The chemical changes taking place in the small intestine are many and varied. The acid chyme, with its admixture of semi-digested food material, as it passes through the pyloric sphincter into the small intestine, is at once brought into immediate contact with bile, pancreatic juice, and intestinal juice, all of which are more or less alkaline in reaction. As a result, the acidity of the gastric juice is rapidly overcome, and the enzyme pepsin, which up to this point could exert its characteristic digestive action, is quickly destroyed by the accumulating alkaline salts. Pepsin digestion thus gives way to trypsin digestion, - most effective in an alkaline medium, and the proteids of the food, already semi-digested by pepsin-acid, are further transformed by trypsin; aided and abetted by another enzyme, known as erepsin, secreted by the mucous membrane of the intestine. These two enzymes are much more powerful agents than pepsin. It is true that they begin work where pepsin left off, but most striking is the character of the end-products which result from their combined action, since they are small molecules and there is a surprising diversity of them. In other words, while gastric digestion breaks down the proteid foodstuffs into soluble bodies, such as proteoses and peptones closely related to the original proteids, in pancreatic digestion as it takes place in the intestine there is a profound breaking down, or disruption of the proteid molecule into a row of comparatively simple nitrogenous fragments, many of them crystalline bodies; such as leucin, tyrosin, glutaminic acid, aspartic acid, arginin, lysin, histidin, etc., known chemically as monoamino-acids and diamino-acids. We have no means of knowing to how great an extent these more profound disruptive changes of the proteid molecule take place in the intestine. Whether practically all of the ingested proteid food is broken down into these relatively simple compounds prior to absorption, or whether only a small fraction suffers this change, cannot be definitely stated.

A few years ago, the majority of physiologists held to the view that in the digestion of proteid food all that was essential was its conversion into soluble and diffusible forms which would permit of ready absorption into the blood. The belief was prevalent that, since the proteid of the food was destined to make good the proteid of the blood and through the latter the proteids of the tissues, any change beyond what was really necessary for absorption of the proteid would be uneconomical and indeed wasteful. On the other hand, due weight must be given to the fact that in trypsin digestion, proteid can be quickly broken down into simple nitrogenous compounds, and that in the enzyme erepsin, present in the mucous membrane of the intestine, we have an additional ferment very efficient in bringing about cleavage of proteoses and peptone into amino-acids. From these latter facts it might be argued that, in the digestion of proteid foodstuffs by the combined action of gastric and pancreatic juice in the alimentary tract, a large proportion of the proteid is destined to undergo complete conversion into amino-acids, and that from these fragments the body, by a process of synthesis, can construct its own peculiar type of proteid.'

This latter suggestion is worthy of a moment's further consideration. As is well known, every species of animal has its own particular type of proteid, adapted to its particular needs. The proteids of one species directly injected into the blood of another species are incapable of serving as nutriment to the body, and frequently act as poisons. Man in his wide choice of food consumes a great variety of proteids, all different in some degree from the proteids of his own tissues. Is it not possible, therefore, that it is the true function of pancreatic and intestinal digestion to break down the different proteids of the food completely into simple fragments, so that the body can reconstruct after its own particular pattern the proteids essential for its nourishment? Or, we can follow the suggestion contained in the work of Abderhalden,1 who finds that in the long continued digestion of various proteids by pancreatic juice there results in addition to the amino-acids a very resistant residue, non-proteid in nature, which is termed polypeptid. In other words, Abderhalden believes that pepsin, trypsin, and erepsin are not capable of bringing about a complete breaking down of proteids into amino-acids, but that there always remains a nucleus of the proteid not strictly proteid in nature, though related thereto, - polypeptid, - which may serve as a starting-point for the synthesis or construction of new proteid molecules, the various amino-acids being employed to finish out the structure and give the particular character desired. This view, however, is rendered somewhat untenable by the more recent experiments of Cohnheim,1 who claims that proteids can be completely broken down by pepsin, trypsin, and erepsin, and consequently polypeptids would hardly be available for the synthesis of proteids. Moreover, Bergell and Lewin 2 have ascertained that there is present in the liver an enzyme or ferment which has the power of digesting or breaking down certain dipeptids and polypeptids into amino-acids. Hence, it follows that if any polypeptids are absorbed from the intestine, they would naturally be carried to the liver, where further cleavage into fragments suitable for synthetical processes might occur. In any event, there is good ground for the belief that the more or less complete disruption of the proteid molecule into small fragments renders possible a synthetical construction of new proteid to meet the demands of the organism; a fact of great importance in our conception of the possibilities connected with this phase of proteid nutrition.

1 Broil Abderhalden: Abbau and Aufbau der Eiweisskörper im thieriachen Organisamus. Zeitschr. f. phytiologitche Chemie, Band 44, p, 27.