■ ,■.■...-•■ ■■■■■■ ■■ .-.■ ■,.■■ ■ .: ■■ ■■: ^■■^,.Jf^v:*{':':-.;i*^/'i The Contemporary Science Series ALBERT R. MANN LIBRARY New York State Colleges of Agriculture and Home Economics AT Cornell University Gift from the CLIVE M. McCAY LIBRARY OF Nutrition and Gerontology Cornell University Library The original of tliis bool< is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003134271 The Contemporary Science Series. Edited by Havelock Ellis. I. THE EVOLUTION OF SEX. By Prof. Patrick Geddes and J. A. Thomson. With go Illustrations. Second Edition. 3s. 6d. "The authors have broiip;bt to the task— as indeed theii- names guarantee— a wealth of knowledge, a lucid and attractive method of treatment, and a rich vein of pictur- esque language."— ^V«(uir. II. ELECTRICITY IN MODERN LIFE. By G. W. de TuNZELMANN. With 88 Illustrations. 3s. 6d. 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" The book is a complete and wonderfully interesting exposition, and as such ouglit to receive a hearty welcome." — Scotsman, XXXIV. THE NATURAL HISTORY OF DIGESTION. By A. LocKHART Gii.i.ESPiE, M.D., F.R.C.P. Ed., F.R.S. Ed. With a large number of Illustrations and Diagrams. 6s. XXXV. DEGENERACY: Its Causes, Signs, and Rksults. By Professor EuGENE S. Talbot, M.D., Chicago. With Illus- trations. 6s. THE CONTEMPORAR Y SCIENCE SERIES. Edited by HAVELOCK ELLIS. THE NATURAL HISTORY OF DIGESTION. THE NATURAL HISTORY OF DIGESTION BY A: LOCKHART GILLESPIE M.D., F.R.C.P. Ed.7'F.R.S. Ed. LECTURER ON MATERIA MEDICA AND THERAPEUTICS IN THE MEDICAL SCHOOL OF THE ROYAL COLLEGES, EDINBURGH; MEDICAL REGISTRAR, ROYAL INFIRMARY, EDINBURGH ILLUSTRATED BY FIGURES, DIAGRAMS, AND CHARTS LONDON WALTER SCOTT, LTD., PATERNOSTER SQUARE //■5 r 352497 PREFACE. Civilised man habitually ill-treats his digestive organs. One who would never venture out on a chilly day without some extra covering to protect his lungs, seldom has any such regard for his stomach, nor hesitates irregularly and irrationally to introduce into this long-suffering organ all manner of articles fancied by his senses. The possession by man of a cultivated palate, of highly though delicately trained senses of taste and smell, and of an appetite peculiar to his species, which can be habituated to all kinds of unusual flavours and bizarre tastes, — in many instances with the development of a craving for them, — is not an unmixed blessing. Stimulation of the appetite with highly-flavoured foods diminishes the natural wish for food for its own sake, it tends towards living to eat, instead of eating to live. It is not only the wealthy and leisured who are guilty in this respect. The use of food accessories has largely extended throughout all classes. In the present volume an attempt has been made to describe in a brief compass the general laws governing digestive processes in all living bodies. A study of these processes as carried out in plants and in lower animals serves to indicate the modes and means by which living protoplasm in all vegetable and animal cells can most easily and economically procure a "Hving wage." The higher animals only differ physically from the lower in being formed of myriads of living cells which have as it were joined a " Trades Union " to enable them to assist each other in the most economical and thorough way, and have for the most part abandoned individualism. Should conditions exist under vi PREFACE. which the cells in any section of this "Trades Union'' are afforded an insufficient wage, their action resembles the "strike" of human members of similar bodies. As their income is given them in kind, not in money, the deficiency may take various forms and affect either one section of the cells of the body or the whole number present in it. For instance, should the cells of the stomach or liver be given too much to do, they often strike for shorter hours and longer holidays; if too little iron be available to pay the red blood corpuscles, or perhaps too much lost to them by leakage from the body, they very soon indicate their wish for a greater supply, or for lessened expenses. The food and air taken in are the paymasters, and if they are unable to grant the amount demanded, stoppage of work ensues; perhaps temporary, perhaps permanent. As cellular units are built up from the elements of the food accessible to them, and as the nature of the food influences their state of well-being, though not necessarily their physical and inherited form, animal and vegetable bodies, forming collective groups of cells, are as dependent upon the elements given them in food as any single-celled representative of individualism. No animal can change his generic form by altering his food, but he can modify, often very profoundly, the attributes of his component cells. But even this is not invariably the case, for some men develop great strength of mind and body on a vegetarian diet {plus milk and eggs), while meat always dis- agrees. In hot climates much meat is harmful, but experience teaches that the more meat eaten by the inhabitants of a temperate clime the greater is their brain power and physical prowess. The great nations of the earth exemplify this very clearly. I have perhaps stated in the text the case against the professional vegetarians — not against those induced or com- pelled to use such a diet from personal idiosyncrasies — in rather too dogmatic a fashion, but we have only to consider the dentition and structure of the alimentary canal in man, and to recall the very varied conditions under which he has from time PREFACE. VU to time lived, to be assured that the beautiful fantasies conjured up by such enthusiasts, in which man appears as a frugivorous and herbivorous animal shedding no blood to gratify his carnal appetite, have little solid basis for their support. The course of history has been and still is controlled by the diet of the nations. The present struggle for commercial supremacy is largely a matter of digestion. The British work- man, better fed and eating more meat, works more rapidly and more energetically than many of his continental confreres ; but working a shorter time and earning higher wages, his standard of diet and dietetic accessories is rising still higher and is exceeding the small margin of profit left by his more rapid working or greater skilfulness. However much the food of one man succeeds in developing his power of action above the capabilities of another subsisting on a poorer diet, there must be a point at which the shorter and shorter hours and the increasing pay of the first must become unable to produce the equivalent of the work of the other, who works longer hours for lesser pay, even though less skilful. The appetite grows with what it feeds on; the worker having once experienced the benefits of a generous diet, craves for more and more, not realising that a narrow line divides the diet best adapted for brain and brawn from the slightly richer diet, which yields to the body rather greater income than suffices to balance its current expenses, and may lower all the bodily functions to a point below that appertaining to the slightly under-fed but not ill-fed individual. But to return to the book itself : I take the opportunity of warmly thanking the Royal College of Physicians of Edinburgh for the leave granted me to prosecute original researches in their laboratory, and of expressing my indebtedness to Dr. Noel Paton, the superintendent, for much kind advice and guidance. In all cases the name of the authority for any dogmatic statement has been inserted in the text, and in the more interesting passages the reference given, but it was deemed of very little use to include a bibliography of all the Vlii PREFACE. works consulted, as such could not be complete without the addition of many unquoted papers, while those who desire to investigate the literature of the subject more fully can obtain every important reference to it from such comprehensive works as the Index Medicus, Schmidt's Jdhfbucher, or from the catalogues of any of the larger medical libraries. Much information may be derived from the perusal of such standard works as : — on historical subjects, Baas's History of Medicine and Withington's Medical History from the Earliest Times; on the physiology of plants, Sachs's different works, especially his Lectures on this subject (in German, Enghsh translation by Marshall Ward), The Natural History of Plants, by Marilaun (English translation by Oliver), Darwin's Carnivorous Plants, and Geddes's article on "CarnivorousPIants" in \h&Encyclopce.dia Britannica, appended to which is a full bibliography of the sub- ject ; on the physiological processes in animals, Meade Smith's Physiology of the Domestic Animals; the text- books on human physiology by Michael Foster, Stewart, and Waller; on physio- logical chemistry by Neumeister (in German), Bunge, Gamgee, and Hoppe - Seyler ; on diet and indigestion. Sir William Roberts's Diet and Digestion, Lauder Brunton's Disorders of Digestion, and Knight's Food and its Functions. From these books much information may be gathered, while Uffelmann and Munk's work on nutrition (German) will give some of the more technical details. The majority of the illustrations have been borrowed from previous works, to the authors and publishers of which I am deeply indebted. My thanks are due for this permission to Professor Schafer and Messrs. Longmans, Green & Co., for several figures from Quain's Anatomy; to Dr. Meade Smith, the author, and F. A. Davis, of Philadelphia and London, the publisher, of The Physiology of the Domestic Animals; to Mr. R. Francis Nesbit and the proprietors of the Strand Magazine, for permission to reproduce four plates ; to Mr. H. J. Veitch and the Royal Horticultural Society, for two figures ; and to Dr. Lovell Gulland, for the use of one photo- PREFACE. IX graph and four original figures of the microscopic appearance of the gastric epithelium and zymogen granules in the fish. The drawing showing pepsinogen granules in the gastric cells of the salmon is, I believe, the first representation of the actual peptic zymin ever published. The original has just appeared in a Report to the Fishery Board- of Scotland, re- ferred to on page 254. For valuable assistance when photo- graphing the movements of Droiera under difficult conditions I am indebted to Mr. J. Hume Paterson, assistant in the Royal College of Physicians' Laboratory, Edinburgh. Most of the charts are original ; one or two have been modified from diagrams previously published, the others con- structed from original data, or from data the source of which is in each instance acknowledged. A. L. G. Edinburgh, May 1898. CONTENTS. CHAPTER I. PAGE Ancient Theories of Digestion ... i In Animals: — Uncivilised Man — Demonology — Ancient Egypt •^The Hindus — The Chinese — The Greeks — Hippocrates — Coction — Plato — Dogmatic School — Aristotle — Alexandrine School — The Empirics — The Romans — Asclepiades — Galen — Mediseval Europe — Arabic School — Paracelsus — latro- chemical School — latro-mathematical School — The Eclectics — Animism — Nervous Pathology — Vitalism — Phlogistics — Brunonian System — Chronological Synopsis. In Plants: — Theophrastus — Dioscorides — Doctrine of Signa- tures — I'Ecluse — Linn^us — Hales — Ingenhouss — von Liebig and Boussingault — Schulze — Pasteur — Sachs. CHAPTER n. Digestion in Plants General Survey : — Chlorophyll — Formation of Starch — Pro- teolytic Ferments — In Seeds — Papain — Bromelin — Action of Roots. Vegetable Metabolism : — Proteids — Carbo-hydrates— Fats — General Principle — Change in Constructive Materials. Comparative Digestion in Plants ; — Bacteria — Form — Food — Lichens — Algie — Higher Plants — Cotyledons — Stores of Food — Annuals, Biennials, Evergreens, etc. — Plants flower- ing only after long periods. xii CONTENTS. CHAPTER III. FACE Carnivorous Plants 4^ Carnivorous Plants with Adhesive Apparatus— 2?wi»- phyllum lusitanicum. Carnivorous Plants which show Movements in the Capture of their ^^%\—Pingidciila vulgaris— %\x\K.\Mxe. — Digestion — Drosera TOfewa'z/o/za;— Structure— Inflection of Tentacles — Aggregation of Protoplasm — Digestion and Absorption— Digestive Power on Different Substances — Dioncea muscipida — Description — Mechanism — Sensitive Hairs — Aldrovanda vesiculosa. Carnivorous Plants with Traps for the Capture of Insects — Nepenthacem — Pitchers — Secretion — Cephalolus folliaclaris — Lalhraa squainaria — Bartsia Alpina — Utricu- laria neglecta — Insects not truly digested by ii—Geiilisea ornata — Heliainphora nutans — Sarracenia purpurea — Sum- mary — Authorities. CHAPTER IV. Digestion in Animals ... ... ... ... ... 8i Development of the Alimentary Canal. — Of the Asso- ciated Glands. Anatomy of the Alimentary Canal. — Mouth — Teeth — Tongue — Salivary Glands — Stomach — Intestines. CHAPTER V. Ferments and Ferment Action ... ... ... 109 Unorganised Ferments : — Action — Theories — Berzelius— Liebig — Fischer — Bunge — Tammann — Pohl — Common Characters — Zymogens— Ptyalinogen — Trypsinogen — Pep- sinogen — Rennet-zymogen — Other possible Zymogens — Glucoside-splitting Ferments — Myrosin — Emulsin. Organised Ferments:— Properties — Products — Unformed Fer- ments produced by Formed Ferments. CONTENTS. xiii CHAPTER VI. PAGE Food Elements 136 Carbo-hydrates : — Composition — Polysaccharides — Disacchar- ides — Monosaccharides. Proteids :— Composition — Properties — Classification — Scheme of Proteid Decomposition. Fats : — Composition — Properties. CHAPTER Vn. Digestive Processes in the Body 150 Action of Saliva on Starch — Dextrins — Maltose — Effect of Acids — Digestion in the Stomach — Production of Hydrochloric Acid — Maly's Views — ICoeppe — Gamgee — Acidity of the Stomach Contents — Determination — Variations — Peptic Digestion of Proteids — Relative Digestive Power of Acids — ■ Combination of Acids with Proteids — Causes of Simplifica- tion of Proteid Bodies. CHAPTER VIII. Digestive Processes (continued) ... ... ... 181 The passage of the food from the Stomach into the Duodenum — Intestinal Digestion — The Pancreatic Secretion — Historical — Characters— Ferments — Action on Proteids — The Liver — Bile— Other actions of the Liver — Digestion in the Small Intestine — Reaction of the contents — Absorption of fluids — Digestion in the Large Intestine. CHAPTER IX. Absorption from the Alimentary Canal ... ... 205 Heidenhain's laws — Proteids — Sugars — Salts — Fat — Osmosis — Vital action — Change in form of absorbed proteids — Course of absorbed material — Lymph duct— Chyle. XIV CONTENTS. CHAPTER X. PAGE The Micro-organisms of the Alimentary Tract OF THE Higher Mammals ... ... ... 213 Micro-organisms in the Mouth — Stomach — Intestines — Chemical Poisons formed by them — Cadaveric Alkaloids — Ptomaines — Leucomaines. CHAPTER XL The Influence of the Nervous System upon the Actions of the Digestive Organs ... ... 223 Influence on the Salivary Glands — Secretory Fibres — Pressure of Secretion — On Gastric Digestion — On Pancreatic Secretion — On the Biliary Excretion — Intestinal Digestion — Paralytic Secretion — Summary. CHAPTER XII. A Comparative Study of Digestion in Animals ... 235 Types of Nutrition — Vegetal — Saprophytic — Specialised — Protozoa — Amoeba — Foraminifera — Metazoa — Spongida — Coelenterata — Annuloida — ■ Arthrozoic Series — Arthro- poda — Crustacea — Polyzoa — Mollusca — Gasteropoda — Tunicata — Summary — Digestion in Pisces — Organisms in Canal^The Salmon — In Aves — In Reptilia — In Ruminantia — In other Herbivora — Vomiting — Digestive Leucocytosis. CHAPTER XIII. The Senses 279 Smell : — Mechanism — Acuity — Taste largely smell, Taste : — Different tastes — Minimum amounts of substances re- quired—Connection of sense of taste with habits and appetite. Hunger : — Nervous origin — Cause. Thikst: — Local sensations — Nervous origin — Connection with hunger. CONTENTS. XV CHAPTER XIV. PAGE Metabolism in Animals ... ... ... ... 288 Metabolism : — Definition — Food Constituents — Metabolism of Proteids^ Living and Dead Protoplasm — Urea — Seat of Urea Formation — In the Liver — Uric Acid — Hippuric Acid — Muscular Work and Metabolism of Proteids. Metabolism of Carbo-hydrates : — Glycogen — Diabetes Mel- litus — Oxidation of Sugar in Active Muscle — Formation from Proteids, from Carbo-hydrates. Metabolism of Fat. Inorganic Salts : — Iron — Metabolism during Starvation. CHAPTER XV. Dietetics and Animal Heat ... ... ... ... 321 The Ideal Physiological Diet : — Relation of Diet to Body Weight — Variations in Diet — Necessary Amounts of Oxygen . and Hydrogen — Diet in Young Animals. Animal Heat : — Amount — Source — Seat of Production. CHAPTER XVI. Stimulants 340 Alcohols : — Nature— Universal Use— Actions on the Body — Influence on Digestive Processes — On Saliva — Peptic and Pancreatic Digestion — Action of Fusel Oils. NON- Alcoholic : — Tea— Coffee — Cocoa— Coca. CHAPTER XVII. Foods 361 Food primarily sought for Subsistence only — Habits vary vifith Nature of Food — Economic Changes produced by the Cultivation of Food-stuffs by Man — And by his placing a Monetary Value on them — The Ideal Food of the Future — Vegetarians — Cost of Foods and Proportional Food-values — The Food of the Working Classes — The Foods of Physio- logical Diets — The Food of Herbivorous Animals — The Common Food Substances used by Man — Milk — Eggs — Bread — Meats — Fruits — Fungi — The Action of Heat on P'oods — Conclusion. LIST OF FIGURES. FIG. ^ TITLE PAGE 1. The Root-Hairs of Plants (Diagrammatic) . . 27 2. The Common Butterwort (Piiiguuula vulgaris) . . .47 3. 'Lea.i oi Drosera rotttndifolia . . . . .54 4- Do. do. five minutes after feeding with Proteid ....... 5. Leaf of Drosera rohindifolia twenty-eight minutes after feeding with Proteid ...... 6. Leaf of Drosera rotundifoKa twenty-eight hours after feeding with Proteid ...... 54 55 55 59 7. The Gland of a Marginal Tentacle of Drosera rohtndifolia. unfed ....... 8. A Gland and Stem of Marginal Tentacles of Drosera rotuudi- folia after feeding with Proteid . . . .59 9. Dionma mtiscipitla (Venus's Fly-trap) (Meade Smith, after Darwin) ..... 62 10. Nepenthes Edwardsiana (Veitch) ... 67 11. h.Y\i(^t\ho'ca Nepenthes Dicksoniana >^€\\.!i}a) . . .69 12. Utricular! a neglecta . . . . . .74 13. The Development of the Alimentary Canal (Quain) . . 83 J^ c Sections across the Upper Jaw of a Foetal Sheep (Quain) . 87 16. Vertical Section of Circumvallate papilla of the Tongue of the Calf (Quain) .... -92 17. Taste-Buds ....... 93 18. Section of the Parotid Gland of the Rabbit in an Inactive State (Meade Smith) . . . . . .95 19. Section of the Parotid Gland of the Rabbit after Stimulation of the Cervical Sympathetic Nerve (Meade Smith) . 95 20. A Gland from the Pyloric Region of the Stomach of the Dog (Quain) ....... 98 21. A Gland from the Cardiac End of the Stomach (Quain) . . 100 22. Glands of the Fundus of the Stomach in Man (Meade Smith) . loi 23. The Superficial Epithelium of the Gastric Mucous Membrane in Salmo salar (GuUand) .... 102 24. The Epithelium lining the Intermediate and Lower Parts of the Gastric Glands of Salmo salar (Gulland) . . 102 25. Micro-photograph of the Gastric Mucous Membrane of the Silino salar, showing Aggregation of Pepsinogen Granules (Gulland) ,,,.., 103 J? LIST OF FIGURES. 26. Cross Section of a Villus of the Small Intestine of the Cat (Quain) . . . . ... 104 27 The Pancreas of the Dog when Inactive during Fasting (Section) (Meade Smith, after Heidenhain) . . 122 28. The Pancreas of the Dog during First Stage of Activity (Section) (Meade Smith, after Heidenhain) . . . 123 29. The Pancreas of the Dog during Second Stage of Activity (Section) (Meade Smith, after Heidenhain) . 124 30. .A-cinus of the Pancreas of the Trout in the Resting Stage (GuUand) . . . . . -125 31. Acinus of the Pancreas of the Trout in the Active Stage (Gulland) 125 32. The Nerve Supply to the Parotid Gland (Meade Smith, after Yeo) . . . . . .224 33. Parania'ciiim bussaria (Meade Smith, after Stein) . . 239 34. The Hydrozoa (Meade Smith, after Jeffrey-Bell) . . 240 35. Diagrammatic Representation of the Digestive Organs in the Earthworm (Meade Smith, after Ray Lankester) . 242 36. The Stomach of a Salmon-Trout (Meade Smith, after Carus) . 252 37. The Alimentary Canal and Associated Glands in Birds (Meade Smith) ....... 256 38. Horizontal Section of the Gizzard of Goose (Meade Smith, after Jarrod) ..... . 257 39. 40, 41, 42. A Snake, Dasypeltis scabra, swallowing an Egg (Photographs by R. F. Nesbit). . . 260, 261 43. Vertical Section of the Rumen and Reticulum of the Ox (Meade Smith, after Colin) .... 264 44. The Rumen and Reticulum of the Ox (Meade Smith) . . 265 45. Outlines of the Stomachs of various Mammals, and of a Turtle (Meade Smith, after Thanhofifer) . . . 269 46. The Stomach of the Horse (Meade Smith, after Colin) . . 272 47. The Terminal Cells and Nerve-Fibres of the Olfactory Region (Quain, after Schviltze) ..... 280 48. The Udder and Nipple of the Cow (in Section) (Meade Smith, after Thanhoffer) .... 382 LIST OF DIAGRAMS. DIAGRAM TITLE PAGE I. The Relative Proportion between the Length of the Body and of the Alimentary Canal in Mammals. (Length of Body = I) . . . . . . 267 IL The Absolute and Relative Capacity of the Stomach and Intestine of Man and the Domestic Animals . . 274 III. The Proportions between the Surface Areas of the Body and Alimentary Canal in Different Animals . . 275 IV. The Loss of Weight of the various Organs after Death from Starvation, i. — Percentage of Total Loss borne by each Organ. (After Voit) .... 316 V. The Loss of Weight of the various Organs after Death from Starvation. 11. — Percentage Loss in each Organ . 317 VI. The Actual Amounts of Oxygen contained in the Three Principal Food Elements, and the amounts necessary for their full oxidation. The food contains 140 grammes of Proteid, 100 grammes of Fat, and 350 grammes of Carbo-hydrate material, and serves to maintain nitro- genous equilibrium ..... 329 VII. The Heat Equivalent of various Dietaries (drawn partly after Stewart) . . . . . -337 VIII. The Capacity of the Pluman Stomach at Different Ages . 365 IX. The Quantities of Food-Stuffs required to yield 20 grammes of Nitrogen and 300 grammes of Carbon, and the excess of one or other element present in each case . 371 X. The Relative Amounts of Fats and Carbo-hydrates, as com- pared with Proteids, in the Diet of Man and the Domestic Animals best suited for keeping up the body-weight. Proteid material represented as one gramme per kilogramme of body-weight. (Based largely on Meade Smith's data) . . . 376 XI. The Nutritive Value of the Milk of various Mammals com- pared with Cow's Milk ( = 100) . . . 384 XII. The Relative Proportions of Proteids, Fat, and Sugar in the Milk of various Mammals, Cow's Milk being taken as 100 ....... 385 LIST OF CHARTS. CHART TITLE PAGE I. The Effect of Temperature on the Activity of Salivary and Pancreatic Ferments (after Roberts) . • 1 1 7 II. Secretion of Pepsin . . . - ■ .126 III. The Quantity of Stomach Contents removed 144 hours after Food, with the frequency of the presence of Free HCl 159 IV. (i.) The Course of the Total Acidity and the Free Acidity of the Stomach Contents in Man after Proteids and after Carbo-hydrates, with Griitzner's data for the relative quantity of Pepsin added . . . .166 (ii.) The Variations in the Amounts of Pancreatic Juice in relative proportion, of its Solids and of the quantity of Bile in grammes, and of Sodium Taurocholate in deci- grammes, at different hours after Food . . . 166 V. The Relative Amounts of Pepsin and Diastatic Ferment excreted in the Urine at different hours of the day . 170 VI. The Spheres of Activity of the different Ferments, and the probable reaction of the contents in each section . 182 VII. The Quantity of Bile secreted after different forms of Food (after Barbera) ...... 195 VIII. The Percentage of Total Solids and Ash of the Filtered and Unfiltered Contents in different divisions of the Ali- mentary Tract of the Dog and Calf fed on Milk alone . 199 IX. The Changes in the Composition of Chyle in Ruminants (after Wlirtz) ....... 212 X. The Actual Number of Bacterial Colonies grown from Seg- ments of the Intestinal Canal of Salmon during different months ....... 217 XI. The Decrease of Solids in the Submaxillary Saliva of the Dog on prolonged Stimulation of the Chorda Tympani (Ludwig and Becker) . . . . 226 XII. Digestive Leucocytosis in Man : the Proportion of ^Vhite Corpuscles to Red throughout the Day (data from Hirt) 277 XIII. Digestive Leucocytosis in Man: Actual Increase in Numbers (data from Limbeck) ..... 27S XIV. Graphic Representation of the Daily Loss of Carbonic Acid and Urea, and of the Decline in Weight of a Starving Cat. (Bidder and Schmidt. ) In grammes . -312 XV. Graphic Representation of the Daily Loss of Urea (A) in a Fat Dog starved for 60 Days, and (B) in a Lean Dog starved for 24 Days (constructed from data by Falck) . 313 LIST OF CHARTS. XXI CHART XVI. XVII. XVIII. XIX. XX. XXI. XXII. XXIII. TITLE PAGE The Loss in Flesh and Fatof " Dog of lo Kilogrammes, fed on Lean Meat alone, in increaiing quanddes (after Voit) . . ■ . . . . 319 The Relative Action of Alcohol, Whisky, Sherry, Hock, and Ale on Salivary Digestion .... 347 The Relative Actions of Absolute Alcohol, Whisky, Sherry, and Ale on Peptic Digestion . . . .35' The Relative Actions of Absolute Alcohol, Whisky, and Sherry on Tryptic Digestion .... 353 The Relative Actions of the "so-called" Fusel Oils on Salivary Amylolysis (from data by Chittenden) . 356 The Relative Action (data from Chittenden) of the "so- called" Fusel Oils on Peptic and Tryptic Proteolysis . 357 The Increase in the Capacity of the Human Stomach, Small Intestine, and Large Intestine at different Ages . 363 The Heat Calories per o.oi gramme of different Vegetables, Grasses, and Fruit, and the Percentages of Proteids, Carbo-hydrates, and Fat contained in them . . 395 LIST OF TABLES. TABLE TITLE PACE I. Digestive Action of Bromelin in Acid, Neutral and Alka- line Media . . . . . .2.6 II. Facts regarding the Secretion of the Glands and the Movement of the Edges of the Leaves of the Fingiiicula vulgaris after Stimulation with various substances (compiled from Darwin) . . .49 III. The Time taken by the Leaves of the Pinguicula vulgaris to Dissolve and Absorb different Proteids . . S° IV. Absorption of Peptone by Pinguicula vulgaris . ■ 5 ' V. Showing the Results of Feeding Pinguicula vulgaris with various Proteids . . . . .5' VI. The Effects of Stimulation of the Leaves of Drosera rotundifolia by various substances (compiled from Darwin) . . . .62 VII. The Reaction of Drosera to Proteid and Proteid Deriva tives ...... VIII. The Different Characteristics of the Carnivorous Plants IX. Dental Formulae in Mammals X. The Average Weight of the Salivary Glands with thi Mean Quantity of Saliva Secreted XI. Electric Conductivity of Ferment Solutions (Nasse) XII. Analyses of Pepsin (Schomnow and Simanowsky) XIII. List of Unorganised Ferments: their Actions and Products XIV. To show the Points of Similarity and Dissimilarity between Unorganised Ferments, Organised Ferments, and Pro teids ....... XV. The Chronology of Ferment Action XVI. The Proteids ..... XVII. The Principal Native Proteids XVIII. The Reactions of Proteid Bodies to various Common Reagents ..... XIX. Table of Fats XX. Composition of the Adipose Tissues of Man and of the Domestic Animals .... XXI. The Amount of Solids found in Saliva per Mille . XXII. Composition of Saliva, per 1000 parts, in Man, Dog. Plorse, SheejD, and Cow XXIII. Soluble Starch ... XXIV. The Carbo-hydrate Group : from Starch to Glucose and Alcohol ...... 155 XXV. The Rapidity of Action of Pancreatic Juice (Roberts) . 157 XXVI. Composition of Gastric Juice (per mille) . . . 158 63 79 90 96 113 "5 118 130 135 141 144 145 148 148 153 153 154 LIST OF TABLES. XXIU TABLE XXVI L XXVIIL XXIX. XXX. XXXI. XXXII. XXXIIL XXXIV. XXXV. XXXVL XXXVII. XXXVIII. XXXIX. XL. XLI. XLII. XLIII. XLIV. XLV. XLVI. XLVII. XLVIII. XLIX. L. LI. LIL \ Schemes of the Peptic Digestion of Proteids 76 179 183 183 18s TITLE PAGE Acidity of Gastric Juice in Horse after different Foods (Ellenberger and Hofmeister) . . . 166 The Relative Action of Pepsin on Egg Albumin in the presence of various Acids — 1. Without Hydrochloric Acid .... 172 2. With Hydrochloric Acid added to each . .173 {:" Percentage of Hydrochloric Acid in its Compounds with Proteids ...... Rate and Proportional Secretion of Pancreatic Juice (Colin) ...... Composition of Pancreatic Juice (Schmidt and Hoppe- Seyler) ...... Digestion of Fibrin by Trypsin (Kuhne) The Products of the Action of Trypsin on Proteid Sub- stances, (A) in the absence of Bacteria; (B) with Bacteria present ..... Mean Percentage Result of Analyses of Bile in Man and various Animals ..... The Proportion of Bile secreted in twenty-four hours in various Animals to each Kilogramme of Body-weight and of the Weight of the Liver The Relationship between the Pressure of the Bile ex- cretion and that in the Superior Mesenteric Vein in the Dog (Heidenhain) .... Urotoxic coefficients of Urine before and after ligature of the Portal Vein (Roger) .... The Percentage of Food-Stuffs passed in the Fteces Composition of the Conlenls entering and leaving the Large Intestine ..... Analyses of Lymph and Chyle in Man and various Animals (per mille) ..... Composition of the Chyle in the Thoracic Duct (per mille) ...... Average Composition of Chyle and Lymph (based on Tables XLIL and XLIIL) . Numbers of Bacteria in Alimentary Canal of the Dog fed on Milk (Gilbert and Dominici) The better-known Animal Alkaloids Parotid Saliva obtained by Stimulation of Jacobsen's Nerve ...... Percentage Composition of the Saliva of the Cat, on Stimulation (i) of the Chorda Tympani, (2) of the Sympathetic ...... The Food of Bees (Griffiths) .... The Salivary Glands and Salivary Constituenls in the Invertebrata ..... The Organs capable of Digestion in the Invertebrata The Maximum Dilution at which the various Tastes are appreciable (data from Valentin) . . . 283 It 190 194 202 203 219 220 227 227 243 250 251 XXIV LIST OF TABLES. LIII. The Relative Accuracy in the Power of Distinguishing Bitter and Saline Substances in various Degrees of Dilution {data from Camerer) . . . 283 LIV. The Periods of Time, in Fractions of a Second, which elapse before the Brain perceives the Taste of various Substances placed upon the Tongue . . . 2S4 LV. The Molecular Composition of Proteids and of several of their Derivatives ..... 297 LVI. The Metabolic Changes of the Food Elements sufficing in Man to maintain a Nitrogen Balance when at Rest and at Work (after Voit) .... 300 LVII. The Influence of Muscular Work upon Proteid Metabolism (Krummacher) ..... 300 LVIII. Percentage Elementary Composition of Htematogen and Hemoglobin (Bunge) . . . . • 311 LIX. The Daily Changes in a Dog of 35 kilogrammes in weight when starved and then fed with increasing amounts of Lean Meat (after Voit) .... 318 LX. Classification of Food Principles .... 323 LXI. Composition of Food Elements . . 325 LXII. The Normal Metabolic Changes in the Nitrogen, Hy- drogen, Carbon, Oxygen, and Salts of a Diet capable of maintaining Nitrogenous Equilibrium in an Adult ...... 326 LXHI. Comparison of the necessary Amounts of the Food Elements with the Weight of the Body .... 327 LXIV. The Metabolic Processes in a Man of 71 kilogrammes given Food after twenty-four hours Fast (after Petten- kofer and Voit) ..... 332 LXV. Calories in the Food of an Adult per Diem (estimated by actual combustion) ... . 334 LXVI. Calories of Food absorbed, those for the parts passed in the fzeces, and in a not fully oxidised form in the Urine, subtracted (Rubner) .... 334 LXVII. Heat Equivalent of Food Elements . . . 334 LXVIH. The Heat Equivalent of the Specimen Diet . 335 LXIX. The Loss of Heat from the Body (Vierordt) . . 335 LXX. The Proportion of Alcohol and other Constituents in Si:)irits and Wines ..... 344 LXXI. The Effects of Alcoholic Liquors on Salivary Digestion (Roberts) ...... 3ij.6 LXXII. The Action of Brandy and Rum oil Salivary Digestion (Roberts) . . . . . .348 LXXIII. Analyses of New and Old Wines (Bertlielot) . . 34S LXXIV. The Constituents of Wine (Wynter Blyth) . . 349 LXXV. Proportion of Theine and Tannin in Teas (Dittmar) . 35S LXXVI. The Food Values of Certain Articles of Food in Terms of Grammes obtainable fora Penny . . . 370 LXXVII. The Weekly Expenditure of an Irish Labourer in 1835, with a wife and two children, receiving a wage of nine shillings per week (Report of Poor Law Commissioners) 372 LIST OF TABLES. XXV TABLE TITLE PAGE LXXVIIL Dietaries of Working Men in various Countries in 1835 (Senior) ...... 372 LXXIX. A Daily Menu for a Labourer, his Wife, and Eight Children, in London (Barnett) . . . 374 LXXX. Comparison of the Carbon and Nitrogen in Food Stuffs 377 LXXXI. Theoretical Diets containing 20 Grammes of Nitrogen per Diem ...... 377 LXXXII. Ideal Daily Diet (Waller) . . . .378 LXXXIII. The Proportional Amounts of the Constituents of Vege- table Foods digested by the Herbivora (Meade Smith) 381 LXXX IV. The Nutritive Value of Milk from Cows of various Countries ...... 386 LXXXV. The Nutritive and Heat-producing Value of Milk com- pared vifith various Articles of Food . . . 387 LXXXVI. The Composition and Nutritive Value of Hen's Eggs . 389 LXXXVII. The Nutritive Value, per cent.. Digestibility, and Com- parative Value of certain Meats . . . 392 LXXXVIII. The Nutritive Value and Digestibility of the Flesh of certain Fishes and Shell-fishes, and their Value compared with Lean Beef . . . 393 LXXXIX. The Percentage of Water and Nutritious Elements in certain Fruits . . . . 394 XC. The Composition of Mushrooms and Truffles . . 396 ERRATA. Page 103, line 9, foi- "aggregations" read "aggregation.'- .' 196, ,, 7, „ "glycoxylic" „ "glyoxylic.'' .. 256, „ 6, ,, "Canals" ,, "Canal." TEMPERATURE. To convert degrees Fahrenheit into degrees Centigrade, subtract 32, multiply by 5, and divide by 9. To convert from Centi^ade to Fahrenheit, multiply by 9, divide by 6, and add 32. Thermometric Scales. i Kiiocarorie"(of heat) =3069. 036 foot-pounds 1 foot-ton =310.00 kiIogramm6tres 1 Kilocalorie (of heat)=4'23.9 kilogramm&trea Missing Page Comparison of the British and the Metric Sys/em of Weights and Measures. WEIGHTS. Metric. 1 Millinramme (0.001 grm.) = 1 Centigramme (0.01grm.) = 1 Decigramme (0.1 grm.) = 1 Gr.amme = 1 Decagramme (10 grm.)- 1 Hectogramme (100 grm.): 1 Kilogramme (1000 grm.)= BBITISH. Bkitish. : 0.0154 grain - 0.1643 grain : 1.6432 grain : 16.432Sgrain.s a64.S235 grains : 1643. 236 grains or 3.6273 ounces avoirdupois 1 Hundredweight 35.273 ounces or 1 Ton 2.2046 pounds 1 grain 1 ounce Ifi ounces (1 pound) 14 pounds (1 stone)' Metric. = 0.0648 gramme = 23.3495 grammes = 463.592 grammes = 6.36 kilogrammes (6360 grammes) = 50.8 kilogrammes = 1016 kilogrammes MEASURES. Metric. British. 1 Micromillimj!tre = 0.00003937 inch 1 Centimetre = 0.39371 inch 1 .Mitre = 39.37079 inches 1 Kilometre = 39370.79 inches or 0.6213 mile To convert centimHres to inches, multiply by if. British. Metric. 1 inch = 2.6399 centimetres 1 foot = 30.4794 centimetres 1 yard = 91.4383 centimetres Imile = 1609.315 metres To convert inches to centimetres, multiply by 3.3_ VOLUME. Metric. British. 1 Centimetre cube = 0. 061027 cubic inch (Cubic centimetre) 1 Litre (1000 c.c.) = 61.027 cubic inches (36 fluid ounces, or 1.76 pint.) British. Metric. 1 fluid ounce =28.8968 centimetres cube 1 pint =567.9 centimetres cube 1 cubic foot =28.316 litres ENERGY (Approximate). Metric. British. British. Metric. 1 Kilogrammetre =7.24 foot-pounds 1 foot-pound=0.1381 kilogrammetre 1 Kilocalorie (of heat) =3069. 036 foot-pounds 1 foot-ton =310.00 kilogrammetres 1 Kilocalorie (of heat)=423.9 kilogrammetres THE NATURAL HISTORY OF DIGESTION. CHAPTER I. Ancient Theories of Digestion. In Animals: — Uncivilised Man — Demonology — Ancient Egypt — The Hindus — The Chinese — The Greeks — Hippocrates — Coction — Plato — Dogmatic School — Aristotle — Alexandrine School — The Empirics — The Romans — Asclepiades — Galen — Mediasval Europe — Arabic School — Paracelsus — latro-chemical School — latro-mathematical School — The Eclectics^-Animism — Nervous Pathology — Vitalism — Phlogistics — Brunonian System — Chronological Synopsis. In Plants: — Theophrastus — Dioscorides — Doctrine of Signatures — I'EcIuse — Linnffius — Hales — Ingenhouss — von Liebig and Boussingault — Schulze — Pasteur — Sachs. I. ANCIENT THEORIES OF DIGESTION IN ANIMALS. Any survey of the views held by mankind in the past on the subject of animal digestion and bodily nutrition must neces- sarily be synonymous with the history of medical science. The savage, or the rudely civilised man, cannot fail to speculate on the causes which underlie his sensations of hunger and thirst. The great business of his life consists in the search for food to satisfy the one, and for water to quench the other. He is, however, more impressed by any interference with the func- tions of the body than by the rationale of processes which in health give little sign of their existence. To him health is an abstract, disease a concrete idea. The majority of ancient medical records which have come 2 THE NATURAL HISTORY OF DIGESTION. down to us are largely occupied with references to diet and descriptions of imaginary causes of ill-health. In the absence of direct testimony as to the conceptions held by the ancients with regard to the manner in which the food they ate nourished their bodies, we are forced to rely on statements throwing a side-light on the general subject. The inhabitants of ancient Egypt believed that all diseases arose from food, and they followed precept by practice in taking a purgative and an emetic regularly three times a month. They regarded sensations of hunger and thirst as quasi-poisonous substances which forced themselves into the body, and required to be subdued, or neutralised, by the taking of food or drink to save the body from destruction. Even the dead were supplied with food to shield them from any inconvenience caused by such sensations. Apart from this, the Egyptians were careful of their digestion, observing strict dietetic rules, many of which can still be read in the Ebcrs Papyrus, probably the oldest medical book extant, dating from about 1550 B.C. The prevalent idea that diseases were due to the entrance of evil spirits, or demons, into the body is well illustrated in the Apocrypha by the story of Tobias. It is there related that Tobias, wishing before his marriage to exorcise a demon which had taken possession of his bride, fumigated her with the burning heart and liver of a fish. The smell thus arising drove the demon away to the furthest parts of Egypt ! This demonological view of disease can be traced in many parts of the biblical narrative, and has survived up to the present date among the ignorant in all countries. The demon was con- sidered of a sufficiently substantial form to require some other habitation after his expulsion, a process of reasoning which we may observe in the New Testament story of the herd of swine. A curious instance of similarity in ideas at very different periods is afforded by the supposition of the Hindus that the demon of a disease such as jaundice, causing a yellow colour- ation of the skin, sought a bird of yellow hue to inhabit after its expulsion from the body, a supposition which was closely paralleled by the much later doctrine of " Signatures." The authors of the great epic poems of the Hindus have given evidence, however, of the existence of other theories related indirectly to demonology, whereby principles or humours were supposed to reside in the body and to direct ANCIENT THEORIES. 3 the natural digestive processes. Tiius the Aryan writers recognised three principal humours : — Fata, situated between the feet and navel, or Wind. Pitta, between the navel and heart, or Bile. Kafa, between the heart and top of the head, or Phlegm. The principle, Kafa, was predominant during the period when the food was in the mouth; Fitta, during the course of digestion; and Vata, after the food in the stomach was fully digested. If from any cause these principles became unduly prominent, the condition of the bowels was affected. Thus when Vata exercised too strong an influence the bowels were costive; when Pitta, or the bile, was in the ascendant, they were loose, while they acted normally when Kafa was the most powerful. These writers describe also a Pachaka, or fire of digestion, situated between the stomach and small intestine, which assisted digestion and imparted heat to the body. Pachaka was able as well to separate the nourishing portions of the food from the dejecta. By some this body was looked upon as being identical with Pitta, or bile, and the author of the Rava-pradipa imagined it to be a minute heating substance situated in the centre of the navel. Another principle was supposed to reside in the liver and spleen. This Ranjaka imparted redness to the essential fluid of the body, changing it into blood. The changes which foods and medicines underwent from contact with the digestive fire were termed their Vipaka. They were thought to become decomposed, although often recognisable in other forms, and their medicinal effects greatly modified. The actions described under the name of Vipaka were evidenced by changes of taste. Thus the six kinds of tastes were, sweet, sour, pungent, saline, astringent, and bitter. The first three remained unaltered, and each characterised a Vipaka. Sweet Vipaka changed saline substances into sweet, promoted phlegm, and lessened wind and bile. Sour Vipaka in- creased bile, and diminished wind and phlegm. Pungent Vipaka altered astringent and bitter substances, giving them a pungent taste, gave rise to disorders of wind, but subdued the principles of bile and phlegm. That is to say, the change undergone at the hands of the digestive fire under the influence of siveet Vipaka increased Kafa, or phlegm, situated in the body above the heart; when sour Vipaka acted. Pitta, or the bile, placed 4 THE NATURAL HISTORY OF DIGESTION. between the navel and the heart, caused symptoms of disease or discomfort; while pungent Vipaka increased the prin- ciple resident below the navel, Vata, or wind. The external elements which brought about the changes in the principles were five in number, ether, fire, air, earth, and water. The literature of the Hindus abounds with references to medicine and the proper treatment of disease. The Rig- Veda (2000-1000 B.C.) and the Atharva-Veda (700 B.C.) contain many passages relating to herbs, diseases, and healing waters, but are mainly concerned with incantations and invoca- tions for use against the demons of disease, or for the help of the deity presiding over the organ affected. Their dietetic rules were extremely minute. Meat was generally forbidden. Among the Chinese the medicine and physiology of yester- . day are the same as that of to-day, and probably will be the same to-morrow. Physiological science is at its lowest among them. The principle of " moisture " is located in six chief organs, the heart, lungs, spleen, liver, and the two kidneys ; " warmth " in the stomach, small and large intestine, gall- bladder, and urinary apparatus. These two fundamental principles, warmth and moisture, whose union is life, whose separation is death, are associated in the Chinese mind with two elements, wood and metal, analogous to the fire and earth of the Greeks. The blood flows out from the lungs five times daily, terminating in the liver. They regard the bile as the seat of courage ; the spleen, of reason, and, with the assistance of the heart, of ideas ; the liver as the granary of the body, and the stomach as the abode of the mind ! From an early period the stomach was generally recognised as the chief organ concerned in digestion, and the earliest writers on medical subjects paid much attention to it. The Greeks of antiquity supposed that food putrified, or was cooked, in it. Anaxagoras of Clazomenre (500-428 B.C.) imagined that the body by means of affinity made use of the results of this di- gestion by appropriating those portions akin to itself The great Hippocrates favoured the idea that gastric digestion was a process of coction, in which heat brought about the necessary changes in the food. He (460-377 b.c.) was the forerunner of what is called the Dogmatic School, or, as Galen said, the founder of the Rational School, the legitimate ancestor of modern scientific medicine. This school held that the science of medicine should be based upon the study of anatomy ANCIENT THEORIES. S and physiology. If its members had not attempted to rear a complete structure of medical science before obtaining a suffi- ciently sound foundation of physiological data on which to rest it, they might have arrived even at that early time at results not far removed from those of the present day. Hippo- crates separated the active agents of the body into four principal humours, blood, phlegm, black and yellow bile, as other Greeks had done before him. Heat, cold, dryness, and moisture were their corresponding qualities. Heat, due to the blood, caused digestion of the food. The liver formed blood and bile, and assisted the process of coction in the stomach by increasing its heat. To certain foods were ascribed qualities corresponding to the four humours, and such foods were recommended for maladies due to excess or defect of humours. Dietetics, as a science, was founded by Hippocrates. The following extract from Baas, in Handerson's translation (New York : Vail & Co., 1889, p. 102), sums up his views on this subject : — " This science regarded the age — 'Old persons use less nutviment than the young,' — the season — ' In winter abundant nourishment is wholesome ; in summer, a more frugal diet,' — the bodily condition — ' Lean persons should take little food, but this little should be fat ; fat persons, on the other hand, should take much food, but it should be lean,' — the habits, etc. In addition, respect was paid to the easy digestibility of food — ' white meat is more easily digested than dark ' — and to its preparation. Water, barley- water, and wine were recommended as drinks. Baths, inunction, gym- nastic exercises, and the frequent use of emetics were also commended as dietetic measures, and the dietetic principles of Hippocrates in febrile diseases are substantially observed at the present day." Immediately following Hippocrates came Plato the philo- sopher (427-347 B.C.). As so often happens, the enunciation of a rational doctrine by Hippocrates tended to cause a re- action, through which men were led to seek for the under- lying principles of physiology and disease, not by the study of facts, nor the evidence of the senses, but in purely mental speculations. Reflection ranked higher than experience. The hold which this method of solving physical problems obtained on the physicians and thinkers of that time put back the advance of physiological knowledge by a period only to be reckoned by centuries, Plato, for instance, gravely propounded the theory that some of the fluid drunk passed through the wind-pipe into the lungs, and there served to cool the heart. Taste, he said, was due to the conveyance of soluble, sapid 6 THE NATURAL HISTORY OF DIGESTION. atoms to the heart and soul by small vesselsj the intestinal tube was long and tortuous, so that the food might not have to be renewed too often and thus incommode the contempla- tions of the soul; the spleen was the abode of blood impurities, the liver of the lower desires. Based upon Plato's fantastic and speculative scheme of humoral physiology and pathology, which disregarded the ana- lytical portion of the scheme of Hippocrates, was founded the Dogmatic School. Praxagoras of Cos, one of their number {circa 335 B.C.), specified no fewer than eleven humours. He regarded digestion as the outcome of putrefactive processes, and recorded the first known case of bulimia, or inordinate appetite. His treatment of intestinal obstruction, as told by Caelius Aurelianus, exhibits evidence of a very correct knowledge of the functions of the bowel. He recommended purgatives, enepiata, emetics, injections ol mx per asium, or, if these failed, massage of the abdominal walls, and finally laparotomy, with division and suture of the intestine. Aristotle (384-321 B.C.), in addition to philosophical works, wrote many valuable treatises on natural science. He was the first comparative anatomist and physiologist. Although his influence among physicians and medical scientists was not so great as that of Plato, he did much to break down the excessive idealism current in the latter's system, and to bring back thought into more realistic grooves. Plato's system, however, appealed more strongly to men's imaginations, and long served to dominate the truer science of Aristotle. Like the " Father of Medicine," he regarded digestion in the light of coction in the stomach, the resulting chyle proceeding to the heart. He looked upon the blood as the medium by which the nutritive material for the growth and warming of the body is conveyed through the vessels, containing in its normal condition neither mucus, black or yellow bile, nor water. Herophilus {circa 335-280 B.C.) and Erasistratus {ci>-ca 330- 280 B.C.) were the founders of the Alexandrine School, and the most celebrated members of it. They, however, differed widely in their tenets. Herophilus discovered the chyle duct, the lymphatics, and the duodenum, which he named, and described the liver accurately. He was a follower of Hippocrates and of the humoral doctrine. Erasistratus also was acquainted with the lymph and chyle vessels, which he said contained air and milk ANCIENT THEORIES. 7 alternately. He, however, discarded the humoral system, and propounded one of his own, based upon the theory that the arterial vessels contained "vital spirits." He ascribed diges- tion to the result of friction of the food between the stomach walls (see latro-mathematical School, p. 12), nutrition to the addition of new particles, and secretion to the action of what he termed the non-attractive force. He thought that the bile was useless, the spleen superfluous, and showed that fluids did not enter the air passages. The Empirics, among whom was Heraclides (230 B.C.) of Tarentum, despised anatomy and physiology, saying with Serapion, "It is the cause, not the cure of diseases, that con- cerns us ; not how to digest, but what is digestible." Among the few fragments of a book written by Heraclides, On Diet and Health, we find such suggestions as that it is always well to eat a little before drinking, and that sheep's trotters, snails, and other glutinous substances will cause dyspepsia if eaten to excess. Few records have come down to us of the medical opinions current among the inhabitants of early Rome. They believed that all the organs of the body were supervised by special divinities, who could be appealed to if anything untoward occurred. Asclepiades of Prusa (about B.C. 200), a Greek physician who had studied in Alexandria and Athens, was the first physician of importance. He introduced a new doctrine into physiology, founded on the atomic philosophy of Democritus and Epicurus. He looked upon the body as composed of atoms, with canals or " pores " between them in which still more minute atoms circulated. Disease was caused by changes in their relationship, and by blocking of the pores. Of Asclepiades and his views on digestion Celsus notes, " And there are rivals of Asclepiades who suggest that all these [pores] are useless and superfluous; for nothing is digested, but from the crude material, such as is ingested, everything in the body is derived." It is probable, however, that Cicero expresses v/ith more truth the opinions of Asclepiades, whose personal friend he was, in the following words : — " In the alimentary canal many wonderful processes take place; . . . the bowel also, owing to its complex and tortuous form, holds together and arrests the material introduced, whether it be dry or moist, and thus 8 THE NATURAL HISTORY OF DIGESTION. it is able to alter and digest it; then contracting and relaxing it, curdles and mixes everything it receives, so that both by the action of heat, of which it has much, by disintegration of the food, and further, by the aid of the 'spiritus,' all is distributed throughout the body, concocted (prepared) and digested." The most famous of all the Roman physicians, Claudius Galen of Pergamus (a.d. 131-201), is worthy of more particular notice, for the world during more than a thousand years followed, more or less implicitly, his teaching and doctrines. He is classed by some as belonging to the Dogmatic School, by others among the Eclectics. He himself denied any asso- ciation with the Eclectics. Following Hippocrates and the Alexandrines, he emphasised the great importance of the study of anatomy and physiology, and as he performed many experi- ments in both of these sciences, his opinions are of great interest. He regarded bodily metabolism as the outcome of the actions of the four elements, water, air, fire, and earth, to which pertained the qualities of moisture, coldness, warmth, and dryness respectively. Thus water constituted the major part of mucus (cold and moist), which was chiefly formed by the brain; fire predominated in the yellow bile (warm and dry) originating from the liver, earth in the black bile from the spleen (cold and dry), while the blood contained the four elements equally mixed (warm and moist). Digestion of the food was brought about in the stomach by means of coction; the results of digestion went to the liver, where they became blood (an error which persisted up to the seventeenth century). The blood, on reaching the heart, was driven by part of that organ to all portions of the body, and was almost entirely used up in transit in their nutrition. There can be no doubt that the extra- ordinary error made by Galen in the ascription of the formation of the blood to the liver, and the origin of the veins to the same organ, hindered him from discovering the circulation of the blood. A convincing token of his acuteness is afforded by his suggestion that respiration serves to heat the blood, not to cool it, as was generally believed in his time, while he even theoreti- cally discovered oxygen by postulating the probable existence of an element in the air which might be the source of this heat. Aretaeus of Cappadocia (a.d. 30-90), an earlier member of the same school, believed that the blood was prepared in the liver. ANCIENT THEORIES. 9 the bile in the gall-bladder, and that a secondary digestion took place in the large intestine.^ That even Galen, the chief authority in medical science for over a thousand years, had peculiar ideas about the nature of digestion is shown by his statement to Marcus Aurelius, who was suffering from dyspepsia. When asked what was amiss he explained that he suffered from a digestive disorder due to the food he had eaten, which required to be turned into phlegm before being excreted. Little of interest or of value can be gathered from the writings of those who followed Galen. For a period to be measured by centuries the cur'rent opinions were only variants of Galen's views. In the dark ages of mediseval Europe the tendency of all knowledge was rather to recede than to advance; indeed, the influence of Christianity tended to hasten the decline of medicine rather than advance its progress. The majority of the treatises which have come down to us from that time deal mainly with diet, ofteri in what may seem to us ludicrous ways. Two of the physicians who flourished in Constantinople in the eleventh century are, however, worthy of mention. Demetrius Pepago- menus wrote an excellent monograph on gout, in which he ^ The following rock inscription, found at Epidorus, and quoted by Miss Jane Harrison (Brit. Med. Journal, 17th July 1897), affords a good example of contemporary tenets: — " M. Julius Apellas, who was suffering from many complaints and from attacks of indiges- tion, was sent for by the god, but on the journey when I was at Arginia he bade me not be so nervous about myself. And when I was come to the precincts, he bade me go about for two days with my head covered. I was to eat bread and cheese, a salad of celery with lettuce. I was to attend to myself in the bath and to drink lemonade, rub myself against the wall of the bath, take e.\ercise in the loggia, practice on the trapeze, rub myself with sand, and go barefoot, pour wine into the hot water of the bath before I got in, bathe myself alone and give the bath attendant an Attic drachma, making offerings to Asklepios and Hpione and the Eleusinian goddess in common, and drink honey mi.xed with milk. And one day when I was drinking milk alone the god said to me, ' Put honey in your milk that you may be easier of digestion.' When I prayed the god to heal me more quickly I felt as if I was rubbed all over my body with salt and mustard, and was going from the shrine to the baths, I saw a youth with a smoking censer, and the priest said: 'Your cure is accom- plished, and now you must pay your fee.' I stayed on still longer and he bade me use anise and oil for my headache, but just then I had no head- ache ; however it happened that from working too hard I got a rush of blood to the head. I used the oil and my headache went. He ordered me to gargle with cold water for a swelling in the glands. He ordered me also to make up the record of this. Cured and grateful I have departed." 10 THE NATURAL HISTORY OF DIGESTION. declares that it should be treated by diet rather than by drugs; a method which, as he confesses, is easier to advise than to adopt. John Actuarius recommended that ointments with purgative ingredients should be rubbed into the skin, showing that he knew of the central action of many of these substances, although it is doubtful if he understood their mode of action when thus applied. Among the celebrated leaders of the Arabic School of Medi- 'cine we find Isaac Ben Solomon (850-950), who lived in Egypt and wrote many highly prized works. A quotation from his work on Diet may prove interesting. " The flesh of pigs is most nutritious; it forms a good chyme, and by its humidity and viscosity preserves the moisture of the stomach. It is diuretic and not suited to those who eat little. It is useful for those of hot and dry temperaments, for they readily digest and thrive upon it ; but for those of an opposite complexion and of weak digestion it is noxious, generating evil humour and a viscid phlegm, which may produce gout, lumbago, sciatica, renal stone, and paralysis. The flesh of old and decrepit pigs is most unwholesome; it is hard, woody, in- sipid; those who habitually eat it fall into melancholy and hectic fevers " (quoted by Withington, Medical History from the Earliest Times, p. 149). Another physician of this school, Averroes, the son of the great Avenzoar, gives in one of his books the following philo- sophical description of barley-water and its uses : — " Barley bread is inferior to wheaten, but is cooling and readily diges- tible, and its coldness is of the first degree. Barley-water is more medicinal than the bread; it is excellent in hot and dry diseases, since it cools, moistens, tempers and wonderfully generates a laudable humour. Nor does it inflate or remain in the stomach; all this we have learnt from experience" (Withington, loc. cit.). Then follows a minute description of the proper mode of preparing it. Some peculiar statements on diet by a much later Arabic physician, Arnald of Villanova (1235-1312), may be quoted here to afford an idea of the advice given in those days to patients. " Let him take moderate exercise before eating, and rest entirely after it, till the food has left the stomach, and then ride horses or gently trotting mules." " He may sleep soon but not immediately after eating, and should lie first on the left side that the food may descend to the fundus of the stomach. ANCIENT THEORIES. II and then on the right that it may pass on.'' He may eat among other things pickled pork, but only rarely. In winter and spring he should take two meals a day, but three smaller ones in summer. He should never eat until he has fully digested the previous meal, " for nothing is worse than to add indiges- tion to indigestion." At the dawn of the Renaissance physiology and other medical sciences again began to develop, although they were among the last of the sciences to be affected by the revival of learning which began in the middle of the fifteenth century. Perhaps we may say that nearly a hundred and fifty years elapsed from the year 141 3 — when a long-lost book of Celsus, De Medidna, was discovered, and started a spirit of research into the history of ancient times — before the sciences of physiology, chemistry, and medicine really felt the influence of the age of reformation in its entirety. Theophrastus von Hohenheim, or Bombastus Paracelsus, as he is otherwise called (1493-1541), was one of the best known of the physicians of that period. We may only note in passing, however (for his reputation was much higher than his merits), that he looked upon digestion as carried out by a presiding; force in the body, which he termed its "archeus." This force separated the nutritious from the poisonous ingredients of the food, and helped to absorb the nutriment. If the " archeus " failed in his duty, or if the excretory organs were unhealthy, the poisonous part of the food remained in the body, meeting with "a spirit of coagulation," which caused its deposition as tartar on the teeth, or as gouty deposits in the joints. The archeus of Paracelsus seems to have been a reminiscence of the demonology of the ancients, and to have caused the digestion of food solely by means of heat. Johan Baptista van Helmont (1577-1644) was the first to look upon digestion as a process ol fermentation. He, indeed, bound up his theory with the doctrine of material life in such a way as to appear ludicrous to us, but he deserves credit for being one of the earliest to break through the artificial fabric of medical science erected by Galen and his successors. To him the whole body was under the influence of an " archeus influus," or vital principle, which resided in the stomach, while every organ had its " archeus insitus " directing the processes natural to it. During the process of digestion in the stomach the local " archeus " generated a ferment whereby 12 THE NATURAL HISTORY OF DIGESTION. an acid was produced to dissolve the food. This " fermentum acidum " caused the necessary changes to take place in the food, not by means of heat, but by some other vital action. Van Helmont was aware that acids alone are unable to act as digestive agents, but require the additional aid of a ferment. He was unable to explain this action in a satisfactory manner, but summed up the properties of his ferment as follows : — "A digestive ferment, therefore, has the essential power, by reason of its vital acidity, of causing transmutations." Such men as Sylvius, Descartes, and Willis belonged to this school, and identified gastric digestion with fermentation. Nehemiah Grew (1628-1711) wrote shortly on digestion at the end of the seventeenth century, and notices the glands in the mucous membrane of the stomach. The following quota- tion from a lecture by him is given in Gamgee's Physiological Chemistry: — " By the joynt assistance of the glandulous and nervous membranes the business of chylification seems to be performed. The mucous excrement provided by the former, as an animal corrosive, preparing, and the excrement of the nerve by the latter, as an animal ferment, perfecting the work." Following the inconclusive attempts of the chemical school to attribute natural processes to purely chemical operations, another school of physiologists arose. The latro-mathematical School based the phenomena of digestion and other physio- logical processes upon the action of mechanical laws, in some cases admitting the co-existence of chemical changes. Their study of the mechanical attributes of the different parts of the body resulted in many and lasting additions to science, even although many of their notions were extremely extravagant. Borelli (1608-79) and Redi (1626-98) may be mentioned as leaders of the school, the latter of whom studied the mechanism of the gizzard of birds very carefully. The mechanical action of the gizzard probably suggested the idea of a similar action by the gastric walls in man. As an example of the extraordinary lengths to which disciples of the latro-matheraatical School ventured to go, I may cite the argument used that, as the pressure capable of being excited by the human thumb is very considerable, and as the super- ficies of the stomach wall is much more extensive, so the pressure which can be exercised by the coats of that organ is correspondingly greater. Dr. Pitcairn estimated this power at 12,951 pounds, or about five tons, and ascribed gastric digestion ANXIENT THEORIES. I 3 to this triturating force alone, a force greater than is generally applied by large millstones. In his essay he states that this explanation is quite sufficient, and that there is no need to call in "the assistance of a Daemon or a Stygian Liquor." The Eclectic System originated with Hermann Boerhaave (1668-1738), who combined in it many of the doctrines of earlier systems. Among other tenets, he held that the acridities of the body fluids played an important part in the functions of life. There were acid, saline, oleaginous, glutinous, alkaline, and mixed acridities originating from the food and causing disease. Digestion of food was ascribed to mechanical forces.^ Under the name of Animism, Georg Ernst Stahl (1660-1 734) ascribed the force used in digestion, along with all the otherbodily processes, to the "soul," or supreme principle. It was not the same as the spirit, but the special life-giving and life-preserving principle of the body. William Cullen {1712-90), reasoning by pure deduction, laid the foundation for modern Solidism in his system of Nervous Pathology. Disregarding all the fanciful principles and forces so often stated to exist in the fluids of the body by the authors of earlier systems, he founded his system on the living solid parts, aided by the nerves. The presiding force over the bodily processes he called "the nervous principle." Another system, that of Vitalism, was introduced by Bordeu, of Montpellier (1722-76), and supported by Barthez, of the same school (1734-1806). In this system the "vital principle" is the cause of all the phenomena of life, carrying on the digestion of food, muscular force, the production of heat, and the mental processes. It is diametrically opposed to our modern conception of force and matter. The Phlogistic Theory merely treated of animal heat, but one of its supporters, Edward Rigby (1747-1821), held that ' Me.iicina Musica, or a Mechanical Essay on the Effects of Singing, Mustek, and Dancing on Human .Saa'/Vj, by Richard Broune, "Apothecary in Oakham," 1729. Of singing, after dwelhng on its efficacy in chasing away "darlc, gloomy ideas," he says: "Singing must mightily conduce to the Dissolution of the Aliments and Prevention of Crudities and Indiges- tions, the ill consequences of which are too well known to want Demonstra- tion ; the Diaphragm also and abdominal Muscles, by this Exercise compressing the Stomach with more force than ordinary, give more Assistance to the Stomach in Comminuting the Food into Chyle, and rendering it fit for its Translation into the Blood." 14 THE NATURAL HISTORY OF DIGESTION. the stomach was the seat of "phlogtstification,' where free heat was produced. The Brunonian System (John Brown, 1735-88) was based on the supposition that life was an artificial result of irritations always in action. Irritations cause excitement, to which all the processes of the body are due. They are of two kinds: external, as food, blood, warmth, etc. ; internal, as thought, feeling, and movement. Such were the different systems and theories formulated from time to time by the greatest physicians, scientists, or philosophers of their day. At the present time we are unable to appreciate at their true value the circumstances under which they were formulated, the advantages or disadvantages attendant on their conception. But it must remain an element of sur- prise and disappointment that the views current up to the close of last century should show so slight an advance on those formulated, with less particular knowledge, two thousand years before. Nor are any of the systems propounded in the days of Hippocrates and his successors less fanciful than many seriously suggested by men of attainment during the eighteenth century. A no less surprising fact is the immense advance which has been made since the beginning of the nineteenth century in the elucidation of physiological problems. Coincident with the onward progress of science our knowledge of the chemical and physiological processes which serve to support life has increased. Thanks to the patient investigation of the infinitely little, and to the gathering together of the results, theories of vital processes can now be based on multitudes of observed facts, giving both positive and negative evidence. By following the teaching of Hippocrates in 400 b.c, teach- ing which was until lately entirely disregarded, or at best spasmodically and superficially followed, we have penetrated many of the mysteries connected with bodily processes, though much still remains to be done. The outcome of more than two thousand years of inquiry into the manner in which the food is altered in the body so that the nutritious part may be used up and the useless portion cast out, and into the processes by which the nutriment is built up into the various solid and fluid constituents of the organism, may be summed up in one sentence. The active digestion of nutritive substances results from inti- mate chemical and physical changes brought about by the action ANCIENT THEORIES. I 5 of a force possessed by the living protoplasm of cells of which we as yet know nothing, but which constitutes the most intimate element underlying that mysterious entity Life. The digestive influence of protoplasmic activity on the food elements may be exerted within the cellular units, or may be applied outside the cell-wall; it may act through direct contact with the protoplasm itself, or be brought to bear on elements not in touch with protoplasm by means of active secretions; and again, a single cell may be capable of all necessary digestive processes within its walls, or groups of cells sub- ordinated to the performance of special digestive functions may serve to support other groups dependent upon them. In the latter case specialised cells are alone able to carry on the processes of actual digestion, although every individual living cell, in virtue of the power inherent in its protoplasmic contents, possesses the faculty of absorbing, and it may be of altering, the substances necessary for its nourishment from the food elements offered it. Synopsis of the Historical Records regarding Digestion in general. 1550 — 7°°B.c....Demonology : Digestion performed by the aid of a " dEemon," or spirit. Indigestion due to anger of the spirit. — Egyptians, Hindus. 460 B.c Dogmatic School : Digestion carried out solely by means of heat and moisture. — Hippocrates. Digestion simply putrefaction. — Praxagoras of Cos. 300 B.c Alexandrine School: The arteries contained vital spirits which performed the bodily functions. — Erasi- stratus. 220 B.c The Empirics: Concerned more with what is digestible than how it is digested. — Heraclides. 200 B.c Early Roman: Gastric digestion due to peristalsis (churning movement), mixing, heat, and a spirit or exhalation. — Asclepiades. 1-200 A. D Roman: Reversion to Hippocratic teaching, the cardinal qualities of foods, medicines, etc. ; heat, cold, moisture, and dryness. — AretEeus, Galen, Celsus. 200-1500 "DarliAges" in Europe: No progress made; variants of Galen's views. 850-1312 Aral)ian School: Temperaments. Properties of tem- perature and moisture affect digestion. — Ben Solomon, Avenzoar, Averroes. 1493-1541 " Archeus" of digestion, employing heat. — Paracelsus. l6 THE NATURAL HISTORY OF DIGESTION. 1550-1675 latro-chemical School: Digestion akin to fermentation, under an " Archeus." Gastric juice found to be acid, gastric glands discovered. — Van Helmont, Sylvius, Descartes, Willis, Grew. 1610-1710 latro-mathematical School: Digestion chiefly mechanical. — Redi, Borelli, Pitcairn. 1659 Rumsseus uses a stomach brush to remove " mucus." 1664 Graaf obtains pure pancreatic juice. 1710 («';ra) Reaumur (1683-1757) obtains pure gastric juice, and finds it acid and antiseptic. 1777 Stevens: First demonstration of digestive properties of the gastric juice outside the body. 17S3 Spallanzani corroborates the last two observers. 1S14 Trevisanus finds a sulphocyanide in saliva. 1824 Prout, and Tiedemann and Gmelin, independently dis- cover the presence of free hydrochloric acid in the stomach secretion. 1825-33 Beaumont: Observations on gastric digestion in the Jiving .subject. 1829 Arnott proposes a stomach syphon. 1831 Leuchs discovers the diastatic action of saliva. 1834 Eberle prepares an artificial gastric juice. 1836 Schwann postulates the existence of pepsin, makes first biliary fistula. 1839 Wasmann isolates pepsin. 1842 Bassow and Blondlot make artificial gastric fistula in the dog. 1846 Claude Bernard: Theory of hepatic glycogenesis, fat- emulsifying ferment in pancreas, etc. 1847 Diibrunfaut isolates maltose. 1 85 1 Heintz finds the milk-curdling ferment. 1851 Lehmann finds lactic acid in gastric contents. 1S52 Bidder and Schmidt prove conclusively that hydro- chloric acid is secreted by the gastric glands. 1S54 Busch finds no secretion in small intestine in absence of stimuli. Ebstein and Griitzner postulate the existence of pepsinogen. 1S59 Meissner investigates the products of the gastric digestion of proteids. 1864 Stadeler investigates the bile pigments. Thiry analyses juice of small intestine by means of an artificial fistula. 1871 Paschutin discovers sugar -inverting ferment in small intestine. Eichhorst shows that ferments are absent in mucous membianc of large intestine. 1S73 Liversidge demonstrates the presence of a zymogen of the diastatic zymine in the pancreas. 1875 Ileidenhain discovers proteolytic zymogen in the pancreas, etc. Darwin describes digestion of proteids by plants. ANCIENT THEORIES. 17 1S7S Kuhne investigates the products of tryptic digestion of proteids. Richet finds that hydrochloric acid in the stomach contents is often combined with organic bodies. 1 88 1 , Vella modifies Thiry's fistula. \'&%2 et seq Langley investigates the influence of reaction on the functions of ferments, etc. II. — HISTORICAL REFERENCES TO THE DIGESTIVE PROCESSES IX PLANTS. In treating of the historical references to the absorption of nutriment in plants, the fact at once becomes prominent that up to a comparatively recent date no one thought of troubling about the subject. At the present time the average man, "the man in the street," classifies plants into those that are good for food and those that are poisonous, or separates them into those that are pleasing to the senses and those that are unobtrusive or repellent. During the earlier periods of the world's history, owing to the necessity which existed for man to satisfy his hunger, or to supply food for domestic animals, coupled with his inherent sense of that which is beautiful, plants came to be regarded almost entirely from such stand- points. The first trace left us from the past of any endeavour to study plant-life apart from its nutritive or medicinal value is contained in the Natural History of Plants by Theophrastus, a book dating from about 300 B.C. Similar books were written by Dioscorides and Pliny. Throughout the whole period of the Roman Empire, and in the dark times of the Middle Ages, no further attention was given to plants save in apprais- ing their value for food or medicine. As an instance, the Doctrine of Signatures, chiefly elaborated by Bombastus Para- celsus, who lived in the earlier part of the sixteenth century (1493-1541), may be mentioned. His doctrine was a species of physical homoeopathy; a leaf shaped like the liver cured hepatic disorders, a heart-shaped flower was a specific for cardiac disease. The numerous and bulky herbals published about this time were filled only with the names of plants, and with the authors' endeavours to identify them with those described by the ancients. Their medicinal or dietetic value was estimated solely from adventitious and empirical imaginings. Charles de I'Ecluse (1526-1609), a Belgian, was the first to Study plants for the plants' sake, and the first, also, to travel 2 1 8 THE NATURAL HISTORY OF DIGESTION. for the purpose of botanising. It was the Swedish botanist Linnteus (1707-78), however, who laid the foundations of the scientific study of plant-life. He evolved short and technical names for the different parts of the plant, many of them arbitrary, but none the less useful. The publi- cation of his classification at once gave an immense stimulus to botanical work, and stimulated inquiry as to the reason for and the use of the various organs described by him. In dealing with the biological significance of the different organs of plants, especially in regard to digestion and assimila- tion, it would be of little use to refer to the theories of the early philosophers. Aristotle and his school held ideas about vegetable life which are merely fantastic dreams. In 1718 Stephen Hales published what appears to have been the first series of experiments on the natural history of plants ( Vegetable Siaiicks, or an account of sot?ie Statical Experiments on the Sap in Vegetables, being an Essay towards the Natural History of Vegetation). It was not till a century later that the secret of nutrition was discovered. Modern science progresses from the individual cell upwards, explaining each step as it goes; previous observers endeavoured to theorise from the functions of the entire plant, and thereby to apportion to each part its role in the economy. The enormous strides which have been made in our knowledge of Nature's processes during the last fifty years are the logical outcome of the modern method. In 1779 Ingenhouss, indeed, proved that carbon, the most abundant constituent of every vegetable body, is obtained from the carbon dioxide of the air; and De Saussure, shortly afterwards, corroborated his statement, and pointed out for the first time the true significance for vegetable metabolism of the mineral matters taken up by the roots. But it was not until Justus von Liebig and Boussingault, in 1840, made re- newed and patient studies into the principles underlying vegetable nutrition that these principles could be said to have been thoroughly established or understood, to the immense advantage of agriculture and forestry. Another step in advance was made by Sachs in i860, when he published the results of his experiments on artificially reared plants, and showed that they could absorb their nutriment from watery solutions without the presence of soil. This discovery ANCIENT THEORIES. 19 facilitated beyond measure the investigation of vegetable metabolism. In 1837 Latour and Schwann proved, independently of each other, that yeast cells, known even to Leeuwenhoeck with his primitive microscope, were vegetable cells capable of growth and increase. Schulze, the year before, made the more important discovery that the fermentation of fluids required the presence of minute vegetable organisms, and he showed, and after him Pasteur and Chevreul, that if air be excluded from sterile fluids they remained sterile. The year 1864 saw the publication of the work of De Bary on these organisms. The foundations of bacteriology, so soon to assume a position of immense importance, were thus laid. More recent years have seen the advent of a host of workers on biological subjects, with an immense amount of literature and an accumulation of so many new facts that space does not permit any historical notice of them in this place. 20 CHAPTER II. Digestion in Plants. General Survey: — Chlorophyll — Formation of Starch — Proteolytic Ferments — In Seeds — Papain — Bromelin — Action of Roots. Vegetable Metabolism : — Proteids — Carbo-Hydrates— Fats — General Principle — Change in Constructive Materials. Comparative Digestion in Plants : — Bacteria — Form — Food — Lichens — .'\lgce — Higher Plants— Cotyledons — Stores of Food — Annuals, Bien- nials, Evergreens, etc. — Plants flowering only after long periods. I. — GENERAL SURVEY. If a seedling of a plant which possesses green leaves, a seedling, for example, of maize {Zea mays), be placed in a glass filled with distilled water it will grow vigorously for some time, and may put forth three or four normal leaves (Sachs). As soon, how- ever, as the reserve materials present in the seed are exhausted this growth is arrested, and, although the plant may survive for a long time, it perishes before attaining maturity.^-'-On examination of the seed all the proteid and starch formerly present in it is found to be absent. The plant, in fact, has elaborated its leaves from the store of proteid, starch, and salts contained in the seed, with the aid of carbonic acid from the air. On the other hand, if the plant be grown in a solution of various salts, it grows vigorously and continuously. A useful solution for this purpose contains in a litre of water i gramme of potassium nitrate and 0.5 gramme of sodium chloride, of calcium and magnesium sulphates, and of phosphate of calcium. In time the small amount of iron in the seed is used up in the formation of the chlorophyll in the first set of leaves, and the later leaves of the plant grown in this solution are quite white. The addition of a small quantity of an iron salt to the solution soon causes the leaves to take on a green hue. A plant DIGESTION IN PLANTS. 21 grown under these conditions flourishes normally and can proceed to its full development. Vegetables bearing green leaves can therefore form proto- plasm, proteids, carbo-hydrates,'^ and an inorganic skeleton from such simple substances as those detailed above, provided that a sufficient supply of carbonic acid gas be supplied them from the air. The roots possess a certain selective power, those of different plants taking up different proportions of salts, but the ash of similar plants contain more or less of a certain salt according as they are grown on a soil rich or poor in it. The elements necessary for the growth of plants are potas- sium, calcium, magnesium, iron, phosphorus, and sulphur in suitable neutral combinations containing oxygen, nitrogen in some combined form, usually as a nitrate, carbon in the atmosphere, and hydrogen in water. Numerous other ele- ments, indeed, have been discovered to be present in plants, such as silica in Equisetacex, iodine in marine plants, and zinc in plants growing on a calamine (native carbonate of zinc) soil, but these are not necessary for the process of nutrition. In this way plants procure the elements which enter into the composition of proteids in an uncombined state, or in the form of simple compounds, and build up from them the complex proteid molecules. The hydrogen and the oxygen come from water, carbon from the air, nitrogen from nitrates contained in the soil, and sulphur from sulphates. Cultivation of plants in water has shown that nitrates are able to afford nitrogen to the roots, and that the view once held that only ammonia salts yielded nitrogen after absorption by the roots is erroneous. No doubt plants may absorb some nitrogen in this way, but it should be remembered that the ammonia formed by the decomposition of vegetable compounds is easily transformed into nitrates in the presence of potassium or other metallic salts in the soil. The method by which a plant with green leaves obtains its supply of carbon from the air is a little more complex. The chief agents are the chlorophyll granules. Embedded in the protoplasm of the cells lying near the surface of the leaf many roundish or polygonal granular bodies of a soft consistence may be found. In some of the lower algs similar structures 1 For the e.^planation of the terms, proteid and carbo-hydrate, see p. 136. 22 THE NATURAL HISTORY OF DIGESTION. take the form of bands or plates. These chlorophyll bodies always lie within the substance of the cell-protoplasm, and possess a bright green colour. If the colouring matter be removed by means of strong alcohol, the bodies still retain their original shape and almost their original size. The colourless matrix responds to the ordinary tests for proteid substances. A chlorophyll granule may therefore be regarded as a portion of the protoplasm of the cell coloured with the pigment chloro- phyll. But these granules conduct themselves in many ways like independent organisms; they can grow, change their form, and even divide by fission. The colouring matter itself, chlorophyll, is soluble in alcohol and ether, and is thought to consist of two yellow and two green bodies possessing distinct optical properties. Although the presence of iron is necessary for its formation, that element has not as yet been satisfactorily shown to compose a part of its molecule. It is only developed under the action of light, and its colour varies with the degree of oxygenation. In the presence of sunlight, chlorophyll is able to detach the carbon from the oxygen in the molecules of carbonic acid gas, to assimilate the carbon, and give off the oxygen in a pure state. The light rays which act in this way are the red and yellow particularly, the blue-violet portion -of the spectrum, the active part in photography, having httle power of decomposing the gas. The evolution of oxygen and retention of carbon is a function of wave-lengths of light which are not larger than n'siitJT) mm., or smaller than -^-^^^-^jy mm. The maximum result is attained with wave-lengths of -j-j-l-^^o" ™™' Again, the action of the light on the chlorophyll, and the consequent absorption of carbon and formation of starch, is purely local in character. If parts of the large leaves of such a plant as tobacco be cut off from the light throughout the summer by means of tinfoil, and if, after a few days, a leaf be removed, the chlorophyll dissolved out with alcohol, and a weak alcoholic solution of iodine poured over it, only those portions which had been exposed to the light become blue-black owing to the development of iodide of starch. In like manner, if one part of a leaf be introduced into a space containing air deprived of its carbonic acid gas, no starch develops in it. During the action of light enormous quantities of carbon are daily removed from the atmosphere by the action of chlorophyll. Thus Sachs found that a tobacco plant rpmnvpd in the course of a hundred DIGESTION IN PLANTS. 23 days as much as 400 grammes of carbon, a sunflower more than 800 grammes in the same time. Now 10 cubic metres of air contain httle more than 2 grammes of carbon. These two plants alone, therefore, removed as much carbon as would be contained in four thousand cubic metres of air. Weber estimated the production of starch per square metre of leaf surface in Helianthus annmts at 5.559 grammes during ten hours of daylight. Such a figure enables one to form a faint idea of the quantity of starch formed and carbon absorbed during the yearly growth of a large tree such as the horse chestnut. Under favourable circumstances the formation of the starch corpuscles within the chlorophyll bodies may be observed by the aid of the microscope. The laminated appearance of these corpuscles is clearly seen to be due to successive depositions of newly-formed starch round the first small portion formed. These starch-grains are rounded, but if they grow out from the periphery of the chlorophyll body, as often happens, the side next the chlorophyll body is more strongly developed. Schimper has also demonstrated the presence of colourless starch-forming bodies in the deeper parts of plants and in their storehouses of reserve materials, as, for instance, in the potato-tuber. These bodies cause the formation of starch by an action of metastasis or transference, not by assimilation, as in the chlorophyll bodies. The actual formation of starch in the latter is probably a very complicated chemical process, not a simple combination of carbon and water in the correct proportions, although the oxygen given off exactly corresponds to that of the carbonic acid used up. It is also probable that the protoplasmic matrix acts on the carbon liberated by the chlorophyll, as starch granules may still be found to increase in some instances after the green colouring matter has disappeared. Sachs suggests that sugar is first formed, and then through protoplastic action it is further converted into starch. The starch thus produced in the sub- stance of the chlorophyll bodies is constantly being dissolved and carried to other parts of the plant. This process continues both in the light and in darkness. As the starch is not formed in these bodies in the absence of light, the process of solution and transportation soon renders them free of it. In daylight the constant assimilation is greater than the removal, and starch accumulates; during the night the stored-up starch disappears. In some plants, as in the common onion {Ailitcm cepa), little or no starch is formed in the leaves, but glucose can be recog- 24 THE NATURAL HISTORY OF DIGESTION. nised as the immediate result of assimilation. The leaves of Strelitzia and Musa generally contain fats, not starch, in their chlorophyll bodies, but if grown in a strong light, and with a larger quantity of carbonic acid, they produce some starch. Probably in them the starch is at once transformed into fat. But among the reserve materials proteids are often found, and they are undoubtedly used up in a similar manner. To allow of this, the presence of proteolytic ferments — that is, ferments which are able to change the characters of proteids — must be admitted, and it appears probable that all germinating seeds making use of proteids produce such a ferment. Gorup- Besanez was the first to discover the presence of peptonising ferments in the seeds of the vetch, hemp, flax, and barley. Krukenberg found a similar body in the protoplasm of a myxomycete {^Althalium sepiicuni), while many of the bacteria peptonise proteids by the same means. The activity of these ferments in seedlings has been proved by the discovery by Schulze of peptones in the lupine. Particularly energetic peptonising ferments have lately been detected in the latex of Carica papaya, and of the common fig, Ficus carica, and in the fruit of the pine-apple, Ananas sativa. The peptonising ferments in these plants differ from those present in animals in being capable of action in acid, neutral, and alkaline media, and it was thought at the time of their discovery that, owing to this fact, their use in medicine should prove of great service. Unfortunately the results obtained have not been equal to the anticipations, and their action in the animal body does not seem to be so marked as in artificial experiments, and is not to be compared with that of pepsin or pancreatin. Papain. — The ferment obtained from the juice of the Carica papaya forms a fine yellow-white powder. It dissolves with some ditificulty in distilled water, almost completely in dilute alkalies and acids. In common with most ferments, its action is greatest at a temperature of about 60° C., but it remains active between 12° and 70° C. Hirsch {Tiierapeut. Monatshefle, December 1894) comes to the conclusion that it is the most powerful peptoniser known in a neutral medium, but that its action on proteid foods is limited. If the proteids are in an indigestible form, as in coagulated white of egg, cooked bacon and ham, and indeed cooked meat of any kind unless very finely minced, its action is nil. It has little action on the proteids of bread, but digests raw white of egg, uncooked DIGESTION IN PLANTS. 25 meat, and milk with much vigour. Albumoses and peptone result from its action. In a weak solution of carbonate of soda, or of hydrochloric acid, its power is increased. As compared with pepsin, however, Dott found a marked in- feriority. Under like conditions, papain digested with the aid of 0.5 per cent, hydrochloric acid only 12.9 and 11.3 per cent, of raw egg albumin, while pepsin caused the diges- tion of 99.6 per cent. In an alkaline medium the same observer obtained still poorer results, These experiments, however, were not performed under conditions tending to scientific accuracy. Papain has been found to act strongly on diseased tissues, leaving the surrounding healthy structures unaffected, and is now largely used in the treatment of ulcers, especially in the back of the mouth and nose. The digestive powers of pine-apple (Ananas saiiva) have been investigated by Chittenden {Journ. of Physiology, xv., 1893). The juice is capable of proteolysis, and the ferment contained in it can act in neutral, alkaline, or in slightly acid media. The juice is acid, its acidity is as high as 0.45 per cent, as hydrochloric acid, and contains albumoses as vv'ell as ferments. The principal ferment, which has been named bromelin, can be separated by saturation of the juice with ammonium sulphate, but the product thus obtained always contains albumoses. The ferment may also be precijntated by alcohol and dissolved up in glycerine and water. It acts most powerfully at 60° C., but, like papain, can digest proteids between 12° and 70° C. The peptonising action of bromelin is more closely allied to that of trypsin than of pepsin, even leucin and tyrosin, bodies significant of profound proteid decomposition, occurring in its products.^ The juice also contains a milk-curdling ferment. If the precipitate which falls on the addition of alcohol to the juice be dried at 40° C. and used in an artificial digestion experiment on raw egg albumin, its action is found to be slower and weaker than that of pepsin or trypsin. Thus 0.1 gramme of the precipitate added to a solution of egg albumin had almost no action at a temperature of 40° C. after one hour and ten minutes. 0.05 gramme had a stronger action on half the quantity of albumin in four hours. The following table (Table I.) shows these points : — > Cf. p. III. 26 THE NATURAL HISTORY OF DIGESTION. Table I. Reaction of Solution. Percentage digested by Bromelin. Ill seventy minutes. In four hours. Alkaline, I per cent, sodium carbonate 6.7 ... 15.4 Neutral - Nil. ... 17.4 Acid, o. 1S2 per cent, hydro- chloric acid - - 2.0 ... 16.9 Acid, 0.182 per cent, hydro- chloric acid with pepsin 70.2 ... 97.0 The reaction of the medium appears to have little effect on the power of the ferment. A glycerine e.xtract of the rind and ptilp after expression of the juice also contains soine of the ferment, and e.\periments showed that its action, though slight as compared with pepsin or trypsin, was exerted both m alkalies and acids. The addition of an organic acid, such as acetic acid, produced as much digestion as the addition of hydro- chloric acid. Although a considerable quantity of glycerine was used, the acidity of the extract, 0.26 per cent, as hydro- chloric acid, shows that the tissues of the fruit of the plant are very acid. The ferment of the fig {Ficus caricd) is of a similar nature but has not been thoroughly investigated at present, though Hansen {Sitzungsher. der Physical. Med. Soc. zu Erlangen, Nov. 1880) detected it as far back as 1880. Action of the Roots. — The roots, as we have seen, absorb all the water required by plants (although some ascribe a slight power of absorption to the leaves in many cases), and also procure the necessary mineral salts. The roots themselves have no part in this process. But if the tip of a vigorous rootlet be examined numbers of fine hairs, the root-hairs, may be seen. (Fig. i.) These are narrow tubes with exceedingly thin walls, and measur- ing only a few miUimetres in length and a few hundredths of a millimetre in diameter. It is through these microscopic hairs that the plant obtains its water, its salts, and its nitrogen. These structures may be regarded as protuberances of the walls of the outermost layer of cells immediately behind the growing tip. They only live for a few days, and as the root elongates the part covered with hairs gradually changes its place in the ground, following the direction of growth. In this way they are able to extract from fresh spots the materials which they are in search of. As each terminal branch of the roots is DIGESTION IN PLANTS. 27 covered with thousands of these minute hairs it can be easily grasped how plants are able to procure sufficient water for their growth even in soil which is apparently parched. They in- FlG. I. THE ROOT-HAIRS OF PLANTS (DIAGRAMMATIC). (From a drawing by the Author.) A. Terminal branches freed from earth by washino-, showing: the distribution of root-hairs. B. Particles of earth adhering to root-hairs. T. Growing tips of the roots free as yet from root-hairs. sinuate themselves between the particles of soil and absorb the minutest drop of water present. Sachs tried to estimate their power of withdrawing water from a dry soil by determining the 28 THE NATURAL HISTORY OP DIGESTION. percentage of water in it when the leaves of the plant drooped in a moist atmosphere. He found that a tobacco plant began to droop while the soil, sand and black beech humus still contained 12.3 per cent, of water. This soil was able to retain by adhesion 46 per cent, of its weight of water, so that 33.7 per cent, was at the disposal of the plant; 12.3 per cent, was held too fast to permit of the roots withdrawing it. In a loamy soil 8 per cent., and in a coarse-grained quartz sand only 1.5 per cent, of water remained when the leaves began to droop. These figures show that plants can still withdraw water from the soil when it is impossible to squeeze any out of it. But not only do the root-hairs absorb water, but they possess the power of taking up inorganic solids as well. This property is due to the presence of an acid juice which permeates the root- hairs. The action of this acid on minerals may be well seen by growing plants so that their roots are in close apposition to the polished surface of marble slabs, or of dolomite. After a time, if the slabs be removed and examined, very fine corrosions may be detected on the polished surfaces corresponding to the form of the roots. In this manner carbonates of calcium and magnesium and phosphate of calcium are acted on and the salts altered to a soluble form. Roots are thus by no means only passive agents ; they exert considerable force in the with- drawal of water clinging fast to the particles of soil, and in the chemical solution of solid minerals. The Path of the Absorbed Material. — In aquatic plants fluids and soluble nutritious substances are absorbed by the entire surface, while the evolution of gases from metabolic processes causes condensation of the cell-contents, leading to endosmotic diffusion of fluids into them. In cellular plants, as in the lichens, mosses, and fungi, the circulation of fluids appears to result from endosmotic action alone. In a vascular plant the fluid absorbed by the roots, or sap, as it is termed, mounts upwards towards the growing parts as a result of endosmotic action, of imbibition or capillary force, and of pressure, constant or intermittent. To endos- mosis must be assigned the principal part in the process. The evaporation of fluids from the external surface, even when regulated by the stomata of the leaves, renders the sub- stance of the adjacent cells less fluid, and produces osmotic action, whereby the neighbouring tissues yield part of their DIGESTION IN PLANTS. 