. 
 
 Beats,^308. 
 
 Beer, 86. 
 
 Bernardin, 133. 
 
 Hile, Characteristic^: of, 119. 
 
 , Quantity of, 120. 
 
 salts. Test for, 120. 
 
 , Uses of, 121. 
 
 Bilirubin, Test for, 120. 
 Binocular vision, 297. 
 Biogenesis, 323. 
 Biuret test for ui'ea, 161. 
 i'lastoderm, 335. 
 iilastodermic vesicle, 333. 
 Blind spot, 276. 
 Blood, 8. 
 
 , Coagulation of, 13. 
 
 , Composition of, 16. 
 
 corpuscles, 11. 
 
 pressure, 36. 
 
 , Quantity of, 10. 
 
 vessels, 34. 
 
 Bottger's test for sugar, 7. 
 Brandy, 87. 
 Bread as food, 84. 
 Bronchial murmur, 66. 
 Brunner's glands, 125. 
 Bulbus arteriosus, 351. 
 
 Calorific rays, 298. 
 Calorimetry, 148, 
 
3S4 
 
 Human Physiology. 
 
 Canal of ITuller, 338. 
 Capacity of cliest, 63. 
 CapilLiries, 49. 
 Carbohydrates, 6. 
 Carholjc acid, 173. 
 Cai-bonic acid of blood, 2?. 
 
 in respired air, 69. 
 
 Cardinal veins, 352. 
 Carnin, 375. 
 Caseiu, 4, 89. 
 
 , Ditrestion cf, 113. 
 
 Catlielectrotonus, 221. 
 Centre, Accelerator, 253. 
 
 , Ciliospinal, 251. 
 
 , Deglutition of, 107. 
 
 , Ejaculation, 251. 
 
 , Erection, 251. 
 
 , Insalivation, 2f2, 
 
 , Mastication, 252. 
 
 , Micturition, 177. 
 
 , Parturition, 251. 
 
 , Suction, 253. 
 
 Cereals as food, 84. 
 
 Cerebral peduncles, 254. 
 
 Cerebrin, 375. 
 
 ChaJazge, 335. 
 
 Chemistry of nervous tissues, 213. 
 
 of respiration, 68. 
 
 Cheno-tauroeholic acid, 375, 
 
 Childhood, 370. 
 
 Cholalic acid, 375. 
 
 Cliolesterin, 375. 
 
 Cholin, 376. 
 
 Chorda tympani, Action of, 105. 
 
 Chorion frondosum, 346. 
 
 Igeve, 346. 
 
 primitivum, 347. 
 
 verum, 347. 
 
 Chi'omatic abeiTatiou, 277. 
 
 Chyle, 130. 
 
 Chyme, 118. 
 
 Ciliary ganglion, 237. 
 
 movement, 179. 
 
 Ciliospinal centre, 251. 
 Circulation, First, 351. 
 Coagulated proteid.<, 5. 
 Coagulation of blood, 13. 
 Coflee, 88. 
 Colorific rays, 298. 
 Colostrum, 89. 
 Colour blindness, 309. 
 Compass of voice, 312. 
 Complementary colours, 298. 
 Composition of blood, 17. 
 
 of body, 2. 
 
 of milk, 89. 
 
 of muscle, 184. 
 
 of ta'iue, lc'8. 
 
 Cominxlsory movements, 256. 
 
 Conjugation, 324. 
 Consonants, 314. 
 Contractility of muscle, 185, 
 Contraction, Ptieuomena of, 191. 
 Co-ordination of movements, 2o2. 
 Corpora qnadrii^emiiia, 254. 
 
 striata, 255. 
 
 Corpus luteum, 3?2. 
 
 spiu'iiini , 332. 
 
 verum, 332. 
 
 Costo-inferior type of respiration, 
 
 62. 
 superior type of respiration, 
 
 62. 
 Coughing, 77. 
 
 Course of fibres in cord, 250. 
 Cutaneous respiration, 76. 
 transpiration, 77. 
 
 Death, 372. 
 
 Decidua menstrualis, 330. 
 
 reflesa, 349. 
 
 seiotina, 349. 
 
 vera, 349. 
 
 Decomposition of light, 29". 
 Def 86 cation, 128. 
 Deglutition, 107. 
 
 centre, 252. 
 
