CHAPTER VI.

DIGESTION.

_The Principal Processes of Nutrition--The General Plan of Digestion--Mastication--The Teeth--Preservation of the Teeth--Insalivation--The Stomach and the Gastric Juice--The Movements of the Stomach--Gastric Digestion--The Intestines--The Bile and Pancreatic Juice--Intestinal Digestion--Absorption by means of Blood-vessels and Lacteals--The Lymphatic or Absorbent System--The Lymph--Conditions which affect Digestion--The Quality, Quantity, and Temperature of the Food--The Influence of Exercise and Sleep._

[Sidenote: 1. Design of food? How accomplished?]

1. NUTRITION.--The great design of food is to give _nutriment_ or nourishment to the body. But this is not accomplished directly, as the food must first pass through certain preparatory changes, as follows: (1), _Digestion_, by which the food is reduced to a soluble condition; (2), _Absorption_, by which, when digested, it is imbibed into the blood; (3), _Circulation_, which carries the enriched blood to the various parts of the system; and (4), _Assimilation_, by which each tissue derives from the blood the materials necessary for its support.

[Sidenote: 2. Sustaining power of food? Simile of the engine? Operation in the human body?]

2. By these means the sustaining power of food is gradually developed and employed, and the vital machinery kept in working order, somewhat after the manner of the steam-engine. To operate the latter, the force imprisoned within the coal and water is set free and converted into motion by the combustion of the fuel and the vaporization of the water. It will be seen, however, when we come to study these operations in the human body, that they are conducted silently and harmoniously, with marvellous delicacy and completeness, and without that friction, and {81} consequent loss of power, which attend the working of the most perfect machinery of man's invention.

[Sidenote: 3. Change of food in digestion? Process of digestion? Describe the alimentary canal.]

3. GENERAL PLAN OF DIGESTION.--The great change which food undergoes in digestion is essentially a reforming process, reducing articles of diet, which are at first more or less solid, crude, and coarse, to a liquid and finely comminuted condition, suitable for absorption into the blood. The entire process of digestion takes place in what is called the alimentary canal, a narrow, tortuous tube, about thirty feet in its entire length. This canal begins in the mouth, extends thence downward through the gullet to the stomach (a receptacle in which the principal work of digestion is performed), and thence onward through the small and large intestines.

[Sidenote: 4. Situation of the stomach and intestines? Action of the food? Mechanical action? Chemical?]

4. The stomach and intestines are situated in the cavity of the abdomen (Fig. 16, C, and Fig. 22), and occupy about two-thirds of its space. The action to which the food is subjected in these organs is of two kinds--mechanical and chemical. By the former it is crushed, agitated, and carried onward from one point to another; by the latter it is changed in form through the solvent power of the various digestive juices. {82}

[Sidenote: 5. Describe the process of mastication? How many and what movements?]

5. MASTICATION.--As soon as solid food is taken into the mouth, it undergoes mastication, or chewing. It is caught between the opposite surfaces of the teeth, and by them is cut and crushed into very small fragments. In the movements of chewing, the lower jaw plays the chief part; the upper jaw, having almost no motion, acts simply as a point of resistance, to meet the action of the former. These movements of the lower jaw are of three sorts: a vertical or cutting, a lateral or grinding, and a _to-and-fro_ or gnawing motion.

[Sidenote: 6. Composition of the teeth? Enamel of the teeth? Interior of teeth?]

6. The teeth are composed of a bone-like material, and are held in place by roots running deeply into the jaw. The exposed portion, or "crown," is protected by a thin layer of enamel (Fig. 17, _a_), the hardest substance in the body, and, like flint, is capable of striking fire with steel. In the interior of each tooth is a cavity, containing blood-vessels and a nerve, which enter it through a minute opening at the point of the root (Fig. 19).

[Sidenote: 7. The milk teeth? The permanent teeth?]

7. There are two sets of teeth; first, those belonging to the earlier years of childhood, called the milk teeth, which are twenty in number and small. At six or eight years of age, when the jaw expands, and when the growing body requires a more powerful and numerous set, the roots of {83} the milk teeth are absorbed, and the latter are "shed," or fall out, one after another (Fig. 18), to make room for the permanent set.

