420 A MANUAL OF PHYSIOLOGY 



has actually been shown that during the digestion of a protein 

 meal the mucosa of the stomach and intestine contains proteose 

 and peptone, while none is present in the muscular coat or in 

 any other organ. They rapidly disappear from a portion of 

 the mucous membrane kept at a temperature of about 40 C. 

 outside of the body, and their disappearance is due, not to their 

 regeneration into serum proteins, as was once supposed, but to 

 their decomposition by the erepsin. We must suppose that the 

 serum and organ proteins are built up from the products of this 

 decomposition. But whether the mucosa of the alimentary tract 

 is especially a seat of the synthesis is unknown (p. 497). It is 

 at least equally probable that it occurs in all the cells of the 

 body, each one building up for itself the particular kind of protein 

 which it needs. The direct way of testing the question would 

 be to examine the blood coming from the intestine during the ab- 

 sorption of proteins, and to determine quantitatively any changes 

 which might have occurred in the nitrogenous constituents. 

 But the flow of blood through the intestine is so great, the absorp- 

 tion of the digestive products so gradual, and their removal from 

 the blood by the tissues, in all probability, so rapid, that there 

 is no reason for surprise that hitherto the results of such de- 

 terminations have been ambiguous. It has, however, been 

 shown that when peptone, albumose, or the final products of 

 tryptic digestion are introduced into a ligated segment of a dog's 

 small intestine, there is always, when absorption occurs, an 

 increase in the nitrogenous substances in the blood, in the form 

 of compounds which are not precipitated by tannic acid, and 

 therefore are neither native proteins nor proteose. Urea 

 accounts for about one-half of the increase ; the rest probably 

 represents amino-acids and similar substances (Leathes). A 

 much more conclusive experiment has been made on the in- 

 testines of certain octopods, which, when excised and suspended 

 in the oxygenated blood, will live for many hours. A solution 

 .of peptone was introduced into the isolated intestine, and after 

 twenty hours the crystalline products, leucin, tyrosin, lysin, and 

 arginin, were found in the blood. In the intact animal none 

 of these bodies could be detected in the blood (Cohnheim) . The 

 inference is that protein in these animals is absorbed in the 

 form of amino-acids, etc., which are then carried to the tissues 

 and utilized there. It may be that some of the proteose and 

 peptone are regenerated by a shorter process, and without having 

 been further split up, but of this, too, there is no definite proof. 

 The regeneration, wherever it occurs, must presumably take 

 place in cells, and the only available cells in the digestive mucous 

 membrane are those which line the tube, or the leucocytes which 

 wander between them. Accordingly, both have been credited 



