In the absorption of proteid products, their passage from the intestine by way of the portal circulation insures exposure to the action of the hepatic cells, before they are distributed by the general circulation throughout the body. It is only under conditions of an excessive intake of proteid foods that their products are absorbed by way of the lymphatics. These points are clearly established, and there is every ground for believing that substantial reasons exist to account for this single line of departure. Just what the liver does, however, is uncertain. In fact, as already indicated, there is lack of definite knowledge as to how far the proteid foods are broken down in digestion, prior to absorption. The combined action of pepsin, trypsin, and erepsin, if sufficiently long continued, can accomplish a complete disruption of the proteid molecule.

1 Franz Hofmeister: Ueber Resorption und Assimilation der Nahratoffe. Arcliir f. d. exper. Pathol, u. Pharm., Band 25, p. 240.

We are inclined to assume in a general way that the "proteids taken as food cannot find a place in the economy of the animal body till they have been, as it were, melted down and recast."1 How far this melting down or disruption extends in normal digestion, we do not at present know. As already stated, neither proteoses and peptones, nor the amino-acids, are found in the blood stream in sufficient amounts, or with that frequency, to suggest absorption in these forms. Possibly, as some physiologists have suggested, the amount of any of these products to be found at any one time in a given quantity of blood is too small for certain recognition, yet in the twenty-four hours the amount passing from intestine to liver might be sufficiently large to equal the total proteid absorbed. We can, however, at present only conjecture, and must rest content with the simple statement that in the digestion of the proteid foodstuffs, proteoses, peptones, and amino-acids are formed, and that by transformation or total reconstruction of these products, special types of proteid are manufactured either in the epithelial cells of the intestinal walls during absorption, or elsewhere in the body after absorption. If this latter is the case, the liver might readily be regarded as a likely spot for the synthesis to occur.

Bearing in mind what has been said regarding the production of specific types of proteid by every species of animal, we can the more readily conceive of a synthesis "out of fragments of the original molecules rearranged and put together in new combinations, by processes in which the intestine can hardly be supposed to play a part." This, the liver might well be assumed as capable of accomplishing, and if we were disposed to accept this view we might use as an argument the fact that the products of proteid digestion are taken directly to this organ, before being cast loose in the tissues and organs of the body. There is perhaps as good ground for assuming that a synthesis or reconstruction of proteid takes place all over the body; that, as suggested by Leathes, "the synthesis of proteids is a function of every cell in the body, each one for itself, and that the material out of which all proteids in the body are made is not proteid in any form, but the fragments derived from proteids by hydrolysis, probably the amido-acids, which in different combinations and different proportions are found in all proteids, and into which they are all resolved by the processes, autolytic or digestive, which can be carried out in every cell in the body." It is certainly a reasonable hypothesis, and since we lack positive knowledge it cannot at present be disproved. All that we can affirm in the light of established fact is that the products of proteid digestion are absorbed from the intestine by way of the portal circulation, and that either in their passage through the intestinal wall, or later on in the liver or elsewhere, there is a construction of new proteid to meet the wants of the body. The liver, indeed, may be effective in both construction and destruction of proteid, but there is no way of telling at present just how far it acts in either direction.

1 J. B. Leathes: Problems in Animal Metabolism. Blakiston's Son and Co., 1906, p. 128.

Regarding the absorption of fats, a single statement will suffice, in addition to what has already been said. Fats gain access to the general circulation by passing from the intestine into the lacteal radicles, thence into the lymphatics, whence they move onward into the thoracic duct, and from there are emptied into the great veins at the neck. A small amount is apparently absorbed in the form of soap by the portal circulation, but by far the larger amount of fat gains access to the blood stream without going through the liver.

In these ways, the blood and lymph are continually supplied with proteid, fat, and carbohydrate from the ingested food, and as these fluids surround and permeate the organized elements of the tissues, the latter are enabled to gain what they need to maintain their nutritive balance. Living matter is essentially unstable; it is the seat of chemical changes of various kinds, anabolic or constructive, and kata-bolic or destructive. The more comprehensive term "metabolic " is applied to all of these changes that take place in living matter. In anabolism, the dead, inert proteids, fats, and carbohydrates are more or less assimilated and made a part of the living matter of the tissue cells, while at the same time a certain amount of the food material, probably the larger amount, is simply stored as such, or left to circulate in the blood and lymph, without being raised to the higher level of living protoplasm. In katabolism, this accumulated material, and in some degree the living substance itself, is broken down or disintegrated with liberation of the stored-up energy, which manifests itself in the form of heat and mechanical work. At times, the anabolic processes predominate and there is a relatively large accumulation of stored-up materials; while at other times, katabolism, with its attendant chemical decompositions, predominates, and the body loses correspondingly. The point to be emphasized here is that the living body, with its multitude of living cells, is the seat of incessant change. Construction and destruction are continually going forward side by side; sometimes the one and sometimes the other predominating, according to existing conditions. The living protoplasm with its attendant storage material is, under ordinary conditions, constantly being made good from the assimilated food, a part of which is raised to the dignity of living matter and becomes an integral part of the living cells, while the larger portion is simply stored for future uses, or circulates in the blood and lymph which bathe them. Doubtless, this storage or circulating material is the main source of the energy which constantly flows from the cells in the form of heat and of work, as a result of the disruptive changes that constitute katabolism.