 Dentals, 314, 
 Depressor iibres, 243. 
 Derived albumins, 4. 
 Development of embryo, 322. 
 Dextrin, 6. 
 Diapedesis, 52. 
 Diastole of heart, 23, 
 Diet, 92. 
 Dietaries, 92. 
 Direct vision, 276. 
 Dissonance, 308. 
 Distribution of blood, 52. 
 Drinks, 85. 
 Drum of the ear, 304. 
 Duct of Miiller, 338. 
 Ductless glands, 128. 
 Ductus venosus, 352, 
 Dyslysiu, 376. 
 Dyspncea, Qo. 
 
 Egg-albmnin, 3. 
 Eggs as food, 83. 
 Ejghth nerve, 240, 
 Ejaculation centre, 251. 
 
 of semen, 329. 
 
 Elasticity of muscle, 185, 
 Electric currents in miujclc, 205. 
 
 in nerve, 218. 
 
 Electrotonus, 220. 
 Eleventh pair of nerves, 244. 
 Embryonic ai'ea, 339. 
 
Index. 
 
 38: 
 
 Emmetropia, 285, 
 Fntoptic phenomena, 291. 
 Epiblast, 339. 
 Epiblastic sphere, 338. 
 Erection, 328. 
 
 centre, 251. 
 
 Erect vision, 293. 
 Enimcea, 65. 
 Eastacliian tube, 335. 
 Expiration, 59. 
 Extensibility of muscle, 1S2, 
 
 Faeces, 127. 
 
 False amnion, 342. 
 
 Falsetto voice, 313. 
 
 Far point, 230. 
 
 Fat, Formation of, 98. 
 
 Fats, 7. 
 
 Fecundation, 333. 
 
 Ferment of blood, 16. 
 
 Fibrin, 415. 
 
 Fibrinogen, 320. 
 
 Fibruio-plastic substance, 19. 
 
 Fick on excretion of urea, 163. 
 
 Field of vision, 273. 
 
 Fifth nerve, 235. 
 
 Fission, 324, 
 
 Food, Classification of, 78. 
 
 , Deprivation of, 95. 
 
 , Excessive supply of, 97. 
 
 , Insufficient sujiply of, 97. 
 
 , qnantitv required, 90. 
 
 yolk, 335. 
 
 Force of heart, 29. 
 Formic acid, 376. 
 Fourth nerve, 235. 
 Fruit, 85. 
 Fiinctions of nerves, 221, 
 
 Gamut major, 307. 
 
 minor, 308. 
 
 Gases of blood, 21, 69. 
 
 of body, 2. 
 
 of intestine, 128. 
 
 Gastric juice, 103. 
 
 , action on cane sugar, 114. 
 
 Gelatin, Digestion of, 113. 
 Gemmation, 324. 
 Generation, 322. 
 Genito-spinal centre, 251. 
 
 ui-inary appai'atus, 364. 
 
 Germ yolk, 335. 
 Germinal epithelium, 333. 
 
 spot, 337. 
 
 vesicle, 337. 
 
 Gin, 87. 
 Globulins, 3, 
 
 Glosso-pharyngeal nerve, 240. 
 Glottis, Function of, 311. 
 
 Glycerin, 376. 
 
 phosphoric acid, 377. 
 
 Glycogeny, 133. 
 
 Gmelins' test for bile, 119. 
 
 Goltz' Qiiarrversuch, 233. 
 
 Green food, 85. 
 
 Grey substance of cord, 248. 
 
 Gutturals, 314. 
 
 Haematin, 18. 
 Hssmatoidin, 19. 
 Hsemin, 19. 
 Hasmochromogen, 18. 
 Hasmoglobin, 17. 
 
 of muscle, 183. 
 
 Halones, 336. 
 
 Haughton on excretion of urea, 
 
 165. 
 Head-fold, 341. 
 Hearing, Cause of, 304. 
 Heart, Action of, 23. 
 
 , Impulse of, 27. 
 
 , Nervous mechanism of, 30. 
 
 , Sounds of, 26. 
 
 Heat, Animal, 143. 
 
 rigor, 189. 
 
 Height and weight, Table of, 371. 
 Hemi-peptone, 122, 
 Hepatine, 133. 
 