29 liquid and in turn attract a compensatory flow from the next series of cells ; the process continued throughout numberless layers of cells at length exerts an influence on those of the terminal rootlets. Pressure of the cell-walls, partly due to endosmosis, partly to alterations in temperature, serves to propel the fluids in the line of least resistance — i.e., to the upper terminal organs. Herbert Spencer has also attributed to the oscillations of the branches and leaf-stalks or petioles an influence on the ascent of the sap by the intermittent pressure thus occasioned. In addition to these factors the chemical actions resulting in the formation of starch, its transformation into sugar and the other metabolic processes of plant life, necessitate a supply of water and thus induce its diffusion. Hales has shown that the sap rises with very considerable force. Experiments on the cut end of branches of the vine showed a force equal to twenty-six inches of mercury. Clarke, also in an experiment on the vine, observed a still greater force. The ascensional force of the sap was capable of balancing a column of water 48 to 50 in. in height. The force exerted by the ascending sap varies with the season and with the hour of the day. It is greatest in spring and during the daytime ; the first because of commencing growth, the second owing to the influence of warmth. The chief flow of sap occurs along the elongated cells connected with the vessels in the fibro- vascular bundles. In spring, before the leaf-buds expand, the absorption of water with its nitrogen and mineral salts exceeds the amount transpired. The excess of water is stored in the stem. After expansion of the leaves in summer transpiration is greater than absorption, and the deficient supply obtained from the roots is rendered adequate by the excess stored in the stem. Even in winter some movement of sap takes place owing to the effect of the higher temperature of the soil than of the air. Ascending sap is a watery fluid, containing unchanged mineral salts, dextrins, and sugar, but no starch or chlorophyll. When it reaches the terminal and active organs of the plant the element carbon, hitherto absent save in the sugars derived from the reserve stores, is added to it by the process already described. The ascending sap is termed crude jrt/, the sap after the further addition of carbonaceous bodies elaborated sap. 30 THE NATURAL HISTORY OF DIGESTION. There is little doubt but that much of this elaborated sap descends from the leaves to the woody parts of the branches and stem, where it is in part used up in the formation of new tissue, partly in the repair and metabolism of the old, and in part deposited in the form of reserve stores of nutriment. It must not, however, be supposed that the only movements of the sap are in an upward or downward direction. It appears to be able to permeate all the tissues of the plant in any direction, diffusing from one vegetable cell to another. In fact, the walls of vegetable cells seem to allow the dialysis of soluble substances and the diffusion of water to a much greater extent than their analogues in the animal kingdom. Mulder suggested that the roots assimilate as well as absorb nitrogenous material, and similarly the leaves carbonaceous substances, the movements of the sap con- veying these products to all parts of the plant. II. — THE PRODUCTS OF VEGETABLE METABOLISM. Before entering upon the description of vegetable metabolism it will be necessary to explain what the term metabolism implies, and to give a brief definition of the substances which take a part in the process. A more detailed account of the different food-elements concerned in metabolic processes will be found in a subsequent chapter. The term metabolism denotes all those processes and interchanges by which a living organism makes use of the inert substances in its food in building up and nourishing its body and in procuring the energy requisite for its bodily activity. Plants are possessed of life as well as animals, and the law of nature, that no kind of action can take place without a corresponding production of energy, applies equally to plants and animals. All living organisms, be they vegetable or animal, are made up of cells, and each active cell is complete in itself as far as vitality is concerned. Every living cell contains more or less protoplasm. Protoplasm is the ground- work of Nature. It cannot be analysed, for analysis implies death, and death occasions intimate changes in its structure. Living it is endowed with the power of initiating structural processes, a power the nature of which is quite unknown to us, while the processes thus initiated serve to form heat and energy with their concomitants, growth and movement. Proto- plasm when dead does not differ materially from inert proteid DIGESTION IN PLANTS, 3 1 bodies, and is absolutely incapable per se of producing energy. Proteids are complex bodies containing the elements carbon, hydrogen, nitrogen, oxygen, and sulphur in varying proportions, and in addition a small proportion of mineral salts. White of egg is a well-known example of a proteid. A large number of atoms of the elements named go to form a proteid molecule, that is, the smallest portion which contains all the elements in proper proportion and retains the properties of any one proteid. The proteids found in nature or incorporated in protoplasm have relatively bulky molecules, which oppose their easy transit from one part of the organism to another. Proteolysis is the term applied to any digestive process whereby these large inutile molecules are simplified and transformed into smaller and more transportable proteid bodies. In plants simpler compounds are made use of to build up the more complex proteid. They are assimilated, and in virtue of the action of the protoplasmic contents already contained in the cells of the plant are combined together in proteid molecules. Carbo- hydrates form another group of chemical substances which are formed or made use of in metabolic processes. Sugar and starch are the best known examples of carbo-hydrates, which contain no nitrogen, while the hydrogen and oxygen present bear the same proportion to each other as in water — that is to say that the atoms of hydrogen are always double the number of the atoms of oxygen. The only other element present is carbon. Fats contain the same elements but in different proportions. Metabolism of plants is connected first with the formation of these substances from their elements, and secondly with the changes brought about in them so that they may be used in nutrition or employed as sources of energy. For the proper development of the fungi the five elements contained in proteid matter are necessary, and, in addition, phosphorus, potassium, or one of the metals of that series, and one of the series, calcium, magnesium, barium, or strontium. The higher plants need both calcium and magnesium, the fungi can exist, according to some (cf. p. 37), with only one of them. Iron is not a necessary element. The fungi, as a class, are capable of obtaining the various substances necessary for their growth from a much more widely extended field than the higher plants, and in procuring these substances produce an enormous number of different chemical reactions, some of them useful, 32 THE NATURAL HISTORY OF DIGESTION. some detrimental to themselves and to others. Dead and de- caying matter is a paradise to many of them; indeed they may be looked upon as the scavengers of the world, and as the ulti- mate transmutation of all living things. The means by which they alter the substances on which they live will be considered in the following section. Changes in the Constructive Materials in Plants — Ferments. — The starch produced in the leaves may be altered and used at once, or may be stored up for future use. It may be stored up in the form of starch itself, as cane-sugar or as inulin, another form of sugar derived from starch. Cane-sugar and inulin are soluble and are also present in the sap, but are not in a form which can directly be made use of by the cells. In order that they may be rendered serviceable these bodies must be changed into more simple and diffusible forms. Thus both starch and inulin are polysaccharides (CgHjgOj)^, and cane- sugar a disaccharide (0^2^22011)- To render them useful they must be changed into monosaccharides (QHj.jOg) (cf. p. 139). In the starchy seeds of all plants, and in their bulbs, tubers, or other reservoirs of reserve-materials, a ferment appears at or just before germination which is called diastase. Diastase is capable of transforming relatively enormous quantities of starch into glucose. It acts most powerfully at temperatures just below 70° C, but is very active at ordinary temperatures (15-20° C). Although found most plentifully in these seeds and bulbs, it is present in smaller quantities throughout all parts of the plant. Baranetzky never failed to find it even in plants that abound in sugar, as the carrot and turnip. Similarly, inuUn in the tubers and root-stocks of the Composites, is transformed into glucose, probably by a fer- ment of the same class, while cane-sugar is split into the monosaccharides, dextrose and Isevulose, by invertin, a fer- ment identical with that formed by the glands of the intestinal mucous membrane in animals. Cellulose, also, another polysaccharide formed from the assimilated starch, and the chief component of the cell-walls in plants, is converted into sugar in the germinating date-stone by means of another ferment. This ferment appears to be the active agent by which those fungi which live on the wood of trees transform cellulose into nutritive sugar. A more particular study of the action of ferments on carbo- hydrates must be reserved until animal digestion is reached, DIGESTION IN PLANTS. 33 Proteids. — As starch is the first visible product of assimila- tion in plants, all ot;her organic compounds contained in them must arise from or be built up upon it through chemical changes. The two elements which are absent in the starch molecule, but present in the proteid, sulphur and nitrogen, are, as we have seen, derived from the soil. That the forma- tion of proteids from carbo-hydrate bodies with the addition of nitrogen and sulphur, through an inherent power of living protoplasm, is possible, follows from Pasteur's experiment on yeast. A few yeast-cells, fed with sugar and the salts men- tioned, give rise to millions of new cells. Where the formation of the proteid molecules takes place is doubtful. The nitrates and sulphates absorbed by the roots penetrate to all parts of the plant, and may thus meet the carbo-hydrates transferred from the leaves in any part. The actual position has not been identified as yet. Some have suggested that the process may occur in the chlorophyll bodies themselves, perhaps during actual assimilation, perhaps during the night. Sachs, the greatest authority on this subject, suggests the sieve-tubes of the younger shoot-axes and leaves as the probable scene of the action. In them we find a proteinaceous slime, neither proto- plasm nor organised proteid, but circulating proteid, evidently on its way to the younger growing organs. In their neighbourhood layers of cells may be seen filled with fine-grained transitory starch and with sugar, and other cells are often present in the immediate vicinity, in which calcium oxalate has crystallised out. Holzner long ago suggested that oxalic acid was formed in plants for the purpose of decomposing calcium sulphate, and thereby setting free the sulphuric acid for the production of proteid substances. It is possible, however, that the proteid molecule can be built up in any cell containing living proto- plasm from another nitrogenous body, asparagin, which abounds in the sap of growing shoots and in stock reservoirs. Asparagin is an amide, amido-succinamic acid, C4HgN20,, containing no sulphur. It is crystallisable and soluble in water, more readily in warm than in cold. Whether it is formed in the plant by synthesis, and converted into proteid by a like process and by the addition of sulphur, or whether it always arises from splitting up of proteid matter, is doubtful. Asparagin is certainly formed from proteids in many ger- minating seeds, to be re-converted into proteid again when the green organs begin to assimilate nitrogenous material, and if 3 34 THE NATURAL HISTORY OF DIGESTION. asparagin arising in this way can unite with sulphur and under- go such molecular changes as are necessary to form proteid, it is exceedingly probable that it can be produced from the carbo-hydrates and salts for the same purpose. In addition to the proteids used up in the growth of plants, many species lay up in their reservoirs a store of reserve material. In most plants this store consists both of living protoplasm and of soluble non-organised proteid. In some plants a portion of the simple proteid, but always a relatively small part, may be present in the form of crystalloids. These remarkable bodies resemble true crystals in every particular save that they are capable of swelling by imbibition. The crystals in starchy seeds differ from those in fatty seeds. In the first they occur between the starch granules, in the second within the so-called aleurone grains. These consist of small masses of proteid substance in an amorphous state, and almost always contain a crystal of pure proteid. Ricinus communis, the castor oil plant, presents very fine examples of these bodies. Crystals of proteid bodies have also been found in other parts of plants apart from the reservoirs of store material, and are to be regarded as temporary deposits of nutriment for special purposes of metabolism. Carbo-hydrates. — There can be no difficulty in assigning the production of the various carbo-hydrates which abound in plants to the results of chemical changes in the starch formed by the leaves. The different agents at work will be considered more particularly later. Fats. — Fats play a very important part in the vegetable economy. All living protoplastic cells contain fat, while the great majority of ripe- seeds contain large quantities, alone or together with starch. The manner in which they are formed from the starch assimilated by the chlorophyll bodies is chemically uncertain, but that they do arise in this way is proved by the fact that an unripe seed, as of Fceonia, which contains no fat but only starch, if detached from the plant and allowed to lie in moist air develops fatty oil, at the expense of its starch. We have now found that the various carbo-hydrates, fats, and proteid bodies in the plant originate wholly or in part from the starch formed by the chlorophyll, and that the portions unused for active growth are stored up as reserve material. The general principles underlying vegetable metabolism are construe- DIGESTION IN PLANTS. 35 tive, in direct contrast to those of animal metabolism, which are, first, largely destructive ; secondly, constructive. In plants devoid of chlorophyll, or containing so little that it is scarcely of any importance, organic substances must be absorbed as such; they cannot be built up within. As there cannot be any assimilation of carbon from the carbonic acid gas in the air, this element must be absorbed in the form of carbonaceous compounds. Hence all plants devoid of chlorophyll are parasitic. They derive their nourishment from living plants or animals, or from the dead bodies of plants and animals, or their dissolved constituents. This use of prepared compounds is parallel to that in the seeds and young shoots of green plants before the chlorophyll has been developed, when they make use of pre-formed reserve materials. All parasitic plants, however, are not neces- sarily devoid of chlorophyll, as, for example, the mistletoe, but supplement the carbon obtained from the air with that drawn from their host. Similarly the carnivorous plants, which are more fully considered in the next chapter, supplement their scanty store of organic matter by the digestion of flies or from other nitrogenous foods. Perhaps among the plan4:s which do not possess chloro- phyll the most important class is that of the Fungi. The distinguishing feature in the processes by which the fungi in general obtain their nourishment consists in the vast amount of apparently useless work performed. They obtain the com- pounds required from living and dead matter, but in the process they alter much of the medium on which they are living, often in a manner inimical to their own growth, and useless for their development. Thus yeast breaks up much more grape-sugar in the process of fermentation than is requisite for its nourishment, and some bacteria form so much lactic acid from sugar that their further growth is in- hibited by it. A still more peculiar method of nutrition pertains to the Lichenes. They are fungi which extract little organic nourish- ment from the trees or rocks on which they grow, probably no carbon, but some nitrogen and sulphur from the salts containing these elements. To make up for their lack of chlorophyll they weave themselves round and imprison small chlorophyll-bearing Algcz, and obtain from these the carbon required. These AlgcR were formerly regarded as part of the 36 THE NATURAL HISTORY OF DIGESTION. lichen itself, and were termed gonidia, but the true relation- ship between them has now been firmly established. III. — COMPARATIVE DIGESTION IN PLANTS. Bacteria, Schizomycetes — Yeasts, Saccharomycetes — Lichens and Fungi. — The various forms of Bacteria occupy the lowest position in the scale of organised matter. Although the majority of the species included under this title are of microscopic size (some, micrococci for instance, as minute as 0.2 micro-millimetres in diameter), each individual is capable of absorbing nutritive material and of transforming it, by appropriate means, into assimilable forms. It is one of the marvels of Nature that such infinitesimal specks of matter — ■ o. 2 micro-millimetre corresponds to 0.00006 inch, or xTTolrcyi) inch — should be able to carry out processes very similar to those in the higher animals. They are made up of a particle of protoplasm surrounded by a thin wall of a carbo-hydrate material closely allied to cellulose. The almost inconceivably small particle of protoplasm, the mycoprotein of Nencki, which forms the bacterium proper, is endowed with the power of secreting ferments, which act both intra- and extra-cellularly. Many of the special properties which are distributed among different tissues in the individual higher forms are distributed in bacteria among different species. All bacteria, however, require the presence of nitrogen in an available form for their growth; they cannot make use of the free nitrogen of the air, but are able to extract it from ammonium salts, nitrates, proteid bodies, and, according to Nregeli, from such bodies as acetamide, methylamine, asparagin, and leucin. But while some can obtain all the nitrogen which they require from ammonium salts or nitrates, and do not make use of any proteid bodies present, others again obtain it by preference from these bodies. As the fungi are unable to obtain carbon from the carbonic acid in the air, owing to their lack of chlorophyll, they must procure it in another way. Accordingly we find that they can extract carbon from various kinds of organic compounds. It is owing to this power of decomposing organic compounds, especially when these are of simple composition, that science has been enabled to gain a clear insight into their action in the acquirement of the requisite carbon. Thus Pasteur, in 1858, using ammonium tartrate, (NH4)2C4H40|3, as a nutrient medium DIGESTION IN PLANTS. 37 in which to cultivate moulds, yeasts, and bacteria, found that they could obtain from it all the carbon and nitrogen they required. Nsegeli considers the nutritive solution suggested by Mayer in 1869 an excellent medium for yeasts. This solution was made up as follows : — ■ Water 100 c.cm. Sugar Ammonium nitrate 15 grms Acid potassium phosphate Tribasic calcium phosphate Magnesium sulphate 0.5 „ 0.05 ,, 0.25,, Theoretically, three to four grammes of new yeast can be pro- duced from the constituents of the solution as described, but Nffigeli found that only one gramme is actually formed. The cause of this incomplete use of the material is no doubt due partly to the formation by the growing yeast-cells of bodies in which the elements are so combined that the cells can make no use of them, leading to a withdrawal of some of the theo- retical nourishment, and partly to the inhibiting action on the cells by these or other newly-formed bodies. As, however, a single yeast-cell weighs only about one two-millionth of a milli- gramme, or one two-thousand-miUionth of a gramme, the formation of one gramme of yeast represents the development of 2000 millions t)f new cells composed of protoplasm and cellulose. This solution is only one of many which may be employed. In nature, glucose is the most suitable source of carbon available for the fungi. Fungi are able to obtain nitrogen from certain substances, as from urea and oxamide, although they cannot make use of the carbon also present Fungi require, however, in addition to carbon, hydrogen, oxygen, nitrogen, and sulphur, the presence of mineral matter. All native proteids contain a small proportion of in- organic salts. When this is removed by artificial means the proteids lose some of their characteristic features (cf. p. 145). The protoplasm of fungi is no exception to this rule. Nsegeli supposed that phosphorus, an element of the potassium-cjesium series, and one of the series calcium, barium, magnesium, or strontium, were sufficient. He thought that iron was unneces- sary, and that they could grow if one of the elements, calcium 38 THE NATURAL HISTORY OF DIGESTION. or magnesium, was available, unlike the higher plants for which both were required. Molisch lately (Sitzungsber. der Kais. Akad. d. Wissenchaft, Wien, 1894) has shown that Nsegeli did not use sufficiently pure salts. Molisch purified all the salts used by several re-crystallisations. The results of his investiga- tions show that after twenty days the growth of yeasts with iron sulphate is three times as great as the growth of yeasts cultivated under the same conditions but without the addition of iron. Neither manganese, cobalt, nor nickel have this effect, and cannot replace the iron. Molisch finds, also, that calcium cannot replace magnesium. Fungi grown in solutions con- taining calcium but no magnesium salts do not produce spores; nor can barium or strontium be substituted for magnesium. He concludes that while magnesium is absolutely essential for the proper development of fungi, calcium may be withheld without detriment to them ; and that in this particular the fungi differ from the higher green plants. The elements necessary, according to Molisch, are nine in number — Carbon. Nitrogen. Phosphorus. Hydrogen. Sulphur. Magnesium. Oxygen. Potassium. Ferrum (Iron). And in higher plants. Calcium (Lime). The arrangement of the conditions appertaining to the supply of nutriment and to its absorption in the lichenes affords one of the few examples to be found in nature where a parasite and its host live together harmoniously and confer mutual obligations one on the other, even although the host exercises some degree of compulsion in the retention of the parasite. Lichens belong to the subdivision ascomyceies of the fungi, but differ from other members of their subdivision in that they weave themselves round microscopic alga, and imprison them in a thallus of densely interwoven hyphse. Before the true structure of algx was known the existence of two different forms of tissue had been detected — fungus tissue producing organs of fructification, and cells containing chlorophyll. These green bodies were termed gonidia. Bornet and Stahl demonstrated that the union of \.)ie. fungus and the alga promoted the vigour of both. The algce serve as organs of assimilation, in the same manner as chlorophyll cells DIGESTION IN PLANTS. 39 elsewhere, and the fungus obtains the inorganic constituents necessary for the growth of the combined individual. It is for this reason that lichens are independent of organic soils. Other fungi must be supplied with organic material from which to abstract carbon and nitrogen. Lichens, thanks to the imprisoned alga, can develop on rocks, even on crystalline rocks, from which they are able to obtain the mineral matter required for their own growth and for the proper nourishment of the chlorophyll cells represented by the captive algce. Owing to their independence of organic substrata the lichens have a much greater choice of sites whereon to establish them- selves than any of the other fungi. Higher Plants. — The general description which has already been given of the phenomena of vegetable metabolism applies to almost all of the higher members of the vegetable kingdom. Chlorophyll bodies present in their leaves or analogous struc- tures decompose the carbonic dioxide^ in the air under the influence of sunlight and heat, and build up more stable com- pounds containing a greater proportion of carbon and less oxygen for the nourishment of the plants. In the dark this process of assimilation ceases, carbonic dioxide is given off and oxygen absorbed. In the daytime the roots and leaves absorb carbonic dioxide, the oxygen of which is set free by the chloro- phyll of the leaves; during the night the roots may still absorb carbonic dioxide, but it is exhaled unaltered from the leaf surfaces. A part of the carbonic acid gas exhaled in the hours of darkness may arise from oxidation of carbon by some of the oxygen absorbed. The first leaves of the higher plants to emerge from the seed are usually fleshy and more solid in appearance than those which appear at a later date. These cotyledons, or store- leaves, contain reserve nutriment which can be made use of by the growing plant at a time when the roots are insufficient to provide it with enough food. When a plant is capable of procuring sufficient nourishment through its roots and leaves the cotyledons atrophy. Many plants lay up stores of aliment in the form of tubers, bulbs, and fruits for the use of the seeds when germinating. ^ Carbonic dioxide, or carbonic acid, is formed by two atoms of oxygen and one atom of carbon ; it is represented by the chemical formula C02- 40 THE NATURAL HISTORY OF DIGESTION. An annual plant, flowering, seeding, and dying in the space of a few months, grows quickly, provides little store of nutri- ment for its seeds, and after fructification commences to decay. The seeds contain some supply of food against their germination, develop chlorophyll-containing cotyledons of a proportionately large size, and rapidly produce roots capable of supporting them. Biennial plants lay up a store of food material during the first year of life towards the accomphsh- ment of maturation in the following year. The drain on the organism occasioned by the production of flowers and seeds is then too great for continued vitality. Perennial plants may be herbaceous, like the daisy and primrose, which when grown from seed germinate in one season, and in addition produce an underground stem or rhizome, from which annual flowering shoots are sent up. Other perennial herbaceous plants produce one shoot, which may flourish for years before flowering, after which decay sets in. Woody perennials may grow for a large number of years, their growth ceasing during the winter months. The annual drain upon their vitality arising from the process of flowering and fructification has so little effect on their power of development, that they continue to produce flowers and fruit each year throughout their life. Some perennial plants retain their leaves unaltered throughout the annual resting period, only shedding them when a fresh crop is ready to take their place, thus receiving the name of evergreens. To summarise the main facts connected with vegetable nutrition, leaving out of account the carnivorous plants, which are fully discussed in the next chapter, we may state that — A. Plants obtain — i. Their carbon by decomposing carbon dioxide. Some plants possibly can absorb organic com- pounds containing carbon, such as are present in decaying vegetable or animal matters in the form of organic acids. ii. Nitrogen from the nitrates and ammonium com- pounds in the soil. A few plants may be able to absorb nitrogen from the nitric acid present in rain water caught in hollows or receptacles in their structure, iii. Hydrogen and oxygen from water, iv. Mineral salts from the soil by means of their root-hairs. DIGESTION IN PLANTS. 4I B. Plants elaborate carbo-hydrate bodies from the carbon separated from carbon dioxide, and the hydrogen and oxygen in water; proteids from the nitrogen obtained by the roots and the already formed carbo- hydrates, probably through protoplasmic action ; and fats from the carbo-hydrates. c. Part of the elaborated material is used up at once, and part is stored up as a reserve stock in the fruit or seed to serve for its nourishment during germination; and in most cases in different parts of the plant itself, this store being formed in summer and made use of during ordinary growth, but especially in springtime during budding. D. Plants produce the complex bodies which go to form their various organs and secretions from simple elements. E. They are constantly absorbing water from the soil and giving it off again into the air from their leaf surfaces. These surfaces, however, are generally supplied with stomata (cellular valves) whereby this transpiration may be regulated. F. Plants possess no true circulatory apparatus. The movements of the sap are chiefly produced by diffusion and follow the laws of osmosis. The actual causes originating osmotic action arise from the exhalation of water, elastic pressure of the cell walls, movement of the branches, alterations in tempera- ture, and the abstraction of water in the elaboration of chemical compounds. Capillary attraction has also some effect on the movement of the sap. G. Numerous bodies, included in the class of unorganised ferments, are present in many plants, by whose agency alterations in the complex substances formed in the first instance from simple elements are brought about, whereby they are rendered more readily avail- able for purposes of nutrition. H. In plants the organism is protected from the growth of harmful agents by the circulation and acidity of its juices; in the animal organism secondary defensive means are supplied by the presence of phagocytes, and by reason of the existence of a nervous system capable of initiating actions tending to the avoidance of danger. 43 CHAPTER III. Carnivorous Plants. Carnivorous Plants with Adhesive Apparatus — Drosophyllum lusitahicuin. Carnivorous Plants which show Movements in the Capture of THEIR Prey — Pinguicula vulgaris — Structure — Digestion — Drosera ra/zHzrfi/b/M — Structure— Inflection of Tentacles — Aggregation of Protoplasm — Digestion and Absorption — Digestive Power on Different Substances — Dionaa tnuscipula — Description — Mechanism— Sensitive Hairs — Aldrovanda vesiculosa. Carnivorous Plants with Traps for the Capture of Insects — Nepenthacem — Pitchers — Secretion — Cephalotus foUicularis — Lathraa squamaria — Bartsia Alpina — Uiricularia neglecta — Insects not truly digested by it — Genlisea ornata — Heliamphora ntdans — Sarracenia purpurea^Summary — Authorities. One of the most wonderful things in Nature is the develop- ment by some genera of the vegetable khigdom of the power of supplementing their nitrogenous intake by the digestion or dissolution of animal bodies. The number of plants which are capable of preparing and assimilating proteids may be estimated at about five hundred. As insects form their staple diet, they have been termed "insectivorous plants"; the names of "flesh-eating" or "flesh-consuming" have also been applied to them. Since, however, the chief and most important part of the process lies in the utilisation of the albumins in the flesh of the prey, the term "carnivorous" is more properly applic- able. But even this term does not cover the whole question, for many carnivorous plants extract the proteids from pollen and seeds as readily as from unwary insects. Carnivorous plants can live without the extra supply of nutriment derived from captured prey, but they thrive better, grow larger, and are more fruitful if chances of procuring proteids from animal bodies be afforded them. It is one of CARNIVOROUS PLANTS. 43 the most difficult questions in connection with the theory of the origin of species to explain satisfactorily how plants have developed such carnivorous tendencies. The first point to note is, that almost all the species live in soil poor in nitrogenous compounds ; secondly, that the work done by very complicated mechanisms appears to be quite incom- mensurate with the results achieved. It is known that the , seedlings of all conifers, monocotyledons, and those dicoty- ledons which are provided with endosperm, are able to absorb organic and nutritive substances through their leaves. The wonderful thing about the carnivorous plants, then, is not the fact of their being able to absorb nourishment from organic compounds, but the marvellous adaptations of simple organs developed by nature for the purpose. Argument, however, along the lines of " the survival of the fittest," will show that plants, growing in soil poor in nitrogen, must thrive best if an extraneous supply of that element be afforded them, and a process of modification of existing organs to that end, through countless ages, has led to an extraordinary development of what at first were doubtless simple contrivances. Nothing stands still in nature, and reckonirig by the faint records of the dim and distant past as seen in connection with other modifications of structure, slight changes which might serve the purpose of capturing a stray insect may in the course of time be specialised into the complicated arrangements which now excite our wonder and admiration. In highly specialised organs, Nature, working under no agreement as to time, is seldom a niggardly contractor. As the number of carnivorous plants is so large it is necessary to divide them into classes for the purpose of description. The first class comprises those plants which are provided with leaves covered with an adhesive substance to which the insects stick, but which exhibit no movements whereby the prey may be held fast. The second group consists of plants which exhibit movements in the capture of their prey, while they secrete, in addition, a sticky fluid which serves in the first place to entangle and then digest the prisoner. The third class of carnivorous plants possesses traps or pitfalls contrived in their substance, which serve to entrap insects. This class may be subdivided into two groups. In the first the captured insects are not digested but decay, and the products of decay are absorbed ; in the second a 44 THE NATURAL HISTORY OF DIGESTION. digestive fluid is secreted which serves to simphfy the organic compounds contained in the bodies of the insects, and render them more easily assimilable. CLASS I. — CARNIVOROUS PLANTS WITH ADHESIVE APPARATUS. The leaves of the plants in this class act like lime-twigs, their surface being studded with glandular cells which secrete a sticky fluid to capture their prey, and juices to digest it when caught. They can also re-absorb these juices along with the proteids dissolved in them. The best known member of this group is the Fly-catcher {^Drosophylluni lusitanicuni) of the order Droseracem. This plant lives in Portugal and Morocco, and, unlike the majority of carnivorous plants, is found in dry, sandy, or rocky soil. The stem is about nine inches in height, and bears at the top a few short branches crowned with flowers from 2 to 3 cm. in diameter. The numerous leaves spring from the stem close to its base, and are long, slender, and filiform, the upper surface being slightly grooved to form a narrow channel. Both surfaces of the leaves, with the exception of the channel, are covered with small glands, shaped like mushrooms, which glisten in the sun like dewdrops owing to the drop of secretion on the summit of each. These glands are supported on pedicels, and are arranged in irregular, longitudinal rows. They differ much in size and shape, in the height of the pedicels, and in their colour; some maybe purple, some a bright pink. In shape the glands resemble those of the Pmgidcula (cf p. 47), in colour the tentacles of the Drosera, while the drop of colour- less fluid covering the top of the gland corresponds to the similar secretion seen in Drosera. The pedicels contain spiral vessels, connected below with the flbro-vascular or circulatory system of the leaves, and above with cells in the glands them- selves, which are marked with flne spiral ridges. The spiral cells of the glands usually number from eight to ten, and are covered by two layers of delicate angular cells. Similar glands are also to be found on the flower stalks and calices. In addition to these pedunculated glands there are numerous sessile, colourless glands smaller in size, which differ from the stalked in that they only secrete fluid after stimulation with albuminoid material. The secretion of the larger glands is very acid, even before excitation ; that of the sessile glands is CARNIVOROUS PLANTS. 45 also acid ; both are viscid. The glands of either kind have the power of rapidly absorbing nitrogenous matter. The drops of secretion are readily removed from the glands. Owing to this circumstance an insect alighting on a leaf of Drosophyllum soon gets clogged by the viscid secretion adhering to its wings or legs ; crawling on, as it is unable to fly, it is soon bathed in the fluid, and sinking down on the sessile glands, dies suffocated. The glands replace the drops of secretion removed from their surface with great rapidity, and can secrete large quantities of acid juice. It is not an uncommon thing to see a plant covered with dead insects, exhausted of their nutriment, and at the same time with numbers of struggling captives which have recently alighted. Darwin noticed that, although the glands absorbed their secretion in about an hour and a half if a proteid or a drop of nitrate of ammonia were placed upon them, no such absorption could be detected when small bits of glass or charcoal were used in the same way. That is to say that nitrogenous bodies have some special action on the protoplasm of the gland-cells leading to the rapid absorption of their secretion with its contained nitrogenous material. In connection with this absorption of nitrogen a marked change in the protoplasm of the cells which compose the glands occurs. This change is termed "aggregation," and will be more fully dealt with in treating of the digestive processes in Drosefa. Aggregation of protoplasm causes the cells to take on a much darker hue, and is probably due to the processes necessary for the secretion of the acid used in the digestion of the proteids of their prey. That this is the probable reason is shown by the fact that, while carbonate of ammonia, if placed on the glands, primarily causes a darkening of the cells, cells which have been fed on meat infusion lose their deepened colour on the sub- sequent application of the same salt in solution. In the first case the application of the solution of the ammonia salt causes secretion of acid which neutralises it; in the second, the con- tents of the glands which have already secreted acid, part of which has probably been used up in digestion of the meat infusion, are rendered neutral or alkaline with destruction of the colouring matter. The Drosophyllum, unhke many of the other carnivorous plants, does not secrete very much more fluid after stimulation by albuminous substances than before, but the fluid becomes more acid and contains more ferment. 45 THE NATURAL HISTORY OF DIGESTION. Owing to the fact that it is already acid before stimulation the secretion of the Drosophyllum digests animal matter more quickly than the Drosera. The glands absorb the results sooner and begin to secrete fluid again in a much shorter time. The sessile glands appear to have more power of digestion than those furnished with stalks; the latter serve chiefly to entangle the insects. Among the other plants which capture insects by means of an immobile adhesive apparatus I may mention Roridula dentata, a native of the Cape of Good Hope, and similar in many respects to Drosophylhiin; Byblis gigantea (Western Australia) ; Sedum villostim ; and many members of Prinnila, Saxifraga, and Setnpervivum. CLASS II. — CARNIVOROUS PLANTS WHICH SHOW MOVEMENTS IN THE CAPTURE OF THEIR PREY. The genus Pinguicula, of which nearly forty species are known, several members of the genus Drosera, the DioruBa muscipula, and some species of the Aldrovanda vesiculosa may be included in this class. They will be discussed in the order in which they are placed, which is based on the complexity of the arrangements for the capture and digestion of insects. Finguicula vulgaris. — This plant is an inhabitant of damp and boggy places on moors, commons, or mountains. The area of its distribution covers the whole of the arctic and sub- arctic regions of the globe, and stretches down as far south as the Baikal Mountains, the Balkans, and the Pyrenees, generally along the mountain ranges. The plant is in the form of a rosette, eight to ten leaves branching out from the centre, and lying fiat on the ground with their under surfaces touching it. The leaves are oblong-ovate, yellowish-green in colour, with scarcely any footstalk, and with their edges slightly upturned and incurved, converting the surface of the leaves into shallow troughs. (See Fig. 2.) The upper surface is studded with numerous glands which are of two kinds, one, the larger and visible to the naked eye, is supported on a unicellular pedicel, the other variety is smaller and practically sessile. A square centimetre is said to contain 25,000 glands, and a rosette of six or eight leaves to bear about half a million of them. All the glands secrete a viscous, colourless, and neutral fluid. The larger stalked glands secrete this fluid in greater quantity than those which are sessile. The stalked glands, which very closely resemble a mushroom when examined under the CARNIVOROUS PLANTS. 47 microscope, appear circular from above, the cap being divided into ten or sixteen cells by radiating partitions. These cells are filled with a pale green, homogeneous fluid. The pedicel is composed of one elongated cell, with a nucleus and nucleolus resting on a slight prominence of the leaf's surface. Towards the base of the leaf, and especially near the midrib, these glands are supported by multicellular pedicels, while the cap is smaller. The smaller glands differ only in being composed of about half the number of cells, the fluid of which is much paler. They are also supported on slight elevations of the leaf surface. Ordinary flat epidermal cells fill up the spaces between the glands, with occasionally the guard-cells of stomata. The writer has found small sessile glands, composed Fig. d.. THE COMMON BUTTERWORT [Pingvicula vulgaris). (From a pen-and-ink sketch by the Author.) Cross section of a leaf (x 20) of Pinguicula viUgaris. A. Vessels of midrib. J3. Stalked glands. C. Shortly stalked glands. D. Sessile glands on dorsum of leaf. U. Incurved edge of leaf over an insect allowing a greater number of glands to come in contact with the prey. of only four cells, on the under surface of the leaf, but they are so few in number that it is impossible to ascertain if they are capable of secreting digestive juices. The under surface, in addition, is thickly studded with stomata. The leaves are richly provided with fibro-vascular systems. The spiral vessels, proceeding from the midrib, terminate in cells marked by a spiral line close to the edge of the leaf. The flower is of a violet-blue colour, with petals covered with long velvety hairs, and with a spur at the back. It is borne on a long, slender, gracefully curved stalk, which rises from the centre of the rosette of leaves. The roots are short, seldom exceeding 1.5 inch. The leaves, when looked at with the naked eye, present a 48 THE NATURAL HISTORY OF DIGESTION. large number of minute shining points, especially if a light be thrown on them at an angle towards the observer. Under a magnifying-glass these shining points produce a remarkably pretty effect, and are then seen to be due to the drops of secretion surmounting each of the glands. When an insect alights on a leaf it is at once held fast, if it be small enough, by these drops of viscid fluid. In like manner grains of pollen, minute plants, and seeds are frequently found attached to the leaves. Darwin examined a large number of wild plants, and found that about 70 per cent, had captured insects, while many had caught various small seeds as well. Shortly after the capture of an insect the glands in contact with it begin to secrete a larger quantity of fluid, which is still more viscous than that exuded in the fasting condition, and which is strongly acid in reaction, the previous secretion having been neutral. In a few hours the edges of the leaf begin to curve in towards the spot where the insect lies, and the glands directly in contact pour out so much fluid that the prey is submerged in it. Although the presence of any nitrogenous particle is sufficient to cause incurvation of the edges of the leaf some distance, it may be, from its actual position, none of the glands except those in direct contact secrete the digestive fluid. If the insect be caught on one side of the leaf, that side alone exhibits any movement. If, however, its position is close to or on the midrib, both edges curve in over it. The apex of the leaf never moves towards the base. The glands themselves have no power of movement. A remarkable fact with regard to the incurvation of the leaves is the comparatively short time which it lasts. Darwin found that in no case was there any marked incurvation before 2 hrs. 17 mins.; after drops of raw meat infusion had been placed on the leaves he thought he could detect traces of movement in i hr. or i hr. 30 mins. As a rule, well-marked re-expansion occurred within twenty- four hours, and always before forty-eight hours had elapsed. Reference to Table II., which has been compiled from Darwin's observations, will show the different periods at which incurvation of the edges of the leaves, and the secretion from the glands, began and ended. Darwin also pointed out that though fragments of glass caused a certain degree of incurva- tion, which began as soon as that following stimulation with proteid bodies, it lasted a much shorter time, and was not accompanied by any secretion. A leaf that has become well CARNIVOROUS PLANTS. 49 incurved and has again expanded does not respond to a fresh stimulus for a considerable time. Table II. — Fads regarding the Secretion of the Glands and the Movement of the Edges of the Leaves of the Pinguicula Vulgaris after stimulation with various substances. ( Com- piled from Darwin.) Substances placed on Leaves. Position. Secretion. Movement of Leaves. Re- action. Be- ginning. Leaves Dry. Be- ginning. Height of Move- ment. Un- curved. Small Flies Fragment of Large Fly Margin. Near Apex. Medial line. Margin. Margin. Margin. Medial line. Margin. Margin. Margin. Acid. Acid. Acid. Acid. Acid. Acid. Acid. ? Neutral Acid. 40 mins. At once At once At once None. 4 hrs. 30 mins. 4 days 4 days Ito 6 dys 3h.20m 3h.20m. ? None. Several days. 4h.20m. 3 hours (both edges). 2h.l7m. 2h.l7ra. 3h.27m. 1 hour. 2h.30m. (very slight). 2h. 26m. 16 hours 4h.20m. 24 hours eh.SOm. 7 hours 10 hrs. 37 mins. 3h.S0m. 2h.30m. 20 hours 48 hours Do. Eoast Meat Meat Infusion on Sponge Infusion o£ Baw 48 hours 3 days Do. Carbonate of Am- monia in Solution (1 gr. to 2oz.) .. Fragments of Glass Cabbage Seeds 24 hours 20 mins. 24 hours 16 hours 30 mins. 48 hours Scratching surface with blunt needle, no effect. The incurvation of the leaves in the Pinguicula appears to serve three purposes : to prevent the prey from being washed away by rain before digestion, to oppose a greater secreting surface to the captured animal, and to gradually push any larger object than usual, over which the edge is not able to reach, towards the centre of the leaf, by which means it is brought into contact with a far greater number of glands than would be the case if it remained in one spot. It is a curious fact that the simpler forms of proteid— for instance, albu- moses — although they cause secretion of an active juice, have little power of producing a marked incurving of the leaves. The author made a number of experiments with different forms of proteids, the main results of which are summarised in the following tables. 4 so THE NATURAL HISTORY OF DIGESTION. As the incurvation after stimulation with egg-albumin and serum-globulin exactly corresponded to that recorded in Darwin's work, no further notice of it need be taken. When small quantities, about 3 m.gr., of proto-albu- mose, deutero-albumose, or pure peptone were placed on the leaves, little if any movement of the edges took place. It will be seen from the table (Table III.), that of the proteids em- ployed serum-globulin, separated and used in a dry state, and blood fibrin, stained with carmine and dried, excited no secretion. Of the others, proto-albumose caused the greatest flow, while peptone (Table IV.) dis- solved almost at once in the secretion already on the glands, excited very little further secretion, was absorbed within twenty-four hours, and killed the part of the leaf on which it was placed in a short time. A solution of raw egg-albumin mixed with alum-carmine and boiled allowed of very small portions of the proteid impregnated with the stain being placed on the leaves. The albumin was absorbed in time, but not so quickly as when unstained, while specimens of the leaves examined under the microscope after mounting in Canada-balsam gave evidence of the absorption of some of the carmine, at any rate by the glands. Table III. — The Time taken by the Leaves of the Pinguicula Vulgaris to Dissolve and Absorb different Proteids. Proteid Given. April 12th, 8 p.m. Apr. 13th, a.m. Apr. 13th, 2 p.m. Apr. 14th, 9 a.m. Apr. 16th, 6 p. m. Apr. 16th, 6 p.m. Apr. 16th, 9 a.m. Raw Egg Albu- min Dissolved, small drop. Mostly absorbed. Dry. - - - Proto-Albumose Dissolved, large drop. Drop smaller. As before. Small drop, brown. Smaller. Dry. Deutero-Albu- mose . . Do. No change. Partly absorbed. Large drop, no colour. As before. As before. Pure Peptone . . Partly absorbed. Absorbed, leaf wrin- kled, dry. Leaf dying. As before. As before. As before. Serum Globulin Not dis- solved, no secretion. No change. Slightly moist, a little less As in last. As in last. As in last. Blood Fibrin stained with Carmine Dry, no change. No action. No action. No action. No action. No action. Coagulated Egg Albumin col- oured with Car- Moist round patches of red. As before. Drier. As before. Albumin absorbed, Carmine Dry. mine . . left. CARNIVOROUS PLANTS. SI Table IV. — The Absorption of Peptone. A small quantity of pure peptone in powder placed on leaf at 6 P.M. 6.5 P.M. — All dissolved, forming a yellow solution in small drops. 6.7 P.M. — A few brown drops, a little larger. 6.14 P.M. — Drops paler again, fewer, some coalescing. 6.19 P.M. — No incuivation of edges, drops running more together. 8 P.M. — No incurvation of edges, one large pale drop. 9 A.M. (next day). — All absorbed, leaf dry and wrinkled. I P.M. (next day). — Leaf dry, becoming discoloured where peptone had been placed, healthy in rest of leaf. Forty-eight hours from the beginning the portion of leaf on which the peptone had been placed was dead ; it was yellow, dry, and sharply separated from the surrounding healthy portions. For the purpose of investigating the rate of growth of plants fed on different proteid substances, I selected fourteen plants and placed them under identical conditions. The greatest breadth of each was measured from tip to tip of the longest leaves. Once a week they were fed with the substances enumerated in the table (Table V.), two individuals with each variety, and each week they were carefully measured. The two plants which received nothing grew to a little more than double their original size in four weeks ; their increase was 109.4 P^' cent. The only other plants which exceeded this rate of growth were those fed with proto- albumose, in whom the increase amounted to 120.6 per cent. It will be seen that peptone had actually a retarding effect. Table V. — Showing the Results of Feeding Pinguicula Vulgaris with various Proteids. Proteid given each Week Growth in Inches (Greatest breadth from tip to tip of leaves). (Beginning April 12th, at 8 p.m.). April 12th. April 19th. April 26th. May 6tb. May lOtll. Per Cent, of Growth. Nothing Do 1.75 0.76 187 0.87 2.62 1.37 S.OO 1.76 3.25 L76 lSf}l09.4 Raw Egg Albumin Do. .... 1.37 0.75 1.62 1.00 2.12 1.26 2.25 1.50 2.25 1.50 "^^ "■! Proto-Albumose . . Do. 1.60 0.76 187 0.87 2.62 1.37 3.00 1.76 3.12 1.76 -|°}120.6 Deutero-Albumose Do. 1.26 1.00 1.62 1.37 2.00 1.60 2.60 1.76 2.60 1.76 ?^;S} -■= Pure Peptone Do. ... 1.12 1.06 1.26 1.12 1.60 1.60 1.87 1.62 1.76 1.37 t2^} -■" Sernm Globulin Do. 1.25 0.87 1.60 1.00 1.75 1.12 2.00 L60 2,00 L75 ""H 80 5 101.1/ *'"■" Coagulated Egg Albumin stained with Carmine . . Coagulated Egg Albumin stained with Carmine . . 1.25 1.62 1.60 1,76 2.12 2.25 2.26 2,62 2.50 2,75 100,01 \ 84.8 69.7,1 52 THE NATURAL HISTORY OF DIGESTION. The secretion of the glands is neutral in the fasting state, but shortly after direct stimulation with a nitrogenous body becomes acid and contains a ferment with an action similar to that of pepsin. The acid belongs to the acetic acid series, and is probably either acetic acid or formic acid. The secretion acts on proteids in the same way as the gastric secretion of animals, while the glands absorb the products of digestion, leaving the in- soluble matter on the leaf. After absorption the glands dry up, and the indigestible material left is blown away by the wind or washed off by the rain. The protoplasm of the cells in the glands exhibits all the phenomena of aggregation which can be seen so well in Drosera, and will be further described under that plant. Pinguicitia probably secretes another ferment in addition to the pepsin. Linnjeus one hundred and fifty years ago mentioned that the Laplanders had a very favourite dish, similar to our junket, called **Tatmiolk." This is made by pouring milk, warm from the cow, over butterwort leaves, when a peculiar tough mass of a cheesy consistency is formed. This may be partly due to the acid secreted, partly to a rennet ferment. The secretion is also strongly antiseptic, and here again popular empiricism foreshadowed scientific discovery, for the shep- herds in the Alps have long used the leaves as a cure for sores on the udders of their cows, the virtue lying in their antiseptic action. Two other specimens of Pinguicula, P. grandiflora and P. lusiianica, behave in the same way. P. htsitanica has naturally much incurved leaf- edges, but they become very strongly inflected over organic bodies, to a greater degree than in P. vulgaris, and remain incurved for a longer time. P. grandiflora is more easily managed imder cultivation than P. vulgaris, and is therefore more suitable for experimentation. The members of the genus Sun-dew {Drosera) have the same habitats as Pinguicula, and are usually rooted in the boggy, dark soil of moors. The Drosera rotundifolia is commonly found on the moors of this country, and will be particularly described. As in the Pinguicula, and indeed in many other carnivorous plants, the leaves of the Drosera form a rosette with their lower surfaces adpressed closely to the ground and grouped radially round a slender flower-stalk. The leaves are peculiar and characteristic. Circular in shape, numerous filaments or tentacles of a delicate claret colour stand out from their upper surface. Each tentacle carries at its free end a terminal gland which, when the leaf is healthy, is surrounded by a glistening drop of fluid, from which the name of sun-dew is derived. Examination of one of the leaves with a lens shows that the filaments project from the upper surface and margin like pins from a pin-cushion, while they are seen to be of different lengths. Although there is a gradual increase in their size from the centre outwards we may separate them into three divisions: (i) Short perpendicular tentacles rising from the centre of the CARNIVOROUS PLANTS. 53 leaf; (2) longer tentacles slightly inclined outwards, arranged round the last; (3) long, slender marginal tentacles springing from the edge of the leaf and directed outwards. The glands which surmount the central and intermediate tentacles are clavate, those of the long marginal tentacles are inserted on the upper surface of the free extremity, and are much more elongated ; a good idea may be gained of their appearance if they be likened to a somewhat elongated hair-brush, the bristles being replaced by a pad. All the glands secrete a sticky, viscid fluid, which, especially after stimulation, may be drawn out into long threads (cf Fig. 7). There are also numbers of small sessile glands on the upper surface of the leaf, and some on the lower parts of the larger tentacles, composed of two or four cells, which stain much more deeply when the leaf is digesting than when it is fasting. The lower surface of the leaf is bare. Both surfaces show the guard-cells of stomata. The secretion of the glands is neutral before stimulation, but if non-nitrogenous substances be placed in contact with it the quantity is augmented and it may become acid, but contains no ferment. If, however, an insect alight on the leaf, or a particle of a nitrogenous substance be placed on it, the secre- tion of the glands is poured out more freely and possesses active digestive properties. The ferment has the same action on proteid bodies as pepsin, and may be practically regarded as similar to that ferment. As in Pinguiaila, the insect, soon after alighting on the leaf, becomes entangled and drowned in the fluid secretion of the glands; but there are two remark- able differences between the plants. In Drosera the tentacles respond to stimulation with all kinds of bodies, and the glands not directly in contact with them secrete more vigorously. The marginal tentacles exhibit the movements most con- spicuously. If a small fly or a particle of albumin be placed on the gland of one of these tentacles, in a few seconds the tentacle may be seen to bend inwards, the movement com- mencing close to the gland. In from two to three minutes it describes an angle of 45°, and in ten minutes an angle of 90°. The most acute bend in its length is situated not far from the base. By this action the fly or albumin is brought into contact with the-glands of some of the short central tentacles, and a greater power of digestion is thus obtained. But the response to stimulation goes further than this, for in about ten w O K H Q iz; H O P^ Pi w o Pi p o 1-1 o a "D ^ ® O S « ft-" s>f ^ aj to 3. S c3 S » S 5^ "^ ^ o t- m o a (n •J3 J^ S- 0313 i! o o-g g 6i;,o same leaf 1 alk dark, a ■-edges. M Defore were ve white m eflected fro nal tentacle Wl c3 OH-,t-i^ P iIhI- S3 mv^ ^i^s-'^i sio.2'3 Sg5l>. ^"■Sl- •pill twenty-eifiht r a large sect tiown foresho ;he leaf is d bumnse has reater area. Cb g g =8 «■§ S ^ o g „- = g g O Leaf Sinal t inwarc whole while spread 56 THE NATURAL HISTORY OF DIGESTION. minutes the tentacles in the immediate proximity of the first commence to bend in hkewise, in another ten minutes ten- tacles further off follow suit, and in two or three hours all the tentacles may have converged on the prey (Figs. 3 to 6). As a rule the whole leaf becomes concave when all the ten- tacles have converged, and resembles a closed fist, and if the stimulation be strong enough, even the leaf and leaf-stalk may become more perpendicular. If more than one fly alights on the leaf the tentacles divide into sections ; for instance, if one alights on one side, the other on the opposite side of the leaf, half the tentacles devote their attention to the one, the other half to the second. In fact the tentacles exhibit a combined action by which the greatest possible quantity of digestive secretion is brought in contact with the prey under all circum- stances. The period during which the tentacles remain inflected depends on the size of the body captured. If very small two days suffice for the digestion and absorption of its nutritive constituents, if larger they may remain inflected for many days. Other circumstances, such as the age of the leaf and the temperature of the air, affect the length of time of inflection, but not nearly so strongly as the size and nature of the sub- stance caught. I have seen a perfectly healthy leaf still tightly folded over a small portion of proto-albumose ten days after feeding it. The sensitiveness of the leaves is entirely confined to the glands and cells closely adjacent. The motor impulse passes down through the cellular tissue (not along the fibro- vascular bundles) to the lower part of the tentacles, which alone is capable of inflection. When the stimulation proceeds from the terminal gland, the tentacle always moves towards the centre of the leaf, but if it proceed from the shorter tentacles on the disc of the leaf, the direction of inflection of the marginal tentacles is towards the point stimulated. If a plant be immersed in a solution of a proteid, all the tentacles, save the short ones on the disc, move towards the centre. If a particle of a proteid substance be placed on the disc, the surrounding short ten- tacles bend in towards it as well as the longer ones. The motor impulse proceeding from one tentacle to another enters by the base, and causes inflection at once, a further impulse being sent up to the terminal gland. A reflex impulse, though unconnected with any nervous action, now proceeds down the tentacle, causing the protoplasm of the cells to aggregate into masses. That the centrifugal impulse extends to the glands is CARNIVOROUS PLANTS. 57 shown by their increased secretion, which also becomes acid. The actual cause of the movement of the tentacles is obscure, but may be due to the contraction of the walls of the cells on the inner aspect during the process of aggregation. That these cells are in a state of contraction of some sort is shown by the fact that if the outer cells be shaved off during the inflection of a tentacle, the inflection is carried further until a complete circle is formed. Re-expansion, in like manner, is probably brought about by the normal elasticity of the outer cells, which is sufficiently active to cause the tentacle to rise up when the stimulus leading to the contraction of the inner cells is with- drawn. Ziegler advanced the theory that the motor impulses were conducted by the flbro-vascular bundles, but Darwin showed this to be erroneous by dividing the main bundles of several leaves without materially influencing the movements of the tentacles. We are forced, then, to conclude that the impulse passes through the cells, and depends on the irritability of the protoplasm in them. The impulse may be of the nature of a slight chemical change, or merely be occasioned by increased movement of the protoplasm. The fact that a stronger stimulus increases the rate of the impulse, and that the process of aggregation of the protoplasm, which follows the movement, can be observed spreading down the tentacles from cell to cell with a pause between each, makes it probable that another form of protoplasmic activity is the agent at work. It is propagated faster and has no power of itself to initiate aggregation, but like an efferent nerve-impulse, stimulates the glands, causing the requisite excitation of their cells for the transmission of afferent impulses to the cells involved. Aggregation of Protoplasm. — Stimulation of the tentacles by any of the methods described above as capable of inducing movement in them also excites changes in the protoplasm of the cells of which they are composed. If the cells of one of the terminal glands be examined immediately after stimula- tion the contents can be seen to become cloudy, even in so short a time as ten seconds. Within a minute granules appear in the cells of the tentacles immediately below the glands, and these soon aggregate into larger masses, which show incessant changes of form. These masses consist of aggregated proto- plasm. The stalk of each tentacle is penetrated by one or two vessels with delicate spiral markings on the inner surface, and around these are the parenchymatous cells. The gland 58 THE NATURAL HISTORY OF DIGESTION. has in the middle several cells with similar spiral markings with which the vessels of the stalk communicate. Grouped round these cells are two or three layers of parenchymatous cells, in which the protoplasm can be discerned streaming round close to the walls; whilst in the centre there is a vacuole filled with a red-violet fluid. The phenomena known as "aggregation of protoplasm" occurs in the parenchyma, the contents of each cell dividing into a dark red-purple mass, as described above, and a clear colourless fluid. This change may be regarded as an expression of the activity of the cells, but does not depend on absorption, as it may be excitedby a few touches; as a rule it is synchronous with the inflection of the tentacles, appearing just before the commencement of movement, and ceasing with full re-expansion. The process begins in the parenchyma of the glands and spreads down the cells of the pedicels; its progress is appreciably checked at the point of junction of the cells. If the stimulus appHed to the gland be very powerful, all the cells down to the root of the tentacle may be affected. If the stimulus be applied to the short central tentacles the process of aggregation in the longer tentacles commences in the gland and travels down to the base. The active impulse communicated to the gland must, therefore, be different from the exciting influence generated by the gland itself. What this influence consists of is as yet unknown. It may be of the nature of molecular change trans- mitted from cell to cell, or it may be carried by the spiral vessels. It is a strange thing that a lowly vegetable, sans muscles, sans nerves, sans everything necessary to animal life, should be able to capture animals, and unerringly to direct its tentacles to one spot by means which apparently must be purely chemico-physical. The author has found that the best method by which to obtain permanent records of the process of aggregation is to place an entire plant in a weak solution of a proteid, coloured with pure methylene-blue, and to remove por- tions of the leaves at different periods. If gentian-violet be used very pretty results may be obtained, the cells of the tentacles actively engaged in digestion being coloured red-violet, those only rcflexly stimulated blue- violet; but the plants only live a short time in solutions of this stain. To obtain the differential stain mentioned, a small piece of any nitrogenous material should be placed on a leaf before immersion, and the leaf examined before all the tentacles have become inflected on it. Fig. 8 shows the masses of aggregated protoplasm in the terminal gland of a tentacle of a plant grown in the manner described. Another point may be mentioned with regard to the colour of the stimulated o P H o w O Q O <1 o Pi H O Q <: O H 6o THE NATURAL HISTORY Ot DIGESTION. cells. Even to the naked eye the glands engaged in digestion appear deeper in colour, due to the aggregation of their contents, while if an entire leaf, on which a small particle of proteid has been placed an hour or two before, be mounted, unstained, in Canada-balsam, after removal of the chlorophyll with alcohol and clearing with a volatile oil, only those glands which have been in direct apposition to the proteid retain their reddish colour. The fact that cells directly engaged in digesting and absorbing food stain red with gentian-violet seems to indicate that they are acid in reaction, and it is interesting to note that the parenchymatous cells of the pedicels exhibit this reaction as strongly as those of the glands themselves. The stronger the stimulation the further down the pedicels does this phenomenon occur. Aggregation of the protoplasm of the cells in Drosera, there- fore, occurs after stimulation of the glands, either directly or indirectly, is synchronous with the movement of the tentacles, does not necessarily indicate increased secretion either of an active or inactive juice, but when the terminal gland is acting accompanies the secretion of a peptic ferment and a greater supply of a vegetable acid, the cell contents apparently be- coming acid in reaction. Probably the process of aggregation has some influence on that of inflection, causing a decrease in bulk in the cells to the inner side of the tentacle, re-expansion occurring through the elasticity of the outer cells whenever the contraction of the inner cells has ceased. It must, how- ever, be stated that no change in the size of the different cells can be made out under the microscope. If the small sessile glands above described, which are scattered over the surface of the leaf, be examined after immersion in a proteid solution, the protoplasm of the two or four cells which go to form them will be found to be aggregated in a similar manner. There is little doubt but that they serve to digest and absorb the portions of the food which have sunk down to the immediate surface of the leaf, out of the reach of the glands at the ends of the tentacles. Both the subjects of digestion and absorption in Drosera have been touched upon in the description given of the inflection of the tentacles and the aggregation of protoplasm in the cells. But a- few points remain which deserve notice. The secretion of the glands before stimulation is neutral, after stimulation acid, but contains no ferment unless the stimulant be of an organic nature containing nitrogenous matter. A very close parallelism exists between the digestive power of the secretion of Droseta and that of the mammalian stomach. Table VI. — The Effects of Stimulation of the Leaves of Drosera Rotundifolia by various Substances. {Compiled from Darwin.) Inflection of Leares. Digestion. Glass of Stimulant. Stimulant. Quantity. Aggregation of Protoplasm. Heigk Notes. Beginning. Degree. Duration. Secretion. Degree. 1. Phyiical One or two light 70 minutes 6 houra Marked 22 hours Present, -but slight None Acid, no ferment Moderate Repeated moderate touches (as rain- .... None None None None Normal, neutral None .... drops) Hot water 100° c. None, cell con- tents granular None .... Killed, tentacles bent back Do. 64 -4' C. None None None .... Paralysed, not killed Do. 48-8*-61-6° C, 2 minutes Several minutes Marked Re-expanded in 10 hrs. in cold water Slight — Leaves more active after Water at ordinary 30 minutes 6-8 hours SUght 7 hours to 2 Slight, in from Often only some of the temperature days 1 hour to 6 tentacles inflected 2. Intolvble. Human hair 000822 m.gr. 30 minutes 2 hours Marked 1-2 days Marked Acid, no ferment Moderate On single tentacles Inactive substances. Fragments of glass A few minutes 2 hours Marked 1-3 days Marked Do. do. Do. Do. do. WUk Small drop 45 minutes 8 hours Great 8 days Marked Active Do. Placed on leaf •taw meat Small cube 10 seconds 10 minutes Very strong 2-6 days Very marked Do. Strong On single tentacle Isinglass ■0296 m.gl'. Some hours 18 hours Slight 26 hours Blood fibrin Incomplete 48 hours .... Syntonin A few minutes 8 hours Very great Slight ^ Very marked Leaves quickly killed Jartilage i in. cube 3 days Active slight 3one Small piece A few minutes A few hours Very great 8-10 days Marked Do. Strong Sresh Casein Do. Within 24 hrs. About 48 hrs. Marked 7-9 days Present Do. Slight Artificially pre- Do. After 48 hours About 72 hrs. Slight gdays Present Do. Very slight Artificiai Casein is pared Casein really insoluble Legumin Do. Within 1 hour 21 hours Strong 3-4 days Present Do. Strong Gluten Do. Within 2 days 3-4 days About 12 hours Slight 7 days Present, marked Do. Slight .... Globulin (Lens) Do. A few hours Strong 3-7 days Present Present Probably inactive .... Heematin Do. Within 2 days Strong Leaves killed Present Active ? Leaves killed Coagulated Egg Cubes of 1'27 A few minutes Within 24 hrs. Strong 10 days Marked Very active Strong Soluble only after Albumin m.gr. digestion Decoction of Cab- bage Urea Small drop A few minutes 4 hours Very strong .... Very marked Active Do. Drop of a Solu- None None None Add, no ferment None tion (1 in 438) = •0674 m.gr. 2. Inorganic. Ammonium Car- bonate = ■0&!B m.gr. Less than 35 minutes 36 minutes Moderate 21-67 hours Very marked ■000482 m.Kr. sufficient — .... With crystals leaves were killed Ammonium Phos- = -0169 m.gr. 12 minutes 6 hours Very strong 48 hours Marked ■ ..• Minimum amount able phate to excite movement O^O0O0O3-28 m.gr. Ammonium Nitrate = -027 m.gr. A few minutes 24-30 hours Strong 61 hours Not well marked b. Ifon-nitrogenotis Cane Sugar Iin73 None Only In stronger solutions Do. do. Acid, no ferment None 1. Organic Starch 1 drop J minim of Solu- None 2. Inorganic. Salts Of Sodium, ra- Various Generally present Many of those causing tion 1-437 pid Heavy Metals, rapid Potassium, etc., none rapid inflection acted as poisons Acids 1-437 About 6 hours From 8-24 hrs Strong Leaves killed in most cases Galliv, Tannic, Tartaric, and Citric had no effect Alkalies Strong Present Present, but an. tftgonised None CARNIVOROUS PLANTS. 6l Substances which are digested by pepsin and hydrochloric acid, as albumin, milk proteids, fibrin, etc., are digested by their glandular secretions. Such substances as cellulose, epithelial cells, mucin, starch, oils, etc., are not digested by either. The process is stopped in both by the addition of alkali, and recommences on the further addition of acid. Both secretions, also, are strongly antiseptic in virtue of the acid contained in them. In addition to their' power of preparing and absorbing animal proteids, both the mammalian stomach and the glands of Drosera can make use of vegetable albumins. When solid inactive bodies are placed on the glands of Drosera no move- ment or increase of secretion occurs until they have sunk through the caps of fluid and actually touched the cells. When the body contains nitro- genous matter a trace dissolved in the fluid serves to stimulate the gland cells, and further to cause the formation of an enzyme, identical in its properties with pepsin. Here again the sequence of events is the same as in the higher animals. Reasoning by analogy, the cells of Drosera contain a zymogen (Droserinogen), which requires a nitrogenous stimulant in the presence of an acid to form the active ferment. (See pp. ll^ et seq.) Rees and Will were the first to separate out the ferment by extraction with gly- cerine, followed later by Lawson Tait. Frankland came to the conclusion that the acid present belonged to the acetic series, either propionic or a mixture of acetic and butyric acids, in addition to which Will found formic acid. Of these acids formic is the only one which does not prove poisonous to the leaves in dilute solutions. As to the digestive power of the acids of the acetic series the reader is referred to the table on p. 172. Absorption of the products of digestion is carried out by the cells of the terminal glands, and, as Bennett first stated, by the small sessile glands on the upper surface of the leaves and the bases of the larger tentacles. Clark demonstrated the presence of absorption by feeding plants with flies soaked in a salt of lithium. The characteristic spectrum of lithium was obtained on burning any part of the leaves. Table VI., which has been compiled from data given in Darwin's book, requires little explanation. The fact that so minute an inactive particle as '000822 m.gr. causes marked inflection of the tentacle on which it is placed, or -00000328 m.gr. (practically Tnr^owo- grain) of phosphate of ammonium brings about movement and marked aggregation of the cell protoplasm, reveals the extraordinary delicacy of the mechanism involved. The result of Darwin's experiments is to show that nitro- genous substances which are digestible excite the secretion of an active juice, while indigestible bodies, even if nitrogenous (with the exception of ammonium salts), only cause an in- creased secretion of an acid but inactive fluid. 62 THE NATURAL HISTORY OF DIGESTION. In Table VII. the results of some observations on the effect of the lower proteids and proteid derivatives are given, from which we may conclude that the glands of the Drosera only respond to proteids of a certain class, and only if further digestion is called for. Serum-globulin, for instance, produces only a slight degree of excitation, nuclein or nucleic acid acts as a poison, and peptone is absorbed so quickly that the leaf on which it is placed soon dies from a surfeit. The albumoses proved to be very stimu- lating and were entirely absorbed in course of time; no poisonous effects, similar to those described as occurring in animals after ingestion of these substances, were observed. Among the class of proteid derivatives urea acts as a poison, except in very minute doses ; kreatinin and tyrosin are not absorljed. Asparagin appears to be absorbed to some extent, but the reaction of the leaves to its stimulus is not so marked as in the case of albumin or albumoses. There are several other species of Drosera, differing much in size and form, but the phenomena of digestion in all of them is practically the same as in D. rotiindifotia. Another member of the Droseracea merits some attention, Dionaa muscipula, commonly called Venus's fly-trap (Fig. 9). Fig. 9. DION^A MUSCIPULA (VENUS'S FLY-TRAP). (Meade Smith, after Darwin.) Side view of leaf, showing marginal spines and sensitive bristles on upper surface of both lobes. A native of North Carolina, it bears bilobed , leaves on a foliaceous footstalk. The two lobes are placed at a little less than a right angle from one another and present two to four small pointed processes directed towards the opposite lobe. The upper margins of the lobes are broken up into many long, sharp, and rigid bristles, so placed that they interlock when the ^'^ ta' j^ £^ S Leaf hou Leaf hou ^ TS rj r-< ' c « '-' S o >> !» o g ii s — J s •3 M 5g -i; 7. n> ■^ y .S <: ^ <; C eg o O -5! ^B JSS os= a t-^ j-iS ^1 i; *_. o c t, jD ^ (3 c 3 o a)_, =J =5 o o HJ3-S o V CO £ -5 ■- — - O M u O F- a ;- c 3 a 1 ^f&MU S .OJ ^ a 3 -laa 1 s ery sm on sin tacle eutero mose 1h m CU P> H c sjs_»._C'S e* ^ , Do. do. at twelve weeks. /, Primitive lungs connected with pharynx ; s, Stomach ; d, Duodenum; i, Small intestine ; i', Large intestine ; c, CBecum and vermiform appendage ; r. Rectum ; cl m A, the Cloaca ; a in .B, anus distinct from si, the sinus uro-genitalis ; v, the jolk sac ; vi, the vitello intestinal duct ; u, bladder urachus ; g, the genital ducts. and laterally, communicating with each other and enclosing mesoblastic tissue in which blood-vessels connected with the umbilical vein develop,. Hollow processes sprout out from the solid cylinders through the liver substance and represent the larger hepatic ducts, the solid processes becoming the lobular tissue of the liver. In the foetus the umbilical vein communi- cates with the vena cava by one branch, a second and larger branch joining the portal vein, After birth the iiriibilical veiq 84 THE NATURAL HISTORY OF DIGESTION. becomes obliterated up to its connection witii the portal vein; the branch to the vena cava also disappearing. The pancreas, like the liver, is first represented by a mass of mesoblastic cells attached to the wall of the duodenum close to but to the left of the hepatic rudiment. It may be detected on the fourth day in the chick. A diverticulum of hypoblastic cells from the bowel wall penetrates the mesoblastic mass, forming the rudi- ments of the pancreatic duct and of its branches. Some observers credit hypoblastic columns of cells pushing in among the mesoblast with the formation of the glandular cells, the mesoblast affording only the vessels and connective tissue. This view is the most probable, but, as in the liver, the question as to how far the hypoblast enters into the composition of the gland, or how much of it is derived from mesoblastic elements, is still a matter of controversy. The anal invagination of the epiblast mentioned above is comparatively slightly marked in mammals. In rabbits the connection lietween the hind- gut and this invagination becomes patent about the tv^elfth day, in birds not until the fifteenth. In all species the uro-genital and anal apertures are at first conjoined in one opening. After the fifth week in all the mammals, with the exception of the Monotretnata, the anal orifice becomes distinct from the urinary and genital opening. SURVEY OF THE AN.\TOMICAL FEATURES OF THE ALIMENTARY CANAL. The anatomical details of the alimentary canal in the various members of Animalia differ with the nature and digestibility of the food. The teeth vary with the actions, gnawing, tearing, and grinding, required to thoroughly masticate the sub- stances taken in ; the form of the lips with the mode of prehen- sion ofthefoodj andthelengthandcomplexity of the alimentary canal with the nature, the digestibility, and the nutritive value of the substances consumed. The method by which the food is seized exerts a marked modifyiiag influence upon the form of the mouth and the structure of the associated organs. Thus, among Monotremata who live on small insects and molluscs, the Ornithorhynchus, or duck-mole, captures the aquatic organisms on which it lives by means of a beak, very similar to the bill of a duck; while the only other member of this order, the Echidna, or porcupine ant- eater, possesses a long and flexible tongue covered with a sticky secretion, by which it captures the insects forming its food. DIGESTION IN ANIMALS. 85 The kangaroo uses its forelegs to bring the food to its mouth. The Sirenia, comprising the manatee and dugong, live on sea- weeds which they obtain by cropping with their fleshy lips. In the Cetacea, the minute marine organisms forming the food of some of the species are strained off from the sea water by means of enormous meshworks of whale-bone or baleen. In the horse the upper lip, in the ox the tongue, and in the sheep and goat both the upper lip and the tongue, are used to bring the food under the action of the teeth. The lower lip in the pig serves to gather the food rendered accessible by the spade- like action of the snout. Carnivorous animals, as the dog, cat, seal, etc., seize their prey with their teeth, while in many the tongue is furnished with horny papilte which serve to remove the flesh from the bones of the animals caught. The rodents use their teeth in obtaining food, their teeth being adapted for cutting and gnawing. One of the most peculiar and specialised organs for the prehension of food is the proboscis or trunk of-the elephant, which must be regarded as a result of evolution based upon the advantage derived by an animal able to reach the foliage on which it feeds, at a height inaccessible to most other species. In the apes and in man the arms and hands serve to lay hold of the food and bring it to the mouth. The Mouth. — The lining membrane of the mouth is, as we have seen, formed by the epiblastic or outer layer of the embryonic cells. The lips are covered with a very sensitive, dry mucous membrane, continuous with the skin. The mucous membrane is supplied with numerous vascular papillae, con- taining nerve filaments and nerve end-bulbs. Inside the mouth the lips and cheeks are covered by a moist, smooth mucous membrane attached in the middle line anteriorly to the mucous membrane of the gums by two folds or frcena, the upper of which is the larger. Numbers of small mucous glands open on the inner surface of the lips and cheeks, termed labial and buccal glands respectively. They appear only to secrete mucus. Corresponding with its epiblastic origin, the mucous membrane Hning the mouth is formed of scaly epithelium on the surface, and of prickle cells, like those of the skin, in the deeper layers. The gums are formed of dense connective tissue firmly attached to the periosteum covering the jaws. In the Monotremata and in some of the Edentata the teeth are wanting, as in the whale-bone whales ; they have no need of them. 86 THE NATURAL HISTORY OF DIGESTION. The Teeth. — Among the mammalian orders provided with teeth three classes may be described : — 1. Carnivorous animals with teeth adapted for tearing and gnawing flesh. 2. Herbivorous animals — A. The Rodentia, with incisor teeth adapted for cutting and molars for grinding. B. The Ruminantia, which are provided with cutting incisors in the lower jaw only, and with grinding molars. C. In the SoUpedes and Pachydermata cutting incisors are present and grinding molars, while in many the lower canines are absent. In the elephant one pair of upper incisors is enormously developed. 3. Omnivora, — The dentition of omnivorous animals forms a link between that of the carnivora and herbivora. The Mammalia generally are supplied with two. sets of teeth, a milk or temporary, and a permanent set. A tooth is developed from the epiblastic or outer layer of the embryo which lines the mouth, by a projection downwards of its epithelium. It is not, as might be imagined, a product of the bone in which it is afterwards embedded. (See Figs. 14 and 15.) At first the downward growth of the epithelium is in the form of a line or groove along the border of the jaw, when it is called the common enamel-germ. The epithelial cells contained in it develop more rapidly at the points correspond- ing to the future milk-teeth; these growths, becoming separate in time, are known as the special enamel-gerins. A papilla rises from below, projects into each special tooth-gum, which forms a cap or covering over it. The epithehal elements become the dental enamel, while the papillae represent the dentine and pulp of the teeth. About the sixteenth week in man a secondary bud develops from the neck of each follicle, composed of epithelial cells, to become later the ten anterior permanent teeth. About the fourth month of fcetal life thin caps of dentine are formed on the pulps of the milk-teeth, covering the outer edges of the papilte, and shortly after enamel begins to be deposited over them. The dentine is formed of large cells, with long filamentous processes situated in the outer parts of the pulp. It is at first uncalcificd, but DIGESTION IN ANIMALS. 87 later nodules of lime salts are deposited in its matrix, forming at length a uniform hard tissue. The epithelial cells of the enamel-germ covering the dentine now become calcified in situ. Fig. 14. SECTION ACROSS THE UPPER JAW OF A FCETAL SHEEP. (Quain, after Waldeyer. x 50.) 1, Enamel germ dipping down into the raucous membrane, where it is half surrounded by a semi-lunar shaped mass of denser-looking tissue, the germ of the dentine and dental sac ; 2, palatin process of the maxilla. Fig. 15. SECTION ACROSS THE UPPER JAW OF A FCETAL SHEEP. (Quain, after Kulliker.) A special enamel germ,/, becoming flask-shaped ; e, c', epithelium of mouth; /, neck of enamel germ ; /', body of flask-shaped enamel germ. the outer cells growing meantime and calcifying progressively from within outwards. The eruption of the temporary teeth is preceded by absorption of part of the bony wall surrounding it. As the tooth emerges bony tissue is again deposited 88 THE NATURAL HISTORY OF DIGESTION. round its neck and fangs, forming a socket. The permanent teeth which succeed the milk-teeth are developed from the epithelial buds already mentioned, in the same way as their predecessors, but growing below and behind them. As the permanent teeth increase in size, the cement and dentine of the milk-teeth are attacked by large cells called osteoclasts and become absorbed. Thus their absorption does not proceed from pressure by the rising teeth alone. The six posterior permanent teeth develop in a similar manner to the milk- teeth. A tooth is thus composed of a vascular pulp, containing blood-vessels and nerves, of dentine secreted by cells in the pulp, and of enamel formed of calcified epithelial cells. The enamel is made up of very hard microscopical columns of phosphate of calcium with a trace of calcium fluoride, 4.4 per cent, of calcium carbonate, 1.3 per cent, of phosphate of magnesium and other salts, only 2 to 3.5 per cent, of animal matter, and 0.2 per cent, of water. The individual enamel columns mea- sure about -s-TffTjth of an inch, .005007 millimetre. On the surface of the enamel in unworn teeth a thin horny membrane is found, "the skin of the teeth," serving as a protective covering. The dentine, on the other hand, consists of much less dense tissue. Its percentage composition is as follows: — Water, 10; phosphate of calcium, 66.7; carbonate of calcium, 3.3; phosphate of magnesium, etc., 1.8; organic matter, 18.2. (Bibea.) Dentine is penetrated by numbers of fine, parallel tubules, about f^au inch in diameter, and twice or thrice that distance apart from each other. Lomas has observed the tubules running into the enamel in the teeth of marsupials, and also, but less distinctly, in man.^ Owing to the fact that the arrangement of the articulation between the upper and lower jaw differs in the various mam- malian orders, the possible movements of the teeth of the lower jaw against those of the upper are dissimilar. In the Carnivora, for example, the lower jaw is narrower than the upper, and the upper canines overlap the molars, so that only an up-and-down motion is possible. No grinding movements can be performed. The meaning of this is not far to seek, as the carnivora feed on easily digested matters, which they tear into pieces of a convenient size and bolt with little or no preliminary mastica- tion. All their teeth are suited for cutting or tearing the food. The incisors are large and sharp, the canines long and pointed, while the molars are irregularly ridged or tuberculated, and, as they overlap like the blades of shears, act as cutting not grind- ^ The description of the development and anatomy of the teeth has been partly taken from Quain's Analoiny. DIGESTION IN ANIMALS. 89 ing agents. The Rodentia are characterised by the back and forward movements of their jaws, and the great size of their incisor teeth. These teeth have a layer of hard enamel anteriorly and a somewhat softer portion of dentine behind. The unequal rate at which these two portions wear away causes the teeth to retain a sharp chisel-shaped free surface. The teeth of the Rodentia are peculiar in that they continue to grow during the whole life of the animals, as do the molars of the ox, while in all other mammals the second teeth reach their full development in a short time after eruption, cease to grow, and even undergo little or no natural repair. The molar teeth in the Rodetitia are marked with transverse ridges across the flat crown. There are no canines. The forward and backward movements of the jaws serve to press the sharp cutting edges of the incisors against those in the opposite jaw and so to cut through the grass used as food, while the same movements cause the trans- verse ridges of the lower molars to grind and bruise it against those in the upper jaw. There are no incisors in the upper jaw of the Rmninantia, the incisors of the lower jaw being opposed by a firm fibro-elastic pad. The incisors have sharp cutting edges, and the molars present flat crowns transversely ridged. The canines are generally absent, or are confined to the lower jaw. The chief point worthy of note as regards the movements of the jaws in the Riuninantia is the extensive power of lateral motion. Strictly speaking, it is circular in nature, the axis of the lower jaw crossing that of the upper, and is alternative — that is to say, the lower jaw is moved to the right, then back to the centre, and to the right again for a certain period of time, for half-an-hour or longer, when the deviation changes to the left. A backward and forward motion is also used. In the other members of the Herbivora, the SoUpedes and Pachy- dermata, a great contrast between the dentition present exists. In the SoHpedes a similar but less extensive lateral motion of the lower jaw occurs, incisor teeth are present in both jaws, canines (in the stallion, but absent in the mare) and irregularly ridged molars are also present. Only the upper incisors are found in the PacJiydermata, taking the form of long tusks which project from each side of the jaw. The molars are very large and few in number, with flat crowns transversely ridged and tuberculated. As the elephant procures its food by means of its trunk no teeth are required for the purpose of seizing or cutting, and all the necessary mastication can be performed by 90 THE NATURAL HISTORY OF DIGESTION. molar teeth adapted for grinding. The Omnivora possess teeth adapted both for cutting and grinding, and can move the lower jaw in the three directions mentioned above. The dentition of man may be looked upon as being midway between that of the Carnivora and the Herbivora. If anything, it inclines more to the carnivorous type than the herbivorous, owing to the constant presence of canine teeth and the character of the molar crowns, which are tuberculated and not ridged. For convenience, the numbers of the various kinds of teeth present in the different classes are generally represented by formulas, the " dental formulae." In such a formula the number of each variety of teeth present on one side only of each jaw is often given, and the number multiplied by two gives the total number present. In the formulae, i. indicates incisor, c. canine, pm. pre-molar, and m. molar teeth. The following table gives a few of the most important of the formula among the Mammalia. Table IX. — Dental FormulcB in the Mammalia. Carnivorous. Dog, 3-3 1-1 4-4 2-2 — c. — ji.m. — I 3-3 1-1 4-4 Cat. 3-3 1-1 3-3 . — c. — p.m. — 1 3-3 1-1 2-2 Herbivorous. Ox. 0-0 0-0,,,^^ .6-6 , = 42 i. — c. — P-™- ^^ — = 32 3-3 3 3 1-1 "■ 6-6 Horse. 1-1 3-3 1-1 3-3 3-3 — = 30 i. — 0. — p.m. — m. — = 40 1-1 3-3 1-1 3-3 3-3 Seal. 3-3 1-1 . — c. 2-2 1-1 p.m. .6-5 = 34 &-D Rabbit. 2-2 0-0 3-3 i. — c. — p.m. — 1 1-1 0-0 2-2 3-3 3-3 :23 Hedgehog. 3-3 0-0 4-4 3-3 . — c. — p.m. — m. — = 36 3-3 0-0 2-2 3-3 Kliinoceros:, 1-1 0-0 0-0 4-4 3-3 i. — 01- — c. — p.m. — m, — ■■ 1-1 0-0 0-0 4-4 3-3 32 Omnivorous. Man. 2-2 1-1 2-2 3-3 i. — c. — - bioiisp. — m. — : 2-2 1-1 2-2 3-3 :32 DIGESTION IN ANIiMALS. 9 1 Hog. 3-3 1-1 3-3 3-3 i. — c. — p.m. — m. — = 40 3-3 1-1 3-3 3-3 The dentitiou of tlie members of the Carnivora given above yields an approximate mean of 3-3 1-1 4-i 3-3 i. — c. — p.m. — m. — 3-3 1-1 3 3 3-3 The liedgoliog being insectivorous, has no need of canines. The Herbivorous animals yield an average formula approaching to : 2-2 /O-O -N 0-0 1-1 6-6 i. — ( — in Eurainantia ) c. — or — p m. & m. — 2-2 \3-3 y 0-0 1-1 6-6 The typical Omnivorous formula contains : 2-2 3-3 1-1 2-2 3-3 i. — or — c. — p.m. — m. — 2-2 3-3 1-1 2-2 3-3 The Tongue. — The tongue is almost entirely composed of muscular tissue covered by mucous membrane. On the under surface this covering is similar to and continuous with that of the inner aspect of the cheeks and lips. The mucous membrane of the upper surface is m'odified to subserve the function of taste. It is covered with papilte of three kinds, which in turn possess numerous secondary papillae under the epithelial cells. The smallest and most numerous papillae, termed^ conical, are most numerous anteriorly, gradually diminishing in numbers towards the base. They are, as their name implies, small conical eminences, and are arranged in lines diverging from the central line or raphe. The conical papillae possess a denser epithelial covering than the other forms, and are of use in the mechanical action of the tongue upon the food. In many of the carnivorous animals these papillae become enormously developed, enabling their possessors to scrape the flesh off the bones of their prey. The second group of papillae, the fungiform, are rounded in form, of a deep red colour, and are most numerous at the apex and borders of the tongue, though also scattered over the anterior two-thirds of the dorsum. Each papilla is abundantly supplied with nerve filaments. 