[Sidenote: 8, 9. Number of teeth? How distributed?]

8. There are thirty-two teeth in the permanent set, as many being in one jaw as the other. Each half-jaw has eight teeth, similarly shaped and arranged in the same order: thus, two incisors, one canine, two bicuspids, and three molars. The front teeth are small, sharp, and chisel-edged, and are well adapted for cutting purposes; hence their name incisors. The canines stand next, one on each side of the jaw; these receive their name from their resemblance to the long, pointed tusks of the dog (Fig. 19). {84}

9. The bicuspids, next in order, are larger and have a broader crown than the former; while behind them are the molars, the largest and most powerful of the entire set. These large back teeth, or "grinders," present a broad, rough surface, suitable for holding and crushing the food. The third molar, or "wisdom tooth," is the last to be cut, and does not appear until about the twenty-first year. {85} The order of arrangement of the teeth is indicated by the following dental formula:--

[Sidenote: 10. Different forms of teeth? Human teeth? The inference?]

10. It is interesting, at this point, to notice the different forms of teeth in different animals, and observe how admirably their teeth are suited to the respective kinds of food upon which they subsist. In the _carnivora_, or flesh-feeders, the teeth are sharp and pointed, enabling them both to seize their prey, and tear it in pieces; while the _herbivora_, or vegetable-feeders, have broad, blunt teeth, with rough crowns, suitable for grinding the tough grasses and grains upon which they feed. Human teeth partake of both forms; some of them are sharp, and others are blunt; they are therefore well adapted for the mastication of both flesh and vegetables. Hence we argue that, although man may live exclusively upon either vegetable or animal food, he should, when possible, choose a diet made up of both varieties.

[Sidenote: 11. Cleaning of teeth? Effects of not cleaning?]

11. PRESERVATION OF THE TEETH.--In order that the teeth shall remain in a sound and serviceable condition, some care is of course requisite. In the first place, they require frequent cleansing; for every time we take food, some particles of it remain in the mouth; and these, on account of the heat and moisture present, soon begin to putrefy. This not only renders the breath very offensive, but promotes decay of the teeth. {86}

[Sidenote: 12. Effects upon the saliva? Formation of tartar? How prevented? How destroyed?]

12. The saliva, or moisture of the mouth, undergoes a putrefactive change, and becomes the fertile soil in which a certain minute fungus has its growth. This fluid, too, if allowed to dry in the mouth, collects upon the teeth in the form of an unsightly, yellow concretion, called tartar. To prevent this formation, and to remove other offensive substances, the teeth should be frequently cleaned with water, applied by means of a soft tooth-brush. The destruction of the tartar fungus is best effected by the use of a weak solution of carbolic acid.

[Sidenote: 13. Destruction of the enamel? How guarded against?]

13. Again, it should be borne in mind that the enamel, Nature's protection for the teeth, when once destroyed, is not formed anew; and the body of the tooth thus exposed, is liable to rapid decay. On this account, certain articles are to be guarded against; such as sharply acid substances that corrode the enamel, and hard substances that break or scratch it--as gritty tooth powders, metal tooth picks, and the shells of hard nuts. Sudden alternations from heat to cold, when eating or drinking, also tend to crack the enamel.

[Sidenote: 14. Mixing of food with the saliva? What is the saliva? How secreted? The salivary glands?]

14. INSALIVATION.--When the morsel of food is cut and ground by the teeth, it is at the same time also intimately mixed with the saliva, or fluids of the mouth. This constitutes the second step of digestion, and is called insalivation. The saliva, the first of the digestive solvents, is a colorless, watery, and frothy fluid. It is secreted (_i. e._ separated from the blood) partly by the mucous membrane which lines the mouth; but chiefly by the salivary glands, of which there are three pairs situated near the mouth.

[Sidenote: 15. The flow of saliva? The thought of food? Anxiety and grief? Animals fed upon dry and coarse food?]