 Bering's theory of colours, 298. 
 Hiccup, 77. 
 Hiijpuric acid, 171. 
 Holoblastic ova, 336. 
 Homoio-thermal animals, 143. 
 Hyptermetropia, 2S7. 
 HyiJoblast, 339. 
 Hypoblastic sphere, 333. 
 Hypoglossal nerve, 244. 
 Hypoxanthin, 171, 377. 
 
 Idiomuscular contraction, 189. 
 
 Impulse of heart, 27. 
 
 Inanition, 95. 
 
 Indican, 172, 377. 
 
 Indigo, 377. 
 
 Indigogen, 172. 
 
 Indii-ect vision, 276. 
 
 Indol, 377. 
 
 Infancy, 370. 
 
 Inhibitory centre of heart, C3. 
 
 nerves, 226. 
 
 of heart, 31. 
 
 Initial contraction, 201. 
 Inorganic compounds, 2, 
 Inosinic acid, 378. 
 Inosite, 378. 
 Insalivatiou , 101. 
 
 centre, 252. 
 
 Inspiration, 58. 
 
386 
 
 Human Physiology. 
 
 Insufficient diet, 97. 
 Internal respiration, 75. 
 Intervals, Musical, 306. 
 Iris, 278. 
 Irradiation, 291. 
 
 Isotropous substance of muscle, 
 184. 
 
 Joints, 207. 
 
 Jugulars, Anterior, 352. 
 
 Keratin, 378. 
 Kreatin, 378. 
 Kreatinin, 171, 378. 
 
 Labials, 314. 
 
 Labyrinth, Function of, 303. 
 
 Lachrymal apparatus, 303. 
 
 - — ganglion, 237. 
 
 ±iacteal fluid, 131. 
 
 Lactic acid, 172. 
 
 Lardacein, 5. 
 
 Large intestine. Digestion in, 127. 
 
 Latent addition, 197. 
 
 period, 195. 
 
 Laughter, 77. 
 
 Leaping, 212. 
 
 Lecithin, 379. 
 
 Leguminous plants as food, 84. 
 
 Levers, 208. 
 
 Lieberkiihn's follicles, 125. 
 
 Liebig's test for la-ea, 161. 
 
 Light, Decomposition of, 297. 
 
 Liquor amnii, 342, 344. 
 
 Longsightedness, 287. 
 
 Lutein, 379. 
 
 Lymph, 54, 130. 
 
 Mai de Montague, 72. 
 Malic acid, 379. 
 Mammalian ovum, 337. 
 Marriotte's blind spot, 276. 
 Marsh gas, 127. 
 Mastication, 99. 
 
 Meat, Digestion of, 82, 113. 
 Mechanism of respiration, 65. 
 Meconium, 362. 
 Medulla oblongata, 251. 
 Medullary folds, 3iO. 
 Membrana tympani, 304. 
 Menstruation, 330. 
 MerobLastic ova, 336. 
 Mesoblast, 339. 
 Mesonephros, 364. 
 Metanephros, 364. 
 Methaemoglobin, 18. 
 Mictirrition, 177. 
 "— ^ centre, 177. 
 
 MHk as food, 88. 
 
 , Digestion of, 113. 
 
 sugar, 88. 
 
 Millon's test for proteids-, 6. 
 Moore's test for glycose, 7. 
 Motor nerves, 223. 
 Mucin, 379. 
 
 Mulder's test for glycose, 7. 
 Miiller's duct, 338, 365, 3Ct). 
 Muscle curve, 195. 
 
 plasma, 182. 
 
 serum, 183. 
 
 stimiih, 190. 
 
 Muscles of the eye, 301. 
 Muscular movement, 178. 
 
 sensibility, 319. 
 
 Musical sounds, 306. 
 Myopia, 286. 
 Myosin, 3, 184. 
 
 Near point, 280. 
 Negative after images, 301 
 
 variation, 193, 2lD. 
 
 Nerve, Tirst, 233. 
 
 , Second, 2.33. 
 
 , Third, 234. 
 
 , Toiirth, 235. 
 
 , Fifth, 236. 
 
 , Sixth, 239. 
 
 , Seventh, 239. 
 
 , Eighth, 240. 
 
 , Ninth, 240. 
 
 , Tenth, 241. 
 
 , Eleventh, 244. 
 