92 THE NATURAL HISTORY OF DIGESTION. The circumvallate papiUce (cf. Fig. i6) form the last group, and are much larger in size, only from seven to twelve in number, and situated in two converging rows at the base of the tongue. Each papilla is placed in a circular depres- sion of the mucous membrane, the smaller end set in the centre of the depression and the broad flat top reaching a little above the level of the surrounding surface of the tongue. In the walls of a papilla a zone of taste-buds opens into the surrounding trench, and in some species another zone is found on the opposite wall. Fig. i6. VERTICAL SECTION OF CIRCUMVALLATE PAPILLA OF THE TONGUE OF THE CALF. (Quaiu, after Engelmann. x 25.) A. Papilla, i?. The surface of the mucous membrane of the tongue, dippinjj; down on each side to form the vallum or trench which surrounds the papilla. In the wall of the papilla, forming one side of the trench, taste-buds may be seen. The nerves of the papilla are shown running upwards and towards the taste-buds. At the bottom of the trench on the left the duct of a gland is seen to open. These taste-buds (Fig. 17) open on to the surface by pores between the epithelium cells. Further in they form globular flask-shaped bodies, their outer covering composed of long flattened cells, their centre of spindle-shaped cells. The central spindle cells are enlarged about the middle to accommo- date their nuclei, while their extremities are prolonged into fine processes, one of which projects through the orifice, the other communicating with a plexus of nerve filaments placed in the tissue below the taste-bud. Flask-shaped bodies of a very similar structure are found scattered over the mucous membrane of the mouth, and even in the skin, of fishes, and are believed to serve the same DIGESTION IN ANIMALS. 93 function. In the Amphibia the taste-buds are replaced by patches of gustatory cells interspersed among the epithelium of the dorsum of the tongue. In the rabbit and hare numbers of taste-buds are distributed in the laminse of a structure called \\\^ papilla foliata situated at the base of the organ. The mucous membrane of the tongue also contains many small glands principally grouped near the base, and some opening into the trenches round the circumvallate papillse. These lingual gla?ids secrete a watery fluid probably contain- ing some mucus, but their function is not clear. The body of the tongue is made up of muscular tissue Fig. 17. TASTE-BUDS. (Quain, after Engelmann. x 450.) Two taste-buds from the papilla foliata of the rabbit. arranged in such a way that free movement of the organ in every direction is attainable. The roof of the mouth is covered by a dense mucous . membrane, which is usually corrugated to a greater or less extent. In some animals these corrugations are very pro- minent and serve to aid the action of the tongue in the disintegration of food, when the morsels of food are pressed against and dragged over them. A continuation of the mucous membrane of the hard palate forms the pendulous structure, or soft palate, which provides a partial separation between the cavities of the mouth and pharynx. 94 THE NATURAL HISTORY OF DIGESTION. On each side, immediately posterior to the soft palate on each side, are found the tonsils. They are composed of lym- phoid tissue in which are enclosed twelve to fifteen crypts. These crypts open by a similar number of apertures on the inner aspect of the gland, and contain in their walls great numbers of lymphoid follicles identical with those described as present over the posterior part of the tongue. The Salivary Glands. — The salivary glands are of three distinct varieties, The first variety secretes a thin, clear, serous fluid, resembling dilute blood-serum ; the secretion of the glands belonging to the second type is a viscid, mucous fluid ; while the third type yields a mixed fluid composed of both serous and mucous secretion. In the Mammalia the parotid gland is almost invariably a purely serous gland, and in the rabbit the submaxillary gland belongs to the same type. In man the submaxillary gland secretes a mixed saliva, but in the cat and the dog it only yields a pure mucous fluid. The sublingual gland is generally a mixed gland, but with the mucous element predominating. The structure of the dififerent salivary glands varies with the characters of the secretion. The serous glands of which the parotid may be taken as the type, are made up of branching alveoli or system of ducts, each main duct progressively sub- dividing until the terminal ducts are reached. The alveoli are united by connective tissue into lobules. On their basement membrane are placed glandular cells, with nuclei at the end next the membrane, but usually in the inactive state so filled with protoplasmic granules that the nuclei cannot be seen (cf. Figs. 18 and 19). After activity the granules disappear from' all parts of the cells except close to the lumen of the tubes. In the submaxillary gland of the dog the alveoli are similarly arranged, but the cells lining the tubules when in- active are large, clear, and bulge into the tubules so as almost to occlude them. The cells are filled with mucigen, or the forerunner of mucin. After discharge the cells appear shrivelled and granular. If they are subjected to prolonged stimulation some cells may be discharged bodily with the mucus. A part of the cell may remain along with the nucleus and serve to regenerate it, or, as Heidenhain held, the cell is replaced by the growth of an adjacent marginal cell. It is possible that during secretion cells are constantly being cast off and new cells developing to take their place. DIGESTION IN ANIMALS. 95 Fig. iS. SECTION OF THE PAROTID GLAND OF THE RABBIT IN AN INACTIVE STATE. (Meade Smith, after Ileidenhain.) The cells are granular, the nucleus often indistinct owing to the number of granules, especially towards the outer part of each cell. Fig. 19. SECTION OF THE PAROTID GLAND OF THE RABBIT AFTER STIMULATION OF THE CERVICAL SYMPA- THETIC NERVE. (Sle.ide Smith, after Heidenhain.) The cells are darker; the outer parts less so than before, the inner parts more so. g6 THE NATURAL HISTORY OF DIGESTION. In the mixed glands in man, such as the submaxillary and lingual, both serous and mucous alveoli are found, although it is not uncommon to find mucous cells and serous cells mingled in the same alveolus. The secretion of the parotid gland is carried into the mouth by Stenson's duct, which opens opposite the second molar tooth of the upper jaw in man, and opposite one or other of the molars in animals. The submaxillary gland occupies a position to the inner side of the lower jaw and on the digastric muscle, and sends its secretion by Wharton's duct to an opening beneath the anterior portion of the tongue. The sublingual gland lies beneath the tongue reaching back from the fr^num, and pours its secretion into the mouth by from eight to twenty ducts, the ducts of Rivini, immediately above the position of the gland. In man the parotid is the largest, the sublingual the smallest of the glands. In all animals save the dog the parotid gland is the largest. In Herbivora, especially the Ruminanda, it is highly developed. As a rule the posterior salivary gland, belonging to the serous type, corresponds in size with the amount of mastication required. The anterior glands, whose secretion serves to lubricate the food swallowed before thorough mastication, are most highly developed in the carnivora and aquatic animals. But the rule is not invariable, for the ox possesses enormous submaxillary glands and relatively small parotids, while in the dromedary the former are small and rudimentary. The volume of the salivary glands in the Herbivora exceeds that in other species, but bears no relative proportion to the amount of saliva secreted. Table X. — The Average Weight of the Salivary Glands with the Mean Quatitity of Saliva Secreted. Animal. Weight of Glands. Saliva Secreted per hour. During Mastication. Horse 509 grammes 5000-6000 c. cm. 0.x 624 do. - 6000 do. In the twenty-four hours. Horse 509 do. 42 kilogrammes. Ox 624 do. 55 do. Dog 25 do. 86 grammes. Man 34 do. 1.5 kilogramme. In the hog the total weight of the salivary glands averages 305 grammes; in the sheep, 83 ; in the cat, 10. The parotid glands of the horse are four times as large as the submaxillary glands, but they secrete twenty-four times the amoimt of saliva; and the submaxillary gland of the ox, though rather larger than the parotid, or equal to it in volume, only secretes one- fourth of the quantity produced by the latter. DIGESTION IN ANIMALS. 97 The (Esophagus. — The structure of the oesophagus is com- paratively simple. It is a tube of varying length formed of three coats. The external coat consists of two layers of muscular fibres, an outer of longitudinal and an inner of circular fibres. The muscular layers are much thicker than in other parts of the intestinal canal except near the anal region. The middle coat is composed of areolar tissue and serves to connect the outer with the inner coat. The inner layer, or mucous membrane, is covered with scaly, stratified epithelium, and contains a number of racemose, branching oesophageal glands, more particularly at the lower end of the tube. They are of the character of mucous glands. Some of the variations in the form of the gullet are mentioned else- where (pp. 255-286), • The Stomach. — In the embryo, as we have seen, the rudi- mentary stomach is placed parallel to the long axis of the body. In the Reptilia and oviparous vertebrates this position persists, though in birds the first indication of a transverse position appears to be more fully developed. In the Mammalia the stomach is a simple or compound sac whose walls consist of four coats — viz., an outer peritoneal covering and three layers corresponding to those of the oesophagus. The muscular coat is formed of longitudinal, circular, and oblique fibres arranged in layers from without inwards. At the lower end of the organ the circular layer forms a thicker annular bundle or pyloric sphincter. The mucous membrane of the stomach is a soft, smooth, pink layer thrown into numerous wrinkles or folds when the stomach is contracted or empty, but with an even surface when the organ is distended. The surface of the mucous membrane is covered with columnar epithelial cells, the type of cell changing abruptly at the point of junction between the oesophagus and stomach. These columnar cells are mucigenous. While inactive they are filled with a clear mucigen — forerunner of mucus — which is discharging during digestion. When the mucous surface of the stomach is examined with the aid of a lens numerous small round openings can be seen amidst the epithelial cells. These are the mouths of the ducts belonging to the gastric glands. The glands are either simple, consisting of one tubule, or compound, and made up of two, three, or even six tubules uniting together at a common aperture. Two types of glands are described. The first type (of. Fig. 20) is represented by the simpler pyloric glands, most 7 98 THE NATURAL HISTORY OF DIGESTION. numerous near the pylorus, and formed by a duct lined with epithelium like that of the mucous membrane of the Fig. 20. A GLAND FROM THE PYLORIC REGION OF THE STOMACH OF THE DOG. (Quain, after Ebstein. Highly magnified.) m, Month of gland opening: into stomach cavity, wider than in cardiac glands, narrowinfj at n to a neck, and expanding below into the active part or fundus; a cross section of this deep portion is shown below, tr. stomach leading down to several terminal tubules lined by a single layer of short, finely granular columnar cells. The DIGESTION IN ANIMALS. 99 second kind of gland — the cardiac^ gland (the fundus-gland of Heidenhain, the oxyntic gland of Langley) — is shorter and composed of one or two tubules lined by coarsely granular epithelium (cf. Figs. 21, 22). These epithelial cells were termed by Heidenhain the principal cells, they are also called the central cells. Between them and the basement membrane of the tubules superadded, parietal, or oxyntic cells are found. These cells are of large size, oval in form, markedly granular, and most numerous at the necks of the glands, though ceasing abruptly when the ordinary epithelium of the stomach begins. They stain deeply with osmic acid and aniline blue, and are readily recognised owing to the bulging of the basement membrane over them. In some animals, as, for instance, the porpoise and pig, the parietal cells are contained in a special pouch of the basement membrane communicating with the main tubule by a narrow orifice. The principal cells in the bird occupy the main tubules, the parietal cells lining accessory branches, while in the Ampliibia the cardiac glands contain no principal cells, parietal cells alone being present, but the lower end of the oesophagus is furnished with cells analogous in form and function to them. During digestive activity the central or principal cells of the cardiac glands and the cells of the pyloric tubules, which when at rest are granular throughout, present changes similar to those described as occurring in the cells of the salivary glands of the serous type; the granules partly dis- appear and partly congregate in the part of the cell next the lumen of the tubule (cf. Figs. 23, 24, 25). The parietal cells enlarge during the earlier periods of activity and then shrink again, but no definite change in the character of their contents has been noted. The principal cells of the cardiac glands and those of the pyloric glands are supposed to secrete pepsin, while the parietal or oxyntic cells are allotted the function of the secretion of the hydrochloric acid. The principal cells of the frog's oesophagus yield only pepsin, while the pyloric region of the stomach has been isolated, and found to afford an alkaline fluid containing pepsin. In neither region are parietal cells to be found. The cardiac glands of the frog's stomach have been found to yield a small quantity of pepsin, although ' The upper part of the stomach where the gullet joins it is known as the cardiac end, the body as the fundus, and the lower as the pyloric end. 100 THE NATURAL HISTORY OF DIG.ESTION. Fig. 21. A GLAND FROM THE CARDIAC END OF THE STOMACH. (Quain, after Klein and Noble Smith. Highly magnified.) 6, Base or fundus of a tubule ; c.t.ecc. (Photograph by Dr. Lovell GuUand.) E-pp. Surface epithehum. zy. Zymogen granules in the cells of the glands. c.t., etc. Connective tissue, hlood-vessels, etc. The photograph was taken from the mucous membrane of a kelt when the glands were growing again on descent from the spawning bed to the sea. layers, and is attached to the raucous membrane by a loose areolar tissue. The surface of the mucous membrane is exactly like velvet pile in appearance, owing to the number of pro- cesses, or vilH, which cover it. It also presents numerous wrinkles, some of which disappear on distension, while others are permanent. The permanent folds, the valvulcB conniventes of I04 THE NATURAL HISTORY OF DIGESTION. Kerkring, are transverse projections of the mucous membrane, reaching in each case from one-half to two-thirds of the cir- cumference of the bowel. The villi cover the whole surface and are about 5V 'o sV inch long, and occur in great numbers in the upper portions of the small bowel, about 10 to 18 per square millimetre, compared with from 8 to 14 in the ileum. This computation yields a total number of about four millions (Krause). (Fig. 26.) Fig. 26. CROSS SECTION OF A VILLUS OF THE SMALL INTESTINE OF THE CAT, (Quain. Highly magnified.) 6, Basement membrane ; c, blood-capillaries ; e, columnar epithelium ; .gr, goblet cell with mucus partly exuded ; i, lymph corpuscles between epithelium cells ; 7w, section of plain muscular fibres ; c, ly central lacteal. Each villus consists of a prolongation of the mucous mem- brane of the bowel, containing blood-vessels, one or more lacteals, and a few unstriped muscular- fibres. The small arteriole which enters each villus runs up the centre, breaking into numerous capillary branches when it reaches beyond the middle point of the villus. The capillaries ramify just below the basement membrane supporting the epithelial cells, and in man unite in a single vein at the tip of the villus. The vein then courses down the centre of the villus to join the venous system of the connective tissue beneath. The lacteal vessel DIGESTION IN ANIMALS. lOS also lies in the centre of the villus. It is usually single in the smaller villi of man, never divided into more than two branches. In the sheep numerous branching lacteals are found. The lacteals communicate with branched cells in the areolar tissue of the villi, and with its cell-spaces. These spaces are in touch with the flattened cells of the basement membrane, from which prolongations extend between the epithelium-cells. The deeper ends of the lacteals join lymph vessels which unite in turn with the central chyle duct. The epithelium covering the mucous membrane of the small intestine is columnar in form and polygonal on cross section. The free end next the lumen of the gut is more highly refracting than the rest of the cell, and shows fine striations. The striae are supposed to indicate either the presence of fine pores or the existence of solid rods. The remainder of the cell has a granular appearance under the microscope from the presence of numerous vacuoles in the protoplasm. Among the columnar epithelium-cells goblet or chalice cells occur which possess a mucus-forming function. The number of these mucus cells varies greatly in different animals. After a meal containing fat the columnar cells become opaque and turbid from the presence of small oil- droplets within them. Shortly afterwards the branched cells in the areolar tissue also become turbid, and finally the central lacteal vessel presents the same appearance. Brunner's Glands. — For one or two inches below the pylorus the walls of the duodenum are studded with small glands similar in structure to the pyloric glands of the stomach, but more complex and extending more deeply into the intestinal wall. Smaller numbers of these glands are found in the lower part of the duodenum. During digestion the epithelium-cells of Brunner's glands are large and clear, becoming granular again when quiescent. In the sheep the glands form a continuous layer round the duodenal wall and appear to secrete a fluid capable of digesting proteids and converting starch into sugars. Lieberkuhri s glands zxq the most numerous of the secretory structures in the mucous membrane of the small intestine. They form minute tubes, opening on the surface between the villi. Their length varies from -j^^th to ^jth inch, and their diameter reaches only to -j^jth inch. The tubes are lined by columnar epithelium resting on a basement membrane, among which are occasional goblet cells. They secrete the succus entericus, and are the main source of this I06 THE NATURAL HISTORY OF DIGESTION. fluid. Other glands present in the walls of the smaller bowel are simply masses of lymphoid tissue; when isolated they are termed solitary glands, when grouped together in oblong patches they receive the name of Peyer's patches. Payer's patches are more frequent in the ileum than in the bowel above it, and are placed longitudinally on the bowel-wall opposite to the insertion of the mesentery, the thin tissue membrane which serves to support the gut and attach it to the walls of the abdominal cavity. Situated in the intestinal wall, there are two fine plexuses of nerve filaments principally connected with the superior mesen- teric nerves. The outer plexus — Auerbach's — lies between the longitudinal and circular muscular fibres of the wall com- municating with the inner and finer plexus, found in the submucous coat — the plexus of Meissner. Tlie Large Intestine. — The large intestine has the same four coats already noted as surrounding the small bowel. Of these only the mucous membrane requires separate mention. It is smooth and possesses no villi, but contains large numbers of the glands of Lieberkiihn, which are larger and longer than in the small intestine. The cells lining them are largely made up of goblet or mucus-cells. In the rabbit all the cells are of this kind, in the dog every alternate one is a goblet-cell, and in man they are fairly numerous. Columnar epithelium of the ordinary type is found between them. Lymphoid nodules are scattered over the surface, similar in appearance to the solitary glands of the small intestine. The vermiform appendix, which juts out from the cscum immediately beyond the ileo-csecal valve, where the small and large bowel join, is a long, slender tube, the walls of which are mainly composed of lymphoid tissue. The appendix is only found in the higher mammalia and the wombat. It is not as yet known what its exact function is, but from its lymphoid structure it has been termed the intestinal tonsil. The large intestine is divided into three portions: the caecum, the colon, and the rectum. The mucous membrane of the caecum and colon is sacculated in many of the vertebrata, but in animals provided with a com- plex stomach capable of causing very complete digestion of the food, as in the ox, both these segments are smooth. The Liver. — The liver in vertebrate animals is a large gland placed so as to intercept the blood coming from the intestinal organs before it reaches the right side of the heart. The blood DIGESTION IN ANIMALS. 10/ is conveyed from these organs by the portal vein, which enters the liver and subdivides into branches ending in a terminal capillary network in each hepatic lobule. These capillaries proceed from the circumference to the centre of each lobule, where they unite to form venous branches of the hepatic veins, which receive as well the blood from the hepatic artery after its circulation through the liver substance. The blood in the hepatic artery serves to nourish the liver tissue. The sub- stance of the liver is made up of a great number of small lobules, ^\ — tV inch (1-2 mm.) in diameter in man. Each lobule consists of hepatic cells occupying the spaces between the capillary network from the portal vein, and separated from one another by fine spaces, the commencing biliary ducts. Small canaliculi in the substance of the hepatic cells com- municate with these adjacent passages. The finer vessels unite to form biliary ducts, and these in turn form the hepatic duct. Attached to the liver there is a membranous sac, the gall- bladder, from which a duct, the cystic duct, proceeds to join with the hepatic duct already mentioned, forming the common bile-duct. The common bile-duct opens into the duodenum. As the liver cells possess no cell-membrane, and are situated close or in apposition to the walls of the portal capillaries, they are in a position to receive and make use of the products of digestion contained in the blood serum with great facility. In a fasting animal the cells are granular and faintly yellow in colour, with one or two nuclei. After a full meal they are more opaque and contain masses of glycogen. The gall-bladder is lined with columnar epithelium and secretes a large quantity of viscid mucus which mingles with the bile from the liver. In the Inverlebrata, the so-called bile is poured directly into the intestine. Among the Vertebrata, the Carnivora and Omni- vora, and most of the Herbivora, Aves and Reptilia, possess gall-bladders. It is, however, absent in the Solifedes (horse, mule, and ass); in the stag and camel; in the elephant, rhinoceros, and tapir; in the wild boar, and some of the Cetacea; and among birds, in the pigeon, cuckoo, parroquet, and ostrich. The opening of the common duct may be separate from or united with that of the pancreatic duct, but is always placed obliquely, so that pressure from the intestinal contents closes it and prevents regurgitation into it. The Pancreas. — The pancreas is one of the most constant of I08 THE NATURAL HISTORY OF DIGESTION. all glands in animals. It exists in most insects and fish, in all reptiles, birds, and mammals. In the Mdtnmalia, and the majority of the Aves and Reptilia, the pancreas is situated in a bend formed by the duodenum. In rodents and in the cat the duodenal mesentery is wide and the pancreas arborescent or scattered; in the dog and other mammals where the mesentery is short or absent the gland forms a thick elongated body. The pancreas communicates with the duodenum by one or two, sometimes several, ducts. The dog and rabbit possess two ducts, the upper one opening along with the bile duct into the bowel. In the cat and seal the ducts are more numerous. The pancreas is an acino-tubular gland closely resembling the salivary glands in structure, though softer and less compact. The ultimate branches of the ducts communicate with the glandular alveoli and are lined with flattened epithelium. As these unite to form the larger ducts the epithelium becomes first cubical then columnar in character. While the gland is inactive and at the earlier stages of activity the alveoli are almost entirely filled by the secreting cells ; at a later stage the shrinking of the cells causes an enlargement of the lumen. The characteristics of the secretory cells are described else- where (p. 122). I op CHAPTER V. Ferments and Ferment Action. Unorganised Ferments : — Action — Theories — Berzelius — Liebig — Fischer — Bunge — Tammann — Pohl — Common Characters — Zymogens — Ptyalinogen — Trypsinogen — Pepsinogen — Rennet-zymogen — Other possible Zymogens— Glucoside-splitting Ferments — Myrosin — Emulsin. Organised Ferments : — Properties — Products — Unformed Ferments produced by Formed Ferments. Those bodies which have received the name of Ferments may be divided into two distinct classes — the unorganised and the organised ferments. The latter class comprises all those minute types usually described as micro-organisms, each indi- vidual representing a complete and living entity capable of producing its kind. The fermentation which is caused by them is merely the result of their growth and development. Their power of digesting or simplifying complex bodies is treated of further on. The unorganised or unformed ferments are the products of living cells, and do not of themselves con- stitute living entities. THE unorganised FERMENTS. Living protoplasm, whether it be in the form of a wandering Amceba, the active part of a vegetable cell, or the living force in an animal protoplast or cell unit, always possesses certain fermentative properties. It has the power of producing from its own constituents highly complex proteid bodies endowed with the faculty, indispensable for the continued life of the protoplast, of initiating and facilitating chemical changes in other bodies of high potential energy, whereby they are transformed into derivatives of less potential energy, without a commensurate expenditure of force. These unformed or unorganised fer- no THE NATURAL HISTORY OF DIGESTION. merits — enzymes, as Kiihne has christened them — are incapable ■in themselves of any further reproduction or increase, in direct contrast to the organised ferments, which increase and multiply in the actual course of their specific action. The organised ferments are killed by the action of alcohol, formalin, chloro- form, and many antiseptics which do not prevent, though they may retard, the processes rendered possible by the unorganised bodies. The action of an unorganised ferment or enzyme may be compared to that of oil in the working of machinery. In fact the words ferment and enzyme (from ^V'?) 3- ferment) are ill-suited to describe the active agents of digestion. A satis- factory term has yet to be found. Lubricant, to the author's mind, describes their action more precisely, especially as the derived substantive denotes instability as well as the property of facilitating the motion of bodies in contact. An enzyme has no power in itself to cause chemical changes in the organic compounds on which it acts. It merely renders them more unstable, allowing the primary agents — as, for instance, acids in peptic, and alkalies in tryptic digestion — to act in a state of greater dilution or at a lower temperature than they require when unaided. For instance, if albumin be boiled for a long period in a dilute solution of a mineral acid, in course of time the same changes occur in it as are much more rapidly pro- duced at a lower temperature if a proteolytic ferment has been added. The acid is capable by itself of decomposing the albumin, but only if added in strong solution, or, if dilute, when a high temperature is maintained for a long period. When even a minute amount of pepsin is added the complex albumin molecule appears to become less stable, and a dilute solution of the acid is able to split up the molecule into smaller and more hydrated bodies. To return to the simile of the lubricating oil : just as, in the movement of machinery, the quantity of oil used to allow of easy motion is extremely small when compared with the results obtained, so in the chemical movements termed fermentative, occurring in both plant and animal organisms, the quantity of ferment destroyed is infinitesimal and out of all proportion to the amount of work which it has enabled the chief factor in the process to achieve. Still another parallel exists; for in time the oil becomes thickened with the results of the friction of the machinery, and the wheels become clogged; while the FERMENTS AND FERMENT ACTION. Ill ferments cease to act if the products of their action are not removed by absorption or dialysis. The underlying principle of all unorganised ferment action in nature consists in catabolic changes, whereby complex organic bodies are split up into simpler compounds, with which additional atoms of the elements of water are united. The chief unorganised ferments which act in the processes of digestion may be divided into four classes. The action of the first class is comparatively simple, the class comprising the diastatic ferments, which act on carbo-hydrates, and the proteo- lytic ferments, which simplify proteid bodies. The chemical changes effected by the members of this class are elementary but extensive. The ferments of the second class, on the other hand, produce complicated chemical changes, and are exempli- fied by emulsin of the almond, and myrosin of mustard. This class has the power of splitting up glucosides into glucose and various other bodies. The fat-splitting ferments form the next class. By their means neutral fats are resolved into glycerine and free fatty acids. Ferments of this class occur both in plants and animals. The last class of which mention need be made is one which is represented by rennet, or the milk- curdling ferment; this also is found in both animals and vegetables. The precise mode in which rennet acts is as yet unknown, the coagulation of casein produced by it appears to be simply a molecular change, the previously soluble casein becoming insoluble in water. The coagulum formed from milk on the addition of acids differs from that produced by the milk-curdling ferment in two particulars — it is much less easily soluble in dilute acids, and contains no phosphate of lime, the coagulated casein due to the action of the ferment dissolving readily in acid media, and always con- taining phosphate of lime ; in fact, rennet is entirely inactive if earthy phosphates are absent. In speaking of the peptonising ferments, the changes brought about by their action are described as simple, regarding them from a chemical point of view. It should be stated, however, that in plants much more profound decomposition of proteid bodies occurs when they are converted into asparagin (CgHgNHg) (COOH,), and tyrosin (QH,(^^jj^ ^^^^^ (COOH), ^°^^ °^ which are crystalline and diffusible, requiring only some carbo-hydrate bodies and a compound of sulphur to again form 112 THE NATURAL HISTORY OF DIGESTION. proteid molecules, an anabolic process which is performed by the living protoplasm of the cells. In animals these bodies, together with leucine and glutamic acid, are formed during tryptic digestion from proteids, but here they are by-products and forerunners of urea, rather than diffusible and soluble means of conveying nitrogenous compounds to the cells. Two additional ferments have been described as existing in the small intestine, namely, the " inverting" and the " maltose- converting" ferments. The first assists in the transformation of cane-sugar into dextrose and lasvulose, the second helps to convert maltose into grape-sugar. Although it is by no means proved, it is most probable that all these actions are only rendered possible by the presence of the different ferments, the various acids and alkalies present being the active agents. All the changes induced by ferment action can be artificially produced by the aid of strong acids and alkalies, of heat, or of both combined. The action of organised ferments cannot be thus imitated. Many of the organised ferments produce unformed enzymes, some of which, as in the Micrococcus urecB, have been shown to act within the body of the cell producing them. As a general rule, unformed ferments act extra- cellularly. Theories of Ferment Action. — Berzelius was the first to ascribe to soluble ferments a "catalytic" action. Reasoning from the various reactions which platinum black — platinum in a state of extremely fine division— is capable of causing, while undergoing no change itself, he argued that mere con- tact in certain cases was sufficient to bring about chemical action in other bodies, with rearrangement of their elements, and without any change in the provocative substance. Subsequent research has, however, shown that the catalysing body does not remain absolutely inert. Indeed, a catalytic force which can exert kinetic, and at the same time, retain all its potential energy, is opposed to every natural principle. Still this theory has its supporters, probably owing to its simplicity. Liebig modified the catalytic theory by suggesting that ferments were complex bodies in a state of instability. They were prone to decomposition, and the changes resulting in the rearrangement of atoms started similar changes in bodies exposed and susceptible to their action. According to Liebig, the action of ferments is purely fortuitous. Others have likened FERMENTS AND FERMENT ACTION. II3 the action of ferments to the force employed in exploding a charge of gunpowder, a change from unstable to stable chemical bodies. A small amount of energy expended may produce effects quite out of proportion. But there is this objection to be taken to such a view, that the action pro- duced by a ferment is strictly proportionate to, though immensely greater than, the quantity of ferment present, while there is no limit to the amount of gunpowder or other explo- sive which can be decomposed by the same initiatory force. Lately, Fischer finds the ground for ferment action in " stereo- chemical relations," that is in the geometrical configuration of the substances acted on. Nasse argues that if ferments are the cause of the transposition of free ions, the " leitungsfahig- keit"^ — conductivity — of watery solutions will be increased over that of water itself, or of watery solutions containing the ferment after it has been rendered inactive. Table XL — Electric Conductivity. {Nasse.) The resistance of watery solutions of a ferment, after being boiled, equals - 2106 Dittp, before boiling - 2556 When the resistance was tested in sohitions containing ferments after boiling, and bodies on which they could act, the figure was - 2124 Ditto, with unboiled ferment 2082 "The increase of conductivity shown above, as occurring in the solution containing the raw ferment and fermentable substances, is to be regarded as due to an increased dissocia- tion of water, and therefore Ihe formation of ions by ferment action must be held as proved." Bunge, in his admirable Lehrbuch der physiologischeti und pathologischen Cheinie, con- cludes his description of the various changes produced by unformed ferments and the analogous changes which can be brought about by metals and living cells, thus : "We see from all these examples that our knowledge of the ferments is limited to calling them catalysing bodies. They are substances whose presence is necessary for the initiation of those chemical changes which give the impulse for an alteration of an atom-complex from an unstable to a stable substance of the same bulk. We speak of 3, catalytic action if the substance whose action is described as being of this nature is a known inorganic compound or a single element. If the substance, on the other hand, be of 114 THE NATURAL IIISTORY OF DIGESTION. unknown organic composition, we speak of ferment-action.'' That is to say, Bunge recognises that we know little about the changes brought about by inorganic catalytic processes of chemistry, and that the sum of our knowledge concerning them is the sum of all that we can confidently assert in connection with the analogous actions of organic ferments. Tammann {Zeitschrift fur fhys. Chem., xvi. s. 271) suggests that the action of hot water and acids in producing changes in complex chemical bodies similar to those brought about by ferments may be due to transference and movements of oxygen ions. The ferments, on the other hand, are not electrolyte, but may contain certain atom-complexes in their molecules which have an affinity for certain atom-complexes in the molecules which are split up. Jacobson has lately contradicted the state- ment, made by Schonbein, among others, that the specific action of unformed ferments is very nearly akin to the power possessed by inert bodies, as, for instance, platinum black, of decomposing peroxide of hydrogen into oxygen and water. Injection of the unformed ferments into the body is followed by the pro- duction of toxic symptoms, with fever and a tendency to haemorrhages or local blood clotting. Many have suggested that these symptoms are due to the introduction of organisms along with the ferments, owing to the difficulty of sterilising them without destroying their specific action. Kionka {Deutsch. Med. Wochen., 1896, No. 38) has lately shown that a solution of invertin, rendered sterile with corrosive sublimate by Geppert's method, remained active but still caused similar toxic symptoms, though to a less degree. E. Buchner was able to produce the alcoholic fermentation of sugar by means of the fluid material expressed from large masses of yeast, and absolutely free from all living yeast cells. Neither chloroform nor a I per cent, solution of arseniate of sodium arrested the action. II. Buchner draws an analogy from this between alexins, unformed ferments, and toxins which may form part of the active contents of cells though not products of their metabolic processes. Pohl {Arch. f. Exper. Path. u. Pharmak., xxxviii., i u. 2, p. 65, 1896) draws attention to the lack of uniformity existing between the different unformed ferments, both of anhnals and of plants. Thus the ferment contained in liver extract can oxidise formaldehyde into formic acid, g>c=o go>c=o, but is unable to form " Indo-phenolblau " from o-naphthol or phenylendi- amin ; while a ferment obtained from the leaves of plants, or from pine- needles, can only act conversely. So amygdalin produces indigo almost at once from the above-mentioned substances, but has no action on formalde- hyde. The indigo-reaction as a test for ferment action is liable to error, *f)d only those ferments which can oxidise aldehydes into members of the FERMENTS AND FERMENT ACTION. US carboxyl group answer to a test whicli is usually thought to be conclusive, i.e., their destruction by heat. In a work of a philosophic cast Arthus (Nature des Enzymes, Paris, 1896) argues that the essential nature and mode of action of enzymes, or unformed ferments, are dependent on properties of material substances rather than materialistic in themselves. He regards unformed ferment action as due to certain modes of motion in matter. The fact, however, that formed fer- ments can secrete enzymes capable of acting furth of their bodies, and that these enzymes gradually disappear as the fermentative action, induced by their presence, proceeds, renders the possibility of this view being in accordance with the actual mode of the action very doubtful. The Common Characters 0/ Unforined Ferments. — No one has succeeded as yet in obtaining any of the unformed ferments in an absolutely pure state. Owing to the difficulty of separat- ing the ferments frorn the accompanying proteid bodies dissolved in an extract of the glandular tissue, or in its secretion, the opinion come to by many observers, that the ferments themselves are similar to proteids in composition, cannot be accepted implicitly. As Hammarsten remarks, it has not yet been determined whether the bodies isolated are pure enzymes, or enzymes in conjunction with albuminous bodies. Meade Smith, in his treatise on the Physiology of the Domestic Animals, hazards the statement that purified ferments do not give the proteid reactions, but his statement must be received with caution, as no proof is advanced of the manner in which the specimens alluded to were purified. Schomnow and Simanowsky have lately published some analyses of pepsin obtained from the gastric juice of a dog in whom oesophagotomy had been performed, and, as well, a gastric fistula made. No saliva could enter the stomach, and the juice was obtained as secreted. The pepsin was obtained by drying the juice in vacuo, saturating with sulphate of ammonium, or by cooling to 0° Cent. The purest pepsin was obtained by the cooling process. It always gave the reactions for proteids, was easily soluble in water, and acid in reaction— giving the tests for free hydrochloric acid ; it digested albumin, especially if hydrochloric acid up to 0.6 per cent, was added, and it was soluble in glycerine. When dry it was of a slight greenish-white colour, and when incinerated it yielded an unweighable quantity of ash. Ultimate analysis gave the following result : — Table XII. Pepsin from Cooling. Carburetted hydrogens 50.73 per cent. Hydrogen - 7.23 „ Chlorine - 1.17-1.01 ,, Sulphur - 0.98 ,, Nitrogen — Pepsin from Sulphate of Ammonium. 50.37 per cent. 6.88 „ 0.89 ,, I-35-I-34 „ 14-55-15.0 „ Il6 THE NATURAL HISTORY OF DIGESTION. These authors conclude that pepsin is a proteid body which has a certain proportion of chlorine in its molecular structure. On the other hand, Hammarsten states that the purest specimens of the milk-curdling ferment which have as yet been obtained do not give the ordinary reactions characteristic of proteids. We may conclude that the true chemical nature of the unorganised ferments is still unknown, but that the majority of them do not obscure by their presence, or even vary, the common proteid reactions exhibited by the extracts containing them, and that, as they are a product of the activity of proto- plasm, they are either very similar in composition to proteid bodies, or are derivatives split off from them. All enzymes are soluble in water, and probably in glycerine. Langley states, however, in a note printed in Gamgee's Fhysiological Chemistry, vol. ii., that it is by no means certain that pure, sti-ong glycerine can dissolve any quantity of an enzyme. In most cases dilute glycerine is employed, the water (or dilute salt solution) being the active solvent. All enzymes are incapable of being dialysed, are carried down by fine precipitates of other substances in a purely mechanical manner, a property which is made use of in their artificial preparation, and when in solution all are destroyed by heat. Solutions of enzymes for the most part are at once rendered inactive by boiling ; the highest temperature at which a ferment can exist and still retain its special properties is variously stated at 65°, 70°, and 80° C, though exposure to these temperatures for a prolonged period ultimately destroys them. Each ferment has an optimum temperature, at which its activity is greatest. Thus pepsin and the other ferments met with in the animal body act most strongly between 40° and 45° C. For diastase and malt ferment the best teinperature lies between 60° and 65° C. (Chart I.) At the ordinary temperature of the air ferments are all capable of some degree of activity, while at 0° C. those of cold-blooded animals and plants, chiefly owing to their great concentration, possess powerful digestive properties. At the latter tempera- ture the ferments of warm-blooded animals when in normal strength lose most of their activity, though they are not destroyed. On the other hand, experiments show that those of cold-blooded anirnals and plants, though sufficiently active at the ordinary temperature of the air or of water to do what is required of them, have their power greatly FERMENTS AND FERMENT ACTION. 117 increased at a higher temperature. If ferments be exposed in an absolutely dry condition to a temperature of even 150-160° C. their activity is not destroyed. Hiifner heated dry pancreas-ferment to 100° C. ; Salkowski exposed dry pepsin to a temperature of 150° C. and invertin to 160° C. CHART I.— Showing the effect of Temperature on the Activity of Salivary and Pancreatic Ferments (after Roberts). Relative Digestive Power. 100 = Maximum 100 90 80 70 60 5° 40 30 TEMPERATURE (Centigrade). 0° 10° 20 30° 40° 50° 60° 70° 80° 90° f '"\ . 1 l\ \ / / \ \ \ / / / / \ \ \ PANCF UIAS EATIC ' rASE _' 1 / \ I \ 1 1 '- 1 ALIVARY (DIAS PTVALIN ASEJ i '; I \ \ , // 1 1 U TRYP J' \ \j / / \ \ \ \ / ' • / / \ '> \ \ r \ \ for several hours at a time and found that they had lost none of their activity. All unformed ferments are precipitated from their solutions by alcohol, but their efficacy is not destroyed nor are they rendered insoluble in water or glycerine afterwards. In this they resemble native proteids such as albumirL and globulin as well as the proteoses and Ii8 THE NATURAL HISTORY OF DIGESTION. peptones derived from these bodies. Unorganised ferments, like most proteid bodies, are precipitated by saturation of their solutions with sulphate of ammonium. The precipitation of ferments by means of the addition of solutions of the heavy metals renders them inert. (Cf. Table XIII.) Table XIII. — Unormnised Ferments. Ferment. Action on Product. Vegetable. Animal. 1. Diastase Starch Maltose In most plants 2. Do. Inulin Do. Compositse, etc. 3. Ptyalin - Starch Maltose In saliva 4. Amylopsin Do. Do. Pancreatic juice B. ? - ... Cellulose Glucose In seeds and tree fungi 6. ? - . - - - Do. Marsh gas, acids, etc. Bacteria Bacteria in the intestine of herbivora 7. Invertin Cane-sugar Dextrose and laevulose Do. do. Fungi 8. Do. Maltose Intestinal glands 9. ? Glycogen Dextrose In the liver? 10. Glucoside-splitting Glucosides Glucose, etc. In plants 11. Steapsin - . - - Neutral fats Glycerine and fatty acids Pancreatic juice 12. Similar body Do. Do. In seeds 13. Proteolytic Proteids Peptones Do. In stomach and ». Active in neutral ■\ - Fruit and seeds pancreas solutions, papain j of some plants &. Active in acid solu- tions, pepsin V Proteids Peptones In carnivorous plants Gastric juice V. Active in alkaline Pancreatic juice solutions, trypsin ) 14. Rennet - - - - Casein ogen Casein and whey-albu- mose Pinguicula, etc. Stomach and pancreatic juice A recent paper by Pekelharing (Zlscht. f. phys. Chein., xxii. 2, s. 233) deals with a new method for the preparation of pure pepsin. The mucous membrane from ten pigs' stomachs is finely chopped up, and digested with six litres of a o. 5 per cent, sohition of hydrochloric acid for five days at 37° Cent. The solution is then filtered and dialysed for 15 to 20 hours in running water. The contents of the dialyser are now filtered, the precipi- tate caught on a filter paper is dissolved in o. 2 per cent, hydrochloric acid and again dialysed as before. The precipitate which forms is collected on a filter paper, washed thoroughly with distilled water, and dried over sulphuric acid. Treatment of the original filtrate by concentration, addi- tion of basic acetate of lead and ammonia, separation of the resulting precipitate by removal of the excess of lead with oxalic acid, and dialysis of the remaining substance, affords a precipitate identical with the first product obtained. The product, which possesses an extraordinarily great peptic power, dissolves with difficulty, when freshly made, in distilled water, FERMENTS AND FERMENT ACTION. II9 is much more soluble in weak salt solutions, and when dried forms a pale yellow powder with a slight power of absorbing water. Unfortunately Pekelharing was unable to procure a sufficient quantity of this pepsin to subject it to ultimate chemical analysis. He ascertained, however, that it gave the proteid reactions, contained phosphorus, and was split up, by heating in acid solutions, into a nucleo-proteid insoluble in acids, a sub- stance containing phosphorus easily soluble in warm alcohol but with difficulty in cold, and into an albumose. If a solution of the pepsin was very gradually heated on a water bath this decomposition did not occur, but at the same time the pepsin lost its digestive power. It is possible that Pekelharing's product is absolutely pure pepsin free from foreign proteid bodies, and this view is supported by the fact that both the loss of peptic activity and the actual splitting up are caused by identical conditions. The statement made by many that, because an artificial gastric juice frequently fails to yield positive evidence of the presence of a proteid, pepsin cannot be of that nature, is shown by Pekelharing to be erroneous. His pepsin answered to all the proteid tests when in a concentrated solution, but when added to a dilute artificial gastric juice gave none of these reactions. The most striking and novel fact with regard to the pepsin thus obtained lies in the discovery of phosphorus compounds, which can be split off from it in the form of a nucleo-proteid, and an undetermined body as well as an albumose. As far as we know, no one before this has suggested that phosphorus is one of the elements of an unformed ferment either combined in the radical of nuclein or in another form. No chlorine is present, sug- gesting that the chlorine found by Schomnow and Simanowsky in their pepsin was derived from the hydrochloric acid of the gastric juice. Hjbrt {Centralbl. fiir Physiol., a. p. 192, 1896) has found that the juice of the majority of the higher fungi — i.e., mushrooms, toadstools, etc. — possesses a proteolytic power resembling that of trypsin. Albumoses, peptone, and ainido-acids can be obtained by its action upon albumin and globulin. The various forms of these fungi contain ferments which differ from each other in minor points.-' Zymogens. — The ferments which are secreted by the different glands engaged in the general process of digestion are not con- tained in the cells of the glands in a fully formed state. For example, the fresh pancreatic tissue may be free from any ready- formed proteolytic ferment, or may only contain traces of it. In a short time, a few hours, the pancreas after removal from the body always contains trypsin, and the time of its appear- ance may be hastened by treatment with dilute acids. Heidenhain, who first discovered this fact, called the forerunner ' In the acephalic moUusca {Artemis exoleta, etc.) Tieri and Patier (Arch, de Phys., 1897, No. 60) have demonstrated the presence of an oxidis- ing ferment. It is only found in the tentacles, feelers, and blood, is soluble in water, and precipitated by alcohol. The ferment is most active at 60° C, and is destroyed at 100°; possesses most power in acid solutions, such as 0.2 to 0.3 per cent, acetic acid, little influence in neutral, and none in alkaline media. I20 THE NATURAL HISTORY OF DIGESTION. of trypsin, zymogen. The term zymogen is now applied to the antecedents of the different unorganised ferments, so that Heidenhain's original zymogen may be termed the zymogen of trypsin, the others zymogens of their several ferments. Reason- ing from analogy, we may confidently assert that all ferments are produced when required by the active cells of the different glands from an inactive precursor, formed by and present in these cells. On receipt of the proper stimulus the cells dis- charge their zymogens, which are probably converted into ferments or enzymes outside their walls. The Secretion of Ferment-yielding Material — Ptyalinogen. — For some time after the discovery and identification of the zymogens of pepsin and trypsin the actual existenceof a zymogen of ptyalin in the cells of the salivary glands could not be sub- stantiated, Griitzner thought that it was present in the saliva of the rabbit, and Goldschmidt, in an incomplete research, suggested its presence in that of the horse. Recently, Latimer and Warren have announced that it is present in many animals, especially in the rabbit, but absent in others, as for instance, in the sheep. They obtained the zymogen by extraction with chloroform water, in many cases after the action of very dilute acetic acid. The microscopic appearances of the secreting cells of the salivary glands differ very markedly in direct relationship to the activity of the cells examined. Mislawsky and Smirnow describe the cells of the parotid gland of the dog, when fasting, as having a single nucleus, protoplasm with fine granules, through which a delicate network could be descried, and a homogeneous ground substance. After section of the fibres of the sympathetic nerve and stimulation of the auriculo-temporal nerve, which conveys the nerve impulses from the brain to the gland, the granules become less in number, larger in size, vacuoles appear especially in the periphery of the cells, con- taining a slightly coloured substance and the remains of the network; while the nuclei become more distinct and larger. Fig. 32, on page 224, gives a representation of the nervous supply of the parotid gland. If, on the contrary, the auriculo-temporal nerve is cut and the sympathetic fibres stimulated, no flow of saliva occurs, the cells shrink, the network shrivels, the granules become small and numerous, and the nuclei increase in size. If the auriculo-temporal nerve is stimulated while the sympathetic fibres are intact, the granules completely disappear, large vacuoles fill the FERMENTS AND FERMENT ACTION. 121 cells, often leaving naught but the nucleus, and even that now and then vanishes, while Boll's star-shaped connective tissue cells become very prominent. During the disappear- ance of the granules, Langley observed that they gradually aggregated in the inner part of the cell, that is, the part next the lumen of the duct to which they were apposed. (Figs. 1 8 and 19.) The formation and extrusion of granules by the cells of the secretory glands are actions of the greatest importance, some indeed look upon these granules as actual zymogens, others are of opinion that the fully-formed enzyme is secreted by the cell. The furthest point to which it is safe to go in this matter is to state that the extrusion of granules by secretory cells implies a power of discharging protoplasmic products. That these products do contain a zymogen, in addition to other substances, is proved by the fact, ascertained by Goldschmidt, that the saliva of the horse does not become active if kept absolutely aseptic ; the moment it becomes in- oculated with an organised ferment, or micro-organism, as when it enters the mouth, the ptyalinogen which it must contain yields the ferment ptyalin, and the saliva immediately exhibits an amylolytic or starch-converting power. Trypsinogen. — The cells of the pancreas exhibit very similar changes during activity. Heidenhain was able to differentiate between two zones in each cell, the outer easily stained, of a homogeneous character, and containing the nucleus; the inner, granular, staining with difficulty, and containing a fine network throughout its substance. The outer zone was regarded by Heidenhain as the protoplasmic, the inner as the " proto- plasm-product " zone. In the fasting condition the inner and granular part occupies about two-thirds of the cell, the outer only about one-third; after activity the proportions are reversed, while the whole cell becomes smaller. Kuhne and Lea have observed during life in the pancreas of the rabbit, which in that animal is scattered in isolated lobules between the layers of the mesentery, the actual process of secretion, and describe the gradual diminution in size of the inner zone, the aggrega- tion of the granules towards the lumen of the tubule, their disappearance, and the increase of the protoplasmic, homo- geneous zone as the granular one decreases. The whole process represents the flow of protoplasm and its products from the part of the cell nourished by the blood to the lumen of the tubule in whose walls the cell is placed. (Figs. 27 to 31.) 122 THE NATURAL HISTORY OF DIGESTION. The pancreas when absolutely fresh contains no trypsin or only a trace of that ferment. If examined a few hours after removal from the body trypsin can always be detected. The conversion of the inactive zymogen into the active trypsin is connected with a change in the reaction of the gland tissue. After removal from the body the gland gradually becomes of acid reaction, and Heidenhain has proved that this acidity has to do with the conversion by showing that the addition of acids hastens the process. Trypsinogen is soluble in glycerine and in water, but splits up when dissolved in water, yielding trypsin, the rapidity of this action increasing with the temperature. Fig. 27. THE PANCREAS OF THE DOG WHEN INACTIVE DURING FASTING (IN SECTION). (Meade Smith, after Heidenhain.) The cells show two zones. The one next the lumen of the tubule clear and only slightly granular, the cuter zone granular in apj'earance and containing the nucleus. If the solution be acid in reaction the decomposition occurs more quickly. It can also take place in solutions of neutral and alkaline salts. In a fresh neutral solution of the pancreatic extract in glycerine trypsinogen appears to remain unaltered for an indefinite period. IClihne found that FERMENTS AND FERMENt ACTION. 123 alcohol causes the appearance of the actual ferment, and Podolinski (quoted by Heidenhain) has stated that the passage of oxygen through its solution sets trypsin free, although hydrogen has no such action. Peroxide of hydrogen and platinum black act similarly to oxygen. Trypsin can be extracted directly from the pancreas with glycerine if the gland is immersed for a short time in one per cent, acelic acid. As acids soon destroy the activity of trypsin the preliminary addition of acid must be of short duration. Pepsinogen. — If a glycerine extract of the mucous membrane of the stomach be added to a solution of albumin and acidu- FlG. 28. THE PANCREAS OF THE DOG DURING THE FIRST STAGE OF ACTIVITY. (Meade Smith, after Heidenhain.) Ttie zone next tlie lumen is now much more granular ; the zone further from it is narrower. lated with hydrochloric acid, and if to another portion of the albumin solution the same amount of an extract, procured by the action of a dilute solution of hydrochloric acid on the gastric mucous membrane, be added, and the proportion of hydrochloric acid present rendered equal to that in the first specimen, it will be found that the glycerine extract contains a much less active ferment than the extract made by means of 124 THE NATURAL HISTORY OF DIGESTION. the dilute acid. This difference occurs whether the mucous membrane of the fundus or of the pyloric region of the stomach is used for the extraction. Reasoning from this fact, Ebstein and Griitzner assumed that the body removed from the gland cells could not be active pepsin, but a forerunner of that ferment which became active under the action of hydrochloric acid. This pro-pepsin, or pepsinogen, as it is called, was Fig. 29. THE PANCREAS OF THE DOG DURING THE SECOND STAGE OF ACTIVITY (IN SECTION). (lleade Smith, after Heidenhain.) The cells are returning to the state present during tasting. thought by them to be a combination of the ferment with the proteids of the gland cells. This suggestion, however, is rendered of doubtful value by the fact that glycerine is unable to extract all the pepsin from proteid bodies should they be contained in the same solution. Schiff found that an extract of the gastric mucous membrane, produced by means of acidu- lated water, increased in digestive power for some weeks. This, he thought, might be due to the presence of pro-pepsin, which FERMENTS AND FERMENT ACTION. 125 Fig. 30. ACINUS OF THE PANCREAS OF THE TROUT IN THE RESTING STAGE. C.t.Tl. Ct.f. (From a drawing by Dr. Lovell Gulland. X 1000.) cA.f. Connective tissue fibre; ci.n., connective tissue nucleus; 2.^., zymogen gr.uiules. Fig. 31. ACINUS OF THE PANCREAS OF THE TROUT IN THE ACTIVE STAGE. ■c.t.r. (From a drawing by Dr. Lovell Gulland. x 1000.) c.tf. Connective tissue fibre ; z.g., zymogen granules much diminished in number. 126 THE NATURAL HISTORY OF DIGESTION. was only slowly converted into pepsin. Later evidence shows, however, that this body is converted into the ferment exceed- ingly rapidly in the presence of even very dilute hydrochloric acid. Although these observers suggested the probable exist- ence of pepsinogen, Langley {/ourn. of Phys., vol. iii. p. 269) was the first to show conclusively that such a body exists. He found that pepsin, when digested at 40° C. with a .5 to i per cent, solution of sodium carbonate, is rapidly rendered inactive; pepsinogen, on the other hand, when similarly treated, is only CHART II.— SECRETION OF PEPSIN. Hours afierl !. last meal. / ^ ^■" . •" \ * ^ ^ \ / X \ I • / / i ^ -. • . / i / { w 1 1 J ^ -J ^ ■ — — \ Lj \dl — 1 l—J L_l tiL iid !"• b 1 10 15 20 25 30 35 . Quantity of Pepsin in Mucoua Membrane of the Cardiac end of the stomach after food. Pepsin in the Mucous Membrane of the Pyloric end. Quantity of Pepsin in the Secretion of the glands of the Cardiac region. slowly affected. In the latter case the carbonate of sodium gradually produced small quantities of pepsin. Another fact which he adduced to prove the presence of this forerunner of the gastric ferment seems to be conclusive. If an active hydrochloric acid extract of the mucous membrane of the stomach be neutralised and treated with the solution of sodium carbonate mentioned above and then acidified again, it loses its peptic power, while a similar watery extract treated in the same way is still capable of proteolytic action on the addition FERMENTS AND FERMENT ACTION. 127 of acid. I.angley concludes from his experiments that the gastric glands contain no active peptic ferment during life, but a quantity of zymogen, or forerunner of pepsin, and that this zymogen can be seen in the chief cells of the glands in the form of granules. During digestion the granules become used up, and under the microscope the outer zone of the chief cells becomes non-granular, the inner zone still contain- ing these particles of zymogen. (Cf. Figs. 23-25.) Langley, working with Edkins ijoc. cii., vol. vii. p. 371), some time later published the results of further researches into these bodies. He found that pepsin was readily destroyed by alkalies, and even to a considerable extent on neutralisation of its acid solution. Pepsinogen is similarly affected, but its destruction is very much slower. All the pepsinogen present in an aqueous extract of the stomach of a cat was converted into pepsin in sixty seconds by the addition of a 1 per cent, solution of hydrochloric acid. Carbonic acid, when passed through a solution of pepsinogen in water, destroys a large portion of it, and this more rapidly than pepsin. The presence of peptone and other proteids delays the rate of destruction by carbonic acid ; certain salts increase the rate. (Chart II.) Rennet-zymogen. — Only from the fresh gastric mucous membrane of the calf and of the sheep can the fully-formed active rennet or milk-curdling ferment be extracted by means of water or other similar solvents. As a rule no substance capable of curdling milk can be obtained by treatment with water from the mucous membrane of the stomach of other members of the Mammalia or of Aves. It is almost never present in fish. If, how- ever, acid be added to the watery solution, the zymogen quickly yields the true ferment. The variations in the amount of rennet-zy mogen in the gastric mucous membrane correspond very closely to those of pepsinogen. As is the case in other zymogens, a more active extract of the rennet-zymogen can be obtained by the use of an acid solution. The extracts procured from the calf and sheep by water alone do not exhibit nearly so powerful an action as those obtained by acidulated solutions. It is possible that a zymogen of the pancreatic amylolytic ferment exists in the pancreatic cells, but if so it must, unlike the zymogens of pepsin and trypsin, be insoluble in water. Liversidge removed all the diastatic ferment present in the minced pancreas with long-continued washing with water until no more could be obtained ; on exposing the gland to the air for a few hours, however, a fresh quantity of very active ferment could be ' extracted with distilled water. It is possible that all bacterial action was not provided against, but allowing for this, it appears to be logical, even in the absence of further proof, to postulate the presence of such a zymogen, especially as the other ferments in the body are preceded by these bodies. 128 THE NATURAL HISTORY OF DIGESTION. The maltose inverting ferment of the intestinal mucous membrane also is probably preceded by a zymogen. As all ferments are the products of cell-protoplasm, it is only natural to conceive that the cells especially set apart for the elaboration of these bodies should in the intervals of rest form and store up substances which can be converted into active ferments in the shortest time possible. And also that they should store them up in an inert and innocuous form corre- sponding to the zymogens which have already been discovered. Protoplasm itself is a zymogen, and the zymogens of the digestive ferments constitute a moiety of the total fermentative properties of protoplasmic action, split off for greater ease in distribution and for the relief of the protoplasm left, which, like a thrifty housewife with her savings in the bank, proceeds to hoard up more to add to the "running account." The formation of zymogens, then, appears to serve the purpose of providing an already prepared store of ferment material, only requiring the necessary stimulus derived directly or indirectly from the food to be discharged into a medium capable of converting them into active agents, while their separation in the form of granules from the general mass of cell protoplasm allows of its recuperation even before the products of its energy have been expelled. Glucoside-splitting Ferments. — Another class of ferments which exist in a small number of plants has the power of split- ting up glucosides into glucose and other bodies. Examples of these are myrosin, from sinapis, and emulsin from amygdala. Myrosin has as yet never been obtained free from albumin or lime : its solution froths on shaking, and coagulates at 60° C. and on the addition of acids and alcohol. If a small quantity of this substance be added to a solution of myronate of potash, a compound present in mustard seeds belonging to the class of glucosides, volatile oil of mustard is formed, along with glucose and bisulphate of potash. The myrosin, hke most other unorganised ferments, does not enter physically into the reaction, it only helps the glucoside to split up into simpler coiTipounds, thus — C,oH,sK.NS20,o = QH.NS + CeHi^Oe + HKSO4. This reaction only occurs in the presence of water. The volatile oil thus formed gives to mustard its irritating qualities. Emulsin acts similarly on the glucoside, amygdalin, of the FERMENTS AND FERMENT ACTION, 129 bitter almond, splitting it in the presence of water into glucose, the essential oil, and hydrocyanic acid, thus — CooHjjNOn + 2H2O = QHjCOH + HCN + aQH jA- ^\gm.\}iX\&. (MonaisJiefte fur Chemie, xiii., 1892) has found that the glucoside-splitting ferments are closely connected with the fat-splitting ferments. The fat-splitting ferments can act on glucosides and on ethers and esters ; the glucoside-splitting ferments on fats and ethers. Fischer {Bericht d. d. Chem. Ges., xxvii. 2, 2031) states that invertin from beer-yeast a.cts on various glucosides of the a class, while emulsin split up the isomeric /3-glucosides. THE ORGANISED FERMENTS. The term organised or formed ferments is applied to those minute vegetable bodies, the moulds, yeasts, and bacteria, by whose growth changes are brought about in the constituents of the media nourishing them, analogous to the chemical phenomena resulting from the action of the unformed fer- ments. The action of the formed ferment differs from that of the unformed in that it is infinite not finite, and in that the changes brought about are accessory to, and greatly in excess of, the needs of the active agent. The action of an unformed ferment is finite ; a certain quantity is capable of chemically transforming a proportionally large but still only a definite amount of material ; the action of a formed ferment is infinite in so far that however large in amount the soil may be, provided the conditions remain suitable, the smallest quantity capable of growth suilSces to act on the whole soil, while a portion removed from the first growth is just as capable of complete action on a second supply of the medium. In short, the one has the attributes attendant on living matter, the other only those appertaining to inanimate bodies. A number of properties are common to the formed and un- formed ferments, while the points of difference between them in regard to their actions are few in tuimber and indefinite in character. I30 THE NATURAL HISTORY OF DIGESTION. Table XIV. — To show the points of Similarity and Dissimilariiy between Unorganised Ferments, Organised Ferments, and Proteids. Unorganised ferments. Organised Ferments. Proteids. Temperature — In solution, 65° C. to 80" 0. In a dry state, 100° 0. to 160° C. Alcohol. Antiseptics. Destroyed. ]Many unchanged. Precipitated, still active, activity diminishes in time. Unaltered if solution be dilute. Destroyed except some with spores. All killed in a few hours. Killed. Killed. Native proteids coagulated, derivative proteids unchanged. Albumins and Globulins altered in time. Precipitated, and in time, if in native form, coagulated. Various. That the points of difference between the two classes of ferments should be small is not surprising if it be remembered that the one arises from the action of the protoplasm of single living cells immersed in the substance on which they grow, while the other is the product of cells fixed in body tissues, capable of further production, but incapable of actual multipli- cation of productive cells. Formed ferments are organised protoplasmic cells with an infinite power of expansion under favourable circumstances, producing chemical changes of a similar character to those brought about by the protoplasmic activity of cells which form part of a composite organism, and are specialised for the purpose of secreting unformed ferments or enzymes. Both are similar in that they are destroyed by heating to the boiling point of water when in suspension or in solution in fluids, although the unformed are less resistant to the action of heat than the formed ferments, as the latter are in part protected by their cellulose coating. When dry either class can withstand, as a general rule, a higher temperature for a limited time without loss of function. Neither can act in the absence of water, while if the solutions containing them be saturated with neutral salts the formed ferments are rendered absolutely inactive, and the unformed ferments checked, or, as in the case of trypsin (Neumeister), rendered almost inert. Perhaps one of the most striking similarities between the formed and unformed ferments is the coincidence in the FERMENTS AND FERMENT ACTION. I3I maximum temperature at which they are most active. The temperature of the body appears to afford most ferments the greatest possible faciUty for the fulfihnent of their functions; they are able to act in higher and in lower temperatures, but not so powerfully. The formed ferments are checked and in a few cases destroyed by a temperature of or below the freezing point of water; a similar temperature slows the action of the unformed ferments, although their activity may often, as in the cold-blooded animals, appear to be as great as when the temperature is higher. The reason of this apparent contra- diction has been shown to be due to the large amount of the ferments present, the increased bulk equalising the lessened power (cf. p. 253). The interesting observation was made by Paul Bert, that the formed ferments are killed if subjected to the action of oxygen under the compression of many atmospheres; the unformed ferments are unaltered. Pasteur, however, found that the spores of certain formed fermentative agents can resist a pressure of from ten to twelve atmospheres. All these facts go to show that the rationale of the fermentative processes initiated by organised agents is only divided by a very narrow line from that which underlies the chemical changes initiated by the unorganised class. Two further examples may be given of the similavity between them. Lechartier and Bellamy found that when chopped up leaves and fruits of phanerogams were placed in an atmosphere devoid of oxygen, as in carljonic acid, they developed alcohol from the carbo-hydrates present, in the total aljsence of yeast ; an observation which confirms a previous statement of Pasteur. This production of alcohol seems to be one of the stages which occur in all plants during the intra-molecular conversion of starch into carbonic acid and water. The sudden deprivation of the oxygen neces- sary to further convert the alcohol into these end-products acts, as Ewald says, as a surprise to the intra-molecular respiration, which, in lieu of oxygen, exhausts the requisite elements present in the cells, but can pro- ceed no further for lack of the oxygen. Those who believe in the doctrine of the chemical nature of all fermentation regard this intra-molecular re- spiration as due to a special unorganised ferment present in the cell, akin to Traube's hypothetical alcoholic ferment in yeast. A second example is given by the self-fermentation of yeasts, by which they are able, in the absence of sugar, under certain conditions, to produce alcohol and sugar from their own substance. Bechamp and Schutzen- berger suggest that in a similar way products of the fermentation of nitro- genous bodies, such as leucin, tyrosin, etc., can be formed from yeast-cells, although Naegeli's explanation is more feasible when he states that such profound chemical alterations are due to bacteria growing at the expense of the decaying yeast-cells. 132 THE NATURAL HISTORY OF DIGESTION. These observations apparently support the view that all fermentative processes, the chief result of which is the simplification and oxidation of complex molecules, are initiated by chemical substances formed by proto- plasmic activity. The objection that has been urged against this applica- tion of Lechartier and Bellamy's observation — viz., that destruction of yeast by the addition of alcohol, or arrest of its activity by deprivation of oxygen, inhibits the fermentation, while unorganised ferments are inde- pendent of the presence of that element — may be met by the fact that the quantity of unorganised ferment produced by the yeast-cells is probably very small relatively to the amount of sugar present, and is used up in great part shortly after production in the fermentative processes which it causes. As has already been stated, a gradual fermentative action has been observed after the death of the cells, an action which is gradual because of the small amount of ferment present. Products of tlu Activity of Formed Ferments. — Starch is not produced as a rule by any of the fungi. From NaegeH's observations the lower fungi can form fat from carbo-hydrates in the same way as fats are produced from starch and sugars in ripening seeds ; these fungi, however, are able to form fat from proteids and other nitrogenous organic com- pounds. Moulds and yeasts placed in distilled water form a certain amount of fat at the expense of their proteid substance. Bacteria flourish in a solution of proteid material, and increasing in number cause an increase of fat, it may be to more than a millionfold ; and when grown in solutions containing little proteid but much sugar or tartaric acid, the increase of albumin is derived from the nitrogenous while most of the fat and cellulose produced comes from the non- nitrogenous, compounds present. For the formation of fat free oxygen must be obtainable; the moulds only grow when they have access to free oxygen, and they abound in fat ; the yeasts and bacteria require little oxygen and are correspondingly deficient in this substance. Those portions of mould-fungi growing on the surface of a solution contain more fat than the portions submerged, and their resting spores only yield much fat when developed on hyphse which reach up into the air. The formed ferments are thus able to form fat from both nitrogenous and non-nitrogenous compounds, but apparently only in the presence of oxygen. They produce no starch, little if any true sugar, are often able to inanufacture mannite (cf. p. 140), and can form a cellulose different from that in other plants in that it does not give a blue colour with iodine and sulphuric acid; indeed Winterstein suggests that fungus-cellulose is not FERMENTS AND FERMENT ACTION. 1 33 a simple molecule of the composition CgHjoOj, but a com- pound of this and a molecule of a nitrogenous substance. Unformed Ferments prodxiced by Fungi. — Yeast-cells form a ferment called invertin which passes out into the solution round about them. Invertin splits up cane-sugar into dextrose and lasvulose, forms which can be acted on by the yeast-cells with the production of alcohol. If sufficient chloroform be now added to kill the cells the solution can still act on cane- sugar in virtue of the invertin present in it, but no further conversion of dextrose into alcohol occurs owing to the death of the cells. If a watery solution containing shreds of fresh fibrin from blood be left for some weeks exposed to the air, especially if at a temperature of about 38° C, the fibrin gradually dissolves, foul gases are evolved, and in time, should the supply of oxygen be sufficient, the complex proteid molecules are reduced to the end- products carbonic acid, water, ammonia, and sulphuric acid, apart from the portions taken up in the formation of the pro- toplasm and cellulose of new bacteria. If such a mixture of fibrin and water be exposed to the air for a shorter period and chloroform water then added to it, a similar dissolution of the fibrin occurs unaccompanied by the production of gas and terminating in the formation of proteid derivatives. In this case the action is carried on by the small quantity of unformed ferment produced by the bacteria before death and unaffected by the chloroform water. Salkowski first described this experiment in 1889, but later investigations have shown the possibility of a fallacy occurring through the use of chloroform water, as some observers have found that the continued action of chloroform on proteids results in an appreciable conversion of these bodies into peptones. The employment of a saturated solution of potassium nitrate in place of the chloroform water by Limbourg has, however, shown the result to be correct, as peptonisation continues in such a solution although the growth of the bacteria is completely arrested. The two instances just given are examples of formed ferments producing diffusible unformed ferments capable of acting outside the cell. Many other bacteria produce similar ferments which only act intra-cellularly on simple and easily diffusible food-stuffs penetrating their outer covering. All substances which coagulate or precipitate native albumins 134 THE NATURAL HISTORY OF DIGESTION. and globulins by so doing arrest the action of the formed ferments. If, however, the solution in which the micro- organisms are growing be highly albuminous, combinations between the proteids and the precipitant fall to the bottom of the fluid and may free the solution from the poisonous sub- stance in an active state, and still allow the growth of bacteria to continue, provided the substance added be not in excess of the combining power of the proteids. In such an instance a quantity of corrosive sublimate may be added without arresting all ferment action which would be quite sufficient to check all growth in a non-albuminous solution of the same bulk. On the other hand, the unformed ferments, although their activity may be diminished in the presence of such protoplasmic poisons as alcohol, ether, chloroform, thymol, etc., on their removal or dilution recover their former power. All mineral-acids in solutions of above 0.2 per cent, destroy sooner or later both the unformed and the formed ferments, with the remarkable exception of pepsin. It does not follow from this that all the formed ferments are unable to flourish in an acid medium provided it be of a lower strength, for many bacteria and yeasts appear to favour a slight acidity — indeed many form acids by their action, acids which in time, should they pass a certain degree of strength, exert an inhibitory effect on the organisms producing them. Similarly many of the enzymes or unformed ferments which are destroyed in time by very weak acid solu- tions are capable of energetic action in the very medium which will later on cause their destruction. A similar arrest of activity in the formed ferments to that occurring by reason of the acid they produce is brought about by the accumulation of many other of their products ; the growth of the yeast is in time checked by the alcohol it forms from sugar ; the organisms of putrefaction cease multiplying when the phenol, or carbolic acid, they form reaches the concentration inimical to them, an interesting example of Nature's application for antiseptic purposes of the substance which was the first to be used by man in his recent departure in treatment, when he unwittingly employed a body formed by bacteria as a check to their growth, and as an agent for their destruction. The unformed ferments are also arrested by an accumula- tion of the products of their activity, although here again we find an exception, as the milk-curdling ferment or rennet does FERMENTS AND FERMENT ACTION. 1 35 not appear to lose any of its efficacy however great the accu- mulation of casein coagulated by it may be. Both classes of ferments possess the property of splitting peroxide of hydrogen (HjOj) into water and oxygen when placed in its solution. Jacobsen [Ueber Ungeformie Fermente, Inaug. Diss,, Berhn, 1891) found, however, that the ferments of the pancreatic juice when heated to 60° C. , and allowed to cool down to 40°, lost the power of decomposing peroxide of hydrogen, although they still retained an unimpaired diastatic action. Precipitation of the unformed ferments by saturation of their solutions with neutral salts, or by the addition of alcohol in excess, renders them incapable of acting on peroxide of hydrogen, although they are still able to produce their special changes on food-stuffs. The property of decomposing peroxide of hydrogen belonging to the unformed ferments is therefore independent of their digestive action. Probably the products of the formed ferments would be found on investigation to contain unformed ferments responding in like manner. Table XV. — The Chronology of Ferment Action. 1833 Payen and Persoz ; On diastase. 1838 Cagniard-Latour : Vinous fermentation. Schwann : Alcoholic fermentation and putrefaction. _ Berzelius : Action by catalysis, digestion caused by contact with ferments which are bodies capable of initiating chemical changes without loss or alteration of their own structure. 1S40 Liebig modified this catalytic theory, supposing that the ferment is a body in a state analogous to decom- position, its own charges starting those in other bodies. i?}'jo et seij . ...Pasteur lays down the law that there are two forms of ferments : — (1) Organised, or capable of reproduction, fer- mentation being due to its living action. (2) Unorganised, incapable of reproduction, action clue to chemical change. Present day... The first class, as described by Pasteur, is recognised to act as he suggested. Tlie second class is looked upon as consisting of complex chemical bodies of unstable composition, contact with which, in the presence of an active agent such as an acid or alkali, causes a splitting or - an isomeric chanije in the size of the molecules of the substance acted on, usually accompanied with hydration. 136 CHAPTER VI. Food Elements. Carbo-hydrates :— Composition — Polysacchavides — Disaccharides — Mono.saccharides. Proteids : — Composition — Properties — Classification^Sclieme of Proteid Decomposition. Fats : — Composition — Properties. CARBO-HYDRATES. The carbo-hydrates are bodies containing three elements, carbon, hydrogen, and oxygen, the latter two in the exact proportions necessary to form water. A carbo-hydrate molecule may be formed by varying numbers of carbon atoms, and of atoms of the other two elements, but two hydrogen atoms are always present for each atom of oxygen. This characteristic is not confined to the carbo-hydrate group ; acetic acid, for instance, C2H^02, shows the same proportion of hydrogen to oxygen atoms. In almost all the members of the carbo-hydrate group, however, the atoms of carbon in each molecule number six or multiples of six. The exceptions, the trioses, tetroses, pentoses, octoses, and nonoses, are of little importance with reference to nutrition. The carbo-hydrates are divided into three classes — • 1. Polysaccharides. 2. Disaccharides. 3. Monosaccharides. Tlie polysaccharide molecule may be regarded as a multiple of tlie mono- saccharide molecule niiniis the elements of water, or conversely the mono- saccharide as a part of the polysaccharide molecule phis water. The disaccharides are equal to two monosaccharide irrolecules minus a molecule of water. The formulae for these groups are : — Polysaccharides {Q,^\%yf)^x={C^-^^^-\^^)x Disaccharides Ci2H2.20n = (C5H12O0 + CgH -J^^ - HjO) Monosaccharides ...C5II12O FOOD ELEMENTS. 137 The Polysaccharides ( Saccharo-CoUoids, Amylases). The members of this group are made up, as we have seen, of multiples of the molecule of the monosaccharides with the subtraction of water. Sabanejeff {Che?/!. Centralblatf, 1891, s. 10) estimated the molecular weight of glycogen to be 1625. If the figure 10 be substituted for the sign x in the formula given above, a molecular weight of 1620 is obtained, agreeing very closely with the physical result. 1 (C5Hj()05)io =(12x6 + 1x10 + 16x5)10 = (i62)jQ= 1620. Brown and Heron double the constituent atoms, and regard a molecule of starch as represented by (Ci2H2qOi(,)io, with a molecular weight of 3240. The principal polysaccharides are — Vegetable starch. Animal starch (glycogen). (In the liver and other animal tissues.) Dextrins. (Exemplified in toasted bread.) Cellulose. (The principal constituent of woody fibre, cotton wool, etc.) Reserve cellulose. Gums. Inulin. The polysaccharides are all tasteless, amorphous, more or less soluble in water, except cellulose which is absolutely insoluble, and are insoluble in alcohol and ether. Starch and glycogen form opalescent solutions in water. All of them when dissolved in water are optically active, and are indif- fusible through dialysing parchment. Saturation of their solu- tions with salts, especially with ammonium sulphate, causes their precipitation. They form no compounds with bases or with phenyl-hydrazin, nor do they reduce alkaline solutions of copper salts, except after long-continued boiling. If the polysaccharides be subjected to the action of steam at high pressure, of boiling in dilute mineral acid solutions, or of ferments, they are converted through the stages represented by dextrins and disaccharides into monosaccharides. ^ In chemistry, the atomic weight of carbon is given approximately as 12, of hydrogen as i, and of oxygen as 16. The sum of the number of atoms of each element multiplied by their atomic weights yields the relative molecular weight. 138 THE NATURAL HISTORY OF DIGESTION. Thus starch is finally converted into glucose, after passing through the different kinds of dextrin, and the form of maltose. Glycogen and cellulose are also split up into glucose, inulin into Isevulose, reserve-cellulose into mannose, and gums into galactose. The dextrins differ from the other members of the group, and indeed may be looked upon as forming a connect- ing chain, made up of bodies with progressively smaller molecules, between the polysaccharides and the disaccharides. They are easily soluble in water and are diffusible, slightly only when akin to the polysaccharides, freely when approaching to the constitution of the disaccharides. Starch subjected to a high temperature, but below the point of incineration, changes into dextrins, as exemplified every day in bread-crust or toasted bread. The Disaccharides (Hexobioses, Sucroses). The Disaccharides comprise : — Cane-sugar (sucrose or saccharose). Milk sugar (lactose). Malt sugar (maltose, ptyalose). Cane-sugar = i molecule of dextrose and i molecule of laevulose {minus i molecule of water). Milk sugar = i molecule of dextrose and i molecule of galactose {iiiinus i molecule of water). Malt sugar = 2 molecules of dextrose {minus i molecule of water). These sugars are soluble in water and alcohol, are optically active, crystallise, are capable of diffusion, and have a sweet taste, sweeter than the monosaccharides. They form com- pounds with bases; hydrazones and ozazones with phenyl- hydrazin. When dry they form caramel on the application of heat. With the noteworthy exception of cane-sugar they reduce metallic oxides in alkaline solutions. Steam at high pressure, boiling with dilute mineral acids and inverting ferments, convert them into monosaccharides. The sucroses do not readily undergo fermentation until converted into glucoses. Bacteria help in this action by secreting an inverting ferment. A peculiar sugar, raf5nose, is known, with a molecule made up of a combination of dextrose, Isevulose, and galactose, thus being a trisaccharide or hexotriose (QiaHsaOie)- I' occurs in barley, in eucalyptus-manna, and in the seeds of the cotton plant. FOOD ELEMENTS. 139 Alonosaccharides (Glucoses or Hexoses). 1. Grape-sugar, glucose or dextrose. 2. Fruit-sugar, Isevulose or fructose. 3. Mannose. 4. Galactose (distinct from lactose). All these sugars have the same chemical composition ; that is, they are isomers of each other, but three are simple isomers; their molecular grouping is the same, but the arrangement of the groups of atoms is different; while one, tevulose, is a stereoisomer, the position of the atom-groups to one another not being the same. Dextrose, mannose, and galactose are supposed to be made up thus (Neumeister) — CH2.OH - (CH.OH), - c o = QHi A ; while Ifevulose is represented as — ■ CH2.OH - (CH.0H)3 - CO - CH2.OH = CeH,/,),;. They are actually oxidation products of steveoisomeric hex-acid alcohols, CeHuOe; or CH2.OH - (CH.OHli- CH2.OH, of which three are known to occur in nature, sorbite, mannite, and dulcite. Sorbite is obtained from the berries of the mountain ash, Sorbus aucuparius, mannite from the manna of Fiaxinus ornus, and dulcite from Madagascar manna, in Eitonymus EiiropcsuSy etc. Dextrose, mannose, and galactose are the aldehydes of sorbite, mannite, and dulcite respectively, while Isevulose is the ketose of mannite. The glucoses are easily acted upon by organisms ; many yeasts form alcohol and carbonic acid from them : QHijOe = 2C2H5.OH + 2CO2. Glucose. Ethyl alcohol. Carbonic acid. Other organisms form lactic acid : C8Hi20fj = 2;CH3-CH.OI-I-COOIi) or 2C3H6O3. Glucose. Lactic acid. Like aldehydes and ketones, all the glucoses can form hydrazones and ozazones from hydrazins. Fischer has by careful oxidation of the hexyl- alcohols succeeded in obtaining mannose and Irevulose from mannite, and has also built up synthetically the simpler sugars from elementary carbon compounds. The far-reaching significance of the work of this German chemist on the relationship between the different sugars and their synthetic preparation can hardly be over-estimated, but an investigation into his 140 THE NATURAL HISTORY OF DIGESTION. results to be of value would require to be exhaustive, and therefore out of place here. It must suffice to state that true mannose can be formed from chemically made akrose, by way of akrit, a hexyl-alcohol, identical with inactive mannite, and of inactive mannose, finally forming true mannose, capable of rotating the plane of polarised light to the right ; that the splitting up of the ozazone of this mannose affords tevulose, rotating polarised light to the left; and that by acting on the synthetic, as well as the natural, mannose with bromine water, the corresponding mannonic acid can be obtained, and from it glycuronic acid, which when reduced by nascent oxygen yields a glucose not to be distinguished from natural grape- sugar. A great number of these isomeric sugars are theoretically possible, and a considerable number have been isolated, as sorbinose, rhamnose, arabinose, etc.^ A substance of great interest, amido-glucose, or glucosamine, contains nitrogen, one atom of amidogen replacing one of hydrogen in the glucose molecule. This body can be obtained from the proteid bodies chitin and chondroitin, and affords glucose when treated with nitrous acid. CgHuOs-NHj + NOOH = CeHjoOe + Nj + H^O Glucosamine. Nitrous acid. Glucose. Nitrogen. Water. PROTEIDS. The most important role in the play of life is undoubtedly that acted by the group of complex chemical bodies bearing the name of proteids. No living cell in either the animal or the vegetable world but plays its part in the drama by means of proteids en- dowed with that mysterious force "vitality." A proteid body by itself is incapable of reproducing its kind, but as the chief constitu- ent of living protoplasm it is, as it were, touched by a magician's wand, and enabled to perform the many different processes which are requisite for the production of the body and frame- work of each being, for its nourishment, and for the continu- ance of its kind. Protoplasm indeed is maintained by some to be only " living proteid," the change from life to death to be only a molecular one. But such a hypothesis is untenable. Protoplasm consists of the three classes of bodies used in alimen- tation with, in addition, a varying amount of mineral matter. ' A sugar — pentose — has been obtained from different members of the nucleo-albumin group of proteids by Blumenthal, who has separated this carbo-hydrate from the nucleo-albumin of the pancreas, liver, thymus, thyroid, spleen, brain, and muscles. Pentose molecules consist of con- densed molecules of hexose bodies, where all or part of the aldehyde groups are shielded from oxidation by condensation, as for example in hemicellulose and cellulose. These arise by oxidation, and by splitting off, from the final alcohol groups of glucose and galactose molecules undergoing disintegra- tion. (Chalmot, Ber. d. d. Chem. Ges., xxvii. s. 2722.) FOOD ELEMENTS. I4I Chiefly made up of proteid, it probably contains some carbo- hydrate and fatty bodies either in close conjunction or in actual conjunction with the proteid, forming exceedingly complex molecules ready to give off various combinations of their atoms when called upon by the needs of the organism of which they form a part. It is one of the most suggestive things possible to watch the movements of the protoplasmic contents in the single cell of the Vampyrella spitogyrm, a microscopic, almost naked mass of protoplasm, and to observe the manner in which it exercises a power of discretion as to what food it should engulf in its substance, and what it should reject, and to think that when devoid of life this simple mass of protoplasm consists only of a minute speck of proteid and carbo-hydrate molecules, with purely chemical attributes. Well might Cienkowski exclaim after watching this phenomenon, " The behaviour of these monads in the search for, and ingestion of, their food is so remarkable that the observer might almost believe their actions to be those of a conscious being." Table XSfl.—The Proteids. I. — Animal (partly after Hoppe-Seyler.) I. Albumins : a. Serum-albumin ; i. Lact-albumin ; c. Egg-albumin. II. Globulins: A. Ptecipitated by saturation wilh NaCl. a. Myosin ; b. Fil")rinogen ; c> Fibrin-globuHn ; d. Serum-globulin. B. Not precipitated by saturation witlr NaCl. a. Vitcllin ; i. Globulin from the Crystalline Lens (Crystallin). IIL Fibrin. IV. Coagulated Albumins and Globulins. V. Amyloid. VI. Acid-albumin or Syntonin. VII. Alkali-albumins, Albuminates, or Albuminic Acids. VIII. Nucleo-albumins : a. Nucleoalbumin in Cell Nuclei ; b. Vitellin ; c. Casein ; d. Spermatin. IX. Mucin from Gall-bladder (probably a Nucleo-albumin). X. Albumoses : A. Insoluble in water. a. Heteroalbumose ; b. Dys-albumose ; c. Anti- album id. B Soluble in water. a. Proto - albumose ; b. Deutero - albumose ; c. Atmid-albumin ; d. Atmid-albumose. XI. Peptones: a. Ampho- or Hemi-peptone ; /;. Anti-peptone. XII. Oxyprolo-sulphonic Acid. 142 THE NATURAL HISTORY OF DIGESTION. XIII. Albuminoids : A. In Verlebrata — a. Keratins, Neuro-keratin. b. Elastins. t. Collagen, Gelatins. V>. Ill Inverleira/a — a. Conchiolin' b. Spongin i. Chitin I Skeletins d. Cornein j (Krukenberg). e. Fibroin / Silk I C. Glucosides — a. Mucin. (/'. Chitin) L. Chondroitic Acid. II. — Vegetable. I. Vegetable Albumins. II. Globulins: a. Vegetable Myosin; b. Vegetable Vitellin (Phyto-vitellin); c. Para-globulin. III. Coagulated Proteids : a. Coagulated by heat ; b. By ferment action — I. Gluten, 2. Anti-vitellid, etc. IV. Acid and Alkali Albuminates : a. Legumin or Vegetable Casein ; b. Conglutin. V. Albumoses : a. a-phyt-albumose (Proto-albumose?); - b. ^-phyt-albumose (Hetero-albumose?) ; c. Insoluble Phyt-albumose (Dys-albumose?) ; d. Vitelloses. VI. Peptones. III. — Pigments of a composition similar to or combined with Proteids. A. An'imal — ■ I. H;Kmoglobin ; Albumin and Hcematin ; Heemocyanin, Histo-hjematins. II. Melanins. IV. Lipochromes. III. Chlorophyll. V. Bilirubin, etc., in Bile. IS. Vegetable — I. Chlorophyll. III. Cyanic Series. II. Xanthic Series. IV. Lipochromes. The class of proteids is a large one, but all the members present certain features in common. First, they may be separated into two broad groups — those which occur as part of the living tissues and fluids on plants and animals, and those which can be derived from the first group by various means. The first class are termed native, the second derived proteids. All proteid substances contain the five elements, carbon, hydrogen, nitrogen,, sulphur, and oxygen. The exact composition of all the varietie'^^ is still uncertain. Proteids are so easily altered in constitution by any of the methods requisite for their isolation, and have so great an affinity for so many chemical substances, that, FOOD ELEMENTS. I43 hitherto, the analyses performed by even the ablest chemists present results which vary within considerable limits. The following figures give the different values obtained by various investigators, with the mean which may be regarded as the approximate constitution of the proteid molecule : — ■ Carbon 50-55 per cent. Mean 52 per cent. Hydrogen, ..6. 