Tables have been prepared showing the length of time which various articles of food take to digest. They are based in part upon observations made upon the living stomach in cases in which it has been possible to examine its contents through a fistulous opening; in part upon the results of artificial digestions carried out in the laboratory. It is hardly too much to say that such observations are absolutely without value as tests of the relative digestibility of the several articles of diet consumed as parts of an ordinary meal. The fact that the commencement of the flow of gastric juice depends upon mental stimuli, and its continuance upon hormones, shows how difficult it must be to reproduce the conditions which obtain in a healthy living body. The most wholesome of foods taken by itself may be longer in digesting, or may produce more irritation, than many less desirable things taken in judicious combination. Crushed chicken, hastily swallowed, sometimes proves more difficult of digestion than meat so cooked and served as to stimulate appetite and to demand mastication.
Returning to the story of a meal, vegetables pass almost unaltered through the stomach. Some of the scanty proteins which they contain are peptonized, but unless they are very well masticated or cooked until they are soft, and therefore easily pulped by the churning action of the stomach, the gastric juice has to reach the proteins through cell-walls. None of the digestive juices are able to dissolve the cellulose of vegetable cell-walls. Blocks of vegetable tissue pass down the whole length of the alimentary canal in the form in which they were left by the teeth. Hence the extreme indigestibility of ill-chewed cucumber or apple. The pyloric valve of the stomach is forbidden to allow any lumps of food to pass until the very last stage of gastric digestion. Pieces of ill-masticated vegetable tissue lie for a long time in the stomach, irritating the ends of the gastric nerves, until at last the time comes for them to be shot through the pylorus into the duodenum. Many salts which vegetables contain, especially the earthy carbonates and phosphates, are dissolved by the acid of the gastric juice.
Meat consists of muscle-fibres supported by connective tissue. In the stomach the gelatiniferous connective tissue is dissolved, setting the fibres free. Further, the fibres being surrounded by a membrane of the same nature—sarcolemma—this is removed; and although it may be hardly justifiable to speak of “Krause’s membranes” ([cf. Fig. 10]) as gelatiniferous septa, the fibres are certainly composed of segments—Bowman’s discs, sarcous elements—into which they break up under the action of gastric juice. As a result, meat-fibre is reduced to a finely divided granular condition. The capacity of gastric juice for dissolving collagen (the substance of which connective tissue is composed) may be regarded as its most characteristic, as it is one of its most valuable, properties. Collagen, when boiled or acted on by acids, takes water into its molecule, becoming gelatin. Under the influence of gastric juice gelatin is rapidly hydrolysed into diffusible gelatin-peptone. Pancreatic juice is unable to act upon collagen, unless it has been previously boiled, or swollen by the action of dilute acids.
Fat is composed of vesicles of oil supported by connective tissue. Gastric juice, by dissolving the connective tissue and the collagenous walls of the vesicles, sets the oil free. The oil, even though it be as firm as suet when cold, is liquid, or almost liquid, at the temperature of the body.
Thus, with the exception of raw vegetables, the hard fibre of cooked vegetables, elastic tissue of meat, and a few other indigestible substances, the meal is reduced in the stomach to a cream-coloured, fatty, strongly acid “chyme.” In this condition it enters the duodenum, where it at once comes into contact with an alkaline secretion. The passage of acid chyme down this portion of the canal provokes the discharge of gushes of bile and pancreatic juice. By precipitating partially digested proteins and “acid-albumin” bile renders the mixture thicker and sticky. It colours it yellowish-brown. Under the influence of pancreatic juice the remaining proteins and proteoses are rapidly converted into peptones, some of which are shaken down by the violent action of erepsin into simpler bodies, such as leucin and tyrosin, etc. The chyme becomes alkaline, grey, and thin. All undigested starch is changed into maltose, and this into dextrose. Cane-sugar is converted into dextrose and levulose. These sugars are absorbed into the blood. Milk-sugar, if not converted into lactic acid, remains as lactose (C₁₂H₂₂O₁₁), in which condition it is absorbed without “inversion.” Fats are split by a ferment of the pancreatic juice into fatty acid and glycerin; some of the fatty acid combines with alkali to form soap, but of this we shall have more to say later on.
