It follows from this so-called peristaltic contraction, that the contents of the intestines are constantly being propelled, by successive and progressive narrowing of their calibre, from their upper towards their lower parts. And the same peristaltic movement goes on in the large intestine from the ileo-cæcal valve to the anus.

The large intestine presents noteworthy peculiarities in the arrangement of the longitudinal muscular fibres of the colon into three bands, which are shorter than the walls of the intestine itself, so that the latter is thrown into puckers and pouches; and in the disposition of muscular fibres around the termination of the rectum into a ring-like sphincter muscle, which keeps the aperture firmly closed, except when defæcation takes place.

23. The only secretions, besides those of the proper intestinal glands, which enter the intestine, are those of the liver and the pancreas-the bile and the pancreatic juice. The ducts of these organs have a

common

opening in the middle of the bend of the duodenum; and, since the common duct passes obliquely through the coats of the intestine, its walls serve as a kind of valve, obstructing the flow of the contents of the duodenum into the duct, but readily permitting the passage of bile and pancreatic juice into the duodenum (Figs. 36, 38, 42).

Pancreatic juice is an alkaline fluid not unlike saliva in many respects; it differs, however, in containing a considerable quantity of proteid material. Bile we have already studied.

After gastric digestion has been going on some time, and the semi-digested food begins to pass on into the duodenum, the pancreas comes into activity, its bloodvessels dilate, it becomes red and full of blood, its cells secrete rapidly, and a copious flow of pancreatic juice takes place along its duct into the intestine.

The secretion of bile by the liver is much more continuous than that of the pancreas, and is not so markedly increased by the presence of food in the stomach. There is, however, a store of bile laid up in the gall-bladder; and as the acid chyme passes into the duodenum, and flows over the common aperture of the gall and pancreatic ducts, a quantity of bile from this reservoir in the gall-bladder is ejected into the intestine.

The bile and pancreatic juice together here mix with the chyme and produce remarkable changes in it.

24. In the first place, the alkali of these juices neutralises the acid of the chyme; in the second place, both the bile and the pancreatic juice appear to exercise an influence over the fatty matters contained in the chyme, which facilitates the subdivision of these fats into very minute separate particles. The fat, as it passes from the stomach, is very imperfectly mixed with the other constituents of the chyme; and the drops of fat or oil (for all the fat of the food is melted by the heat of the stomach) readily run together into larger masses. By the combined action, however, of the bile and pancreatic juice the large drops of fat which pass into the intestine from the stomach are emulsified, that is to say are broken up into exceedingly minute particles, and thoroughly mixed with the rest of the contents; they are brought in fact to very much the same condition as that in which fat (¿.e. butter) exists in milk. When this emulsifying has taken place the contents of the small intestine no langer appear grey like the chyme in the stomach but white and milky; in fact it and milk are white for the same reason, viz., on account of the multitude of minute suspended fatty particles reflecting a great amount of light.

The contents of the small intestine, thus white and milky, are sometimes called chyle; but it is best to reserve this name for the contents of the lacteals, of which we shall have to speak directly.

The emulsifying of the fats is not, however, the only change going on in the small intestine. The pancreatic juice has an action on starch similar to that of saliva, but much more powerful. During the short stay in the mouth very little starch has had time to be converted into sugar, and in the stomach, as we have seen, the action of the saliva is arrested. In the small intestine, however, the pancreatic juice takes up the work again; and indeed, by far the greater part of the starch which we eat is digested, that is, changed into sugar, by the action of this juice.

Nor is this all, for, in addition to the above, the alkaline pancreatic juice has a powerful effect on proteids very similar to that exerted by the acid gastric juice;

it converts them into peptones, and the peptones so produced do not differ materially from the peptones resulting from gastric digestion.

Hence it appears that, while in the mouth amyloids only, and in the stomach proteids only, are digested, in the intestine all three kinds of food-stuffs, proteids, fats, and amyloids, are either completely dissolved or minutely subdivided, and so prepared for their passage into the vessels.

As the food is thrust along the small intestines by the grasping action of the peristaltic contractions, the digested matter which it contains is absorbed, that is, passes away from the interior of the intestine into the blood vessels and lacteals lying in the intestinal walls.

A great deal of this absorption takes place in the small intestine (though the process is continued on in the large intestine), and there can be no doubt that it is largely effected by means of the villi. Each villus as we have seen (§ 22), is covered by a layer of epithelium, and contains in the centre a lacteal radicle, between which and the epithelium lies a network of capillary bloodvessels embedded in a delicate tissue. Now in some way or other, not even yet thoroughly understood, the majority of the minute particles of the finely divided, emulsified fat, pass through the epithelium, past the capillary bloodvessels, into the central lacteal radicle; so that, after a fatty meal, these lacteal radicles of the villi become filled with fat. The lacteal radicle is continuous with the interior of the lymphatic vessels which ramify in the walls of the intestine, and which pass into the larger lymphatic vessels running along the mesentery towards the thoracic duct. Into these vessels the finely divided fat passes from the lacteal radicle of the villus, and, mixing with the ordinary lymph contained in these vessels, gives their contents a white, milky appearance. Lymph thus white and milky from the admixture of a large quantity of finely divided fat is called chyle; and this white chyle may after a meal be traced along the lymphatics of the mesentery to the thoracic duct, and along the whole course of that vessel to its junction with the venous system. After a meal, in fact, this vessel is continually pouring into the blood a large quantity of chyle, ¿.e. of lymph made white

and milky by the admixture of fats drawn from the villi of the small intestine.

The peptones and sugar, being soluble and diffusible, pass, by a process which may be broadly described as osmosis, through the epithelium into the substance of the villi, and here they appear to be taken up by the capillary blood-vessels of the villus, so that very little if any of them gets to the lacteal radicle. From the capillaries of the villi the peptones and sugar are then carried along the vena porta to the liver, where they probably undergo some further change. So that while the fat, though it gets for the most part into the general blood current by a roundabout way, viz., by the lymphatics, reaches the blood, as far as we know, very little changed, the peptones and sugars on the other hand, though also taking a roundabout course, viz., by the liver, are probably altered before they are thrown into the general blood stream; for the portal blood in which they are carried is acted upon by the liver before it flows through the hepatic vein into the general venous system. But concerning both the process of absorption itself and the changes undergone by the absorbed products before they reach the heart, ready to be distributed all over the body, we have probably much yet to learn.

25. As the food thus passes along the small intestine, digestion and absorption go on hand in hand. All the way down, the proteids, amyloids, and fats of a meal are being dissolved or freely divided, or otherwise changed, and passing away into the lacteals or blood-vessels. So that, by the time the contents of the intestine have reached the ileo-cæcal valve, a great deal of the nutritious matter has been removed. Still, even in the large intestine, some nutritious matter has still to be acted upon; and we find that, in the cæcum and commencement of the large intestine, changes are taking place, apparently somewhat of the nature of fermentation, whereby the contents become acid. In herbivorous animals it is probable that very considerable changes are effected in this part of the alimentary canal.

One marked feature of the changes undergone in the large intestine is the rapid absorption of water. Whereas in the small intestine, the amount of fluid secreted into

the canal about equals that which is removed by absorption, so that the contents at the ileo-cæcal valve are about as fluid as they are in the duodenum; in the large intestine on the contrary, especially in its later portions, the contents become less and fluid. At the same time a characteristic odour and colour are developed, and the remains of the food, now consisting either of undigestible material, or of material which has escaped the action of the several digestive juices, or withstood their influence, gradually assume the characters of fæces.