5-7.3 ,, ,, 7 Nitrogen ...15-17.5 ,, ,, 16 ,. Sulphur 0.3-2.4 ,, ,, 2 ,, Oxygen 19-24 ,, ,, 23 ,, Other elements are present in some proteid bodies, but only in small quantities, as for instance, iron in hsematin, and phos- phorus in the group of nucleo-proteids. Two atoms of sulphur are present at the least in each molecule, for if albumin be heated with a dilute solution of potassiutn hydrate, part of the sulphur forms a sulphate, and can be precipitated and removed by the addition of acetate of lead, leaving a part, probably united with other substances, in the original molecule, and that more firmly fixed, though it can be transformed easily into sulphuric acid by heating with the hydrate and nitrate of potassium. In the same way, part of the nitrogen of the proteid molecule, when heated with a weak solution of potassium hydrate, forms ammonia and is driven off, the larger portion remaining in the solution. It is plain, therefore, that we have to deal with a very complicated molecule in which the various atoms are united to one another with different degrees of force, while in all probability the whole molecule is made up of con- geries of atoms, each a molecule in itself, which are prone to separate from the parent molecule on receipt of the proper stimulus. The typical proteid molecule, then, is a large, un- stable composite body, and there is reason to believe that, as we have seen when discussing the molecule of starch, several of these compound molecules are combined together in native proteids to form a still larger, doubly-compound body. It is only on this supposition that the formation of differently constituted substances from albumin during digestion, with at the same time the continued presence of proteid bodies with practically the saine composition per cent, though with other physical v^jalities, can be satisfactorily explained in the present state of our knowledge. In addition to the elements described above as entering into the composition of the proteid molecule, we must not forget to 144 THE NATURAL HISTORY OF DIGESTION. add that the native proteids are looked upon as being in a state of low hydration, that is, they have very few molecules of water attached to them in their natural condition. Indeed, the alteration in their characteristics, brought about by digestion, or by similar artificial means, is to be regarded mainly as a splitting up and cleavage of the compound molecules accom- panied by the addition of the elements of water. The lower we go in the proteid scale the larger we find the proportion of water connected with the typical proteid molecule to be All pro- teids are insoluble in alcohol, and all are precipitated from their solutions by it; although only the native proteids become coagulated by the prolonged action of alcohol and are then insoluble in water; the derived proteids can always be redis- solved, however long they may be exposed to its action. All proteids are precipitated by soluble salts of the heavy metals, but can be separated from the metallic combinations thus formed by appropriate means. They are also precipitated by potassio- mercuric iodide and hydrochloric acid, by picric acid (derived proteids are soluble on heating with this acid, though re-precipitated on cooling), by Millon's reagent with the formation of a red colour, and by phospho-tungstic acid and hydrochloric acid. They all reduce sulphate of copper in the presence of a caustic alkali, the higher members striking a violet, the lower a pink colour. On heating one of the group with nitric acid and, after cooling, adding ammonia, usually known as the xantho-proteic reaction, a yellow-red colour appears in the solution. Finally, all proteid bodies yield nitrogen on ultimate analysis. The native proteids, that is, those which occur in living tissues, although they respond to these common tests, are by no means exactly identical one with another. Table X^ll.—The Pi-incipal Native Proteids. Albumins. — Serum-albumin. Egg-albumin. Lact-albumin. Muscle-albumin. Globulins. — Fibrinogen (metaglobulin) and fibrin. Serum-globulin (paraglobulin). Myosinogen and myosin. Muscle-globulin. Vegetable globulins. Missing Page FOOD ELEMENTS. 145 Artificially altered Native Proteids. Albuminates. — Acid-albumin. Alkali-albumin. Ash-free-albumin and globulin. Coagulated and precipitated albumins and globulins. Proteids combined naturally ivith other Sttbstances. Nuclein (albumin combined with phosphoric acid or a nucleic acid). Nucleo-albumin (nucleo-proteid), a proteid combined with nuclein, as casein. Glycoproteid (proteids combined with a carbo-hydrate), mucin, mucoids, hyalogens. Hemoglobins (combined with iron), hfemoglobin, oxy-, meta-, and reduced hcemoglobin. In addition to these proteids there is a large class of substances which may be called albuminoids, and which correspond very closely in composition and reaction to the true members of the proteid group. In this class keratin, elastin, and gelatin may be placed, the first rich in sulphur, the second free from that element, and the third containing a small quantity. Of the three, gelatin is the most unlike a true proteid. It would be out of place in this volume to give an account of the numerous differential tests to which the various members of the proteid group respond. A short statement of their principal characteristics must suffice. (Table XVIII.) Albumins are soluble in distilled water, and are not precipitated by saturation of their solutions with magnesium sulphate; while, on the con- trary, globulins require a certain proportion of a neutral salt in water before they can dissolve, and they are precipitated in a saturated solution of magnesium sulphate, or in a half-saturated solution of ammonium sulphate. Similarly, acid and alkali albumins are not soluble in neutral solutions, but rerjuire a certain amount of acid or alkali respectively to keep Ihem in a soluble form. Ash-free albumins are insoluble in water (Btilow, PJlilger's Arch. f. d. ges. Phys., Iviii. s. 207), easily dissolved in solutions of acids and alkalies, the acid solutions being very readily precipitated by the addition of neutral salts, the alkaline solutions indifferent to these. Solutions of albumins in distilled water, containing a minimum of mineral matter, are not coagulated by heat even on the addition of acetic acid, but the addition of a very small quantity of a neutral salt to the mixture at once causes the proteid to fall in a coagulated form. The 10 146 THE NATURAL HISTORY OF DIGESTION. different proteids do not all coagulate at the same temperature, and this difference has been made use of, especially by Halliburton, to identify and separate individual proteids from a solution containing several members of the group. For example, if the plasma which can be obtained from the juice of raw muscle be heated gradually, at 47° C. paramyosinogen, at 56° myosinogen, and at 63° myo-globulin are precipitated (these are all globulins) ; if the. solution be further heated, at 73° C. myo-albumin coagulates out. The method of fractional heat-coagulation of proteids is, however, rendered of less service as a means of identification of these bodies than was at one time anticipated, owing to the variations in the coagulation point when different amounts of salts are present. All native proteids are capable of being split up into less complex and more highly hydrated derivatives. This may be achieved by simply heat- ing their solutions in distilled water at a high temperature and under pressure, or by the action of dilute acids at medium temperatures for a prolonged period, or of stronger acid solutions for a shorter time. When heated with baryta water, or after very prolonged treatment as just described, the process of decomposition goes further, and ends in the for- mation of various amido-acids, belonging both to the fatty and the aromatic series. These changes are effected in animal and vegetable forms at the temperature of the air, or of the body, by dilute acids and alkalies aided by special ferments. The further reduction to amido-acids is not effected by digestion with acids, but pertains only to that form of digestion in which an alkali is the active agent. The action of a dilute acid or alkali in excess, unaided by a ferment, can go no further at the ordinary or body tempera- ture than the formation of an acid or alkali proteid. These bodies, often termed albuminates, appear to be compounds of the particular acid or alkali in action and the original proteid molecule. The alkali-albumin, however, contains less nitrogen, and the part of the sulphur which is easily split off from native and acid albumins appears also to be absent from its molecule (Neumeister), though this may be due to the action of the combined alkali. A further account of the properties and significance of these bodies will be found in the description of peptic digestion. So far, then, and no further, can unaided dilute acids and alkalies act on proteids at ordinary temperatures. With the assistance of ferments the process goes much further. All proteids are decomposed by peptic or acid digestion, save only mucin, keratin, and nuclein, and these are simplified by alkaline digestion. The first step in all digestive processes is the formation of acid or alkali albumin, and these bodies, as digestion goes on, are split up into albumoses, and these in turn into peptone. Kiihne, in his classic work on the products of proteid digestion, enunciated the theory that all proteid bodies when split up by the action of hydrolytic agents form two groups of derivatives, a hemi- and an a«//-group. By the continued action of the active agent the meinbers of both groups are ultimately transformed through progressive stages FOOD ELEMENTS. I47 into molecules of hemi- and a«//-peptone respectively. The peptone of the /zf/;«'-group is less stable than that belonging to the other, and can be further decomposed by tryptic digestion into chemical bodies of comparatively simple constitution, such as leucin, tyrosin, and glutamic acid. The anti-pepfone cannot be further split up. The antecedent albumoses to each of these peptones are termed hemi-albumose and anti-albiimose. FATS. Fats are either of animal or vegetable origin, and are found within the cells of the organism or suspended in the circulating fluids. A fat or hydro-carbon contains carbon, hydrogen, and oxygen in a proportion different to that found in a carbo-hydrate. The natural fats are compounds of a monatomic fatty acid, such as stearic, oleic, or palmitic acids, and a triatomic radical propenyl or glyceryl. Three molecules of the acid combine with one of the radical. The fats derived from these acids are termed stearin, olein, and palmitin. Similarly, butyrin is derived from butyric acid. Tripalmitin C15H31O2I Water. Glycerine. Palmitic Acid. CieHjiG^ ^C3H5 + 3H,0 = C3H5(OH)3-^3 (QsHs^Oj) Tristearin ...C3H5 (Ci8H3502)3 Triolein C3H5 (CigHj^Oojj Tributyrin ..C3H5 {Q,iyi^pi^\ Beeswax is an animal fat formed from a cetyl alcohol radical, from which spermaceti is also derived. Waxes can be saponified, but are incapable of digestion. Saponification of fats on boiling them with an alkali results from the combination of the alkali with the fatty acid, the glycerine being set free. Fats are insoluble in water, but may be suspended in the form of very minute droplets as an emulsion if such bodies as gum or albumin be added. When some free fatty acid is present a permanent emulsion may be formed by the addition of sodium carbonate, a reaction which normally occurs in the small intestine. Fats vary much in consistence. Vegetable fats are usually fluid, with only one or two exceptions. Animal fats are fluid at the temperature of the body, but solidify on cooling. Depositions of fat vary in composition in different parts of the body, and vary in their properties in relation to the proportion of olein, stearin, and palmitin present. 148 THE NATURAL HISTORY OF DIGESTION. E^ 1— ( w 1 -u'^ ^ ee 2 ll y;r3 M . -S ci i| -^'f •% +3 ■5 " '^^^ •'S ^ ;i, a. — rH S-, O a a* F^ S 03 _M — - -> C a> .^ cS S 5 Hi? — 3 > o P -;; !D 3J 0) " ce - 5 f« ^ ^ ^ 3 o 3 K "o M ih _o 3 . . w '6 8 ;n o >>5 II • ■ >, IJ2 M I2 ■ 'S 'T! S "U ■ ■ G 1 :5i|3 : : ■^ <2 S ^ § .v» 5^ ■^ § S; ■^ ^ ^ 0) 33 03 _o ; _o t^ :t: .ti : ; t^i '^i^i^':^ ^ ?5 5- . • -d ■^'o -* : b ^5 Ho ■* : ; :^ -a< s C3 ^ "^ .CO -M ■ -rC CO » n T-1 CO ■^ ="3 -* I-H iH iH ^ 11 "ij & -f tH •ClOM-f 100 t* r^ t- L^ ■« ^— 1 — ' ^ ■+3 t^ 00 ■OOOOiH . . ^ It; ■ 00 ;o -M • • -^ oi :-ot^'N* : : ■V» ?. ■ CO 00 ■OOQOOT ^ a zn Crt -CO^ 10 • • e3 in c a (m" o^ \Vi^^ : : .■^ ^* c'^ 5 ^ 9" -cox-* . . 1 0^ Ji c: J2 d : oi c; -o : : ^ _^.,^ir. a 1 C « X .=1 ■ i^-i 03 ^ d £ 1 1; FOOD ELEMENTS. 149 In the formation of adipose tissue, i.e., the laying on of fat, the protoplasmic contents of the cells are gradually replaced by globules of oil, the nucleus being pushed to one side. In vegetable cells fat may arise directly through protoplasmic activity from carbonic dioxide and water, but it also may be formed from starch. An interesting fact has been noted by Noel Paton in connection with the fat of the porpoise. The layer of fat immediately under the skin of that animal remains iiuid at a lower temperature than the fat deposited round the internal organs. The significance of this will at once be seen when it is remembered that the external fat is more exposed to the low temperature of the surrounding water. Table XIX. gives a list of the chief individual fat.s, and Table XX. the composition of the compound fats found in the animal body. Lecithin is a phosphorised fat found in almost all growing cells and in the white corpuscles of the blood. Unlike ordinary fats, it contains nitrogen and phosphorus, but when boiled with baryta water yields barium stearate, neurin, and glycero-phosphoric acid. Margarin is not a special fat but a mixture of stearin and palmitin, often occurring in the form of fine crystals in fat cells. ISO CHAPTER VII. Digestive Processes in the Body. Action of Saliva on Starch — Dextrins — Maltose — Effect of Acids — Digestion in the Stomach — Production of Hydrochloric Acid — Maly's Views — Koeppe — Gamgee — Acidity of the Stomach Contents — Deter- mination — ^ Variations — Peptic Digestion of Proteids — Relative Digestive Power of Acids — Combination of Acids with Proteids — Cause of Simplification of Proteid Bodies. The Action of the Saliva on Starch. — The action of the diastatic ferment of the saliva on starch has been so thoroughly investigated that this appears to be the proper place to take up in detail the changes which starch undergoes during its conversion into true sugars. In 1831 Leuchs ascertained that when starch was mixed with saliva it gradually dissolved with the production of a body possessing most of the properties of grape-sugar. Some years later, Mialhe, by the addition of alcohol, precipitated an organic body from saliva, which pos- sessed the same power as the original fluid. This body, he found, was able to convert two thousand times its weight of starch into sugar. Before this it had been known that when starch is boiled with dilute sulphuric acid a sugar is formed. Then it was found that the first stage in the conversion of starch into sugar, either by means of dilute acids and heat, or by the action of diastase, is the formation of an isomer of starch, dextrin; so called because of its optical properties. Up to the time of Dubrunfaut, in 1847, and of the researches of O'Sullivan, the sugar formed from dextrin in the second stage of the process was regarded as dextrose. These observers showed it to be a sugar of the cane-sugar or disaccharide class, and gave it the name of maltose. Soon after this, the identity of the sugar formed by the action of diastase, with that produced under the influence of ptyalin in the saliva, and amylopsin in the pancreatic juice, was estabhshed. Musculus and von Mering in 1878 {Zeitschrift f. phys. Chemie, vol. i.) showed DIGESTIVE PROCESSES IN THE BODY. 151 that the action of dilute sulphuric acid was not, as had been thought, of the same nature as that of the diastatic ferments. Musculus supposed that the phenomena caused by the action of these ferments are not those simply of hydration, but of decomposition, in which the starch molecule, a molecule of very complex structure, split up into dextrin and maltose, further action splitting up the dextrin into a less complex dextrin and sugar, and, during the continued action of the ferment, this process was repeated until the starch had been converted for the most part into sugar, leaving a small amount of dextrin unacted on. Brown and Heron confirmed this view in 1879, ^nd enlarged it; while the first-named, along with Morris, has thrown much additional light on the process of conversion of the complex starch molecule into simpler mole- cules of sugar. The best method of testing the action of the saliva, or any other liquid containing a diastatic ferment, consists in the use of a standard starch mucilage containing one gramme of pure potato-starch in 100 c. cm. of dis- tilled water (Roberts). The starch should first be washed with water, treated with a very weak solution of potassium hydrate, then with a one per cent, solution of hydrochloric acid. It should then be washed until all trace of the acid has disappeared, and dried at from 25 to 30° C. It is then mixed with a little cold water in a mortar, and thrown into boiling water with frequent stirring. The mixture should be kept boihng for two or three minutes (Brown and Heron). Liquefac- tion occurs almost immediately on the addition of a diastatic ferment to some of this starch-paste kept at a temperature of about 30-50° C. If the ferment be small in amount the change occurs more slowly, and if the ferment be added in less and less proportion, a point is reached at which this formation of soluble starch is the only change taking place, no dextrin or sugar being formed. This soluble starch may be regarded as the first step in the simplifying of the large and composite insoluble starch molecule, without any change in its elementary composition. Soluble starch is coloured blue by iodine, and is precipitated from its solutions by alcohol and by tannic acid, like insoluble starch, but unlike dextrins and maltose. If iodine be added to the mixture after the action has gone on for a short time, a red colour appears ; if there is still some soluble starch present, the presence of the red alters the blue colour previously produced by iodine to a purple, more and 152 THE NATURAL HISTORY OF DIGESTION. more reddish in tint in proportion to the amount of erythro- dextrin present. The soluble starch can be removed by pre- cipitation with tannic acid, the erythrodextrin remains. If alcohol be added, it, in like manner, may be got rid of, leaving a sugar, maltose, in the filtrate. Two erythrodextrins have been described, but their existence as actual substances is regarded by some as doubtful, the reactions presented by them being ascribed to varying mixtures of lower dextrins and unconverted starch. Further action of the ferment produces other dextrins from erythrodextrin, probably progressively, the one from the other, with at the same time the formation of maltose. The addition of iodine to solutions of these lower dextrins, achroodextrin and maltodextrin, is not followed by any colouration, and forms Roberts' achromic point. The time taken by a certain amount of a diastatic fluid to render a constant quantity of starch-paste achromic is used as an indication of the power of its ferment. If a large quantity of ferment be added to a sufficiently dilute starch-paste, the achromic point may be reached practically instantaneously. Roberts has shown that the amount of starch converted varies directly with the quantity of the ferment employed, while the time taken to reach the achromic point is practically in inverse ratio to the quantity of ferment. Table XXI. — The amount of Solids found in Saliva per mi I. Parotid Saliva — Total Solids. Organic Matter. Ash. Man 13-485 6.220 7.265 Sheep - 11.000 1. 000 10.000 Cow 9.300 0. 440 8.860 Horse 8.540 4.300 4.240 II. Dog Subiiiaxillary Sa - 7-184 Itva — 1.743 5.441 Dog - Cow 11.700 S.860 6.432 3-530 5. 268 5-330 Horse 7.500 4-925 2-575 III. Man - 4. 100 Sublingual Saliva — 2.900 1.200 IV. Mixed Saliva — iS-750 3-450 12.300 Sheep 11.000 1. 000 10.000 Dog 10.4CO 3.610 6 790 Cow 9.300 0. 440 S.860 Horse 8.000 2.000 6 000 Man 5.880 3-S60 1.920 DIGESTIVE I'ROCESSES IN THE BODY. 153 4 -t in < .^ -0 _.; •* "-^ i-l L-l M ?1 ^- •^rJH 1 n •\' la ^ r-i Oi CO 1 1^ -rt- Z) ;; CO 1 ^ ^ Tf y. 1- "S ^M 'o7;^o;2;^a6d >H ' -iji d M """^ 10 ' c^^ c: 31 i-i « -J3 6 -ij iri '-^. '^. r-t (M ^ i^ H ^ ■ 1 00 '*'"-« (N ^ 1 tri TO ;3 1 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 i-H ■■-' 1 i-i OT M 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ^ -J 1 d'S ! CO (U "* 'E 00 iH m •# ifj :ooo ' rH ■* 5-1 u^ :C i ifi i~ /) i-I Ifi CO ■?! iri GO 01 i-< IM I-* rH r-( a: ^ "^ «J "^ "^ CO Tti >n ^ ^: 1^ 3 S ■ (M g i-H M to 0; (M c: I- q Si,.,.,^^,r.^ (M - = 00 1- COM rH 3 1- -^ ^ '^biccci'^c:osaiS'5) 4. I- 3C :c L- -f 1) rH CO -* XM--^ -)^ ■^i C -■ C-; " r: 7-j » >D Ci t- »a) X a: 3; cr. GO » 1? COOiOSOlCOOiCSOiCSC^ 3i a: C-. c: CL C. CJ 35 c: Oi C. C: 01 C-. C: CO 01 OS C3 '0 * ' ^ i"' ij r^ = ■;;; 53 X :ti Q tT ri v~. F ^^ <~ «a-^l?|g5|£ i '1 -=■ j; -.^'ri --.Q --0 "O'- _ll ^ |- 50 _ 1 ^ i> a, - _£^ ^ C ^ £3 ft " 'a zjrj Isa " ' " 's5 iia " ' ' _^ S S Q S ^ U^l '^j n ^ W M 154 THE NATURAL HISTORY OF DIGESTION. If one molecule of soluble starch be looked upon, with Roberts, as io{C-i^^U^fij^i^), 8(H20) or ten simple starch mole- cules with eight molecules of water, the actual changes occur- ring may be thus represented : — Soluble starch equals — Table XXIII. 1. Erythrodextrin a. 9(Ci2H,oOio) (C-^^U.^fi^^) (Maltose). 2. Do. 6. 8(Ci2H;oOio) 2(Ci2H2.30ii) do. 3. Achroodextrin a. 7(Ci,H,(,0]o) 3(Ci2H220n) do. 4. Do. i. 6(Ci2H2„Oi„) 4(Ci2H220„) do. 5- Do. c. S(C,2H2oOio) S(C,2H220„) do. 6. Do. d. 4(C,2H2„Oi„) 6(Ci2H220„) do. 7. Do. e. 3(Ci2H2oOio) 7{Ci2H220n) do. 8. Do. / 2(Ci2H2oOio) 8(Ci2H220ii) do. The total equation may be represented as — I0{C,,U,fi,,) 8H2O = 8(Q2H220ii) 2(Ci2H2oO,o). Soluble Starch. Water. Maltose. Achvoodextrin. Represented in this way, each successive step consists in the removal of one of the primary starch molecules, which, added to the elements of water, becomes maltose. The accompanying table, after Sir William Roberts, explains the process in detail (Table XXIV.). The achroodextrin left unconverted into maltose at the end of the reaction is only with difficulty acted on by ferments, but is converted into sugar by the action of dilute sulphuric acid and heat. No notice is taken in the above scheme of malto-dextrin, a body found during the action of diastatic ferments under certain conditions. Unlike achroo- dextrin it possesses a slight reducing power, and is regarded by Brown and Morris as being composed of one molecule of maltose and two primary dextrin molecules, thus — ■ Malto-dextrin. Water. Maltose. DIGESTIVE PROCESSES IN THE BODY. ISS "oOOOOOOOOOCOCOjO O m N ro tJ- xj^kD r~^00 0^ O ^ ^ ^ O O ^ rt CI CTi ro g XXXXXXXXX; ?„ + + + + + + + + + + !i - ' u . t-^vo u^ ■^ ro CI ' [S X X X X X X ] ooooooooooooaoo " 18 CO H en IF (^ ■§ co i>.MD Lo r^ f^ « S fccTr . 9o .2 to L? .^+ + ^<< ,-C— !? ^ 1 S'iq Ph t3 BE a? o S S'^^ « ^u O w II II II H ^ < stare starch tarch w o ^■5 3 c go u ^ °'o w dj ^ _aj in S 3 :=! H sss M TJ^ >-« X o flJ X It w m ,2 <; H ^=5';::'^ ?? XXXXXXXX ,N(NNNC^C1C^MM ' d ro ro ro CO CO ro rn f^ N CO 'vh UTsD r-^co 1^ o 5 + + + + + + + + + + I + + + + + + + + + + t ooc?c5'ooooo « « N C-1 (N N CI CI (M CI ^ .- Q\oo r^-o i-n ^ ro N M -^ oQ'Ho o'dddd I , ^ oQQQQPQ I w <; i-I pi CO -ti- Lo^o t^od d^ o ■^ O + o ?3 O U o X \a •^ ^ aj T3 fi CJ ^~. ^ to o ■go O ^^ s-»- o o + M ■^'o + ^ IN .2U 5'§- «ll ^ .o IT) O "1 O + + + + ^ OJ o ^ O ' OJ c = -S ^ to 5 ^ ■tr' ^ O O "3 >^ u 5 ^ O u i2 ^'3 d o c o c! -^ °^ o g iii S .... o -ti c o o o o ^ H IS "^ ■"grtooooooo 3 -Jo sue 5*^00000000 g -"i-^^Tl-Tj-uoOo ^^ CO -^ u^^o r^oo So Soo O T3 IS6 THE NATURAL HISTORY OF DIGESTION. The end product of the digestion of starch consists of the sugar known as maltose. It is isomeric with cane-sugar, but loses one molecule of water when its crystals are dried at 100° C. It is dextro-rotatory, rotating the plane of polarised hght to the right, and reduces Fehling's copper solution, but to a much less extent than glucose. If the reducing power of glucose be regarded as equal to loo, that of the same amount of maltose is only equal to 65. It is easily fermented by yeast. It is said that in a mixture of maltose and dextrose the maltose disappears under the action of yeast before the dextrose is affected. Boiled with dilute sulphuric acid it yields 98 per cent, of its weight of dextrose. The diastatic ferment of the saliva produces this change in starch more energetically as the temperature rises from 0° to 30° C. ; it is most active between that point and 45° C, and becomes less powerful above this, until from 65° to 70° C. all conversion ceases. (See Chart I.) Although the products of vegetable diastase and the animal amylolytic ferments are the same, the ferments exhibit several points of difference. Thus malt-diastase is most active at 60° C., and only ceases to act at 80° C, while the presence of .05 per cent, of salicylic acid arrests its action, although it requires the proportion of .1 per cent, of this acid to cause the least retardation of the action of the salivary ferment. The form in which the starch is used has some influence on the ease with which it is converted by ptyalin. Hammarsten found that while maltose could be detected in a few minutes when saliva was added to rye- or maize-starch, a period of two or three hours elapsed before its appearance when raw potato- starch was used. It must also be borne in mind that the granules of raw starch are surrounded by an envelope of cellulose on which the salivary juice has no action. If in the process of boiling these capsules are ruptured, or if in mastica- tion the same thing occurs, the saliva is able to act. Whole granules of raw starch, when swallowed intact, are almost indigestible in man; in the Herbivora, however, they are acted on in the rumen and stomach. Glycogen, or animal-starch, an isomer of starch, is acted on by the diastatic ferments in exactly the same way as starch itself. As in the case of other un- formed ferments, the action of diastase and ptyalin is retarded, and at length arrested, if the sugar formed be allowed to accumulate in the solution. DIGESTIVE PROCESSES IN THE BODY. 157 Rapidity of Action of Ptyalin. — Roberts gives 4 minutes as the normal period of time which elapses before the achromia point is reached, when i c. cm. of human saliva is added to 10 c. cm. of a standard starch mucilage (t per cent.) at blood- heat. The following table from Roberts shows the rapidity of action in the case of the pancreatic juice : — Table XXV. IOC cm. standard starch mucilage, with 90 c. cm. water, and 0.1 c. cm. pancreatic extract, at 40° C. Time. Reaction with Iodine. 10.30 Commencement. 10.31 - Blue. 10. 32 A^olet. 10.33 Brown. 10.34 Vellowish-brovvn. 10.35 VzXt yellow. 10.36 No reaction, achromic point. 6 minutes. The action of all diastatic ferments proceeds best in a neutral or slightly alkaline medium. It is still active in a faintly acid solution, but is incapable of converting starch in a solution containing o. i per cent, of free hydrochloric acid. Tlie authorities on this suliject are, however, by no means at one with regard to the strength of acid required to inhibit or retard the action of ptyalin. Thus Chittenden and Ely (foam, of Physiology, vol. iii. ) state that o 05 per cent, of hydrochloric acid rather increases the action of the ferment, a statement which Astachewsky {Centralblatt f. med. Wissenschaft, 1878) upholds. Langley, on the other hand (fourn. of Physiology, vol. iii. p. 246), says that ptyalin is destroyed by hydrochloric acid above 0.005 P*^"^ cent., and is distinctly inhibited by 0.0014 ps"" cent. According to this observer, carbonate of sodium above 0.5 per cent, stops the action of ptyalin. Again, Chittenden and Griswold {Aiiier. Chan, fourn., vol. iii.) state that a very small quantity of acid added to saliva increases its power, a statement explained by Langley, who found that neutralised saliva is more active than if not neutralised. But another factor has to be considered. Saliva contains a small quantity of proteid bodies, and free acids combine to a certain e.xtent with proteids. The first portion of the free acid added neutralises the alkali present, the second combines with the proteid, and it is only after the affinities of the proteids present have been satisfied that free acid appears in the solution. Langley thought that the combined acid acted prejudicially on the ferment, but Chittenden and Smith showed that up to a certain point it rather stimulated it. The author found (fourn. of Anat. and Phys., vol. xxvii. p. 201) that while .018 per cent, of hydrochloric acid, mostly in the free state, entirely stopped the action of ptyalin, .0318 per cent, of that acid, when combined with proteids, allowed some conversion to take place, while IS8 THE NATURAL HISTORY OF DIGESTION. .015 per cent, of combined acid had only a slight retarding effect. This point is of value when the question of the digestion of starch in the first stages of gastric digestion is considered, as during the early part of digestion in the stomach the hydrochloric acid secreted by its glands is entirely com- bined with proteids or inorganic bases. Aitchison Robertson {Edin. Med. Journ., May 1896) used filtered gastric contents in an investigation into this subject, and found that small quantities of acid contents hindered or inhibited the action of ptyalin. As he made no observations on the state of the acid contained in the specimens employed, and neglected to take into account the alkalinity of the saliva added, his conclusions in many particulars require revision. After the introduction of starch mucilage into the stomach by the stomach tube, however, he found that some conversion took place under normal conditions, but that it was very slight in amount, whereas in disease, with a lessened acidity, most of the starch introduced was transformed into dextrins and maltose. The Gastric Juice. — The composition of the gastric juice of man, and of the dog and sheep, is depicted in Table XXVI. The greater part of the organic solids given in it may be taken to represent the ferment pepsin. The proportion of hydrochloric acid present varies in different animals, the gastric juice of the dog containing about .3 to .4 per cent.; of man, .2 to .3 per cent.; and of the sheep, only from .1 to .15 per cent. The amount of gastric juice secreted varies so much under different conditions that little value can be placed on the esti- mates of observers. Schmidt reckoned the flow at 500 grammes per hour in man. The total gastric secretion per diem of the dog has been stated to be iV of the body-weight, and if the same proportion obtains in man the total daily secretion will equal 6 to 6.5 kilogrammes. Table XXVI. — Composition of Gastric Juice (per mille). Man, Dog. Sheep. Water .... 994.40 973.062 9S6. 14 Total solids 5.60 26.938 13.86 Organic solids (Burnt off) . 3-39 20.645 5.78 Ash ... . 2.21 6.293 8.08 Carbonate of potassium . 0-5S 1. 125 1-52 Chloride of calcium 0.06 0.624 O.II , , sodium 1.46 2.507 4.37 ,, ammonium — 0.468 0.47 Phosphate of calcium 1 fl.729 I.I8- ,, magnesium \ OI2S \ 0.226 0.57 ,, iron . J [0.082 0-33 Hydrochloric acid 0.2 (?) (2 !.0) 3 050 1.26 Chlorine 1.368(3. 078) 5.720 4-97 Volatile . 0.19 ? (1 [.9) 3.270 I 53 Inorganic . , 1.178(1. 178) 2.450 3 44 DIGESTIVE PROCESSES IN THE BODY. I 59 Gintl, working under Riegel {Miinchener Med. Wochenschrifl, 1897, No. 23), found, as the result of measuring 189 specimens of fluid obtained through the stomach tube from the human stomach in health after fourteen hours abstinence from food, that the quantity was usually small, averaging, about 9 c. cm. ; while in 40 per cent, no free hydrochloric acid could be identified. Chart III. shows the chief points of Gintl's investigations. It is apparent from it that the most frequent quantity obtained varied from o to i c. cm., while in only one case was it above 100 c. cm. Further, it is evident that those specimens with the smaller amounts of fluid more commonly contain no free hydrochloric acid. CHART III.- — Showing the quantity of Stomach Contents removed 14J hours after Food, with the frequency of the presence of Free HCl. Number of observations, 189. Percentage Proportion of Cases.' 3S ja 25 20 15 10 5 J^mounts qbtained in Cubic Centimetres. Percentage without Free HCl to number at each amount, 70 60' SO 40 30 20 JO Over From fo J — 5 — 10—20 — 50—100-100 i \ J \ \ \ 1 Y / \ \ / N s ^ \ ^ Percentage nunjliet yielding the various quantities of Fluid Perieniage number which did not s'hov/ the presence of FreeHCl Percentage of cases without Free HCl to total numbers yielding the different amounts of fluid l6o THE NATURAL HISTORY OF DIGESTION. The Production of the Hydrochloric Acid of the Gastric Juice. — Since it was proved that hydrochloric acid formed the principal acid present in the gastric juice, various attempts have been made to explain the chemical method by which it is separated from the blood serum by the gastric glands. It may be first assumed that the acid is derived from the chlorides of the body, especially as it has been shown that the formation of the acid ceases when food is given containing no chlorine com- pounds. Griitzner {Neue Untersuchungen iiber die Bildung und Ausscheidung der Pepsin, Breslau, 1875) has observed that the gastric mucous membrane contains, most pepsin when it is richest in chlorides. Bence Jones, Roberts, and Maly have found that the acidity of the urine diminishes or may disappear during active digestion. Among the various theories which have been broached to explain how the hydrochloric acid is formed, Briicke's hypothesis that, under the influence of the secretory nerves of the stomach, the gastric glands possess the power of decomposing chlorides electrolytically, and of directing the resulting acid towards the outer surface of the mucous membrane, the bases towards the blood-stream, may be looked upon as purely speculative and to rest on no firm foundation. Ralfe showed, however, that if a weak current of electricity was passed through a U-tube, one limb of which con- tained a solution of sodium bicarbonate, the other a solution of sodium chloride, with a dialysing membrane between the two solutions, the liquid at the positive pole became acid, and that at the negative became alkaline. The reaction which occurs in this experiment depends upon the formation of hydro- chloric acid and carbonate of sodium from the two salts employed. That such a force is in active operation in the body rests only on theory, and has not been proved in fact. Maly found that if chloride of an alkaline metal is mixed with lactic acid and the mixture dialysed, free hydrochloric acid diffuses through, the result of the chemical action being the formation of a lactate and hydrochloric acid, in addition to the remainder of the chloride and lactic acid un- acted upon. But he found that there is no formation of lactic acid in the body by which such a chemical reaction can be brought about, and concluded that the free hydro- chloric acid of the gastric juice arises from some other form of decomposition of chlorides. Maly, again, has published a more recent theory. He regards the formation of the DIGESTIVE PROCESSES IN THE BODY. l6l hydrochloric acid as brought about by interaction between carbonic acid, disodic phosphate, monosodic phosphate, and chloride of sodium both in the blood-stream and in the cells. Hydrochloric acid possesses a remarkably high power of diffusion, and we have only to look upon the glands of the stomach as forming a very delicate diffusion apparatus to account for the separation of the acid set free by the above chemical process. Against this theory we may advance the objection that if hydrochloric acid be already formed in the blood we should expect it to be secreted by the kidney as well as by the stomach. Also, the interactions suggested by Maly are not all located by him in the cells of the gastric glands, and if his proposition be correct free hydrochloric acid will be present in the general blood-stream, as well as in the serum which passes through the cells of the gastric glands. Koeppe {Ffliiger's Arch., Ixii. s. 567) has suggested that the osmotic pressure of the blood-plasma has to do with the formation of hydrochloric acid. He estimates the osmotic pressure of blood by Hedon's hsematokrit against a solution of cane-sugar. He notes that salts give off no warmth themselves, but do work when passing into solutions. He finds that the osmotic pressure of the blood rises on the addition of salts to it, and in this way water may be secreted into the stomach. He says that the acid of the gastric juice is not formed in the gland cells themselves but in their walls, in virtue of their specific properties, which prevent, as a semi- permeable wall, free hydrochloric acid ions passing through, while they allow free hydrogen ions to pass in the other direction. The presence of a secretory stimulus is necessary for the formation of the acid, and this may consist in free chlorine ions on the inner side of the cell wall. He refers also to the formation of hydrobromic and hydriodic acids when chlorine is abstracted from the food, and salts of these acids given in its place. Gamgee has modified Maly's theory by suggesting that the acid-forming cells of the stomach have peculiar selective powers in reference to certain salts of the blood, as we know the cells of other secreting glands to have. They may be supposed, for instance, to have an affinity for the phosphates of sodium, both alkaline and acid, and for chlorides. If this is granted, we have only to surmise that l62 THE NATURAL HISTORY OF DIGESTION. the same reactions may occur inside the cells of the gastric glands which take place in similar solutions in the laboratory. 2 Na„HP0,+ 3 CaCl2 = Ca3(P04)2 + 4 NaCI + 2 HCl. Acid + Chloride = Piiosphate + Chloride + Hydro- phosphate of calcium of of chloric of sodium. (Chloride calcium. sodium. acid, of lime). The result of this reaction is the production of hydro- chloric acid, which, from its great diffusibility, can pass, on being formed in the body of the cells, through the walls into the stomach cavity. This hypothesis removes the seat of the for- mation of the acid from the general blood-stream to the cells, and is a feasible adaptation of Maly's theory of the chemical processes which probably occur. The theory of Koeppe.may be easily brought into Hne with Gamgee's by supposing that the acid is formed in the walls of the cells and not in the cells themselves. As the hydrochloric acid of the gastric juice must arise from the chlorides in the blood, and as the reaction sug- gested by Maly occurs outside the body, the gastric juice should contain the salts corresponding to those found in artificial experiments. If we regard the reaction as an interchange between acid phosphate of soda and chloride of sodium or calcium, we should expect to find phosphates of sodium and calcium, chloride of sodium and free hydrochloric acid in the juice secreted, in like manner to that which we find /« vitro. And analysis of the gastric juice corroborates this in every particular, for it contains the chloride of sodium and calcium, phosphate of sodium, and phosphates of calcium and magnesium. The magnesium phosphate may be formed, of course, from the chloride in the same manner as that of sodium and calcium. The secretion of free mineral acid by animal cells at first sight appears to be an anomaly. We have seen before, however, that the saliva of a gasteropod, Dotium galea, contains free sulphuric acid in very considerable proportion. The total acidity of the stomach contents may be determined on either filtered or unfiltered specimens. Titration of the unfiltered contents gives the most correct results unless the solid matters have a marked coloura- tion which may interfere with the end reaction. As a rule the acidity of an unfiltered specimen is higher than that of the filtered, owing to the presence of coagulated proteid with its attached acid. On the other hand, if the suspended matter be largely carbo-hydrate in nature, the total acidity DIGESTIVE PROCESSES IN THE BODY. 163 may be lower and give an erroneous impression of the state of digestion. In such cases the determination of the acidity on the filtered contents affords thfe more reliable results. It is a question whether the greater accuracy obtainable by determining the end of the reaction in the filtered contents does not compensate for the slight difference observed between their acidity and that of the unfiltered contents. To put it in another way, filtration removes the coagulated proteids, with the hydrochloric acid combined with them, and the other insoluble portions of the food, chiefly vegetable in nature ; the filtrate containing all the soluble proteids, the acids, free or combined with them, the acid salts, and the soluble carbo-hydrates. The principal object in estimating the total acidity of a sample of stomach contents is to get a figure from which the free acidity can be calculated after determination of the acidity which has combined with proteid bodies. The mere fact of the gastric contents having a certain total acidity is of little consequence unless the component constituents of that acidity are known. In this way it matters little whether the whole analysis is undertaken on an unfiltered or on a filtered specimen, if the fact be noted and the nature of the preceding meal indicated. Undoubtedly the investigation of the unfiltered contents gives a result more akin to the actual state of digestion in the stomach, but, as mentioned above, the colour reactions are of necessity less delicate. If the unfiltered contents are used they must be well shaken beforehand to divide the insoluble particles and to render the fluid uniform. The actual determination of the total acidity is performed by means of a deci-normal, or, if great accuracy is required, a centi-normal solution of sodium hydrate. A normal solution of any chemical substance signifies a solution which contains in each litre as many grammes of the substance as there are figures in its atomic weight. For instance, hydrochloric acid has an atomic weight of 36.5 (H i, CI 35-5), and a normal solution of HCI contains 36.5 grammes of the acid in each litre. The exact weight of HCl is 36.46, but for ordinary purposes 36.5 is sufficient. In the same way a normal solution of sodium hydrate is made by putting 40 grammes into a litre measure, and adding distilled water up to the mark. An equal quantity of an acid normal solution exactly neutralises an equal portion of an alkaline normal solution. A deci- or a centi-normal solution is made by diluting the normal solution ten or a hundred times respectively. The in- dicators used are litmus, cochineal, or phenol-phthalein. Phenol-phthalein is the most delicate, but gives very erroneous results in the presence of ammonia or its salts. Neither litmus nor cochineal can be used with any approach to accuracy if the contents are highly coloured. Phenol-phthalein is colourless in acid solutions, bright red in alkaline. It is unnecessary to point out the advantage gained by the use of an indicator which absolutely changes its colour at the end of titration, over one whicli merely alters its shade. The strength of the solution of phenol-phthalein is of little con- sequence, a 2 per cent, solution in alcohol is generally recommended. Litmus may be used in the form of papers, or dissolved in alcohol. Cochineal should also be dissolved in alcohol, care being taken to ensure that the solution is neutral in reaction. The actual process of titration is as follows : — A measured quantity of the stomach contents is poured into a beaker ; the amount is immaterial, but 5 c. cm., 10 c. cm., or 100 c. cm. are the most convenient amounts. 164 THE NATURAL HISTORY OF DIGESTION. A Mohr's burette is filled with a deci-normal solution of caustic soda, and the point on the scale to which it reaches duly noted. The soda is then added drop by drop to the stomach contents, to which a few drops of the phenol-phthalein solution have been previously added. Whenever the stomach contents exhibit a permanent pink tinge the titration is over. If the contents are tested without filtration the end reaction is often very indeterminate, and it is well to leave the specimen standing for some time after a decided pink tinge has appeared, and to stir it occasionally, as the alkali added takes some time to neutralise the acid bound up in the coagu- lated proteids. For example, 10 c. cm. of stomach contents are taken, two drops of phenol- phthalein added, and deci-normal soda dropped in until a permanent pink colour appears in the solution, and 9. 5 c. cm. of the soda solution are requi- site. As each cubic centimeter of a normal solution of sodium hydrate exactly neutralises 0.0365 grammes of hydrochloric acid, a similar quantity of a deci-normal solution will represent 0.00365 grammes of the acid, 9.5 c. cm, corresponds to (0.00365 X 9.5) 0.034675 grammes of HCl. But 10 c. cm. were taken, and 0.0346 grammes in 10 c. cm. equals 0.346 in a hundred. The total acidity of the specimen is therefore 0.346 per cent, expressed in terms of hydrochloric acid. To express it in terms of any other acid the atomic weight of the acid required is substituted for that of HCl, care being taken in the case of dibasic acids, such as H2SO4, to divide the atomic weight by two before multiplying. Another method of expressing the acidity has been recommended by Ewald. It has the merit of simplicity, but is hardly so scientific. Ewald suggests that the number of c. cm. of the deci-normal solution of sodium hydrate required to neutralise 100 c. cm. of the gastric contents should be taken as the symbol of its acidity. By this method the acidity of the illustrative sample given above would be 95. In health, the reaction of the various fluids secreted by the glands concerned in digestion is fairly constant, varying within small limits. Still it is a noteworthy fact that the percentage of acid or alkali in the pure juice, although almost constant under identical conditions in one individual, varies appreciably when samples from different individuals are analysed. How do the' cells of the secreting glands know when to cease pro- ducing either acid or alkali in the act of digestion ? The pro- portion of free hydrochloric acid, to take gastric digestion as an example, in the pure gastric juice produced by reflex or local mechanical irritation, depends upon the amount of acid secreted by the oxyntic cells and mixed with the secretion of the pepsin- producing glands. The acidity of the mixture thus obtained may be regarded as the normal for the individual. In man it varies from .15 per cent, to .22 per cent. ; in the dog, froin .23 to .33 per cent; ; in the sheep it is about . 1 2 per cent. If the gastric contents of any of these animals be examined shortly after food has been taken, the acidity present will be found to be much DIGESTIVE PROCESSES IN THE BODY. 165 less than that of the gastric juice, provided the food does not contain much acid in itself, while the acid present at this stage is either in the form of an inorganic acid, such as lactic acid, or as hydrochloric acid combined with the proteids of the food. In this condition hydrochloric acid does not give the reactions peculiar to a free mineral acid, although its acid value towards alkalies remains the same. As time goes on the acidity of the contents gradually increases, according to the nature of the food, and if the food consists largely of proteids may even ex- ceed the acidity of the pure gastric juice before the presence of free hydrochloric acid can be detected. If the food be almost wholly composed of carbo-hydrates, free hydrochloric acid appears at an early period and soon reaches its maximum. From some experiments made by the author on artificial peptic digestion in dialysers {Journ. of Anat. and Phys., vol. xxvii.), in which the substance experimented upon was placed inside the dialyser with some pepsin, and a solution of hydrochloric acid of known strength placed outside the parchment, it is clear that the presence of proteid bodies has a great influence on the distribution of hydrochloric acid. When the substance used consisted almost entirely of carbo-hydrates the acidity in- side the dialysing tube after a few hours was almost the same as that outside, and was mostly in the free condition. If proteids alone were placed inside the dialyser within the space of from one to two hours the acidity inside was greater than that of the solution outside the tube, although free acid could not be detected in the digesting mixture. Later on, the acidity of the contents of the tube often rose very markedly above that in the surrounding fluid, while the free acid present in it was less. When proteids constituted the bulk of the substance digested in this way the free hydrochloric acid inside the dialyser never equalled the proportion of free acid outside it. That is to say, that however great the percentage acidity of the digesting mixture might be, if it was composed entirely of hydrochloric acid in a state of combination with proteids, free acid could still dialyse through, but not in sufficient amount to equalise the free acidity of both solutions. If a solution of this acid is placed round a dialysing tube filled with water, in time, by the law of osmosis, the acidity outside and within the tube will be equal. If a solution of a pure carbo-hydrate, such as soluble starch, be similarly used, the same thing occurs. Put a proteid body, either in solution or in bulk, into the water in the tube, l66 THE NATURAL HISTORY OF DIGESTION. the acidity within soon rises above that without. The acid which combines with the proteid must, ipso facto, lose all, or nearly all, its dialysing power, and allow more free acid to diffuse through the membrane to preserve the osmotic balance. When the solution in the tube contains proteid bodies and has had pepsin added to it, as digestion proceeds and diffusible products are formed the condition changes. Many of the products of proteid digestion are diffusible and occasion the presence of combined acid outside as well as within the tube, and, in time, the amount of proteid outside may equal that remaining, when the acidities of both approxi- mate. (Chart IV.) The same phenomenon occurs in the healthy stomach. The greater the proportion of proteid material in the food ingested, the higher does the acidity of the gastric contents rise. In the Edinburgh Medical Journal for 1893 the author published some observations bearing on this subject, in which he investigated the effect of carbo-hydrates and proteids on the gastric acidity. The results serve to show that the course of the gastric acidity corresponds to the nature of the food, and to the conditions described above in digestion in dialysers. An observation by EUenberger and Hofmeister on the horse corroborates these statements. They found the following acidities to be present in the stomach of that animal : — Table XXVII. Food. Hydrochloric Acid. Organic Acids. 1. Oats and chopped straw o. 163 per cent. 0.287 per cent. 2. Oats 0.490 ,, 0.610 ,, 3. Hay 0022 ,, 1.798 ,, The amounts of hydrochloric acid obtained exactly corre- spond to the proportion of proteid in the food. Oats contains from 12 to 14 per cent, of proteids, hay from 5 to g per cent., and straw about 3 per cent. Allowing, then, that the acidity of the gastric contents varies with the character of the food, we are forced to admit the necessity of there being some controlling mechanism through which these changes can be brought about. As in experiments with dialysers, so in the healthy stomach the combined acid appears to have little or no effect on the equilibrium, which must exist between the agents at work in supplying sufficient acid and the amount Missing Page Missing Page DIGESTIVE PROCESSES IN tllE SOUV. 167 of acid in the contents. In the case of dialysis, simple diffusion supplies the active agent. In the stomach diffusion may have some little part to play, but the vital processes in the cells of the acid-producing glands are much more nearly concerned. The different views taken of the actual formation of hydrochloric acid are detailed above (page 160), and do not concern us here. It appears to be most probable that there is what may be termed a "chemical reflex" at work. By that is meant the presence in the mucous membrane of the stomach of nerve fibrils possessing a power, probably through the ganglia in the stomach wall, of reflexly stimu- lating the secretory glands when food is introduced into the stomach; and that this power persists until there comes to be a definite percentage of free hydrochloric acid in the stomach contents. When this point is reached the nerve- terminations become irritated, and supply inhibitory im- pulses to the ganglia in place of accelerating messages. The presence of combined hydrochloric acid may be regarded as inert to a great extent as regards this mechanism, free acid being still poured out, although the total acidity of the con- tents be greater than would serve to arrest its secretion if the acid were all in the free state. In man the natural percentage of free hydrochloric acid of the gastric juice may be regarded as the maximum possible under ordinary circumstances, although if much combined acid be present the maximum proportion of free acid is usually less. The natural total acidity is from .15 to .22 per cent., while the normal free hydrochloric acid after food, containing, as most foods do, a considerable quantity of proteid, is seldom more than .10 per cent., and may be lower. When carbo-hydrates alone are taken, the absence of proteids allows the free acid to reach its physiological maximum in a short time, and we find that this maximum is not exceeded unless the food be particularly irritating, or the gastric functions out of order. In the dog the maximum percentage of free hydrochloric acid permissible by our hypothetical mechanism is higher, .3 to .4 per cent, are not uncommon proportions in the stomach of this animal. Again, in those conditions in man in which excess of this acid in a free state is found, causing pain and discomfort, the nerves which act as regulators of the amount of free acid are unable to respond to the normal percentage of acid, and do not 1 68 THE iSTATURAL HISTORY OF DlGEStlOX. arrest its secretion until an abnormal acidity obtains. The ordinary nerve-endings, sensible to pain, on the other hand, are irritated by the abnormal proportion of free hydrochloric acid, and notify the fact by representing it as discomfort and pain. The presence of free organic acids in the stomach contents would seem to irritate the sensory nerve-termina- tions almost as vigorously as that of hydrochloric acid ; their influence on the mechanism for regulating the acidity is rendered more difficult to estimate owing to the fact that the organic acids, which take their origin in fermentation, can only occur in any quantity when the supply of free hydrochloric acid is deficient. They, in all probability, act on the nerve-endings after fermentation, owing to some dis- eased condition of the stomach walls, has begun, and diminish the stimuli sent to the secreting glands. Thus a temporary cause may bring about a prolonged deficiency of hydrochloric acid if the primary cause be neglected. Another fact bearing on this question is the rapidity with which an alkaline solution introduced into the stomach is not only neutralised, but frequently rendered more acid than the normal contents from the increased stimulus given by the acid to the secretory glands; while acid solutions appear to be- come less acid if their acidity is above that of the normal stomach contents or juice, owing to dilution with a gastric fluid containing little or no acid. Hitherto the influence of the central nervous system has been ignored in the discussion of this hypothesis. The fact that a flow of gastric juice has been observed in the dog (after the formation of a gastric fistula) at the mere sight of food, even when the saliva is prevented from reaching the stomach by the establishment of a salivary fistula, proves the existence of some communication by way of the brain between the nerves of sense and the gastric glands. Richet observed, in a patient with a gastric fistula, where the oesophagus was completely closed by a stricture, and not even saliva could reach the stomach, a copious secretion of gastric juice coincident with an increased flow of saliva, when savoury articles of food were chewed. Section of the pneumogastric nerves, however, has no effect on the secretion of gastric juice, although it arrests all movements of the walls of the stomach. The sympathetic nerves may also be cut without altering the secretion. We are forced to the conclusion by these facts that the local effects DIGESTIVE PROCESSES IN THE BODY. 169 depending on the presence of food are the chief agents at work; that the local mechanism stimulated by it, and probably by the products absorbed, is governed by the ganglia which are so numerous in the stomach walls; that it is possible that this mechanism includes a nerve arc for the regulation of the proportion of free acidity; and that the local centres can act independently of the central nervous system, but at the same time can be influenced by it on occasion. If the existence be granted of a mechanism for preserving as far as possible under normal circumstances the maximum pro- portion of free mineral acid in the stomach contents proper to each individual after food of various kinds, it can easily be understood how the same proportion of free acid may prove irri- tating should the individual suffer from any disease in which the supply of nourishment to the stomach walls is deficient. For instance, women suffering from ancemia or chlorosis frequently complain of symptoms obviously due to the action of free hydrochloric acid, while examination of the stomach contents fails to reveal any excess, perhaps a slight diminution from the average amount. Here there is a proportional excess of acid, not a percentage above the average. The secretion of saliva is governed entirely by a centre in the central nervous system, probably in the medulla, and the chemical reflex concerned with its reaction is not so apparent, nor, considering the short time during which it can act, so important. But here again the flow varies with the reaction of the food; after acids are taken into the mouth the rate of secretion is accelerated. The pancreatic juice is practically secreted continuously though in varying quantities in dogs and man. Twenty-four hours elapse after a single meal before the flow ceases. In the rabbit, in Herbivora, and in Ruminantia, the flow is constant; in Carnivora, though practically constant, it varies in amount and in digestive activity with the time after, and the nature of, the food. The moment food enters the stomach the pancreatic juice begins to be secreted, but the rate of flow soon decreases until the chyme enters the duodenum, when a further increase occurs. The first flow is reflexly caused through the nervous system, the second in all probability by local stimulation to some extent at least. The fact that the contents of the stomach are acid stimulates the pancreas to neutralise it, while the constant presence in the bowel of organic acids pro- I70 THE NATURAL HISTORY OF DIGESTION. duced by bacteria must serve to excite the gland to neutralise them, only with the result of further facilitating the growth of the organisms. < c v Jc TJ ^ rt ^ ^ t> M C s c: o .Cti rn ij (J CW f^ c 0) o H K fl> )_, i. >■ S3 hQ Pi < X u ~" ■" "■" ■^ "■ ^~ oo m 2 * a z z o SI 00 < (9 + z. lU 2 tr UI u. o in < 5 - •o U) *" •' N ^^ ... S ' , ^ * •.. ^ ' ^ '■• / [^ ^ X .. / ■•.. y '•■ to t^ ' nj V, ^ *v z in Q. UJ a o o / . "O ^-0 \ .• s .* .03 .. < •. o to ••. ys> in -■ ■ufe 1 h Q: W O goooooopooo During the period of gastricdigestionthe passage of chlorine intothegastric glands, and its secretion as hydrochloric acid into the cavity of the stomach, render the blood more alkaUne, the urine less acid. When the digestive process has been transferred to the intestine, per contra, the blood becomes less alkaline, the urine more acid. In the first period acid is practically withdrawn from the blood and passed out of the body, leaving organic and DIGESTIVE TROCESSES IN THE BODY. 17I alkaline salts of the bases previously combined with the chlorine. In the second period the chlorine is being reabsorbed as chloride of sodium, while alkali is passed out, the carbonate of sodium formed in the production of hydrochloric acid serving to re-form chloride of sodium ; when secreted by the intestinal or pancreatic cells into the small intestine, it neutralises the acid coming from the stomach, and the base is again united to its former partner. Some of the pepsin is absorbed by the walls of the stomach or intestine, or enters the blood-stream from the cells of the gastric glands with- out entering the stomach. The chart (Chart V.) shows the variations in the amounts of pepsin compared with those of the starch-converting ferment excreted in the urine at different periods of the day. The amount of pepsin increases till shortly after midday, diminishes until about half- past four, and then increases again as evening advances. The diastatic ferment, on the other hand, increases m amount up to half-past four, with a minor rise about 9 a.m. The Digestion of Proteids by the Action of Acids aided by Pepsin. — The class of ferments which act on proteid bodies in an acid medium may be iitly represented by pepsin, the ferment of the animal stomach, and by hydrochloric acid, its coadjutor in its work. For convenience, and in accordance with general custom, all proteolytic ferments which act only in an acid solu- tion will be termed pepsin. Pepsin can aid all the mineral acids and many of the organic acids to digest proteids, but the digestive power of the different combinations varies greatly, and the strength of the acid necessary to produce the maximum amount of digestion also differs with each. In all probabiUty, pepsin derived from different sources acts differently, though the variations in the process are as yet quite unknown. Wro- blewski {Ztsch. f. fhys. Chem., xxi. s. 1) found that casein from cow's milk and fibrin were more easily and more com- pletely digested by oxalic than by hydrochloric acid. He also obtained varying results from the digestion of a native proteid with acetic acid and pepsin made from different animals. In carnivorous plants the acid present invariably belongs to the fatty series, and is generally formic acid, either alone or along with other acids. In the Protozoa, in whom digestion is intra- cellular, the reaction of the protoplasm before proteid food has been engulfed is alkaline. Immediately after the entrance of any proteid-containing body within the cell the reaction of its contents becomes acid. Greenwood and Saunders found that AmabcB changed the colour of litmus, alizarin, congo red, etc., from that representative of an alkaline reaction to that of acid 172 THE NATURAL HISTORY OF DIGESTION. conditions when fed with bodies impregnated with these stains. If the body was indigestible it was extruded and the reaction of the cell again became allcaline. If digestible, as soon as it was dissolved, the alkaline reaction returned. The acid secreted is unknown, but the necessary ferment is probably akin to the pepsin of higher organisms. Although hydrochloric acid possesses the greatest power among acids of bringing about peptic digestion, the power of several others is by no means small. Thoyer gives (^Mhn. de la Soc. de Biol., February 20, 1891) a list of acids arranged in their order of digestive power — 1. Hydrochloric acid. 2. Sulphuric acid. 3. Acetic acid. 4. Oxalic acid. 5. Tartaric acid. 6. Citric acid. 7. Ivactic acid. 8. Hydrofluoric acid. On the Other hand, Hubner (yForischritte des Med., xxii. s. 163, 1894) arranges the digestive power of the halogen acids exactly in inverse order to their molecular weights, hydrofluoric first, hydrochloric next, hydrobromic third, and hydriodic last and least powerful — Acid. Molecular Weight. I. HFl. = 20.0 2. HCl. = 36-5 3. HBr. = 80.0 4. HI. = 12S Table XXVIII. — The Relative Action of Pepsin on Egg Albumin in the presence of various Acids. I. — Duration of Digestion 2\ hours at 38° Cent. Acid. Acidity per Cent, as HCl. I'evcentage oi Albumin renders uncoagulable. I. Oxalic Acid ■IS 633 2. Hydrochloric .146 47-7 3. Trichloracetic .036 4I.I 4. Nitric ... .169 34 4 5. Sulphuric .146 33 3 6. Citric .139 30.0 7. Phospho-wolfraniic .003 8. Benzoic .05S4 9. Sulphanilic .0438 10. Acetic .21 DIGESTIVE PROCESSES IN THE BODY. 173 n. — The same Adds added to a Solution of Egg Albumin, Pepsin, and Hydrochloric Acid. Duration of Digestion 15 hours at 38° Cent. Total HPi not due to "^'- HCl. Proportion Albumin Acid added. Acidity. of HCl to Acid. Digested per cent. 1. Sulphuric . .31 .1014 2036 1—2.05 97.7 2. Acetic .43 .192 283 1—1.5 95.6 3. Sulphanilic .16 .1014 0686 C.6J9— 1 91.1 4. Benzoic .18 .1014 0786 0.77-1 84.4 6. Hydrocliloric . .116 .115 82.2 6. Nitric .44 .1325 2575 1—1.4 71.1 7. Tricliloracetic . .233 .152 086 0.56-1 67.7 8. Citric . .25 .114 186 1—1.19 63.3 9. Oxalic .2.33 .1014 1216 1—1.19 44.4 10. Pliospho-wolframic . .15 .1014 0486 0.479-1 Table XXVIII. gives the results of a series of experiments upon the relative proteolytic action of pepsin in the presence of various acids on egg-albumin, alone and when added to a solution containing hydrochloric acid of constant amount, though the mixtures were not of constant proportional acidity value after their addition to the acid solutions. In each case a known quantity of egg-albumin was used, and after the expiry of the experiment the weight of the coagulable remainder determined. As the formation of acid-albumin may be re- garded as the preliminary stage of proteolysis, this body was not separated from the lower proteid derivatives. From the first part of the table it is seen that the greatest loss of albumin after digestion for two and a half hours occurred in the experiment in which oxalic acid was used. Hydro- chloric acid of 0.146 per cent, caused the second greatest loss, closely followed by 0.036 per cent, trichloracetic acid with 41. 1 per cent, uncoagulable. The result obtained from trichloracetic acid was always entirely due to the amount of acid-albumin formed. Nitric and sulphuric acid were almost of the same activity, and were followed at a little distance by citric acid. The other acids tested had no action. The position occupied by oxalic acid corroborates AVrdblewski's observations, already noted, on the peptic digestion of fibrin. The results depicted in the second part show that the power of individual acids of digesting albumin bears little relation to their action on the proteolysis occasioned by 174 THE NATURAL HISTORY OF DIGESTION. pepsin-hydrochloric acid. The addition of sulphuric, acetic, sulphanilic, and benzoic acids rather increased the action of hydrochloric acid and pepsin. The presence of the others diminished it, notably oxalic acid with only 44.4 per cent, of albumin altered after fifteen hours, and phospho-wolframic acid, which arrested all digestive action. Although the action of pepsin, whereby an acid is able to act on proteid molecules, is exceedingly obscure, the action of the acid facilitated by pepsin is better known, but even this know- ledge is by no means based on a sound footing. The changes which occur in the intimate chemistry of the large composite molecules of native proteids, when subjected to the action of hydrochloric acid and pepsin, must now be con- sidered in some detail, as they constitute some of the most important features in the natural history of digestion. Their action on egg-albumin may be taken as the general type of the changes caused by them in all proteid bodies. If the whites of several eggs be taken and freed from their fibrous shreds by clipping with scissors, shaking up with small bits of broken glass, and filtering through muslin, the globulin and alkali-albumin in the filtrate may be removed by cautiously adding a small quantity of dilute hydrochloric acid and catch- ing the resulting precipitate on a filter-paper. The further filtrate must now be carefully neutralised by the addition of a weak alkali, and any precipitate which may appear during the process again filtered off. The clear neutral fluid left may be still further purified by dialysis into distilled water to remove the salts. We have now a solution of egg-albumin in water, practically in as pure a state as is possible without proceeding to very elaborate methods. If a dilute solution of hydrochloric acid be added to this fluid, and the fluid tested from time to time, an increasing amount of acid-albumin will be found to be present, and, until a certain proportion of acid has been added, no trace of free hydrochloric acid can be discovered ; until, indeed, as much acid has combined with the albumin to form acid-albumin as is required. Whenever the proportion of acid by weight reaches about 8-9 per cent, of the albumin in the solution, the further addition of it at once appears as free volatile acid. The addition of more acid brings about no further change in the proteid. The action of dihite adds in the cold transforms albumin into acid-albumin, but goes no further. DIGESTIVE PROCESSES IN THE BODY. 1 75 If a small quantity of pepsin be added to the solution of egg-albumin at the same time as the hydrochloric acid, the resulting change goes much beyond that caused by the acid alone. Acid-albumin is to be looked upon as a molecule of albumin in which some of the water-molecules attached to it have been replaced by molecules of acid. It should be men- tioned here that prolonged action of dilute hydrochloric acid (0.25 per cent.) at 40° C, boiling with sulphuric acid (3-5 per cent), or the prolonged action of dilute acid with an inadequate quantity of pepsin, converts the acid-albumin into — first, anti- albuminate, a variant of acid-albumin which cannot be further digested by acid and pepsin j and secondly, transforms another portion into albumoses and peptones. The changes which follow the formation of acid-albumin in the ordinary course of gastric digestion consist primarily in the splitting up of the acid-albumin into smaller and more hydrated molecules. In the presence of pepsin the acid is enabled to cleave the unstable and unwieldy parent molecule, already combined with as many molecules of hydrochloric acid as it can retain in the absence of pepsin, into smaller mole- cules, each with a greater affinity for the acid, and, when the acid has been removed, containing more of the elements of water. These smaller proteid molecules are acted on by the acid in a precisely similar manner, yield still more hydrated and less bulky molecules, while these in turn suffer the same fate until the limit of acid and peptic digestion is reached by the formation of peptones. The process may be represented by this schema — Table XXIX. ALBUMIN I ACID-ALBUMIN \ I 1 rroto-albumose. ITetero-albumose. I I Deutero-albumose. Deutero-aUmmose. I I Peptone. Peptone. Kiihne and Chittenden have proved, however, that the process is by no means as simple a one as represented in this schema. They have shown that in all artificial digestion 176 THE NATURAL HISTORY OF DIGESTION. experiments two groups of very similar bodies are formed, the one, which is composed of more stable substances, they terra the anti-group, the other, less resistive to hydrolytic agents, the hemi-group. A modification of their scheme is as follows : — Table XXX. ALBUMIN I ACID-ALBUMIN I I Anti-albumose. Hemi-albumose. I I I I .1 Anti-peptone. Anti-peptone. Hemi-peptone. Hemi-peptone. I \ ! ^1 AMPHO-PEPTONE. Consideration of the " Anti " compounds would lead us too far afield, and will be omitted. The percentage composition of different derivatives is prac- tically the same as that of the original proteid. But they can- not be the same in reality. Their properties are very dissimilar, and all point to the lower members of the series having a smaller molecule than the members above them. The different bodies derived from albumin are more soluble in various men- strua, more easily diffusible, and less easily precipitated by alcohol and other reagents as we go down the scale. Their compounds with acids and metals contain a greater proportion of these bodies than the compounds of those above them. The evidence in support of a series of proteid bodies as the result of peptic digestion, presenting a progressive diminution in the size of the mole- cules, though it maybe insufficient for the purpose of estimating their exact size, is extensive and conclusive enough to enable us to hazard a surmise as to their relative bulk. Both native and derived proteids form compounds with acids and with salts of the heavy metals. If these compounds be analysed, the lower down the proteid scale the higher is the proportion of acid or metal in the salt. These compounds of proteids with acids and metals are by no means uniform bodies, the strength of the reagents used, and the temperature at which the reaction takes place, cause variations in the products. Thus Harnack (Zeitschrift f. phys. Ch., Bd. v., s. 198) obtained two compounds of egg-albumin and copper, in which the copper was as 1.35 per cent, or 2.64 per cent, to the albumin. The composition of the albumin remained the same in both. So in the analogous silver compounds Loew [Ai-ch. f. Phys., Bd. xxxi., s. 393) could vary the DIGESTIVE PROCESSES IN THE BODY. 177 amount of silver present in his albuminate of silver by altering the strength of his solutions. If strong solutions of silver nitrate, copper sulphate, gold chloride, mercuric chloride, and other metallic salts, be added in excess to solutions of proteids a precipitate forms, which is as a rule flocculent and lighter in the case of the native albumins, finer and heavier in the lower derivatives. The colour of the lower compounds is usually more pronounced than that of the higher. The proportions between the amounts of the different proteids and of hydrochloric acid in their combinations are of great interest. The writer has performed a number of experiments intended to determine the relation- ship between this acid and various proteid bodies, with fairly constant results. The variations, however, are sufficiently large to suggest that our present methods of separating proteids from one another are very incomplete, and that in all probability the broad distinctions of the simpler proteids serve to cover a much more numerous series of these bodies, distinct from one another yet differing only in minute details. The usual method by which proto-albumose is separated from deutero- albumose by saturating the solution containing them with chloride of sodium, if carried out in stages yields at least three if not four precipitates answering to the general tests for proto-albumose, but giving slightly different reactions. Thus, if a solution containing both of these albu- moses be evaporated after the addition of chloride of sodium, before absolute saturation is reached a precipitate appears in masses, floating on the surface of the water, or adhering to the sides of the vessel. After filtering this off and evaporating further until crystals of the chloride begin to form, a further portion of the proteid can be removed. Continued heating of the second filtrate to facilitate the removal of the excess of salt yields a further crop, and sometimes a fourth crop may be obtained before all the primary albumoses have been isolated. From each of these pre- cipitates some hetero-albumose can be separated by the action of dialysis. The four substances were found, after purifying, to present certain points of difference; the series gradually became more akin to the secondary group. Thus the precipitate formed on the addition of chloride of platinum to a watery solution of the first was light yellow in colour, insoluble in cold water and hardly soluble in boiling water; of the second, was darker in colour, and only slightly soluble in hot water; of the third, large and flocculent masses, yellow in colour, and soluble in cold water but more readily in hot water; while the precipitate occasionally obtained after this formed darker and finer flocculi which were very soluble in water, in alcohol up to 70 per cent, but insoluble in ether. The first two precipitates appeared to be able to combine with from 9-10 per cent, of hydrochloric acid, the third with 13 per cent., and the last with about 14 per cent. Pekelharing has described two precipitates obtainable, after removal of the primary and secondary albumoses by the saturation of their solutions with ammonium sulphate, first slightly acidifying and making sure that the fluid has been thoroughly saturated with the salt at the boiling point. If, after the precipitated albumoses are filtered off, the filtrate be further concen- trated so that when at the boiling point a portion of the ammonium sulphate remains crystalline, be then rendered alkaline with ammonia and allowed to cool, a deposit of small particles of proteid gradually forms. In some instances, after filtering this and treating the filtrate in a similar manner, but with the addition of acetic acid, a further precipitate can be obtained. Any I? 178 THE NATURAL HISTORY OF DIGESTION. proteid substance remaining after this may be regarded as of the nature of pure peptone, but it is not improbable that in time further members of the long series dividing true peptone from native proteid will be discovered. The alterations which occur in the highly complicated albumin molecule before it becomes, it may be only in part, the very much simpler though still complex peptone molecule through the action of dilute acid or alkali, and ferments, possibly consist of a continued and lengthy series of small changes similar to but much more numerous than those which intervene between a molecule of starch and a mole- cule of maltose, changes which together result in an alteration in chemical relations, and remain at present beyond the power of the chemist to appreciate. All deter- minations of the chemical composition and reactions of pro- teid bodies have hitherto yielded very varying results, not only because of the difficulty experienced in freeing them of salts, as has been already mentioned, but also because, except in the case of the artificial crystalline globulins, there is no doubt but that few analyses have been performed on any class of proteid absolutely free from proteids of another group, or belonging to contiguous members, in the proteid series. So many analyses, however, have been carried out on the different and larger groups that their mean may be taken as conveying a fair estimate of the average composition of each group, and probably may be regarded as the actual composition of the individual proteid body which occupies the position midway between the lowest member of the group immediately above, and the highest member of that below the group in question. If, therefore, proto-albumose really includes four or more different bodies, all of which differ from the deutero-albumoses, for instance, in that they are insoluble in hot saturated solu- tions of sodium chloride, and are precipitated by trichloracetic acid, the values obtained for proto-albumose as an entity will represent the average values for its individual members. The average percentage of hydrochloric acid which the writer has found to combine with proto-albumose is from ir to 12; the figures obtained from the four separate precipitates yield a mean of 11.5 per cent.; while the highest member gives a figure close to that of acid-albumin, the lowest one is only slightly removed from that obtained from deutero-albumose. Blitm has laid it down that the percentage of hydrochloric acid which DIGESTIVE PROCESSES IN THE BODY. 179 can combine with the series of proteid bodies formed during digestion increases as the proteids decrease in size and complexity. Gurber, on the otlierhand, by estimating the.nitrogen of such compounds by Kjeldahl's process, and the hydrochloric acid with Gunzberg's reaction, found that acid-albumin gave i molecule of hydrochloric acid to 5 atoms of nitrogen, or if nitrogen be taken as 15 per cent, of the albumin, the acid formed about 7 per cent, of the compound by weight ; that, similarly, proto-albumose combined with only 4 per cent, of the acid ; while deutero- albumose united with about 10 per cent. , and peptone with 8 per cent, of the acid. Nitrogen estimations, however, of bodies obtained through the use of ammonium salts are seldom trustworthy, owing to the great difficulty attendant on the complete removal of ammonia, and the consequent fallacy introduced into the nitrogen values. Cohnheim has also determined the proportion of hydrochloric acid which combines with the various proteids. He only used albumoses and peptone in his experiments. He found — Pioto-albumose combined with 4.3 per cent, hydrochloric acid. Deutero-albumose ,, 5.5 ,, ,, Hetero-albumose ,, 8.2 ,, ,, Anti-peptone ,, 16.0 ,, ,, The proportion between the figures for proto-albumose and those for deutero-albumose, though they are actually smaller, is the same as between those obtained by the author, vifhile the percentage of acid combining with peptone is in both series much above the percentage combining with the other proteids. The author, by drying solutions containing known quantities of different proteids after the addition of hydrochloric acid, and by checking the results obtained by the use of Gunzberg's reagent for determining the exact moment at which, with the addition of a deci-normal solution of the acid drop by drop to similar solutions, free hydrochloric acid could be detected, obtained the following results : — Table XXXI. — Percentage of Hydrochloric Acid in its Com- pounds with Proteids. (Expressed in terms of HCl per cent, of total.) Proteid. HCl per cent. Egg-albumin o — 8 Serum-albumin o — 9 Serum -globulin o — 10 Acid-albumin 8 — 9 Hetero-albumose 8.84 Dys-albumose - 12. i Proto-albumose 11 — 12 Deutero-albumose 13 — 14 Peptone 19 —20 An albumin molecule does not appear to lose its characteristic properties until 8 or 9 per cent, of hydrochloric acid has com- I So THE NATURAL HISTORY OF DIGESTION. bined with it, when it affords the reactions of an acid-albumin. Hetero-albumose corresponds very closely to acid-albumin in its reactions, and combines with the same amount of acid. The other proteid derivatives combine with more of the acid as they become simpler in character.^ The size of the various proteid molecules may be suggested by the percentage of the metal present in these compounds. In considering this question it must be remembered that the metals are combined in the form of their salts. Thus when silver nitrate is used to precipitate proteids the resulting com- pound is formed by the proteid and the actual silver salt. The percentage of metallic silver recovered by incineration refers therefore to the weight of the total compound, not of the proteid and silver alone. The hydrochloric acid com- pound is similarly a hydrochloride, not a chloride ; while the chlorides of gold and of platinum, and the sulphate of copper form integral parts of the precipitates. The doubtful point is the behaviour of the molecules of water. Probably mole- cules of native proteids coagulated by these reagents lose all their water, while it is also probable that the compounds of the lower proteids with metallic salts retain part of their attached water molecules, as they are much more soluble in warm, and some even in cold water. The hydrochloric acid molecules may be regarded as ousting a similar number of water molecules attached to the proteid without disturbing the remainder of the water molecules, unless it be present in strong solution sufficient either to coagulate albumin or split it and other proteids into non-proteid derivatives. Drec\i%e\ (Journ. f. frakt. Chem., N. F. Bd. 19, a. 331) analysed a crystalline compound of a vegetable globulin with magnesium oxide, and found 1.43 per cent, of the metaUic oxide in it when dried at 110° Cent. This gives the molecular weight of the proteid present as 2757, A similar sodium compound gave a weight of 1496. Harnack's copper albumin, with 1.35 per cent, of copper, affords an albumin-molecule weighing 4576, or a little more than three times that of the sodium preparation; his second copper compound with 2.64 per cent, of copper yields a proteid molecule of 2266, or just one-half of the first. Similarly, Loew's (PJliiger's Archiv., Bd. 31, s. 393) silver compounds with 2.3 per cent, and 4 3 per cent, of silver, correspond to 4587 and 2403 respectively. ^ Paal {Bericht. d. d. Chem. Ges., xxvii., s. 1827) has obtained compounds of albumoses and peptones with hydrochloric acid containing from 2.2 to 19.88 per cent, of the acid, with molecular weights ranging inversely to the proportion of acid, from 252 to 2120. l8l CHAPTER VIII. Digestive Processes {continued). The passage of the food from the Stomach into the Duodenum— Intestinal Digestion — The Pancreatic Secretion — Historical — Characters — Ferments — Action on Proteids— The Liver — Bile — Other actions of the Liver — Digestion in the Small Intestine — Reaction of the contents — Absorption of fluids— Digestion in the Large Intestine. The Passage of the Chyme from the Stomach into the Duodenum. — Up to recent years it was believed that, at the end of gastric digestion, the contents of the stomach were rapidly discharged almost eri masse into the bowel. From the observations, however, of Cahn and von Mering on the dog by means of duodenal fistula, it is clear that some of the stomach contents are propelled into the bowel shortly after food has been taken. For instance, these observers found that after the animal drank plain water fluid began to escape from the stomach in a few minutes. When the dog was given solu- tions of various substances in water, such as alcohol, peptones, and albumin, as much fluid, and often more, was discharged through the pylorus as had been taken ; while the peptones and the alcohol were found to be diminished in amount. That is to say, that while it has little or no power of absorbing water, the stomach is able to absorb a considerable quantity of such substances as peptone and alcohol. Unfortunately, they did not investigate the conditions under which the stomach contents are discharged after a meal of mixed fluids and solids. Schiile (Zeit. f, Klin. Med., xxviii. , p. 87) found in a dog with a similar duodenal fistula that the temperature of the food given had a great influence on the time when the stomach emptied. Thus 300 c.c. of water at 18° C. were passed through the pylorus in 10 minutes, a physiological salt solution at the same temperature in 2i minutes. The same amount of water at 28° C. and at 40° C. passed through in less than 10 minutes, while at 0° C. 15 minutes elapsed before the first portion appeared. If milk were given instead of water the stomach emptied almost as quickly, while after solids were given the pyloric discharge was delayed, and after both fluids and solids, the fluids passed through first. The greater the amount of fluid Missing Page DIGESTIVE PROCESSES IN THE BOtlY. 1S3 it is viscid, gelatinises on cooling, and coagulates easily; as digestion proceeds it becomes less viscid, more dilute, and sparingly coagulable. The changes in the cells of the gland before and during active secretion are described in connection with its proteolytic ferment, trypsin, and the zymogen, trypsin- ogen. The pancreatic juice, unlike the saliva, is secreted at a low pressure — in fact, its pressure only reaches about tV of that of the saliva, or below 20 mm. of mercury. Quantity. — Owing to the alterations in the flow of the juice shortly after the establishment of a pancreatic fistula, as men- tioned above, the estimation of the normal quantity is beset with difficulties. Observations, however, upon animals serve to show that there is no ratio between the size of the gland and the amount of secretion. The following table shows the quantity per kilogramme of body-weight secreted in the twenty- four hours, as determined by Colin : — Table XXXII. — Rate and Proportional Secretion of Pancreatic Juice. Man...... Dog Horse . , Cow Sheep ... Hog Weight of Pancreas. G ramnies. 300 300 50-60 140-180 Secretion per Kilo, in 24 hours. Grammes. 3.01-5.0 2.4 16.8 14.4 12.0 7-2 Maximum Secretion per hour. Grammes. 9.1-20.8 265 200-270 8 15 Table XXXIII. — Composition of Pancreatic Juice. Dog. (Schmidt.) Horse. (Hoppe- Seyler.) 1. Iimnediately after Operation. 2. Permanent Fistula. Water 900. 76 99.24 90 44 8.80 0.58 7-37 7-73 0.12 980.45 I9S5 12.71 6.84 3-31 3-43 09 O.OI 982 50 17-50 8 88 8.62 Total Solids Organic Solids ... Ash Na (Sodium) Mg (Magnesium) — 1 84 THE NATURAL HISTORY Ot" DIGESTION. Pancreatic juice is alkaline in reaction, equal to about 0.3 per cent. NaHO, with the high specific gravity, when ob- tained from a recent fistula, of 1030, but falling to loio in course of time (see Table XXXIII.). It coagulates on heating into a firm white mass, from which an alkahne opalescent fluid separates out containing alkali-albumin. When dropped into water a precipitate forms which is soluble in solutions of sodium chloride or dilute acids; a similar precipitate is formed if it is dropped into dilute acid solutions, but clears up on agitation of the fluid. An abun- dant white precipitate follows the addition of alcohol, which is largely soluble in water, and not reprecipitated by acetic acid until its action has continued for some time. The alcoholic precipitate contains, or carries down with it, the different pan- creatic ferments. Pancreatic juice readily undergoes putrefac- tion, yielding the red reaction with chlorine water indicative of tryptophan during the earlier stages of this process, and, after this reaction has disappeared, the red colour formed by indol when impure nitric acid is added. Pancreatic Ferments. — Three ferments are undoubtedly pre- sent in pancreatic juice. 1 . Trypsin ; proteolytic in neutral, faintly alkaline or acid (where the acidity is not due to mineral acids) media, and able to decompose proteids into amido- acids as well as into peptones. 2. Amylopsin; a diastatic ferment, acting similarly to ptyalin. 3. A fat-decomposing ferment, steapsin, forming gly- cerine and fatty acids by hydrolysis from neutral fats. A fourth is probably present in the form of a milk-curdling ferment, and still another is hypothetically assumed to be contained in pancreatic secretion and to act not upon the actual intestinal processes so much as upon the activity of glycogenesis or glycolysis in the liver and body generally. A fat-emulsifying ferment was formerly thought to be present by Claude Bernard, but this view has been shown to be erroneous. The emulsification of fats by the pancreatic juice is probably brought about by the carbonate of sodium and the proteids contained in it. Digestion of Proteids by Trypsin. — Trypsin acts energetically on albumins and globul'"= Jn ^ t npr rpnf inlntion of sodium DIGESTIVE PROCESSES IN THE BODY. l8S carbonate and at a temperature of 40° C. For the accurate observation of the changes brought about by the ferment, the mixture must be rendered aseptic by the addition of a small quantity of salicylic acid or thymol. Fibrin digested by trypsin does not swell up as in peptic proteolysis, but its edges are gradually eroded away, until there remains nothing save a powdery residue. The first products of tryptic proteolysis are identical with those resulting from the action of pepsin and acids ; hemi-albumose and anti- albumose are formed, to be changed in turn into henii- and anti-peptone. Further action transforms the hemi-peptone into such amido-acids as leucin, tyrosin, and glutamic acid. Kiihne found that of 382 grammes of dried fibrin, with 55 grammes of pancreas (15.2 grammes when dried), 343.7 grammes was dissolved, yielding 2 11. 2 grammes of albumoses and peptones, 13.3 grammes of tyrosin, and 31.6 grammes of leucin. (Table XXXIV.) Table XXXIV. — The Digestion of Fibrin by Trypsin. ied fibrin and gland tissue 397.2 grammes. Dissolved 343-7 Unaffected 53-5 Peptone and albumoses 2 11. 2 grammes = 53. 1 per cent. Tyrosin 13.3 ,, = 3.3 „ Leucin 31.6 ,, = 7.9 ,, Other soluble pi oducls - 87.6 ,, =22.0 ,, Unaltered - - 53.5 „ = 13.7 Tryptic proteolysis, in the absence of bacteria, is unaccompanied by the evolution of gases. Hiifner found, indeed, that oxygen was absorbed, while carbonic acid gas was developed, but in very minute quantity. In Kiihne's experiment the albumoses and anti-peptones were not individually separated from each other. In another observation this observer, along with Chittenden, obtained 120 grammes of pure anti-peptone from 300 grammes of fibrin, or 40 per cent. The amido-acids resulting from the action of trypsin on proteids in the absence of bacteria are bodies in which an atom of hydrogen contained in the alcohol-radical of various acids is replaced by amidogen. (Table XXXV. ) Thus acetic acid, CH3COOH, becomes CHjiNH^jCOOH, or glycocoll; and caproic acid, C5H11COOH, becomes C5Hi(|(NH2)COOH, or leucin. THE NATURAL HISTORY OF DIGESTION. ^ to '^- 'm "« I XI w m X X X w H a o OS CO CO CO (M .£3 CO o "« (N us "* O O 00 O CO d >> CJ h 1) O CD O O 00 ' O in W to CO 01-<«i-(CO Oi-*CNiffl o OJCOOO -i-COfNC*] oiwor-' O rHrH rHi>J001r~ ^— . — "— .— ' "—Y— '■■—.— ' ^— .— - '— T— ' ig ■^ Ol Oi , r 1 > 1 W t— CO ^ O ^n rH 00 wa ^ d "^^ <= ;* ^ Oi M CO ■"• r ( 1 1 O fcb O r-l CO O C-I 00 f^ d rH* 00 .-^ ' ' ' ' W 00 .H 00 ^— "-^^ — ■— ^ O O CO to -* M CO O 1 N 1 1 1 1 IQ "* O d 00 1 M 1 1 i 1 oj d i cooo'i'ior^o ooiooo cd 3 miHI^P0(MCO 1 OOOCDCOQO | I OiHOCOrHTt. I-S-WOOO I 1 <£ <6 <6 <6 ci (^zidoci i- B -*'Ma3i-liOOS CBlO-^h-Tjl •^"^ inoi-rx-jiicjio .OJ^ioOifN , 1^ Tp^DTjiCOrHOS r-IOOOO 1 dcs, , ,-* in tN O O OrH ococoin t- N ddddddddd i-i § 'it _ COMiracOOI^M •-*iiOC0E~MC0O| 1 I 1 1 1 t r-idddr-ldm I ' 1 ■ 1 1 22i^^ SF° '0,1-1,0 o c-c CO (N Tti 1 Oi t-; . ^ i 1 \ r ', (' ^ // 1 \ ^f ' ~1 I I N •^ ' n (UNFILTEREDl) (FILTERED.) „,, r*- (UNFILTERED.) ^*'^^~ ^ AVERAGE PROPORTION OF SOLIDS AND ASH IN MILK. A To show the Total Solids per cent, in the Contents of the different divisions of the Alimentary Canal in the Dog and Calf fed on Milk alone. R To show the Inorganic Salts in the same. * The Large Intestine in the Dog had been emptied shortly before Death. 200 THE NATURAL HISTORY OF DIGESTION. constant, but to be less during the night than in the day-time. Macfadyen, Nencki, and Sieber found that the time elapsing between the ingestion of food and its appearance at the iko- ccEcal valve varied greatly in the same person. Thus in the case of a patient with a fistulous opening at the junction of the ileum and the colon, from 2\ hours- up to sj hours elapsed between the swallowing of green peas and their first appearance at the opening, while from 14 to as many as 23 hours passed before the last of the peas were discharged on the different occasions on which the experiment was performed. The contents as they enter the colon are generally of an acid reaction, and contain albumin, mucin, a small proportion of the derivatives of proteid digestion, and of the conversion of starch, along with lactic and volatile fatty acids, bile acids, and biUrubin. Macfadyen, Nencki, and Sieber found in a man on a mixed diet that the total solids of the contents passing through the lower end of the ileum amounted only to 5 percent.; when food yielding a more solid residue was given, the solids rose to about 10 per cent. On one occasion 26.95 grammes, and on another 26.05 grammes of solid matter were passed in 24 hours. In a dog fed on sterilised milk the writer obtained much higher figures from the estimation of the solids in the lower part of the ileum — viz., 25 per cent., and in a sucking calf as much as 32 per cent. The constituents of the solid matter entering the large intestine comprise about 30 to 42 per cent, of proteid bodies, 8.5 per cent, of fats, 45 per cent, of carbo-hydrates and other alcoholic extractives, and about 8.5 per cent, of inorganic salts. These proportions are subject to very considerable variations. In health neither indol, leu- cin, nor tyrosin are present. (Chart VIII.) In the small intestine the action of micro-organisms on the contents is confined almost exclusively to the carbo-hydrate moiety present. In the large intestine putrefaction of albu- minous substances may occur. (Cf p. 219.) The secretion of the glands of the large intestine is strongly alkaline, even more so than the succus entericus of the small bowel. The contents, however, remain acid in reaction owing to the products of bacterial activity. The continued growth of bacteria is possible, notwithstanding the presence of acids formed by them, by reason of the constant addition of alkali by the glands of the mucous membrane. But for this DIGESTIVE PROCESSES IN THE BODY. 201 the acidity would soon rise above a point inimical to bacterial life. The secretion of the large intestine contains neither a proteolytic nor a diastatic ferment. Any digestive change which can occur in its contents must proceed from the presence of ferments added above, or, and this more especially, from the action of micro-organisms. The mucous membrane, however, is capable of absorbing soluble albumins, peptones and albumoses, sugars and water. This property is made use of in the rectal alimentation of patients. Uncoagulated albumins in solution do not appear to be absorbed except in the presence of a neutral salt. Although the secretion of the large intestine is devoid of ferments, and in the carnivora and man the processes carried on in that part of the bowel are chiefly absorptive, in the herbivora, other than ruminants, the large intestine, especially the ccEcum, is of great size, and acts as a receptacle for partially digested food, where it undergoes the same fermentative change as in the rumen of the ruminaniia, where cellulose is digested through the agency of bacteria. In the animal kingdom, the development of the large intes- tine, and the importance of the processes carried on in it (and this applies more particularly to the ceecum), vary inversely to the size of the stomach or the completeness of gastric digestion. The stomach of the horse is small, and only partially able to cope with the material given it; of the cow, large, and capable of very thorough digestion of the food; and of the dog, of moderate size, but endowed with active properties and aided by vigorous digestive processes in the small intestine : so in the horse the large intestine is enormously developed, in the cow much less so, while in the dog it is small and short. The Faces. — The characters and composition of the faeces vary with the amount and nature of the food taken, and especially with the proportion of indigestible matter consumed. In carnivora the small residue left after the digestion of meat causes the faeces to be scanty and firm. For the same reason a milk diet leads to dry and small evacuations. In herbivora the ingestion of large amounts of food with a low nutritive value, and containing much indigestible matter, accounts for the copious evacuations of these animals. In them, too, the amount passed is proportionately greater when compared with the food taken, owing to an increased percentage of water dis- 302 THE NATURAL HISTORY OF DIGESTION. ^ -^ 13 .