Bile also contains bile-pigments. Their colour varies in different animals, and changes according as the bile is exposed to the air, or subject to the action of reducing agents. If oxidized, the colour is green (biliverdin); if reduced, brownish-yellow (bilirubin). Bile-pigment is formed from hæmoglobin, the colouring matter of the blood, after the removal of its iron. Worn-out red blood-corpuscles are destroyed in the spleen, in the manner already described, but it is uncertain whether the conversion of the hæmoglobin thus set free into bilirubin occurs in the spleen, or whether this chemical change is reserved for the liver. Physiologists incline to the view that the liver is the seat of the change.
Intestinal Juice.—The mucous membrane of the alimentary tract, as far down as the middle of the rectum, is, as previously stated ([p. 102]), studded with tubular glands. They secrete a light-yellow fluid, alkaline in reaction, and opalescent. Its most important property is due to a ferment which converts cane-sugar into a mixture of dextrose and levulose, and changes maltose—the sugar produced by the action on starch of saliva and pancreatic juice—into dextrose. It is in the form of dextrose that sugar is carried about the body and assimilated by the tissues.
Intestinal juice also contains a ferment, erepsin, which shakes to pieces the heavy molecules of peptones and partly formed peptones. Under its influence they break up into comparatively simple bodies containing the radicle of ammonia. Substances containing an NH₂ group—one H of NH₃ (ammonia) having been given up, in order that the group may have a “free arm” with which to link on to the other component parts of the molecule—are termed “amides.” The amides which are most characteristic of the action of erepsin are leucin, an amidated fatty acid; and tyrosin, an amidated aromatic acid. The tendency of proteins to break up along these two lines—the fatty acid line and the aromatic acid line—is of considerable interest. The one line is represented by acetic acid, CH₃COOH; the other contains the hexone radicle, C₆H₆. Benzoic acid, C₆H₅COOH, is representative of the latter. It used to be thought that proteins which were shaken into simple bodies such as amides were lost to the economy. Their downward career was a foregone conclusion. There could be no arresting it before they brought up at the bottom—as urea, CO(NH₂)₂—the diamide of carbonic acid. It was even supposed that this disintegration of proteins was a provision for getting rid of the surplus animal food which we consume. Physiological chemists now take quite a different view. They believe that the epithelial wall of the intestine through which these substances are absorbed, or the liver, to which they are carried by the portal blood-stream, has the power of recombining these fragments into the complex protein edifice. It is even thought that disintegration is a necessary preliminary to the rearrangement of the sub-groups. A large variety of proteins is ingested as food. Many of them, especially the vegetable proteins, are quite foreign to the body. By the activity of pancreatic juice and erepsin, they are broken into small and relatively stable groups of atoms, which are again fitted together into the particular forms of protein which are of use to the economy.
The Story of a Meal.—The chemistry of digestion will be understood most readily if the constituents of a meal are traced from their entrance into the mouth to their absorption through the wall of the alimentary canal, or abandonment as indigestible.