{87} 15. These glands consist of clusters of very small pouches, around which a delicate network of blood-vessels is arranged: they empty into the mouth by means of little tubes, or ducts. The flow from these glands is at all times sufficient to maintain a soft and moist condition of the tongue and mouth; but when they are excited by the presence and taste of food, they pour forth the saliva more freely. Even the mere thought of food will at times cause the saliva to flow, as when the appetite is stimulated by the sight or smell of some savory article; so that the common expression is correct that "the mouth waters" for the favorite articles of food. Anxiety and grief prevent its flow, and cause "the tongue to cleave to the roof of the mouth." In the horse and other animals, that feed upon dry and coarse fodder, and require an abundant supply of saliva, we find large salivary glands, as well as powerful muscles of mastication.

[Sidenote: 16. Importance of the process? The first place? The second? The third?]

16. The mingling of the saliva with the food seems a simple process, but it is one that plays an important part {88} in digestion. In the first place, it facilitates the motions of mastication, by moistening the food and lubricating the various organs of the mouth. Secondly, it prepares the way for other digestive acts: by the action of the teeth, the saliva is forced into the solid food, softens the harder substances, and assists in converting the whole morsel into a semi-solid, pulpy mass, that can be easily swallowed, and readily permeated by other digestive fluids. The saliva also, by dissolving certain substances, as sugar and salt, develops the peculiar taste of each; whereas, if the tongue be dry and coated, they are tasteless. Hence, if substances are insoluble, they are devoid of taste.

[Sidenote: 17. Its final importance? Starch? How effected? Ptyalin?]

17. Finally, the saliva has the property of acting chemically upon the food. As we have before stated (Chap. IV.), starch, as starch, cannot enter the tissues of the body; but, in order to become nutriment, must first be changed to grape sugar. This change is, in part, effected by the saliva, and takes place almost instantly, whenever it comes in contact with cooked starch. This important function is due to an organic ingredient of the saliva called _ptyalin_. This substance has been extracted from the saliva by the chemist, and has been found, by experiment, to convert into sugar two thousand times its own weight of starch.

[Sidenote: 18. Each of the processes? Why is a knowledge of the digestive functions important? How shown?]

18. IMPORTANCE OF MASTICATION AND INSALIVATION.--Each of these processes complements the other, and makes the entire work available; for, by their joint action, they prepare the food in the best possible manner for further digestive changes. The knowledge of these preliminary functions will appear the more important, when we reflect that they are the only ones which we can regulate by the will. For, as soon as the act of swallowing begins, the food not only passes out of sight, but beyond {89} control; and the subsequent acts of digestion are consequently involuntary and unconsciously performed.

[Sidenote: 19. Rapid eating? Describe the process and effects.]

19. It is generally known that rapid eating interferes with digestion. How does this occur? In the first place, in rapid eating, the flow of the saliva is insufficient to moisten the solid parts of the food, so that they remain too hard and dry to be easily swallowed. This leads to the free and frequent use of water, or some other beverage, at meals, to "wash down" the food,--a most pernicious practice. For these fluids, not only cannot take the place of the natural digestive juices, but, on the contrary, dilute and weaken them.

[Sidenote: 20. Loss of taste? Another effect of rapid eating? Mistakes?]

20. Secondly, the saliva being largely the medium of the sense of taste, the natural flavors of the food are not developed, and consequently it appears comparatively insipid. Hence the desire for high-seasoned food, and pungent sauces, that both deprave the taste and over excite the digestive organs. Rapid eating also permits the entrance of injurious substances which may escape detection by the taste, and be unconsciously received into the system. In some instances, the most acrid and poisonous substances have frequently been swallowed "by mistake," before the sense of taste could act, and demand their rejection.

[Sidenote: 21. Effect of imperfectly broken food in the stomach? Dyspepsia? Overeating?]

21. Thirdly, the food, being imperfectly broken up by the teeth, is hurried onward to the stomach, to be by it more thoroughly divided. But the task thus imposed upon the stomach, it is not at all adapted to perform; so that the crude masses of food remain a heavy burden within the stomach, and a source of distress to that organ, retarding the performance of its legitimate duty. Hence persons who habitually eat too rapidly, frequently fall victims to dyspepsia. Rapid eating also conduces to overeating. The food is introduced so rapidly, that the system has not {90} time to recognize that its real wants are met, and that its losses have been made good; and hence the appetite continues, although more nutriment has been swallowed than the system requires, or can healthfully appropriate.