^^ X w u S ■c I- 0^ 1 I S 1 1 M S H . tH (N -J [ -■ S OS 2: t~^ 1 br _ "~~^ 1 ' '^ 1 1 . ^D mS £ in l-QO W iH to -* SI It:; [ 1 ^'j in 00 oci ct-r -aJ i-^j>:co d gq-^-" 0, ^3 3 II . C5 -■3 " s •la |a a " S^in(N u:i CO 1 ^-6 yj D -H lo o m 5:J£ "i-. EO ^-^ =;«■;';: 7, 1 - Tf c; i S^ii'' " Ih "* I^ I^ ' =^-= ^ d =■ .ri k and Ch . 218 grm Cheese. 2050 grm. Milk. ^X 33 r- t- . c'_j ^ 5 L- b- :o w ^•^r CO » d 1-: gp ^ ^ ~ a ^ d -d ^ ^M ^0 t~r- r-l X' rt . aicri^ig^ M C* _; ^ J C1 t~ ^ ^ Ss 0-* :o .a 1 r-^oJ-+^ 1 M of 3075 grm. jjfM b- (M ^ r-oi>:iriQ6 a- r-< ^ tf 1^ ■— ^ '^ <^ , (M Mill 133 f-m. ^OOWMCO 1 '5 ".iu,- Id Cl r rH CO (M tfl = i>I 0" CO ad 1 9 a ^TKOrHOO^H ?-^ = CO i> 1-: to ici •*- 03 ti-i id o-OCOS-TM 1 r-((N 1 .^ Ss ^I-IO tHO I it' M °S5 ■*(>] r-iiO 1 § -S m 3 'C tJ .5 q rf : : :« S b nH ^ ess-: -a I ^ .2i3-j3a> .SS-Sxl ftgl^^O &Z,6< DIGESTIVE PROCESSES IN THE BODY. 203 charged along with it. The horse empties his bowel once in about every three hours, and on an ordinary diet will pass in the day about 17 kilogrannmes of feces containing 2.6 kilo- grammes of solids. Thirty kilogrammes of faeces with 4.6 kilogrammes of solid matter forms a fair average in cows, while dogs on a purely meat diet may only discharge a few grammes every second day. In man one-seventh to one- eighth of the solid food taken in is evacuated from the bowel, or about 150 grammes per diem. The more rapid the course of the food through the bowel, the greater is the proportion discharged. (Table XL.) A comparison between the average composition of the con- tents of the ileum as they pass into the colon, and that of the fjeces in man, will serve to illustrate the changes which take place during their progress through the large bowel. Table XLI. — Composition of tiie Conlenls entering and leavins the Large Intestine. Contents at ileo- 1 csecal valve . . . / Fteces ... Water. 90-95% .8.7-82.6%. Contents at ileo- cfecal valve ... Fteces ... Pi'oteids and Mucin. - 30-42% ■ o - ;o% . Total Solids. Organic Solids. Salts. •■ 5-10% ..,4.58-9. 15%. ..0.42-0.85% ■•l7-4-3l-7%--- 10. 7-25.0%... 6.77, Solids. Fats. Carbo-hydrates and Extractives. Sa'ts. 8-5% - 45% .- 8.5% 11.5-15%.,. 15.6-22%, .24.2-38. 5% Defacation. — Defaecation is partly a voluntary and partly a reflex act. The contents of the intestine are constantly pro- pelled in a direction from the mouth towards the anus by the peristaltic or worm-like movements of the walls. At the lower extremity of the gut their progress is arrested by the resistance offered by the anal sphincter or muscular ring, which normally remains contracted and closed. The sphincter is fonned of two layers, an internal composed of involuntary fibres, and an external of voluntary muscle fibres. A nerve-centre in the lower or lumbar part of the spinal cord, by a constant passage of nerve impulses, maintains the contraction of these muscles. When fasces enter the rectum, their presence causes a desire 204 THE NATURAL HISTORY OF DIGESTION. to defecate, and leads to the arrest of the nerve impulses to the sphincter muscles, both reflexly from the rectum and voluntarily from the brain. When the muscular rings are relaxed the peristaltic movements of the rectal walls alone are sufficient to evacuate the contents, but they are usually aided by voluntary contractions of the muscles in the abdominal wall. 20S CHAPTER IX. Absorption from the Alimentary Canal. Ileidenhain's laws — Proteids — Sugars — Salts — Fat — Osmosis — Vital action — Change in form of absorbed proteids— Course of absorbed material — Lymph duct — Chyle. The Process of Absorption in the Alimentary Canal. — At one time it was generally supposed that absorption of the products of digestion, and of the fluids taken with the food, was simply the outcome of a simple act of diffusion, as first described by Graham. The researches of Cahn and von Mering, and of others, upon absorption in the stomach show clearly that the laws of osmosis cannot apply to the process of absorption of digestive products in that organ. Little or no water is absorbed by the mucous membrane of the stomach, while sugar, salts, and peptones can be taken up. The rate of absorption from the stomach has been found to be increased when such substances are given dissolved in alcohol, while alcohol itself rapidly passes through the gastric mucous mem- brane into the blood-stream. These observations are sufficient to negative the supposition that simple diffusion plays the greater part in the absorption of fluids, or of the bodies which they contain in solution. Heidenhain {Pfliiger's Archiv., Ivi. s. 579) placed the laws of absorption in the alimentary tract on a securer basis. The chief laws of diffusion are as follows : — 1. Watery fluids of equal osmotic value on either side of a dialysing membrane retain the same volume. 2. Fluids of unequal osmotic value alter in volume; some of the water of the fluid with the lesser passing through the membrane to the side of the fluid with the greater value until the value becomes equalised. 206 THE NATURAL HISTORY OF DIGESTION. 3. The osmotic value of each solution is equal to the sum of its single constituents. 4. If the total osmotic value in a mixed solution be equal, but that of the single constituents different, and portions be placed on either side of a dialysing mem- brane, the individual constituents become equalised, although the volume remains the same. If absorption in the alimentary canal were the result of simple diffusion, it would necessarily follow these conditions. In such a case, fluids corresponding in osmotic value to blood, or with a value which is smaller, could not be absorbed. Heidenhain found that blood-seium placed in the intestine of a dog was abs.iibed by the mucous membrane, although by the addition of intestinal secretion its osmotic equivalent had been rendered less than that of the blood circulating in the walls. Again, dog's blood-serum placed in the in- testine lost water and salts in a ratio corresponding to their original propor- ti'>ns, while the organic substances diminished in much smaller proportion. Using solutions of common salt, he found that whether the osmotic equi- valent was above or below that of the blood, interchange took place. When, therefore, the osmotic equivalent of a salt solution is higher than that of the blood and absorption occurs, the cells lining the mucous mem- brane must have some action in the process. On the other hand, as water and the salt dissolved in it are absorbed in proportion to the amounts present, osmotic force must also have some influence. As the proportion of salt increases in the contents of the bowel, the absorption of fluid becomes larger, and as the osmotic force of the fluid is increased by the greater per- centage of salts contained in it, the salts are also absorbed in greater quantity. If the laws of simple diffusion were followed, water would flow from the blood into the contents; but this does not occur, owing to an inhibitory power, placed by Heid- enhain in the epithelial cells lining the mucous membrane. The greater the proportion of salts in the contents the greater is the opposition exercised by the cells to the flow of water from the blood. In certain concentrations the absorption of water ceases, while srtlts are still removed. With 2 per cent, of common salt the absorption of salt stops, while with 0.3 to o. 5 per cent, of salt the process proceeds both by the absorption due to osmotic force and by physiological action. If fluoride of sodium in small quantities be added to a chloride of sodiurrr solution introduced into the bowel, the physiological path of absorption is blocked, while the water absorption from the salt ABSORPTION FROM THE ALIMENTARY CANAL. 20/ solution \z increased. Thus when the strength of the fluoride of sodium solution be above 0.04 to 0.05 per cent., the epithelium of the gut is altered, water absorption is increased, and removal of salt lessened. Other circumstances point to the fact that absorption from the alimentary tract cannot solely be due to simple diffusion. For instance, cane-sugar, which is highly diffusible through dead animal membranes, is scarcely taken up at all from the bowel. Grape-sugat, which is not nearly so difTusible as sodium sulphate, is absorbed from a loop of intestine much more rapidly than that salt. Albumin, again, which does not pass through dead animal membranes, is, to a certain extent, absorbed by the mucous membrane of the living bowel without change. Thus, Friedliinder [Zeitschriflfiir Biologic, xxxiii. 2) found that both egg and serum albumin were absorbed up to a pro- portion of 20 per cent, in an isolated loop of the small intestine, while 60 per cent, of alkali-albumin, 90 per cent, of commercial peptone, and 70 per cent, of albumoses were similarly absorbed. The albumin seemed to be absorbed as such, not in the form of any of its derivatives. Neither casein, acid-albumin, nor myosin were absorbed to any appreciable extent, although the acid used to dissolve them was rapidly removed. The absorption of fats is carried out by the epithelial cells of the villi in the upper part of the small intestine. They are not absorbed in solution but in a state of fine division, and, to some extent, in the form of soaps. If an animal be killed during digestion of a fatty meal the epithelial cells in the villi may be seen to contain globules of different sizes which stain black with osmotic acid, are dissolved by ether, and which are there- fore fatty. We do not know how the cells take up the fat particles from the bowel contents, but they certainly pass through and not between the cells. The particles have, in- deed, been described as pushing their way between the cells and then being taken up by wandering white blood corpus- cles or leucocytes. But, in the first place, the fatty granules discovered between the cells had, in all probability, been forced between them by shrmkage of the tissue during its histological preparation. In the second, the granules looked upon as fat in white blood corpuscles or leucocytes are not soluble in ether, as true fat is. A dog normally absorbs 9 to 21 per cent, of the fat swallowed with food in three to four hours, from 31 to 46 per cent, in seven hours, and 86 per cent, in eighteen hours (Harley). After extirpation of the pancreas the small intestine has been found four to seven hours after a meal to contain more fat than was taken in with the food. This seemina; 208 THE NATURAL HISTORY OF DIGESTION. impossibility is explained by the fact that excision of the pancreas partly prevents the absorption of the fats present, while the mucous membrane of the intestine normally ex- cretes fats into the bowel. The extirpation of the pancreas does not entirely prevent the absorption of fatty particles, because the lacteal vessels running from the bowel are filled with a milky chyle, but the amount absorbed is less than the amount re-excreted into the bowel. Proteids are absorbed for the greater part after their alteration into simpler derivatives, although there is no doubt that native albumins and globulins may be absorbed as such. The physiological factor has much to do with the absorption of proteid bodies, more than with the absorption of salts or of water. The epithelial cells lining the villi are the chief agents in the process, and they probably have a second function in addi- tion to that of absorption. That is to say, the proteid derivatives absorbed from the contents of the bowel by the one side of the cells, reach the blood-stream on the other side of the cells in the form of native albumins and globulins again. ^ How this change is brought about is unknown. There may be some connection between the amount of carbonic acid in blood and the alteration of the proteids absorbed by the stomach and bowel. Lahoussen found that if he injected 0.3 gramme of peptone per kilogramme of the body-weight into the blood-stream of a dog, the amount of carbonic acid in the blood sank to a half ; o. 5 gramme per kilo, caused a decrease of one quarter. The loss in the amount of carbonic acid lasted from one to two hours. There was no diminution in the temperature of the animal. The peptones and albumoses absorbed by the mucous membrane of the alimentary tract may in this way cause a diminution in the amount of car- bonic acid in the blood, or blood-serum, circulating through it. May it not be possible that the anabolic metamorphosis of peptone and albumoses resulting in the re- formation of albumins and globulins is due to gaseous substances present in the blood, interchanging with gaseous elements in the lumen of the tract ? The constant presence of carbonic acid gas in tlie stomach (Schierbeck, Skandin. Arch, f. Physio!., iSgr, Bd. iii. s. 437) is of importance with regard to this question. In the empty stomach this gas is present at a pressure of from 30 to 40 mm. of mercury. During the height of digestion the pressure rises to 130 or 140 mm. If the stomach be thoroughly washed ^ Waymouth Reid introduced in(o an isolated loop of the intestine of goose 50 c. cm. of a i per cent, solution of albumoses and peptones free from albumins and e.\tractives, made from Grtibler's commercial peo'ton by precipitation with trichloracetic acid. 48 c. cm. were recovered iff three minutes, containing 98.41 per cent, of the proteids introduced ABSORPTION FROM THE ALIMENTARY CANAL. 209 out and all the carbonic acid removed, in a short time it is again secreted, until the pressure once more equals 30 to 40 mm. Part of the gas may be the result of an interchange between the oxygen of the air swallowed and the carbonic acid in the blood. As, however, carbonic acid gas passes into the stomach after the organ has been emptied, reaching to svhat we may assume to be its normal pressure, while this pressure increases enormously during active digestion, simple interchange with the oxygen of any air which has been swallowed is insufficient to account for its presence in such quantities. Some function, as yet undiscovered, probably pertains to the changes of pressure of carbonic acid in the blood and within the alimentary tract during digestion, and may have to do with the proteid metamorphosis in the intestinal walls. The Course of the Absorbed Materials. — ^The water, salts, and sugar absorbed through the alimentary canal pass nor- mally into the smaller branches of the portal veins, and by them are conveyed to the liver. There is no increase in the quantity of those substances in the chyle during digestion, while sugar increases in quantity in the portal blood. In man about i per cent, of the sugar, corresponding to i per cent, of the carbo-hydrates in the food, may be discovered in the contents of the lymphatic duct. (See Table XLII.) Chyle. — Fats chiefly enter the small terminal branches of the lymph vessels which pass into each villus of the intestinal mucous membrane and are thence carried to the chief lymphatic vessel in the body, the thoracic duct. The junction of the numberless small lacteal vessels into intermediate larger ducts, and finally of these into the thoracic duct, provides a channel by which fats and any of the other products of digestion which have not entered the blood-stream of the portal system can be added to the circulating blood at a spot where their transit through the liver before arrival at the heart is avoided. Veins of the neck fulfil this condition, and it is into them that the duct empties. When a ineal rich in fat has been given to an animal shortly before death, on opening the abdomen numbers of bright glistening lines can be seen running from the upper part of the small intestine across the mesentery which attaches the bowel to the posterior wall of the abdomen. These are the lymph vessels filled with chyle, and owing their white colour to the presence of fats in a state of very minute sub-division within their walls. The fats of the food which have been saponified by the action of the alkalies in the intestinal secretions are absorbed by the veins and carried to the liver, but at least two-thirds of the fat given enters the 14 2IO THE NATURAL HISTORY OF DIGESTION. C~5 r-i -y) ■«*r-I^'» " C Z:; M '^ ^ ift -O ci 00 K ;i Oj " W5 -^ ^ b ■S M ffi t- CO fH 1(5 (D -^Jt M in ^ , t- (N r-( O CO CO Oi a 5^ . -, « ■ d* • • ■«^ c« •VJ ■^ H-jS SS |SS!g3 •if o HI O T-J l~ lO -J -Ji CI _ r-; «3 Ti CO Z) ^ -i- rsi OtSCOr-ici— ^jit-C H . ,H 11 -* CN ~ ^ 1* o t-l ^ i^Mt-Oini-^Oi >4 1 O o;o««»aoco'0 •§ ^ °''§g^?j°'^'^ S c B "^ S e 5 ;o « i^ in s; ton^ ^ m d S 1-1 X ° ? s fflOO-* 0=) 1- 8 C -^ -+■ O '^^ CO «D r J c3i d ifi -# ci ? O ^ CO CO (M =0 CO ^-7 -* C ,■ -)J 1^ O (N Ci ■*" C 5t >* C; i-< irt t- 00 t-; c J c; d o ■:•! oi ic' 1 is S^ o iH (?j t- in M iH 00 i-l IN CO O -v>t- CO io i-i m "^ 4> r- IM^ l-H rH O CO ^ d CO C-i r-i r-J -ji a) o oj (?] (>] CO (N O CO b-lO r-l O O O O O (H CD ■* Ci CO iri 6 r-' C-: O J5 i^Mt-lOtNCOCD . E-^ oi d oi 10 ■* (n" •^ ^: : : : : : : : : : cu j/ (DCJiHMiot^oo r-|GOco ^ . , . . ^ 2 Cy ^i'^ ovnirio^o^^^^xin^i-* i^-M t-CO-Mft-C'lCJ^lCin'Mm 5j .g^(MCMCDC;l--CO^:O30'MJO^'-| ci '-'^dcid*;c{a:i>^r-(d-*">]Cii- ■&1 ^-1 < 3 cS O m i^ S i Oil's .c£1 1- ^^ .i^ .•;; ^ ® H 5 ^ y '-^ oi ri rt -; "-^ rt' ^o PHA fa K-r-i^ 96 72 92 69 8.8 6(5 84 63 8.0 60 76 57 7.2 54 6.8 51 6.4 48 6.0 45 5.6 42 5-2 39 48 ,16 4-4 33 4.0 30 3-6 27 32 24 2.8 21 2.4 18 2 15 1.6 12 1.2 9 08 6 04 3 IX.— Showing the Changes in the Composition of Chyle in Ruminants (Wiirtz). OX. COW, Fed on Before After Hay and Straw and Rumination. Rumination. Straw. Clover. ~ ~ "" "~ " — ~ / - /S / / s / / \ , / / \ s / / y / \ ^\ \, / - y / \ s. / A >H / / / \ s, / T( TAl tID ^ 2^ ■^ -.K^ ^ L _ — 'n ^ ^^ — — — — — 7 — V- — — = — 'x -^ ■^ ^ ^ F RO ■Eri 5' ■-^ s •v * V > ^ ^.^ ^ ■\ ^ /■ /- \ ^ y /- \ _F AT L^ / _ :i. — — — — — — — — — — — — — — — — — — — — — — — — — ^ mixture, or from the gas contained in tlie blood. It may be of the nature of a soap, but substances which are easily saponified, such as the ester of phenol-benzoic acid, are not affected by the action. 1 ^ Harriot (Bullet. Acad, de Mid., November 10, 1896) states, how- ever, that a soluble saponifying ferment can be isolated from the blood- serum capable of acting on emulsified fats, apparently from any source, and of doubling the amount of fatty acids and glycerine. The presence of this " liZ-ase," analogous to the i^^fl:/«'» of the pancreatic secretion, explains the mechanism of the intimate nutrition (so Harriot) of the tissues by fatty substances, as its action proceeds until the fatty acids become more and more simple in character — i.e., butyric, acetic, and carbonic acid, until finally the salts formed by them become double compounds, leaving carbonates and giving off carbonic acid. CHAPTER X. The Micro-organisms of the Alimentary Tract OF THE Higher Mammals. Micro-organisms in the IMouth — Stomach — Intestines — Chemical Poisons formed by them— Cadaveric Alkaloids — Ptomaines — Leucomaines. Large numbers of bacteria flourish in the mouths of all animals. It would not be worth while to enumerate here all the different varieties which have been grown from the secre- tions and accretions thus found in the mouth. The organisms chiefly flourish round the bases of the teeth, and form a large part of the tartar so common in that locality. The food, when uncooked, contains in the great majority of cases a large number of micro-organisms. If fresh food be eaten hot, after thorough cooking, very few bacteria are swallowed. Raw food, especially the green food of the Herbivora, permits of large numbers of bacteria finding their way into the alimentary tract. In man also, meat which has been too long kept, and foods which have been insufficiently preserved, contain large numbers of these fungi which are thus introduced into the bod)'. When the organisms swallowed with the food reach the stomach in the Carnivora or Oinnivora, they can grow unchecked or only slightly checked during the first quarter- or half-hour. In the Herbivora, such as the horse, the growth persists throughout a considerable period of the time during which the food stays in the stomach, while in the Ruminantia a large part of the digestion of cellulose proceeds in the rumen through fer- mentative processes due to bacteria ; some say as much as 60 to 70 per cent, is thus digested. In the Carnivora the 214 THE NATURAL HISTORY OF DIGESTION. inhibitory action of tlie hydrochloric acid secreted by the gastric cells begins to act about half-an-hour to three-quarters of an hour after the food is taken, varying, as we have seen, with the quantity of proteid contained in the food, and, there- fore, with the time at which the free hydrochloric acid appears in the contents. In the Herbivora, the action of the hydrochloric acid on the bacteria begins when the food enters the fourth part of the stomach. The acidity of the contents proceeding from hydrochloric acid reaches, as a rule, 0.3 to 0.4 per cent, before the end of digestion. Early observers, finding that 0.3 to 0.4 per cent, of free hydrochloric acid destroyed most bacteria, argued that one of the chief actions of the gastric juice was an antiseptic one. The discovery, however, that when the hydrochloric acid after secretion combines with the proteids of the food taken, and is then much less antiseptic in its powers, has altered this conception of the bactericidal action of the gastric juice. The amount of free hydrochloric acid present rarely exceeds o. i to 0.15 per cent. Few of the organisms, except those very susceptible to the action of acids, are killed. On the other hand, the presence of this amount of free acid inhibits their active growth in the stomach. MacFadyeti has proved by direct experiment that the Staphylococcus pyogenes aureus and the Micrococcus tetragenus can pass through the heallhy stomach without harm, while the spores of other bacteria are seldom affected. Capitan and Moreau found in a bacteriological investiga- tion of the stomach contents of thirty individuals only three kinds of micro- organisms, two of these being moulds and one a bacillus. Abelous found sixteen different forms in the stomach-washing in his own case. Seven of these were identified, viz. — Sarcina ventriculi. Bacillus pyocyaneus, Bacterium lactis serogenes, Bacillus subtilis, Bacillus mycoides, Bacillus amylobacter. Vibrio rugula, while of the remaining nine, eight \verc bacilli and one a coccus. Most of these organisms were found to be able to withstand the action of .17 per cent, hydrochloric acid for a long time, especially if they contained spores. Four of these organisms peptonised the casein of milk without coagulating it. Milk was coagulated by thirteen of the number, nine of these dissolving up the coagulum on further growth. All the organisms obtained acted more or less upon carbo-hydrates. The author, in an investigation into the bacteria present in the stomach in man [Journal of Pathology and Bacteriology, February 1893), was able to isolate twenty-four separate organisms, many of which were able to grow in a high percentage of hydrochloric acid, if most of it, however, was com- bined with proteids. Rummo and Ferranini have found that 0.05 per cent, of hydrochloric acid delays the alcoholic, the lactic, and the butyric acid fermentations, while o. i to 0.2 per cent, of the free acid — the normal amount of free acid generally present — stops these fermentations entirely. MiCRO-ORGANlSMS. 21 5 Hirschfeld observed that 0.07 to 0.08 per cent, of hydrochloric acid is sufficient to arrest the action of the Bacillus acidi lactici on milk. In the Herbiyora, and especially in Ruminantia, the organisms which flourish during the early stages of gastric digestion are almost entirely those which cause acid fermentation of the carbo-hydrates of the food. The lactic acid fermentation, which occurs in the stomach during the earlier part of digestion, is mainly the result of the growth of Bacillus acidi lactici. This organism appears under the microscope as a thick bacillus .1 to 1.7/xi long and .3 to .4/^ broad. It often occurs in pairs, and forms spores in milk. It grows in solutions of milk-sugar, cane-sugar, and dextrin, form- ing lactic acid and carbonic acid. Hiippe states that when about 0.8 per cent, of lactic acid has been formed the action of this organism ceases, but can commence again on the solution being neutralised. Starch is first converted by it into sugar, from which the acid is after- wards formed. Another organism which acts in much the same way upon carbo-hydrates is the Bacterium lactis serogenes of Escherich. Butyric acid is formed by many bacteria from carbo-hydrates. The organism which probably evolves the butyric acid found sometimes in the gastric contents during the first stage of the digestion of milk is the Bacillus butyricus. This organism is also able to decompose cellulose with the formation of marsh gas, carbonic acid, and sulphuretted hydrogen, and to change the lactic acid formed by the bacilli mentioned above into butyric acid. Other organisms, especially the yeasts and the moulds, can form acetic acid, alcohol, and other products from the carbo- hydrates of the foods. A peculiar organism often met with in the stomach-contents of both man and animals may be mentioned here. This is the Sarcina ventriculi, first described by Goodsir in 1842. It grows in groups of eight, arranged in a characteristic manner like that of bales of wool — in fact, the groups are known by the name of " wool-packs." Little is known about the exact role which this organism plays in tlie fermentative processes in the stomach. It undoubtedly produces an acid during its growth, and is capable of living in a solution of a very considerable acidity. The autlior has obtained Sarcinse in a living condition from stomach contents with an acidity as high as .48 per cent. One or two other Sarcinje occur, 2l6 THE NATURAL HISTORY OP DIGESTION. hut thcbe are only occasional forms derived from Ihe air. They are of much smaller size than the Sarcina ventriculi. To summarise : the growth of micro-organisms in the stomach of animals and man is checked by the gastric juice during the later processes of normal digestion, but the forms which are able to produce organic acids by their growth, manufacture more or less of this acid during the earlier parts of the digestive process. The acids thus formed are chiefly lactic and butyric acids. In addition, carbonic acid and hydrogen gases may be produced by their action. The Bacteria of the Small Intestine. — Those micro-organisms which have not been killed by the acid of the stomach juice soon grow again when they reach the small intestine; and, as the acid-forming organisms are those which can the more readily and with the least harm withstand the action of the acid in the stomach, it is those forms chiefly which flourish in this part of the alimentary canal. In addition to these organisms, however, many putrefactive bacteria or bacilli may be present. They, however, do fiot flourish in the small intestine unless the acid-forming organisms have been overpowered by their numbers, or by some abnormal condition of digestion. (Cf. Charts VI. and X.) Nencki, Macfadyen, and Sieber identified seven chief micro- organisms from the normal contents of the small intestine. Only two of them had any action upoii proteids, the Strepto- coccus liquefaciens ilei and the Bacterium liquefaciens ilei ; both liquefied gelatine and decomposed meat, evolving an odour of stale cheese but producing no indol or skatol. With the exception of the second mentioned, they all acted strongly on dextrose, five producing alcohol and six lactic acid. Frey found that one of these organisms, the Bacterium ilei, pro- duced succinic acid, sarcolactic acid, alcohol, large quantities of carbonic acid gas, and of hydrogen. There is no doubt also that fats are acted on by some of the bacteria present in the small intestine, and are decomposed into fatty acids and glycerine, though there is little evidence to support the view that this decomposition occurs in health. Nencki, Macfadyen, and Sieber failed to find either leucin or tyrosin in the contents of the ileum, and thought that the explanation might be due to the lactic acid generated by micro-organisms so diminishing the activity of the proteolytic ferment of the pancreas that such a profound decomposition of the proteid molecule is prevented. MICRO-ORGANISMS. 217 CHART X. — Showing the actual number of Bacterial Colonies grown from Segments of the Intestinal Canal of Salmon during different months. >'o. of MONrns. Ct'lonie 350 325 300 27s 250 225 200 175 150 125 100 75 SO 25 MAR. MAY. JUNE. JUIY. AUG. SEP OCT. ^NOV. 1 1 ( 1 1 I \ 1 I \ 1 ', 1; \ \ 1! \ \ M f i w \ i— — - -- i j \ ; II 1 i \ 1 \ / / \ \ L / \ "1 \ ■'a ,' / '/ ; i // '' '^ I h 1 ;,■ b ;,' i \ r /.' \ jl \ If \ ' N ^ ■ /. V ^r =^ 4 ■- ^ -CEsophagus -Stomach -Intestine 2l8 THE NATURAL HISTORY OF DIGESTION. Owing probably to the acid reaction of the contents, the so- called ptomaines have not been found in the contents of the small intestine under ordinary circumstances. Neither Brieger nor Baumann and Udransky were able to separate these bodies from the intestinal contents in healthy individuals, even twenty- four hours after death. The contents, however, contain bacteria which are able to produce ptomaines when grown upon gelatine, though, as said before, their growth in the bowel is inhibited in health by the acids present. Perhaps, also, the non- production of these poisonous bodies may be due to the antiseptic influences of the bile acids and to the diffusion of oxygen between the blood of the mucous membrane and the intestinal contents, although no free oxygen has ever been detected in them. The inhibitory action of the oxygen on these ptomaine-producing organisms is due to their anaerobic conditions of growth — i.e., they flourish best when oxygen is absent. The usual method of estimating the amount of putrefaction present in the small intestine is by the estimation of the ethereal sulphates in the urine. In cases of intestinal indi- gestion, the putrefactive organisms are able to form many of the poisonous and non-poisonous derivatives of proteids, many of which belong to the class of ethereal sulphates. Any of the measures taken to diminish this putrefaction, such as the administration of antiseptic drugs or the giving of a purely milk diet, at once diminish these bodies in the urine. Stern (Zeitschrift f. Hygiene u. Dimifections Kranchestiis, xii. s. 88) notes that observations on the number of organisms actually grown from the intestinal contents are of little value, owing to the fact that even under normal conditions their number varies much. Microscopically also an enormous number of bacteria may be observed, while only a few may grow on gelatine culture. Taking into consideration this fact, the estimation of the amount of decomposition in the small intestine is perhaps better arrived at from the amount of the products formed by bacteria and passed in the urine. On the other hand, Gilbert and Dominici {Compt. Rend. Soc. de Biologie, April 1894) found that the micro-organisms in the fasces fell to rVst of the previous number in man when fed on milk alone. Similarly, they found in the dog on a milk diet a great decrease in the numbers present in all parts of the alimentary canal — viz,, MICRO-ORGANISMS. 219 Table XLA''. Stomach 100 pro milligramme, instead of 50,000 Duodenum 50 ,, ,, 30,000 Ileum 1300 ,, „ 100,000 Large Intestine, 1275 ,, ,, 30,000 Rovighi (Ztschrft. f. phys. Che/n., xvi., 1/2, s. 20) found that he could diminish the ethereal sulphates in the urine of the dog to a marked degree by the administration of turpentine or camphor, but that the effect of these drugs was not nearly so pro- nounced in man. Mineral waters, such as Carlsbad or Marien- bad, increased the output of sulphates in the first twenty-four hours, while after that they were diminished in quantity. He recommended kefir, a fermentation product of milk, as the best means of subduing intestinal putrefaction, ascribing its action to the lactic acid which it contains. An interesting observation was made by Mester {Zeitschrift f. klin. Med., xxiv., 5/6, s. 441), who, in a series of experiments, excluded all chlorine from the food given to dogs. In a short time he found that the hydrochloric acid in the stomach disappeared, and that, with a meat diet, decomposition of the intestinal contents soon set in, with coincident increase of the sulphates in the urine, although if the diet given was poor in organisms no evidence of excessive decomposition could be found. If common salt was added to the meat diet the sulphates in the urine did not increase, showing that excessive decomposition of the contents of the alimentary canal had not occurred. Chemical Poisons formed by Micro-organisms in the Alimentary Canal — We have already seen (p. 186) that the digestion of proteids with the aid of the pancreatic ferment trypsin does not produce those poisonous alkaloids and bases which are commonly termed ptomaines (tttuijio., a corpse) or leucomaines, when its action takes place under strict aseptic conditions. Some of the final products of aseptic tryptic digestion consist of the amido-acids, tyrosin, leucin, and asparagin, and of the pigment tryptophan. Many bacteria are able to decompose the proteid molecules still further, although the first stages produced by their action are very similar to those produced by trypsin, i.e., albumoses and peptones are formed. Later on, the amidoacids, fatty acids, carbonic acid, ammonia, amines, and in some cases, indol and skatol, result from their activity. 2 20 THE NATURAL HISTORY OF DIGESTION. Ptomaines are compound ammonias, ammonias in which the hydrogen atoms are replaced in part or altogether by radicals. The simpler forms are amines. Thus in trimethylamine all the hydrogen atoms in ammonia are replaced by the radical methyl, thus : N H fCH, N-^ ( H N-^ CH3 .H ICH3 Ammonia. Trimethylamine. The next class are diamines, in which the hydrogen atoms are replaced by two radicals in place of one, as in : CH„ — CH2 — NH2 r CHa — CHj — NH2 Tetra-methylen-diamine or Putrescin ; or with an additional radical, as : CHj — CH3 — NHj I CH2 I CH2 — CHa — NH2 Penta-methylen-diamine or Cadaverin. Some of these bodies are not very poisonous, but bases with highly toxic properties can easily be formed from them. Thus piperidin may be pro- cured from cadaverin, reurein from cholin ; both exquisitely poisonous, although the mother substances possess little physiological action. Another base possessed of highly toxic properties has been separated by Brieger from putrid horse-flesh four weeks after the commencement of decomposition. It appears to be an oxidation product of'creatin, thus : NII2 NH2 / / C(NH) ■f30 = C(NH) -J-H30-^2C0., \ \ NH — CH3 — CH2 — COOH NH — CH3. Guanidin-propionic acid (creatin). Methyl-guanidin. Methyl-guanidin, therefore, is represented by CH3NH — C = (NH) — NH.^. Table XLVI. — The better knoivn Animal Alkaloids. Non-oxygenous : Neuridin, CjH^N.j - Non-toxic. Saprin, , C5HJ4N2 • - Non-toxic. Cadaverin, C.HjjN, 1 „,. , ,, , . Putrescin, C4H12N; | Shghlly toxic. Mydalein Highly toxic. IVIethyl-guanidin, CjH^N:, - Toxic. I'iperidin, CjHuN - - Highly toxic. Oxygenous : Neurin, Cholin, Muscarin, Mytilotoxin, Betain, QHisNO C5H15NO, QHijNOs CHisNOj MICRO-ORGANISMS. 221 Toxic. Non-toxic. Toxic. Toxic. Xon -toxic, A distinction has been drawn between the animal alkaloids produced by the action of bacteria and those basic bodies arising from the metabolic processes of the body. The first are termed ptomaines, the second leucomaines. Many of the leucomaines belong to the uric acid series, as xanthin, hypo- xanthin, adenin, etc. ; they are non-toxic. Other leucomaines are recognised as being connected with creatin — such as creatinin, xanthocreatinin, crusocreatinin, etc. ; they are toxic. The monamine trimethylamine is also found in the body, and a number of amido-acids, as leucin and tyrosin, which may be also classed under the term of alkaloids. The simpler compounds formed in the intestinal canal, when the putrefactive processes do not exceed their normal limit, the ainido-acids, the skatol, indol, and phenols, are derivatives of that portion of the proteid molecule which corresponds in arrangement to the bodies comprised in the fatty series. When true putrefactive decomposition of proteid bodies occurs, basic bodies, very similar to the alkaloids found in plants, are formed. The substances derived from animal proteids by the action of bacteria differ, however, from the corresponding vegetable alkaloids, in that they belong to the series with a molecular arrangement which characterises the fats; the majority of the vegetable alkaloids, on the other hand, have an arrangement of atoms based on the formula of pyridin. The vegetable alkaloids have very similar properties. In conina, the alkaloid of hemlock, two atoms of hydrogen in ammonia are replaced by N4H7, thus— rcji, N-^ C4H- : CsHi^N; or in nicotine, where all three atoms of hydrogen are replaced by the triatomic N5H1,, and whose molecule corresponds to two molecules of ammonia: 2N{C^il'j)2=Ci„^IiJ^2- The compounds of the vegetable alkaloids correspond to salts of ammonia, not of ammonium^that is, they combine directly and do not lose ihe atoms included in the molecule of water. iMorphina = C]fHjc|N05, Hydrochloride of morphia, CjjHjqN0.,C1, 222 THE NATURAL HISTORY OF DIGESTION. The mucous membrane of the bowel appears to constitute one of the bulwarks which defend the animal organism from out- ward infective agents. Thus Chunin (Arc/u'ves de J'/iys., 1896) showed that the toxines produced by the growth of bacteria rapidly lost their poisonous properties if introduced into an isolated loop of intestine, although their direct injection into the portal vein was followed by toxic symptoms. After destruc- tion of the mucous membrane of the bowel by heat or by the action of tannic acid, they lost none of their toxic power when introduced into the intestinal loop. Charrin also found that the toxic symptoms produced by the injection into a vein of some of the contents of the ileum could be inhibited by the pre- vious administration of an extract of the mucous membrane of the bowel 223 CHAPTER XL The Influence of the Nervous System upon the Actions of the Digestive Organs. Influence on the Salivary Glands — Secretory Fibres — Pressure of Secretion — On Gastric Digestion — On Pancreatic Secretion — On the Biliary Excretion — Intestinal Digestion — Paralytic Secretion — Summary. Most of our knowledge with regard to the influence of the nervous system upon the digestive organs has been gained by the study of the processes occurring in the salivary glands, by reason of their superficial and exposed position. Lately, how- ever, many observations of great interest have been made upon the nervous mechanism of both the gastric and the pancreatic secretions. The Influence of the Nervous System on the Salivary Glands. — All the salivary glands are supplied with nerve fibres from the brain directly, and from the sympathetic system (see Fig. 32). These fibres are of three classes — secretory, vaso- constrictor or vessel-contracting, and vaso-dilator or vessel- dilating ; the first and third of which are contained, as a rule, in branches of the nerves proceeding directly from the brain. These branches are the chorda tympani to the submaxillary and sublingual glands, and the auriculotemporal, which com- municates also with the otic ganglion, to the parotid gland. The second class, the vaso-constrictors, run in branches from the sympathetic system, along with some secretory fibres. Ludwig {Zeitschrift f. rat. Mediz., N.F., p. 259), in 1851, showed that if one of the cranial branches supplying a salivary gland is stimulated, two actions are produced, namely, secre- tion, and dilatation of the blood-vessels. These two actions, however, are not produced by the same fibres, as was proved 224 THE NATURAL HISTORY OF DIGESTION. later, when it was shown that such drugs as atropine paralyse the secretory fibres, leaving the vaso dilators intact. If the filaments from the sympathetic are stimulated the blood-vessels of the glands contract, and a small quantity of saliva is secreted Fig. 32. DIAGRAM OF THE NERVE SUPPLY TO THE PAROTID GLAND. ^Vll. (Meade Smith, after Yeo.) AT Auriculo-temporal nerve arising from V, the inferior branch of fifth cranial nerve' SCO superior ganglion of the sympathetic nervous system in the neck, sending a branch to carotid artery (S) and thence to gland. VII, Portio dura of the seventh nerve. which differs physically and chemically from that obtained on stimulation of the cerebral nerves. Heidenhain later advanced the proposition that there were two sets of secretory fibres — one dealing simply with the secretion of water, the other, to DIGESTIVE ORGANS. 225 which he gave the name of the metabolic fibres, with the secre- tion of the chemical bodies contained in the salivary juice. He also ascribed the conversion of insoluble into soluble substances and the increased production of protoplasm to the action of these metabolic fibres. If a canula be placed in Wharton's duct, which leads from the parotid gland into the mouth, and the saliva is collected, its flow is found to increase on stimulation of the peripheral end of the divided chorda tympani nerve, though the saliva is very much more watery. At the same time, the blood-vessels of the gland become dilated. The action of atropin, above referred to, in causing dilatation of the vessels with a diminished flow of saliva, shows that the increase following on stimulation of the chorda is not due to increased blood pressure. Indeed, it has been found that the maximum pressure in the salivary duct may much exceed the blood pressure in the arteries during stimulation of the chorda. Thus in one experiment the blood pressure in the carotid of a dog was only equal to 125mm., while the pressure in Wharton's duct reached 195 mm. of mercury. The dilatation of the blood-vessels may be looked upon as an instance of the general law that an organ when active always receives more blood than when it is in repose, to make up for the increased activity of its cells. Chauveau states that the quantity of blood which passes through the carotid of a horse during mastication may be four times that which obtains when it is not feeding. (Chart XI. ) The endings of the secretory fibres of the chorda in the gland are as yet undiscovered ; they probably, however, come in direct connection with the secreting cells themselves. Nerve fibres have been shown to end in the cells of the so-called salivary glands of the cockroach. The action of nicotin, which paralyses nerve cells much more readily than nerve fibres, has been employed to demonstrate the fact that many of the fibres of the chorda become connected with ganglion cells inside the submaxillary gland. Stimulation of the nerve, after the injection of nicotin into the local venous supply between the brain and the gland, produces no effect ; although a similar stimulation of the gland itself is followed by a copious flow. The same has been found to hold good with regard to the vaso-dilator fibres. The stimulation of the sympathetic fibres, as has been said above, after division of the main trunk of the sympathetic nerve in the neck and moderate stimulation of the part connected with the glands, produces a thick viscid and scanty flow from both the submaxillary and sublingual ducts, the current of blood passing through these glands being at the same time diminished. From the parotid no visible secretion may escape, but on microscopical examination of the 15 226 THE NATURAL HISTORY OF DIGESTION. gland its fine ducts will be found to be filled, or, as Langley expresses it, plugged with a thick secretion. If both the chorda and sympathetic nerves are stimulated, the action produced by the first, if the sympathetic be only moderately CHART XI. — Showing the Decrease of Solids in the Sub- maxillary Saliya of the Dog on prolonged Stimulation of the Chorda Tympani. (Ludwig and Becker.) The curves are plotted for four equal periods after the commencement of stimulation. GRAMMES 12 3 4- 20 20 18 1-8 16 1-6 14 14- 12 12 10 10 8 S 6 -6 4 -4- 0- O — / ** ^*« • - ,,^ / \ > ^^ rs / 1 s / > — 1 \ f i ^' ^ / '*' ^. — _/ \ \ \ — / ,... ..• •••' ►^... .."Tl 1 ' — — L _ _ — ^— — Total Saliva Secreted. • Total Solids- ] Organic Solids j-Ten times tha •••• Inorganic SaltsJ 'scale. DIGESTIVE ORGANS. 227 Stimulated, prevails to such an extent that the submaxillary saliva is secreted in considerable quantity, but it is not so watery as when the chorda has been stimulated alone. Strong stimulation of both nerves causes a plentiful and dilute secretion. Table XLVII. — Parotid Saliva obtained by Stimulation of Jacobsen^s Nerve. Water Without. . 99.44 98.97 09.43 99.61 With simultaneous stimulation of the sympathetic. 97.68 98.26 Percentage of DilTerence to first figures. 99.839 98.40 Total Solids 0.56 1.03 0.67 0.49 2.42 1.74 0.64 0.661 1.60 Ovganic Solids 0,24 0.76 0.21 0.16 2.06 1.41 0.33 + 142 0.342 Inorganic Solids . . 0.319 1.283 +275 0.317 -0.6 Table XLVIII. — Percentage Composition of the Saliva Cat, on Stitnulation {i) of the Cliorda Tymfani, the Sy77ifathetic. of the {2) Of Per- centage of Ash to Total Water. Total Solids. Organic Inorganic Solids. Solids. 1. stimulation of Chorda .. 98.796 2.. Stimulation of Sympathetic— a. Weak 99.205 h. Stronger 99.299 c. Strong, after 5 mgrms. ofAtropin .. .. 99.021 1.205 . 0.795 . 0.701 . 0.979 .. 0.865 . 0.339 . .. 0.S53 .. 0.442 .. 0.426 .. 0.275 . .. 0.525 .. 454 . . 28.1 65.6 . 39.2 . 46.3 nifference of 1 from 2a . . . . -0.41 Percentage Difference from 1 . . . +0.41 + 34 ..+0.512 ..-0.103 ..+69.1 ..-30 3 The Influence of the Neivous System on Gastric Digestion. — The influence of the nervous system on the gastric glands appears to be exerted in two distinct ways. The secretion of gastric juice can proceed when all the nerve branches going to the stomach have been severed, although stimulation of the divided nerves influences it. There must, therefore, be a 228 THE NATURAL HISTORY OF DIGESTION. peripheral secretory mechanism, capable of independent action, as well as a central apparatus. Heidenhain has suggested, indeed, that the flow of juice which follows direct mechanical irritation of the hning membrane of the stomach may, like the analogous phenomenon in Drosera, be independent of nervous control. Again, when the oesophagus is entirely occluded, as by a contracted scar (Richet), or disconnected, as by division in dogs (Pawlow), the presence of food in the mouth, or even the sight or smell of food, is sufficient to cause a flow of gastric juice. After both the nerves which run from the brain to the stomach, the two vagus nerves, and the sympathetic fibres also supplied to it, have been divided below the branches of the vagi, which go to the heart and lungs, the entrance of food into the stomach induces secretion and dilatation of the gastric vessels. Leubuscher and Schafer (^Centralblatt. f. Innere Med., XV. 33) could find no alteration, after section of the vagi, in the contents of the stomach of a rabbit fed on milk, except that the stomach-wall was slack and dilated, the result probably of some disturbance of motility. A similar observation on the dog seemed to point to the absence of any disturbance of secretion, but to an altered motor-power. All the evidence at present in our possession fails to prove the existence of true secretory nerves for the stomach, but the fact that the secretion can be influenced reflexly from the mouth, or by the character of the stimulus applied to its mucous membrane, and the circumstance that mental con- ditions can affect it markedly, go to show that, however independent of central control the gastric secretory mechanism may be, the greater nerve centres possess some power over it, although they may be seldom called upon in health to exert it. The reflex impulse from the mouth to the gastric glands has been shown by Pawlow and Schumowa-Simanowskaja in dogs, in whom the oesophagus was divided, to be interrupted by section of the vagi. When these nerves were divided, the passage of food through the mouth and out at the open part of the gullet caused no reflex secretion of gastric juice. These observers maintain that the negative results of others have been due to coincident circumstances, such as the influence of the narcotic or aneesthetic given, etc., because they could assure themselves of the occurrence of a secretion of gastric juice on irritation of the peripheral parts of the vagi divided as above described. In curarised fasting dogs Schneyer ligatured the oesophagus in the neck, doubled the duodenum back against the stomach, divided both the vagi, and obtained the following results on the electrical stimulation of the vagi DIGESTIVE ORGANS. 229 and splanchnics. Stimulation of the vagi in the neck when undivided, or of their peripheral portions, caused a flow of gastric juice. The secretion began at once, but was most marked 7 to 10 minutes after stimulation. Stimulation of the central portions of the vagi or of either portion of the splanchnic nerves caused no secretion. In dogs deprived of food by the mouth for 48 hours, but given nutrient enemata, the fluid secreted by the gastric glands on stimulation of the vagi contained free hydrochloric acid and inorganic chlorides, and possessed active peptic properties. Hayem and Winter suggested that at the commencement of the act of secretion after the ingestion of food the gastric glands only afforded a solution containing inorganic chlorides, while the stimulation of the food on the mucous membrane caused a subsequent liberation of hydrochloric acid from the cells in place of the salts. Schneyer holds that stimulation of the secretory mechanism by food or other means induces a flow of chlorides from the blood into the gland cells, while the cells through their specific action secrete the corresponding acid. By an ingenious application of the X-rays and the addition of small portions of sub-nitrate of bismuth to the food, Roux and Balthazard have been able to observe the movements of the stomach in frogs and in dogs. They have found that the functions of the stomach in both animals are identical. The food accumulates in the greater curvature. No movements of the walls could be seen in this part. The pyloric region exhibited marked peristaltic movements and is regarded by these observers as the true motor agent of the stomach. They have been able to show that the move- ments of the walls close to the pylorus are due to the active peristalsis of the neighbouring parts, contrary to Hofraeister and Schutz's views, and corroborate Moritz in stating that the presence of food in the duodenum does not arrest the pre- pyloric peristalsis. The Influence of the Nervous System on the Pancreatic Secretion. — As in the submaxillary gland, section of all the nerve fibres going to the pancreas is followed by a "paralytic" secretion of a thin, watery juice. Electrical stimulation of the part of the brain known as the medulla oblongata causes the gland to commence secreting if inactive, and increases its secretion if it is actively secreting at the time. Stimulation of the central end of the vagus, the part going to the brain, or of any sensory nerve, arrests pancreatic secretion even for some time after the stimulation has ceased, possibly through contraction of the blood-vessels. When food is taken into the stomach the pancreatic secretion at once com- 230 THE NATURAL HISTORY OF DIGESTION. mences, while if ether be injected into the stomach of an animal with a pancreatic fistula secretion is at once set up, corresponding in character to the time which has elapsed since food was taken, viscid if within six hours, watery if fifteen hours, after a meal. The available evidence as to the influence of nerve fibres on the pancreatic secretion is Httle if at all more conclusive than that relating to the gastric secretion, and consists of very similar facts. Actual experi- ment fails to reveal positive proof of the existence of vaso- motor nerves of the pancreas, but the reflex secretion following upon ingestion of food and stimulation of the stomach, and the " paralytic " flow on section of all the pancreatic nerves, suggest the presence of some central nervous control. Some recent observations, which, however, require further corroboration before they can be absolutely accepted, tend to show the existence of nerve fibres in the vagi (Popielski, Centralbl. f. Phys., x. s. 405, 1896) of the dog, some of which excite while others arrest pancreatic secretion. This observer induced a flow of juice by introducing some 5 per cent, hydrochloric acid solution into the duodenum, or by using pilocarpine, and then arrested the secretory action produced by this drug by stimulation of the vagus. The latent period before this action took effect was from 5 to 7 seconds. Popielski considers it probable that a reflex centre for the control of the different secretory apparatus adjacent to the pyloric end of the stomach is situated in the neighbourhood of the pylorus. Shirokikh found that local stimulation of the stomach with mustard oil or cayenne pepper did not induce a flow of pancreatic juice in dogs unless large enough doses were given to cause vomiting ; while Dolinsky, experimenting on dogs in whom both pancreatic and gastric fistula had been made, found that weak mineral acids, acetic and lactic acids, and sour drinks reflexly induced the secretion both of the gastric and pancreatic juices. Alkalies caused a flow of gastric secretion but no flow from the pancreas until the gastric juice had altered the reaction, or unless gastric secretion had been previously induced. When the oeso- phagus was cut through in addition to the establishment of the fistulas, the excitation of the pancreas was only indirect — i.e., induced by the flow of acid gastric juice, following the law that acid fluids call forth alkaline secretion. The secretion from the pancreas followed the ingestion of fats ; alcohol brought about only a moderate flow. The Influence of the Nervous System on the Secretion and Expulsion of the Bile. — Bile is constantly secreted, although only expelled into the duodenum at intervals. Thus the CtGESTlVE ORGANS. 23! influence of the nervous system may affect it in two ways — by influencing the secretion or the expulsion of the bile. As the amount of bile is closely connected with the quantity of blood passing through the liver, and as this is dependent upon the blood-supply of the other abdominal organs, any condition which constricts the abdominal vessels will slow bile secretion. In this way stimulation of the central brain centres, the spinal cord, the splanchnic nerves, and any sensory nerve diminishes its formation. Section of the cervical spinal cord, by lowering the general blood-pressure, acts similarly, but section of the splanchnic nerves, causing dilatation of the abdominal vessels without a marked decrease of general pressure, increases biliary secretion. No evidence exists as yet as to the existence of any special nervous mechanism, local or central, governing the secretion of bile. When the spinal cord is stimulated the muscular fibres of the gall-bladder and the larger bile-ducts contract, and expel the bile contained in them. The natural expulsion of bile which occurs on the entrance of the acid contents of the stomach into the duodenum is probably due to a reaction, similar to that following stimulation of the spinal cord, reflexly induced by impulses from the duodenal mucous membrane. The passage of food into the stomach leads to an increase of secretion (Chart IV.), the presence of the food in the duodenum to expulsion. Intestinal Secretion. — The influence of the nervous system on the secretion of the intestinal mucous membrane has been very fully investigated, and these investigations have yielded more fruitful results than those performed in connection with the stomach and pancreas. Stimulation of the vagi was found by Thiry to have no efiect on the secretion, while Budge and Lamansky observed an increase after removal of the creliac and mesenteric nerve plexuses. Lauder Brunton, Pye-Smith, and Moreau isolated three contiguous segments of the small intestine of the dog with ligatures, and divided all the nerves going to the middle segment. In a short time this middle portion was distended with secretion, while the other two remained empty. The absence of secretion in the segments with intact nervous supply must have been due to a nerve mechanism capable of restrain- ing secretion, and Brunton located this mechanism in the lower ganglia of the solar plexus and in the superior mesenteric branch from these ganglia. Destruction of the ganglia, though 232 THE NATURAL HISTORY OF DIGESTION. the other nerves be intact, causes secretion, while if they are left untouched, removal of the spinal cord and semilunar ganglia, and section of the vagi and splanchnics, fail to pro- duce it. The movements of the intestinal wall in the rabbit are not influenced by section of the vagi or splanchnic nerves (Pohl), but can at once be stopped by stimulation of the mesenteric nerves, unless the causal irritation is very powerful, or when this has been locally produced. Judging by the effect of local irritants on the muscular tissue of the walls, the mucous mem- brane of the small intestine is very insensitive. The plexuses of Auerbach and Meissner (cf. p. 106) are formed of ganglion cells, each with an axis-cylinder process and many protoplasmic ofishoots. Fine varicose fibres, which Dogiel (Anat. Anzeig., x. 16, s. 517) regards as derived from the cerebro-spinal system, form networks round the ganglia. The recent neuronic theory of the relationship of one cell or neurone with another, in which all actual direct connection between them is denied, may be applicable to these ganglionic plexuses, as it seems to be reason- able when applied to the cerebro-spinal system. Under these circumstances the neurones composing the plexus may be regarded as capable of com- municating impulses one to another, while each transmission must at the same time cause some slight change in the adjacent terminations of the neurones in communication with the central system or with the larger sympathetic centres, insufficient under ordinary circumstances to cause a transmission of the impulse to the higher neurones forming the connection between the cord, or brain, or sympathetic centre, and the neurone in juxtaposition to the intestinal ganglia. Forcible impulses from the neurones forming the intestinal plexuses, or from the central neu- rones, are able, on the other hand, to modify the action of the terminal members, central or peripheral, of their chains. When the innervation of the stomach and intestines is regarded in the light of this theory the diffi- culty arising from the frequent isolated action of local ganglionic systeins, if looked upon as being in direct unbroken communication with the higher centres, is surmounted. Each nerve-cell with its processes is an individual isolated unit or neurone, while its fine terminal processes, round the cell at one extremity and given off from the axis cylinder at the other, interlock with and closely adjoin the fine branches of other neurones, but principally with that one forming a link in the neurone chain of communication. It is more probable, however, that the ganglia of the intestinal plexuses com- municate directly with one another, but are only connected by neuronic intercommunication with the cerebro-spinal centres. If this suggested arrangement of the nervous supply of the alimentary canal be accepted in rough detail, the continuous actual inter-communication of the nerve elements composing the two plexuses will explain the transmission of impulses, inhibitory or exciting, from one segment of the tract to another. DIGESTIVE ORGANS. 233 which we have seen constantly occurs during the course of normal digestion. That this may be so is the more probable because any abnormal change in one part of the canal speedily leads to sympathetic changes in another. If there be intestinal discomfort gastric digestion suffers ; while often diarrhcea may be produced when irritating matters have been swallowed, although vomited before any part could have reached or been re-excreted into the bowel. During the course of the healthy digestion of a good meal the associated nerve elements in the intestinal walls give no sign to their host as to what is going on; unless it be that the contiguous neurones signal to the brain the general feeling of satisfaction locally pervading the presiding nerve cells, to mingle with the similar message directly impressed by more purely chemical means on the brain cells by reason of the absorption of food products into the blood-stream. But if too much or too rich food be eaten, or it may be too often, the normal stimulation of the local apparatus passes into irritation, raising sufficient energy for the transmission of a warning message to the neurones contiguous to it, and thence to the central system ; although owing to the line of least re- sistance lying along the joined ganglia the message, by the time it reaches the brain, is frequently indefinite and diffuse. An intense pain, as that caused by a gastric ulcer, may be able to telegraph its exact site, but even in such a case there is great uncertainty. The slowly increasing pain of subacute appen- dicitis signals most contradictory messages, until it has become sufficiently acute to influence predominantly those central neurones connected with the spot affected. Such a rough hypothesis, however, requires qualification in so far that the spinal cord appears to contain centres inhibiting the excessive peristalsis of the intestinal walls, not only in the splanchnic centre but in the lower part of the cord. Thus section of the cord below the splanchnic centre causes the peristalsis produced by stimulating the vagi to become greater, and after removal of the lower dorsal and lumbar sections of the cord the bowel only exhibits a " pendulum- movement " (Pal) unaltered on dividing the splanchnic nerves, while stimulation of the vagus causes strong peristaltic move- ments, which can be arrested by stimulation of the peripheral part of the splanchnic nerve. The vagi appear to supply motor fibres to the bowel as far down as the rectum, the splanchnics, 234 THE NATURAL HISTORY OF DIGESTION. ■ and other nerves from the cord, inhibitory fibres throughout the same extent. Such experiments, however, only apply to conditions in which the stimuli applied are much greater than probably occur in a normal state of digestion, with the exception of the inhibitory property of the spinal cord centres. If the neurone theory is applied here, it may feasibly be postulated that the neurones connected with these spinal centres are constantly charged with messages providing that the partly independent ganglionic system shall not excite too great a peristaltic movement of the bowel wall at the instance of the vagus nerves. Moreau, in 1868, maintained that the fluid obtained from a loop of bowel, all the nerves of which had been severed, was identical with the normal in- testinal secretion. Others since that time have regarded it as a so-called " paralytic intestinal juice," a transudation not a secretion. Mendel lately has shown that Hanau, Lauder Erunton, and Pye-Smith are correct in looking upon the fluid as being the result of a true secretion, and, contrary to Leubuscher and Tecklenburg, has found that it possesses an amylolytic power, and can form dextrose from cane-sugar and maltose, though in- active towards lactose. Courtade and Guyon [Arch, de Physiologic, 1897, p. 422), by using an apparatus devised to register the contractions of the circular and longi- tudinal muscular fibres in the wall of a segment of the bowel of the living dog immersed in warm physiological salt solution, found that normally the circular fibres acted more effectively in causing peristalsis than the longi- tudinal. Their action was synchronous, while the contractions of one of the layers followed an upward, of the other a downward direction. Stimu- lation of the large splanchnic nerve arrested peristalsis for a moment owing to paralysis of the longitudinal fibres, whether the peripheral or the central stump of the nerve was irritated. The circular fibres were only affected when the stimulation of the nerve was slight. Section of both vagi did not alter this result. Very energetic electrical stimulation of the splanchnic in a few instances caused contraction of the longitudinal and paralysis of the circular fibres, and was accompanied by vaso-constriction of the blood- vessels in the walls of the intestine. The sympathetic nerves, therefore, are concerned in the regulation of intestinal peristalsis, and can act on it directly or reflexly. 235 CHAPTER XII. A Comparative Study of Digestion in Animals. Types of Nutrition — Vegetal — Saprophytic — Specialised — Protozoa — Amoeba — Foraminifera — Metazoa — Spongida — Coelenterata — Annu- loida — Arlhrozoic Series — Arthropoda — Crustacea — Polyzoa — Mol- lusca — Gasteropoda — Tunicata — Summary — Digestion in Pisces — Organisms in Canal — The Salmon — In Aves — In Reptilia — In Ruminantia — In other Herbivora — Vomiting — Digestive Leucocytosis. We have now to trace the gradual ascent manifested in the phenomena of digestion in the lowest and most general form in unicellular animals, towards the complexity of the process as it occurs in man. All animals, even the lowest and least specialised, may be divided into herbivorous, carnivorous, or omnivorous forms. No sharp line of demarcation can be drawn between the animal and vegetable kingdoms with regard to digestion. The lowest animal forms (of the class Gre- garinidcB) practically obtain the elements necessary for their existence in the same manner as the Bacteria. Although the lowest forms of animal life resemble each other in the minuteness of their bodies and the simplicity of their structure, they by no means invariably present evidences of uniform digestive processes. Indeed, throughout the whole animal kingdom, three chief types of nutrition may be identified — 1. Vegetal. 2. Saprophytic. 3. Animal. I. Vegetal nutrition. — In some animals chlorophyll is con- tained in the cells, and serves in them, as in vegetable forms, a nutritive purpose as well as a respiratory one. Chlorophyll is found chiefly among the Flagellata. In them the digestive 236 THE NATURAL HISTORY OF DIGESTION. organs are called chromatophores, and consist of granular masses of protoplasm tinted by a colouring substance. 2. Saprophytic nutrition is the simplest type, and occurs in the Gregarinidm, Protoplasta, etc. The organism absorbs its food through any area of its outer surface, extracts those parts capable of yielding nourishment, and rejects the remainder. 3. In all animals, save those which come under the pre- ceding types, the function of nutrition is served by a specialised digestive canal, which may be of the simplest or of the most complicated description, and by which the animal seizes its food and extracts the elements necessary for its life. As we ascend the scale we pass from cells, forming com- plete organisms and endowed with many different properties, but performing the functions allotted to them in the most perfect manner with what appear to be most imperfect means, to animals in whom separate organs, made up of highly speciaHsed cells, perform the work demanded of them in a more and more finished manner, although the single cell of the lowest is as competent to subserve the laws of life in its own singular person as are the different congeries of myriads of cells in the complicated beings, evolved in the process of time from those very units. THE INVERTEBRATA. The Gregarina. — This unicellular parasitic animal occurs in the alimentary canal of both vertebrates and invertebrates, especially, however, in the intestines of cockroaches and earth- worms. It absorbs predigested substances from the alimentary canal of its host by every part of its surface, through which it can also excrete the effete matter. As it practically exercises no digestive function save the absorption of nutriment already prepared for its needs, it can hardly be said to digest, only to assimilate some of the food predigested by its host. The Amceba. — The Anixba consists simply of a mass of living, contractile protoplasm with a nucleus. Miss Green- wood has investigated the process of digestion in Anmba proteus with great care, and has added largely to our knowledge of its life-history. An Anmba when in active movement pro- gresses in a definite direction; a fairly constant hind end can then be distinguished. It is this part that most actively ingests. "The ectosarc is drawn like a funnel round the prey, and the opening which corresponds to the mouth of the funnel is A STUDY OF DIGESTION IN ANIMALS. 237 eccentrically closed." If the prey be in active motion when engulfed in the cell protoplasm of an Arnceba, a considerable area of water surrounds it; if immobile, little fluid is included. This space within the protoplasm, or "vacuole of digestion," remains to a greater or less extent until full digestion is accomplished. The movements of the Amoiba, which serve both for progression and for the prehension of food, are produced by temporary protrusions of parts of the cell protoplasm, termed pseudo-podia. The protrusion and retraction of these pseudo- podia initiate motion, while food particles too large to be directly drawn in are first enwrapped by pseudo-podia, and thus engulfed in the cell substance. The lateral pseudo- podia may sometimes engulf the prey, the anterior and most actively moving part never. The length of time which the act of ingestion lasts is comparatively short, generally not more than a few minutes. Fat, pure carbo-hydrates, and innutritious substances, such as litmus, are rejected unaltered after ingestion. Starch grains, for example, remain for some days in the body of an Amoeba, but when ejected are unchanged either in shape, size, or reaction. Unshielded proteid matter and proteid surrounded by a cellulose wall, is rapidly digested, although in the second case the cellulose covering is not dissolved nor visibly per- forated. When protoplasmic food is enclosed by an Amceba, the "vacuole of digestion," mentioned above, persists until all the nourishment has been removed. If the food be such that the Amceba cannot digest it, the water occupying the vacuole soon disappears, and is not replaced by a digestive fluid, such as is secreted by the protoplasm under the former conditions. Engelmann, Meissner, and Favre-Domergue have thought that they could detect an acid reaction in the fluids of some Amabm during digestion, but Miss Greenwood was unable to corroborate these observations, either by means of methyl- violet or tropfeolin. The actual agent employed is unknown, but it appears likely that in Amceba the digestion of proteids is carried on by an acid and a ferment, as in so many other animals. Metschnikoff has noticed, in the course of his work on the phagocytic power of various Amxbcs, that the bacilli killed by them do not change their shape, although they react to stains in a totally different manner than when living. For instance, they acquire the power of taking up vesuvin, which does not affect them in their natural condition. The digestive 238 THE NATURAL HISTORY OF DIGESTION. power of the Amoeba is their chief defence from their enemies, and by it they are able to protect themselves and to obtain their food at one and the same time. Speaking of the Rhizopoda, to which family the Amxba belongs, M. Richet says that "irritability is their life com- plete." The presence of particles of food irritates the proto- plasm of the cell, excites digestion and absorption, and causes the rejection of innutritions material. The life-history of the Anmba provides a practical illustration of the unaided powers of Protoplasm. In those members of the order of Amxbiz in whom the body is protected by a carapace, ingestion is carried on by the portion uncovered by it, the process being identical with that in the forms in which the body is naked. The members of the Foraminifera may be regarded as complex Ammbce, united together and enclosed in a shell. When the shell is perforated, the food is seized and taken in by the pseudo-podia protruded through the apertures ; when imperforated, by the pseudo-podia of the youngest and as yet uncovered segment. The Infusoria show a marked advance on the conditions so far described. They possess a mouth and a distinct but short oesophagus, which does not end in any gastric dilatation, but is simply a closed tube. Particles of food are brought to the oral aperture by means of flagella {Infusoria flagellaia), ten- tacles (/. ieniacuiifera), or cilia (/ ciliaia). In Monas vul- garis the food is dashed against the mouth by a sudden jerk and pressed into it by the base of the flagellum, a whiplike process. In Infusoria ciliaia a further differentiation of the alimentary canal can be traced, as it possesses an anal region, which is, however, not directly connected with the oesophageal tube. No actual anal aperture is present except when effete matter is ejected, but this always takes place at one well-marked region. In Paramecium (see Fig. 33) the particles of food are carried round the body of the organism in vacuoles of the endosarc, or inner part, by contractile movements of the ectosarc, or outer part of the protoplasm. Digestion goes on during these movements, and the products are absorbed by the contiguous protoplasmic substance. In the Protozoa, digestion is endosmotic in the Gregarinidcs, in the other classes true digestion of soHd particles is per- formed by the entire cellular body, while there is a gradual advance from the Amoeba, where the points of entry and exit A STUDY OF DIGESTION IN ANIMALS. 239 of the food are not clearly marked, to the Infusoria, with an oesophagus and, in some, a distinct anal region. When we turn from the Protozoa to Metazoa, or many- celled organisms, we find that the Spongida, or sponges, the lowest in the scale of the Metazoa, possess a body cavity which serves both for digestion and circulation. Water bearing food particles passes through minute pores on the surface of the body, circulates through the gastro-vascular space, and finds its way Fig. 33. PARAMCECIUM BUSSARIA. 12 3 (Meade Smith, after Stein.) 1. Viewed from the Dorsal side. A, Cortical layer; £, Nucleus; C, Contractile vesicle; D, D\ Food particles; Jl, Chlorophyll. 2. Viewed from the Ventral side, A leading to B, The mouth ; JD, Nucleus with nucleolus D' ; E, Central proto- plasm. In 1 and 2 the arrows indicate the direction of protoplasmic flow. 3. Transverse division of Paramcecium. A, A', Contractile vesicles ; B, B', Nucleus dividing ; C,C', Nucleolus divided. out again by a smaller number of larger apertures, or oscula, along with any effete products. The body cavity is lined with flagellate cells, which possibly have the power of digesting. It forms a rudimentary type of a true digestive system. The Cmlenteraia differ from the sponges in having no inhalent or exhalent pores, while they possess a distinct oral aperture and a complicated gastro-vascular system, ending in an anal open- ing. The mouth communicates directly, or by a short canal, 240 THE NATURAL HISTORY OF DIGESTION. with a dilated sac or stomach, from which numerous gastro- vascular canals, by which both the lymph and blood-vessels of higher animals are represented, run out to the margin of the body, to end in a peripheral, circular canal (see Fig. 34). The slimy secretion of Medusa has been found not to contain any digestive action, but Krukenberg has observed the digestion of fibrin introduced into their body cavities. In Actinozoa the mesenterial filaments are said by the same observer to secrete an active digestive fluid. Probably the chief mode of digestion in Cmknterata is intracellular, the cells of the inner layer Fig. 34. THE HYDROZO.\. (Meade Smith, after Jeffrey-Bell.) A. Eydra viridis attached to Duckweed. B. One of the same magnified. C. Diagram of a section of Hydra. EC, Ectoderm ; Mlf, Endoderm ; M, Month : SC, Somatic cavity ; T, Tentacles. of the protoplasm digesting and absorbing the food, thus pro- viding suitable nourishment for themselves and the other cells of the body. In Echinodermata the digestive canal is further differenti- ated. It consists of a mouth, oesophagus, and an intestinal canal, in which there is no division into stomach and bowel, and which ends in an anal opening. The canal is suspended in the body or somatic cavity, and is distinct from the water- A STUDY OF DIGESTION IN ANIMALS. 24I vascular and true vascular systems. In Urasier rubeiis Griffiths found no digestive ferment in the cells lining the tract, except in those forming the blind processes or pyloric cfeca which run from the intestine along the rays. This ferment acts on oils, emulsifying and splitting them, converts starch into sugar, and digests albumin. The digested part of the food transudes through the walls of the canal into the somatic cavity, and is absorbed by the cells lining it. The first class of Annuloida exhibits a reversion to a more rudimentary type of digestive apparatus, in accordance with the rule that true parasites have their digestion performed vicariously. The members of this class possess mouths and simple intestinal canals, generally without an anus. The Cestoidea, as shown by Fredericq, secrete no ferment of any kind, but absorb pre-digested material from the fluids sur- rounding them. The Rotifera form an exception as they are not parasitic. In addition to a simple but complete aUmentary canal, many of them have a well-developed muscular pharynx and two glandular tubes opening into the anterior part of the stomach, which possess a pancreatic function. The second class of Annuloida, Annelida, includes the leeches and the earthworms. The leech used in medicine, Hirudo medi- cinalis, has a mouth armed with chitinous or horny teeth opening into a pharynx, which is provided with glands. The sucker at the anterior part of its body is brought into action by muscles surrounding the pharynx. A slender gullet passes .from the pharynx into a long stomach, furnished with a number of lateral diverticula, from which a narrow intestine runs to a dorsally placed anus. The earthworm possesses a still more elaborate canal (see Fig. 35). A small opening forming the mouth leads into a cavity furnished with true salivary glands, which adjoins a pharynx. This passes into a straight oesophagus with three sets of diverticula or glands, and is continued into a crop or proventriculus about the fifteenth segment of the worm's body. This in turn opens into a muscular gizzard or stomach, which is succeeded by the in- testine, terminating in an anus. The secretion of the salivary glands acts on starch but has no power of digesting proteids. The fluid poured out by the glands on each side of the oesophagus is rich in calcium carbonate, serves to neutralise the organic acids of the food, and thus aids the digestive fluids which are only active in alkaline media. The gizzard usually ;6 242 THE NATURAL HISTORY OF DIGESTION. contains small stones, and serves the same purpose as the gizzard in birds. A yellowish glandular tissue envelops the intestine and is called the liver, but it secretes a pancreatic Fig. 35. DIAGRAMMATIC REPRESENTATION OF THE DIGESTIVE ORGANS IN THE EARTHWORM. - OES -CG (Meade Smith, after Ray Laiike-ster.) JIf , Mouth ; PH, Pharynx ; OES, (Esophagus ; CG, Calcareous glands : CP. Ci-OD ■ ff, Gizzard; /, Intestine. • > f fluid, and must be regarded as the representative of the pancreas of higher animals, and not of the liver. Digestion in leeches and earthworms is essentially the sarne as pancreatic digestion A STUDY OF DIGESTION IN ANIMALS. 243 of vertebrates, and can proceed as far as the formation of amido-compounds. Among the larger series of the Metazoa termed the Arthrozoic Series the digestive apparatus in the first class of Nematoidea, or "thread-worms," is very similar to those already described. The class Arthropoda comprises the animals usually known as insects, and the crustaceans. A general survey of the group will suffice. The majority of its members have well-defined digestive tracts. The mouth is provided with suckers, and organs for mastification, or organs for biting. The epithelium of the intestine is often furnished with a covering of chitin (CjgHggNgOjQ), a hard, horny sub- stance designed to crush the food. The lower arthropods have so-called livers, which are blind or caecal prolongations of the intestine; the higher groups possess a "liver" of consider- able size, especially Crustacea, though a better name for it is a " pancreas." The salivary glands are better marked as a rule in Inseda than in the other classes. The bee is provided with a cylindrical tongue which serves to lap up honey, and a crop, or honey-bag, separated from the stomach by a valve which must be opened by special muscular action before any of the contents can enter the stomach. In the bee's crop a change occurs in the nectar, which serves to arrest any acetic acid fermentation. On reaching the hive the nectar can be regurgitated into the wax cells prepared for it. The juice which is stored in the hives for the use of the larvae has been found to be the regurgitated contents of the true stomach, the valve at its entrance being raised by muscular action to allow of their ejection. The food stored for the larvfe destined to become queen-bees is free from any admixture of pollen ; the food of the drones contains pollen grains, partially digested, after the fourth day. The following table (Griffiths) gives the average composition of these food-stuffs : — • Table XLIX.— The Food of Bees. Food Stuff of Female or , , , ■nmnpc. Neuter or Queen Bees. Male 01 JJi ones, -working Bees. Water - 69.38 per cent. 72.75 per cent. 71.63 per cent. Total solicl.s - Per cent, j^it-genous ,° J I Glucose =°1'<'=- Ush 30.62 27.25 28.37 45.14 43-79 51-21 13-55 8.32 6.84 20.39 24.03 27.65 4.06 , 2.02 , — 244 THE NATURAL HISTORY OF DIGESTION. All these foods are rich in nitrogen. The queen-bee's food is the stickiest, that of the working-neuter the most fluid. The sugar present is always invert-sugar. The web, by means of which the spider catches its prey and preserves its young, is produced by more than a thousand glands with ducts opening to the surface in six projections or mammillse on the posterior part of the abdo- men. The secretion is insoluble in water, and has a nitro- genous basis. The products of digestion in the insects and crustaceans are removed by the blood-vessels, chiefly venous in character, distributed over the outer surface of the intestinal canal. In many Crustacea the digestive organs are surrounded by cells filled with yellow or blue oils, which probably serve as stores of nutriment, to be used during the moulting season. The colouring matters in these oils are termed liJ>ochromes. The stomach of the fresh-water crayfish, Aslacus fluviatilis, is much more complex than any of those hitherto described. The cardiac portion contains two hard nodules, or gastroliths, weighing about three grains each, imbedded in the walls. These are cast in summer with the carapace. This portion of the stomach is also furnished with minute hairs. In the pyloric half the walls are strengthened by calcified ossicles moved by four principal muscles which are attached to the carapace, and unite to form a powerful gastric mill. The walls of this part are also provided with long hairs which act as a strainer by preventing any coarse particles from entering the intestine. The Polyzoa, though mainly microscopic, possess complete alimentary canals, with an anus opening close to the mouth. Absorption of the products of digestion, however, is conducted in a similar manner to that in Actinozoa and Echinodermata. The digestive tract is suspended in a somatic cavity, the pro- ducts of digestion penetrate the visceral walls, and are absorbed by the cells lining the body cavity, or the organs situated in it. The Brachiopoda are characterised by a similar arrange- ment. The Molhisca. — The order of Lamellibranchiata is charac- terised by an oral cavity supplied with cilia, a short oesophagus, an expanded stomach, surrounded by a so-called liver, and furnished near the pylorus with a crystalline style, and a long intestine, much convoluted, which passes through the heart and terminates in an anal projection. There are no salivary glands, and the so-called liver possesses the properties of a pancreatic gland. Fredericq claims to have isolated glycogen A STUDY OF DIGESTION IN ANIMALS. 245 from the "liver," but it is probable that it is only present during the periods of rapid growth when moulting. The contents of the digestive canal are acid, due to hydrochloric acid. This is especially the case in the contents of the stomach, and it is possible, as Griffiths suggests, that this is the first suggestion of a separate digestive act performed by the stomach. Absorption takes place through the medium of blood-vessels. In Gasteropoda the snail may be taken as the type. The mouth is bounded by lips, and is situated just in front of the foot. Two large salivary glands open into it. The oesophagus dilates into a crop leading to the stomach, which is provided with a blind CEecal appendage. A long and coiled intestine leads into a wider rectum, which opens into the rrtantle. Round the intestine is a large organ with many lobes called a liver, but exercising a pancreatic function. The ducts from this organ open into the stomach and anterior part of the intestine. The salivary fluid acts on starch, as does the secre- tion of the " liver," while the latter can also enable the diges- tion of proteids to take place in alkaline media. The Cephalopoda present analogous features. Krause finds that their salivary glands secrete a poison with which they can kill crabs, etc., before they commence to eat them. When it is injected into frogs, these animals die. The Timicata. — The Tunicaia have a mouth leading into a pharyngeal-respiratory chamber, from which the food particles are selected by small tentacles round the opening into the oesophagus and passed down to an internally-folded stomach. The intestine is covered with tubules, called Savigny's tubules, which may be present over the stomach wall as well. These tubules open by a common duct into the stomach, and appear to have a pancreatic function. A general survey of the various facts relating to digestion as it occurs in the Invertebrata will not be out of place here. In the purely parasitic types, Gregarina and Cesioidea, no organs which subserve a digestive function have been dis- covered ; they simply absorb and utilise pre-digested foods. The Rhizopoda ingest, digest, and excrete by their unicellular bodies in virtue of the properties of their protoplasm. Their digestion is absolutely intra-cellular. Some of the Infusoria may be able to secrete a digestive fluid which can act in the short oesophagus (Paramcecium), but the main digestive action' occurs within the food-vacuoles which develop in the proto- 246 THE NATURAL HISTORY Of DIGESTION. plasm. A slight advance takes place in the sponges, where the flagellate cells lining the gastro-vascular cavities act for the cells of the mesoderm, and possess the power of ingestion, digestion, and excretion of waste products, while the products of the digestion are transmitted to the rest of the body. These lining cells are able to form a ferment akin to that of the vertebrate pancreas, or, more correctly, to form three ferments — diastatic, proteolytic, and fat-splitting. Among the lower_ or Zoophytic members of the Metazoa the processes of digestion and absorption are performed in a manner very similar to the processes in Rhizopoda and Infusoria. They possess, how- ever, gastro-vascular cavities lined with flagellate cells, into which the food particles pass through apertures in the body- wall, as in Spongida ; or through a distinct oral aperture, as in the Cceknterata, into a common digestive and somatic cavity. In the two divisions of the Ccelenterata, the Hydrozoa and the Actinozoa, a further advance is marked by the fact of digestion of food taking place in the fluid contained in the central cavity and absorption of the products by the endodermic cells. In the lower forms the food is brought directly into contact with the protoplasm of the cells, which exercises both a digestive and an absorptive action on it. In the Echino- dermata the digestive canal is distinct from and suspended in the somatic cavity, and the products of digestion transude through the walls of the alimentary canal and are absorbed by the cells lining the somatic cavity. In the Annuloid Series, except in the lower parasitic members, the digestive apparatus is much more highly specialised, and is divided into a pharynx, a gullet, a stomach, an intestine, and an anal aperture. The leech possesses a stomach furnished with numerous diverticula ; the earthworm, carbonate of calcium glands on each side of its oesophagus, a stomach divided into crop and gizzard, and a pancreas or so-called liver enveloping the bowel. The digestion of food is carried on in these species in p.n alkaline medium, and is capable of performing all the changes brought about by the pancreatic secretion of man, and in addition to attack cellulose. In the Arthrozoic Series the Inseda and Crustacea possess well-defined alimentary canals, from which the products of digestion are absorbed by the cells lining the walls and received into venous vessels and receptacles which not only serve the purposes of circulation but act also as absorbent agents for the A STUDY OF DIGESTION IN ANIMALS. 2^f chyle. This method of conveying the nutriment to the cells marks the Insecta and Crustacea from the Inverlebrata already considered, in which the products of digestion transuded directly through the endoderm in the more specialised forms, or originated within the cells themselves in the simpler, and from the Vertebrata, in which the absorbent and circulatory vessels are distinct from one another. The members of these orders possess salivary glands secreting an active diastatic ferment, a "liver" or pancreas, the secretion of which acts on starch, proteids, and fats, and can produce amido-acids from proteid bodies. The Polyzoa in the Malacozoic Series revert to the mode of digestion and absorption characteristic of certain of the lower forms, in that their digestive canal though complete is suspended in a somatic cavity into which the products of digestion transude and are absorbed by the lining cells. In the MoUusca the further advance of an acid secretion especially provided in the stomach for the purposes of digestion may be noted as the first indication of a separation in function of that organ from the bowel. As before, the "liver" of the Mollusca possesses all the characteristics of a pancreatic gland whose secretion can act on starch, albumin, and fats. Barfurth calls it a hepato-pancreas, but there appears no evidence to show on physiologico-chemical grounds that the gland possesses any of the functions of a true liver. It has been lately termed the " Mitteldarmdrlise," the mid-gut gland, and, according to Krukenberg, acts as a compound gland with the properties possessed by the salivary, gastric, and pancreatic glands united, He has found that its secretion acts in one instance in an acid solution, in another in an alkaline medium, and again in a neutral fluid with the same power; while in those molluscs in which acid secretions are wanting the secretion of the gland is inert in the presence of acids, and conversely, as in Helix Pomaiia, its secretion can only act in the presence of acids. The Tunicaia present very analogous features. A study of the arrangements provided for the preparation of food, and the extraction from it of the needful constituents in an assimilable form among the various members of the Li- vertebrata, reveals a regular and progressive advance towards sub-division of function and increase in complexity of the apparatus employed. The lowest type of digestive power may be ascribed to the purely parasitic forms inhabiting the alimentary tracts of higher 248 THE NATURAL HISTORY OF DIGESTION. species, and deriving their nourishment from the already digested substances dissolved in the fluids which bathe them. Such forms possess no alimentary canal, no digestive organs, nor do they secrete any active digestive ferments; they absorb all their nourishment through their outer covering. The next stage is represented by those animals which ingest their food either by their external surface or by a rudimentary aperture, but possess no other means of digesting the food thus obtained than the intrinsic action of protoplasm. Though physically much more simple in formation tlian the preceding class they possess a higher digestive power. Immediately above this class may be placed those animals in which, though possessing some form of alimentary canal, digestion is per- formed within the cells of the endoderm, and distribution of the products is carried on by simple intracellular diffusion. Further progress is shown by the secretion of a digestive fluid into the alimentary canal, and by the absorption of the products of its action by the cells lining the tract or the somatic cavity. This stage proceeds through many additional stages until the highest type of invertebrate digestion is reached. At first the fluid secreted for digestion is common to the whole tract; little by little separate segments become adapted for special functions; pyloric cseca, attached to the stomach, secreting a pancreatic juice, salivary glands, the so-called liver or pancreatic gland round the intestine, and the differentiation of the alimen- tary canal itself into definite portions mark these advances. The methods of absorption of the products of digestion and their circulation through the tissues can also be traced from the simplest form to one in which a complex arrangement of organs and vessels is present. In the lowest members of the Inverte- brata the single cells which constitute their entire bodies require no means for the distribution of the nourishment save the intracellular movements of the sarcode forming them. In slightly higher forms the products of intracellular digestion diffuse through from the active cells to those vfhich do not come into direct contact with the food, as in the compound Protozoa, or pass by a similar process from the endodermic cells to the cells of the meso- and ecto-derm as in the Hydrozoa, or, as in the Actinozoa, transude through the endo- derm of the alimentary canal and are absorbed by the cells lining the somatic cavity from which they pass by a process of diffusion throughout the tissues. The nutriment absorbed by A STUDY OP DIGESTION IN ANIMALS. 