 , Twelfth, 244. 
 
 stimuli, 214. 
 
 Nerves, Functions of, 221. 
 
 , Inhibitory, 226. 
 
 , Motor, 223. 
 
 , Secretory, 224. 
 
 , Sensory, 223. 
 
 , Trophic, 225. 
 
 ■ , Vaso-constrictor, 226. 
 
 ■ , Yaso-dilator, 226. 
 
 , Vaso-motor, 226. 
 
 Nervous mechanism of heart, 
 30. 
 
 system, 212. 
 
 Ninth nerve, 240, 
 
 Nitric acid test for proteids, 5. 
 
 Nitrogenous compounds, 2. 
 
 Noises, 306. 
 
 Non-azotised compounds, 2. 
 
 Non-miisical sounds, 306. 
 
 Non-nitrogenous compounds, 2. 
 
 Notochord, 340. 
 
 Nuclein, 379. 
 
 Nucleolus of ovum, 337. 
 
 Nucleus of ovum, 3137. 
 
iNDEk. 
 
 387 
 
 Cblique muscles of the eye, 302. 
 Oculo-motor nerve, 231. 
 Old age, 372. 
 Olfactory nei've, 233. 
 Optic axis, 273. 
 
 reiTe, 133. 
 
 thalami, 255. 
 
 Optogxaios, 295. 
 Ossicula, 305. 
 Otic ganglion, 238. 
 Oviduct, 338. 
 Ovum of bird, 834. 
 
 of mammal, 337. 
 
 Oxalic acid, 172. 
 Oxahiric acid, 172. 
 Oxyg-eu of blood, 21. 
 Oxy-hsemoglobin, 17. 
 
 Pain, 818. 
 
 Pancrea=, Innervation of, 121. 
 
 Pancreatic juice, 122. 
 
 Parablastic formations, 340. 
 
 Paradox of Weber, 198. 
 
 Paraglobulin, 4, 19. 
 
 Paralactic acid, 379. 
 
 Parkes on excretion of urea, 155. 
 
 Parthenogenesis, 324. 
 
 Partial presence of gases, 68. 
 
 Parturition centre, 251. 
 
 Pepsin, 110. 
 
 Peptones, 3, 111. 
 
 Percussion sounds of chest, tJj. 
 
 Peristalsis, 115. 
 
 Pettenkofer's test for bile, 120. 
 
 Phenol, 173, 380. 
 
 Phosphenes, 292. 
 
 Physiology of new-boni child, 3G9. 
 
 Pietrowski's test for proteids, 6. 
 
 Pitch of sound, 308. 
 
 Placenta foetnlis, 346. 
 
 uteriua, 346. 
 
 Plasma of the blood, 19. 
 
 of r uscle, 1S4. 
 
 'Pleuro -peritoneal cavity, 341. 
 Pueumogastric nerve, 241. 
 'Poikilo-thermal animals, li3. 
 Pons varolii, 2.54. 
 Portio dura, 239. 
 
 intermedia, 239. 
 
 mollis, 240. 
 
 Positive after-images, 301. 
 Potatoes as food, b5. 
 Presbyopia, 285. 
 Pressor nerves, 243. 
 Pressiu'e in air passages, 67. 
 
 of blood, 86. 
 
 of urine, 177. 
 
 , Sense of, 315. 
 
 Primitive aortse, 851. 
 
 Primitive streaB:, 340. 
 Primordial ovules, 338. 
 Projection, Law of, 293. 
 Pronephros, 864. 
 Proteids, 3. 
 
 , Coagulated, 5. 
 
 — — , Digestion of, 111. 
 
 , Tests for, 5. 
 
 Protista, 1. 
 
 Protoplasm, 1. 
 
 Pi-oximate principles, 2. 
 
 Pseudo-nuclear bodies, 837. 
 
 Ptyalin, 108. 
 
 Pulmonic circulation, 23. 
 
 Pulse, 41. 
 
 , Frequency of, 28. 
 
 - — — , Tracing of, 43, 
 Purkinje's figures, 292. 
 
 images, 292. 
 
 Pyrocatechin, 380. 
 
 Quantity of the blood, 10. 
 
 Pain-water, 85. 
 
