While absorption through a permeable animal membrane outside the body may be a simple physical process, following the ordinary laws of pressure, diffusion, osmosis, etc., the ease is far otherwise in the alimentary tract, where the selective activity of the living cell becomes a positive factor. The pressure in secreting glands rises above blood pressure; substances pass inward to the body through the mucous membrane without a corresponding movement in the opposite direction. Thus blood serum, placed in a loop of intestine is promptly absorbed, but if the loop be surrounded with serum, it does not pass in a similar manner into the lumen of the section of intestine.

The stomach is not primarily an absorbing organ, but a few substances, such as sugars and peptones, pass to a limited extent through its walls. The most important seat of absorption is the small intestine, and second, the large intestine. Absorption in the small intestine is affected through the innumerable little projections with which the intestine is lined, known as villi, which greatly extend the amount of surface. These villi contain a capillary network and a so-called lacteal space. The capillaries by converging into larger vessels eventually form the portal vein which passes through the liver. The lacteal spaces are prolonged into lymphatic vessels which similarly converge to form the thoracic duct. The lacteals are intended almost wholly for the absorption of the fats, all other digestive products passing through the liver before reaching the general circulation.

Absorption continues throughout the length of the large intestine, being especially flavored by antiperistalsis in the upper portions. Constipation sometimes results from the great absorptive power of this region, the feces being left dry and hard and hence difficult to evacuate.

Absorption Of Protein

This question still continues to be a riddle. At some point, either in the intestinal walls and villi or blood vessels, or in the blood itself, the amino acids formed in digestion are united to form body protein. We do not know how or where this transformation occurs. It is certain that a small amount of humanized albumin reaches the blood through the thoracic duct. The rest must enter the portal circulation and pass through the liver into the general blood stream. The end products of peptic digestion seldom are found in the blood, and when present, tend to pass at once into the urine. When food albumin for any reason enters the blood, it does not act as blood albumin, but is at once excreted by the kidneys as a poison. Proteoses and peptones behave similarly. As animals may be made to live in health on a diet of end products alone, we know that these represent the utilizable portion of food albumin.

Absorption Of Fat

It is now commonly believed that fat is absorbed in the form of the end products of its digestion, i. e., fatty acids and glycerine, and that somewhere in the intestinal wall these are recombined into neutral fat. It is possible that' some of the finely emulsified fat is able to pass unchanged through the intestinal wall. In either case, minute fat droplets pass into the lacteals, and finally into the blood stream by way of the thoracic duct. A small portion finds its way directly into the blood.

Fats having a low melting point are absorbed more quickly than solid fats, and pure fat more readily than fat intermingled with other food principles, as in fat meat, for example.