[Sidenote: 22. Gullet? Describe the stomach and its location. Effects of gormandizing?]

22. THE STOMACH.--As soon as each separate portion of food is masticated and insalivated, it is swallowed; that is, it is propelled downward to the stomach, through a narrow muscular tube about nine inches in length, called the _oesophagus_, or gullet (Fig. 23). The stomach is the only large expansion of the digestive canal, and is the most important organ of digestion. It is a hollow, pear-shaped pouch, having a capacity of three pints, in the adult. Its walls are thin and yielding, and may become unnaturally distended, as in the case of those who subsist on a bulky, innutritious diet, and of those who habitually gormandize.

[Sidenote: 23. Heart-orifice? Gatekeeper? Coins, etc.? Indication of the soft and yielding texture of the stomach?]

23. The stomach has also two openings; that by which food enters, being situated near the heart, is called the _cardiac_, or heart orifice; the other is the _pylorus_, or "gatekeeper," which guards the entrance to the intestines, {91} and, under ordinary circumstances, permits only such matters to pass it as have first been properly acted upon in the stomach. Coins, buttons, and the like are, however, readily allowed to pass, because they can be of no use if retained. The soft and yielding texture of this organ--the stomach--indicates that it is not designed to crush and comminute solid articles of food.

[Sidenote: 24. What is meant by the gastric juice?]

24. THE GASTRIC JUICE.--We have seen how the presence of food in the mouth excites the salivary glands, causing the saliva quickly to flow. In the same manner, when food reaches the stomach, its inner lining, the mucous membrane, is at once excited to activity. (At first its surface, which while the stomach is empty presents a pale pink hue, turns to a bright red color, for the minute blood-vessels which course through it, are filled with blood. Presently a clear, colorless, and acid fluid exudes, drop by drop, from its numerous minute glands or "tubules," until finally the surface is moistened in every part, and the fluid begins to mingle with the food. This fluid is termed the gastric juice. {92}

[Sidenote: 25. What is the office of the gastric juice? Acidity of the gastric juice? Quantity of gastric juice used? What becomes of it?]

25. The gastric juice is the proper solvent of certain articles of food, especially those belonging to the albuminoid class. This solvent power is due to its peculiar ingredient, _pepsin_; in digestion, this substance acts like a ferment; that is, it induces changes in the food simply by its presence, but does not itself undergo change. The acidity of the gastric juice, which is due to _lactic acid_, is not accidental; for we find that the pepsin cannot act in an alkaline solution--that is, one which is not acid or neutral. The quantity of gastric juice secreted daily is very large, probably not less than three or four pints at each meal. Though this fluid is at once used in the reduction of the food, it is not lost; since it is soon re-absorbed by the stomach, together with those parts of the food which it has digested and holds in solution.

[Sidenote: 26. Muscular coat of the stomach? Expansion and contraction of its fibres? Action of the fibres?]

26. MOVEMENTS OF THE STOMACH.--The inner coating of the stomach is the mucous membrane, which, as we have seen, furnishes the gastric juice. Next to this coating lies another, called the muscular coat, composed of involuntary muscular fibres, some of which run circularly, and others in a longitudinal direction. These expand to accommodate the food as it is introduced, and contract as it passes out. In addition, these fibres are in continual motion while food remains in the stomach, and they act in such manner that the contents are gently turned round from side to side, or from one end of it to the other.

[Sidenote: 27. Peristaltic movements? What is said of our consciousness of and power over these movements? Describe the movements of the pylorus.]

27. By these incessant movements of the stomach, called the _peristaltic_ movements, the gastric juice comes in contact with all parts of the food. We are, however, not conscious that these movements take place, nor have we the power to control them. When such portions of the food as are sufficiently digested approach the pylorus, it {93} expands to allow them to pass out, and it closes again to confine the residue for further preparation.

[Sidenote: 28. How has the knowledge and the workings of the stomach been ascertained? St. Martin? How else?]