 Eapidity of conduction in nerves 
 227. 
 
 of reflex acts, 232 
 
 Eecti muscles of the eye, 301. 
 EeciuTrent sensibilitv, 224. 
 Eed corpuscles of blood, 11. 
 Eeflex action. Laws of, 231. 
 
 • acts, 230. 
 
 Eenuet fennent, 113. 
 Ee.sonators, 809. 
 Eespiration, 56. 
 
 , Cutaneous, IQ. 
 
 , Internal, 75. 
 
 , Movements of, 57. 
 
 , Sounds of, 66. 
 
 EesiDiratory nerve-centres, 64. 
 
 Ebtoacope, Physiological, 21;D. 
 
 Ehodopsin, 295. 
 
 Ei2-or mortis, 187. 
 
 Eit-er-Talii's law, 220. 
 
 Eiver-water, 85. 
 
 Eoots of si)inal nerves, 247. 
 
 Hum, 87. 
 
 Euuge's tost for sugar, 7. 
 
 Eimning, 212. 
 
 Eut, .333. 
 
 Eye as food, 84. 
 
 Saliva, Characters of, 101. 
 
 , Uses of, 1U2. 
 
 , Varieties of, 101, 102. 
 
 Salts of blood, 20. 
 
 of urine, 175. 
 
 Sarkin, 171, 880. 
 Sarkosin, 380. 
 
iSS 
 
 IJUMAN PhYSIOLOCV. 
 
 Schemer's experiment, 2S l. 
 Sebaceous matter, 142. 
 Secondary wave, 200. >■% 
 S'^cretory nerves, 224. 
 Segmentation of ovum , 338. 
 Self-digestion of stomach, 117. 
 Semen, 326. 
 
 Semicircnlair canals, 305. 
 Senihty, 378. 
 Sense of pressure, 316. 
 
 of smell, 321. 
 
 of space, 315. 
 
 of taste, 320. 
 
 of temperature, 317. 
 
 of touch, 315. 
 
 Sensibility, Mtiscnlar, 319. 
 Sensory nerves, 223. 
 Serum, 20. 
 
 albumin, 3. 
 
 Seventh nerve, 239. 
 
 Sexual generation, 325. 
 
 Sharpness of vision, 275. 
 
 Short-sightedness, 286. 
 
 Sighing, 77. 
 
 Silver test for glycose, 7. 
 
 Single vision with two eyos, 
 
 294. 
 Sinus terminalis, 351. 
 Sitting, 210. 
 Sixth nerve, 239. 
 Skin, Functions of, 139. 
 
 , Eespiration by, 76, 140. 
 
 , Transpiration through, 77. 
 
 Small intestine, 125. 
 
 Smell, Sense of, 321. 
 
 Smooth muscle, 182. 
 
 Snoring, 77. 
 
 Sodium sulphate test for proteids, 
 
 5. 
 Somatopleure, 341. 
 Sound of muscular coiitractiou, 
 
 193. 
 Sounds, Division of, 303. 
 
 of heart, 26. 
 
 of respiration , ^, 
 
 Space, Sense of, 315. 
 
 Spectrum of blood, 8. 
 
 Speech, 310. 
 
 Speed of blood-current, 46. 
 
 Spermatin, 327. 
 
 Spermatozoa, 327. 
 
 Spheno-palatine ganglion, 238. 
 
 Spherical aberration, 276. 
 
 Sphincter aui, 128. 
 
 Spinal accessory nerve, 244. 
 
 cord, 247. 
 
 Spirits, 87. 
 
 Splanchnic nei-ves, Action of, 125. 
 
 Splanchnopleure, 341. 
 
 Spleen, Functions of, 13!?. 
 
 Spring- water, 85. 
 
 Standing, 209. 
 
 Starch, 6. 
 
 Steapsin, 122, 124, 126. 
 
 Stenson's experiment, 1^9. 
 
 Stomach, Movements of, 115, llo. 
 
 , Nervous Bn]Dply of, 116. 
 
 Submaxillary gaijghon, 333. 
 Subzonal membrane, 342. 
 Succinic acid, 172, 380. 
 Suction centre, 253. 
 Sugars, 6. 
 
 Summation of stimuli, 197. 
 Suprarenal capsules, 138. 
 Sweat, 140. 
 
 centre, 251. 
 