249 the endodermic cells in the Annelida is conveyed to the tissues partly by inhibition and diffusion, partly by means of pseudo- hcemal vessels. The classes following the Annelida possess vessels which serve the double function of carrying the blood and of conveying the absorbed nutriment to the cells of the body. There are thus in the Invertebrata no absorbent vessels analogous to the lymphatics of the Vertebrata and distinct from the vessels carrying the blood, and it is only in the higher forms that such vessels are found. In tracing the development of the alimentary canal we have found that in forms below the Actinozoa it is identical with the body cavity, but that in all the higher forms, save the Tunicata, it is distinct from this cavity. As low down as in some of the Protozoa a rudimentary oesophagus marks the first appearance of a digestive tube, but it is not until the higher Echinoder- mata are reached that the alimentary canal shows any trace of a specialised portion corresponding to a stomach. In the Annelida, Arthropoda, Polyzoa, and Mollusca a true stomach is present. In the Insecta a crop is also provided, and in the Roiifera, OligochcBta, the higher Insecta, Polyzoa, Gasteropoda, and Cephalopoda a gizzard is observed. In connection with the apparatus provided for mastication of the food in addition to the gizzards in the Echinodermaia, the Echinidea possess five hardened epithelial jaws each bearing a tooth. A further advance may be noted in the hardened buccal epithelium in Lumbricus and Hiruda, forming analogous structures to the teeth of the Vertebrata. The Arthropoda present more highly-developed structures. In the majority of the species the mouth is furnished with hard crushing jaws, or mandibles, and more delicate maxillae, while in many these structures are developed into masticatory and prehensile claws. In the Crustacea indeed two mandibles, four maxillcE, and three pairs of feet-jaws (inaxilli- pedes) are all adapted for the prehension and mastication of food. Various modifications of these structures are met with in the Insecta, while the Mollusca possess a radula or odonto- phore which may be regarded as a still further advance in that it seems to combine the properties of both teeth and tongue. Among the numberless members of the numerous classes of the Invertebrata, from the microscopic and apparently insensible Amceba, the delicate and sensitive Actinia or sea-anemone, to the wasp or bee, and to the molluscs, the apparent means of digestion are widely dissimilar, although the results are the 250 THE NATURAL HISTORY OF DIGESTION. same; the lowest in the scale is as capable of choosing the food suited for its nutrition, and of rejecting the unsuitable, as the highest organism. Whatever be the particular mode by which the food-elements are obtained and absorbed, the great under- lying principle consists in the properties of living cell-proto- plasm, properties combined in a single cell or specialised in different series of cells, by which it can so alter the chemical attributes of native food substances as to render them assimi- lable with its own constituents. Those forms which do not require to digest their food, as in the Cestoidea, possess no power of digestion; those which do not require more carbon than that supplied in proteids, or obtained from water or air through the agency of pigments akin to chlorophyll, do not possess an amylolytic ferment, nor are they stimulated to exert any of the associated acts accompanying the digestion of carbo- hydrate substances. In others, as for instance in the sponges, where the digestive processes are entirely intracellular, the cell- protoplasm forms a ferment capable of acting on proteids, carbo-hydrates, and fats. The protoplasm of one series of identical cells is thus capable of transforming each of the three elements of the food into the same derivatives as the secretions of specialised groups of cells in the higher animals. Table L. — The Salivary Glands and Salivary Constituents in the Invertebrata. Salivary Glands/ Oligo- Ortlio- Lepido-Hymeno- Ai-ach- Caster- are present in \ cliwta. ptera. ptera. ptera. nida. Neutral. ! Diastatic ferment -V -h + Mucin ? Sulphocyanates ? + + Calcium ? + ? Reaction Aliia- Alka- Ali;a- Une. line. line. faster )poda. - Cephal- opoda. + ■1- -1- -1- + -i- + Alka- line. -1- Alka- line. A STUDY OF DIGESTION IN ANIMALS. 251 Table LI. — The Organs capable of Digestion in the Inveriebrata. P - r \r \ 7 z ^v ^ >.-.^ Breakfast. Dinner. Supper. Ascoli [II PoHclinico, Nov. 15, 1896), are the increased stimulation of the abdominal circulation during active assimilation, and the consequent con- gestion of the blood-vessels distributed to the intestinal organs ; and secondly, and a little later, the stimulation of the blood-forming apparatus by the absorbed material inducing an increase of chemio-taxis. As the white corpuscles are practically unicellular organisms contained in the circulating fluids of multicellular animals, there can be no inherent obstacle to the acceptance of the hypothesis that they can assimilate directly, even while in transihi, properly prepared nutriment should it be afforded them, nor can it be denied that, given such increment, the white 2;8 THE NATURAL HISTORY OF DIGESTION. cells possess the power of karyokinetic or intrinsic multiplication. The point which as yet remains undecided is the site of this multiplication, but it appears probable that, although the chief sites are bone-marrow, lymphatic glands, and adenoid tissue, whether diffuse or aggregated into Peyer's Patches and similar bodies, the hyper-leucocytosis which accom- panies active digestion originates largely in the blood-stream itself. CHART XIII. — Showing Digestive Leucocytosis in Man. Actual Increase in Numbers. (Data from Limbeck.) Hour. A.M. '10 11 NOON. 12 ] 2 P.M 4 s 6 ! 8 15,000 13,000 ^ ^ \ \. y ^ s \, 11,000 / s \, i — -- X / v S 9,000 / / N \ i / / \ 7,000 K / / \ ^ y \ S,ooo -- ■ - -- ■ - 3,000 1,000 Both subjects had fasted foi 18 hours; at 11. ij (i) and 11.30 (2) were given food. Actual number of Leucocytes per cubic millimetre, ' Kirst Subject. — — Second Subject. '^w^mmmmm Meal of First at JLlJ. •■—■• ™-« Meal of .Second at 11.30. 279 CHAPTER XIII. The Senses. Smell : — Mechanism^Acuity — Taste largely smell. Taste : — Different tastes — Minimum amounts of substances required — Connection of sense of taste with habits and appetite. Hunger : — Nervous origin — Cause. Thirst ; — Local sensations — Nervous origin — Connection with hunger. Smell. — The sense of smell is comparatively slightly developed in civihsed man. The savage, and even a civilised man who has lived from his earliest youth in the open air, have much more sensitive olfactory organs. But even their power of smell is insignificant when compared with that of the dog or the deer. Kant labelled smell " taste at a distance," and he was justified in doing so by the close connection which exists between the two. The stimulation of the terminations of the nerve fibrils supplied to the mucous membrane of the nose from the olfactory lobes of the cerebrum by volatile and soluble substances, constitutes the sensation known to us as smell. The exact intra-cerebral course of the fibres is as yet doubtful, but they may come from a centre in the tip of the uncinate gyrus on the inner surfaces of the cerebral hemi- spheres. The olfactory fibres are distributed over an area of the nasal mucous membrane known as the regio olfactoria; the remaining part is termed the regio respiraioria. The entire nasal mucous membrane is called either the pituitary or the Schneiderian membrane. The mucous membrane which covers the regio olfactoria, generally comprising the upper part of the septum and of the middle turbinated bone, is thicker than elsewhere. It is lined with a layer of single cylindrical epithelium cells containing a yellow pigment, and between which peculiar spindle-shaped cells are placed, terminating in slender processes. The process which reaches the surface of 2§0 THE NATURAL HIStORY OF DIGESTION. the mucous membrane ends in some animals in a projecting filament, very similar to that in the taste-buds (see Fig. 47 and Figs. 16 and 1 7). The inner extremity forms a thin beaded fila- ment which is supposed to communicate indirectly with fibres from the olfactory nerve. Some of the cylindrical cells appear Fig. 47- THE TERMINAL CELLS AND NERVE-FIBRES OF THE OLFACTORY REGION. (Quain, after Schultze. Highly magnified.) 1, From the frog; 2, from man ; 3, from the dog. a, Epithelium cell with blunt outward end, and long inward process branching at its extremity ; in 3, a shows the division of nerve-fibres into fine fibrils ; 6, olfac- tory cells, showing peripheral rods, c ; outer extremities, e (in 1 prolonged into fine hairs), and central filaments, d. also to be connected with nerve terminations, and the special name of olfactory cells, which has been applied to the spindle cells, would seem to be equally applicable to both forms. In all mammals a subsidiary organ of smell is to be found, Jacobson's Organ; though rudimentary in man, it is highly developed in some animals. Both in the rabbit and guinea-pig THE SENSES. 28l It is of relatively large size, and is supplied with fibres from the olfactory nerve. The olfactory nerve is absent in Cetacea, and other aquatic mammals, and the nose is simply of use in respiration. A sensation of smell is caused by contact with odorous particles, which must be volatile to enter the nasal cavity, and soluble to pass into the fluid which always covers the nasal mucous membrane. As the olfactory area is situated over the upper part only of the nose, the best means of detecting the pres- ence of odorous particles is by a sudden sniff, or by a series of short inspirations. The sensation is only developed at the first contact, the perception of the odorous particles lasting only for a short time, unless the air containing them be drawn in afresh in separate amounts. It is a well-known circumstance that the continuance of a pleasant or even of a very foul odour soon causes a loss of its perception in one subjected to it, though departure from the impregnated atmosphere for a brief period suffices to again permit of its cognisance on return. Odorous substances dissolved or suspended in non-irritating fluids can be detected if sniffed up the nostrils, or if the nose be filled with them. This fact was long denied, but must at least hold good in fish, who can find their food, or a bait, in the dark by the sense of smell. Much misconception exists with regard to the different classes of substances which possess the power of stimulating the olfactory nerves. The only substances which can cause strictly olfactory sensations are the non-pungent scents or perfumes. All others affect the tactile nerves as well as the olfactory, or act on the tactile fibres alone. Thus the odour of menthol can be detected by the olfactory nerves at the same time as the tactile fibres record first irritation, then numbing of their terminations. Acetic acid acts in the same way; while carbonic acid gas produces what seems to be the result of olfactory stimulation, but is really an excitation of the tactile fibres only. Electrical currents passed through the olfactory membrane give rise to a smell like that of phosphorus. In Invertebrata no organ of smell has been accurately recog- nised, but many of Insecta possess the sense. Perhaps it is located in their antennae. The nostrils in fish lead into cul- de-sacs, not communicating with the alimentary canal. The lower of these sacs are thrown into folds, to the cells over which nerve-fibres pass from the olfactory lobe. 282 THE NATURAL HISTORY OF DIGESTION. In Amphibia the nasal cavity is of simple design, but Jacobson's organ is well developed, while in Repdlia two canals open externally and internally into the mouth by passing through the palatine arch. In Aves the high develop- ment of the sense of sight compensates for comparatively poor olfactory power. The nares communicate posteriorly by sht-like apertures with the auditory cavities. In birds of prey and fish-eating Palmipedes the olfactory lobes are larger than in other species. Of Mammalia little need be added to the foregoing description. In many mammals, the hog, elephant, and tapir for instance, the nose is adapted for the prehension of food as well as for olfaction. The sense of smell aids animals in their search for food and in their endeavours to avoid enemies. The dog or the ante- lope maybe said to live in "a world of smells." The domesti- cated dog has been enabled to increase his sense of sight, but this still remains very inferior to his olfactory powers. Smell is concerned as much as taste in the perception of flavours. The volatile odorous particles of the food ascend by the pos- terior nares to the olfactory region and are there perceived, but, as is perhaps natural, the resulting stimulation is ascribed to the gustatory organs. The professional " taster " of teas or wines is aware of this, and takes care that the volatile bodies producing the flavour or " bouquet " have free access to the nasal chambers. The child who cheerfully swallows a dose of nauseous medicine when his nostrils are closed, and the man who is unable to detect the flavour of his food owing to nasal catarrh, afford practical illustrations of this fact. Taste. — The structure of the lingual apparatus for the recep- tion and transmission to the brain of the gustatory sensations produced by various substances has already been described. The lingual nerve contains the nerve-fibres of taste for the tip of the tongue, the glosso-pharyngeal those for the posterior part. Section of either nerve abolishes the sense of taste from the portion supplied by it. Four varieties of taste can be distinguished, bitter, sweet, acid, or saline. What is commonly known as "flavour " is composed of one or more of these "tastes" coupled with stimulation of the sense of smell. For a substance to cause gustatory stimu- lation it must be soluble. The tip of the tongue appreciates sweet and acid tastes better than the base, bitter tastes not so acutely. Experiment THE SENSES. 283 has shown that some tastes are produced by more dilute solutions of their causal agents than others. An acid taste is appreciated in solutions of the greatest dilution ; bitter, saline, and sweet tastes require the presence of progressively larger amounts of the active ingredients before they can be dis- tinguished. Table LII. — Showing the Maximum Dilution at which the various Tastes are Appreciable. {Data from Valentin.') Substance. Charac- ter of Taate. Strength. Amount of Fluid necessary. Total Substance in Solution Tasted. Intensity of Taste. c. cm. Grammes. Sugar Sweet l/ss 20 0.24 Faint Salt -Saline I-S 0.007 Pronounced J, 1/426 12.0 0.027 Faint Aloes Bitter 1/32.3 0.25 0.0008 Very pronounced ,, ^/l2500 10 0.0008 Perceptible ,, jj ^/gooooo — — Faint after-taste Quinine Sulphate J J ^/ssooo — — Pronounced j» )j ,, I A 000 000 — — Trace Strychnine - „ 1/40000 — — Strong »> ,, 1/400000 — — Pronounced )> )) '/640000 — — Perceptible Sulphuric Acid Acid l/lOO 000 — ■ — Pronounced J) 3J 3> I /l 000 000 — — Absent Table LIII. — Showing the Relative Accuracy in the Fuwer of Distinguishing Bitter and Saline Substances in Various Degrees of Dilution. 30 c. cm. taken each time. {Data from Camerer.) Amount of Substance in 30 0. cm. Degree of Dilution. Percentage of Correct Perceptions. m.grrns. Quinine Sulphate ...0.089 ,, ...0.074 ,..0.059 ,, ...0.044 ,, ...0.029 1/34000 1/40000 '/si 000 i/esooo 1/103400 S9 88 77 62 32 Chloride of Sodium... 28. 600 „ ...19.100 , , ... 1 4. 300 ... 9.500 ... 4.S00 l/l049 l/l562 '/2O98 '/3125 l/e230 100 86 80 49 9 284 THE NATURAL HISTORY OF DIGESTION. Quinine is about twenty times more powerful a gustatory stimulant than common salt. The time which elapses, the "reaction period," between the contact of substances with the taste papilte and their appreciation varies with their characters. Table LIV., — The Periods of Time, in Fractions of a Second, which elapse before the Brain perceives the Taste of various Substances placed upon the Tongue. Reaction-period when Substance. Taste. placed on tip of Tongue. Seconds. Chloride of Sodium Saline 0.1598 Sugar Sweet 0.1639 Acids Acid 0.1676 Quinine Bitter 0.2195 Direct Touch — 0.1507 When the cathodal end of a galvanic circuit is placed on the tongue, the passage of the current produces an acid taste; the anode similarly applied gives rise to a metallic bitter taste. The sense of taste in Invertebrata, when present, must be located about the mouth, in the proboscis, suckers, or other processes, for they do not possess any organ analogous to a tongue. Fishes have either little or no sense of taste. Their tongues are rudimentary, horny or even covered with small teeth. Their isolated olfactory cavities may serve as taste organs as well as for smell. Judging by what such a fish as the cod will swallow, its taste sense must surely be of a most catholic character.! ^g birds swallow their food whole, an acute sense of taste is unnecessary. In graminivorous birds the tongue is hard and horny and precludes the probability of it being of use for tasting; carnivorous birds, with fleshy tongues, may be supposed to possess some power of taste. The senses of taste and smell in Mammalia exhibit numerous variations, the reason for which remains unknown. The Herbivora assuredly take pleasure in the taste of grass, hay, or corn; a horse prefers oats to hay, and hay to straw; but normally to all Herbivora the taste of flesh is abhorrent, save during the period following on the birth of their young. Curiously, it is during the same period that women so often display likings for articles which at other times would prove nauseating. The rabbit and the sow will devour their off- ' The author has personal knowledge of boots and puppies being found in the stomach of large cod 1 THE SENSES. 285 spring; women will crave for or eat ashes, slate pencils, and filth of every description. The general phenomena of the likes and dislikes of an animal towards articles of food are closely correlated to the nutritional needs of its body and to the digestive powers of its alimentary canal. But even in members of the same species there is a marked difference in the pleasure or dislike originating in the smell or taste of different substances. In man, in whom civilisation has often rendered the balance of the nervous system unsteady, a certain smell or taste has been known to cause fainting and nervous upset on every occasion on which it has been encountered. The intimate relationship of these two senses to the alimentary system and to cerebral processes; their adaptation to the re- quirements of bodily metabolism; their development as means of defence against danger; and their peculiar variations in different species or individuals, mark them out as the outcome of mechanisms developed by long-continued habit and need, which are closely bound up with the inner life of beings, and differ with the individual requirements of each. The eye and the ear differ little in structure and arrangement, or in their powers, save in acuity, among the higher animals; taste and smell vary as to the mental processes originated by them in almost all species. Hunger. — As sleep is the cerebral indication of a general sen- sation of weariness, so hunger is a signal, localised near the region of the stomach, but given by the brain, indicative of a general impoverishment of the circulating fluids in the body. Over-exertion may inhibit sleep, notwithstanding intense weariness, through an excess of metabolic products in the blood; various states of mental excitement act similarly. The sensation of hunger arises whether the stomach be full or empty. In the rabbit and guinea-pig the stomach never empties, the newly-swallowed food serving to drive the con- tents downward, but the starved rabbit exhibits signs of hunger. The Carnivora do not suffer from the sensation of hunger until hours have elapsed after the stomach has become empty. Many general conditions influence the sensation. Many drugs act on the nerve centre involved, and diseases often act in the same way, preventing the onset of the sensa- tion, while other maladies cause increase of the hunger sense, or even bulimia. All these conditions act by influencing the blood-supply to the central nervous system, causing con- 286 THE NATURAL HISTORY OF DIGESTION. sciousness of a state of bodily nutrition difierent from, or opposite to, the actual facts. Many have sought to prove that hunger arises locally; that it is due, principally at any rate, to immediate influences acting on the stomach walls. Nearly all the evidence obtainable is against such a view. Solid indi- gestible bodies, or water, introduced into the stomach often do arrest the feeling of hunger, but only temporarily; while injec- tion of nutritious fluids into the blood, or as enemata into the rectum, abolish it. Pigeons and frogs deprived of their cerebral hemispheres for long exhibit no desire for food ; but Ewald observed a pigeon treated thus begin to take food voluntarily after a period of eleven months, and saw frogs, devoid of their cerebra, catching and devouring flies in a normal manner. Section of the vagi and sympathetic nerves going to the stomach does not abolish hunger, so that no nervous connections between the brain and that organ are necessary for causation of the sensation. Hunger, then, may be regarded as a manifestation by the central nervous system below the cerebrum, probably by a centre in the medulla, of an impoverished state of the blood; exhibited more or less locally in the gastric region, and arrested, temporarily by mechanical filling of the stomach, or by pressure over it, gradually by the ingestion of food, and rapidly by the injection of nutriment into the blood- stream. The controlling hunger-centre in the medulla regu- lates the excitation of the centres by which the symptoms are initiated. Thirst. — The mechanism involved in the causation of the feeling of thirst very closely approximates in its arrangement to the apparatus which brings about hunger. The resulting sensation is more localised, only being perceived over the fauces and pharynx. Application of water to this region temporarily quenches it, as does the passage of saliva over it. As the period of abstention lengthens, the secretion of saliva diminishes, while it becomes less fluid and less capable of satis- factorily acting on the mucous membrane. It is well known that a pipe of tobacco, by increasing the flow of saliva, often proves of use when thirst is present and cannot be quenched for some time ; but its influence lasts only during the act of smoking. Fluids, when drunk alone, rapidly pass through the stomach, and are absorbed by the bowel. Solids do not enter the blood- THE SENSES. 287 Stream so quickly. Thirst, therefore, is quenched sooner than hunger is satisfied. Fluid taken with solids does not satisfy thirst so soon as if taken alone; but as it passes out of the stomach in small quantities sooner than the mass of the soHd food, the sensation of thirst, though not so quickly abolished as when fluid is taken by itself, tends to disappear before that of hunger. Continued deprivation of water and solid food together does not rouse so acute a feeling of thirst as depriva- tion of fluid when dry solid food-stuffs are taken. As hunger advances, the water excreted only corresponds to that con- tained in the tissues actually broken down; the percentage of water in the remaining tissues does not alter. A dog during the later stages of starvation will not drink the water offered to it. Thirst can be alleviated by the subcutaneous injection of water, or by rectal injections, and is only a local expression of the irritation of a centre in the brain owing to a diminution in the proportion of water in the blood. All conditions tending to withdraw water from the blood favour the appearance of thirst; warmth, exercise, fevers, diabetes, loss of blood, or ingestion of much salt act thus. 288 CHAPTER XIV, Metabolism in Animals. Metabolism : — Definition — Food Constituents — Metabolism of Proteids — Living and Dead Protoplasm — Urea — Seat of Urea Formation — In the Liver — Uric Acid — Hippuric Acid — Muscular Work and Meta- bolism of Proteids. Metabolism of Carbo-hydrates: — Glycogen — Diabetes Mellitus — Oxidation of Sugar in Active Muscle — Formation from Proteids, from Carbo-hydrates. Metabolism of Fat. Inorganic Salts: — Iron — Metabolism during Starvation. The term metabolism includes all the chemical processes taking place in the body by which the available elements of the food are prepared for nutrition, changed for incorporation into the tissues and fluids, used up for the purposes of movement or vitality, and finally altered into forms capable of excretion. To understand fully the processes of metabolism in the animal body we must first consider the part which the different elements of the food play in them. As the food ingested is intended to provide all the necessary constituents of the different tissues of the body, and as different animals and even different races of mankind use different food-stuffs, the subject perforce is a very complicated and difficult one. It has already been shown that the necessary constituents of the food must be capable of supplying a sufficient quantity of the elements, nitrogen and carbon, and of mineral salts and water, to satisfy the wants of the various formed tissues and fluids of the body, while the necessary food substances have already been divided into five classes : proteids, fats, carbo- hydrates, salts, and water. If we consider the part which each class of food-stuff plays we find that — METABOLISM IN ANIMALS. 289 1. The proteids are a source of energy, and contribute to form and keep up the primary or active protoplasm of cells, and their accessory or inactive constituents. 2. The fats are also a source of energy, and may supply a portion of the inactive material composing the cells. 3. The carbo-hydrates have similar functions to those of the fats. 4. The mineral salts are not producers of energy in the body, but are indispensable both for the active or protoplasmic part and for the inactive framework of the cell. 5. Water acts as a general solvent and necessary agent for the different metabolic processes. Proteids, water and mineral salts are absolutely necessary for the preservation of the life of all animals. Proteid substances can be so altered in the body as to supply both fats and carbo-hydrates when required; the converse does not apply. The animal body is unable to produce proteids from either fats or carbo-hydrates. On the other hand, as these two classes of food-stuff act as sources of energy, if they be given along with proteids they will serve to diminish the amount of albuminous food necessary. That is to say, they act as proteid-sparing bodies. An animal fed upon proteid alone requires more proteid than one fed upon a diet of proteid mixed with either fats or carbo-hydrates. With such a mixed diet the animal draws some of the energy required for its various bodily functions from the fats or carbo-hydrates instead of from the proteids assimilated from its food. Neither of these two classes of food elements contain nitrogen ; they can serve, however, as sources of energy. Nitrogenous sub- stances are not essential in the performance of physical bodily functions. The carbon in fat and carbo-hydrates, uniting with the oxygen conveyed in the blood, can produce the requisite amount of energy required for animal life ; but, as nitrogen is absolutely essential for the formation of protoplasm, and as protoplasm is the primary agent in all living processes, the fats and carbo-hydrates alone are not sufficient to support life. When an animal is said to be in a state of nitrogenous equilibrium the output of nitrogen in the various excretions is exactly the same as that taken in with the food. In this con- dition of nitrogenous equilibrium, however, it is necessary to give two and a half times the amount of proteid corresponding to the nitrogen which would be eliminated in an animal after it 19 290 THE NATURAL HISTORY OF DIGESTION. had been starved for a short time. This fact shows that all the proteid in the food cannot be used to replace the nitrogenous tissues disintegrated in the body. To explain this difference in the amount of nitrogen arising from tissue waste and elimi- nated, and the nitrogen contained in the food, it has been suggested that part of the proteid taken goes to form the living cells of the body or to replace their waste, and part to form the unorganised proteid circulating in the blood and other fluids of the body. The part which is supposed to circulate in the juices and to bathe the protoplasmic elements of the cells is said to be rapidly broken up into derivatives of a non- proteid character without having ever formed part of the protoplasm of the cells. The disintegration of this circulating proteid is very much more rapid than of that which goes to form the tissue cells. Voit found that a large dog during starvation only eliminated each day as much nitrogen as corresponded to i per cent, of the living cell material of its body. In men of normal adult weight, kept without food, the daily destruction of proteid is generally about 33 grammes, corresponding to a loss of 5.25 grammes of nitrogen. If we consider that the destruction of the tissue proteid during starvation is more than double the normal, a fact which has been shown to be true in the dog, the amount of tissue proteid used up each day in the healthy adult man will be 16 grammes. The circulating proteid, under normal conditions, must there- fore yield 16 grammes to the tissues each day for the purpose of rebuilding this loss. In addition to this a certain amount will be required for the growth of new cellular elements. Thus we may presume that in man when the amount of proteid assimilated exceeds 16 or 17 grammes per diem, the excess may be transformed into fats, carbo-hydrates, and nitrogenous derivatives, of which the first two are made use of as sources of energy. Moleschott has estimated the total amount of water in the tissues of the human body as 68 per cent., the proteid and other nitrogenous substances as 20 per cent., while Voit gives the amount of fat as 18 per cent. The different organs contain very varying proportions of water : thus the enamel of the teeth contains only 2 per cent., the muscles 75.7 per cent., and the blood 79 per cent. Roast beef contains as much as 72 per cent, of water, milk 87.4 per cent., and white of egg 85.7 per cent. The metabolism of a single living cell, whether it forms an entire individual, as Amoeba, a congeries of practically inde- METABOLISM IN ANIMALS. 29I pendent cells as in the Polyps, or one of the myriads which go to make up the higher animal beings, is exactly alike. In the unicellular organism, such as Amceba, the food taken in is assimilated by the single cell and applied by it in the production of energy for the growth of its body and the repair of any of its substance used up during the outcome of vital activity. The same is true of all the cells which, when segregated together, go to make up a multicellular organism. As we ascend the scale of the animal kingdom we meet with complex organisms formed of numberless cells, some of which are capable of one function, some of another, and others which are practically passive. The single cell of an Amaba requires the same proportion of food elements as the entire body of a higher animal type, the same, indeed, as is necessary for the maintenance of the life of man. In such a single living cell the elements of nutrition are used to nourish and vivify the cell itself. In more complex animal organisms the food elements absorbed are used by the protoplasm of the various kinds of cells in very different ways. There is no rule by which we can say definitely that such a molecule of proteid matter will go to aid in the nourishment of, say, a brain-cell, or that such another particular molecule will find its resting-place in the protoplasm of a bone-forming, a gastric, or a splenic cellular unit. In whatever form the proteid or the carbo- hydrate molecule is present in the food, it may serve, thanks to the changes brought about by digestion and to the vital power of protoplasm, as an addition to the body-proteids in any kind of cell, or be used up in the production of heat or energy. The subject of animal metabolism is so complex that it will facilitate matters to consider the changes which take place in each of the classes of proximate principles seriatim, and then to sura up the information gained in a review of the general process. Metabolism of Proteids. — The different proteids contained in the various tissues and fluids of the body are derived from the proteid products of digestion which have been absorbed from the alimentary tract. The two chief proteid bodies in the blood-plasma are serum-albumin and serum -globulin. The exact site of their formation is supposed to be in the intestinal mucous membrane during the passage from the canal into the blood-stream. The proteids absorbed from the alimentary 292 THE NATURAL HISTORY OF DIGESTION. canal are chiefly in the form of albumoses and peptone. Although they are absorbed in those forms by the mucous membrane, the blood which nourishes the walls of the tract does not contain much more peptone or albumose than the blood passing through other parts of the body. These substances then, it is clear, must be altered during their passage through the epithelium lining the alimentary tract, and be transformed into serum-albumin and serum -globulin. The importance of this change will be recognised when it is stated that the presence of peptone and albumose produces grave alterations in the blood, causing it to lose the power of coagulation, and inducing symptoms of active poisoning throughout the body generally. A description of the difference between living and dead proteids contained in Dr. Stewart's Manual of Physiology, 1895, i^ ^o apt that I need make no apology for quoting part of it : — " Now and again a living proteid molecule in the whirl of flying atoms which we call a muscle-fibre or a gland-cell, or a nerve-cell, falls to pieces. Now and again a molecule of proteid, hitherto dead, coming within the grasp of the mole- cular forces of the living substance, is caught up by it, takes on its peculiar motions, acquires its special powers, and is, as we phrase it, made live. But it is not any difference in the kind of proteid which determines whether a given molecule shall become a part of one tissue rather than of another. For it is from the serum-albumin and serum-globulin of the blood that all the proteid material required to repair the waste of the body must ultimately be derived ; and a particle of serum- albumin may chance to take its place in a liver-cell and help to form bile, while an exactly similar particle may become a constituent of an endothelial scale of a capillary and assist in forming lymph, or of a muscular fibre of the heart and help to drive on the blood." Indeed, members of the same species are fashioned in each other's likeness from very different foods, or again, different species present strongly contrasting attributes though subsist- ing on the same diet. In the first case, we can find a parallel by taking as an example men's habitations ; all those built of stone have a certain resemblance to each other, and differ from those constructed from wood or other material. Using the same illustration, those buildings constructed with stone may differ in detail; one may be a dwelling-house, another a church, and yet another a lighthouse. Similarly, various METABOLISM IN ANIMALS. 293 breeds of dogs may be reared on the same diet, while the same diet, given to another animal, suffices for its nutrition in a perfectly normal manner. We may safely affirm that no change in the character of the food is able to radically alter the structure of any of the more highly organised tissues of the body. The individual cells may be fed with different food-stufifs, may be overfed, or may even be starved, without any change in their essential features as long as they possess life. This leads to one of the most difficult questions in all physiology. The further the science advances the more remote does the possibility of our being able to determine what the difference between a living and a dead cell consists in. Pfiiiger has suggested an ingenious theory to account for the alterations in the characters of living and dead protoplasm. Living protoplasm is a complex substance of great instability ; the molecules forming it possess great powers of movement and of dissociation. When dead the same substance is more stable and inert. Pfliiger ascribed the instability of living protoplasm to the presence of the nitrogen in the form of cyanogen radicals, the atoms of which are able to move about in the molecule with great facility; while in dead proteid the nitrogen takes the form of amides. Similarly, Loew and Bokorny have suggested that the charac- teristics of living protoplasm depend on the presence of the unstable aldehyde group, H - C = O. Whatever the cause may be, the proteid of living protoplasm can split up into smaller molecules and can reconstruct other forms of matter, faculties which dead proteid bodies do not possess. The living proteid breaks down in the body more easily than dead proteid molecules outside it, but it falls to pieces in a fairly constant manner, giving rise to carbonic acid, water, and simple nitrogenous substances such as urea, uric acid, and ammonia, with the help of the oxygen present in the blood- stream. It is an interesting question, which goes, however, beyond the province of this book, how far living protoplasm corresponds in chemical composition to uncoagulated native proteids removed from the tissues. It is impossible, of course, to analyse living protoplasm, as the chemical processes which it has to undergo during analysis necessarily involve its death. Protoplasm and native albumins, however, correspond very closely in their elementary constituents: they both present all the reactions 294 THE NATURAL HISTORY OF DIGESTION. which indicate a native proteid, and both invariably contain a certain percentage of inorganic salt. The fact that so-called ash-free albumins and globulins have been artificially obtained outside the body in no way detracts from the accuracy of this statement, as the reactions of these ash-free bodies present many points of difference from those of the ordinary native albumins. (i.) TIu Formation of Urea. — By far the largest proportion of the excreted nitrogen leaves the body of the mammal in_ the form of urea, although in cold-blooded animals, and in birds, uric acid, which is probably a preliminary stage in the forma- tion of urea, takes its place.^ In the 'Herbivora hippuric acid may be said to take the place of uric acid in its turn. Urea is contained in small quantities in the blood, from 2 to 4 parls per 10,000. Some of the urea excreted by the kidney may therefore be simply separated by' that organ from the blood, and this has been shown to be the case in so far as the blood in the renal vein running from the kidney always contains a little less urea than the blood of the renal artery. If we could exactly measure the difference between the proportion of urea in the renal vein and renal artery, and, therefore, the loss occurring in the kidney during the course of twenty-four hours, we should be able to tell exactly the amount eliminated in that period of time, and to tell by the amount of urea found in the urine whether a process of separation is the only one at work. Voit found that a dog of 35 kilogrammes weight, and receiving a sufificient proteid diet to maintain its weight (500 grammes of lean meat per day), excreted 40 grammes of urea in the twenty-four hours. He then calculated the total quantity of blood which would be circulating in a dog of this weight, and, assuming that the average time required for the blood propelled by each beat of the heart to pass through the kidney was 10 seconds, he arrived at the conclusion that 300 kilogrammes of blood passed through the kidneys in twenty-four hours. ^ Tschlenoff has found that the increase in the excretion of nitrogen in the urine does not proceed uniformly after a meal rich in proteids, but shows two maxima. Veraguth lately (journ. of Phys., xxi. 2 and 3, p. 112, 1897) has repeated TschlenoiTs experiments, and finds three maximal periods during which the nitrogen excreted in the urine increases in a more marked manner than during the intervening periods. The first occurs soon after food is taken ; the second two to four, and the third six to seven hours later. If the food taken contains less proteid these maxima are still perceptible, but are not so prominent. METABOLISM IN ANIMALS. 295 Therefore, if the blood contain .3 per 1000 .of urea, the total weight of urea which would circulate through the kidney during that time would amount to 90 grammes. But only 40 grammes were excreted in the urine. The kidney epithelium actually came in contact in the twenty-four hours with more than double the quantity of urea that it actually separated from the blood. The urea may be formed, to some extent, in the kidney it- self, but ligature of both renal arteries in the dog does not prevent the accumulation of urea in the blood, and the amount of accumulation appears to be very similar to that which the animal would excrete under normal circumstances. Little urea, therefore, can be actually formed in the kidney substance. The liver is the chief seat of urea formation from the debris of broken-down nitrogenous bodies brought to it by the blood. It acts, in fact, as a factory in which the nitrogenous remains of the proteid waste of the body are simplified through several stages into the final product of urea.i The facts which point to the liver acting in this way are — I. Leucin, if taken in large quantities, is excreted by the kidneys as urea, while in certain diseases of the liver, such as acute yellow atrophy and advanced fatty degeneration, where the liver tissue is much altered, the urea excreted in health may be almost entirely replaced by leucin and tyrosin. ^. If a solution of ammonium carbonate be mixed with defibrinated blood and passed through the vessels of a liver recently excised, urea is formed in quantity, although none appears to be produced from ammonium carbonate when similarly sent through an excised kidney or through muscle. ■^ Recent observations by Nencki and Pawlow {Arch. f. Experim. Pathol, u. Pharmakol., xxxviii., 3 u. 4, p. 215, 1897), however, serve to throw doubt upon the liver being the exclusive source of urea in the body. These observers experimented on healthy dogs abundantly fed with meat. They established in them a communication between the portal vein and the in- ferior vena cava (Eck's venous fistula), and then extirpated the liver as com- pletely as possible. This proceeding should be followed by an increase of the ammonia excreted by the urine, and a diminution of the urea if the liver be the sole source of urea. In the first dog operated on, the ammonia in the blood and urine was increased in amount, and the urea decreased ; but in the second dog the quantity of urea in the blood, and of ammonia in the blood and urine, was increased. In a third dog, in which the blood of the portal vein was diverted into the inferior vena cava, the liver was not removed, but the hepatic arteries ligatured ; the proportion of urea in the blood remained unchanged. Nencki and Pawlow conclude that the liver cannot be regarded as the sole urea-forming organ or tissue of the body of mammals. 296 THE NATURAL HISTORY OF DIGESTION. 3. If blood taken from a dog, killed during the process of digestion, be passed through an excised liver, some urea is formed, while the blood of a fasting animal treated in the same way yields none. This experiment shows that the blood of an animal during active digestion must contain certain substances in considerable quantity and in excess of the amount which the liver is able to convert into urea during their progress through it until this has been repeated many times. For in birds uric acid, which, as we have seen, takes the place of urea and forms the chief ultimate product of the metabolism or breaking-down of proteids, is certainly pro- duced almost exclusively in the liver. Minkowski, taking advantage of the fact that there is a direct communi- cation in birds between the portal and the renal-portal veins, extirpated the entire liver in living geese. After this operation the blood from the alimentary canal passes by a devious route through the kidney to the inferior vena cava, and the animals can survive from 6 to 20 hours. The uric acid excreted after this operation contained only 3 to 6 per cent, of the total nitrogen excreted in the urine, the nitrogen given off as ammonia from 50 to 60 per cent, of the nitrogen. In a normal goose, the uric acid eliminated contains 50 to 60 per cent, of the total nitrogen in the urine, and 9 to 16 per cent, is in the form of ammonia. The small amount of urea, which is normally present in the excretion of the bird, is not affected by the excision of the liver, but urea injected into the blood after such an operation is excreted unchanged, although it appears as uric acid in a normal animal. The foregoing evidence is sufficiently convincing to allow of our definitely stating that the liver is the chief seat of the final disintegration of proteid molecules. It is not, however, the only organ or tissue in which proteid changes can occur. In all the parts of the body which contain proteid — and every part contains it in some form or other — proteid molecules are ever breaking down ; the local products of this destruction of proteids are less oxidised and more complex than urea, and are probably intennediate stages between the complete proteid and urea, stages which are progressive and result in a series of nitrogenous bodies which become more and more simple and contain more and more oxygen, until they pass, through the action of the hepatic cells, into the final form of urea. Some of these substances are given in Table LV. It will be seen from the table that the proportion of nitrogen to oxygen is about 1:1.4 in native proteids; and that this proportion rises rapidly in the series from guanin to uric acid from I :o.228 to 1:0.857. Kreatin is contained in the body in much greater amount than any of the others, muscle containing as much as 0.4 per cent, of it. Bunge has suggested that it may be one of the stages on the way to urea, as it forms one of the series represented by the substances which constitute the medi- METABOLISM IN ANIMALS. 297 *!l B S ■a « ° 00 u-1 00 |13 C^ ^ o^ ■o OJ^ W3 a) -^ •O oT -. CO 10 11^ 7 l-H 1 1 1 1 1 r i i 1~« d 1 d 1 "-I ^ §•=> 2 a." c " ?i ■^ C\ U1 ^ rn C> CO un ■LTi x-^ q U-) ^ 0\ 'O ■2 -so r^ ro VO ^ ^o 1-3 CO 6 ^ CO 'rj- 00 vD N CI N M m ot hH '^ '-' M cs Cl 01 0.3 £ » B 2:^ oj T^h CO "5 &>■? 0^ N I^ VD N HH q IT) CO *-< CO CO 10 ^ S't 1^ CO t^ d t-^ Ov CT^ 0\ ^ ^ ^ CO oi MD VO 't, M W ■<;t- -^■ CO CO CO CO ^ ■ ■ -s 00 M ■o t>~ 'O 01 CO 10 •SJm 00 m tn ■^ ^3- •^ m ro U-) VD ro ^O Sg| l-H hH l-H &■ = l-O h-l m M fO M ^ Tj- ^ l-l 01 CO 01 ^ fi t^ QJ "^ Tj- =0 M w « M 1 1 1 1 1 1 1 1 1 1 1 1 1 „'"'S- . S-g ^ M M N IS CO l-l 10 ■<^ ^ ':^ CO ro 01 |g« M M ^0 . U r- 00 hH ro 00 ■!h 0^ U-) ';^ Tj- ^ ON r^ ■^ rn 2 •"" 00 On vO ^ ^ \o »j-i LO LO "1 in TT ■"t '- p! rS C 3 iH c C r2 C 'So S .3 1 "u "Eo a 5 5 5 Oh 'a Irf M w ro ^ 10 VO I^ 00 ON - M - Trf »J-t 298 THE NATURAL HISTORY OF DIGESTION. ate members of the table. Kreatin, however, does not appear to be changed in any quantity into urea in the body. The only member of this class about which there is conclusive evidence is uric acid, and it appears certain that it is one of the stages lying between the complex proteid molecule and simple urea. Amido-acids have also been said to form intermediate substances in proteid destruction, but Bunge has shown that there is not enough carbon in the proteid molecule to convert all the nitrogen present into amido-acids. When these sub- stances are given by the mouth, however, the output of urea is increased, although the increase is less than corresponds to the amount of their nitrogen. Some of it must therefore be used up in other ways in the body, and Lea has suggested that amido-acids act as agents in the liver, taking part in the synthetic or constructive processes in that body, and so are not on the downward series of substances derived from proteid metabolism, but rather, as he terms it, on the upward grade. An observation by Lieblein (Arch.f. Exp. Path. u. Pharm., 33-4-5, s. 318), who destroyed the liver tissue in dogs by injecting acids into the common bile-duct, shows that under such conditions an increase occurs in the amount of uric acid excreted by the kidneys. The proportion between the nitrogen excreted as ammonia and the total nitrogen in the urine varied slightly, if at all, in direct contrast to the experi- ments noted above on geese. Gaglio {Du Bats' Archiv., 1886, s. 400) has found that blood in the dog during the digestion of flesh contains 0.3 to 0.5 per thousand of lactic acid. During fasting this figure falls — for instance, it falls, forty-eight hours from the last meal, to o. 1 7 per thousand — but it never wholly disappears. He also observed an increase in the quantity of lactic acid in blood which had been passed through a recently excised kidney or through excised lungs. It is interesting to note that blood-serum is unable to remove lactic acid from the tissues, although, whenever the corpuscles are not removed from the blood, the amount of lactic acid invariably increases. In the experiments of Minkowski {loc. cit.) on geese, the ammonia excreted in the urine was found to be in the form of lactate, not as the usual carbonate, while the lactic acid and the ammonia were always present in amounts equivalent to their powers of combination. It is probable from these observations that ammonium lactate plays a part in the forma- tion of uric acid in birds. It is also noteworthy that in METABOLISM IN ANIMALS. 299 animals whose liver has been removed the quantity of lactic acid excreted is independent of the amount of carbo-hydrates in the food, but varies directly with the proteids taken. Araki (Zeit. f. phys. Chem., Bd. 15, s. 335) has thrown still more light on this subject. He found that dogs, rabbits, and hens kept in an atmosphere poor in oxygen soon excreted ammonium lactate in the urine. We may safely say, there- fore, that one of the forerunners of urea consists in lactate of ammonium, which can be transformed by oxidation into ammonium carbonate and then into urea. (2.) Uric Acid. — Little need be added to our previous de- scription of the formation of urea in dealing with its close ally, uric acid. In cold-blooded animals and in birds most of the nitrogen of the broken-down proteids in the body is ex- creted in this form. It seems best to look upon the formation of uric acid in man as a result of a certain type of proteid metabolism, resembling processes in animals in whom different physiological methods take place. The fact that in man a distinct uric acid diathesis (which is often hereditary) is met with, may perhaps point to some physiological reversion towards those lower forms in which this body is so abundantly formed under normal circumstances. (3.) Hippuric Acid. — In the Herbivora this body replaces uric acid as one of the results of proteid metabolism. In man the ingestion of benzoic acid is followed by excretion of hippuric acid by the kidneys. Hay contains a benzoic acid compound, and it is not difficult to find in it one of the causes leading to the appearance of hippuric acid in the urinary excretion of the Herbivora. (4.) Kreatinin. — We have little knowledge of the origin of this substance in the body. Kreatin can be very easily trans- formed into kreatinin by artificial means, but there is no evidence to show that such a chemical change really takes place in life in the body. (5.) Carbonic Acid. — It cannot be definitely said whether carbonic acid, split off from the proteid molecule, is at once produced when the nitrogenous part of the molecule breaks down, or whether the carbon not utilised in the formation of the proteid derivatives forms other intermediate products, which may finally be excreted as carbonic acid. Of course, as we shall see shortly, some of the carbon may be retained in the body as glycogen or fat, and this, perhaps, may point 300 THE NATURAL HISTORY OF DIGESTION. to a series of changes before the carbonic acid is finally excreted in its simple form. (Table LVI.) The effect of muscular work on proteid metabolism, as shown by the observations of Krummacher (Zeit.f. Biol, xxxiii. s. loS, 1896), is indicatea by a slight increase in the destruction of body proteid, an increase not in proportion to the amount of work done, and least when the food is rich in non-nitrogenous material. Consideration of the calculated energy corre- sponding to the amount of proteid used up in the body shows that it cannot be produced to any great extent by proteid combustion during muscular work, and not at all from proteids under ordinary conditions. Table LVII. — Krummacher's Observations. Giving the results obtained in three series of observations upon the day on which work was done by subjects kept on uniform diet for eight days, with muscular work on the fifth. Percentage increase Diet in Grammes. Work in of Albumin Albumin, Fat. Carbo-hydrates. Kilogrammetres. destroyed on fifth day. I- 95 88 303 153,070 23 2. 137 168 709 324,540 22 3- 89 17s 903 401,965 7 Percentage of Percentage of increase in Albumin in Total Diet Proteid Diet. in grammes. Calories. Destruction. 1. 19 495 2,389,726 23 2. 13 1,014 4,854,618 .. 22 3- 7-5 •■• 1,167 5,444,906 7 A recent paper by Noel Paton and others {Journal of 1 hysiology, xxii., 1897) shows that increased amounts of proteid are made use of by the body under conditions of excessive muscular work. This proteid is chiefly derived from the muscles, because no increased excretion of uric acid or phosphorus takes place, muscle having little nucleo-proteid in it from which these substances can be derived. If the individual be in poor train- ing, other tissues as well as the muscles are called upon for some of their proteid. We may sum up our present knowledge of the metabolism of proteid bodies as follows : — 1. Nitrogen does not pass into the form of urea in the tissues theiTiselves, but appears in the urine for the most part in this form as the result of changes effected principally in the liver. 2. Some of the nitrogenous elements of the food may be excreted as urea without having formed a part of the proteid constituents of the body. That is, nitrogen may be absorbed Missing Page METABOLISM IN ANIMALS. 3OI from the alimentary canal as a constituent of proteid deriva- tives, such as leucin and tyrosin, which are changed into urea in the liver. 3. Some of the proteids in the blood may break down and be excreted as urea without having previously formed part of the tissue-cells. 4. The chief site of the breaking up of proteids for the pro- duction of physical work is in muscular tissue. Other tissues probably can supply proteid to make up the loss in the muscles when the individual is in poor training. Metabolism of Carbo-hydrates. — We may begin this subject by giving shortly the main points of Bernard's glycogenic theory. The carbo-hydrates taken in as food pass into the blood of the portal vein as dextrose ; are carried to the liver, and in part, probably in chief part, are arrested in that organ, where they are stored up as glycogen in its cells. From the liver, again, the glycogen is gradually given out in the form of sugar in the intervals of digestion, as required for the needs of the organism. The nature and properties of the individual carbo-hydrates have already been described, but the leading characteristics of glycogen and sugar may be repeated. Glycogen is a polysaccha- ride corresponding in the animal to starch in vegetables, while dextrose and Isevulose are simple monosaccharides capable of diffusion and of easy solution. Carbo-hydrate taken in the form of dextrose or Isevulose may be immediately oxidised, yielding carbonic acid, or may be stored up as glycogen in the liver. We have no evidence that carbo-hydrate bodies can form proteids, and although a carbo-hydrate diet favours the deposition of fat in the body, it has not as yet been absolutely proved, though it is almost certain, that the one can be changed directly into the other. Carbo-hydrates may form one of the sources of the lactic acid found in muscles, which yields carbonic acid on further oxidation ; but this view has not been confirmed by recent observations. Whatever form the carbo- hydrates acquire before ultimate oxidation to carbonic acid and water, there is no doubt of the fact that the liver has, much to do with the regulation of their stay and distribution in the body. Thus the blood of the portal vein during digestion contains more sugar than that of the hepatic vein, especially after a meal rich in carbo-hydrates ; while, on the other hand, in the 302 THE NATURAL HISTORY OF DIGESTION. intervals between the active digestion of food, the blood of the hepatic vein contains more sugar — 2 parts per 1000 — than the blood in the portal vein, or than the general blood-stream, which contains on an average i to 1.5 per 1000. Minkowski found that sugar rapidly disappeared from the blood in the goose when the circulation through its liver had been arrested. The facts which go to prove that the sugar stored up in the liver is in the form of glycogen are as follows : — 1. Glycogen has been found in the livers of a large number of species of animals, including some invertebrates. 2. In mammals it is most abundant in those feeding princi- pally on carbo-hydrates. 3. It is found, however, in the livers of the carnivora and in those of the omnivora when feeding on flesh exclusively. 4. The quantity of glycogen in the liver gradually disappears in a fasting animal. 5. Fat alone does not give rise to an increase in the quantity of glycogen stored in the liver, nor can it arrest the disappearance of glycogen if no other food be given. 6. During early fcetal life glycogen can be recovered from all the tissues ; later on it becomes more and more localised in the liver, although even in adults it may be found elsewhere, as for instance in the muscles. If a rabbit be killed after a large meal chiefly consisting of carbo-hydrates, a meal of carrots for example, its liver rapidly excised, cut into small pieces, and thrown into boiling water which has been slightly acidulated, no sugar can be found in the extract filtered off from the coagulated proteids, even after the pieces of liver have been rubbed up in a mortar and boiled again. In this extract, however, glycogen is present in large quantity. Again, if the liver of an animal, killed some time previously, be excised, and treated as before, the watery extract contains very little glycogen but a proportionately large amount of sugar (dextrose). We may infer from these two experiments that boiling the Hver destroys the agent which con- verts the hepatic glycogen into dextrose. And further, that the agent may be an unformed ferment or the change be occasioned by a direct action of the liver cells. Boiling destroys the activity of both such agents. After a meal containing much carbo-hydrate, the liver cells may be actually seen under the microscope to be loaded with some substance which is either glycogen itself or a closely allied body. The liver is then METABOLISM IN ANIMALS. 303 large, soft, and friable, while after starvation it is small and firmer, and under the microscope the cells may be found to be shrunken. An interesting observation has been made upon the glycogen contained in the liver of the frog. In winter the frog hibernates and for months takes no food, and during this period the liver cells contain a considerable amount of a substance which responds to tests in the same way as glycogen, although in summer, when in full activity, little or none can be detected. At first sight this fact seems inconsistent with the theory that glycogen is stored up in the liver, to be gradually distributed to the body in the form of sugar in proportion to the needs of the organism. But the condition of the liver of a winter frog can be changed into that usually found in summer by merely warming the atmosphere in which it is placed. At 20 or 25° C. the glycogen soon disappears ; while on cooling a frog in sum- mer by artificial means, glycogen accumulates in that organ. The rationale of these phenomena appears to be similar to that which is thought to explain the storage by plants of their surplus starch. In the frog in winter metabolism is slow, and some substance akin to sugar may be produced in the body in greater quantity than can be used up. The surplus is stored in the liver as glycogen, just as it is reserved by plants during autumn in the form of starch, for use in the coming spring. If an animal be made to do severe muscular work while fasting, the glycogen rapidly disappears from its liver. If fasting, but kept at rest, the glycogen takes a longer time to leave the liver. In fasting animals there may not be a trace of glycogen left in the liver or the muscles, although there is still an abundance of fat in the body. From these facts we may argue that the glycogen stored in the liver and present in the muscles forms the readiest and most easily accessible source of energy for the organism. Carbo-hydrates can be utilised at once, fats require a longer time, while proteids are not utilised for some time further. As Stewart ingeniously puts it, glycogen resembles consols which can be realised at once, fats are like a good security but which requires some time for realisation, while tissue proteids are "long-date bills which can be only discounted sparingly and almost with a grudge." An interesting question arises in connection with this subject. Where is the seat of the lesion in diabetes mellitus? We have seen that the 304 THE NATURAL HISTORY OF DIGESTION. blood contains normally from i to 1.5 parts per thousand of sugar, much of which is used up during its circulation in the body, but a trace of which appears in the urine. One theory of the causation of diabetes mellitus .ascribes it to the inability of the liver to store up carbo-hydrates in the form of glycogen, or to its too rapid distribution of the glycogen as sugar. It has been shown experimentally that any increase over the normal proportion of sugar in the blood at once causes its appearance in the urine. If the liver fails to regulate the passage of sugar into the blood- stream the symptoms of diabetes must necessarily follow. Observations which have been lately made upon the occurrence of this disease after excision of the pancreas or following disease of that organ, have suggested to some the presence of a ferment produced by the pancreatic cells which is able to convert sugars into glycogen. If this ferment be not produced owing to removal or to disease of the pancreas, the sugar absorbed after digestion will pass straight into the general blood-stream and at once be excreted by the kidneys. It would be a mistake, however, to ascribe all the causes of diabetes to the liver alone. In all its functions the liver is influenced by the conditions of other parts of the body, especially of the nervous system, and, although it is certain that in many instances the liver has much to do with its causation, in others the disease may be due to very diverse causes. Although much of the hepatic glycogen is converted into sugar, we have no proof that the whole of it is so changed. Oil globules can often be seen in the liver-cells side by side with glycogen, and we know that fat may be formed from carbo-hydrates. Glycogen may then in part be a half-way house between sugar and fat. Proteids, however, are able to produce glycogen, so that as glycogen can be formed from proteid and fat from glycogen, glycogen may also act as an intermediary in the production of fat from proteid bodies. Pavy has sought to prove that the glycogen stored up in the liver is never reconverted into sugar in health, but becomes other substances, such as fat and proteid ; the sugar in the urine of diabetics arising from splittino- of proteid molecules. But although he has expanded this theory with a wealth of illustration and many ingenious arguments, he has few supporters, and the doctrine of Bernard, stated shortly above, is that accepted by most modern observers. The fate of the sugar circulating in the blood appears to consist in its oxidation during the metabolism of the tissues into carbonic acid, which passes off in the venous blood. Some of it may be transformed into fat, but only a small proportion. The recent results of Chauveau, from direct analysis of the blood before and after its passage through the muscles of the upper jaw of the horse during activity — i.e., while chewing serve to show that 3.5 times as much grape-sugar is oxidised by the active muscle as in the same muscle at rest. That is to say, that the amount of sugar in the blood coming from these METABOLISM IN ANIMALS. 305 muscles is less than in the blood supplied to them, while the decrease of grape-sugar corresponds with the loss of oxygen in the blood entering the muscle, and the increase of carbonic acid in the blood which has passed through it. So, also, more grape-sugar is destroyed in the parotid gland of the horse when active than in a passive condition. In the gland, how- ever, the proportion of sugar oxidised is far below that in a muscle of corresponding size. The fact that the diabetes caused in an animal by puncture of the medulla oblongata at or near the region of the vaso-motor centre does not occur if the animal has been starved beforehand, when little or no glycogen is stored up in the liver, or if the splanchnic nerves, or the spinal cord above the third or fourth dorsal' vertebra, be previously cut, shows : first, that the presence of an excess of carbo-hydrate is necessary, and, secondly, that nervous influences are involved. A substance called phloridzin, when given to an animal, produces diabetes, differing from that caused by puncture of the medulla, in that it can occur in an animal free from glycogen. In diabetes caused by this substance proteids are extensively destroyed. Another point which may be mentioned is that lasvulose may be entirely used up in the tissues of a diabetic patient and cause no increase of sugar in the renal excretion, while dextrose, which is so similar to it, appears at once in the urine. We may sum up the facts regarding the metabolism of carbo- hydrate substances in the body as follows : — 1. They are absorbed by the mucous membrane of the alimentary canal in the form of dextrose or grape-sugar. 2. The dextrose is carried by the portal vessels to the liver, where the excess not required to sustain the normal proportion of sugar in the blood is stored up in the liver cells in the form of glycogen. 3. In turn, part of the glycogen is re-converted into grape- sugar corresponding to the calls upon the blood for carbo- hydrate material by the tissues of the body, and is then dis- tributed to all parts of the organism. 4. In the tissues it serves as a source of energy and of heat, owing to its oxidation and conversion into water and carbonic acid. 5. The carbonic acid resulting from this decomposition is carried by the pigment of the corpuscles in the venous blood to the lungs, and there exhaled. 6. Some of the grape-sugar may also be converted into fat in the liver. 7. Carbo-hydrates are never a source of proteid bodies. 39 306 THE NATURAL HISTORY OF DIGESTION. The Metabolism of Fat.—T\\& fat absorbed through the mucous membrane of the intestine passes along the thoracic duct and is thus conveyed directly into the blood- stream without passing beforehand, like the carbo-hydrates, through the liver. Normal blood, however, contains only the merest traces of fat save during active digestion, so that fats must be rapidly removed from the circulation in some part or other of the body. As fat contains a large proportion of carbon, it is clear that it is capable of serving as a powerful source of energy. It was at one time thought that the fat absorbed from the food was stored up in the fat-cells of adipose tissue, just as glycogen is in the liver, and given off as required for the needs of the body. But it appears probable that most of the fat taken is rapidly burned up and oxidised into carbonic acid and water, although any excess may be stored up in the form of adipose tissue. The question of the fate of the fats in the body is rendered difficult by the fact that fat may be produced both from carbo-hydrates and from proteids in the body. A cow will produce more fat in her milk than can be accounted for by that contained in her food. A bitch fed on lean meat may gain weight owing to the deposition of fat in its body, and even gain in weight by the deposition of fat while suckling her young. Analysis of the fat contained in the food of pigs has shown that it may only bear the proportion of loo to each 472 parts of fat deposited in its body. So, again, bees can construct the wax of their cells from proteids, or use other substances for the purpose, such as sealing-wax. The fat of one animal differs in properties and in composition from that of another, and under ordinary circumstances retains its peculiar characteristics whatever the food may be or in whatever form fat itself may be given. The fat of a dog consists of a mixture of palmatin, olein, and stearin. If a dog be starved for some time and then given lean meat and a fat containing no stearin, the fat accumulated in the body is still composed of all the three principles. The stearin which must thus be formed may be either derived from the palmatin or olein in the food, or and this is more likely, may be formed directly from the proteids of the food or of the tissues. It has been found that in such animals as the porpoise the fat deposited underneath the skin and forming the blubber has a different melting-point from that situated around the internal organs. There is no doubt that the fat stored up in the body is not the result of METABOLISM IN ANIMALS. 307 the simple deposition of fats taken in with the food, but the outcome of complex chemical changes within the body. These changes result in the deposition of different fats characteristic of particular animals, or varying as to the part of the body in which they are located. Urea contains 46 per cent, of nitrogen and 20 per cent, of carbon. The dry proteids from which it is derived only con- tain on an average 15 per cent, of nitrogen and 50 per cent, of carbon. That is to say, when the nitrogen of proteids has been completely eliminated in the form of urea only one- seventh of the carbon is got rid of by the same means. The carbon which remains is, under certain circumstances, stored up as fat. To illustrate this point we may refer to an experiment of Pettenkofer and Voit on a dog excreting as much nitrogen as it was taking in its food. Two thousand grammes of lean meat were given each day. The nitrogen equilibrium remained constant, while 17 per cent, of the carbon of the food remained behind, representing about 58 grammes of fat. Again, as the administration of phosphorus causes the well-known fatty degeneration of many of the tissues of the body owing to decomposition of their proteids, so phloridzin hastens the disintegration of proteids, and, besides causing the appearance of sugar in the urine, produces an accumulation of fat within the cells. It is a fact of great practical importance that an animal during lactation requires more proteid in its food than is necessary for the ordinary proteid metabolism of the body. Common experience has already taught all races the truth of this statement. The increase called for is rendered neces- sary by the large quantity of proteid bodies broken up in the formation of the fat secreted in the milk. The possibility of the arrest of a nitrogen-free portion of the albumin molecule in the form of glycogen or of fat is of great importance for the economy of the organism. Glycogen and fat thus produced serve as albumin-sparing bodies. Fats also seem to be formed in the body from the carbo- hydrates present in it. It has been found that if proteids are added to a diet of carbo-hydrates in greater quantity than is necessary for nitrogenous equilibrium, fat is stored up. If the amount of proteid given be just sufficient to sustain the nitro- genous equilibrium, much less fat is produced. From this we can argue that the giving of carbo-hydrates for fattening purposes is based on the part taken by them in replacing 3o8 THE NATURAL HISTORY OF DIGESTION. proteids and fats in the ordinary metabolic processes, acting, in fact, as a more ready source of energy, and allowing body fat to be stored up from the other two food elements. Voit has denied that fat is ever formed directly from carbo- hydrates, but Rubner has shown that if a dog is fed on an almost purely carbo-hydrate diet, containing a little fat but no proteid, the carbon excreted is less than can be ascribed to the destruction of proteids in the body or to the fat of the food. Schulz's observations on the proportion of fat in the blood of normal and fasting animals are of great interest. According to his results, the blood of fasting animals — he used rabbits and pigeons — contains very much more fat than the blood of animals on a full diet. The excess in the rabbit may rise from 50 to S3 per cent., and in the pigeon from 30 to 100 per cent. This observation points to the abstraction during hunger of the elements of fat, or of fat itself, from the cells of the tissues and organs of the body rather than from the larger deposits of fatty tissue. Incidentally Schulz found that the percentage of fat in the blood of different animals varies to a remarkable extent. The blood of pigeons contains double the amount of fat present in the rabbit's blood. Kaufmann, one of the most recent observers on the origin of fat in the body, contradicts Voit's dictum that the proteids of the food are the sole source of the fat formed, and regards the other food-stuffs as capable of supplying the necessary elements. If during digestion a large quantity of proteid is absorbed, much of it breaks down rapidly and forms fat. He states that the fat arising from chemical changes in the food-stuffs is employed in three ways. A part is oxidised, forms glucose, and supplies energy by its further combustion. A part is oxidised, but less completely, forming glycogen, which is stored up in the body. The remainder is deposited in the tissues as fat. When the amount of glycogen in the body is large the fat produced from the food is chiefly burnt off, or remains as fat in the tissues. If the body is poor in glycogen nearly all the fat not burnt off at once may be transformed into that carbo-hydrate. During the decomposition of albumin in the absence of free oxygen, with the production of fat and other bodies, it is probable that no heat is produced. We may summarise the preceding paragraphs dealing with the metabolic processes in the body by stating that the three great groups of organic substances in the diet — proteids, carbo- hydrates, and fats — do not directly enter in an unaltered state into the composition of the tissues. The proteids may form in the organism similar proteid substances, and they may when undergoing destruction be so split up as to yield either carbo- hydrates or fats. The fats of the body may be derived, probably after intimate chemical decomposition and recon- struction, from the fat present in the food, or they may be formed in the body from proteids and from carbo-hydrates. The carbo-hydrates of the body may be derived from the METABOLISM IN ANIMALS. 3O9 carbo-hydrates of the food or may be split off from proteids. It is probable that they cannot be produced from fat. Neither fats nor carbo-hydrates can form proteid substances in the body, but act as proteid-sparers, shielding them from a too rapid metabolism. Perhaps we might suggest as an illustration of the use of these three classes of food-stuffs in the body the simple one of an ordinary fire in any of our dwelling- houses. The carbo-hydrates might be likened to the paper, which is readily consumed; the fats to the wood, which- requires a little longer and gives off more heat in combustion; while the coal might represent the proteid molecules, which con- tain a large store of potential energy, and are burned up more slowly than the others, but afford heat and energy for a much longer time. Such an illustration can only be applied to the physical aspect of the subject; chemically there is no relation- ship between the three fuels and the contrasted food-stuffs as regards the carbon available for combustion. Salts. — A certain quantity of inorganic salts is required for the proper maintenance of the bodily functions. Each day about 30 grammes of inorganic salts are necessary. The practice of taking common salt with the food is largely due to the desire for a condiment, as the food itself contains sufficient or almost sufficient for the bodily needs. Bunge has shown, however, that the proportion of potassium to the sodium salts in the food exercises some influence on the quantity of common salt required. In flesh, the proportion of potassium to sodium is exactly that required, but in most vegetables the proportion is much too great. This excess of potassium salts in vegetable foods explains - why vegetable-feeding animals and vegetarians wish for and require greater quantities of sodium chloride than meat-eaters. Should the salts be removed from the food as far as possible, and such salt-free food given to an animal, in a short time it becomes unhealthy, and in time, if salts are not added, death ensues. Organic salts appear also to be necessary for health, but to a much less extent. If the food for a long time contain no organic acids or salts, such as citric or tartaric acids, and the salts formed by these acids, the blood becomes changed in character and quality, while symptoms of scurvy may appear. One per cent, of common salt is sufficient to arrest gastric digestion, while one-half per cent, retards it markedly. The salts of the organic acids have a slightly greater retarding effect. Von Liebig enunciated the " law of the minimum " in relation 3IO THE NATURAL HISTORY OF DIGESTION. to vegetable metabolism. This law is to the effect that a plant is unable to make use of a soil of the richest nutritious value unless a certain minimum quantity of mineral salts is available. This law holds good also for the animal organism. An animal deprived of all mineral salts in its food dies as quickly as when starved. Lunin fed mice on ash-free cheese and cane-sugar, with distilled water. The mice died in from ii to 21 days. When common salt only was added to their food they lived for about the same length of time. Dried milk and distilled water sufficed to support mice indefinitely, but ash-free cheese and sugar mixed with inorganic salts in the same proportion to, and in the same relation with, the usual constituents of the ash of albumin, did not prevent death ensuing from the 20th to the 31st day. The remarkable result of the last experiment can only be explained by the supposition that inorganic salts in an absolutely pure condition are unable to take the place of the same salts when intimately combined with proteid in the pro- teid molecule. The probability of such a suggestion is rendered greater by the analogous contrasts between the pharmaco- logical actions of some natural and artificial drugs, apparently identical in composition, on the physiological processes of the body in health. It is a peculiar circumstance that post-mortem, no change, or one which is barely perceptible and doubtful, can be demon- strated in the proportion of mineral salts contained in the organs of an animal which has died from the effects of "salts- hunger," from that obtaining in the organs of an animal given a full allowance of salts. Lime-salts are indispensable. If withheld the bones become altered, and symptoms of rickets appear. The Ingestion and Metabolism of Iron. — Bunge found 0.047 grammes of iron per kilogramme in the body of a cat, and, calculating from this figure, the body of a man of 70 kilogrammes should contain .32 grammes. Stockman {Journ. of Pkys., 1895) has shown that the amount of iron ingested each day in an ordinary dietary only amounts to about 6 to 1 1 milligrammes dV to ^ grain), while the daily loss closely corresponds to these figures. As the intake is so small a reserve store of iron is rendered necessary for the organism to guard against emergencies, hasmorrhages, excessive destruction of red blood corpuscles from any cause, etc. This store is provided by the liver, whose cells rapidly take up any excess of the metal. Quite one-half the amount of an inorganic iron salt injected into the blood is detained by the hepatic cells. Zaleski and Vay showed that the iron arrested by the liver combined with various organic bodies in the cells, METABOLISM IN ANIMALS. 3II a part in a much looser state than others. A part combines witli the nucleo- albumins, and can only be separated from them by incineration ; another portion forms as it were a "floating" (Stockman) store, more easily separated from its compounds, and therefore more available for the needs of the organism . The iron set free by the breaking down of red blood corpuscles is in all probability stored up in the liver and not excreted, at least the greater part is thus detained. The actual iron metabolism must be very small, less than 10 milligrammes per day. Bunge has shown that artificial peptic digestion of the yolk of eggs yields an indigestible nuclein con- taining iron, all the other portions of the yolk being iron-free. As the yolk contains no hfemoglobin, and as during incubation no iron can enter from without, the iron-holding nuclein must afford the iron for the haemo- globin of the chick. This nuclein Tie calls hjematogen. Table LVIIL Haematogen. Hcemoglobiu in the Hen. Per cent. Per cent. C 42.11 c 51-15 H 6.08 H 6.76 N 14-73 N 17.94 31-05 23.425 S 0.55 S 0.389 P 5-19 Fe 0.336 Fe 0.29 If the molecular arrangement of these two substances be worked out, Hsematogen=C6j7 H 1174 N206 O374 PsaFe 83=19,310 H£emoglobin = Cn2 H-uso '^2U O245 Fe 82=16,710 in molecular weight. While if the phosphorus in the hsematogen be removed in the form of 32 molecules of phosphoric acid (H3PO4), the percentage of iron in the remainder closely corresponds to that found in heemoglobin. The figures obtained do not tally exactly, the weight of the remainder being 16,174 instead of 16,710, but the hsematogen analysed by Bunge was of doubtful purity. 8ubtracting again the molecule of heematin from one of hemoglobin, we obtain that of albumin. Haemoglobin C^a H1130 N214 Oajj Fe 8,= 16,710 Hasmatin - C32 H32 N4 Oj Fe = 592 Albumin Cjso Hms '^m O241 ^2 = i6,iiS From these data we can understand hov^ the organism has no need of much iron, important though the presence of the metal is for oxygenation in the blood and tissues. The iron absorbed is combined as hsmatogen or haemoglobin, the excess is to a certain extent laid up as hsematogen, the re- mainder is excreted. Frequently the faeces contain more iron than is represented by the food taken, owing to its excretion into the intestinal canal by the lining epithelium, which has been known for some time past to possess this power. Very httle is 312 THE NATURAL HISTORY OF DIGESTION. Id ti) tJ ■p a a c OH S2 c B OJ3 J3 u ! ) •X3 <4-. o y tfi iJ -O ^ CQ n1 a n1 U hn t... O }-• nt n1 W n oi w^ en o (U 4_, Pi 1) t> ^ .n a P, nt (i> r! O O Q ri p Q .c ^ (3 c QJ ^^. tuO &0 In W 4_l U pi u c 1-1 o id Hi d M % -- i>j CO .• / .•• 1^ z o / ..«■■ iS < / ( / ••■' m / m f £ a / t * / • 5 / / £ 1 r >- o o / •.. OI 1 • 1 7 *• r / f" o / •* 01 / i" CO /' • > / / / f S'^ J 1 / • lO / y * .' i' 1 ^y ••"'* ss ^ ? J3 U= % "^ J CO ^^ y 10 \ / -^ * r / / w -^ '0 V 00 t ■^ U) > ^ * ^ \ (VI •-^ ^ in > I < < o in E < o: < 12 314 THE NATURAL HISTORY OF DIGESTION. excreted in the bile. It is a remarkable fact, and one which has led to much controversy of late years, that although the necessary amount of iron for the maintenance of health is absorbed from organic compounds, when given for disease arising from a poor state of the blood relatively large doses of inorganic iron salts are followed by better results than the administration of iron in organic combinations. Why this should be is doubtful. Injection of non-irritating organic iron compounds into the blood of ansemic persons is followed by rapid improvement. Bunge has suggested that the inorganic salts combine with sulphuretted hydrogen in the intestine, and forming the insoluble sulphide, are not absorbed but allow the organic compounds a better chance of gaining an entrance into the blood unchanged. But Stockman has shown that the sulphide of iron itself causes an increase of haemoglobin in the blood of patients given it. An observation by Socin has an important bearing on the question. He found that mice not only survived but had gained in weight at the end of loo days during which they were fed on iron-free food. In the normal course of events only a small intake of iron is necessary both for vegetable and animal organisms; the quantity excreted if no iron be taken in the food is small; the reserve of the metal in the form of hamatogen is relatively large. States of anaemia, or poorness of blood, seem to arise either from long-continued insufKciency of the supply of iron salts, from constant, though often small, losses of blood, or from inability of the individual, owing to continued lack of iron in the food, to make use of organic iron supplied in small or moderate amounts, while able to benefit from the adminis- tration of large quantities of soluble inorganic compounds of that metal. Metabolism during Statvation. — During starvation the out- put is greater than the intake. If the carbon and nitrogen excreted by a starving animal be estimated day by day dimin- ishing amounts of each element can be recovered from the excretions as time advances. As there is no intake, the loss of each element continues to be proportional throughout the period of starvation to the available store in the body, until a time is reached when the processes necessary for the main- tenance of life can no longer be supported by the elements left, and the death of the animal ensues. METABOLISM IN ANIMALS. 31S A starving animal fulfils one of the common laws of nature, it makes the most of the material at hand. It spins out the inevitable loss as far as possible, indeed those substances which are absolutely essential for the preservation of the vital spark, or for the continuance of the action of such necessary organs as the heart and central nervous system, are only used up when the supply from other organs has almost entirely failed. Fats and any store of glycogen are first used up, along with part of the proteid, until, when from a quarter to a half of the total body weight has been lost, the machine stops for want of motive power. Chart XIV., representing the actual loss in grammes of body weight and of carbon and urea in a fasting cat, as esti- mated by Bidder and Schmidt, shows the daily variations in the values of each factor. The amount of urea does not diminish during the first twenty-four hours, owing to excretion of the nitrogen contained in the food last taken. Indeed, if the animal be of lean habit the proportion of urea excreted rises during the first few days because of the increased consumption of proteid in the absence of fat (Chart XV.). When deposi- tions of fat are present the daily loss of fat is nearly constant, but however much be stored up in the tissues some proteid waste must occur to allow the vital processes to continue. The fat only serves as a proteid-sparer economising the neces- sary use of the nitrogenous substances. As has already been mentioned, the loss is not borne by all the organs aUke. Generally stated, we may say that the less important an organ or tissue is for the preservation of hfe the greater is its waste during starvation. Diagrams IV. and V. have been constructed from data given by Voit as to the loss of weight in the individual organs and tissues after death from starvation. The proportion which the loss in each bears to the weight of a similar animal killed when in good condition is also included in it. From these figures it appears that although the waste of the muscular tissue of the body forms the largest part of the total loss of weight, it only reaches 3 1 per cent, of the total muscular substance, while, though the actual proportion of weight lost is less, 97 per cent, of the fat present is used up. The other organs which lose much of their weight are the spleen with 67 per cent., the liver with 54 per cent, and the testes with 40 per cent. The central nervous tissue and the heart only 3i6 DIAGRAM IV.— Showing the Loss of Weight of the various Organs after Death from Starvation. 7o !• — Percentage of Total Loss borne by each Organ. SO 20- '£ M '^ 5 C3 3 ; ; : « ; ; \n 00 VO O vo lo \r\ 'I- CO w d d 1-1 PL, E-H ., Oh -co ; 317 DIAGRAM v.— Showing the Loss of Weight of the various Organs after Death from Starvation. 7(, 11. — Percentage Loss in each Organ. lOO — 80 60 40 20 3i8 THE NATURAL HISTORY OF DIGESTION. -s •* N On ,o grms. Fle rms. Fat. 1 66 grms. grms. Fl rms. Fat, 78 grms oss of fle Dss of fat, t. oss of fle grms. of 59 grms 1 1 + + II II II II .2 a II -S-M .---irg c , , ^ 'rt " " " grms. N "1 _ grms. C J ~ 4 grms. C=i! Loss in weig grms. N. N grms. C. N Weight cons grms. N. N 4 grms. C, or Gain in weig cu 8 * N " Tf ^o fn ^ B S S2 "'on si c II II II II m M i-t M 00 --< 00 ■* ri :: :: " 1 1 1 1 1 1 +1+1 +1 + 33^ '■£3 PH a,o\ s p ; : : h M N I^ ON OJ cn ■^ VO 00 -G ^ 4- rt 3 ^ M u u^ H -§ CJ CD S S S ■a ^ :; : " o 0\ LT) ro -o r^ 3 s > CJ g^" M '3 S oT ^ 0000 d. o r^ m O " ^ r* t^ ^ 'r- LO M o ^ >-< N M i° 1-t M U-) 3 II 11 II II o « m r^ J o 3.5 1:: ^ - :: r c4 ^vo E " " " P '"' .« s . _ „ s - - - 1'^ vO u-i .0 rt o O I-" ^^ f^ '^ ^ f CD ^ 0000 o ^O CD \.o >-' n M "^ 1 . 1 1^ o -2 1 2 c . . . • © (U ^ to .14 rt a J ; cC . "" -w c-i i- *G 1— 1 bj] ai c -d 00 'S ^ ^ " o o JJi 00 o T - 2 8 ri * « 0000 OJ ^ „ lJ-1 ^ i-i u-1 f-H -^ ri >^ h-;t:ri> n ■"• (-( i-i P "mm ^ — ^5 -^ S « OJ ^ g ■*-!'''-' •5 c -n '^ ^ (J 'O-^ ^ a g J3 fill u II u mi § a'^s § "'0 §" c « c|^ -g II ■0.5 a u C) §§.25 Hit ).. c-5 est! 11 ;2i ■^-i! MJ3 "S-o s; ^§t" r-e.S3 (C " >- ° a Oh f^d-aS ■5 J1 ?i CHART XVI.— Showing the Loss in Flesh and Fat of a Dog of 10 Kilogrammes, fed on Lean Meat alone, in increasing quantities. (After Voit.) Loss of Grammes of Lean Mrat per Diem. Nitrogen _ooo o o o o o o o ' .a _OOiOOOOOOOO O t 11^ Oml^^-^(^lC0■^lOCOI^^00 0ll Grammes. I I I I I I I I I I I L I I I I I I I I I | | | | Grammes. Loss of Carbon 35 33 31 29 27 25 23 21 19 17 15 13 II 9 7 5 ' " "lA " ±. .._ _^r_ ji i'// ^^ .. ^1 __ _ _.. " " _.' " ^^^''" "~ __.<-/ 4^ AS"// ■¥ ^^ -^^^^ iS* ,(ir •*^ ^\r ^- ^ _.^^^?^"" fy- " . T'" ' ' 7-^-1 ■'^7 ^^^ ^"^$'1 A & ^ aW >ft-H CO ® a 550 s^ .■I lasa^ ■^ .« M -tJ H) o S Qi a5 at o E- tn o rt 5 ^ ©^ rt ©^ gj .3 s -35 ^3 )-'-' ■^-*^ c" WJC crt •03 H s H tie's ?-«5 3 gs coWMHfe S ►:] ft > O !il C! c.Sc.S 1 3 cj 3-3 > 03 =5 ' O © c goM^Otc P4 5oW = 324 THE NATURAL HISTORY OF DIGESTION. must be recorded and illustrative examples from the lower animals incidentally alluded to; but the subject of diet in animals is discussed in its relation to the general laws laid down for man in the sequel. (Table LX.) In considering the normal diet of man most observers have calculated the necessary proportions of food-elements required to maintain bodily equihbrium in an adult of 70 kilogrammes — 11 stones or 154 lb. — in weight. The average discharge of carbon and nitrogen from a man of this weight is 230 grammes and 15 grammes daily, provided he be healthy and performing little bodily labour; m addition 2250 c.c. of water and 24.0 grammes (Voit), and 30 grammes (Moleschott), or 25 grammes (Ranke) of inorganic salts are excreted. For the purpose of oxidising the carbon taken in in the form of food-stuffs, corresponding to the output given above, 85 litres of oxygen will be required. Theoretically, while a man of 70 kilogrammes remains in health and of constant weight, his diet need only contain the same number of grammes of carbon and nitrogen. But such a diet would leave no margin between the intake and output, and the normal diet should be calculated to contain 20 grammes of nitrogen and 300 grammes of carbon. If with this increased diet the weight still remains constant, the output rises in direct proportion ; while, as the smaller amounts are sufficient to maintain the body-weight, the excess taken in is excreted practically as a waste product. If, however, any increase in the amounts of carbon and nitrogen over those contained in the minimum diet capable of maintaining a constant weight be given, and less carbon and nitrogen excreted than taken in, the body-weight will be found to be greater ; and, conversely, should more carbon and nitrogen be given off than taken in in the food, the body-weight falls. The diminished output of carbon signifies an increase in the bodily fat, an increased output a loss of fat from the body. When less nitrogen is excreted than is taken in, the difference remains in the body as proteid, that is, flesh; when more nitrogen is lost than is taken, the body loses flesh. Knowing the quantities requisite, it is easy to calculate the amounts of actual food-stuffs which are able to furnish the nitrogen, carbon, salts, and water for the bodily metabolism. The following table represents the proportion of nitrogen, carbon, hydrogen, and oxygen in proteids, carbo-hydrates, and fats when free from water and salts : — DIETETICS AND ANIMAL HEAT. 325 Table LXI. — Composition of Food Elements. ft ft 5 m '3 ^ d £ a h g"^ o'bjoaj □,-Ig C ^ C -w a> •A '^ ^ tn (BOO r 1 Pi tq c = II <«■" <+^ Pioteid 15-16 50-53 • .. 7 •• . 21-22 140 grm.. . . 600 grm . . .1 to 3.3 or 3-53 Fat — .., ■ 77 . .. 12 .. . II .. — ■■ SSgg""-- Carbo-hydrate — •• • 44-4 ..6.1 .. •49-S- . — .. 680 grm.. • — The proper proportion between the nitrogen and the carbon in the food is 20 : 300, or 1:15. The figures given in the table show that a diet composed solely of proteids requires 140 grammes per diem to afford the necessary 20 grammes of nitrogen, while this amount would only yield 70 grammes towards the requisite 300 grammes of carbon. Rather more than four times that amount is necessary to provide 300 grammes of carbon, and therefore the necessary number of grammes of carbon can only be supplied in the form of pure proteid along with 90 grammes of nitrogen, or 4.5 times that which is wanted. Neither fats nor carbo-hydrates contain nitrogen, while 389 grammes of fat and 680 grammes of carbo- hydrates can supply the 300 grammes of carbon. But few people either care to live on a physiologically correct diet, or can obtain the proper articles of food required to form it. Thus the Chinese labourer has to consume normally 2000 grammes of rice to obtain 20 grammes of nitrogen, and at the same time takes in 700 grammes of carbon, or twice as much as he requires. More work is, therefore, laid upon the tissues engaged in their metabolism than should be physio- logically. The Irish peasant is situated in even a worse plight : to obtain his daily 20 grammes of nitrogen he has to consume 5 kilogrammes of potatoes, which contain nearly double the requisite 300 grammes of carbon. On the other hand, a man living on flesh alone would require 2400 grammes of meat to yield the necessary amount of carbon, although 600 grammes would be sufficient to yield the nitrogen required. Oatmeal and wheaten flour contain nitrogen and carbon in almost the normal proportions, i to 15; oatmeal contains a little more nitrogen than wheat flour. (Table LXII.) Remaining Unaccounted for. 1 Oxygen In air. +709 Excess. +8S.8 4.3 Oxygen in water given. +1718.8 Deficit. -626.2 Addition to body equal to Fat. Water. CO GS 1 g CO CO 4.18 77.12 41.8 86.76 Oxygen deficit or surplus. +9.6 -2353.0 -2343.6 Oxygen present infoodandavail- able (in proteid). 22.21 184.3 206.61 o Differ- ence from intake. CS 1 8 CO -81.3 +164.44 +0.1 1 H Quantity of Oxygen necessary to oxidise the elements. 13.71 1873.281 664.02 2537.3 - J 2661.01 k 1 g 1 1 i CO CO o d 3 3^ ID ?H rHOC 1 -100 Skin. 0.12 CO eo 0.93 asCOo 442.4 water. 443.33 497.7 5.3 So ja.2 0^ 0.46 1.5 1.96 Lung. 36.8 36.8 sis 00 mco 683.09 asC02 294.4 water. 957.49 331.2 1 Skin. 0.12 56.3 0.35 448.33 5.3 604.69 Kidney. 17.36 2.76 solids. 142.06 water. 144.81 13.71 solids. 1136.48 water. 1150.19 1278.64 12.8 Lung. 36.8 248.66 967.49 1241.96 &H K Number of grammes of Elements Total elements available, of the Weight of 1 2 3 Food ingested. Foods. In food. Excreted Total in faeces available Nitrogen — unused, in body. Proteids - 140grms. 19.47 2.0 17.47 HVDROGEN— Proteid - - 140 „ 9.8~l Fat - - 100 „ 12.0 } 2.17 Carbo-hydrate 350 „ 21.6J Water - - 2016.3 „ 224.0 9.2 TOTAL - 267.4 11.37 256.03 Carbon — Proteid - - 140 „ 75.0-\ Fat - - 100 „ 79.0 U4.3 Oarbo-hyilrate 350 „ I66.4J TOTAL ■ 309.4 14.3 296.1 Oxygen — 29.4-1 7.19 11.6 ■ 172.8J 1792.0 73.7 0' expired, res 44662.0 43963 46667.7 44033.89 2633.81 2016.3 82.9 1933.4 23.9 6.9 18.0 Kidney. 17.35 144.81 12.6 1160.19 12.8 1337.75 Absorbed. 17.47 250.03 295.1 2633.81 18.0 3220.41 Unused. Fjeces and air. 2.0 11.37 14.3 44033.89 5.9 44067.46 Proteid - - 140 Fat . - 100 Carbo-hydrate 360 ^Vater - - 2016.3 Air - . - 16000 lit TOTAL - Water— ■■JALTS— Totals— Taken in Nitrogen - 19.47 Hydrogen - 267.4 Carbon - 309.4 O.iygen - 46667.7 Salts - - 23.9 Total - 47287.87 DIETETICS AND ANIMAL HEAT. 327 A physiologically correct diet might be formed of 140 grammes of proteid, and either 515 grammes of starch, or 300 grammes of fat; but experience and experiment have shown that, for civilised man at least, a mixture of the two is conducive to, indeed is necessary for, health. Some races of men subsist for long periods almost entirely on flesh without apparent detriment to their nutrition ; others, the Eskimo for instance, on meat and fat, and dogs remain in health on a diet of proteids and fats. To cite a well- known modern example, Nansen and his companion suffered little inconvenience from an enforced exclusion of carbo- hydrates from their food during the course of many months, or rather may be said to have benefited by it, as they put on weight while on a diet of meat and fat, notwithstanding the arduous nature of their enterprise. As a rule, however, it is more conducive to health if the carbon necessary, over and above that contained in the nitrogenous elements, be supplied partly as fat and partly as carbo-hydrate in the food. Should our man of normal weight be doing no work, he should be given at least 50 grammes of fat, or 37.5 grammes of carbon; but twice this quantity would be more beneficial, especially if he be engaged in moderate labour. One hundred grammes of fat yield about 75 grammes of carbon, and added to the 70 grammes contained in 140 grammes of proteid, leave 155 grammes of carbon to be taken in the form of a carbo-hydrate. This amount can be obtained from 350 grammes of starch. To these must be added 30 grammes of inorganic salts, and 2.25 to 2.75 litres of water. Comparing these figures with the body weight, we find the following : — Table LXIII. — Comparison of the necessary amounts of the Food Elements with tJie Weight of the Body. Food Element. Amount in Grammes. Proportion to Body Weigtit, I. Proteids 140 ... 