748 PHYSIOLOGY 



which was kept alive by passing defibrinated blood through its vessels. 

 At the end of some hours the loop was found to contain a certain amount 

 of coagulable protein, but no trace of peptone, nor was any trace of the 

 latter substance found in the blood which had been passed through the 

 vessels. These observations were interpreted as pointing to a regeneration 

 in the intestinal wall of coagulable protein from the proteose and peptone 

 taken up from the gut, and opinions were divided whether the most important 

 part of this regeneration was to be ascribed to the leucocytes of the villi 

 (Hofmeister), or to the epithelial cells of the mucous membrane itself. 



It is evident that such a conclusion was not justified by the experiments. 

 All that these experiments showed was that the proteoses and peptones 

 disappeared, i.e. were converted into something which did not give the 

 biuret test. The discovery of the ferment erepsin by Cohnheim led this 

 observer to repeat the experiments of Hofmeister and Neumeister with a view 

 to testing the conclusions drawn by these physiologists. Cohnheim found 

 that, although it was perfectly true that proteose and peptone disappeared 

 when intestinal mucous membrane and peptone were placed together in the 

 presence of either blood or of Ringer's fluid, this disappearance was due, not 

 to a regeneration of coagulable protein, but to the fact that the erepsin of 

 the mucous membrane carried the process of hydrolysis a step further, con- 

 verting the proteoses and peptones into the ultimate crystalline products of 

 protein hydrolysis. Similar observations were made by Kutscher and See- 

 mann, who showed that at anytime after a protein meal these end-products, 

 especially leucine, tyrosine, lysine, and arginine, were to be found in the 

 contents of the small intestine. A repetition of Salvioli's experiment by 

 Cathcart and Leathes deprived this also of much of its significance. It was 

 found that the artificial circulation, although sufficient to maintain the 

 activity of the muscular wall of the intestine, as evidenced by the peristaltic 

 movements, was insufficient to keep the mucous membrane alive. After one 

 hour's experiment the loop contained a mass of epithelial cells mixed with the 

 products of the action of erepsin on the introduced peptone solution. In 

 no case was there any diminution in the amount of uncoagulable nitrogen, 

 i.e. there was no formation of coagulable protein, while the processes of 

 absorption had been brought by the desquamation entirely to a standstill. 



These additional experiments caused a complete revolution in the attitude 

 of physiologists towards the problem of protein absorption. All this evidence 

 went to show that protein, however introduced, whether as coagulated protein 

 or as albumose and peptone, underwent complete hydrolysis either in the 

 gut or in the wall of the gut before entering the blood stream. If this were 

 the case it should be possible to feed an animal on a diet in which the neces- 

 sary protein had been replaced by the corresponding amount of ultimate 

 products of protein hydrolysis, i.e. by a mixture which would give no biuret 

 reaction. Such a possibility had previously been negatived on theoretical 

 grounds by Kuhne and by Bunge. It was thought by these observers 

 either that the animal body lacked the power of synthesis of proteins from 

 these crystalline products (hydration products), or that any complete 



