Digestion in the small intestine.—The chyme does not pass all at once from the stomach into the small intestine. It has been found that food already acted on by the gastric juice, if allowed to remain in the stomach, im pedes the continuation of the process. The food seems to be digested in the stomach in detachments, and as soon as a portion has be come sufficiently digested the pyloric valve, that we have noted (p. 199) as guarding the communication between stomach and small in testine—the pyloric valve opens, permits that portion to escape into the small intestine, then closes, and opens again as soon as a further quantity of food is ready. The stimulus of the food passing over the openings of the bile and pancreatic ducts causes a discharge of bile and pancreatic juice, which proceed to mix with the food and act upon it.
The pancreatic juice is alkaline, and con tains several ferments: one of them, like saliva, converts starch into sugar; another, like the gastric juice, converts albuminates into pep tones, and may even proceed further and split up peptones into other bodies; and a third acts upon fats, making them into an emulsion or mixture, which is to some extent ca pable of passing through animal membranes, while fat cannot. Pancreatic juice seems also to split fats up into a fatty acid and glycerine, both of which can be absorbed. Thus starch which has escaped the saliva, and albumin which has escaped the gastric juice, are acted on by the pancreatic juice, and prepared for admission to the blood, the fat unacted on by either of the former juices not escaping the third.
The bile is also alkaline, and of a reddish yellow colour. When it has been vomited it is distinctly yellow, because of the action on it of the gastric juice. As much as 2i pounds weight of it may be poured into the small intestine of man in 24 hours. It contains a considerable quantity of colouring matter ; and its chief in gredients are two salts of soda, the taurocholate and glycocholate of soda. Owing to the presence of these two salts the bile is capable of forming a soap with fat, and thus largely aids the pan creatic juice in enabling oil to become fit for absorption.
Owing to the action of these four juices, aided by the intestinal juice secreted by the tubular glands of the intestine (p. 199), the chyme be comes transformed into chyle. ' The chief dis tinctions between chyme and chyle are that the former is acid, the latter alkaline. In the former the oil floats in large globules; in the latter it is evenly diffused throughout the liquid as in an emulsion, and this gives to chyle its milky appearance. The chyle is propelled along the intestine by spiral (peristaltic) contractions of its muscular walls. A function of the bile not yet mentioned is to stimulate these movements, and at the same time to prevent putrefaction of the contents of the intestine. This explains how, when, in diseased conditions, bile is pre vented passing into the small intestines, con stipation results, and when the stools are passed they are badly smelling, and very light in colour, owing to want of the bile-colouring matter.
The special purpose of digestion in the small intestine is, then, the digestion of fat, while at the same time all the other food-stuffs are acted on.
Absorption by the small food is propelled along the small intestine, as we have seen, by movements of the muscular walls. The length of the tube is considerable— at least 20 feet—and it is richly supplied with blood along its whole course. Hence what we have observed to occur in the stomach will also occur here, water containing in solution salts, starch converted into sugar, and albumin con verted into peptone, will be taken up directly into the blood. In addition some amount of changed fat will enter the circulation. We have noticed, however, special structures in the small intestine, namely, the vinous projections (p. 199) containing a loop of blood-vessels, and another vessel--the lacteal. These are specially for the absorption of fat. They dip like fingers into the chyle, and the minute particles of oil pass through their cellular covering and gain entrance to the lacteal. The folds of the small intestine permit a large number of the villi to be present, and a large surface for fat absorp tion is therefore provided. Thus in the small intestine the nourishing portions of the food are withdrawn in two ways, (1) by blood-vessels and (2) by lacteals. The material that enters the lacteals joins the blood current later. It is conveyed by lacteal vessels through the me sentery to reach glands where it undergoes cer tain changes, and finally reaches a vessel—the thoracic duct—which passes up the front of the backbone to reach the root of the neck, where it opens at the junction of the great veins of the left side of the head and left arm. (See fig. 126, p. 278.) We see, then, that all the nourishing material obtained from the food sooner or later enters the blood.
Digestion in the large intestine does not occur to any great extent. The food enters this portion of the digestive tract through the ileo crecal valve situated in the right groin (p. 199). Although the great intestine is much shorter than the small, the remains of the food travel through it slowly, the pouches into which the walls are thrown preventing their speedy pas sage. Time is thus given for fluid matters to be abstracted by the blood-vessels of the mucous membrane. The remains of the food become consequently less fluid, and they acquire their characteristic odour. They are called feces, and consist of undigested or indigestible mate rials, and of substances derived from the bowel itself, with a part of the bile from the liver. Propelled onwards by the contractions of the muscular walls, they at last reach the rectum, where their accumulation gives rise to the sensation that ends in the voluntary effort by which they are expelled from the body.
The nervous relations of digestion are not thoroughly understood. That digestion is con• trolled by the nervous system is quite certain. The salivary glands, for instance, have their activity regulated by nerves, some fibres of which find their way to the very cells of the glands. Such direct relationship is not known