0.2 % or 3-^^^ z. Fat 3. Carbo-hydrates 4. Salts ... 5. Water 100 350 30 2500 0-5 7oOr -^TS 0.0427^ or ^1^ 3.6 7„ or 5V To put it in another way, a sufficient diet for man will contain 2 grammes of proteid, 1.4 gramme of fat, 5 grammes of carbo-hydrate, and 0.4 gramme of salt to each kilogramme 328 THE NATURAL HISTORY OF DIGESTION. of body weight, or nearly i per cent, of solid food, in which nitrogen bears the proportion to carbon of i to 1 5. A cardinal point in all dietaries consists in the fact that the bbdily processes cannot normally go on without the ingestion of a certain quantity of nitrogenous food. The proportion of fat and carbo-hydrates may be varied very considerably. Man can exist on a spare diet, provided he is not engaged on severe manual labour. An emaciated body can do little work, and requires less proteid. Playfair found, thirty years ago, that a London needlewoman lived on a daily regimen con- taining 54 grammes of proteid, 29 grammes of fat, and 292 grammes of carbo-hydrates. This must surely be the absolute minimum on which hfe can be supported. Indeed, Voit remarks that the Trappist monk, who practises the severest form of asceticism, consumes each day 38 grammes of proteid, 1 1 grammes of fat, and 469 grammes of carbo-hydrates. Young animals, which have not reached their full growth or maturity, require more food than their weight would suggest. Their income must exceed their outgo to allow them to grow. Again, the outgo from the body is much more nearly proportional to the superficies than to the mass. As a rule, the cube of the surface of an animal varies directly as the square of the mass; when the weight becomes double, the surface therefore is only one and a half times greater than before. A boy of from six to nine years, and weighing. 18 to 24 kilogrammes, presents an extent of skin two-fifths to one-half that of the adult weighing 70 kilogrammes. He requires about as much as one- half of the food of an adult. So Voit states than an infant of four months, weighing 5.3 kilogrammes, consumes in its daily food .6 gramme of nitrogen for each kilogramme of its body weight, compared with only .275 gramme of nitrogen per kilo- gramme in an adult of 71 kilogrammes. The Requisite Amounts of Oxygen and Hydrogen. — As has already been stated in the consideration of general metabolism, the nitrogen leaves the body chiefly in the form of urea, or as uric acid in some animals, while the carbon is oxidised and passes off mainly as carbonic acid. The amount of oxygen taken in by the lungs is greater, however, than that which reappears in the form of carbonic acid. Some of it goes to oxidise other elements, and especially to act on hydrogen with the formation of water. (Diagram VI. — The amount of each element passed in the faeces is represented by diagonal shading, and in the DIAGRAM VI. — Showing the actual amounts of Oxygen contained in the Three Principal Food Elements, and the amounts necessary for their full oxidation. The food contains 140 grammes of Proteid, 100 grammes of Fat, and 350 grammes of Carbo-hydrate material, and serves to maintain nitrogenous equilibrium. (For explanation see text, foot of page 328.) C = Carbon. H = Hydi-Oien. N=Nitrogen. = Oxygen. S = Sulphur. ■a s a So rt a 'V S to Jroo rt Fat, CT3 ( 'itrbo- s Protoid, 100 Clj.^ hydiates, ti,'3 '5 s 140 grms. ue. 350 K'^ !B a s" n grammes. CO 0^ grins. 0^ ft p ^ M P "rt ft fl S C H N S -!j C H < C H < S S 0] S « 3 6S - - - - -- J__ -- -- - - - '^ 600 - - -| 55° - - ^i)t - - Coo 450 — - i7e 9 - - - 350 — - — — ^ - - z :I r 1 — - - 260 " - - -^ 240 220 - - -_ -- i: -: :; 7 1 - - : - -- -- -- - - -l 6i ) - - 1 1 r - 1 80 " - ^- -- -- ~ - i 160 - % «j-. ■ 1 1 i '1 — - -- ^'^ pi _! ] g - - 1 ■*! - t is '1 - 1 1 i _ - - ! -ir \ 1 1 rj 100 316 • 11- i- i - 1 1 !- - 1 ; ! - "i 80 - - p. 69 - -" 1 - 60 i B„ - 1 ff - - - - M rrl s i '-ip 40 H M ■^ _ : i - H 507 -!i i- r r r ■J 1 J!- 1^ - F 1 20 i 'R s " "' w lip^'.-,. i i ■!i 1 - S^ i^ i ",i J- 1 1 |S goo 800 700 -E 600 500 400 300 C H N S 330 THE NATURAL HISTORY OF DIGESTION. urine by horizontal lines. The actual quantities oxidised in the body are given in vertical lines. The oxygen available in each class of food for oxidation is finally repeated in solid black, with the additional oxygen needed for complete oxidation of all the elements in white columns. The last three columns are drawn to a reduced scale, and denote in a similar manner the total oxygen required, the oxygen obtained from the inspired air, and the amount of oxygen from the food used up. The figures inserted in the body of the diagram indicate the amounts of oxygen required to oxidise the hydrogen and carbon of the food separately. Added together, minus the oxygen in the food, the sum corresponds to the scale. The figures throughout are in grammes.) > A more minute study of the question reveals several impor- tant facts. In the first place, the carbo-hydrates are bodies which contain the two elements hydrogen and oxygen in the proper proportion for the formation of water, and these elements may unite in the form of water in the body, leaving the carbon to unite with the oxygen in the blood. The changes here cannot account for any of the oxygen deficit. Secondly, the hydrogen of the fats is much more than can be completely oxidised by their oxygen. Hydrogen requires eight times its weight of oxygen to form water, while fat only contains about weight for weight of the two elements (r 2 : ii), so that loo grammes of fat require about 80 grammes of oxygen to be sup- plied before all the hydrogen can be completely oxidised. In proteids, again, there are 15 grammes of nitrogen, 7 grammes of hydrogen, and 21 grammes of oxygen in each 100 grammes. Thirty-three grammes of urea correspond to 100 grammes of dry proteid, and contain 15 grammes of nitrogen; but the hydrogen in this quantity of urea is only about 2 grammes, the oxygen about 9 grammes; .5 grammes of hydrogen and 12 grammes of oxygen remain unaccounted for. Bearing in mind the proportion of oxygen required to oxidise hydrogen, it is clear that 40 grammes of oxygen are necessary to form water, of which only 12 grammes can be obtained from the proteid, leaving 28 grammes to be provided by the oxygen taken in in respiration. (Table LXIV.) Alimentation with the antipeptone obtained from digestion of glandular tissues, especially that from the auto-digestion of the pancreas, has been found by EUinger {Zeit. f. Biol., xxxiii., p. 190) to be of less value in the preservation of nutrition than DIETETICS AND ANIMAL HEAT. 33 1 an equivalent ingestion of albumin. The epithelium of the alimentary mucous membrane appears to be unable to trans- form all the peptone introduced into albumin during its absorption, and the unconverted part of the peptone acts in the blood as a toxic agent. Food elements taken in a less soluble form, or even when insoluble in water, are of greater nutritive value than the rapidly soluble substances. Animal Heat. — The higher animals — mammals and birds — have, with a few exceptions, a practically constant internal body temperature. Such animals are termed homoiothermal. A few hibernating mammals, the marmot for instance, have a constant body temperature during the summer, but a varying temperature in winter during hibernation, when the temperature follows closely that of the environment. When the body tempera- ture varies in this manner the animal is termed poiliilothermal. In the lower forms the body temperature always varies with the surrounding degree of warmth, but is usually slightly above it. In a healthy man the internal body temperature remains constant at about 98° Fahr. (38° Cent.) under all conditions, and in a frog it may be 30° Cent, in summer and only 5° Cent, in January. In both the man and the frog, however, heat production is constantly going on, although, in relation to each unit of weight, the amount produced by the frog is much less than in the case of the man, and more easily lost. The tissues of the lower animals are unaffected by changes in the internal body temperature which would suffice to disorganise the tissues of the higher forms. It is impossible to enter into all the details concerning animal heat in this book; only that part of the subject germane to the discussion of bodily metabolism will be described — that is to say, the role which the products of digestion play in the production of heat. All the heat produced in the body arises from the combustion of the food taken in. Relatively small amounts of heat enter the body with the food, by radiation from the sun or from fires. The heat arising from the food is formed by true combustion, the carbon in it is used up exactly as it is in a fire with the production of carbonic acid and water. The combustion is slower, that is all. Perhaps it may be said that combustion in the body is more thorough than in an ordinary fire, and more akin to the processes which take place when gas is burnt in, a Bunsen burner. As has been stated in the section on metabolism and diet, the combustion or oxida- 332 THE NATURAL HISTORY OF DIGESTION. I 8 f^ •Is ^ Co > l-H X w PQ H ^ O M .H CO O o in < cr4^ d ' r-^ ci ' 01 1 CO OD CO O f-< -t- oino tCJ^OOrH CO r-i COCO 1 ? O -Jf CO OS t-OD -* GO CO CO o^^.'^.l 00 tJO ai t^i rH . oo o o CO M C-l o CO CO rH o iH iH (N 1 -t= S= o rf fi o I' — < aj " 00 CO ll r-" iH ^j C! iJ ^ ?2 1 1 I r-l in 1 CO S a :: - : -^ 1 in 1 11 rA c4 r-* H iH fcM S H _., =ot-oi M t., fl 53 03 S" lO OJ C31 -rH 30 s o o "^ 1 in rf S 1 ^ 1 uri ^ ' CO >1 ' rH ' i-i C^ o t-r -< oi J;.^ ^ . . . . O o I S -,CJ1 CO ojd j>; S 8 ^ir- CO cd »« oj r^ t-Oi CO rt* O -* iH ^K 6 C cai>i t- o 'JtO s ri CO d jp 1 l-i J^ ^ 03 in 'Sii H d o 7^ a> O. ■^ '-' 1 a ^1 W3 u w m si in iH N Oi lr~ in(M_ [^ O TO c4 in t-^CJ -'d< o CO rHrHCN t- CIO « E 01^- t-( lO I- CO Ti IH iH M 1-1 05 S a c3 rt M 4J M §g '* CM a fcO c -^ c «S3oO a "> O) CD [M in 00 ce -y oitN^CO ^ rt t3 coo cow )-3 H ^ ^ (N(N eS oCJBd 1? ^ ^ '- >> ^CC >q to o iJ o O o pi 1 1 DIETETICS AND ANIMAL HEAT. 333 tion of the carbon and hydrogen in carbo-hydrates and fats is complete in the body. The nitrogenous derivatives of the proteids, however, can be made use of in the production of heat by further oxidation, and any calculation of the heat-producing power of food substances must include the amount yielded by them. Before treating of the power of heat-production of food substances it will be necessary to explain the standard scale which is used to denote it. The unit of heat, termed the calorie,^ is the amount of heat required to raise i gramme or cubic centimetre of water 1° Centigrade. Thus 20 calories represent the amount of heat necessary for increasing the heat of 20 c. cm. of water by 1°, of 10 c. cm. of water 2°, or of 2 c. cm. of water 10°, etc. The amount of water in grammes or cubic centimetres when multiplied by the increase of heat in degrees gives the corresponding number of heat-units or calories used in raising its temperature. Further terms are employed by physiologists, one to express temperature changes in large bodies, a second in recording minute variations. The amount of heat requisite to raise 1000 grammes of water 1° is called a kilo-calorie, and that requisite to raise i milligramme of water 1° a milli-calorie. If the temperature of i kilogramme of water be the same as that of another portion only measuring half a kilogramme, the number of heat-units present in the first portion is double the number in the second. An animal of 50 kilogrammes possesses twice as much heat as one of 25 kilogrammes whose body temperature is the same. Heat-production. — Experiments on men enclosed in a chamber, or calorimeter, designed for the direct estimation of the heat given off, show that during sleep 40 kilocalories are produced; while at rest during the daytime 100 kilocalories are formed, rising to 150 kilocalories with moderate, and to 250 or 300 with active exercise. The average heat-production in the twenty-four hours by a man of ordinary weight is about 105 kilocalories per hour, or 2500 kilocalories altogether. Of this, 80 per cent, is lost by the skin, made up of 15 per cent, due to evaporation of water, 30 per cent, due to radiation, and 35 per cent, to conduction and convection; 17.5 per cent, is lost by the lungs, 15 per cent, in evaporation of water, 1 Calorie is used throughout in this sense; great calories or kilocalories where advisable for brevity. Thousands and millions are marked off with commas, fractions with dots. 334 THE NATURAL HISTORY OF DIGESTION. and 2.5 per cent, in heating the expired air; 2.5 per cent, is lost with the excreta. Nebelthau found that in the rabbit 16 per cent, of the total loss of heat arose from the evaporation of water. Table ISKV. — Calories in the Food of an Adult per diem. Estimated by Actual Combustion. Playfair 3,133,000 calories. Moleschott 3,159,000 ,, Wolff - ■ 3,031,000 Voit 3,055,000 ,, Mean 3,094,000 calories. Table LXVI. — -Calories of Food absorbed, those for the parts passed in the faces, and in a not fully oxidised form in the urine, subtracted. Rubner - 3,094,000 - (8. 11 per cent.) 251,000 = 2,843,000. If we consider, in relation to the body- weight, the equivalent heat-calories contained in the standard physiological diet for a man of 70 kilogrammes, which has been already stated to consist of 140 grammes proteid, 100 grammes fat, and 350 grammes carbo-hydrates, we find that it affords 2,878,500 calories, or 41,121 calories per kilogramme. This figure is arrived at as follows. By direct experiment Stohmann found that I gramme of dry egg albumin when burned in a calori- meter yields 5,735 calories of heat, a gramme of grape-sugar 3,742, and a gramme of animal fat 9,500 calories. But from the total heat obtainable from a gramme of proteid the equivalent value of the constituents of the urea derived from it and excreted in a form which does not represent full combustion must be subtracted, along with the small quantity which is passed through the alimentary tract and voided in the fjeces (Rubner). Table LXVI I. — Heat Equivalent of Food Elements. One gramme of albumin 5,735 calories One gramme of albumin minus urea and the portion passed in the feces 4>420 „ One gramme of fat 9,500 ,, One gramme of glucose 3>742 ,, If reckoned as starch 4,182 ,, ,, cane-sugar 3.955 ,, DIETETICS AND ANIMAL HEAT. 33S Table LXVIII. — The Heat Equivakni of the Specimen Diet. 140 grammes of proteid- x 4,420=618,800 calories. 100 „ fat X 9,500 = 950,000 ,, 350 ,, carbo-hydrate (as glucose) X 3,742 = 1,309,700 ,, 590 ,, total diet 2,878,500 ,, Body-weight, 70 kilogrammes ; csXoxies, per Jiilogramme, 41,121. If the heat-equivalent of the 350 grammes of carbo-hydrate bodies be reckoned as starch the total calories yielded amount to 3,031,800, or 43,311 per kilogramme. A sufficient diet for a man doing a fair day's work should therefore yield about 40 kilocalories of heat for each kilogramme of his body- weight. To separate the proportion of the energy afforded by this diet into the heat-equivalent of mechanical work performed and the actual amount of heat generated, the heat-equivalent of the mechanical work must be estimated and deducted. As a general rule it may be said that one-fifth of the total energy evolved is expended on mechanical work, and four-fifths appears as heat. Deducting one-fifth from the total heat-equivalent, 2,878,500, we obtain 2,302,800 calories as the sum of the heat produced, and 575,700 calories as the heat-equivalent of the work done. Or if a day's work be considered as equivalent to 150,000 kilogramme-metres — i.e., the raising of 150,000 kilogrammes one metre — -and each kilocalorie as equivalent to 424 kilogramme-metres of work (Joule), the day's work can be represented in terms of heat as 150,000 -12°/, or 353,000 calories. Deducting this from the total heat- equivalent, we get 2,525,500 calories as the amount of actual heat-production. Both these results correspond fairly well with the calculated heat-loss of the body. Vierordt gives the following figures for the daily loss of heat in man ; — Table LXIX.— Z^m of Heat. Calories. 1,822,500 Evaporation of water Ip. 87.5 per cent. 364,000 Radiation J ' ^ ' 182,000 Evaporation' of water i „ , 84:500 Warming e.'cpired air /^■■°"''l""g= '°■^ " " 47,500 Warming urine and faeces 1.8 ,, ,, 2,500,500 Total heat-loss of the body. Probably one-sixth of the total energy available is the pro- 336 THE NATURAL HISTORY OF DIGESTION. portion to be assigned to the production of mechanical work rather than one-fifth. The production and loss of heat in the body varies in close relationship to the state of the body. If the day be divided into three periods of eight hours, one for work, one for rest, and one for sleep, we find that dur- ing the working hours as many as 216,960 calories of heat may be produced in each, 140,000 during each of the resting hours, and only 40,000 during each hour of sleep. The total of these figures amounts to 3,175,680 calories, a sum rather above that afforded by the specimen diet, but the proportion between the figures for the three periods applies equally to both. Thus we find that only about 10 per cent, of the total heat-production occurs during the eight hours of sleep, about 35 per cent, in the eight hours of rest, and 55 per cent, during the eight hours of work. Careful observations by Ott, by D'Arsonval, and by Rubner tend to show that most adults in health produce from 2,000,000 to 3,000,000 calories in twenty-four hours when resting or only engaging in light work ; when performing a hard day's labour the heat-equivalent rises to a figure between 3,000,000 and 4,000,000. In Diagram VII. the heat-equivalent of the diets taken by various classes of men, as calculated by different observers, is graphically shown. Rubner has calculated that, given a standard weight of 67 kilogrammes, a fasting man produces per diem 2,303,000 calories, biainworkers only a little more, 2,445,000 calories, while soldiers in times of peace and day-labourers (probably non-energetic types) produce 2,868,000 calories. Skilled labourers manufacture 3,362,000, and miners 4,790,000 calories. A further class attains a heat-production of 5,360,000 calories, and is represented by lumberers and out-of-door labourers, especially when they have to face intense cold during the execution of their work. It may be stated as a general rule that the food, to be sufficient for the proper production of the energ}', whether it be transformed into mechanical work or into heat, should under ordinary circumstances be equal to from 35 to 40 kilo- calories per kilogramme of body-weight. The Seat of Heat-production. — A few words may be added with regard to the tissues in which the oxidation of the calorific elements, derived from the food and conveyed to them by the blood, chiefly takes place. When a muscle con- tracts, a certain amount of chemical change, chiefly consisting in oxidation, precedes the active movement. The heat-equi- valent, however, of this chemical reaction is much greater than the heat-equivalent of the work done. At least four-fifths of the energy or heat produced takes no part in the mechanical action ; only one-fifth or one-sixth suffices, the remainder appears as actual heat. The amount of heat given off by the chemical changes which are necessary to initiate the total DIETETICS AND ANIMAL HEAT. 337 DIAGRAM VII. — Showing the Heat Equivalent of various Dietaries (drawn partly after Stewart). Food in Grammes o 200 400 600 800 1000 Potential Energy — Calories o 1000 2000 3000 4000 5000 DIETS. 1. Theoretical — ■ For Subsistence (Play fair). For Moderate Work (Voit) For Hard Work (Atwater) For Moderate Exercise (Playfair) Specimen Diet (p. 327) 2. Actual — Lawyer — Munchen Physician — Miinch en Well-fed Blacksmith— England ... German Soldier— Peace Footing.. German Soldier— War Footing ... U.S. Army Ration U.S. Navy Ration British Soldier English Prize-fighter PROTEIDS. FAiS. CABBO-HVDMTES. HEST EOUIVJLENt IN C4L0BIES. 22 338 THE NATURAL HISTORY OF DIGESTION. muscular work of the body during an ordinary day's labour forms the greater part of the heat produced in that period. For instance, the work of the heart muscle has been estimated as equal to 30,000 kilogramme-metres in the twenty-four hours. If the energy produced by chemical changes to effect such an amount of work is equivalent to five times the visible result, the total energy expended will be equivalent to 150,000 kilogramme-metres, or 353,000 calories of heat; and, as most of the mechanical work of the heart is used in over- coming the friction of the blood-vessels, a great part of its energy is in the end transformed into actual heat. The energy evolved by the heart of an adult during the twenty- four hours is equal to that necessary to raise a man of eleven stone about 7000 feet, or about twice the height of Snowdon. If we add to this the 13,000 kilogramme-metres of work done by the respiratory muscles, or rather the equivalent energy production of 65,000 kilogramme-metres, equal to 153,000 calories, during the day, the total of 215,000 kilogramme- metres is obtained, or 506,000 calories of heat produced by the circulatory and respiratory mechanisms alone, about one-fifth of the total heat-production during ordinary labour. But we have seen that about 320,000 calories are produced during the eight hours allowed for sleep. If we divide the total heat given off by the heart and the respiratory muscles during the twenty-four hours by 3, we get 168,000 calories as the result. The circulatory and respiratory movements are, however, slower and less possible during sleep, and 150,000 calories may be taken as their heat-equivalent. If the move- ments of the involuntary muscular tissue of the intestinal canal be also taken into account, it is easily seen that during sleep a large proportion of the heat produced in the body is derived from the chemical changes antecedent to and -necessary for muscular contraction. Hein found that a man gave off 283,000 calories of heat when performing 27,700 kilogramme-metres of work per hour. But as 27,700 kilogramme-metres of work are represented by about 65,300 calories of heat, the amount of the heat- equivalent produced, even if the striped muscles only pro- duce four times as much heat as work, is almost equal to the total evolved. Again, even when at rest muscles produce some heat. The muscles of a dog's leg have been found to remove 150 c.c. of oxygen per kilogramme per hour from DIETETICS AND ANIMAL HEAT. 339 defibrinated blood. It has been calculated that one-tenth of the total heat-production of the body at rest may be afforded by quiescent muscular metabolism. The glands of the body also produce heat, but to a much less extent than the muscles, Many of the chemical changes occurring in the glandular organs are heat-producing, but some of the reactions which take place in them are synthetic in nature and tend to absorb rather than evolve heat. The central nervous system ranks as a heat-producing centre, but its metabolism is very feeble when compared with that of the muscles. The main source of the energy which appears as heat and as muscular work is undoubtedly the oxidation of food substances; but it is as yet unknown how part of the energy is converted into mechanical work, and part into heat. It has been suggested that the energy derived from chemical reactions is first converted into electrical energy, which is in large part squandered in the production of heat. Pembrey and Hale White {Jo2irn. of Fkys., xix., p. 477) conclude from a consideration of experiments on hibernating animals that their body temperature is maintained by the heat produced from the muscular contractions of the heart, the friction resulting from the passage of the blood through the vessels, and the movements necessary for respiration. During their winter sleep any increase in body temperature is accom- panied by the disengagement of carbonic acid gas, and at the time when activity again begins, this development of the gas is so great as to resemble a sort of explosive action. 340 CHAPTER XVI. Stimulants. At.COHOLS : — Nature — Universal Use — Actions on the Body — Influence on Digestive Processes — On Saliva — Peptic and Pancreatic Digestion — Action of Fusel Oils. NoN -Alcoholic : — Tea — CoflTee — Cocoa — Coca. Alcohols. — The monatomic alcohols are neutral hydrates in which an atom of hydrogen in certain hydrocarbons has been replaced by the radical hydroxy! (OH). In the methyl com- pounds the radical CH^ becomes CH'-OH, or methyl alcohol, one atom of hydrogen in the hydride (Methane, CH^H) being replaced by hydroxyl. Similarly, ethylic alcohol is ethyl hydrate or C2H«-0H, propyl alcohol CSH^-OH, butyl alcohol 0*^-0 H, and amyl alcohol C^Hi'-OH. The higher alcohols in the series above butyl alcohol have several isomeric modifications. Ethyl alcohol, C2H«0 = CH8-CH2-OH, is the ordinary alcohol of wines and spirits, and when absolutely pure is a colourless, mobile liquid, with a density of 0.8095 at 0°. It boils at 78.3° Cent., mixes with water in all proportions, causing the evolu- tion of heat and contraction of volume, and, because of its lower boiling point, when heated distils over more quickly than water, and can thus be separated from it. Methyl alcohol, or wood-spirit, when pure is also colourless, but has a nauseous taste and odour. It boils at 66.5°, and has a density of 0.8142 at 0° Cent. Amyl alcohol of fermentation (that produced by chemical decomposition is different in its properties, although isomeric) is a colourless liquid, having rather an unpleasant pdour. It boils at 132°. Its density at 0° is 0.8184. It is better known by the name of fusel oil. Spirit produced by the fermentation of grain or potatoes, in- stead of from malt, contains much of it. STIMULANTS. 34 1 It is doubtful if there has ever been in this world a race of men ignorant of the stimulating and exhilarating effects of the products of the fermentation of starch and sugar. At least all races, from the earliest times to which we can go back, possessed some form of fermented liquor. The Teutonic races drank mead, made from a mixture of honey and water, and -fermented. The Japanese make use of Saka, a sort of beer made from rice. The South Sea Islanders, cut off for ages from intercourse with their fellow-men, contrived to extract a similar beverage from the juice of the cocoa-nut. Nearly all those early products contained only a small per- centage of alcohol, and were drunk in large bulk. It is almost unnecessary to observe that all civilised nations are adepts in preparing seductive fluids, which contain alcohol as a conditio sine qua non. One of the questions which have been discussed for ages, and which will probably long remain a subject of contention, " Is alcohol a food or a poison ? " has been satisfactorily answered by physiologists during the last few years. But calm and dry statements of fact by scientists rarely carry conviction to the public, who are too often misled by deluded fanatics and their theatrical and biassed harangues. As in most issues the truth lies between the two extremes: alcohol is a food under many and varied cir- cumstances, a poison under others, and it serves as a valuable stimulant in many cases where its use, without the least doubt, materially contributes to save life. A poison if taken in too large amount at one time or in smaller quantities too frequently; a food when the diet taken contains too small a proportion of the elements required to equalise the waste of bodily metabolism; it is invaluable as a restorative on such occasions as the crisis of a fever, or the shock which follows an accident or an operation. The effects ascribed to alcohol by mankind are protean. It is drunk in the tropics to cool the body, in the cold of a northern winter for the sake of warmth ; it is taken to drive away muscular fatigue or to rouse the flagging brain, to steady the trembling hand, or to drown the memory of misfortune. Many of these effects appear to be contradictory and impos- sible, but they are all in a sense founded on physiological actions. It is a somewhat difficult task to sketch in a few words the various effects of alcohol on the different organs and functions of the body. For the sake of clearness we may first 342 THE NATURAL HISTORY OF DIGESTION. note its actions on the general bodily metabolism and the nervous system, and then glance at its more local effect upon the processes of digestion. Alcohol is rapidly absorbed by the mucous membrane of the stomach, but even before absorption, if in a concentrated form, reflex stimulation of the heart and circulation occurs. The heart beats more rapidly and with greater force, the blood-pressure rises, even although the blood-vessels, especi- ally those of the skin, dilate, causing a feeling of increased warmth. After the entrance of alcohol into the blood- stream — it is absorbed more readily by the blood-vessels than by the lacteals — a change may be observed in the corpuscles, the amoeboid (intrinsic) movement of the white blood-corpuscles being first increased but soon after markedly diminished, while the red corpuscles part with the oxygen contained in their oxy-hasmoglobin much less readily. As this power of readily yielding oxygen governs the metabolism of the body and the consequent production of its heat, alcohol serves as a tissue-sparer ; but, again, it accelerates the rapidity of the circulation and thus counteracts to some extent the diminution in its oxidising power. Small quantities taken at moderate intervals do not interfere with oxidation ; in larger doses, and if taken frequently, imperfect combustion of fats is manifested by the deposition of fat in the tissues, and by the occurrence of fatty degeneration of the organs. In health we may fairly state that, taken infrequently and in small quantities (2 oz. in the day the maximum), it can do no harm, and probably does some good in stimulating the appetite, as we shall see later. If indulged in too freely it acts dele- teriously. In disease, however, especially in cases of pyrexia, its power of diminishing oxidation renders it a most valuable adjuvant to other treatment. It also acts in such cases as a food, owing to its oxidation in the tissues. For long the question of the oxidation of alcohol in the body remained doubtful. Lallemand, Perrin, and Duroy asserted that it was all excreted unchanged by the emunctories. Anstie, on the other hand, proved that only a slight proportion was passed out by the kidneys, and then only if much had been taken. Hammond at length placed the matter on a secure basis by showing that the addition of alcohol to a diet insufficient to maintain the body-weight, not only prevented the loss, but converted it into a gain. STIMULANTS. 343 We may state three propositions as to its action as a food — ■ 1. With a diet greater than necessary to Iceep the body-weight steady, alcohol does harm, is superfluous, and decreases the RATE OF INCREASE IN WEIGHT. 2. With a diet sufficient to maintain the body-weight, the addition of alcohol to it may cause some loss of weight. 3. With a diet insufficient to keep the weight steady, alcohol is of service, the weight remaining the same or even INCREASING. Little of the alcohol taken by the mouth appears in the urine unless the amount drunk be excessive. The tests which were relied on up to a recent date for the detection of alcohol in the urine are very untrustworthy. The most rigid teetotaller with their aid can be shown to excrete large quantities of alcohol. The small amount excreted by the kidneys com- pared with that taken in arises partly from exhalation with the expired air, partly from oxidation of the alcohol in the body. After absorption the chief action of small quantities of alcohol is exercised upon the circulation. The nerves which carry the fibres set apart for conveying accelerating impulses from the brain to the heart are stimulated, the heart beats more rapidly and powerfully, the blood-pressure in the arteries rises in spite of their increased calibre, owing to the stimulation of the vasomotor nerve fibres which influence the size of the vessels, and all parts of the body, for the time, are better nourished. Thoughts become clearer, the brain more acute; the misan- thrope brighti-ns, the timid becomes bold, the bold still more venturesome, even "pot-valiant." But in some individuals alcohol has an opposite effect : they become, even after small quantities, sleepy, heavy, stupid, and often cross. The first and larger class experience the exhilarating effects which follow an increased flow of blood through the vessels of the brain, the second the depressing results of a diminished supply of blood to the brain owing to the large quantity required to fill the dilated vessels of the skin. Such results, if brought about at intervals, do no harm, but if frequently repeated the necessary amount of alcohol for their production has gradually and pro- gressively to be increased, while, although the effects are pleasant and stimulating at the time, the heart is left more exhausted after they have passed off than before, and all the organs Of the body experience the inevitable rebound which follows abnormal stimulation. Probably the occasional indulgence 344 The Natural history of digestion. in a sufificient amount of alcohol to cause the earlier symp- toms produced by its use may be beneficial rather than harmful, if not repeated too often, from the general stimu- lation and temporary "rousing up" of the organs and tissues. But alcohol should never be taken by those exposed to great cold, as by increasing the transpiration of heat from the dilated vessels of the skin much of their body-heat is lost, and they are less able to withstand the cold afterwards from exhaustion of their powers of producing body-heat. The same mode of reasoning applies to those doing continuous and heavy work, and to those living in very hot climates ; alcohol at first stimulates them, but also cools them by increasing perspira- tion, then renders them less capable of withstanding the toil or the heat from the consequent exhaustion. Table LXX. — Showing the Proportion of Alcohol and other Constituents in Spirits and Wines. Liquor. Alcohol 7. SoUds 7, Ash 7, Sugar. Acidity. Absolute Alcohol 99.5-100.. . __ Proof Spirit 49 •• — — — Whisky (Parkes) 50-SS •■ 0.6 Trace. .. . • 0,04% (Chittenden) 50-51 .. 0.3284 ... 0.004 .. . . Acid Brandy (P) 45-55 •■ 1.2 0.05-0.2 .. . 0-27o (C) 47-48 .. 0-43 0.0054 .. . . Acid Gin (P) 49-57 •■ 1.2 0.1 .. . • 0.047, (C) SI •■ 0.29 0.009 .. . . Acid Rum (P) 50-60 .. I.O 0.1 I . ■ 0.1% (C) 50-51 •■ 0-3 0.007 .. — . . Acid Red Wines (K)- - Alcohol. Tannin and ^ Colouring Matter. *"S ar. Free Acid. Rhine Wine 10.08 0.16 - 0.52 Hungarian 9-65 0.13 0.59 Burgundy - 11.15 0-53 Bordeaux 9,07 0.22 0-59 (Chittenden 9.7-IO.C ... Total Solids 3. 37o> Ash Stror 0-67o- igly acid.) White Wines (K)— Rhine "•45 — ... 0.3 7 0.46 Moselle 12.06 — ... 0.2 0.61 Riesling 12.90 — ... 0.0 I 0.65 Sweet Hungarian (K)— Tokayer 12.74 18.34 ... 14.9 3 0.52 Ruster 11. oS 23.64 ... 21.7 4 0.51 STIMULANTS. 345 Fortified Wines (K)— Port '6s 21.91 Sherry '70 22.9 Madeira '70 19. 11 Marsala 20.44 Malaga '72 16.14 Sparkling Wines (K) — Champagne ii.7S Rhine 12.14 8.83 . 6.42 0.45 3-78 . . 1.88 0.44 5.22 .. 3.46 0.48 4.94 .. 3.48 0.39 21.23 . 16.47 0.42 13.96 . ■ 11-53 0.58 12.14 . 8.49 0.57 malt. Abh. Sugars. Albu- minoids ... 7.09 .. . 0.36 ... 6.15 .. 0.57 ... S.42 ., . 0.24 ... 4.22 .. 0.16 ... 6.74 .. • 0.43 ., 5-77 .. 0.26 ... 5.7 .. . — ... — .. — ... 4.425.. . 0.35 .. — •• 0.73 ... 5-4 ■• . 0.36 ... — ■■ 0.73 Beers — Alcohol. Miinchener Lowenbrau (L) 4.45 English Bitter (L) 6.78 English Mild (L) 8.45 Berliner Weiss (W) 1.9 Bass (C) 4.0 Stout (C) 5.5 (P) = Parkes. (Acidity by Parkes in terms of tartaric acid per cent.) (C) = Chittenden. (K)=K.bnig. (Free acidity per cent., probably as oxalic acid.) \L]=Lancet Office (1895). (W)=Willoughby. Influence of Alcohol on the Processes of Digestion. — It is apparent from a glance at Table LXX. that there are very pro- nounced differences between the various wines and spirits, both in the proportion of alcohol which they contain and in their other constituents. From the experiments of Sir William Roberts, and of Chittenden and Mendel, on the action of these different drinks on the chemistry of digestion, it may be assumed with confidence that spirits influence it only in respect to the proportion of alcohol they contain, and that, with wines and malt liquors, the effects produced depend upon the solid matter and extractives contained by them, or upon their acidity. Spirits when taken " neat " act on the mucous membrane of the mouth, if kept in it any length of time, by precipitating the albumin of the cells and blood serum. If swallowed quickly, this action is inhibited by the increased flow of saliva induced. When it reaches the stomach in a concentrated form and in large quantity, the reflex results, already mentioned, may be so greatly intensified as to rapidly cause insensibility or even death. If sufficiently diluted by the saliva or by the contents of the stomach, little effect on the mucous membrane of that organ is produced; if more concentrated, and taken on an empty stomach, irritation of the mucous membrane ensues, and if the irritation be repeated frequently, passes into a state of catarrh. (Chart XVII.) 34<5 THE NATURAL HISTORY OF DIGESTION. As has been said, alcohol stimulates the flow of saliva, but when we look into its action on the special properties of that secretion some important facts are obtainable. The following table from Roberts will sufifice to illustrate this point in a brief but striking manner: — • Table LXXI. — The Effects of Alcoholic Liquors on Salivary Digestion. liormal time taken, 4 minutes. Time taken before the Achromic Point was reached. Amount of Alcoholic -^rooi Liquor* per cent, in the a^.vif Digesting Mixture. ''P'™- Whisky. Brandy. Sherry. Hock. 0.25 — — — 8 min. 16 min. 0-5 — — — 30 ,, 80 ,, I.O — — — No action beyond soluble starch. 2.0 _ — — No action. 5.0 4.0 min. 4 min. 4 min. — — lO.O 4.0 ,, 30 .. Very slow. — — 20.0 40 ,, Very slow. Nil. — — 40.0 S.o „ Nil. )) 4 minutes (after neutralisation). 60 14.0 ,, ,, ,, — — 70.0 20.0 ,, ,, ,, — — 90.0 Very slow. ,, ,, — — * These figures represent the percentage of alcoholic liquor in the mixture during digestion, not the proportion of pure alcohol. Alcohol in the form of proof spirit has no retarding influence even when present in a strength of 20 per cent., or 10 per cent, of absolute alcohol; when present as 40 per cent, the amylo- lytic power is only half as great, although 90 per cent, or 45 per cent, of alcohol, does not entirely abolish all action. Whisky and brandy with practically the same alcoholic strength do not affect the action of ptyalin when present in the pro- portion of 5 per cent. Whisky present as 10 per cent, causes a greater retardation than 70 per cent, and brandy in the same proportion acts as strongly as 90 per cent., of proof spirit. Still more striking are the results obtained when sherry and hock are added. These wines, and especially hock, delay the conversion of starch by ptyalin most markedly, but this cannot be due to the amount of alcohol contained in them, and evidently depends on the acids present Neutralisation of a solution containing 40 per cent, of each of these wines causes a return to the normal period of conversion. As sherry contains about 22 per cent and hock only 11 per cent of alcohol, 0.25 per cent of sherry only represents about STIMULANTS. 347 CHART XVIL— Showing the Relative Actions of Alcohol, Whisky, Sherry, Hock, and Ale on Salivary Digestion. PERCENTAGE OF ALCOHOLIC LIQUOR, o 0.250.5 I 2 3 4 5 10 20 30 40 50 60 70 80 90 lOG 90 80 70 60 50 40 3Q 20 10 P ;r ...» "" »oo< '" 000 000 °°t \ ■f \ \ 1 *, : i • \ 1 1 \ 1 • \ \ i *, ; ' \ \ > > \ '•. \ \\ ° \ \\ \ % ^, \\ % \ I % \, \\ '•_ s \ \ 1 '■.. \ \ \ \ I \ y '\ \ \ \ \ s \ \ •... ■>.' .c„ \ 0070000 0.0 D -^Proof Spirit <5o per cent, alcohol). Hock (11 per c^nt, alcohol). oooooooooo Whisky (50 per cent, alcohol). "~' — • — • — • Sherry (^r per cent, alcohol). • Ale (4 per ceht. alcohol). 343 THE NATURAL HISTORY OF DIGESTION. 0.055 per cent, of alcohol, and of hock about 0.027 per cent. Forty per cent, represents 8.8 per cent, of alcohol in the case of sherry, and 4. 4 per cent, in the case of hock, percentages of pure alcohol which do not affect the rapidity of salivary diges- tion in themselves. The curves given in Chart XIX. (p. 353) make it clear that the increase in the action of whisky and brandy in retarding salivary amylolysis over the action of corre- sponding solutions of pure alcohol is not due to the presence of the higher alcohols or "fusel-oils." In fact, amylic alcohol is the only one in which a i per cent, solution does not tend to increase rather than decrease the action of ptyalin; methylic alcohol up to 5 per cent, aids amylolysis. Whisky and brandy up to 5 per cent, do not retard the salivary digestion of starch, and in higher proportions up to 10 per cent, exert no deleterious action if their acidity be neutralised. Rum acts more adversely, 5 per cent, reducing the relative digestive power to 50, but 10 per cent, has no such retarding action after neutralisation. Table LXXII. — The Action of Brandy and Rum on Salivary Digestion. Brandy. Appearance of Proportions Achromic Point Present. (Saliva 1 : 5). o % 2 minutes 5 ., 12 ,, 5 ,, (Neutralised) 2 ,, lo „ Do. 2 ,, (S ,, Absolute alcohol) 2 ,, The inarked retarding action of rum is therefore not due to the alcohol in it but to the large amount of volatile acid bodies. An interesting analysis showing the effect of age on wines is the following by Berthelot {Comptes Rendus, 1879) : — Table LXXIII. — Analyses of New and Old Wines. Port Wine. 100 Years Old. 45 Years Old. Specific gravity (at lo°) Total residue at 100° % Sugars, reducing, % Sugars, after dilute acid, % Acid (as tartaric; gms. per litre), % Tartaric ether, % Cream of tartar, % Alcohol, % Examination of the deposits showed^ Sugar, reducing, % Cane-sugar, % Rum. Appearance of Proportions Achromic Point Present. (Saliva 1 : 10). % 3 minutes. ■; ,, 16 hours. 10 , , No conversion. 10 ,, (Neutralised) 3 minutes. (S ,, Absolute alcohol) 3 , , 0.988 0991 3-360 5300 1.250 3.150 1.290 3.680 5.170 5.460 I. no 1. 170 0.270 0.420 5-900 16.100 1.250 3.150 0.040 0.530 STIMULANTS. 349 The total residue of old wine is less than that of new, and contains scarcely any cane-sugar, probably due to the slow inverting action of" the organic acids in it. The tannin also decreases, while the glycerine increases. All these facts explain why an old, fully matured wine is less apt to cause digestive disturbances than a new and raw sample. Table IXXlV.—TAe Constihients of Wine. {Wynter Blyih.) Water. Grape-sugar (o to several per cent.). Alcohols, mainly ethylic, but also small quantities of the higher alcohols, as propylic, butylic, amylic, and others. Albuminoid residues. Aldehydes (mainly ethylic). Isobutylglycol. Acetal. Furfurol. Acetic acid. Succinic acid. Tartaric acid. Calcium tartrate. Gum. Malic acid (in bad seasons). Colouring matters. Glycerine. Organic acids in combination. Extractive matters. Esters — acetic, caproic, caprylic, butyric, and tartaric. Tannin, Mineral matters. A few ferment cells. The action of alcoholic liquors on peptic digestion has been studied exhaustively by Sir William Roberts, and, more recently, by Professor Chittenden and L. B. Mendel in America. Their results do not apply in all respects to true gastric diges- tion, as, owing to the experiments having been exclusively performed in vitro, no evidence could be obtained of the influence of alcohol on the actual gastric secretion or on the movements of the stomach walls. The action of alcohol and alcoholic liquors on the proteolytic power of pepsin and hydrochloric acid outside the body must, however, have much to do with their power in the body, and results obtained from such observations have a decided value. These observers found that the action of alcohol in all alcoholic liquors depended largely on its percentage, entirely so in the class of spirits, and to a less extent in wines and malt liquors. One or two per cent, of absolute alcohol — that is, about 2 to 4 per cent, of proof spirit — tends shghtly to increase the peptonising power of the gastric juice. Up to 15 per cent, of absolute alcohol no perceptible retardation occurs; with 15 to 18 per cent, digestion may be reduced one-quarter or even one- third, although the retardation varies greatly with the strength and activity of the gastric juice, and the digestibility of the proteid. The greater the power of the gastric juice is, the less the retardation. A percentage of 20 or more strongly inhibits the digestive process. The common 3 so THE NATURAL HISTORY OF DIGESTION. spirits, whisky, brandy, rum, and gin, act proportionally to the quantity of alcohol contained in them. In the presence of 3 per cent, of whisky digestion appears to be slightly increased, 20 per cent, reduces it one-fourth. The common idea, which indeed is almost universally held, that the higher alcohols, as amylic, methylic, and propylic, the so-called fusel-oils, act deleteriously on the gastric processes, is not confirmed by actual experiment. Indeed, small quantities increase the pro- teolytic power of the juice. Wines and malt liquors with less than 10 per cent, of alcohol delay gastric digestion rather because of the various solid matters contained in them than from the amount of alcohol. This is especially the case in malt liquors when used in large quantities, and arises from the large proportion of extractives present. Small quantities of wine may somewhat increase digestion, similar amounts of beer or ale seem to have no influence on it. (Chart XVIII.) Pure spirits can therefore be taken in small quantities and well diluted, without harming peptic proteolysis, and possess the advantage over wines and malted liquors of not hindering that process while containing a somewhat larger proportion of alcohol. It must be kept in mind in judging of the probable effect of spirits on gastric digestion in the body that the water generally added to them brings the percentage of alcohol in the solution below that of many wines, and even beers, which are taken undiluted. Thus an ounce of whisky with 50 per cent, of alcohol, with 14 ounces of water, makes a drink containing only .5 of an ounce of alcohol in 15 ounces, or 3.3 per cent. This is exactly the proportion which appears to aid gastric digestion, and even it is lessened by the addition of the secre- tion of the salivary and gastric glands. Sherry and port when undiluted act much more deleteriously, owing to their per- centage of alcohol (21-23), and to the effect of the solid matters contained in them. The information which we possess regarding the action of alcohol on the pancreatic digestion of proteids has necessarily been obtained from artificial experiments, either by the use of an artificially prepared pancreatic juice, or by the use of pancreatic juice itself, obtained by means of a fistula. Roberts has found that the action of wines and beer on the activity of the tryptic . ferment is slight, and that stronger spirits only retard its action when present in a proportion of 5 per cent, of absolute CHART XVIII.— Showing the Relative Actions of Absolute Alcohol, Whisky, Sherry, and Ale, on Peptic Digestion. Drawn from data by Chittenden. Normal peptic activity is taken as loo, and the increase or diminution of power in the pepsin-hydrochloric acid solution on the addition of the different liquors inserted in terms of the normal figure. The percentage figures do not refer to the amount of alcohol in the resultant mixture, but to the proportion of alcohol containing liquid added. AcHo™ PEKCENTAGE OF ALCOHOLIC LIQUOR. 100 90 80 -5 c a 5 4 S 6 8 ID 12 15 18 20 25 50 SO | ,-«:: »»,^ ^ ^ ? -v ■•-. ^^. ^ v -<• 70 s \ A \ 60 50 \ \ \ _ V \ ■ \ : \ \ \ w ■; \ 40 50 \ : \ 1 1 \ \ \ I 20 10 . \ \ \ - 0'Sl234SeS Absolute AlcoTioI (loo per cent alcohol). - Whisky (50 per ceni. alcohol). Sherry (21 per cent, alcohol). Burton Pale Ale (4 pet cent, alcohftl). 10 la 15 la £0 25 30 50 % 352 THE NATURAL HISTORY OF DIGESTION. alcohol or above it. With lo per cent, of alcohol digestion goes on to full completion, but the normal duration of the process is considerably lengthened. As alcohol is rapidly absorbed by the mucous membrane of the stomach, it is very unlikely that any such proportion can ever be present in the duodenum during life ; therefore alcohol taken by the mouth can have little direct effect upon tryptic digestion in the body. Chittenden and Mendel found, in the investigation referred to above, that the retardation of digestive action by trypsin from the addition of wines in artificial experiments is due almost entirely to the acids present. The alcohol plainly exerts some retarding influence upon the proteolysis, but the delay is exactly proportional to the amount of alcohol contained in each specimen, as these observers showed by using the dis- tillates from different classes of wines. For instance, claret, with a small percentage of alcohol, and sherry, which contains double the amount, react exactly contrary to what we should expect. The relative digestive action in the presence of lo per cent, of sherry is 61.5, and with 10 per cent, of claret only 36.2. Sherry contains a considerably larger amount of solid material than claret, and the difference in the retarding action is ascribed by these observers to the higher acidity of claret. (Chart XIX.) Alcohol acts upon the pancreatic ferment, amylopsin, in a similar manner to its action, already described, upon the corresponding salivary ferment. That is to say, that beer and wines retard its action owing to the acidity contained in them, while the alcoholic strength of spirits in solution requires to be above 10 per cent, of absolute alcohol before any great slowing can be observed. Chittenden 1^ Journal of Physiology, xii. 3, p. 220) found that 2.5 c.c. of absolute alcohol per kilogramme of the weight of a dog could be given for 8 to 10 days without causing any marked effect upon the dog's proteid metabolism. As alcohol does not contain nitrogen it slightly diminishes proteid ex- change, but, although the results are not very definite, it must have some action on the proteids of the body, as the amount of uric acid excreted rises above the normal, even if the total outgo of nitrogen be lessened. Duogany and Tibald ( Ungar. Arch. f. Mediz., iii. s. 189), experimenting on dogs, corroborate Chittenden's observation except in so far that they found the nitrogen exchange to remain equal to the normal with large CHART XIX.— Showing the Relative Actions of Absolute Alcohol, Whisky, and Sherry on Tryptic Digestion. The arrangement of the Chart is similar to that of No. XVIII., already fully explained. REU^TIVE ACTION. PERCENTAGE OF ALCOHOLIC LIQUOR. j 025 OS 1 a 3 4- 5 10 15 20 100 -^. sj ^ 90 80 ^ -x'' ^ ■■•. 70 60 \ \ I 50 \ 40 30 20 10 ^ JIBSOLUTE ALCOHOL (ALCOHOL, 100%) WHISKY (ALCOHOL, 50 %) SHERRV (ALCOHOL, 21 %) 23 354 THE NATURAL IIISTORY OF DIGESTION. doses of alcohol, while after small doses its outgo increased. The quantity of uric acid is increased after snnall doses of alcohol, but unchanged when large amounts are given. The excretion of phosphorus varies similarly to that of nitrogen, while in small or moderate amounts of alcohol the sulphur is increased, but is diminished when much alcohol is given. Tichen, of Jena, places the maximum daily amount of absolute alcohol which can be taken without detriment to the nervous system at 30 to 40 grammes (i-i.S oz.), represented by about a litre of beer (1.5 pint), with 3.5 per cent, of alcohol, or Toths to Toths litre of white wine (-| to f pint), with 10 per cent, of alcohol. On each occasion when the portion drunk contains no more than 25 grammes of ethyl-alcohol diluted with ten times its weight of water, or equal to about 9 per cent, of alcohol, a stimulating effect follows, lasting for fifteen to thirty minutes, succeeded by a diminution of energy which continues for several hours. Larger amounts taken at one time shorten the first stage and prolong the second. The quantity noted above as the daily maximum, when taken at different times during the twenty-four hours, can bring about the greatest possible duration of the stimulating effects. Allusion has already been made to Hammond's observations, in which the metabolic balance of the body shows an increment when alcohol is taken with an insufScient diet, but suffers loss when it is taken as an adjunct to sufficient or excessive food. Somewhat similar results have been obtained by Duogany and Tibald (Ungar. Arch. f. Mediz., iii. s. 189) from meta- bolic experiments upon dogs. They find that when dogs are kept for some days on a diet on which their weight remains constant, the addition of small doses of alcohol (9 c. cm.) to their daily food increases the output of nitrogen, phosphoric acid, and sulphur in the urine, which is at the same time increased in quantity. Uric acid is also excreted in greater amount. With larger doses (30 c. cm.) the output of these bodies, save luic acid and the quantity of urine passed, are diminished ; the uric acid shows no change from the normal. Comparing the effects of the higher alcohols or fusel oils on digestion of starch in the mouth, and of proteids in the stomach and bowel, as figured in Charts XX. and XXL, it may at once be seen that they exert rather a favourable action, except in the case of amylic alcohol, on the salivary ferment, but retard both peptic and tryptic proteolysis, the latter especially. The dele- terious effects are generally in direct relationship to the weight of the alcohol, isobutylic being the heaviest and the most active, methylic alcohol the lightest and least harmful. The STIMULANTS. 355 amount of fibrin digested by pepsin and hydrochloric acid is actually increased when these alcohols are present in small quantities, an increase which is most marked with propylic and methylic alcohols. Trypsin in the presence of small quantities of these two alcohols is slightly increased in power, but a trace of amylic or isobutylic alcohol, or the presence of the other two above 0.25 per cent., serves to diminish its activity. From these data it is permissible to state that the presence of fusel oils in spirits does not cause derangement of digestion per se. As, however, the classes of spirits containing them are not so palatable as more mellowed brands, and as their partakers seldom dilute them to any extent, the evil effects of raw, neat spirits, which are usually ascribed to the higher alcohols, are more probably due to the non-dilution of the spirit. We may briefly summarise the present state of our know- ledge as to the action of alcohol and its uses as a food accessory. 1. It is to some extent a food, but is seldom taken except as a stimulant. 2. It can be used up in the body as a source of energy and heat, but the heat produced by it is rendered less than useless owing to the greater heat-loss brought about by the dilatation of the vessels of the skin. 3. Alcohol is of no use to healthy men who take sufficient food for their daily wants. 4. In moderate doses, and well diluted, it is not harmful to healthy men under ordinary conditions. 5. Alcohol acts injuriously in all cases of exposure to great cold, or of severe and continuous exertion, such as in Arctic expeditions or mountain-climbing. 6. If taken with meals in moderate doses it is often bene- ficial, as, after much of the alcohol has been absorbed, the secretion of hydrochloric acid increases to often more than double the proportion to which it would otherwise attain. The secretion of acid also continues longer than when no alcohol has been taken. 7. Alcohol seldom has this .stimulating effect if the stomach be diseased. The Action of Tea and Coffee. — The beverages obtained from the tea-leaf or the coffee-berry are not, in the proper sense of the word, foods ; they act purely as stimulants, although in a different manner from alcohol. The stimulating effect is in each 356 THE NATURAL HISTORY OF DIGESTION. case due to an alkaloid, theine in tea and caffeine in coffee, which are identical in chemical structure. The flavour and aroma are due to the presence of volatile oils. Neither the alka- loid nor the volatile oils seem to have any inhibitory effect on the CHART XX.— Showing the Relative Actions of the " so-called " Fusel Oils on Salivary Amylolysis. (From data by Chittenden ) RELATIVE ACTION. PERCENTAGES OF AI.C^HOI.S U?^ED. 0-os 01 oa 0-3 a* os 06 c o-s o-^ lo so 115 114 113 UZ III 110 109 108 107 IDS 105 104 103 102 101 100 99 98 97 96 95 94 93 — ^ / \ / / \ / \ / \ \ . \ \ ISO BUT f\. J % ^ ^ PR( PYl i^ :> •'// x\ ^y ■ y' / \ ^v y \ / •^ S^, \ S s. " \ s \ S \, AM-) L 1 digestion of starch or on the peptic digestion of proteids. Strong infusions of tea or decoctions of coffee exert a very marked inhibitory action on all processes of digestion. This is almost solely due to the presence of tannin. As weak a solution as i in STIMULANTS. 357 5000 of tannin is sufficient to arrest the action of saliva on starch. Roberts has calculated that the amount of tannin contained in a 5 per cent, infusion of tea fully accounts for its inhibitory effect on salivary digestion. For, if the tea used contains 8 per cent, of tannin and three-quarters of this are dissolved in the infusion, the resulting fluid will contain 0.3 per cent, of tannin. The results obtained by experiment 3S8 THE NATURAL HISTORY OF DIGESTION. show that the actions of a 0.3 per cent, solution of tannin and a 5 per cent, infusion of tea-leaves closely correspond. When tea-leaves are infused with boiling water for as short a time as two or three minutes, both the allcaloid theine and tannin are dissolved in the solution. It is a popular error that a short infusion prevents the extraction of tlie tannin, while permitting the solution of a sufficient quantity of the alkaloid to render the beverage of the wished-for stimulating power. In some teas, however, especially the Indian teas, which are highly fired and much fermented during prepara- tion, long infusion causes a continued extraction of tannin after all the theine has entered into solution. The following table, compiled from analyses made by Ditt- mar, shows the proportional amount of these two substances yielded by China, Ceylon, and Indian teas after infusion for five minutes and ten minutes : — Table LXXV. Length of Infusion. 6 Minutes. 10 Blinutes. Theine. Tannin. Theine. Tannin. China 2.58 ... 3.06 ... 2.79 ... 3.78 Ceylon 3.15 ... 5.87 ... 3.29 ... 7.30 Indian 3.63 ... 6.77 ... 3.73 ... 8.09 The table shows that Indian teas contain 25 per cent, more theine than the teas which come from China. They also contain 100 per cent, more tannin. The Indian and Ceylon teas are now more widely used in this country than the Chinese varieties, and it is obvious that, while long infusion of a China tea can yield an amount of tannin which is far less than that obtained during the course of a short infusion of Indian teas, the common practice so prevalent among the middle and lower classes of allowing the teapot to stand on the hob for hours together, with an occasional addition of fresh tea-leaves, results in the production of a fluid containing a proportion of tannin much above that capable of arresting the digestive processes in the stomach even if diluted by the presence of other fluids. The physiological action of theine consists in a stimulation of the circulation of the brain and a decrease in the sense of hunger. It "cheers but does not inebriate." It is a more valuable restora- tive than alcohol in long-continued and fatiguing exertion, princi- pally because no symptoms occur of any diminution of power or of collapse of the heart or nervous system after the actual STIMULANTS. 359 Stimulating power iias passed off. In tliis it differs marlcedly from alcohol. Tea retards the action of the salivary juice upon starch and upon the pancreatic diastase. It slows the peptic digestion of proteids, but does not appear to exert much in- fluence upon trypsin. To obviate the inhibitory effect of tea on salivary digestion a pinch of bicarbonate of soda may be added to the tea. This does not help in the extraction of the bodies contained in the leaves, but renders the infusion neutral or slightly alkaline. Tea infused for tv.'o minutes is really not inferior in stimulating qualities or in its power of retarding sali- vary digestion to tea infused for as long as thirty minutes, unless those varieties be used which contain a large excess of tannin. To minimise the inhibitory action of tea on digestion it should be taken in very weak solution and sparingly, and should not be drunk with, but after meals. Coffee. — Coffee has a less marked inhibitory action on salivary digestion than tea, unless taken in the form of the French " cafe noir." The reason for this diminution in the power of causing retardation of digestion is found in the fact that coffee contains caffeo-tannic acid in place of tannin, a body which has less effect on digestion. Roberts found that a solution of coffee of 5 per cent, caused no slowing of the salivary digestion of starch until the coffee had been added to above 20 per cent, of the mixture, while the same proportion caused a considerable retardation of peptic digestion, its power in the stomach being almost equal to that of tea. Coffee has little effect on any of the digestive processes carried on through the agency of the pancreatic ferments. The physiological effect of coffee is exactly similar to that of tea. Cocoa. — Cocoa is derived from the seeds of a plant called the Theobroma cacao. Unlike tea or coffee, cocoa contains a large proportion, about 50 per cent, of fat, and in addition, 20 per cent, of nitrogenous material. It contains an alkaloid, theobromine, which is almost identical in composition and action with the alkaloids of tea and coffee. Cocoa, however, contains much less of this alkaloid than tea or coffee, so that when drunk as an infusion it is less stimulating ; but, as its constitution shows, it is very nourishing. Most cocoas in the market have had much of their cocoa-butter extracted, and sugar, and sometimes starch, added to them. These prepared cocoas form a very nutritious food in a small bulk, and may be 360 THE NATURAL HISTORY OF DIGESTION. advantageously taken during arduous expeditions, to serve both for food and drink. Food accessories of the class of tea, coffee, mate, guarana, and coca act somewhat differently from one another on the muscular energy of the body. Benedicenti ascribes to all of them the power of lengthening the period before which active movements cause muscular weariness, but in various degrees. Coca acts most vigorously, both increasing and conserving muscular power; mate is the least active. Tea, coffee, and matd only cause slight actual increase of power, but enable muscular exercise to be continued for a longer time. Coffee may exercise this property for one hour after it has been taken ; but tea, mate, and guarana lose their effect by the end of half-an-hour. 36i CHAPTER XVII. Foods. Food primarily sought for Subsistence only — Habits vary with Nature of Food — Economic Changes produced by the Cultivation of Food-stuffs by Man — And by his placing a Monetary Value on them — The Ideal Food of the Future — Vegetarians — Cost of Foods and Proportional Food-values — The Food of the Working Classes — The Foods of Physiological Diets — The Food of Herbivorous Animals — The Common Food Substances used by Man — Milk — Eggs — Bread — Meats — Fruits — Fungi — The Action of Heat on Foods — Conclusion. The primary use of food is for subsistence. The plant pos- sesses no active choice of food ; if its wants cannot be supplied where the seed fell and germinated, it withers. Plants, therefore, are distributed throughout the world in absolute accordance with the possible food supply afforded by their habitat. It is true that slight variations in the composition of the soil may produce minor differences in the same species of plant, but the possible limit is circumscribed. Man "has been able, through the artificial use of the necessary foods, to grow plants on soil incapable unless enriched of affording the requi- site elements of nutrition ; and, again, to modify their natural growth by giving them a superabundance of nutriment. Nearly all forms of animal life, on the other hand, although they may be restricted owing to their physical conformation to the use of some special class of food-stuffs, can voluntarily seek for their appropriate food. They seek it, not from any desire to eat for eating's sake, but for subsistence. When their hunger is satisfied, the search for food is suspended ; when their appetite returns, they again seek for it. If the physical development of the animal is such as to necessitate the ingestion of flesh alone, the food is consumed in large bulk at one time, whenever an opportunity presents, while digestion 362 THE NATURAL HISTORY OF DIGESTION. and assimilation are carried on at leisure. The curious cir- cumstance of poisoning from an excess of proteid food is often exhibited by the members of the carnivora in the course of their natural habits. So in man (and here it is unnatural, save in members of savage tribes with limited opportunities for dining off flesh) an unusually large amount of meat causes sleepiness, lethargy, and mental hebetude. The Carnivora obtain a " sub- sistence-diet" from infrequent and large meals. Their food contains a greater proportion of nitrogen to carbon than the ideal 1:15; in reality it is as high as i : 5 or more, and the excess of proteid over that needed for the bodily economy has to be got rid of at the expense of a passing attack of poisoning by derivatives of the surplus proteid, formed during the process of their elimination. (Chart XXII. and Diagram viir.) In the vegetable kingdom the nature of the food has as much significance. A hardy plant is not only one which can resist changes of temperature, of drought or moisture, but one which can procure its food from more primitive and ele- mentary forms than tenderer plants. A plant which can obtain sufficient nourishment from unpromising materials is hardier than one which is dependent upon special forms of food. Plants also which have developed accessory means of replenish- ' ing their larder, such as Drosera, Nepenthes, can live where others wither. In animals, the nature of the food eaten is a sure index to the character of the species. The carnivorous animal is quarrelsome, assertive ; the Herhivora shun combats save in the rutting season, and seek by flight to avoid the attacks of enemies. If it is asked what precautions should be taken as to diet and food, the answer may be given, " Eat what you find agrees with you, when you desire it, and how you like it cooked. Vary your diet as much as possible, and take your food in moderation, neither allowing too long a time to elapse between meals nor too short an interval. Eat slowly, and chew well. Let breakfast be the largest meal, should you be hungry enough in the morning." The stomach has had a night's rest and can digest more than in the evening, when it has had little time to recover from the day's exertions. The Continental practice of working in the early hours upon a roll and a cup of caf^-au-lait is not to be recommended. If you have to rise early and work before breakfast, drink a tumbler of milk CHART XXII. — Showing the Increase in the Capacity of the Human Stomach, Small Intestine, and Large Intestine at different Ages. =3 oj qj Months. Years. > K* '^ B ^ ^ G I 1 3 5 9 12 2 6 I 5 20 26 47 Sooo 7000 £^- l''--'^ 6000 ', ■., ^. 5000 / ,' 1 4000 / 3000 / ^' 2000 / ^^ / ' ^■^ -^ / / / gpo / / / 800 / , / / / / / ^ / i / K. 4. u'/ /T" 500 ■^ / ,;<■ ^ / ■i- 450 J V ■u, ''/ j 400 / ■ r 1 ^ / /(. 35° ' ^ /a 300 / / ^ 250 / 1 / .' / / / / / y 150 ' _- — / ' 1 ' 90 80 70 60 / / / / SO 40 / / / 3° 364 THE NATURAL HISTORY OF DIGESTION. with some bread or biscuit, or a plate of porridge. Those possessed of energy enough to cause early-rising, unless neces- sary in the course of their employment, take more out of them- selves than an early bedtime can compensate for, unless, indeed, they breakfast fully after rising and not after two or three hours fast. If early rising is accompanied by early retiring, and the meals be taken as usual but proportionately advanced in time, no harm accrues. Physical work before breakfast is perhaps not so hurtful as severe mental work, but both make use of material which has been assimilated before or during sleep, and which has not been reinforced by additional food since the last meal of the evening before. Any tissue employed for physical or mental work at a time when no reserve of material has been lately introduced, from loss of energy, and hence of its own elementary constituents, is much more prejudicially affected than if the blood could supply its needs at once. The food taken the night before is largely used in replacing the losses of the day-time; by morning the brain is refreshed by sleep, the mental powers are more active, but though seemingly more " wide-awake " and active than before sleep, the use of either brain or muscle implies the working of organs which cannot be supplied with the elements needed to replace those used up, except at the expense of other tissues. After a full meal the brain may be heavy and the muscles disinclined for work, owing to the action of the products of digestion in the blood supplying the brain, but apply the incentive of work and both brain and muscle work more powerfully, and are less soon tired. The frequent occurrence of drowsiness and lethargy after a full mid-day meal, or a late dinner, and the almost entire absence of these symptoms after a big breakfast, indicate that Nature can cope with the digestion and the assimilation of con- siderable amounts of food in the morning without causing any sensible disturbance in the body, while her task on the other occasions gives evidences of its magnitude in various ways.^ ^ Lambinius in his notes on Plautus gives the following information as to the meals of the Romans : — " Jentamlum was their first meal (d/cpdTt(r^a of the Greeks) ; Praudiuin the mid-day repast, Vespcnia the afternoon, and Ca:na the evening meals, corresponding to the dpiaToVf eairipiafia, and Selirvov of the Greeks." Actuarius discussed the merits of two over only one meal in the day. Galen recommended two, Cicero only one full meal, and Haly Abbas thought a late dinner better than a large mid-day meal, thus coinciding with many authorities at the present time. FOODS. 365 The herbivorous and ruminating animal, on the other hand, when untamed by man, usually feeds in the evening or even at night on a form of food which necessitates its ingestion in large bulk, rapidly stores this in a distended portion of the stomach DIAGRAM VIIL— Illustrating the Capacity of the Human Stomach at Different Ages. Mil The central black square (■sV'x^'u") represents i ounce, or 28.4 c.cm. The small white square shows the capacity of an infant's stomach — 2 ounces, or 56.8 c.cm. The square closely shaded represents an Imperial pint — 20 ounces, or 568 c.cm. The square dotted represents the capacity in child of six years— 36 ounces, or 1022.4 c.cm. The large white square represents the ordinary capacity in adult — 64 ounces, or 1817.6 c.cm. The outer shaded square represents the ordinary capacity in adult when more fully distended — loo ounces, or 2S40 c.cm. 366 THE NATURAL HISTORY OF DIGESTION. for further masticatory treatment, and retires to pass the day in quiet rumination and sleep. In other members of the Herbivora the food is more finely comminuted in the mouth, while the absence of a paunch renders rumination impossible. In them the food undergoes a certain amount of digestion in the true stomach, while the processes carried on in the rumen of the first class take place in the csecum of the second. Man, in the earliest stage of his life-history of which we have any cognisance, combined the habits of the carnivorous and of the non-ruminating herbivorous animals, except in so far as, not being in possession of either a rumen or capacious caecum, he was unable to make use of plants rich in cellulose, which he could not digest, and whose use occasioned great irritation of the bowel. The primitive man procured the necessary nitrogen from flesh or fish, or from nuts; the carbon came from animal fats, berries, nuts, and fruits. His food, though of uncertain and precarious supply, yielded all the elements needful. Whenever he sustained life on flesh alone he was compelled to devour a much greater quantity than sufficed for the demand by his body for nitrogen ; if he lived on berries and fruit an even greater excess of carbon over the physiological amount had to be consumed ere his need of nitrogen was satisfied. If food was scanty and insufficient for all the members of the tribe or settlement to which our primitive man belonged, he fulfilled the laws of supply and demand by killing the least useful members of the community, or allowed few only of the female children born to him to survive. Such customs are still in vogue in out-of-the-way parts of the globe, even at the end of the nineteenth century. In like manner the Carnivora equalise the number of consumers with that of the prey by devouring members of their own order when food becomes scarce. Among plants and vegetable-eating animals the food- supply more directly influences their survival ; the stronger plant ousting the weaker by appropriating the nourishment available; the stronger animal obtaining more food and eluding with greater success the attacks of enemies. As civilisation commenced, an entirely new element influenced the laws of supply and demand. Before cultivating plants or rearing animals to act as reserves or stores of food, as well as for the purpose of physical help, man began to cook what he ate. He was very near akin to the Anihropojnorpha, the FOODS. 367 higher apes, until he discovered the benefits conferred by the action of fire on foods. The day upon which a prehistoric man first sowed the seed of a vegetable in order that he might obtain from its growth a food-stuff belonging to himself marked a profound alteration in the economic value of matter. Henceforward he had con- ferred a fictitious value upon food-stuffs, fictitious in relation to all other commodities. Up to this time the forms of animal life were equal, their food was common property, its supply governed their habitation, their ability to secure it was their only guarantee of survival. After the prehistoric man com- menced to provide artificially for his wants, no other animal species could compete with him on level terms. In time the cultivation of vegetable foods and the breeding of animals for food purposes led to the recognition of food values, and the influence of fluctuating prices for food-stuffs led to artificial variations in diet. Save in the case of those races which even now have failed to advance beyond the stage of trusting to nature for their supply of food, the prices at which the peoples of the world can afford to buy the principal articles of food, involving, of course, the actual amount of the articles available, govern the kind of food eaten by the majority. The rich and the comparatively well-to-do only vary their food to a slight extent with fluctuations in value, they curtail their luxuries and food accessories, and by so doing can afford to pay an enhanced price should the supply of any of the common food-stuffs be short. Thereby they render it impossible for the needy, who ordinarily live at the starvation point, to purchase enough food material to yield them the 18 to 20 grammes of nitrogen and the 300 grammes of carbon which are necessary to maintain good health. They may be able to obtain enough to keep body and soul together, but their food under such circumstances is very far removed from the physiological ideal. Affluence is by no means synonymous with a physiologically correct diet. The ideal condition is one under which man can just afford to provide himself and his family with an amount of food containing a slight excess of the necessary elements and no more, provided he can continue to do so under what might form adverse circumstances to others. The disadvantages of a richer diet to men of sedentary habits are counteracted by active manual employment or exercise. A farm labourer able to provide 18 grammes of 368 THE NATURAL HISTORY OF DIGESTION. nitrogen and 300 grammes of carbon for his own consumption, with corresponding amounts for his wife and children, is fulfilling the laws of nature much more closely than the millionaire, who gluts his digestive market with food-stock unable to yield a tithe of the interest he is most in need of, although pro- viding deferred payments in the shape of gouty or other metabolic disorders in the future ; not to speak of the con- stant speculations indulged in by the use of champagne and other wines, speculations which afford a speedy but temporary return, followed by the inevitable "slump," and leading to bankruptcy, chronic dyspepsia, diabetes, obesity, or gout. The ideal food of the future, so fondly dreamt of by the scientific chemist, synthetically prepared, containing the different constituents necessary for the support of the animal body in exact proportions and in very small bulk, as well as allowing of easy and rapid assimilation — such a food will never satisfy the appetite of man. An ox or a sheep is ever content to feed on grass throughout its existence. Men are so con- stituted that some change in their diet is essential for their well - being. It almost appears to be true that man has developed a capricious appetite, pari passu with the education of his higher mental faculties, to which hunger serves only for a time as the best sauce, but which requires to be titil- lated by different foods for its maintenance at a healthy level. To one able to procure the variations craved for, subsistence upon a single food-stuff soon becomes intolerable, while those who are so unfortunately placed as to be unable to procure other than one article of food soon exhibit evidences of mal- nutrition. The infant thrives on milk and on milk alone. The adult can maintain his health, and may even put on fiesh, on an exclusively milk diet for some weeks or months. In the course of time, however, he begins to lose flesh, and it becomes clear that he is not procuring sufficient nourishment from the milk to keep his bodily functions at their normal level. A vegetarian may appear to be in perfect health, but his muscles are generally flabby, his power of resistance small. No human being, however, could sustain his bodily powers upon vegetables alone. He has not been supplied with either the rumen of the ox or the caecum of the horse. A vegetarian devotee, therefore, includes eggs and milk in his dietary, basing this indulgence upon the argument that neither of these ever possessed life. In regard to milk this statement is correct, but FOODS. 369 it cannot be accepted as true when eggs are concerned. An egg is endowed with life. Under favourable conditions it will develop into a living animal. Is it any more reprehensible to eat the flesh of a bird than to partake of a living germ along with the nutriment provided for it by its parent, a parent whose flesh is forbidden ? Life is destroyed just as much, perhaps rather more, when an egg is eaten than when a chicken forms part of the diet. Munk and Uffelmann (Die Erndhrung der gesunden und kranken Menschen, Wien, 1887) estimate the relation between vegetable and animal albumin, which should not be increased as far as the plant albumin is concerned, at seven to three — that is to say that no diet can be sufficient for an adult under ordinary circumstances which contains albu- minous constituents derived from vegetables in a greater proportion than seven parts to every three of albumin pro- vided by animal foods. The vegetarian follows out this law by supplying himself with animal albumin from milk and eggs, but has to eat a greater quantity of food, bulk for bulk, to obtain the other substances he requires. Man is not built to deal with the diet of a herbivorous animal ; the arrangement of his alimentary canal proves his disability to cope satisfactorily with large quantities of vegetable foods. The use of the proper food-stuffs is influenced by the very important consideration of cost. Unfortunately many persons are unable to procure those articles which serve best to supply the necessary carbon and nitrogen for the upkeep of the body, and have to content themselves with cheaper food. In Table LXXVI. details are given as to the values of some common articles of diet represented in terms of the purchasing price of one penny. These values have been calculated from the per- centage composition of the food substances. The percentage figures for the nitrogenous constituents regarded as grammes have been multiplied by 5, and of the fatty bodies by 3, while those for the carbo-hydrate material have been added unaltered to the figures obtained from the first two (Konig). For instance, oatmeal contains about _i4.5 per cent, of nitrogenous matter, 6 per cent, of fat, and 65 per cent, of starch or carbo-hydrates. Multiplying as above, we get 14.5x5 = 72.5, 6x3 = 18, and 65, which added together give the food-value of 100 grammes of oatmeal as 155.5. Lean beef in the same way (21 x 5 -i- 1.5 X 3) gives 109.5. -^s a pound corresponds to 453.5 grammes, the food value of i|d. worth or a pound of oatmeal will be 24 370 THE NATURAL HISTORY OF DIGESTION. ^ ^ SO) ^ •« ^.a O O "-^ O 00 vrj o. f 'J-; 00 irioo f^ ON r^OOr^w-^i-''-' t^ 1^00 00 \o 10 M hi vo rr^^O r^ M •-« d <-« ci M l-i .S"i I O 00 t-^ Q\ f woooa\a* o«M ^ONt^ f^ > X X 1-1 w F-l pa o OJ , rt rt 5^ (U w So "S H c "5 v< O O vS B ° Opq cqQpq pq o 1 £ c < bn M rt Missing Page FOODS. 371 453.5 X 155.5 -=-100, or 705.2, or the value of the amount which can be bought for a penny is 470, while the value of a penny- worth of lean beef at lod. a pound will be 49.7. It is at once apparent from the figures given in the table that — (i) carbon is cheaper than nitrogen; (2) that more nitrogen can be bought for a penny when contained in vege- tables than when in flesh or other animal foods ; (3) that the starches and sugars are cheaper sources of carbon than fats ; and (4) that carbon can be obtained at a less cost from fats and oils in the form of butter or olive oil than when bought along with flesh. Another factor, however, has to be considered, the comparative digestibility of these classes of foods. Thus, although 12.48 grammes of nitrogen contained in about 10 ounces of dried pease can be purchased for one penny, the much smaller amount present in a pennyworth of beef — 1.6 grammes — is probably more easily digested and absorbed, and may serve better for the purposes of nutrition. (Diagram IX.) Nowadays the kind of food eaten by man, as has been noted, is largely influenced by monetary considerations. A working man too often looks at the question from the point of " How much can I spend on luxuries and still leave enough for the sufficient supply of food for my household ? " The wife discouraged, taught at the board-school many things, but little cooking, can only provide articles which require a minimum of preparation and afford a minimum of nourishing power. Tea and bread with tinned meats form the staple food of too many households. Fifty years ago the country labourer, although he may have had to pay a little more for his staple articles of diet, lived much more logically than his present-day successor. The food of the English labourer consisted principally of wheaten bread, cheese, and potatoes, with bacon or butcher's meat once or twice a week ; the Scottish worker obtained a sufficiency of nutritive elements from milk, oatmeal, barley-bread, and, occa- sionally, salted fish or flesh; while the Irishman could only afford to live on potatoes, and was often unable to buy salt to eat with them. The Reports of the Poor Law Commissioners prior to 1838 contain many allusions to the food eaten by the inhabitants of Ireland about that date. The following table may prove interesting : — 372 THE NATURAL HISTORY OF DIGESTION. Table liXXMU.—Tke Weekly Expenditure of an Irish Labourer, with a wife and two children, receiving a wage of nine shillings iter week. Two pecks (28 lb., 12,692.4 grms.) of oatmeal at gd. i 6 Five pecks (70 lb., 31,730 grms.) of potatoes at 5d. 2 i Milk (say 8 pints or 4536 c. cm.) at \\&. (?) - - \ o Loaf of bread {4 lb. or 18 1 3. 2 grms. ) - -06 Ilalf-an-ounce of tea (14. 17 grms.) and half-a-pound of sugar {226.6 grms.) - - 05 One pound of bacon (453.3 grms.) - o 6 Herrings or other fish (say 2 lb. or 906.6 grms.) - 06 Coal, oil, and soap - - - i oj Tobacco - o 3 Rent - - 10 Total per week 8 9J Per head per week, 2s. 2.375d. Per head per day, 5. 2d. About 415 grammes of nitrogen would be afforded by such a diet, allowing 20 grammes per diem for the labourer, 18 grammes for his wife, and 10.5 grammes daily for each of his children, while 9625 grammes of carbon distributed in the same proportions in the relationship of i gramme of nitrogen to 20 of carbon would give the labourer about 400 grammes a day, his wife about 360, and each child about 280 grammes of carbon daily. The diet is bulky and sufficient, but contains 2524 grammes of carbon in excess. The good old custom of the Scottish people in making free use of oatmeal in the form of porridge or oatcake gives one a creditable instance of their foresight in the care of their "bawbees." Porridge andjnilk, with perhaps a little cream added, provides the requisite pro- portions of nitrogen and carbon, and has the advantage of cheapness. Of late, however, the increased facility afforded to the inhabitants of the towns and the country of obtaining bread, tea, and meat is tending to cause a deterioration in the race. Porridge is thought not to be good enough, while the money spent on tea and butcher-meat leaves only enough over to provide for the cheapest of the other food-stuffs. Table LXXVIII. — Dietaries of Working Men in various Countries in 1835. In a " Statement of the Provision for the Poor, etc." (1835), Senior gives the following particulars, among others, of the usual food of a working man and his family in different countries : — • FOODS. 373 America. — New York — Tea, coffee, meat twice a day. Massachusetts — Poultry, meat or fish, twice or thrice a day. Mexico — Maize as porridge, or thin cakes and beans. Columbia — Chiefly animal food. Venezuela — Maize, vegetables, and fruit. Uruguay — Animal food. Hayti — Plantains, sweet potatoes and other vegetables. Europe. — Norway — Herrings, oatmeal porridge, potatoes, oatmeal bread, bacon or salt beef twice a week, fish. Sweden — Potatoes and salt fish in the south, porridge and rye bread in the north. Russia — Rye bread, buck wheat, sour cabbage, soup with salt and lard. Denmark — Rye bread, coffee, groats, potatoes, butter, cheese, and milk. Germany — Rye bread, potatoes, meat once or twice a week, soups, beer, peas porridge. Belgium — Bread, potatoes, milk, butter, and an occasional bit of pork. France. — Havre — Meat seldom, bread, vegetables, cider, coffee, and treacle. Marseilles — Soups, farinaceous chiefly, now and then meat soup and bouillee. Piedmont — No meat, more maize than wheat. Greece. — Meat seldom, maize, bread, olives, pulse, vegetables, and salt fish. From this table it is apparent that in the thirties only in North America could the working man afford to eat meat twice or thrice every day. In the other countries noted meat was seldom obtainable oftener than once or twice weekly. In London, at the present time, the barest subsistence diet, which is quite inadequate should hard manual labour be imperative, consists of three pounds of butcher meat and one of fat, either that on the meat or taken in the form of lard, dripping, or butter, added to which are two quartern loaves and some salt. If meat cannot be afforded, either two more loaves, or twenty-one pounds of potatoes, or about six pounds of oatmeal are consumed (Hart). Four quartern loaves and one pound of fat seem Httle to support life upon during a week, but they will yield 13 grammes of nitrogen and 320 grammes of carbon per diem, compared with 12.6 grammes of nitrogen and 202 grammes of carbon per diem when three pounds of meat replace the two extra quartern loaves, or 14 grammes of nitrogen and 230 grammes of carbon when six pounds of oatmeal are sub- stituted. The influence of the cost on the available articles of diet can be well illustrated in connection with the table of the Irish labourer's weekly bill in 183S for food on the preceding page. The rise in the price of potatoes from the figure there given to the figure at present ruling in the markets of this country amounts to the difference between -j'jjths of a penny per pound and I Jd. for the same amount, or i^^ths of a penny per pound. The cost of the weekly five pecks of potatoes would therefore be 7s. 3|d. instead of 2s. id., a rise of 350 per cent., and making the weekly expenditure from this cause alone 14s. instead of 8s. gjd. Similarly, he would now pay i^d. for each pound of oatmeal instead of M'hs of a penny, a difference of 2s. weekly. As in its original form the cost of the food provided for four 374 THE NATURAL HISTORY OF DIGESTION. persons was almost the lowest possible at that time, and as prices of many of the articles included have risen, a wage of gs. a week cannot now be sufficient for the provision of an adequate food supply for four individuals. An interesting paper by Mrs. Barnett, in the National Review for July 1886, contains several daily menus drawn up from experience of the working-class diet, and for their guidance in the search for the cheapest adequate diet. One of these daily menus is given below with some addenda. (Table LXXIX.) Table LXXIX. — A Daily Menu for a Labourer, his Wife, and Eight Children, in London. Quantity of Food Cost. uaroor Total. laceous. i\ur Carbon. Totaf. ogenous ^Jitroi^eu. "• Oz. Grms. 1 Oz. Grms. Breakfast— Oatmeal Porridge. Oatmeal lilb. 567 grms. 2i 14 226 3 "■3 Tinned milk 1 4 pint 850 c. cm li 2i 340 I 12.5 Treacle lib. 226 grms. A 7 III — Di.NNER — Irish Stew. Meat li lb. 567 „ 8 3^ 88.75 3i 9-75 Potatoes 4 ,, 1712 ,. 2i 14 180 2 4.82 Onions li„ S67 ,. I 5i 34 74 li 1.41 Carrots i„ 226 ., I i 11. s i 0.33 Rice 4 m 226 „ I 7 41.5 4 i.oS Bread i^„ 680 ,, 2i i3i 18.44 2i 8.4 Tea — Bread and Coffee. Bread 2|lb. 1134 >> 3i 22i 3074 3i 14.1 Coffee 2i OZ. 70.75 ,. 2| i 12 9 i 1-37 Tinned milk li pint 850 c. cm . li 2.i 340 I 12.5 Total - 17.01b. 7676 grms 2 5 92 1878.19 i8i 77.56 Per individual 1.7 ,, 76.76 ,, 029 9.2 187.8 1.85 77.5 Amount obtained for one penny 9.3 oz. 26.4 ,, o I 3.1 64.7 0.64 2.67 The cost therefore comes to less than 3d. a day for each — say 2i^d. for each of the children, 4d. for the wife, and 5d. for the labourer. To this must be added two shillings and sixpence a week for rent and sundries, bringing the total for the week up to 19s. sd. In the case of the Irish labourer with two children, with proportionate expenses, he would spend at least IDS. sj^d. each week, or is. 5>^d. over his earnings. The Irishman obtained 5.1 grammes of nitrogen and 120.3 grammes of carbon in his food for each penny spent on it, but the London worker only could obtain with this menu 2.67 grammes of nitrogen and 64.7 grammes of carbon for each penny spent on food. A rise in the price of bread of as FOODS. 3^5 little as one farthing for each quartern loaf, or a penny per peck of potatoes, sounds little, but means much to the poor. The price of nearly all staple articles of diet rises in sympathy with those of bread or potatoes. The question to the well- to-do or the better middle-class is only one of a change from one adequate diet to another and cheaper one ; to the poorer classes it often represents inability to purchase the ordinary subsistence diet previously obtainable, and an ability to purchase food in amount or in character insufficient for their daily needs. (Diagram X.) We have already seen that a diet, to be adequate, should contain at least 18-20 grammes of nitrogen and 300 grammes of carbon, in the relation, therefore, of one part of nitrogen to fifteen of carbon. If the diet consist solely of lean meat, the nitrogen will correspond to five times its weight only of carbon, and to procure enough of the latter, thrice the quantity of nitrogen necessary will have to be ingested. Peas and beans also contain less carbon in proportion to the nitrogen than is advisable if they form the only food taken. On the other hand, oatmeal yields 20 grammes of carbon for each gramme of nitrogen, and bread similarly yields 22 grammes. To obtain 20 grammes of nitrogen during twenty-four hours from oatmeal alone, 400 grammes of carbon require to be ingested, or 100 grammes more than needful; if bread alone be eaten, the excess of carbon reaches 140 grammes. Still more striking is the extra amount of carbon which has to be consumed by any one endeavouring to obtain sufficient nitrogen from potatoes, as 660 grammes of carbon, or an excess of 360 grammes, are necessary before 20 grammes of nitrogen are taken in. An exclusive diet of rice or of apples necessitates the consumption of io6o grammes and 1414 grammes of carbon respectively before 20 grammes of nitrogen can be obtained from them. 376 THE NATURAL HISTORY OF DIGESTION. DIAGRAM X.— Showing the Relative Amounts of Fats and Carbo-hydrates, as compared with Proteids, in the Diet of Man and the Domestic Animals best suited for keep- ing up the Body-weight. Proteid material represented as one gramme per kilogramme of body-weight. Sheep. Fattening. The columns in this Diagram are largely based on Meade Smith's data, and represent the normal proportion of fat and carbo-hydrate to proteid bodies m the food best suited for the upkeep of the body-weight, and which entails the least loss of energy required to deal with the surplus'. FOODS. 377 Table LXXX. — Comparison of the Carbon and Nitrogen in Food Stuffs. Excess or Defect of Amounts necessary to Cost Carbon ingested in ■ Food-stuffs. afford 20 grammes in grammes. of Nitrogen. Pence. Above 300 grms Below 300 grms. Beef, lean 602 grms. (I J lb.) . . 13d. . — 230 „ moderately fat 634 „ (lilb.) . . I2ld. . . — 196 Oatmeal 1750 „ (2ilb.) . . 3id. . 100 Bread 1600 „ (341b.) . . Sid. . 140 — Cheese 384 ,, {0.8 1b.) . . 7id. . — . 166 Potatoes 6500 „ (Hi lb.) . . i8d. . . 360 — Rice 2666 „ (5* lb.) . . li\A. . 760 — Milk 3330c.cm. (sfpt.) . . ii4d. . — ■ 65 Eggs - iooogrms,(2.2lb.) . . 44d. . — ISO Dried Peas 606 „ (4 lb.) . . I4d. . 57 Butter 13.333 .. (291b-) ■ . 464d. . • 9033 — Dried Cod - 166 „ (ilb.) . . id. . 219 Illustrative diets capable of furnishing 20 grammes of nitro- gen, and approximately 300 grammes of carbon, can be easily constructed from the figures given in the preceding table. The details of three theoretical dietaries are given below. Table LXXXI. — Theoretical Diets containitig 20 Gra7nmes of Nitrogen per Diem. Grammes Grammes Articles of Diet. Amount. Coat. of of Nitrogen Carbon. /-Beef, moderately fat .. 226 grms. (ilb.) .. .44d. . • 7-1 ■• 35-5 Bread •• 453 ., (I lb.) .. . lid. . • 5-7 .. 123.0 Milk .. S67 c- cm (I pint).. , 2d . • 3-4 .. 40.0 1 .] Fat, as Butter (at 1 2d. ) 1 1 3 grms (ilb.) .. . 3d. . • • 79-1 Cheese l- Total ■• 74 ,. (ilb.) .. . Id. . 1 2d. '. 3^8 20.0 •• 2S.9 1433 grms. 303-5 /'Oatmeal .. 226 grms. (ilb.) .. . Sd. . ■ 4.52 .. 90.4 Milk .. 567 c. cm. (I pint).. . 2d. . ■ 3-4 .. 40.0 Bacon .. 151 grms. (ilb.) .. . 4d. . . 0.68 .. 94-8 2. " Eggs (Cooking) •■ "3 ., (ilb) .. . 2d. . . 3-6 .. 16.4 Bread •• 339 „ (Sib.) .. . Id. . . 4.2 .. 92.2 Cheese '^ Total ■• 74 „ (ilb.) .. . Id. . . 3-8 20.20 .. 25.9 1488 grms. io|d. 359-7 'Milk .. 850 c. cm. (14 pint) ,3d. .. • SI . . 60. Potatoes .. 679 grms. (141b.) Tjd. .. . 1. 81 •■ 67.S J Butter •• 151 .. (ilb.) .. . 4d .. . 0.24 •■ 105 7 3-1 Dried Fish ... • • "3 .. (i lb.) .. . *d. .. • 1356 20.71 ..'115.0 Total 1793 grms. 9(A)id. 348.2 378 THE NATURAL HISTORY OF DIGESTION. The first of these three diets would cost a shilling a day, and would even then be just sufficient to sustain the adult body at a constant level, provided that the consumer of it did not indulge in severe bodily exertion. If the amount of nitrogen is desired to be raised, along with no great change in the amount of carbon, more meat can be given and less cheese. The second dietary scheme is largely based on substances other than flesh. It costs less, but contains an excess of carbon. The third scheme is the cheapest of the three, but the fact that the nitrogen is present for the most part in the form of dried fish renders it less agreeable and less assimilable. A sufficient quantity of nitrogen and carbon could be obtained from 1^2 pound of oatmeal, i^ pint of milk, and i egg (2 ounces), at a cost of only about Sy^d. It has been pointed out in a previous chapter that a diet, however fully it satisfies the theoretical conditions as to the amount of nitrogen and carbon which can be obtained from it, answers the purposes of nutrition best if the constituents be present in all the- three forms — nitrogenous, starchy (or sugary), and fatty substances. A theoretically perfect diet, yielding its nitrogen from flesh and vegetable substances, and its carbon only from starch or sugar, may not be so conducive to the maintenance of bodily health as a diet containing the same quantities of these elements in which the carbon is present in fats as well as starches. Waller, in his Human Physiology, gives as an ideal complete diet— Table LXXXIL— /^e^/ Daily Diet. ( Waller.) Carbon. Nitrogen. ' \. — I pound bread .. 117 grammes . •• 5-5 grammes. Foundation- 2. — i ,, meat ■ ■ 34 •■ 7-5 l3-i „ fat .. 84 „ 4. — I ,, potatoes Ar-rps^nrip.; S-— 4 pint milk Accessories -, g_ j p^^nd ggg^. .7. — -J- ,, cheese ■■ 45 •• 1-3 20 ■■ IS •■ 1-7 .. 2.0 ,, 20 ,, •■ 30 Total ... 3I pounds .. 335 grammes . .. 