28. The knowledge of these and other interesting and instructive facts has been obtained by actual observation; the workings of the stomach of a living human being have been laid open to view and examined--the result of a remarkable accident. Alexis St. Martin, a Canadian _voyageur_, received a gun-shot wound which laid open his stomach, and which, in healing, left a permanent orifice nearly an inch in diameter. Through this opening the observer could watch the progress of digestion, and experiment with different articles of food. Since that occurrence, artificial openings into the stomach of the inferior animals have been repeatedly made, so that the facts of stomach-digestion are very well ascertained and verified.

[Sidenote: 29. What was formerly thought? What do we now know? What else do we now know? Water, salt, and sugar? Absorption?]

29. GASTRIC DIGESTION.--What portions of the food are digested in the stomach? It was formerly thought that all the great changes of digestion were wrought here, but later investigation has taught us better. We now know that the first change in digestion takes place in the mouth, in the partial conversion of starch into sugar. We also know that, of the three organic food principles (considered in Chapter IV.) two--the fats and the sugars--are but slightly affected by the stomach; but that its action is confined to that third and very important class, from which the tissues are renewed, the albuminoids. A few articles need no preparation before entering the system, as water, salt, and grape-sugar. These are rapidly taken up by the blood-vessels of the stomach, which everywhere underlie its mucous membrane in an intricate and most delicate network. In this way the function of absorption begins.

[Sidenote: 30. Albuminose? The process? Chyme?]

30. The albuminoid substances are speedily attacked and {94} digested by the gastric juice. From whatever source they are derived, vegetable or animal, they are all transformed into the same digestive product, called _albuminose_. This is very soluble in water, and is readily absorbed by the blood-vessels of the stomach. After a longer or shorter time, varying from one to five hours, according to the individual and the quantity and quality of his food, the stomach will be found empty. Not only has the digested food passed out, but also those substances which the stomach could not digest or absorb have passed little by little through the pylorus, to undergo further action in the intestines. At the time of its exit, the digested food is of a pulpy consistence, and dark color, and is then known as the _chyme_.

[Sidenote: 31. What are the intestines? The small intestines? The large intestines? Their structure?]

31. THE INTESTINES.--The intestines, or "bowels," are continuous with the stomach, and consist of a fleshy tube, or canal, twenty-five feet in length. The small intestine, whose diameter is about one inch and a half, is twenty feet long and very tortuous. The large intestine is much wider than the former, and five feet long (Fig. 23). The general structure of these organs resembles that of the stomach. Like it, they are provided with a mucous membrane, or inner lining, whence flow their digestive juices; and, just outside of this, a muscular coat, which propels the food onward from one point to another.

[Sidenote: 32. Peritoneum? The work of digestion?]

32. Moreover, both the intestines and stomach are enveloped in the folds of the same outer tunic or membrane, called the _peritoneum_. This is so smooth and so well lubricated, that the intestines have the utmost freedom of motion. In the small intestines, the work of digestion is completed, the large intestine receiving from them the indigestible residue of the food, and in time expelling it from the body.

[Sidenote: 33. The presence of food in the intestines? Bile?]

33. INTESTINAL DIGESTION.--As soon as the food passes the pylorus and begins to accumulate in the upper {95} part of the intestines, it excites the flow of a new digestive fluid, which enters through a small tube, or _duct_, about three inches below the stomach. It is formed by the union of two distinct fluids--the _bile_ and the _pancreatic_ juice. The bile is secreted by the liver, which is the largest gland of the body, and is situated on the right side and upper part of the abdomen (Fig. 22). The bile is constantly formed, but it flows most rapidly during digestion. During the intervals of digestion it is stored in the _gall-bladder_, a small membranous bag attached to the under side of the liver. This fluid is of a greenish-yellow color, having a peculiar smell, and a very bitter taste.

[Sidenote: 34. The pancreatic juice? The joint action of these fluids?]

