acts especially on secondary proteoses and peptone, breaking them down quickly into amino-acids. This enzyme is naturally present in the intestinal secretions, becomes mixed with pancreatic juice, and is able to reinforce the latter in the complete destruction of the food proteins in the intestine, the final products being simple amino-acids and their combinations known as polypeptides.
Now, we understand why proteoses and peptone are not normally present in the circulating blood, even of the portal vein. We see that it is no longer necessary to assume a construction of blood proteins from absorbed proteoses and peptone. The old dictum, so often quoted, that the proteins of our muscle tissue, for example, are simple transformation products of the various food proteins no longer satisfies us, since it is so out of harmony with observed facts. We see opening up before us a totally different conception of the process of digestion so far at least as it relates to protein food. It has been a long period of time since the discovery of the proteolytic enzyme of the gastric juice by Schwann, or the early work by Claude Bernard on pancreatic juice. Slowly, but surely, however, our knowledge has progressed, until to-day we are on the threshold of a new vista; new paths spread out before us filled with the light of truth and they bid us hasten to clear away the accumulated misconceptions of the preceding years.
Think for a moment what the new facts lead to in their bearing on nutrition! Recall how we have come to understand that the specific immunities and specific reactions of the blood of different species are properties which reside in the individual blood proteins, and that these peculiarities are associated with the chemical constitution of the proteins. Every physiologist knows how greatly blood from different species varies, and we may safely say that each species of animal probably possesses blood characterized by a personal coefficient in the constitution of its proteins, upon which rests in some measure at least its physiological individuality. In the light of such suggestions is it not difficult to conceive of the varying food proteins of our daily diet being merged into the specific proteins of blood and tissue through simple transformation into closely related proteoses and peptones? Can their individuality be so easily lost by such a superficial alteration? No, the facts at our disposal to-day clearly indicate that the proteins taken as food can not find a place in the economy of the animal body until (as aptly expressed by Leathes) they have been, as it were, melted down and recast. Stated in different language, it is apparently the purpose of digestion, through the enzymes, pepsin, trypsin and erepsin, to thoroughly dismember the protein molecule so that no vestige of its original structure remains; while out of the many fragments or chemical groups so split apart the body can reconstruct proteins adapted to its own particular needs. This means synthesis of a most marked kind, quite