21.0 grammes. (1324 grammes). This diet is both liberal (giving rather more nitrogen and carbon than necessary) and somewhat costly ; at the current prices it would cost i4^d. Any one accustomed to the food provided for him in a fairly well-to-do household who FOODS. 379 compares the items which should suffice for his physiological requirements with those which he is pampered with, may well resolve to eschew much that he has been in the habit of consuming, when he realises that getting rid of the excess over the necessary quantities imposes a strain upon his organs. Most people who can afford it eat too much, too often, and too much at one time. A savage who gorges to repletion when he happens upon an abundance of food, but who probably lives upon nothing save a tightened waist-band for days between, is not an example applicable to dwellers in this country, save, unfortunately, those who do not, except at rare intervals, possess the wherewithal, from their own fault or from misfortune, to purchase enough to sustain life at a normal level. The well-to-do — including in this term all those who can afford to buy a variety of foods, especially if they be dwellers in cities, as too many are nowadays, where vitiated air, too little exercise, and deficient air cause diminished metabolic processes — are prone to eat too much, and what they do eat is often in a much too stimulating, and therefore irritating form. The Food of Herbivorous Atiimals. — The various plants which serve Herbivora for food may be divided into three classes : a, plants which are nutritious in all parts ; b, plants of which only a part may be eaten ; c, plants which can only be eaten by certain species, acting as poisons to other animals. Many of the plants which form the sole food of the Herbivora are also consumed by members of the Omnivora. As a general rule the parts of the plant which appear above ground are most suited for animal re ; liu-ments, especially during the period which elapses between cj 'y but well-commenced growth and flowering. When too young they are too watery, when they have flowered they are too dry. On the contrary, edible roots are most nutritious at a later period, after the flower has blown, when they in fact have progressed some distance towards their appointed goal, the provision for growth in the follow- ing spring. Most fruits are only edible when ripe. The bark, stems, and woody roots are more indigestible. Many animals, however, feed and thrive upon such stubborn material. The beaver, for instance, and numerous species of insects make use of it. The Red Indian in New Mexico regards the inner bark of a species of pine in the light of a delicacy, and delights to chew what must seem to others a far from delectable sweet- 380 THE NATURAL HISTORY OF DIGESTION. meat. As has been already described, the majority of animals dependent solely upon plants for their nourishment are pro- vided with some modification of the alimentary canal whereby they can make profitable use of cellulose, a carbo-hydrate which is absolutely indigestible by the carnivora. The food of cattle, horses, and sheep may be taken as the type of a her- bivorous diet. Their foods may be divided into green and dry fodders, roots, grains and fruits, food-products, and milk. Green fodder and roots differ from the others principally in the amount of water present. Grass contains as much as 75 per cent, clover 80.2 per cent., of water, while in hay water only forms from 14 to 17.5 per cent, of the total. Turnips contain 88 per cent, of water. Green grass and clover ordinarily contain about 3.5 per cent, of vegetable albumin, 0.75 per cent, of fat, and about 8 per cent, of carbo-hydrates, 5. 8 per cent, of which is in the form of cellulose. This relative nutritive value is therefore 27.75, compared with 40.61 of milk and 109.5 of beef. Clover-hay with only 16 per cent, of water affords 13.4 per cent, of albuminous matter, 3.2 per cent, of fats, and 25.4 per cent, of cellulose, corresponding to a nutritive value of 102, or almost equal to beef. The straws are much less nutritious, wheat-straw having a value of 59.1 and oat- straw of 65.7, but these values are rendered smaller in actual use from the greater difficulty experienced in digesting them. Grasses alter much in composition in accordance with the season. Pott found that while bay grass at the beginning of May contained 27.9 per cent, of albumin, and at the end of June 12.8 per cent, in the middle of August the proportion had fallen to 7.8 per cent During the same period the proportion of cellulose rose from 17.7 to 29.7 per cent. Young grass is therefore more nutritious than old, although containing more water, while owing to the large proportion of albumin the addition of some chopped straw to the diet is advisable. Green fodder is also more digestible than dry fodder, experiments showing that an excess of 2.7 to 3.2 per cent of proteids, of 4.1 to 5.6 per cent, of carbo-hydrates, and of from 2.4 to 4 per cent, of fats is digested from green fodder over the amounts made use of from dry food. Proteids Fats per cent. per cent. Oats — Ruminants 77-3 82.4 Horses 86.0 ■ 77.6 Oat straw — Ruminants 40.7 30.1 Wheat straw ,, 27.0 35-0 Horses 36.0 84.0 Barley — Ruminants - 77.0 1000 Horses 80.3 42.4 (Hogs) - 7S.2 ., 684 Dried Peas — Ruminants 88.0 74-7 Horses 83.0 69.0 (Hogs) 87.0 36.0 Esparcet clover — Ruminants 72. 5 66.7 Meadow grass — Ruminants 70.0 60. Horses 69.0 13-4 Oil-cake — Hulled cakes — Ruminants - 84.7 87.6 UnhuUed cakes ,, 73-4 90.8 FOODS. 381 Table LXXXIII. — The Proportional Amounts of the Con- stituents of Vegetable Foods digested by the Hei-bivora. {Afeade Smith.) Carbo- hydrate per cent. 73-7 76.3 45-5 44,0 24.0 87.0 87.3 90.0 93-3 89.0 97.0 78-3 73-0 66.0 95 I 46.2 As a rule horses can digest more proteid hut less fat and carho-hydrate than ruminants. Horses make better use of oats than ruminants, but generally are unable to digest as great a proportion of the nutritious elements of fodder. COMMON FOOD SUBSTANCES USED BY MAN. Milk is the most digestible of all foods if consumed in a reasonable way. The sole nourishment of young animals, it contains every element required for a typical diet. Many individuals think that milk does not agree vpith them, " It curdles on the stomach, and becomes very acid." Such state- ments accord with fact, for milk should naturally curdle in the stomach. We have already seen that there is a special ferment secreted by the gastric glands for the sole purpose of curdling milk. The curds, however, which form in the stomach of the adult are usually firmer, of closer texture, and more indigestible than those in the child, owing to the addition of water in varying quantity to the child's portion. Acids curdle milk as surely as the curdling ferment, although the resulting clot is of a somewhat different character. Cow's milk forms large clots in the human stomach, and is therefore unsuited for giving to infants unless it is treated in some way or other beforehand. Human milk clots as a granular mass, similar in appearance to 382 THE NATURAL HISTORY OF DIGESTION. that produced by the addition of acid to cow's milk. The majority of adults who believe that cow's milk is to them more or less of a poison would change their opinion were they to Fig. 48. THE UDDER AND NIPPLE OF THE COW (IN SECTION). (Meade Smith, after 'riiatiliuffer.) B, Nipple; Ufa, Glandular tissue; ni,s', Acini of gland; tj. Milk-ducts; C, Milk cistern ; r, Folds in the wider milk-ducts ; z. Sphincter muscle ; kb, External skin ; H, Milk-duct in nipple. dilute it with lime water, or if they refrained from drinking more than a claret-glassful of pure milk at a time, in place of the tumblerful so often gulped down. The admixture of FOODS. 383 simple aerated water to milk renders it more digestible owing to the passage of bubbles of carbonic acid gas through the fluid and the consequent softer curd. Boiled milk is more indigestible than raw milk owing to the increased toughness and density of its curd, but the process of boiling destroys any bacteria which may be present. Bacteria revel in milk — a perfect food for them — and flourish exceedingly. The addition of some phospho-carbonate of calcium (lime) will make the clot more digestible. Milk splits up in the stomach into an insoluble curd and a fluid termed the whey. The nitro- genous body originally present, a nucleo-globulin called casein, is split up into an albumose which dissolves in the whey, and an insoluble body, para-casein (Hammarsten, Maly's Jahres- bericht, vol. ii., 1872), which forms the clot, but only if lime salts are present. Removal of all lime salts prevents the curd- ling of milk by ferment action. Hammarsten found that one part of the "lab-ferment" or rennet causes the curdling of at least 400,000 to 800,000 parts of casein. (Fig. 48.) In Diagrams XL and XII. the relations between the milk of various of the Mammalia are graphically shown. In Diagram XL the nutritive value of the different milks is given as com- pared with that of cow's milk. It is apparent that the milk of the cat, the sow, and the sheep yield more nourishment, bulk for bulk, than cow's milk ; and that human milk falls short of it, but not by so much as the milk of the mare or ass. The variations in the individual constituents of the milk of these animals, as shown in Diagram XII., also arranged in terms of the amounts present in cow's milk, are very startling. The amount of proteid in the milk of the only carnivorous mammal included — the cat-^is 5.4 times greater than the amount present in mare's milk; the highest figure for fat, occurring in sow's milk, is 4.4 times that of the ass ; the figures for the amount of sugar present do not show so great a divergence, the sugar of the mare's milk, the highest in value, being 1.7: i compared to goat's milk. The milk of the cat owes its high nutritive value to the large proportion of proteid present in it ; it is very deficient in fat, and contains a little more sugar than cow's milk. Mare's, camel's, and asses' milk afford a direct contrast to the milk of the sheep and the sow. The milk of the first series contains less proteid and fat and much more sugar than cow's milk, while the proportion of proteid and fat in the milk of the second is 384 THE NATURAL HISTORY OF DIGESTION. ^^ ■^~' ■- "■■" ■— — — — _ _ _ _ _ O J3 FOODS. 38S DIAGRAM XII. — Showing the Relative Proportions of Proteids, Fat, and Sugar in the Milk of various Mammals, Cow's Milk being taken as 100. ^ T ■~ 1 1 1 j — _ ~r- S- -[-^ - - - --' j Irt £ - s ■ * '^^- i o3 dI- ^s.^ - 3- 1 uJ E iA £ 3 i_ 5 . 1 J" ^ < s _ s u^ X s £; S 1 ^ y S £ Ln 3 . § 0^ — s Lis X hsj: °g ~^ as '■i'T^ S 1 -___, 1 1 i 1 25 386 THE NATURAL HISTORY OF DIGESTION. above, the proportion of sugar below, that in cow's milk. Human milk, as compared with cow's milk, is deficient m proteid and fat but contains more sugar. When cow's milk is used for the nourishment of infants it should therefore be diluted with water and some sugar added. The sugar should be in the form of lactose or milk sugar, not of cane-sugar, as the latter is much more liable to undergo acid fermentation. The popular idea that the milk of one cow can be advantageously employed in infant feeding is incorrect. The milk of one cow is much more likely to vary in quality owing to adven- titious circumstances than the mixed milk of a herdj the cow selected may be the subject of incipient disease ; and, again, as the period of time which has elapsed since the dropping of a calf lengthens the milk of the cow becomes poorer. As care is taken to spread out the time of calving among the different members of a herd of milk cows the deterioration of the mixed milk from this cause is avoided. Sterilised milk, or milk heated near to but not up to the boiling point, is less dangerous than fresh milk as regards infection, and it acquires little of the pronounced taste of boiled milk, but even then it is often not relished and is not so digestible. The composi- tion of cow's milk varies very markedly with the food given, and in connection with the manner in which the cows are kept. The following table shows the differences between the milk of cows of various breeds, and indicates that the Dutch and Breton cows give the most nutritious milk : — Table LXXXlV.—A^ufrifive Value of Milk from Cows of va?-ious Countries. Nutritive Value. Proteid Value. Fat Value. Sugar Value. Holstein 60.126 26.760 29.640 3-76o Brittany 48.536 26.870 17. 112 4.554 Dutch 45-983 2I-095 20-58 4.350 Durham 45.000 21.800 19.230 3-970 Belgian 42.252 20.100 18.660 3-292 Swiss 38.694 12.820 21.264 4-590 Normandy 37-59 7 23.665 9.720 421 2 Mean - 45-455 (3-89 per cent. Proteid, 7.3 per cent. Fat, and 4.09 per cent. Sugar). The milk from Holstein cows has by far the greatest quantity of cream, 9.88 per cent. (29.64-^-3), the Normandy breed the least, only 3.24 per cent. The mean figures for the selected FOODS. 387 series show an excess of fat — 7.3 per cent, instead of 4.53 per cent. — over the figure given as the usual proportion of fat in millc. The Holstein and Brittany cows give a millc with much casein in it, the Swiss cows a millc which is remarkably poor in this substance. The milk of the Normandy cow is peculiar in that it contains normal quantities of proteid and sugar, but very little milk-fat. 1 here is a popular idea that in adults a diet of milk alone is synonymous with slow starvation. This is by no means the case. Granted that adults cannot sustain life indefinitely upon milk alone, it is nevertheless true that from five to six pints of milk drunk during the twenty-four hours provide the necessary nitrogen and only a small excess of carbon {vide Table LXXX.). The value of milk as a diet is clearly shown by the accompany- ing table (Table LXXXV.), in which the nutritive value of a pint (567 c.c.) of milk is contrasted with the nutritive values of various food articles. The values are given in terms of Konig's factors — i.e., the amounts per cent, of proteids, fats, and carbo- hydrates multiplied by 5, 3, and i respectively and the results added together. To render the table more complete, the corresponding quantities of those foods which are necessary to yield the same amount of heat as that afforded by twenty ounces of milk are added. Table LXXXV. — A Pint of Milk, 20 minces, corresponds i?i- Nutritive Value to Heat I Production to I. 3.2 ounces of Butter 1. 1.8 ounces c f Butter 2. 34 „ Cheddar Cheese 2 2.8 Cheddar Cheese 3- 3.5 ,, Ham J- 2.9 Ham A- 4.7 ,, Beans 4. 4-3 Beans 5- 7.0 Hare 5- 4.8 „ Cream 6. 7.69 Beef 6. 5-7 Bread 7- 8.2 ,, Cream 7. 8.3 ,, Eggs 8 8.3 ,. Eggs 8. 10. ,, Salmon 9- 8.9 ,, Salmon 9. 12-5 Beef 10 9-5 Bread 10. 12.5 Hare II. 30.0 Skim Milk II. 15.0 Potatoes 12. 30.0 ,, Oysters 12. 26.0 Beer 13- 32.0 ,, Potatoes 13. 28.0 Skim Milk 14. 36.0 Butter Milk 14. 36.0 Butter Milk IS- 42.0 Beer 15- 54.0 Oysters 16. 84.0 Whey 16. 70.0 ,, Whey 17. 86.0 Strawberries 17. 128.0 Strawberries 18. 203.0 Beef Tea 18. 200.0 ,, Beef Tea 19. (6.6? Whisky) 19. 3-7 Whisky 388 THE NATURAL HISTORY OF DIGESTION. It will be seen that in nutritive value a pint of milk is equal to, among other foods, 7.69 ounces of beef, 8.3 ounces of eggs, 3.4 ounces of cheese, 30 ounces of oysters, 32 ounces of potatoes, 42 ounces of beer, and 203 ounces of beef-tea; while the combustion in the body of 20 ounces of milk affords as much heat, everything being equal and the combus- tion complete, as 1.8 ounces of butter, 2.9 ounces of ham, 5.7 ounces of bread, 12.5 ounces of beef, 128 ounces of strawberries, and 3.7 ounces of whisky. It must be kept in mind in this connection that the nutritive value of foods and their relative power of yielding energy by combustion do not run parallel with the foods required by the body. Butter, for instance, affords a large number of heat-calories when oxidised, but no one can live on it alone. The ease with which the different foods mentioned can be digested, and the proportion of their elements absorbed, also complicate the question. It would take up too much space to correct the figures given by the factors for their digestibility. Of the casein in milk 92 per cent, is absorbed, all of the sugar, and 95 per cent, of the fat, so that practically all of the chief constituents of milk are made use of under ordinary circumstances. On the other hand, only 75 per cent, of the small proportion of albumin in the potato is digested, and 93.5 per cent, of its starch. ■Eggs. — The average weight of a hen's egg is 51. i grammes, or nearly 2 ounces, 11.9 per cent, of which consists of shell, 55 per cent, of white, and 33.1 per cent, of yolk. The white is composed of egg-albumin dissolved in water and surrounded by a dehcate membrane lining the inside of the shell. The yolk contains fats, olein, palmitin, cholesterin, and lecithin (cf. p. 149), extractives, sugar and salts, much phosphorus, sulphur, and iron. The fresher the egg is, the more nutriment does it contain; the more completely cooked, the more indigestible does it become. Raw eggs are easily digested, soft-boiled or poached eggs less so, while hard-boiled or fried eggs take longer to quit the stomach than any other food which has been hitherto tested save salt-pork and cabbage. Raw eggs remain 2 hours, soft-boiled 3 hours, and hard-boiled eggs 5 hours in the stomach. To obtain a boiled egg in perfection the usual method of placing it in boiling water or steam for a short period is faulty. Whenever the white of the egg next the shell be- comes heated above 73° to 75° Cent, it coagulates and prevents the ready passage of heat to the parts within. When very fresh FOODS. 389 the white within the shell is often coagulated with difficulty, it rather becomes creamy. The same condition may be brought about by cooking eggs not so newly laid in water kept at or about 73° Cent. (163° Fahr.) for a considerable time, whereby the heat penetrates the egg substance equally, when raising the temperature of the water for a brief period will give a soft- boiled and creamy character to it. As the albumin or white of egg, in solution, does not coagulate on boiling in the absence of an appreciable proportion of neutral salts in the water, the addition of some common salt in poaching eggs is desirable. When the white is in a concentrated form, however, as in the egg itself, coagulation occurs. The com- position of, and other details about, the white and yolk of eggs are tabulated below. Table LXXXVI. — The Cotnposition and Nutri/ive Value of Water. Yolk 50.82 White 85.75 Total Egg ... 73.67 6.2 oz. of yolk, 17 oz. of white, or 10.9 oz. of total egg equal 10 oz. of lean beef. One hen's egg of 2 oz. corresponds to 1.8 oz. of beef. The table shows that the yolk is vastly more nutritious than the white, while the total nutritive value of an egg is very slightly below that of beef. The amount of energy obtainable from an egg is greater than that of lean beef owing to the greater proportion of fat present, 163 calories to loi calories. Breads. — The " staff of life " can only be properly made from wheat and rye, owing to the presence in these cereals of a nitro- genous body, gluten, which enables wheat and rye flour to be kneaded with water into a tenacious (glutinous) dough. This dough is divided into two portions, one twice the size of the other. To the larger portion salt is added, to the smaller the ferment, yeast. The " leavened " dough is set aside in a warm place until it has become porous, owing to the production of carbonic acid by the action of the yeast on the sugar present in it. It is then thoroughly mixed with the larger portion set aside beforehand, the whole allowed to ferment until the risen mass tends to sink again. It is then fired in an oven. The changes which occur during fermentation consist of a lletis Eggs. ^roteid. Fat. Carbo- Nutritive hydrate. Value. 16.24 31-75 0.13 176.68 12.67 0,25 64.10 12-55 12 II 0.55 99-63 390 THE NATURAL HISTORY OF DIGESTION. partial change of starch into dextrin and sugar, and of sugar into carbonic acid gas and alcohol. The action of heat changes much of the starch into dextrins, especially in the crust, while the previously soluble albumins are rendered insoluble. The process of "raising" bread by the action of fermentation has now been largely supplemented by the chemical formation of carbonic acid in the dough from the reaction between bicarbonate of sodium and tartaric acid, or by forcing carbonic acid gas into the dough under pressure. New bread to some persons is indigestible, but it has the advantage over older bread of being practically free of germs. Rusks are pieces of bread slowly baked until all the starch has changed into dextrins, while biscuits are forms of unleavened bread toasted until the same change has occurred. Bread made from fine flour contains less albumin and more starch and sugar than bread made from coarser varieties. Whole meal bread gives a much higher percentage of proteids, a slight increase of fat, and a lower proportion of starch and sugar. The digestion of whole meal bread, however, is not so complete, and there is a much greater quantity excreted unchanged than in the case of white bread. The composition of bread-stuffs, along with their additional merit of cheapness, render them of great value for food; and, as bread forms the larger part of the food of the working classes, especially of those resident in towns, it is a matter of great importance that it should be made in such a way and of such materials as to render it as nourishing as possible. About 60 ounces of moist bread will yield sufficient nitrogen, and more than double the quantity of carbon necessary for the daily wants of the body, but it affords far too little fat. The addition of milk will remove the greater part of these defects, and with some ham, bacon, or salt fish will serve to form a correct diet. Meats. — The muscular tissue of animal bodies forms the class of foods known as meats. Along with the muscle fibres are the nerves, blood-vessels, and connective tissue which serve in life to nourish and support them, and, what is still more important, a greater or less amount of blood. The proportions between the various constituents of meat, are greatly altered by the presence or absence of fat and blood in the muscle. Blood is usually drained off as freely as possible before a carcase is cut up; in veal, indeed, the blood is almost completely removed. Absence of blood renders FOODS. 391 the flesh paler, more delicate, and less liable to decompose. When an animal has been killed, after a variable period, dependent upon the mode of death and condition of the body, all the muscles become stiff. This is the result of a change in one of the proteids of the muscle fibre. During life this proteid is supposed to be present in the form of myosinogen, which is altered into myosin upon the setting free in the muscle substance of a ferment akin to that causing coagulation of the blood. However it may be caused, the alkaline reaction of living muscle becomes acid. Meat is seldom eaten until this rigor mortis or stiffening has passed off As the characteristic proteid of muscle or myosin belongs to the class of globuUns, it is insoluble in pure cold water (cf p. 145), but is slowly and partly dissolved in dilute salt solutions. Cold water extracts of flesh therefore can abstract no myosin, and will contain only about 2 to 3 per cent, of soluble albumin of the blood in the meat. If salt be added some of the myosin may be dissolved out of the meat, but the maximum proteid strength of a watery solution of meat can never be high. Whenever the water is heated above the point at which proteids coagulate, no further extraction of albumins or globulins can occur. A temperature of from 70° to 80° Centigrade (i58°-i76° Fahr.) will suffice to coagulate most proteids, if there be some salt in the water round them or in their substance. Once they have coagulated they are absolutely insoluble in water, and may be boiled indefinitely without the extraction of any further proteid. The flesh of young animals is more tender and sometimes more digestible than of full-grown beasts, but it is not neces- sarily more nutritious for those reasons. Veal is less nutritious than beef, lamb than mutton. Salted or pickled beef and mutton are rendered less digestible, while ham, tongue, and bacon are more easily digested in the cured state than when used in a natural condition. Figures in connection with the theoretical nutritious value of different meats, the time taken for their digestion in the stomach, and the quantities of each required to afford as much nutriment as 10 ounces of lean beef, are given in Table LXXXVIT. The value of fat beef is seen to be high, that of lean beef less than of many other forms of flesh-food. Bacon and ham head the list, excluding beef powder as an artificial product ; only 4.2 ounces of bacon or 4.5 ounces of ham are necessary 392 tllE NAtURAi. HISTORY OF DIGESTION. to supply the same amount of nourishment as lo ounces of lean beef. Lean mutton, and veal, and kidneys contain less nutriment than lean beef. Fat beef, however, is more nutritious than the majority of the other meats. Beef-tea is credited with only 0.5 per cent, both of proteid and of fat, giving a value far below that of beef, and rendering the absorption of 274 ounces necessary before the same amount of nourishment contained in 10 ounces of lean beef is equalled. With only 0.5 per cent, of proteid, which is equal to 0.08 per cent, of nitrogen, no less than 5.5 gallons, or 25 litres, would be required before 20 grains of nitrogen could be supplied to the body. Table LXXXVII. — The Nutritive Value, per cent.. Digestibility, and Co7nparative Value of certain Meats. Beef (fat) ,, (lean) Mutton (fat) ,, (lean) Veal (fat) „ (lean) Pork (fat) ,, (lean) Ham Game Hare Chicken Liver Tripe Sheep's Kidney Sweetbread Bacon Tongue - Powdered Meal Beef Tea Liebig's Extract of Meat Bovril - Nutritive Value. 165.09 .. 109.50 .. 183.17 .. 102.80 .. 116.62 .. 101.66 .. 184.72 121.68 240.79 118.00 120.29 113.00 117.69 Duration of stay in stomach before complete digestion or removal. ... / Steak, 3 hrs. ... 1 ,, ihr. 30m. to 5hrs. 30m. -[ Stewed, 3 hrs. -j Roasted, 4 hrs. ... f Boiled, 2hrs. 30m. ... \ Roasted, 4 hrs. Boiled, 3 hrs. Roasted, 4hrs. 30m. 2 to 4 hrs. 115.20 91.00 . 113 00 . 256.50 . 123.50 . 380.00 . 4.00 . / Roasted, 2 hrs. \ Raw, 2hrs. 15m. Roasted, i hr. Friedl'^^f^'Slirs. (tat) 4 hrs. 3-03 127-51 ■■■ The nutritive values are given in terms of Konig's fact' Amount of each, equal to 10 oz. of lean beef. 6.6 OE. 10 oz. 6.2 OZ. 10.6 OZ. 9-4 OZ. 10.7 OZ. 5.8 OZ. 9.0 oz. 4-5 OZ. 9-3 OZ. 9-1 oz. 9-7 OZ. 9-4 oz. 9-5 oz. 12.0 oz. 9-7 oz. 4.2 oz. 8.9 oz. 2.9 oz. 274,0 oz. 361.0 oz. 8.6 oz. mentioned (cf. p. 369), Beaumont's researches. s, already the duration of the stay in the stomach from FOODS. 393 The flesh of fish, considered in the same way, as depicted in Table LXXXVIIL, is in many instances of less value, weight for weight, than lean beef, but mackerel, sprats, shell-fish, salt herring, and especially dried cod, yield higher values. The calculated nutriment in lo ounces of lean beef, supposing complete combustion in the body to occur, is contained in only 2.7 ounces of dried cod, while 42.3 ounces of oysters are necessary to give the same amount, or about five dozen of these bivalves, while twelve dozen would suffice to supply 20 grammes of nitrogen. The flesh of white fish is more easily digested than ordinary butcher-meat, chiefly because of the absence of fat. The flesh of oily fish is not so digestible, but has a much greater nutritive value, a value which is much enhanced when the relative prices are taken into account. Table LXXXVIIL — The Nutritive Value and Digestibility of the Flesh of certain Fishes and Shell-fishes, and their Value compared with Lean Beef Nutritive Duration of stay Amounts equal in value Value. in stomach to 10 OZ. ot lean beef. White Fish 99.20 .. Boiled, 3 hrs. .. II.OOZ. Mackerel 137-78 .. .. 8.3 OZ. Herring (fresh) 71.88 .. .. 15.0 OZ. Salt Herring 149.00 .. .. 7.6 OZ. Salmon 97.16 .. Boiled, 3 hrs. .. 11.2 OZ. Sole 60.45 •• Boiled, 3 hrs. .. 18.0 OZ. Eel 89.90 .. .. II.9 OZ. Pike 94-75 ■• .. 10.5 OZ. Sprats 163 00 .. .. 6 9 OZ. Dried Cod 403.00 .. Boiled, 2 hrs. .. 2.7 OZ. Caviar 196.80 .. .. 5-5 OZ- Oysters 25.86 .. Raw, 3 hrs. ■ \ Cooked, 3hrs. 30m. .. 42.3 OZ. (58 oysters) Shell-fish (mean ) 11.3.47 ■• .. 9.7 OZ. Fruits. — Fruits possess very varying food-values; most of those growing in this country are of little use for nutrition, and consist of little more than flavoured water. The most valuable constituents of fruits are their acids, which cannot serve as food, but which appear to be most beneficial to metabolic processes generally. The grape contains the least water, with 78 per cent. ; the strawberry, with 87 per cent, the most. 394 THE NATURAL HISTORY OF DIGESTION. Table LXXXlX.^T/ie Percentage of Water Elements in certain Fruits. and Nutritious Nutritive Value. (Carbo-hydrates Fruit. Water. Proteid. Fat. Free Acid. Sugar. ' tlier than Suffar nob Fresh included.) Apple 83.58 .. 0.39 .. — . . 0.84 .. 7-96 .. 9-91 Pear S3. 03 ,. 36 .. — . . 0.20 .. 8.78 .. 10.58 Plum 81. iS .. 0.37 .. — . . X.50 .. 6.44 .. 8.29 Cherry - So. 26 .. 0.82 . — . . 0.91 .. 11.72 .. 15.82 Peach 80.03 .. 0.65 .. — . . 0.92 ., 4.4S .. 7-73 Strawberry - S7.66 .. I 07 .. • 045 . ■ 0-93 .. 6.28 .. 12.98 Apricot 81.22 .. 0.49 .. — . . 1. 16 .. 4.69 ... 7-14 Damson 81.18 .. 0.78 .. — . . 0.85 .. 6.15 ... 10.05 Grape 78.17 .. 0.59 • — . • 0.79 .. 14.36 ... 17-31 Gooseberry 85.74 •■ 047 . — . . 1.42 ■• 703 ... 9-38 Greengage 80.28 .. 0.41 . — . . 0.91 .. 3-16 5.21 Orange Pulp 89.01 .. 0.73 • • — • • 2-44 .. 4-59 ... 8.24 Dried Apple 32.42 .. 1.06 . — . . 2.68 .. 41.61 . . . 46. 64 Pear 29.41 .. 2.07 . ■ 0.35 . . 0.84 .. 29.13 -• 40.53 Damson 29.83 .. 2.55 . ■ 053 . ■ 2.77 ., 42.65 ... 56.99 Cherry - 49.88 .. 2.07 . . 30 . . — .. 31-22 ... 42.47 Grape 32.02 .. 2.42 . ■ 0-59 • . — •■ 54-56 ... 68.43 Fig 32.21 .. 5.06 . — . • — .. 45-28 ... 70.58 Sugar and Including other non- other nitrogenous Carbo- Nuts, etc. bodies. hydrates. Almonds (Sweet) 5-39 •* 24.18 . . 53-68 . . — ... 7-23 ...289.17 Walnut - 4.68 .. ■ 16.37 • . 62. S6 . .. — ... 7-89 ... 277.32 Hazel-nut - 3-77 ■• 15.62 . . 66.47 • , — ... 9-03 ... 2S6.54 Fresh Chestnut 51.48 .. 5-48 . ■ 1-37 ■ . — - 38.34 ... 69.85 It is at once apparent that the nutritive value of even dried fruits, except of those which may be classed as nuts, is very low. The banana, which is not included in the table, is much more nutritious than the greater majority of fruits. Three dozen are reckoned as equivalent in value to a week's supply of white bread. The pine-apple and the fig contain active digestive ferments, and are therefore more digestible than might be anticipated. The juice and soft parts of the pine- apple are easily tolerated if the fruit be ripe, even by those of weak digestion. (Chart XXIII.) Fungi. — A number of the fungi are edible but only a few species are eaten, owing to fear of the evil results which follow FOODS. 395 CHART XXIII. — Giving the Heat Calories pero.oi gramme of different Vegetables, Grasses, and Fruit, and the Percentages of Proteids, Carbo-hydrates, and Fat contained in them. a O/ M P ^ ■ " -■ Pro- u £■ teids ?-^ A OJ and -B | Fats, ■g ■§ ^ S ^^ 3mh WW - kj d M c OO -^^ X 0) w ^ P d CI- > f- 7^ o -e -S rt 5 eS ^-r: rt O t- ^ fu & C ! S S g S a . 3 59 55 50 25 20 15 10 5 -0- ■ ... ' ■ ■■ 1 — — — — — - - 915 •>•» 90 S7-5 85 625 80 77 i lb 7a S 70 67 & 6i &a-!> 60 57 5 55 53 5 50 47 5 *5 42 5 4-0 57 5 55 525 30 27 5 25 225 20 17 5 15 12 5 10 7 5 5 a 5 _ ,. ir ■ ^i 35 - "■., ,^ A •' -- ^ 31 *■ ^ ■'■ •'\ 1 \ P 1/ \ 1 ' =. ,; [ \ 1 I i \ I ; V /^ k ^ \ \ 20 - 1 -; 1 ,^1 18 - - i < /;\ ic \/ ; N :l \ 15 - ! ' ^ * ; - 1- " '' ; I 1 1 li. 1 " ( 9 ' ■ \ '^~t ■- ,' ! l\ ° ',/ M ' > ■" I' ' ' \ 1 »r :^ ' D \ _, _. _.. t|" '•■, / i,^ ■ '' ^' >■. -n- - _ , ■ _il ..' '-' CAtORIES PER 0.01 GRM -■% Of CARBOHVDRATES (EX- CLUDING CELLULOSD. PROTEIOS f*TS 396 THE NATURAL HISTORY OF DIGESTION. the use of poisonous members of the order. Fungi are more nutritious than fresh fruit, but it is doubtful if the proteid in them is digested to any great extent, while the amount by weight of fungi which can be safely taken is so small compared with the total requirements of the body that they can never form an important item in diets. Table XC. — The Composition of Mushrooms and Truffles. „T i Ti t -^ -¥:<*■ Sugar and Nutritive Water. Proteid. Fat. ceUulose. Value. Muslirooms /Fresh 91. ii 4.68 0.40 3.45 28.05 ■\Dry 17.54 23.84 1. 21 34.56 157.39 Tn,fflp« J B'^'='^ 72°° ^-T^ °-5^ '^'59 62.17 irumes - -j^-v^hite 72.34 9,96 0.44 15.16 66.28 Sugar is present as dextrose and mannite in these fungi. The truffle is found in two forms, the black and the white truffle. The black variety is esteemed for its flavour over the white, but both are extremely indigestible. A lichen, Ceiraria islandiia, or Iceland moss, is given as a food to convalescent patiezits or to those who are unable to tolerate the ordinary articles of sick-room cookery. A decoction of Iceland moss forms a soft jelly on cooling. Cetraria contains wheh dried 70 per cent, of a peculiar starch, lichenin or lichen-starch, which is capable of affording nourishment. The name Iceland moss is a mis- nomer, the plant being actually a seaweed or lichen. THE ACTION OF HEAT ON FOODS. Man, as has been said, is a cooking animal, and the state- ment is true of practically the whole human race. Only a few savage tribes remain who do not find it absolutely necessary to subject their food to the action of heat before eating it. Cook- ing fulfils several functions. The first is one which the man who discovered and began to practise the art, in a manner which doubtless would dismay our present-day sybarites, can scarcely have realised, that is the avoidance of the un- necessary exertion implied by the body being compelled to expend some of its heat in warming the food supplied to it. Secondly, cooking softens many hard tissues and hardens many tough tissues, rendering the first soft enough, the latter brittle enough, for the purposes of thorough masti- cation. Again, many chemical changes are brought about. FOODS. 397 Almost everything expands on heating, and in foods this expansion renders them more porous, and if boiled allows thorough permeation of their substance with water. Starch grains swell and burst their envelopes, cells of all kinds absorb water by imbibition. Connective tissue is altered in part to gelatin; albumins, save the legumin present in many vege- tables, are coagulated; fats rendered fluid, and some of the starch changed into dextrins. In addition, heat causes the develop- ment both in meat and in vegetables of complex substances which serve to flavour them. Raw meat and raw cabbage have practically no smell, but when cooked give off very character- istic volatile, odorous particles which serve, as has been pointed out, to give a flavour to the food quite apart from the actual taste. Another result of thorough cooking, by which man empirically forestalled the discoveries of later centuries, lies in the destruction of all micro-organisms or parasites in the food. Even now wherever the custom prevails of eating some forms of meat in an uncooked state, as, for instance, raw ham in Germany, parasites are frequently taken in with the food in a living condition and produce disease in the body of the consumer. The T?-ichina spiralis not uncommonly is the cause of disease in Germany, here it is practically unknown. Similarly, a greater number of animals suffer from intestinal parasites than man, because the uncooked state of their food permits larvae to enter their alimentary canal with their vitality unimpaired. The successful passage of larvae through the acid digestive fluid of the stomach is rendered possible by the indigestible nature of their outer covering. The disadvantages attendant upon the action of heat upon food-stuffs are few and of small consequence when compared with the advantages which have just been detailed. Several articles of diet are more easily digested before cooking than after it. Raw meat, if pounded to break down the outer more indigestible part of the muscle-fibres, is more easily digested than when cooked. Raw eggs and raw milk are similarly more digestible than when boiled. All methods of cooking may be divided into two classes. The one class aims at the retention of all the juices of the meat or vegetable within the food itself; the other seeks to abstract the juices. The simpler the method, the more easily digested is the food. Few cooks properly understand the rationale of the various forms of treating food by heat. The 398 THE NATURAL HISTORY OF DIGESTION. cardinal factor in most cases is the coagulation of albumin at a temperature much below the boiling point. The actual object aimed at is the adequate heating of the food material in all parts ; the usual result obtained is an excessive heating of the outer portion, with a deficient heating of the interior. In roasting meat it should be placed close to the fire for a brief period, about eight minutes, until the outer albumin has firmly coagulated, and formed a skin which can retain the juices of the inner portion. Then the meat should be removed to a greater distance from the fire, where it becomes heated through at a lower temperature without cracking the surface skin of coagulated albumin. Even when the temperature to which meat is exposed is much above ioo° Cent., the temperature of the inner portion seldom rises much above the coagulation point of albumin. If the process of roast- ing be properly carried out, the longer period during which the meat is exposed to a lower temperature suffices to destroy any organisms or parasites present as effectually as exposure to a higher temperature for a shorter time. Most of the constituents of meat should therefore remain in situ when it is properly roasted, and this mode of cook- ing should result in a minimum of loss. Even here the loss of weight by evaporation of water reaches to 25 percent., or as much as a quarter of the original total. Baking is the same process as roasting but performed in an oven, and produces the same result; indeed, on the Continent meat is invariably roasted in the oven, not before the fire, and many good authorities pronounce it to be the better plan. Broiling is only a variant of roasting, in which the meat is cooked over the fire not before it, and the same results are obtained by it. Roast, baked, and broiled meats are characterised by their delicate though very perceptible flavour and odour occasioned by the retention of much of the fat of oil in the meat, and by the chemical decomposition of some of the fat of the external parts through the agency of direct heat. In boiling meat, also, a skin or outer layer of coagulated albu- min is indispensable for the retention of the soluble parts of the meat. The water should therefore be boiling actively before the meat is put in. A coating of coagulated insoluble albumin at once forms round the meat, and this may be hastened by adding some common salt to the water before putting the meat in, and thereby raising the temperature at which the water boils (Knight). In a short time the outer skin has formed, when the FOODS. 399 temperature of the water should be lowered to just above 75° Cent. (167° Fahr.), and kept at that point. Knight recommends the addition of three pints of cold water for every gallon whenever the liquid boils. Cooking by boihng is usually practised at too high a temperature, and results in the progressive coagulation of the proteids from the outside inwards. Properly an insoluble skin should first be formed, and then the whole joint raised to an equal temperature close to the coagulation point, while it may be afterwards subjected to greater heat for a brief period. Treatment of the meat with water at the boiling point throughout the process overcooks the outer part before the inner part has had time to become sufficiently warmed. In spite of all precautions, a considerable loss of the soluble part of the meat occurs in the process of boiling, but the loss is chiefly made up of extractive matter which is not in itself a source of nutriment. Cooking vegetables by steam instead of by boiling them in water is based upon sound scientific principles. The proteid constituents of vegetables are frequently soluble in boiling water, as also is the case with much of their carbo- hydrate moiety. Hence to boil vegetables and then to throw away the water is simply wasting much of their useful consti- tuents. Many vegetables are valued as much for the salts and acids contained in them as for the common food-stuffs. The water in which they are boiled contains the greater part of these salts and acids. If cooked by steam under pressure, the heat apphed to them is sufficient to burst the envelopes of the starch grains and to soften the cellulose, without occasioning any appreciable loss of soluble matters. The vexed question of the nutritive value of beef-tea and soups is too complicated for complete discussion here. It has been stated on page 391 above that the chief proteid of flesh is insoluble in pure water, and that only about 2 per cent. of albumin, soluble in cold water, and derived from the blood, is present in meat. Beef-tea, made by plunging a portion of lean beef into boiling water and cooking it for some time, affords a fluid, after removal of the beef, about as nourishing as the water in which an egg has been poached. If made by prolonged soaking in tepid water and by gradually raising the temperature, beef-tea contains a little more proteid, but the proteid cannot be in greater proportion than is represented by the 2 per cent, of soluble albumin in the beef, dissolved in an excess of water. Beef-tea, however, made from pounded beef 400 THE NATURAL HISTORY OF DIGESTION. soaked in a little water, cold or tepid, to which lo parts of salt have been added to every loo of water, strained, 'more water added, and the solution then heated to the boiling point, will contain much of the albuminous material coagulated in a fine form throughout the fluid, in addition to all the ordinary constituents of the usual beef-tea. The term beef-tea is a singularly accurate one. Tea is not nutritious, it is merely a nerve stimulant. Beef-tea, as commonly made, is likewise a stimulant, and by no means a food. Many of the meat extracts in the market are practically solutions of beef-tea concentrated to a syrup or to a paste. Consequently they are devoid of any great value as foods, but act well in many cases as a food accessory or stimulant. They consist largely of extractive bodies. The digestion of proteids is rendered more complete and is facilitated by the presence of extractives, partly no doubt through the stimulation of the appetite by their physical properties, their odour and taste, partly by direct stimulation of the gastric mucous membrane. They do more good than harm, but the commercial prepara- tions believed to be foods, but chiefly formed of extractives, should not command so high a price as they do in virtue of their supposed nutritive value. Some of the meat preparations, on the other hand, are really foods of great value. Often this value depends, not so much upon the actual proportionate nutritive value compared with that of beef, as upon the ease with which the proteids (present in a fine state of division) can be digested and absorbed, and upon the small space which the extract occupies. In several of the preparations lately offered to the public a greater or less part of the albumin is pre- digested, rendering absorption still more rapid and the pre- parations of greater utility. Frying ought simply to be boiling in oil in place of water. The oil may be provided by the food itself, as for instance by herrings and sausages, or may have to be added. Fried food is strongly flavoured, and is easily made unpalatable by the presence- of the products of fat decomposition. Stew- ing aims at cooking the food in a strong solution of its own juices, and should be carried on at a low temperature until close upon the time for serving, when the temperature should be quickly raised. Stewing and, provided the bouillon and bouillee are both made use of, boiling are the most economical methods of cooking food, nothing being lost. FOODS. 401 Soups from which all the coagulated albumin has been removed, and which contain no vegetables, are no more nutritious than beef-tea. If made in the same way as has been recommended in the case of beef-tea, at a low tempera- ture, with finely cut or minced meat, soup is of more value as a food. When a soup is intended to afford nourishment derived both from animal and from vegetable articles of food, the vege- tables used require to be subjected separately to a higher temperature beforehand to soften them. They may then be added to the cooler meat soup. The dilution of the contents of the stomach with fluids during active digestion tends to retard the processes in that organ. The percentage of acid present is diminished, the quantity of fluid in the stomach is rendered larger and more difficult of manipulation. Other things being equal, the amount of fluid secreted by the glands of the stomach is sufficient to render the digesting mixture sufficiently dilute. It is better for this reason not to drink fluids until after the solids at a meal have been swallowed, and perhaps better still to drink little immediately after a meal, but to take it four or five hours after one and before the next, so that the stomach may be practically washed out and made ready for the food of the next repast. The necessarily brief survey of digestive processes set forth in the preceding pages leaves untouched much that is of great interest and value. The rapid advance in recent years in our knowledge of the physiological and chemical details of vital phenomena renders it difficult, almost impossible, in a small space to do justice to any subject connected with these sciences. Many of the observations which have lately thrown light upon the problems of digestion and nutrition are still uncorroborated, or are based upon so limited a number of experiments that it would savour of rashness to embody them among the generalisations treated of here. And again, many of the details of physiological chemistry are eminently technical and would serve only to confuse those who have no intimate acquaintance with this science, and who would have a difficulty in grasping the value to be attached to some small fact embedded in a matrix of chemical nomenclature. The subject of the digestion and absorption of food has received less attention than the importance of the subject merits. It may be that, as so commonly occurs, what we are constantly doing without a thought proves a matter of little interest. 26 402 THE NATURAL HISTORY OF DIGESTION. Familiarity breeds contempt. This unthinking contempt is too often followed by sincere repentance when the digestive organs revolt against unlawful treatment. Digestion plays an all-important part in the life-history of individuals and of species in every organism endowed with vitality; from the lowest members of the vegetable kingdom to the most highly developed and specialised animal, man, the struggle for existence centres upon the acquisition of food and the propagation of offspring, of which the first is indispensable for the second, the second for the continuance of the first. Plants flourish and multiply, furnish other plants and animals with food ; these animals in turn may be devoured by others, only in time to die and return to the earth the nutriment originally removed by vegetable growths, whose seeds and saplings again remove it to furnish another gene- ration of animals with food nourished upon the elements which supported their forerunners, and so the circle con- tinues. Matter is indestructible. Living things are simply congeries of elemental atoms arranged to form tissues, and endowed with life. Remove the element of life and they resolve into these atoms, again capable of combining anew, it may be with very different comrades, to form other living tissues or some part, may be, of a rock. Elemental atoms undergo many more incarnations than were ever ascribed to Buddha. These ultimate atoms of matter /<■/■ se have no connection with the inheritance of form or character. These must arise from a combination of atoms along with an inherited form of life. The combinations of atoms forming the seed require to be formed by their parent organism from certain elements. If these elements are only available in scanty amount, as when the parent is ill-nourished, diseased, or suffering from the effects of excess of physical work, of mental work, from excesses of any kind, some of the actual elements available for the forma- tion of the germ may be withheld, apart altogether from the effect which all these conditions have upon the vital characteristics of the germs already fully formed. The greater the energy, health, and strength of the progenitors, either vegetable or animal, the greater will be the physical prowess of the offspring, and the more the food partaken by them approximates to the physiological ideal proper for their actions, processes, and environment, the more pronounced will be the vigour of their progeny, should the food FOODS. 403 be employed with discretion. In these statements no allusion is made to the vexed question of the propagation of characteristics in virtue of any concrete quality of the vital force transmitted from parent to offspring by the germ. It is clear, however, that the nutrition of an embryo must influence most radically all the characteristics of its maturer growth. Agri- culturists have long grasped this fact. The finest seeds yield the best crops, the animal in the best condition propagates the finest progeny. Man, however, is content to leave such matters to luck, and many of the puny, unhealthy children unhappily born into this world are practical and sad examples of the offspring of ill-fed, or over-fed, parents, who either cannot obtain sufficient food, or who indulge in the pleasures of the table to an unnatural extent. The nutrition of the parent affects not only the vital characters of the germ, but alters its elementary composition. Neuralgia has been said to be the cry of the nerves for more blood; rickets is an indication of an inability to use phosphates in the making of bone; ill-developed, faulty children are frequently due to a deficient supply of the substances needful to form normal abodes for the living germs in utero ; ante-natal nutrition must profoundly influence the course of future life. Observers have shown that variations in the food taken are capable of influencing the form and characteristics of the off- spring of animals. Such variations are commonly brought about by gardeners in plants. By analogy there is every reason to suppose that the food taken by the higher animals influences their progeny in a similar manner, though not so markedly. Agriculture is based upon a knowledge, more or less empirical, of the nutritive powers of plants ; they are regularly supplied with those substances which they stand most in need of, and which encourage their growth. Domestic animals are trained, fattened, their habits even are modified, to improve their stamina, to increase their fat, and to allow other foods to be relished. IVIan, partly owing to education of his senses, and partly to the changes produced by the introduction of money-values, is content or constrained in most instances to subsist on a diet much below that supplied to domestic animals in point of nutritional value and appropriateness ; or if able to procure an unlimited amount of food, disregards his own rational rules for feeding his domestic animals, and indulges in all kinds of superfluous foods which only yield him the nitro- 404 THE NATURAL HISTORY OF DIGESTION. gen and carbon requisite under the penalty of the disposition and expulsion of much useless, irritating, and often dangerous matter. '^ Has the nutrition of mankind benefited by civihsation? Probably not. But the question is fraught with so many complex influences that no definite answer can be given. Possibly the selection of nutriment through the course of long ages has tended to develop some parts of the body to a greater extent than others. The brain tissue may have been rendered more capable of mental processes by the increased absorption of food suitable for its growth just as much as by hereditary trans- mission of mental characteristics. To be added to this, we must remember that this increase of brain nutrition is often accompanied by greater sensibility; the civilised man is con- stitutionally less able to bear pain and fatigue than the savage, although he may safely pass through trials, by reason of his increased power of inhibition, which would prove disastrous to the other. It is true that the civilised man of good antecedents, by training and healthy exercise, and by avoidance of mental strain during this period, may excel the untutored savage in feats of strength and of temporary endurance, but in doing so he imitates the savage's mode of life. Nevertheless, pit the one against the other in continuous effort and the savage will win, provided he has not tasted the sweets of civilisation. Probably no member of any savage tribe ever developed such physical prowess as, for instance, Sandow j but the ordinary savage, when able to procure sufficient food, is a very different being from the day-labourer or the office-clerk of civilised countries. The finely-prepared dishes indispensable for the more leisured classes entail little trouble in mastication, taxing digestion by their bulk more than by their nature. The decay in the teeth of so many of the population may be largely put down to disuse. Foods are now so well prepared, even among many of ' Within recent times a discovery has been made by Schenck, of a character similar to many others which have previously been announced from time to time by zealous enthusiasts. The manner in which this "discovery" has been heralded is enough to raise suspicion about its value. No great discovery was ever be-pufifed beforehand in this way, but it was found on subjection to analysis to be no mountain, but a very minute mouse. Schenck believes he can influence the sex of the offspring by dieting the mother, especially by giving her saccharine material for some time before and for a month or two after conception. The only people probably who will benefit will be the confectioners. FOODS. 405 the labouring classes, that the teeth are not so necessary as of yore. In time, with the perfecting of our food preparation, teeth will be superfluous and man will become an edentulous animal. The old adage, " After breakfast walk a mile, after dinner rest a while," is only applicable to those who have to give all their powers to the digestion of their food. After breakfast the stomach is refreshed and strengthened by the period of quiet, and needs no help from its host; after dinner, the stomach, only recovering from a previous meal, may require a greater supply of blood to obtain enough digestive juice. Resting would supply this, allowing, by the disuse of other organs, more blood to flow to the alimentary canal. If the meal, however, be of food easily digested and absorbed, and consisting of much nitrogenous matter, work or exercise in moderation after it rather does good than harm by withdrawing blood from the stomach and helping to retard too great and too rapid an absorption of proteid bodies. This retardation is aimed at, according to Sir William Roberts, when tea, or coffee, or malt liquors are taken with food. Shakespeare makes Petruchio say — " For 'tis the mind that makes the body rich; And as the sun breaks through the darkest clouds, So honour peereth in the meanest habit." The mind, however, will not expand and enrich the body if impoverished from poor sustenance. Genius and poverty are seldom combined. A rustic or lowly genius is raised on honest food. Many of those credited with lofty powers, if brought up in poverty, have either inherited mental abihties and perspicuity whereby they are led early in life to properly nourish their brain and body, or belong to the class of eccen- tric geniuses whose powers are based on diseased mental con- ditions, and who soon exhaust their stores of energy, unable to continue an unequal and unnatural existence. The couplet in Lovers Labour's Lost — ■ " Fat paunches have lean pates; and dainty bits Make rich the ribs, but bankrupt quite the wits," illustrates the reverse side of the question. Over-indulgence in food is as ruinous to the brain as an inadequate supply. Medio tutissimus ibis is as true a maxim in the use of diet as in most other things. 406 THE NATURAL HISTORY OF DIGESTION. A translation of the words written by Paulus ^gineta in the sixth century a.d. will fittingly close the discussion of the subject : — " A lowering diet is safer than an excessive one for the pre- servation of health, but fails to impart tone and energy to the body from its lack of nourishment. A moderately nutritious diet should be taken by one suffering from the results of deficient nourishment. They who are wont to take exercise, and are able to rest when it pleases them, can do this with the least danger. . . . The articles of food which lie midway between the lowering and the stimulating are the best, and produce blood of a proper quality." INDEX OF AUTHORS. Abelous, 214 Actuarius, 10, 364 ^gineta, Paulus, 406 Anaxagoras, 4 Anstie, 342 Apellas, 9 Araki, 299 Aretaeus, 8 Aristotle, 6 Arnald, lo Arnott, 16 Arthiis, 115 Asclepiades, 7 Ascoli, 277 Astachewsky, 157 Atwater, 337 Auerbach, 232 As'enzoar, 10 Averroes, 10 Baas, 5 Baldi, 189 Balthasard, 229 Baranetsky, 32 Barbera, 195 Barfurth, 247, 254 Barnett, 374 Baithez, 13 Bary, de, 19 Bassow, 16 Baumann, 218 Beaumont, 16, 392 Bechamp, 131 Becker, 226 Bellamy, 131 Benedicenti, 360 Bennett, 61 Bernard, 16, 1S2, 301 Bernstein, 182 Bert, 131 Berthelot, 34S Berzelius, 112, 135 Bibea, 88 Bidder, i5, 312 Blondlot, 16 Blum, 178 Blumenthal, 140 Boerhaave, 13 Bokorny, 293 Borelli, 12, 16 Bordeu, 13 Bornet, 38 Bouchard, 194 Boussingault, iS Brieger, 218 Broune, 13 Brown, John, 14 Brown (and Heron), 137 Briicke, 160 Brunton, 231 Buchner, ¥., 114 Buchner, H., 114 Budge, 231 Billow, 145 Bunge, 113, 296, 310 Burdon Sanderson, 64 Busch, 16 Ccelius Aurelianus, 6 Cagniard-Lafour, 135 Cahn, 181, 205 Camerer, 283 Capitan, 214 Carus, 252 Celsus, 7 Chalmot, 140 Charrin, 222 Chauveau, 225, 304 Chevreul, 19 Chittenden, 25, 157, 345 4o8 Cicero, 7i 364 Cienkowskij 141 Clark, 61 Clarke, 29 Cohn, 66, 75 Cohnheim, 179 Cohnstein, 211 Colberg, 210 Colin, 182, 264, 272 Courtade, 234 CuUen, 13 Dahnhardt, 210 D'Arsonval, 336 Darwin, C. , 16, 48, 66 Darwin, F. , 80 Delpino, 66 Demetrius Pepagomenus, 9 Democritus, 7 Descartes, 12 Dioscorides, 17 Dittmar, 358 Dogiel, 232 Dolinsky, 230 Dominici, 218 Dott, 25 Drechsel, 180, 187 Dubrunfaut, 16, 150 Duogany, 352 Diiroy, 342 Eberle, 16 Ebstein, 16, 98, 124 Eck, 295 TEcluse, 17 Edkins, 127 Eichhorst, 16 Ellenberger, 166 EUinger, 330 Ellis, 64 Ely, 157 Emicli, 193 PJngelmann, 237 Epicurus, 7 Erasistratus, 6 Escherich, 215 Ewald, 164, 286 Falck, 313 Favre-Domergue, 237 Ferranini, 214 Fischer, 113, 139 INDEX OF AUTHORS. Flaum, 252 Flourens, 269 Frankland, 61 Fraustadt, 66 Fredericq, 241 Friedlander, 207 Gaglio, 298 Galen, 8, 364 Gamgee, 12, 116, 161, 193 Geppert, 114 Gilbert, 218 Gillespie, 157, 165, 214, 254 Gintl, 159 Gmelin, 16, 266 Goldschmidt, 120 Goodsir, 215 Gorup-Besanez, 24 Graaf, 16, 182 Graham, 205 Greenwood, 171, 237 Grew, 12 Griffiths, 241 Griswold, 157 GrtUzner, 16, 120 Gubler, 210 Gulland, 125 Glinzberg, 179 Gurber, 179 Guyon, 234 Hales, 18, 29 Hale White, 339 Halliburton, 146 Haly Abbas, 364 Hammarsten, 115, 3S3 Hammond, 342, 354 Hanau, 234 Handerson, 5 Hansen, 26, 210 Harley, 207 Harnack, 176, 180 Harrison, 9 Harriot, 212 Hart, 373 Hayem, 229 Hector, 153 Heidenhain, 16, 94, 119, 205, 22S Hein, 338 Heintz, 16 Helmont, 11 Heraclides, 7 INDEX OF AUTHORS. 409 Heron, 137 Herophilus, 6 Hippocrates, 4, S, 14 Hirsch, 24 Hirschfeld, 215 Hirt, 277 His, 254 Hjbrt, 119 Hoffmann, 170 Hofmeister, 166, 229 Holzner, 33 Hoppe-Seyler, 141, 210 Hubner, 172 HUfner, 117, 1S5 Hunter, 259 Huppe, 215 Huxley, 262 Ingenhouss, 18 Jacobsen, 135 Jarrod, 257 Jeffrey-Bell, 240 ones, Bence, 160 Joule, 335 Kant, 279 Kaufmann, 308 Kerkring, 104 Kerner, 70 Kionka, 114 Kjeldahl, 179 Knight, 398 Kceppe, 161 Konig, 345, 369-392 Kossel, 187 Krause, 104, 245 Krukenberg, 24, 240, 252 Krummacher, 300 Kuhne, 17, 121, 146 Lahoussen, 208 Lallermand, 342 Lamanski, 231 Lambinius, 364 Landvvehr, 192 Lang, 210 Langley, 17, 99, 116, 157 Lankester, Ray, 242 Lassaigne, 153 Latimer, 120 Latour, 19 Lawson Tail, 61, 71 Lea, 121, 298 Lechartier, 131 Leeuwenhoeck, 19 Lchraann, 16, 153 Leubuscher, 22S, 234 Lcuchs, 16, 150 Levy, 251 Liebig, 18, 112, 135, 309 Lieblein, 29S Limbeck, 276 Limbourg, 133 Lindenberger, 193 Lindsay, 65 Linnaeus, 18 Liversidge, 16, 127 Loew, 176, iSo, 293 Lomas, 88 Luchsinger, 271 Luciani, 276 Ludwig, 182, 223 Lugli, 194 Lunin, 310 lUacfadyen, 200, 214 Maly, 160, 191 IMarchand, 210 Marcus Aurelius, 9 Mayer, 37 IMeade Smith, 115, 376 Meissner, 16, 237 Mendel, 234, 345 Mering, von, 150, 205 Mester, 2ig Metschnikoff, 237 Mialhe, 150 Michaelis, 211 Miescher-Ruesch, 254 Minkowski, 296, 302 Mislavvski, 120 Mitscherlich, 153 Moleschott, 290, 324 Molibch, 38 Moore, 193 Moreau, 214, 231 Moritz, 229 Morris, 151 Mulder, 30 Munk, 65, 211, 369 Musculus, 150 Naegeli, 36, 132 4IO INDEX OF AUTHORS. Nasse, 113, 192 Naimyn, 193 NebeUhau, 334 Nencki, 36, 200, 295 Nesbit, 260 Neuraeister, 130, 139, 146 Odenius, 210 Oliver, 70 O'SuUivan, 150 Ou, 336 Paal, iSo Pal, 233 Paracelsus, 1 1 Parkes, 344 Paschutin, 16 Pasteur, 19, 33 Patier, 119 Paton, 149, 210, 300 Pavy, 304 Pawlow, 228, 295 Payen, 135 Pekelharing, 118, 1 77 Pembrey, 339 Persoz, 135 Perrin, 342 Pettenkofer, 307, 332 Pflliger, 293 Pitcairn, 12 Plato, 5 Plautus, 364 Playfair, 328, 334 Pliny, 17 Podolinsky, 123 Pohl, 114, 196 Popielski, 230 Pott, 380 Praxagoras, 6 Prout, 16 Pye- Smith, 231 Quain, 83 Quevenne, 210 Ralfe, 160 Ranke, 324 ■Reaumur, 16 Redi, 12, 1 5 Rees, 61 Reid, 208 Reinert, 276 Reis, 211 Remak, 82 Richet, 17, 168, 228, 238, 252 Rieder, 276 Riegcl, 159 Rigtiy> 13 Roberts, 151, 160, 345, 357, 405 Robertson, 158 Rockvvood, 193 Roger, 194 Roux, 229 Rovighi, 219 Rubner, 308, 334 RumsKus, 16 Rummo, 214 Sabanejeff, 137 Sachs, 18, 20 Salkowski, 117, 133 Sanderson, Burdon, 64 Saunders, 171 Sassus, 66 Saussure, 18 SchEefer, 228 Schenk, 82, 404 Scherer, 210 Schierbeck, 208 Schiff, 124, 189, 276 Schimper, 23 Schmidt, 16, 153, 312 Schneyer, 228 Schomnow, 115, 119 Schbnbein, 114 Schule, 181 Schulze, 19 Schulz, 308 Schultze, 280 Schumowa-Simanowskaja, 22S Schulz, 229 Schutzenberger, 131, 187 Schwann, 16 Senior, 372 Setten, Van, 153 Shakespeare, 405 Shirokikh, 230 Sieber, 200 Sigmund, 129 Simanowsky, 115, 119 Smirnow, 120 Smith, 157 Socm, 314 Solomon, Isaac ben, 10 Spalianzani, 16, 25S INDEX OF AUTHORS. 411 Spencer, 29 Stadeler, 16, igi Slahl, 13, 38 Stevens, 16 Stein, 66, 239 Stern, 218 Stewart, 292, 303, 337 Stockman, 310 Stohmann, 334 Sylvius, 12 Tait, 61 Tammann, 114 Tecklenburg, 234 Thanhoffer, 269, 382 Theophrastus, 17 Thiry, 16, 231 Thoyer, 172 Tibald, 352 Tichen, 354 Tiedemann, 16, 266 Tieri, 119 Traube, 131 Treat, 75 Trevisanus, 16 Tschlenoff, 294 Udransky, 218 Uffelmann, 369 Valentin, 283 Vay, 310 Veitch, 67 Vella, 17 Veraguth, 294 Vierordt, 335 Vines, 71 Voelcker, 71 Voit, 290, 294, 307, 318, 324, 328 Waller, 378 Warren, 120 Wasmann, 16 Weber, 23 Weismann, 1S2 Werther, 153 Wertheinier, 189 Will, 61 Willis, 12 Willoughby, 345 Winter, 229 Winterstein, 132 Withington, 10 Wolff, 334 Wroblewski, 171 Wiirtz, 212 Wynter-Blyth, 349 Yeo, 224 Zaieski, 310 Ziegler, 57 Zinofifski, 297 INDEX OF GENERIC NAMES. Actinia, 249 Actinozoa, 240 /Ethalium septicum, 24 Aldrovanda vesiculosa, 66 Algae, 35-39 Allium cepa, 23 Amoeba;, 72, 109, 236 Amoeba proteus, 236 Amphibia, 93, 282 Amygdala, 128 Ananas saliva, 24 Animalia, 84 Annelida, 241 Annuloida, 241 Aiachnida, 250 Aristolochia;, 70 Artemis exoleta, 119 Arthropoda, 243 Arthropomorpha, 366 Ascomycetes, 38 Astacus fluviatilis, 244 Aves, 107, 127, 255 Bacillus, acidi lactici, 215 ,, amylobacter, 214 ,, butyricus, 215 ,, mycoides, 214 ,, pyocyaneus, 214 ,, subtilis, 214 Bacteria, 36, 186, 214, 235 Bacterium lactis ierogenes, 214 ,, liquefaciens ilei, 216 Bartsia Alpina, 73 Brachiopoda, 244 Bromeliaceae, 25, 76, 80 Byblis gigantea, 46 Carica papaya, 24 Carnivora, 86, 169, 190, 362 Cephalopoda, 245 Cephalotus folUcularis, 71 Cestoidea, 241 Cetacea, 85, 281 Cetraria islandica, 396 Ccelenterata, 239 Composite, 32, 80 Crotalus, 262 Crustacea, 243 Cyclops, 75 Cypris, 75 Daphnia, 75 Darlingtonia, 78 Dasypeltis scabra, 260 Dionsea muscipula, 62 Dipsaceae, 80 Dolium galea, 162 Drosera rotundifolia, 52 Droseracese, 44, 152, 362 Drosophyllum lusitanicum, 44 Echinidea, 249 Echinodermata, 240 Echnida, 84 Edentata, 85 Equisetaceae, 21 Euonymus Europxus, 139 INDEX OF GENERIC NAMES. 413 Ficus carica, 24, 26 Flagellata, 235 Foraminifera, 238 Fraxinus ornus, 139 F»nS'. 35-39 Gasteropoda, 245 Genlisea Africanus, 77 ,, aurea, 77 ,, filiformis, 77 ,, ornata, 77 Gregarina, 236 GregarinidEe, 235 Heliamphora nutans, 77 Helianthus annuus, 23 Helix pomatia, 247 Herbivova, 86, 169, 191, 299, 362, 379 Hirudo medicinalis, 241 Hydra viridis, 240 Hydrozoa, 240, 246 Hymenoptera, 250 Infusoria, 72, 238 ,, ciliata, 238 ,, flagellata, 238 ,, tentaculifera, 238 Insecta, 243, 281 Invertebrata, 107, 236-251 Lamellibranchiata, 244 Lathrsea squamaria, 7 1 Lepidoptera, 250 Lichenes, 35-39 Lumbricus, 249 Mammalia, 86, 127, 383 Medusa, 240 Metazoa, 239, 243 Micrococcus tetragenus, 214 ,, urese, 112 MoUusca, 244 Monas vulgaris, 238 Monotremata, 84 Musa, 24 Nematoidea, 243 Nepenthaceje, 67, 362 Nepenthes albo-marginata, 69 ,, ampullaria, 68 ,, Dicksoniana, 69 ,, Edwardsiana, 67 ,, rajah, 68 ,, sedeni, 71 Oligochteta, 249 Omnivora, 86, 379 Ophidia, 259, 273 Ornithorhynchus, 84 Orthoptera, 250 Pachydermata, 86 Paaonia, 34 Palmipedes, 2S2 Paramoecium bussaria, 239 Pinguicula grandiflora, 52 ,, lusitanica, 52 ,, vulgaris, 46 Pisces, 251 Polyps, 291 Polyzoa, 244 Porifera, 251 Primula, 46 Protoplasta, 236 Protozoa, 171, 238 Rachiodon, 262 Raptores, 258 Reptilia, 259 Rhizopoda, 72, 238 Ricinus communis, 34 Rodentia, 86 Roridula dentata, 46 Rotifera, 241 Ruminantia, 86, 257 Salmo salar, 253 Sarcina ventriculi, 215 Sarracenia purpurea, 6i Saxifraga, 46 Scrophulariacece, 73 Sedum villosum, 46 Sempervivum, 46 Sinapis, 128 Sirenia, 85 Solipedes, 86, 272 Sorbus aucuparius, 139 Spongida, 239 .78 414 INDEX OF GENERIC NAMES. Staphylococcus pyogenes aureus, 214 Strelitzia, 24 Streptococcus Hquefaciens ilei, 216 Theobroma cacao, 359 Tillsandsia, 76 Trichina spiralis, 397 Tunicata, 245 Uraster rubens, 241 Utricularia, 66, 74-77 ,, amethystina, 76 ,, casrulea, 76 Utricularia, Griffithii, 76 montana, 76 multicaulis, 76 neglecta, 74 nelumbiiblia, 76 orbiculata, 76 Vampyrella spirogyrs, 141 Vertebrata, 106, 251 Vibrio rugula, 214 Zea mays, 20 INDEX OF SUBJECTS. Abomasum, 265 Absorption, in animals, 205 ; diffu- sion, 205-226; epithe- lial cells, 2o5; by pro- toplasm, 227 ,, of alcohol, 342 ; carbo- hydrates, 207, 305 ; fats, 207 ; iron, 311 ; proteids, 164-167,203- 208; salts, 206; water, 196, 205 ,, course of products, 209 ; by blood-vessels, 245, 249 ; chyle, 209-212 ; by lacteals, 209, 249 ; from stomach, 196, 205 ,, in plants, 28 ; of carbon, 21 ; in carnivorous plants, 45, 61 ; roots, 26 ; rise of sap, 29 ; of water, 26 ; oscilla- tion of branches in, 29 ; percentage from soil, 28 Achroodextrin, 152 Acids, acuity of taste for, 283-284 Acids, in digestion, in animals, 158- 180; actions of, 162, 172; comparison of different, 172- 173; of bile, 190; function, 192; glycocholic, 190; taurocholic, 190; in in- testine, 196-203 ; action, 216; source, 196; in saliva, 162 ; in stomach, 158 ; acidity, 162 ; hydrochloric acid, 163 ; antiseptic action, 134, 214; combined, 165, 172; free, 167; individual maximum, 167-169; with pepsin, 174; without pepsin, 115,175-180; on pepsinogen, 123-128 ; production of, 160- 162; proteolysis by, 174; relation to alkalies, 230; to food, 166-167; to salivary digestion in horse, 272 Acids, in plants, with bromelin, 25 ; in carnivorous plants, 44- 73 ; as means of defence, 41 ; nature, 61, 'Jl ; with papain, 24 ; in roots, 20-28 ,, production of, by bacteria, 134, 197, 214 ; in intestine, 196 ,, varieties, acetic, 61 ; amido, 185, 219; amido-caproic, 1 87 ; amido-isobutyl-acetic, 1 87; amido-succinic, 187, amido - valerianic, 186 ; butyric, 216; carbolic, 134; carlsonic, 127, 20S, 301 ; fatty. III, 129, 251; formic, 52, 65, 1 14 ; hydrocyanic, 129; glutamic, 187; lactic, 139 ; malic, *Jl ; oxy-proto- sulphonic, 141 ; propionic, 61 ; sarcolactic, 216 ; suc- cinic, 216; sulphanilic, 172; sulphuric, 172 ; tannic, 35S Adenin, 221 Adipose tissue, 148 Aggregation of protoplasm, 57 Albumins, 141 ; ash-free, 145 ; mole- cular composition, 166, 297 (see Proteids) Alcohol, actions, in body, 341-343 ; on digestion, 345-355; pancreatic, 250 ; peptic, 249; salivary, 346 ; fusel oils on, 354 ; metabolism 4i6 INDEX OF SUBJECTS. after, 342 ; oxidation of, 342 ; work and, 343 Alcohol, proportion, spirits, 344 ; wines, 344 ,, varieties, 339-340 Alcoholic fermentation, 114; in absence of yeast-cells, 131 Aldehydes, 139, 293 Alexins, 114 Alimentary canal, in invertebrates, 236, 247-251 ; in vertebrates, 251 ; anatomy, 84 ; capacity, 274 ;. development, 81 ; body length as to length of, 267 ; surface area as to skin surface, 275 ; divisions : caecum, 63, 106, 201 ; colon, 106, 209 ; vermiform appendix, 106 ; crop, 243 ; duodenum, lOI ; jeju- num, 102; ileum, loi ; oesophagus, 97 ; rectum, 106 ; stomach, 274 ; relations between divisions, 166, 182 (see also under Digestion, and individual sections) Alkalies, in digestion, 146 ; biliary, 193 ; pancreatic, 184 ; salivary, 157 ; stomach, 168 ; reflex from, on pan- creas, 230 „ action on bacteria, 134; pepsin, 127 ; proteids, 141 Alkaloids, animal, 220 ; leucomaines, ptomaines, 220 ; vegetable, 221 Amides, 33, 293 ; amido-compounds, 243 ; amines, 219 ; diamines, 220 Ammonia, albumose, 177; preparation of pepsin, 118; from proteids, 186; compound, 220 ; ammonium salts, in soil, 21, 40, 80; carbonate, in butterwort, 49 ; drosera, 62 ; urea, 295 ; lactate, in metabolism, 29S ; nitrate and phosphate, in drosera, 62 ; sulphate and proteids, 144 Amygdalin, 128 Amyloid, 141 Amylopsin, 197 ; alcohol and, 352 Ancient theories of digestion, I -19 ,, in animals, 1-16; animism, 13 ; Brun- onian, 14 ; coction, 4 ; demonology, 2 ; dogmatic, 6 ; eclec- tic, 13 ; Empiric, 7 ; latro-chemical, II, 13; latro-mathema- tical, 12; mediaeval, II ; nervous path- ology, 13 ; phlogistic, 13 ; solidism, 13 ; vitalism, 13; synop- sis, 15 Ancient theories ainong nations, Chin- ese, 4 ; Egyptians, 2 ; Greeks, 4 ; Hin- dus, 2 ; Romans, 7 ; uncivilised races, i ; in schools : Alexan- drine, 6 ; Arabic, 10 ; Aristotle, 6 ; Ascle- piades, 7 ; Galen, 8; Hippocrates, 4; Para- celsus, II ; Plato, 5 ,, in plants, 16-19; cel- lular, iS ; discovery of carbonic acid, 18 ; fermentation, iS; Linnaeus, 18; Pas- teur, 18; Pliny, 17; Signatures, doctrine of, 17 Animal Heat, 331 (see Heat) Appreciation of tastes, 283 ; flavours, 282 ; of smells, 279 Arabinose, 140 Archeus, 11 Aromatic bodies, 1S6 Asparagin, 33, 62 Azerin, 71 Bacteria, characters, 36 ; digestive processes in, 36 ; dis- covery of, 18; food- elements necessary for, 38 ; nutritive media for, 37 ; salts, 37 ,, in digestion: in carnivor- ous plants, 76-79 ; on cellulose, 213 ; as or- ganised ferments, 129; producing unorganised ferments, 133; com- parison between unor- ganised, proteids and, 130 ; self-fermentation, INDEX OF SUBJECTS. 417 131; in the digestive tract, 213 ; mouth, 213 ; stomach, 213 ; small in- testine, 2i5; numbers, 2i8 ; products in bowel, 219 ; in ruminants, 268 Bacteria, products, 132; acids, 134; alcohol, 139 ; fat from starch and sugar, 132; from proteids, 132; amido-acids, 219; cadaveric alka- loids, 219; bases, 220 ; mannite, 132 Bases, poisonous, 219 Beef, composition, 369; food-value, 370 ; nutritive value, 392 ; beef- tea, 399 Beer, 345 ; nutritive value, 389 ; retarding digestion, 346 - 350 ; strength, 345 Bees, 243 ; bees-wax, 147 ; food of different sexes of, 243-244 Betain, 221 Birds, digestion in, 255 ; crop of, 255- 257 ; gizzard, 258 ; milky fluid secreted by crop, 259 ; variation with habit, 255 Bile, 187 ; analysis, 188 ; amount of, 187 ; acids of, 190 ; characters, 189 ; pigments of, 191 ; secretion of, 189 ; influence of food on, 195 ; of nerves, 230; pressure of, 189; rate of flow, 194 Bile, action of, 192 ; amylolytic, 192 ; on fats, 193 ; precipitation of pro- teid, 192 Bladderworts, 74 ; bladders of, 74 ; cells, 74 ; quadrifid processes, 75 ; secretion in, 76; valve, 75 Bladders, in plants, 74 Blood, absorption of, by bowel, 2o5 ; lipolytic property, 211; saponifying ferment, 212; vessels, 249 Body-cavity, 239, 240; absorption by, 239 Brandy, 348 Bread, 389; "raising" of, 39a; nutritive value, 387 Breakfast, 364 Brownian movements, 76 Brunner's glands, 105 Bulimia, 6, 285 Butter, 370 Butterworts, 46 ; glands, 46, 74 ; growth, 51 ; leaves, incurvation, 48 ; rate of movement, 49 ; se- cretion, characters, 47 ; antiseptic, 52 ; ferments, 52 ; tatmiblk, 52 Butyric acid, 215; butyrin, 147 Cadaverin, 220 Calories, 333 ; of food, 334 ; kilo- calories, 333 ; milli-calories, 333 Camel, stomach of, 268 Cane-sugar, 33, 128, 207 ; absorp- tion, 207; characters, 138; con- version of, 138, 198 Caramel, 138 Carbo-hydrates, characters, 32, 137 ; composition, 136; divisions, 137 ,, in animals, absorp- tion of, 207 ; ac- tion of bacteria on, 215, 267; diges- tion of, in intes- tine, 198 ; in mouth, 154, in stomach, 158, 182, 272 ; in herbivora, 267 ; ptyalin on, 150; tests for, 151 ; achromic point, 152 ,, metabolism of, in animals, 301 ; energy-value, 335; food-value, 369 ; heat-value, 334 ; in liver, 301 ; gly- cogenesis, 301 ; relation to dia- betes, 303 ; normal daily quantity, 327 ; a proteid- sparer, 309; ulti- mate products of, 305 ,, m food, 334; pro- portion in meats, 377 ; milk, 386 ; vegetables, 395 ; relation to pro- teids, 376 ,, in plants, 34; de- composition of, 27 4i8 INDEX OF -SUBJECTS. 155 ; diastase on, 32 ; formation of, 22 ; metabolism of, 41 ; reaction to, in carnivorous plants, 62 Carbo-hydrates, akrose, 140 ; alde- hydes, 139; arabinose, 140; cane- sugar, 32, 128 ; cellulose, 137, 215, 267 ; hemi-cellulose, 140 ; dextrins, 151 ; dextrose, 139; di- saccharides, 32, 128; dulcite, 139; fructose, 139; galactose, 139; glu- cosamine, iresence of, in stomach, 208; in blood-vessels, 20S ; an ultimate product in metabolism, 301-305 ; in respiration, 330 Cardiac end of stomach, 97 ; glands of, 98 ; pepsin in, 126 Carnivorous plants, 42 ; absorbing putrefactive products only, 74 ; acidity of secretion, characters and variations in, 44, 52, 65, 70 ; table of, 7_9 ; acids, nature of, 61 ; ac- tively digesting, 44 ; with adhesive apparatus, 44; aggregation of proto- plasm, 45, 57 ; bladderworts, 75 ; butterworts, 46 ; digestive pro- cesses, 59-71 ; ferments, 61- 71 ; zymogens of, 61 ; growth with or without proteid food, 51 ; movements of, 46 ; prey of, 42 ; pitcher plants, 67 ; secretion, 44, 60 ; sensitive hairs, 64 ; sundew, 52 ; theory of evolution, 43, 80 ; theories as to cause of movements, 56-60, 64 ; toothworts, 74; Venus's fly-trap, 62 Carp, 252 Casein, 145 ; ferments on, 52, III Cat, milk of, 384; starvation, 315; stomach in, 274 Cell, digestion in, 237, 245, 250 ; living and dead, 293 ; metabolism in, 290 ; nuclei, 141 ; primitive layers of, 81 ; protoplasm in, 237 ; movements, 141 Cellulose, 32 ; bacteria on, 132 ; fer- ment in plants on, 32 ; marsh gas from, 215 ; value for man, 366 ; for ruminants, 213 ; from starch, 32 ; worms and, 216 Chemio-taxis, 277 Chitin, 243 Chlorides, source of hydrochloric acid, 162 ; exclusion from food, 219; sodium chloride, 309 Chlorophyll, 20, 142 ; granules of, 21 ; iron in, 20 ; light upon, 22 ; slarch formed, 22, 39 Cholates, 193 ; cholalic acid, 190 ; cholin, 220 ; choletelin, 191 Cholesterin, 19 1 Chondroitin, 142 Chorda tympani nerve, 223 ; stimu- lation of, 226 Chyle, 209; composition, 210 ; changes in, 212; injection into blood, 211 Chyme, 181, 196 Circulation, schemes of, in inverte- brata, 247 Civilisation and digestion, 404 Cloaca, 63 Coagulation of ' proteids, 144 ; in cooking, 397 INDEX OF SUBJECTS. 419 Coca, 360 Cocoa, 359 Coction, theory of digestion as, 4 Cod, sense of taste in, 284 Coffee, 359 ; caffeine in, 356 ; caffeo-tannic acid, 359 Collagen, 142 Conchiolin, 142 Conglutin, 142 Cornein, 142 Cotton plant, laflinose in, 138 Cotyledons, 39 Crocodile, mesentery and stomach of, 259 Ciop, in bees, 243 ; birds, 255 ; snail, 245 ; worms, 241 Crystalline style, in molluscs, 244 Cuckoo, gall-bladder, 107 Curd of salmon, 235 Cyanogen, molecules in proteid, 293 Deathfrom "salts-hunger," 310; from starvation, 314 Defa;cation, 203 Dentition, 86 ; formulae, 90 ; dentine, 86 Deutero-albumose, 141, 179, 186 Dextrins, 151-156; achroodextrin, 152 ; erythrodextrin, 152-156 ; maltodextrin, 153 Diabetes mellitus, 303 Diastase, 32, u6, 150, 156; malt- diastase, 156; temperature upon, 116 Diet, 321 ; physiological, 321, 327, 378 ; constituents, 328 ; heat-equi- valent, 326-337 ; dietaries — black- smith, 337 ; Irish labourer, 372 ; lawyer, 337 : soldiers, 337 ; for work, 337, 372 ; for the young, 328; subsistence — in carnivora, 362 ; in man, 337 Diffusion, laws of, 205 ; in absorp- tion, 207 ; carbo-hydrates, 137-139; of native proteids, 166, 207 ; of lower proteids, 164, 205 Digestibility of meats, 392 ; of fish, 393 ; vegetables, 381 Digestion, definition of, 14 ; in ani- mals, 81; in aves, 253 ; in carnivora, 362 ; herb- ivora, 271; invertebrata. 236 ; mammalia, 263 ; pisces, 251 ; reptilia, 259 ; ruminantia, 263 ; vertebrata, 251 ; ana- tomy of organs, 84 ; de- velopment of organs, 81; ferments of, 109 ; ner- vous supply to, 168, 223; relation between, 169 Digestion, in plants, 20; compara- tive, 36; ferments of, 32 ; products, 30 ; in carnivorous, 44 , with acids, 45, 48, 61, 70, 160; alkalies, 150, 182; by bacteria, 36; of carbo-hydrates, 152 ; fats, 184 ; proteids, 175-1S6; in intestine, 182; mouth, 150; stomach, 158 ,, effect of dilution of stomach contents, 407; fluids at meals, 401 ; in- fluence of alcohol on, 245 ; cocoa, 359; coffee, 359 ; cooking, 396 ; foods, 361 ; tea, 355 ; lime of day, 364 ; vege- tarianism, 368 Dinner, 364 Disaccharides, 32, 138 Dog, 85 ; dentition in, 90 ; pancreas of, 122 Ducts, bile, 107 ; chyle, 105, 277 ; cystic, 107 ; hepatic, 107 ; pan- creatic, io8 ; Rivini's, 96 ; from Savigny's tubules, 245 ; for spider's threads, 244 ; Stenson's, 96 ; vi- tello-intestinal, 83 ; Wharton's, 96 Dugong, prehension of food in, 85 Duodenum, 107 ; passage of chyme into, 181 Duration of digestion in man, 182 Dys-albumose, 141, 179, 186 Ebers Papyrus, 2 Eggs, 388 ; food of snakes, 263 Egg-albumin, 172 Electricity, action on bile, 231 ; in- testines, 231 ; pancreas, 229 ; peristalsis, 263 ; 420 INDEX OF SUBJECTS. salivary glands, 225 ; stomach, 228 ; in pro- duction of HCl, 160 Electricity, conductivity in ferments, , , currents in Dionaea leaves, 64 , , sensory effects — smell, 281 ; taste, 284 Elephant, 107 ; teeth, 89 ; trunk, 85 Emulsin, iii, 128 Emulsion of fats, 198 Epiblast, 87 Equilibrium, nitrogenous, 290 Erythrodextrin, 156 Ethereal sulphates, 218 Excretion, of nitrogen, 289 ; urea, 294 ; after excision of liver, 296 ; uric acid, 298 ; of carbon, 301, 305 ; of iron, 311 ; of ptomaines, 219 Extra-cellular digestion, 251 Fffices, 201 Fangs of snakes, 262 Fasting, 298 ; blood in, 276 Fattening animals, diet for, 376 Fats, 147 ; in plants, 31 ; composi- tion, 147 ; metabolism of, 34 ; produced from carbo-hydrates, 41, 305; by bacteria, 132; by moulds, 132 ,, in animals, absorption, 105; adipose tissue, 148 ; bacteria upon, 211 ; blood upon, 211 ; in chyle, 211 ; emulsion of, 198 ; in echinodermata, 241 ; in fseces, 202 ; proportion in animals, 308 ; varieties, 147 ,, metabolism in animals, 305 ; calorific value, 334 ; energy equivalent, 335 ; food value, 369 ; proteid -sparer, 309 ; source, 307 ,, of foods, cost, 371 ; inmilk, 383 ,, ferments, bacterial, 132; lipo- lytic in blood, 211 ; in plants, 41 ; in protoplasm, 149 ; splitting. III, 129, 251; in Uraster, 241 Fatty acids. III ; in bowel, 251 Ferments, organised, 129 ; actions, 130; products, 132; acids, 134, 197 ; alco- hol, 133; alkaloids,2i9; aromatic bodies, 186 ; fat, 132 ; sugar, 131 ; relation to plants, 129; in soil, 21 Ferments, unorganised, actions, 109 ; characters, 11 1; com- position, 130; electric conductivity of, 113; salts on, 309 ; strength of, 117; temperature in, 116, 130; toxicity of, 119 ,, unorganised, production of, 119; theories as to action, 112; catalysis, 112; mode of motion, 115; stereo-chemical, 115; source of, 119; from organised fer- ments, 133 ; action separate from, 131 ,, diastatic, 32 ; amylopsin, 184; in bile, 192; in carp, stomach of, 254 ; in date-stone on cellu- lose, 32 ; in fungi, 32 ; in leech, 246 ; diastase, 32 ; inulin converting, 32 ; ptyalin, 150 , , fat, emulsifying, in inverte- iDrates, 241, 251 ; split- ting, III ; in blood, 211; in bowel, steapsin, III, 129, 251 ; sapon- ifying, 147, 212 ,, glucoside-splitting, 128 ,, glycogenetic or glycolytic, 184 ,, inverting, 198; in plants, 32 ,, milk-curdling, rennet, 134; in birds, 259 ; butter- wort, 52; pancreas, 184; pine-apple, 25; stomach, 127 ,, oxidising, in molluscs, 119 ., proteolytic, in animals, pepsin, IIJ, 171 ; tryp- sin, 116, 184; in plants, bromelin, 25 ; droserin. INDEX OF SUBJECTS. 421 61 ; in fig, 26 ; papain, 24; in carnivorous plants, 52, 61, 71 ; lupine, 24 ; mushrooms, 119; vetch, 24 Ferments, various, altering surface tension, azerin, 61 ; has- tening decay, 76; proto- plasm, 30, 109, 132, 290 Fibrin, bacteria on, 133 ; digestion in niedusje, 240 Fish, 251 ; dentition, 251 ; vomiting, 273 ; flesh of, 343 Flagella, 238 ; flagellate cells, 239 Fly-catcher, 44 ; aggregation of proto- plasm, 45 ; ammonia, 45 ; glands, 44 ; leaves, 44 ; secretion, 45 Fly-trap, Venus's, 62 ; hinges, 64 ; leaves, 62; spines, 64; stimulation, 64 Foods, of plants, 28 ; of animals, 361 ; characters, 366 ; digestibility, 391 ; effect on habits, 362 ; ele- ments, 136; cost of, 371 ; effect of economic changes, 369 ; of heat on, 396; proportions digested, 381; value for heat, 337 ; for nutrition, 370 ; for work, 335 Formic acid, 61, 65, 71 Frog, stomach, 99 ; vomiting, 273 ; influence of food on, 259 ; hiber- nation, 303 ; heat production in, 303 Fruits, 393 Fungi, 31 ; as food, 394 (see Bacteria, and Ferments) Fusel oils, in digestion, 354 ; nature, 340 Gall-bladder, 107 Gastric juice, 158 (see under Diges- tion, Stomach, etc.) Gastric mill in crayfish, 244 Gastro-vascular canals, 240 ; system, 239. 244 Gelatin, 142, 145 ; gelatoses, 323 ; metabolic value, 323 Gizzard, in birds, 257 ; in worms, 241 ; crocodile, 259 Glands, digestive, in plants, 46-64 ; in animals, buccal, 85 ; Brunner's, 105; gastric, 97; intestinal, 104; labial, 85 ; lingual, 93 ; mid-gut, in invertebrates, 247 ; cesophageal, in worms, 241 ; salivary, mucous, ^ 94 ; poisonous, 245 ; serous, 94 Glands, antitoxic action, 194 ; heat ftom, 339 ; liver, 194 ; suprarenal, 194; thyroid, 194; lymphatic, 277; tonsils, 94 ; poison, in snake, 262 ; spinning, in spider, 244 Globulins, 141 ; vegetable myosin, 141 ; vitellin, 141 Glucosides, proteid, 142 ; in almond, 129 ; in mustard, 128 Glutamic acid, 112 Gluten, 142 Glycerine, from fats, 147, 185, 212; by bacteria, 216 ; as solvent for ferments, n6 ; in wine, 349 ; gly- ceryl, 147 Glycolic, glycoxylic, glyo.xylic acids, 196 ; glycoproteids, 145 Glycocoll, glycocholic acid, 190 Glycozen, 137, 156 Gonidia, 36 Goose, 256 ; gizzard, 257 ; extirpa- tion of liver in, 296 Grape-sugar, 138 ; absorption, 207 Grasses for food, 30 Growth, of animals, and diet, 328 ; rapid in moulting, 245 ; of plants, 20 Gums, 85 Gustatory nerves, 284 Heat, action on unorganised ferments, 116, 130; on formed, 130; on carbo-hydrates, 137-138; fats, 147; proteids, 144-145 Heat, body, 311; production, 333; seat of, 336; source of, 321 ; value, 328 ; equivalent in metabolism, 334 ; in work, 335 Heat, body, relative cost of elements, 371 Heat, bodily, alcohol on, 342 ; hibernation, 303, 339; man, 331,; homoiothermal, 331; poikilother- mal, 331 Heart, worli of, 338 ; heat equiva- lents, 338 Hetero-albumose, 141, 179, 186 Hexyl-alcohol, 139 27- 422 INDEX OF SUBJECTS. Hibernation, 303, 331, 339; frog, 303. 331; marmot, 331 Horse, cKcum, 273 ; oesophagus, 271 ; stomach, 271 ; vomiting, 276 Hunger, 285; alcohol in, 342; centre in medulla, 286 ; after removal of cerebrum, 286 ; stomach in, 285 Hydrochloric acid, 160 (see Acids) Hydrogen for plants, 21 ; in dietetics, 328 ; in water, 330 Ileum, loi Indol, 186 Intestine, development, 82 ; glands, Lieberkiihn's, 105 3, Large, anatomy, 106; bac- teria in, 200, 218; con- tents, 198-203 ; faeces, 201 ; ferments, 201 ; ileo- coecal valve, 106 ,, Small, anatomy, loi ; bacteria, 216; contents, 197-199; ferments, 197; succus entericus, 105 ; valvuls conniventes, 103 ; villi, 103 ,, in invertebrates, crusta- ceans, 243 ; chitin, 243 ; echinodermata, 240 ; in- sects, 243 ; leech, 241 ; molluscs, 244 ; snail, 245 ; worm, 241 ,, in vertebrates, 251 ; birds, 256 ; fishes, 252 ; horse, 271 ; reptiles, 259 ; rumi- nants, 263 Inulin, 32 Iron, absorption of, 314; amount necessary, in animals, 314; in plants, 22, 314 ,, in chlorophyll, 20 ; in blood, 311 ; metabolism, 310; in proteids, 145 Irritability, leaves, 48-62; protoplasm, 238 Jacobson's organ, 280 Jejunum, 103 Keratin, 145 ICilogranime-metres, 335 Kreatin, 187-221 Kreatinin, 187-221 Lacteals, 105 ; absorption of fat, 105, 204 ; in villi, 104 ; pseudo- htemal, 249 Leaf-buds, 29 Leaves, function, 21 ; fibro-vascular bundles, 29 ; stomata, 28 Leech, sucker of, 241 Lecithin, 149 Leucin, 185-187, 216 Leucocytosis, digestive, 277 ; after fasting, 276; sites, 278 Leucomaines, 219-222 Lipase, 212 Lipochromes, 142 Lithium, in sundew, 61 Liver, 107, 187, development, 83 ; extirpation, 295 ; urea, 295 Llama, stomach, 268 Lymph, 209 ; lymphatic ducts, 105, 209, 277 ; glands, in bowel, 106 ; leucocytes, 278 ; lymphoid, ton- sils, 94 ; vermiform appendix, 106 Lysatinin, 187 ; Lysin, 187 Manyplies, 263 Margarin, 149 Marsh gas formed by bacteria, 215 Meat for food, 390 Mechanical work, 335 ^fesoblast, 87 Metabolism, in animals, 2S8 ; of carbo-hydrates, dia- betes, 303; glycogen, 301 ; oxidation in muscles, 303 ; sugar from, 302, 304; from proteid, 308 ; fat, 307 ,, in animals, of fats, from carbo-hydrates, 307 ; from proteids, 306 ; characteristic in animals, 307 , , in animals, of proteids, 291 ; liver, 295 ; muscular work on, 300 ; products, 296 ,, in plants, 30 ; forma- tion of compounds, INDEX OF SUBJECTS. 423 21 ; salts, 31 ; sap, 29 Methyl-guanidin, 220 Milk, 381 Monosaccharides, 32, 138 Moulds, 129 Mouth, 85 Moulting, 244 Mucin, 145 Muscarin, 221 Myrosin, 1 1 1 , 128 Nervous system, central, 168 ; on hunger, 286 ; thirst, 286 ; plexuses, coeliac, 231 ; Auerbach's, 232 ; Meissner's,232; solar,25i; splanch- nic nerves, 231 ; sympathetic sys- tem, 223 ; vagus, 230 Neurone, application to intestinal digestion, 232 Neurin, 221 ; neuridin, 220 Nicotin on salivary secretion, 225 Nitrogen, 21, 33, 36, 143 ; meta- bolism, 289 ; equilibrium, 290 Nucleic acid, 141 ; nuclein, 141 ; nucleo-albumin, 145 Nucleus, 236 Nutrition, animals, 32 1 ; plants, 30 Odontophore, 249 CEsophagus, 97 Olein, 147 Omasum, 266 Osmosis, 205 ; osmotic pressure of blood, and hydrochloric acid, 161 Oxygen, 31 ; in metabolism, 328 ; moulds and, 132; under pressure on ferments, 131 ; source of heat, 330 ; combustion and oxidation, 331 Ozazones, 138 Palate, 93 Palmatin, 147 Pancreas, development, 84 ; diges- tion by, 184; juice, 1S2 ; analysis, 183 ; ferments, 184 ; relation to food, 182; gastric juice, 167; liver, 184; structure, 108, 122 Papain, 24 Parroquet, liver of, 107 Pentose, 140 Pepsin, III, digestion by, 171; in urine, 170; pepsinogen, 123 Peroxide of hydrogen and ferments, 135 Peristalsis, 229 Peyer's patches, 106 Pharynx, 93 Phloridzin, glycosuria from, 305 Phosphorus in proteids, 145 ; in pep- sin, 119 Pigi 99 ; vomiting, 273 Pigeon, 107 ; without cerebrum, 286 Pigments : Bilirul)in, 191 ; bilicyanin, igi ; biliverdin, 191 ; chlorophyll, 20, 142; choletelin, 191; cyanic series, 142; htematin, 311 ; haemo- cyanin, 142 ; haemoglobin, 311 ; histo-hfematins, 142 ; hydrobili- rubin, 191; lipochromes, 142; melanins, 142 ; tryptophan, 219 ; urobilin, 191 ; xanthic series, 142 Pine-apple, ferment in, 25 Piperidin, 220 Pitchers, 67; digestion in, 71; fer- ments, 70; secretion, 68-70 Plants, annuals, 40 ; comparative, 36 ; digestion, 20-40 ; historical, 16 ; actions, 33 Platinum black and ferments, 112 Polysaccharides, 32, 137 Pores, exhalent, 239 ; inhalent, 239 Porpoise, 99 ; fats of, 149 Prehension of food, 84, 282 ; in- fluence of, 85 Pressure, of secretions, bile, 189 ; pancreatic, 183 ; saliva, 225 ,, of gas, in stomach, 208 Proteids, individual forms, 294 ,, in animals : ash-free, 145 ; chemical composition, 142 ; classification, 141 ,, native: albumins, 141- 144; egg, 141-144, 172; lact-albumin, 141-144 ; muscle, 144; serum, 144 j amyloid, 141 ; globulins, 144 ; crystallin, 141 ; fibrin, 141 ; fibrinogen, 144 ; myosin, 144 ; para- myosin, 141 ; vitellin, 141 ; nucleo-albumins, 424 INDEX OF SUBJECTS. 145; casein, 141-145; spennatin, L41 Proleids, derived : acid - albumin, 141; alkali - albumin, 141 ; albumoses : atmid-, 141 ; deutero-, 14I ; dys-, 141 ; hetero-, 141 ; proto-, 141 ; coagulated, 141 ; peptones, 141 ; ampho-, 141 ; anti-, 141 ; hemi-, 141 ; oxyproto-sulphonic acid, 141 ,, albuminoids: elastin, 142; gelatin, 142 ; glucosides, 142; lieratin, 1 42; skele- tins, 142 ,, in plants — native: al- bumin, 142 ; aleurone granules, 34 ; globulins, 142 ; gluten, 142 ; myco- protein, 36 ; myosin, 142; para-globulin, i42;phyto- vitellin, 142 ,, in plants — derived : albumin- ates, 142 ; albumoses, 142; coagulated, 142; precipitated by ferments, 142 ; peptones, 142 ,, in animals, 140-147; chief constituent of protoplasm, 140, 239 ; pigments, 142 ; reactions of, 144 ; native, 144 ; artificially altered, 145 ; fractional heat co- agulation, 146 ; peptic digestion, action of acids and heat on, 146 ; changes therefrom, 146 ; changes due to ferment action on, 147, 171, iSo; combination with acids, 178-iSo; relative diges- tive power of various acids on, 1 72; combina- tion with heavy metals, 176-177, 180; final de- rivatives of peptic diges- tion of, 178-179 ,, tryptic digestion of, 184- 187, 198 ; influences of bacteria on, 185 ; deriva- tives of, 187 Proteids, changes in intestine of, 196; putrefaction in large in- testine of, 200 ; products, 219-222 ,, absorption of, 203 ; change during passage through wall of gut, 20S ; rela- tion of carbonic acid to peptone, 208-209 ; toxicity of peptone and albumose after injection into blood-stream, 292 ; bacteria attacking, 216 ,, relation to ferments, 131 ; to protoplasm, 293 ,, as food, 322; value, 369; necessity for, 322 ; calori- fic equivalent, 334 ; amount required, 327 ; proportion in different articles of food, 376, 381, 389-396 ; relative cost, 371 ; in milk, 384 ,, metabolism of, 291 ; nitro- genous equilibrium, 289, 326 ; urea derived from, 295 ; other derivatives of, 296-300 ; muscular work on, 300 ; carbo-hydrates and fats from, 308 ; in starvation, 314 in plants, 33, 140-142; formation, 34 ; digestion of, 24 ; decomposition of, 33 ; peptonised by vege- table secretions, 24, 34 ; by papain, 24 ; bromelin, 25 ; in crystalloid form, 34 Protoplasm, 14 ; in plant cells, 21 ; endowed with life, 30-31, 293 ; ferment action of, 109 ; ferments produced by, 110-112; formation of zymogens by, 120-129 ! relation to, 140 ; and to proteids, 140 ; direct digestion by, 236-239 ; move- ments of, 236 ; effect of alcohol on, 342 ; reaction of, in amoeba, 171 Proventriculus, 257 Psalter, 263 Pseudo-podia, 237 INDEX OF SUBJECTS. 425 Ptomaines, 219 Ptyalin, 1 50 ; digestion of starch, 'S3> ptyalinogen, 120 Putrescin, 219 Putrefaction, bowel, 198; products of, 219 Pylorus, 97 ; pyloric casca, 251 Radula, in birds, 258 ; molluscs, 249 Rennet, in herbivora, 265 Respiration, 330 ; intra-molecular, 131; heat from, 338; requisite oxygen from, 330 Reticulum, 263 Romans, meals of, 364 Roots, in plants, acid of, 28 ; action, 26 ; hairs, 26 ; water removed, 28 Rumen, 263 Rumination, 263 ; cud, nature, 268 ; number of acts of, 270; size of bolus, 270; mechanism, 271; abomasum, 265 ; manyplies, 263 ; oesophageal gutter, 265 ; omasum, 266 ; psalter, 263 ; rennet, 265 ; reticulum, 263 ; rumen, 263 Saliva, actions, 154; analysis, 152; ferments, 152; poisonous, 245; in stomach, 158; temperature and, 156; glands, in fish, 252; inverte- brates, 251; man, 150; reaction, 157; acid, 162; ruminants, 270; worm, 241 Salmon, curd, 235; digestion, 253- 25s ; micro-organisms in, 216 Salts, metabolism, 300 ; requisite for animals, 309; fungi, 31, 38; plants, 31 ; proportion of sodium and potassium, 309 Salts-hunger, 310; chlorides, 161; law of the minimum, 309 Sap, ascent of, 29 ; composition, 29 ; crude, 29 ; elaborated, 30 Saponification of fats, 147 ; ferment, 212 Saprin, 220 Sarcosin, 323 Savigny's tubules, 245, 251 Sea anemone, 249 Sensitive organs in plants, 65 Sheep, 269 Skatol, 186, 221 Skeletins, 142 Skin, excretion, 326 ; loss of heat, 335 ; relation of surface to digestive organs, 275 ; to food, 328 ; to weight, 328 ; in starvation, 316 Smell, 279; birds, 282; elephant, 282 ; fish, 281 ; whales, 281 ; organs of, 280; tactile sense in, 281 Snail, digestive organs, 245 Snakes, 262 ; swallowing eggs, 264 Specimen diet, 335 Solar plexus, 251 Spider, web, 244 Spinal cord, 231 Splanchnic nerves, 231 Sponges, 239 ; spongin, 147 Starvation, metabolism, 314; organs in, 315; proteids in, 290, 320 Starch, 22 ; formation of, chlorophyll, 23 ; grains of, 23 ; action of amy- lopsin, 197 ; bile, 192 ; diastase, 32; ptyalin, 150; digestion, in crop of birds, 258; in ruminants, 261; in small intestine, 198; in stomach of carp and tench, 253 ; products of digestion, 153 ; insol- uble, 151 ; soluble, 151 Stereo-chemical, 113; stereoisomer, 139 Stimulants, alcohol, 340; caffeine, 358; coca, 360; cocoa, 359; coffee, 359; fusel oils, 354 ; guarana, 360 ; mate, 360 ; mead, 341 ; saka, 341, tea, 355 ; theine, 358 Stomach, absorption from, 205 ; bac- teria in, 213 ; emptying of, 196; nervous action on, 227-229 ; peristalsis, 229 ,, capacity of, 271; in man, 274 ; increase with age, 365 ; relation of area to that of skin, 275 ; re- lation between man and animals in, 274 ; between the intestines and, 274 ., development of, 81 ; form, 269 ; invertebrata, 248 ; vertebrata, 251 ; aboma- sum, 265 ; axis, 258 ; in birds, 256 ; cardiac end, 99 ; carnivora, 273 ; in 426 INDEX OF SUBJECTS. fishes, 251; fundus, 99; gizzard, 257 ; herbivora, 271 ; manyplies, 264 ; omasum, 263 ; psalterj 263 ; pylorus, 99; re- ticulum, 264 ; rumen, 263 ; in ruminants, 264 ; glands of, 97 ; cells in, 99 ; changes on secretion, 162 Stomach, juice, 158 ; acid, 160 ; analysis, 158; ferments, 170; milk-curdling, 11 1 ; in birds, 259; pepsin, 115; products, 171; re- lations to other secretions, 168 ; secretion, 98 ; production, 160 ; theories as to, 160; variations of, 167 ; relation to foods, 168 ; rapidity of digestion, 182 ; rate of flow, 159; sight-secretion, 228; temperature of food on, 181 Subsistence diet, 328, 362 ; of London needlewoman, 328 ; of Trappist monk, 328 Sugars (see .Carbo-hydrates) Sulphur in proteids, 14S ; acid, 172 Sundew, 52 Swan, crop of, 256 Sympathetic nerves, 225, 234 Tapir, prehensile organ of, 282 Taste, 282 ; accuracy of, 283 ; buds, 92 ; dilution, 283 ; time required, 284 ; tongue in, 283 Tatmiolk, 52 Taurocholic acid, 190 Tea, infusion of, 358 ; tannin in, 358 Teeth, 86 Temperature of body, 331 ; loss of, 335 Temperature on ferments, 116; of food in stomach, :8i Tentacles, in sundew, 56 Thirst, 2S6 Thread-worms, 243 Tooth wort, 71 Toxins, 194 Traps in plant, bladder, 74 ; pitcher, 67 ; recesses in leaver, 73 ; spiral canals, 77 Trimethylamin'c, 220 Truffles, 396 Trypsin, 116; digestion by, 184; trypsinogen, 121 Tryptophan, 219 Tyrosin, 185-1S7, 216 Udder of cow, 382 Unicellular digestion, 237, 251 Units, of energy, 335; of food-value, 369 ; of heat, 333 ; of work, 335 Urea, 294, 323; in birds, 296; con- stitution, 297 ; daily variations, 294 ; lactate of ammonium, 298 ; leucin, 295 ; liver, 294 ; oxygen, 299 ; seat of formation, 296 ; source of, 295 ; starvation, loss in cat, 312; in fat dog, 313; in lean dog, 313 Uric acid, in birds, 296 ; in cold, blooded animals, 299 ; extirpation of liver, 296 ; hippuric acid, 299 ; benzoic acid and, 299 Urine, ammonia in, 295; ethereal sulphates in, 218 ; loss of heat by, 335; pepsin in, 170; sugar in, 305 Urotoxic coefficient, 194 ; diet and, 194 Vacuole of digestion, 237 Vagus, effect of section on peptic digestion, 168, 228 ; of stimulation, on the pancreas, 229 ; on the intes- tine, 231-233 \'aso-constrictor nerves, of salivary glands, 223 ; of intestine, 234 ^'egetables, as food in herbivora, 379 ; amounts digested by, 381 ; com- position of, 395 ; heat value of, 395; metabolism in, 30; alkaloids of, 221 Vegetarians, 368 Vinous fermentation, 135 Vipaka, 3 Vitellin, 141; vegetable, 142; vitel- loses, 142 Vomiting, 273 Wasp, 249 Water, in metabolism, 289 ; thirst, 287 ; not absorbed by stomach, 205 INDEX OF SUBJECTS. 42; Weight of body, food elements re- lative to, 327 ; in relation to heal, 333 Work, heat equivalent of mechanical, 335 ; unit of, 335 Wines, 344 ; alcoholic strength, 344; constituents of old and new, 348 ; effects on digestion, 346-355 ; con- stituents of, 349 ; glycerine in, 349 ; tannin in, 344 ; decrease with age, 348 Xanthin, 221 Xantho-proteic reaction, 144 X-rays, in movements of stomach walls, 229 Veasts, 36 ; medium for, 37 ; self- fermentation, 131 ; alcoholic fer- mentation, 131 Yolk of egg, 388 Zymogens, 119; discovery of, 119; preparation, 120 ; secretion of, 120 ; ptyalinogen, 120 ; pepsin- ogen, 123 ; trypsinogen, 121 ; of rennet, 127 ; supposed zymogen of pancreatic diastase, 127 ; and of invertin, 128; granules of pepsin- ogen, 127 ; of trypsinogen, 121- 125; influence of bacteria on for- mation, 121 ; protoplasm a zymo- gen, 12S THE END. 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