 Sj'mpathetic nerves, 245. 
 Systemic circulation, 23. 
 Systolo of heart, 23. 
 
 Tail fold, 341. 
 
 Tapewoi-m, Development of, 325. 
 
 Taste, Sense of, 320. 
 
 Tea as food, 88. 
 
 Tears, 303. 
 
 Teeth, Order of eruption of, 103. 
 
 , Uses of, 99. 
 
 Temperature of the body, 144, 
 
 145. 
 
 , Sense of, 317. 
 
 Tenth nerve, 241. 
 Test for glycose, 6. 
 Tests for grape sugar, 6. 
 
 for proteids, 5. 
 
 Tetanus, Complete, 201. 
 
 , Incomplete, 200. 
 
 Third nerve, 234. 
 Thiry's fistula, 126. 
 Thymus, Functions of, 13S. 
 Thyroid, Functions of, 138. 
 Tim'.jre,of a note, 309. 
 Tongue, Movements of, 106. 
 Tonicity of muscle, 190. 
 Touch, Sense of, 315. 
 Training, Diet in, 94. 
 Transpiration of vapour by skin, 
 
 77. 
 Traumatic degeneration of nerves, 
 
 218. 
 Trigeminus nerve, 235. 
 Trochlear nerve, 235. 
 Trommer's test for glycose, 6. 
 Trophic nerves, 225. 
 Trypsin, 122. 
 
 Twelfth pair of nerves, 244. 
 Twin birth. Frequency of, 331. 
 Types of respiration, 62. 
 Tyrosin, 380. 
 
Index:. 
 
 3B9 
 
 tlitimate elemeuts of the bocly, 2. 
 Umbilical cord, 350. 
 
 ■ vesicle, 340. 
 
 TlniiDolar induced action, 217 
 Unstriated muscle, 182, 205. 
 Urea, Characters of, li/0. 
 
 , Composition of, 159. 
 
 , Excretion of, 162. 
 
 , Or.'gin of, 167. 
 
 , Separation of, ICO. 
 
 , Tests for, 159. 
 
 Uric acid, 168. 
 
 , Origin of, 170. 
 
 , Tests for, 169. 
 
 Urine, 1.55. 
 
 , Composition of, 156. 
 
 , Quantity of, 155. 
 
 Urobilin, 173. 
 Urochrome, 173. 
 Uroerythrin, 173. 
 
 Vagus nerve, 241. 
 
 , Influence of ,on stomach, 
 
 116. 
 Vaso-constrictor nerve?, 226. 
 
 ■ -dilator nerves, 226. 
 
 motor centre, 251. ' 
 
 nerves, 226. 
 
 Vegetable food, 84. 
 Veins, 51. 
 
 Velocity of blood current, 46. 
 Vena terminalis, 351. 
 Ventilation, 74. 
 Vemix caseosa, 143. 
 Vesico-spinal centre, 177 251. 
 Vesicular respiratory sound, 6j. 
 Visible diro'-tion, Law of, 293. 
 
 Vision, Field of, 273. 
 Vision, Sharpness of, 275. 
 Visual angle, 273. 
 
 line, 273. 
 
 Vital capacity, 63. 
 Vitellin, 3. 
 
 membrane, 335. 
 
 Voice, 310. 
 
 Vomiting centre, 252. 
 Vowel sounds, 313. 
 
 Wallrins', 210. 
 Water, 2, 85. 
 
 rigor, 189. 
 
 Wave of muscular contraction, 
 
 194. 
 Wheat as food, 84. 
 Wliisky, 87. 
 White substance of spinal cord, 
 
 249. 
 Wines, 87. 
 Wolffian body, 338, 366. 
 
 ^ duct, 366. 
 
 Woorara, 201. 
 
 Work done by muscle, 204, 207. 
 
 Xanthin, 171, 381. 
 Xantho-proteic acid, 5. 
 reaction, 6. 
 
 Young's theory of colours, 298. 
 Youth, 371. 
 
 Zoamylin, 133, 381. 
 Zona ijellucida, 336. 
 
 radiata, 337. 
 
 Zones of pancreatic cells, 123. 
 Zymogen substance, 123.^ 
 
 CASSEIL AKr COWPAlfY, LIMITED, BELLE SAUVAGE WOEKS, LOXDOS, E.C. 
 

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