We may describe as a typical meal one consisting of bread, vegetables, cane-sugar, meat, milk, fat, and cheese. In the mouth the various foods are crushed and mixed with the alkaline secretions of the salivary glands. A certain amount of the cooked starch contained in the bread is changed into maltose. In the stomach the digestion of starch is continued for a time, but a large part even of the cooked starch awaits the action of pancreatic juice. A certain amount of cane-sugar is converted into dextrose and levulose, which are rapidly absorbed into the blood; but this action is due to hydrochloric acid, and probably affects a comparatively small part of the cane-sugar swallowed. Fat is quite unaltered in the stomach. All proteins are attacked by pepsin, but some yield to digestion more readily than others. Gluten of bread, like all vegetable proteins, is comparatively resistant; but since it is presented to the action of pepsin in small quantities and in a spongy form—very suitable for digestion—it is probable that most of it is peptonized in the stomach. Chemists experimenting with gastric juice taken from the stomach, and reproducing the conditions as to temperature, removal of products of action, etc., as closely as it is possible to reproduce them in the laboratory, find that the various foods take different times to digest. The proteins of meat are more quickly peptonized when raw than after coagulation by heat. The same is true of white of egg. Amongst different varieties of cooked flesh, beef is more quickly peptonized than fish. The casein of milk is more quickly peptonized than any other protein; and it also is no exception to the rule that digestibility is diminished by cooking. Similar data may be obtained for all foods. They are no doubt useful indications of the course of action which we may expect to occur within the stomach, but we can never be sure that my lord will obey the ruling of the chemist. Practice with a captive golf-ball is a useful preparation for the game; but there are conditions on the links which cannot be reproduced on the lawn. In an artificial stomach the clean fibre of raw fish digests more slowly than raw beef. Even when the beef is roasted and the fish fried or boiled in the ordinary way, the beef disappears through the dialyser (the bag of membrane suspended in a vessel of warm water in which experimental digestion is carried out) more quickly than the fish. Nevertheless, the living stomach is better disposed towards a mixed meal containing a certain weight of fish than towards a meal in which, the other constituents remaining the same, beef takes the place of fish. Important conclusions may, no doubt, be drawn from observations of the time occupied in the peptonization of pure food—i.e., fibrin, white of egg, clean meat, etc.—under conditions simulating those which are present in the stomach; but they must be accepted with many reservations. In the stomach it is not pure substances, but mixtures, that the gastric juice has to deal with. And here a most important factor comes into play, to which further reference will be made later on. The amount and quality of the secretion of the gastric glands depends upon the nature of the food. Hence a food, or a combination of foods, which digest readily in the laboratory may take a long time to disappear from the stomach, and vice versâ. Digestibility depends upon the nature of the food. It depends also upon its physical state. To take simple illustrations: Cheese contains coagulated casein, one of the most easily digestible of proteins, but the casein is intimately mixed with fat, upon which gastric juice can make no impression. Even when finely divided, the particles of casein are protected from the action of the juice by fat. In the same way the meat of pork is as digestible as mutton, but the fat of pork is quickly melted and very liquid. In the process of cooking the muscle-fibres become saturated with fat.
It is not the function of the stomach to complete digestion. Its business is to initiate it. Food which reaches the stomach in fragments is reduced to a condition in which its digestion will be readily completed by pancreatic juice. Gastric digestion produces a much larger proportion of intermediate products, proteoses or propeptones, than does digestion in the duodenum. Such intermediate products are quickly dealt with by pancreatic juice. Artificial tests of relative digestibility do not, as a rule, take the amount of propeptones formed in a given time into account. When considering the digestion of a typical meal, we must bear in mind that it is not the duty of the stomach to pass as much sugar, peptone, and fat as possible into the blood. In fact, very few of the products of digestion are absorbed by the bloodvessels of the stomach. The impermeability of its mucous membrane is shown by the fact that hardly any of the water swallowed passes through the stomach-wall. Practically all the water ingested leaves the stomach through the pyloric valve. Various salts, some sugar, and peptones are taken up by the vessels of the stomach; but the bulk of all the different kinds of food passes into the duodenum in a semi-digested state. The function of the stomach is to carry digestion through a preliminary stage. The process will be completed in the small intestine. It is to be noted that, although water is not absorbed by the stomach-wall, alcohol passes through it with great rapidity. The same is true of the various crystalline nitrogenous bodies found in meat-extracts, and also of the essential principles of tea and coffee, which chemically belong to the same class. All these substances are degradation products of proteins produced by oxidation, far advanced along the road to urea. In this selective absorption we see proof of the activity of the cells of the mucous membrane. They take up the substances which it is desirable to remove from the contents of the stomach. Some may be wanted by the body for its immediate use; others are better out of the way, because they are prejudicial to the progress of digestion.
When contemplating the activity of the cells of the gastric mucous membrane, we feel the need of an adjective which shall express our recognition of the fact that they have a power which we cannot confer upon our clumsy mechanical imitation stomach. They can discriminate. “Vital” is the only term available, though much abused. Using it without prejudice, as lawyers say, we speak of the “vital activity” of the cells when we wish to imply that things happen in a living stomach for which we cannot make provision in a model. Of the many substances which make their appearance as digestion proceeds, some are absorbed, others left in the mixture.