34. The pancreatic juice is the product of a gland called the _pancreas_, situated behind the stomach. This fluid is colorless, viscid, and without odor. Like the digestive juices previously described, it owes its solvent power to its peculiar ferment principle, called _pancreatin_. By the joint action of these fluids, the fatty parts of the food are prepared for absorption. By previous steps of digestion the fats are merely separated from the other components of the food; but here, within the intestines, they are reduced to a state of minute division, or _emulsion_, resembling the condition of butter in milk, before it has been churned. There results from this action a white and milky fluid, termed the _chyle_, which holds in solution the digestible portions of the food, and is spread over the extensive absorbent surface of the small intestines.

[Sidenote: 35. The mucous membrane? Experiments on inferior animals?]

35. The mucous membrane of the intestines, also, secretes or produces, a digestive fluid by means of numerous "follicles," or minute glands; this is called the intestinal juice. From experiments on the inferior animals, it has been ascertained that this fluid exerts a solvent influence over each of the three organic food principles, and in this way may supplement and complete the action of the {96} fluids previously mentioned, viz.:--of the saliva in converting starch into sugar, of the gastric juice in digesting the albuminoids, and of the pancreatic juice and bile in emulsifying the fats.

[Sidenote: 36, 37. How much thus far has been done with the food? The next process? Give the first way.]

36. ABSORPTION.--With the elaboration of the chyle, the work of digestion is completed; but, in a certain sense, the food is yet outside of the body; that is, the blood is not yet enriched by it, and it is not in a position to nourish the tissues. The process by which the liquefied food passes out of the alimentary canal into the blood is called absorption. This is accomplished in two ways; first, by the _blood-vessels_. We have seen how the inner membrane of the stomach is underlaid by a tracery of minute and numerous vessels, and how some portions of the food are by them absorbed. The supply of blood-vessels to the intestines is even greater; particularly to the small intestines, where the work of absorption is more actively carried on.

37. The absorbing surface of the small intestines, if considered as a plane surface, amounts to not less than half a square yard. Besides, the mucous membrane is formed in folds with an immense number of thread-like prolongations, called _villi_, which indefinitely multiply its absorbing capacity. These minute processes, the villi, give the surface the appearance and smoothness of velvet; and during digestion, they dip into the canal, and, by means of their blood-vessels, absorb its fluid contents, just as the _spongioles_ which terminate the rootlets of plants, imbibe moisture from the surrounding soil.

[Sidenote: 38. How is absorption effected in another way? Describe it. Name of the lacteals? Thoracic duct?]

38. Secondly, absorption is also effected by the _lacteals_, a set of vessels peculiar to the small intestines. These have their beginnings in the little villi just mentioned, side by side with the blood-vessels. These two sets of absorbents run in different courses, but their destination is the same, {97} which is the right side of the heart. The lacteals receive their name from their milky-white appearance. After a meal containing a portion of fat, they are then distended with chyle, which they are specially adapted to receive: at other times they are hardly discernible. The lacteals all unite to form one tube, the _thoracic duct_, which passes upward through the _thorax_, or chest, and empties into a large vein, situated just beneath the left collar-bone.

[Sidenote: 39. The absorbents? Lymph? What further of the lymph?]

39. THE ABSORBENTS.--The lacteals belong to a class of vessels known as absorbents, or lymphatics, which ramify in nearly all parts of the body, except the brain and spinal cord. The fluid which circulates through the lymphatics of the limbs, and all the organs not concerned in digestion, is called _lymph_. This fluid is clear and colorless, like water, and thus differs from the milky chyle which the lacteals carry after digestion: it consists chiefly of the watery part of the blood, which was not required by the tissues, and is returned to the blood by the absorbents or lymphatics.

[Sidenote: 40. What can you state as to the time required for digestion?]

40. CIRCUMSTANCES AFFECTING DIGESTION.--What length of time is required for the digestion of food? From observations made, in the case of St. Martin, the Canadian {98} already referred to, it has been ascertained that, at the end of two hours after a meal, the stomach is ordinarily empty. How much time is needed to complete the digestion of food, within the small intestines, is not certain; but, from what we have learned respecting their functions, it must be evident that it largely depends upon the amount of starch and fat which the food contains.

[Sidenote: 41. Circumstances affecting duration of digestion? Fresh food?]

