[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 pa.s.ses 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 ma.s.ses of food remain a heavy burden within the stomach, and a source of distress to that organ, r.e.t.a.r.ding 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 appet.i.te continues, although more nutriment has been swallowed than the system requires, or can healthfully appropriate.

[Ill.u.s.tration: FIG. 22.--SECTION OF CHEST AND ABDOMEN.

A, Heart. B, The Lungs. C, Stomach. D, The Liver. E, Large Intestine.

G, Small Intestine.]

[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 ca.n.a.l, and is the most important organ of digestion. It is a hollow, pear-shaped a 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 circ.u.mstances, permits only such matters to pa.s.s it as have first been properly acted upon in the stomach.

Coins, b.u.t.tons, and the like are, however, readily allowed to pa.s.s, 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.

[Ill.u.s.tration: FIG. 23.--THE ORGANS OF DIGESTION.

O, Oesophagus. S, Stomach. L, Liver. M, Pylorus. C, Large Intestines.

P, Pancreas. I, Small Intestines. N, Spleen. G, Gall-bladder. ]

[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? Quant.i.ty 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 alb.u.minoid cla.s.s. 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 quant.i.ty 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 pa.s.ses 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 pa.s.s 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 cla.s.s, from which the tissues are renewed, the alb.u.minoids. 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. Alb.u.minose? The process? Chyme?]

30. The alb.u.minoid 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 _alb.u.minose_. 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 quant.i.ty and quality of his food, the stomach will be found empty. Not only has the digested food pa.s.sed out, but also those substances which the stomach could not digest or absorb have pa.s.sed 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 ca.n.a.l, 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 pa.s.ses the pylorus and begins to acc.u.mulate 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 b.u.t.ter 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 alb.u.minoids, 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 pa.s.ses out of the alimentary ca.n.a.l 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 ca.n.a.l, 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 pa.s.ses upward through the _thorax_, or chest, and empties into a large vein, situated just beneath the left collar-bone.

[Ill.u.s.tration: FIG. 24.--THE LACTEALS.