The mucous membrane shows its power of controlling digestion in yet another way. In the neighbourhood of the pylorus its structure is unlike that which it presents elsewhere. The gastric glands are short, and tend to branch. Their lining cells are all of the same kind. Over the greater part of the inner wall of the stomach the tubes are long. They do not branch. The cells which line them are of two kinds: small cubical cells (the term refers to their form as seen in section), similar to those of the pyloric glands; large oval cells, placed with their longest axes in the same direction as the axis of the gland-tube. These oval cells do not project into the bore or lumen of the tube, but are displaced from it by the cubical cells. They rest on the investing, or basement, membrane. All parts of the gastric mucous membrane secrete pepsin, although the pyloric portion produces very little; the area which contains oval cells alone secretes hydrochloric acid. If a short time after a meal an extract is made from some of the mucous membrane near the pylorus, by pounding it with salt-solution and sand to break up its cells, this extract, when filtered and injected into the blood, stimulates the glands of the cardiac end of the stomach. Under its influence they pour out both pepsin and hydrochloric acid. The extract contains a substance which acts as a chemical messenger. It is a representative of a class of bodies which play a most important part in co-ordinating the activities of the various organs. Hitherto physiologists have concerned themselves with the visible or “external” secretions of glands. They have shown how the production of these secretions is controlled by the nervous system. Recently they have discovered that another set of influences has to be taken into consideration. Glands, and possibly all other tissues, take from the blood the materials out of which they make their characteristic secretions, or, if they do not discharge secretions, the substances which they require for the building of their own structures, and return to the blood “internal secretions” which act as stimuli to other tissues with which they are linked in harmonious co-operation. The active principles of internal secretions have been termed “hormones”—from ὁρμάω, I announce. The glands of the pyloric mucous membrane secrete a hormone which calls upon the rest of the membrane to pour out gastric juice ([cf. p. 89]).
What induces the cells of the pyloric mucous membrane to produce the gastric hormone? Their activity in this respect evidently depends upon the presence in the stomach of partially digested proteid substances. The cells judge, as it were, when these substances come into contact with them, that there is more work for the great bag of the stomach to do. They call upon the part which is most active in secreting gastric juice to pour it out quickly and get the business of digestion over. Meat-extracts, which contain the products of protein disintegration, have a similar influence in promoting the formation of the hormone. Hence, no doubt, the general custom, found from experience to be beneficial, of commencing dinner with soup; although it must be remembered that the rapid absorption of meat-extracts makes them peculiarly valuable as restoratives. They afford very little energy, but what they have to give is quickly placed at the disposal of the economy. Persons whose stomachs are unduly irritable are advised to avoid soup. It leads to undesirable activity on the part of the gastric glands, and especially of the acid-secreting cells. Well chewed bread also encourages the production of the hormone.
Here it may be well to call attention to the evident division of the stomach into two parts—the large bag, or cardiac portion, which hangs down; and the smaller, funnel-shaped pyloric end, which is almost vertical. The distinction between these two parts is faintly visible in the resting stomach, but even opening the abdomen tends to obliterate it. That it is much more evident during active digestion has been shown by adding subnitrate of bismuth to the food, and throwing the shadow of the stomach on a screen with Röntgen rays. When this is done, it is seen that the two parts work in different ways. Food is churned round and round in the cardiac portion, and pressed towards the pylorus. Its fluid products, mixed with the abundant secretion of the gastric mucous membrane, are wrung out of it by the pyloric funnel. They are squeezed towards the pylorus, which opens at intervals to let them through. If lumps of solid matter reach it, the pyloric valve closes tightly, until the undigested food has fallen back into the dependent bag. Dyspeptics are sometimes unpleasantly conscious of the contractions of the pyloric funnel. In fact, putting aside pain due to gastritis, all the discomfort of dyspepsia is felt on the right side. Flatus accumulates beneath the pyloric valve. The valve will not open to let it pass. The pyloric portion of the stomach contracts strongly. Notwithstanding the general trend of movement in the opposite direction, the gases are squeezed back into the larger bag, and escape through the cardiac orifice.