41. In addition to the preparations which the food undergoes in cooking, which we have already considered, many circumstances affect the duration of digestion; such as the quality, quantity, and temperature of the food; the condition of the mind and body; sleep, exercise, and habit. Fresh food, except new bread and the flesh of animals recently slain, is more rapidly digested than that which is stale; and animal food more rapidly than that from the vegetable kingdom.

[Sidenote: 42. Food in concentrated form? A large quantity of food? Experiment on the dog? Ice-water? Variety of articles?]

42. Food should not be taken in too concentrated a form, the action of the stomach being favored when it is somewhat bulky; but a large quantity in the stomach often retards digestion. If the white of one egg be given to a dog, it will be digested in an hour, but if the white of eight eggs be given it will not disappear in four hours. A wineglassful of ice-water causes the temperature of the stomach to fall thirty degrees; and it requires a half-hour before it will recover its natural warmth--about a hundred degrees--at which the operations of digestion are best conducted. A variety of articles, if not too large in amount, is more easily disposed of than a meal made of a single article; although a single indigestible article may interfere with the reduction of articles that are easily digested.

[Sidenote: 43. Strong emotion? The tongue of the patient?]

43. Strong emotion, whether of excitement or depression, checks digestion, as do also a bad temper, anxiety, long fasting, and bodily fatigue. The majority of these {99} conditions make the mouth dry, that is, they restrain the flow of the saliva; and without doubt they render the stomach dry also, by preventing the flow of the gastric juice. And, as a general rule, we may decide, from a parched and coated tongue, that the condition of the stomach is not very dissimilar, and that it is unfit for the performance of digestive labor. This is one of the points which the physician bears in mind when he examines the tongue of his patient.

[Sidenote: 44. Eating between meals? Severe exercise? Sleep after meals?]

44. The practice of eating at short intervals, or "between meals," as it is called, has its disadvantage, as well as rapid eating and over-eating, since it robs the stomach of its needed period of entire rest, and thus overtasks its power. With the exception of infants and the sick, no persons require food more frequently than once in four hours. Severe exercise either directly before or directly after eating retards digestion; a period of repose is most favorable to the proper action of the stomach. The natural inclination to rest after a hearty meal may be indulged, but should not be carried to the extent of sleeping; since in that state the stomach, as well as the brain and the muscles, seeks release from labor.

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QUESTIONS FOR TOPICAL REVIEW.

PAGE

1. What do you understand by nutrition? 80 2. How is the process of nutrition carried on? 80 3. What further can you state on the subject? 80, 81 4. Describe the general plan of digestion. 81 5. How is the process of mastication carried on? 80, 82 6. State what you can in relation to the formation of the teeth. 82, 86 7. What, in relation to their arrangement? 83, 84 8. What, in relation to the process of "shedding?" 82, 83, 84 9. In relation to the different forms of teeth in different animals? 85 10. What causes operate to injure or destroy the teeth? 85, 86 11. What suggestions and directions are given for the preservation of the teeth? 85, 86 12. What do you understand by insalivation? 80, 86 13. How is the process of insalivation carried on? 86, 87, 88 14. Of what importance is the saliva to the process? 87, 88 15. Of what importance are mastication and insalivation? 88, 89 16. Describe the consequences of rapid eating. 89, 90 17. What becomes of the food directly after it has undergone mastication and insalivation? 90 18. Describe the location and formation of the stomach. 90, 91, 92 19. Describe the process by which the gastric juice is formed. 91 20. What are the properties and uses of the gastric juice? 92 21. What are the movements of the stomach, and what their uses? 92, 93 22. What further can you state on the subject? 93 23. What portions of the food are digested in the stomach? 93, 94 24. What are the first changes of digestion? 93 25. Describe the location and formation of the stomach. 94 26. What further can you state in relation to the stomach? 94 27. Describe the process of intestinal digestion. 94, 95, 96 28. What do you understand by absorption? 80, 96 29. How is the process of absorption effected? 96, 97 30. What are the lacteals and of what use are they? 96, 97 31. What length of time is required for the digestion of food? 97, 98 32. What circumstances, of food, affect digestion? 98 33. What circumstances, of emotion, affect digestion? 98, 99 34. What suggestions and directions are given upon the subject of eating and drinking? 98, 99

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