The duct common to the liver and the pancreas opens into the second part of the duodenum. The organs which produce bile and pancreatic juice are comparatively remote from the place where their secretions come into contact with the food. By what mechanism are they thrown into activity when the assistance of their secretions is required? As in the case of the stomach, the agent is a hormone, a chemical messenger. The hormone, termed “secretin,” is formed by the cells of the mucous membrane of the duodenum when acid comes in contact with them. It is absorbed by the blood, which carries it to the pancreas and the liver. When it reaches the pancreas, it acts as a most powerful stimulant to the discharge of accumulated ferments, and to the production of an additional supply. It stimulates the liver to pour forth bile. At present we are in ignorance as to the chemical nature of this hormone. It is not a proteid substance, nor is it a ferment. If scrapings from the mucous membrane of the duodenum be crushed with sand and hydrochloric acid, the mixture boiled, neutralized with carbonate of soda, and filtered, the clear, colourless liquid which results has a powerful effect upon the pancreas, when injected, in even small quantities, into the blood. Apparently, the cells of the duodenal mucous membrane are constantly producing and accumulating a substance which is converted into secretin when acted on by acid. It is not necessary for the acid to stimulate the living cells. If the mucous membrane is ground up with sand and salt-solution, the filtrate is inactive but an active extract is obtained by treating the crushed cells with HCl. It changes some substance which they contain (provisionally termed “prosecretin”) into the efficient hormone.
In the lower portion of the small intestine any maltose that remains is converted into diffusible dextrose. A very large amount of water has been poured into the canal in the various digestive juices. This, together with water drunk, is absorbed in the large intestine. At the lower end of the alimentary canal nothing remains but indigestible substances taken with food, chiefly cellulose, and the pigments and other bodies which, as already said, are eliminated in bile.
The absorption of water is checked by the ingestion of extremely soluble salts, such as sulphate of magnesia, the heavy molecule of which diffuses with difficulty. We attribute the fact that sulphate of magnesia remains in the intestine, and prevents water from diffusing out of it, to its slowness in passing through a membrane, because this is what would happen in dialysis;[2] but we must remember that the living wall of the intestine is not a membrane. The cells which line the intestine take up substances far less easily diffusible than the sulphate of magnesia which they refuse. Nevertheless, speaking generally, it is the less diffusible salts which act as aperients, the more diffusible which are absorbed. The forward passage of the contents of the alimentary canal is hastened by castor-oil. The peristalsis of the intestines is stimulated by certain drugs, such as jalap or the burnt products of tobacco. Another class of drugs, of which aloes is an example, increases the secretion of the intestines, small or large. Certain purgatives, such as calomel, podophyllin, etc., used to be regarded as cholagogues. It was supposed that they increased the flow of bile. This is an error. Their action is complicated, but it affects chiefly the peristalsis of the intestine. The poor misunderstood liver still suffers from the libels of primitive medical science. It is the most innocent of organs, in no way responsible for derangements of digestion. It carries out its functions without haste and without delay. With the possible exception of salicylate of soda, no drug is known which can stimulate it to a more rapid output of bile.
Absorption.—All the cells which line the alimentary canal are capable of absorbing food, if it is presented to them in a suitable form. In a suitable form means, speaking generally, in a diffusible condition, although it must not be supposed that the epithelial cells are incapable, under certain circumstances, of taking up non-diffusible substances, just as a unicellular organism—an amœba—can take in food. If soluble proteins, such as white of egg or acid-albumin, are injected into the large intestine, a very considerable proportion of the substance so injected is absorbed. It is possible, indeed, to supply in this way the whole of the nitrogenous food needed by the system, none entering by the mouth. If milk is injected, a certain amount of the fat also is retained. It can be shown that such absorption takes place when no digestion of the food occurs in the colon. The food is taken up by the epithelial cells in the form in which it is injected.
The organs specially devoted to absorption are the villi, which project into the contents of the small intestine. Each is a conical process about 0·5 millimetre long. The villi are longest in the upper half of the small intestine. Below this level they decrease in number and size. A villus is completely covered with epithelial cells of short, columnar form. The free border of each cell is slightly hardened, forming a disc or cap which appears striated in optical section—an indication, as some think, that it is traversed by pores. Others hold that the appearance of striation is due to minute cilia-like projections which beset the free border of each cell. In worms and other invertebrates the cells carry motile projections of not inconsiderable size, which no doubt free their surfaces from the unassimilable matter which tends to accumulate upon them. Possibly they help to fix particles which are suitable for absorption. In mammals the presence of cilia has not been demonstrated. The extreme minuteness of the striæ seems to point to their being merely indications that the border is permeable to fluids, including droplets of fat.