From the time the food enters the stomach, during nearly its entire course along the alimentary canal, there is a constant production of soluble compounds, which progressively disappear into other channels, so that when the anus is reached only a small portion of the original dry matter is found in the residue. In some way, not wholly explainable in all its details, the digested food has been absorbed and received into vessels through which it is distributed to the various parts of the body.

114. Function Of Lacteals And Blood Vessels In Absorption

A merely casual observation shows us that the inner surface of the walls of the digestive organs are covered by numerous projections. The anatomist, by a careful study of these, has learned that imbedded in their tissue, especially in the intestines, are the minute branches

Fia. 5. - Cross section mucous membrane of small intestine, showing capillaries and lac train. (Gerrish.) of two systems of vessels. One set is the lacteals, belonging to the so-called lymphatic system, and the other set is the capillaries of the blood system. The lymphatic vessels or tubes all lead to a main tube or reservoir, the thoracic duct, which extends along the spinal column and finally enters one of the main blood-vessels. Any material, therefore, taken up by the lacteals ultimately reaches the blood. The capillaries all converge to a larger blood vessel, known as the portal vein, which enters the liver, carrying with it whatever materiaf the capillaries have absorbed.

The manner in which the soluble food is absorbed has been explained in part on common physical grounds. When two solutions of different densities, containing diffusible compounds, are separated by a permeable membrane, diffusion through this membrane from the denser to the lighter liquid will always occur. Such a condition as this prevails in the intestines, we may believe. The intestinal solution, the denser one, is separated from a less concentrated liquid, the blood, which is constantly flowing on the other side of a thin dividing membrane. Under these conditions there occurs the passage into the blood of certain parts of the digested food. It is held that in this way water, soluble mineral salts, and sugar pass directly into the blood-vessels. The peptones are also taken up by the capillaries, and the fats enter the blood through the lacteals.

115. Changes In The Walls Of The Intestinal Tract

In the absorption of peptones and fats, at least, we encounter forces other than the osmotic transference of substances in solution, the operation of which is still more or less unexplained.

Fig. 6. - Intestinal villus, showing a. epithelium; b, capillaries; c, lacteal vessel.

As we have learned, the ingested proteins are changed in the stomach and intestines to peptones, and in part, perhaps mainly, to simpler compounds resulting from the cleavage of peptones. The fats are split partly, or entirely, into fatty acids and glycerin, with the subsequent formation of soaps by the union of the free acids with the alkaline bases of the bile. There is good evidence that in the passage of these new compounds through the walls of the intestine changes occur of a synthetical character, with a partial or total reconstruction of the proteins and fats into forms similar to those in the ingested food. The rebuilding of fats and their transference into the lacteals is regarded as being accomplished through the activity of cells lying in the mucous lining of the intestine. It seems, then, that the vital forces residing in the living cells play a part in transferring the nutrients into the blood •circulation, and that this absorption can no longer be explained wholly on the basis of osmotic pressure.

116. Place Of Maximum Absorption

Absorption of digested food undoubtedly takes place in the stomach, but the main transference of the products of digestion into the blood is from the intestines, particularly the small intestine. Much of the water that passes into the large intestine is absorbed there, together with the products of digestion not already absorbed and those products that result from bacterial action. It is a question of importance whether there is an absorption of proteins when the lower intestine is flooded with an enema containing proteins such as the white of an egg. The evidence is that under these circumstances protein absorption occurs and furnishes a very fortunate means of nourishing a patient when the ordinary method of digestion is not possible.