There are two main paths by which the food constituents are absorbed into the circulation - the blood-vessels and lymphatics. The blood-vessels pour their contents into the radicles of the portal vein, which carry them to the liver. The lymphatics in the submucous coat of the intestine join to form larger trunks, which run between the two layers of the mesentery to a collection of lymphatic glands at the back of the peritoneal cavity. The lymph, after flowing through these glands, is collected into a large vessel, the receptaculum chyli, from which it is carried in the thoracic duct to be dis-charged into the blood-stream at the junction of the left jugular and subclavian veins.
The most recent investigations render it probable that proteins are not absorbed in the form of peptones, as was formerly supposed, but in the form of amino-acids and bases, products which represent an even more complete disintegration of the proteins. These amino-acids are absorbed as such into the blood, and built up again into proteins in the living cells of the tissues, the extent to which any individual protein is broken down during digestion being determined by the degree in which its composition approximates to that of the normal proteins of the digesting animal.
It has been shown that the greater part of the fat is absorbed by the chyle, and 60 per cent, of the absorbed fat can be obtained from the chyle through a canula placed in the thoracic duct. It has not been found possible to trace the mechanism of absorption of that portion of the fat which does not enter the blood by way of the lacteals. It is believed that it may be utilised or built up into more complex compounds in the tissues of the intestinal mucous membrane. In what form is the fat taken up by the epithelial cells covering the intestinal cells? It was formerly thought that the digestion of fat essentially consisted in a splitting up into particles fine enough to be taken up by the epithelial cells, but this is now disproved, and it is now believed that fats, like proteins and carbohydrates, are absorbed in a state of solution. In the digestion and absorption of fat there is a concerted action of the pancreatic juice and the bile. The pancreatic juice splits about 6 per cent, of the neutral fat - which forms the great proportion of the fat of food - into fatty acids and glycerine. In the presence of excess of alkalis the fatty acid forms a soluble soap, which, together with the glycerine, is absorbed by the epithelial cells, and reconverted in the body of the cell into neutral fat. If, as is often the case with a highly fatty diet, the reaction of the small intestine be acid, the formation of soaps can no longer go on. The fat-splitting action of the pancreatic juice, however, continues, and the fatty acids set free are dissolved by the bile acid and taken up by the epithelial colk Here the synthesis of the neutral fat once more occurs, and the bile acid is carried by the portal blood to the liver, to be resecreted with the bile into the intestine, where it may aid the absorption of a further amount of fat. As the free fatty acids and soaps are absorbed, the pancreatic juice is able to split up a further portion of the neutral fat until the whole of the neutral fat of the food has been absorbed by the epithelial cells of the intestine in a state of solution as soaps or fatty acids. It is partly on this account that, after extirpation of the pancreas, fats are not absorbed, even if administered to the animal in the form of a fine emulsion containing neutral fat suspended in a solution of soap. If, however, to this emulsion chopped-up pancreas is added, a large proportion of the fat is absorbed.
It is generally believed that the blood in the capillaries of the intestinal wall takes up sugar in the form of dextrose, and carries it to the liver, where the excess of sugar is taken up by the hepatic cells, and converted by them into the colloid carbohydrate glycogen which is deposited in the substance of the cell. In this way the liver acts as a storehouse of carbohydrate material, and prevents the sugar in a rich carbohydrate meal from escaping into the general circulation. According to Pavy, this view of the manner of absorption of carbohydrates is an erroneous one. This author believes that the main channel of absorption of carbohydrates is the lacteal system, the sugar being absorbed in a combined state, a synthesis between the carbohydrate and protein radicle taking place in the intestinal villi through the agency of the leucocytes. Our knowledge of the exact mechanism of absorption of carbohydrate foods is as yet uncertain. Viewed from the clinical standpoint, there is much to be said in favour of the general correctness of Pavy's view.
Water and salts are absorbed almost entirely by means of the blood-vessels. The rate of absorption of water by the alimentary canal increases from above downwards. No water or dilute saline solution appears to be absorbed by the stomach. In the small intestine the process of absorption is much more rapid in the ileum than in the jejunum. In consequence, however, of the continual secretion of the Succusentericus the intestinal contents reach the ilco-caecal valve in a fluid condition, and the excess of water is only absorbed in the large intestine-. The absorption depends on two factors, a physical and physiological one. When the salts are of small or of no physiological importance the cells appear to have little-power of physiological absorption, and the salts exert their full physical influence, absorbing water or retaining that by which they are dissolved, so that they act as saline purges, e.g. sodium or magnesium sulphate.