RESOEPTTON OF PRODUCTS OF DIGESTION. 189 



the fact that both substances cannot be regarded as true food-stuffs, 

 viz., they are unable to maintain the nitrogenous equilibrium of the 

 higher animals when exclusively used. The possibility that this may 

 be due to the absence of aromatic groups in the gelatin has been ren- 

 dered highly probable through the studies of Kauffmann, who found 

 that its nutritive value could be materially increased if tyrosin and 

 tryptophan were at the same time administered. The diamino-acids 

 are manifestly of no moment in this connection, as gelatin, at least, 

 yields rather more arginin than any of the common albuminous food- 

 stuffs, and, as I have pointed out, Ellinger has shown that the 

 diamino-acids alone are likewise not capable of maintaining nitrog- 

 enous equilibrium. 



A digestion of other albuminoids, notably of the keratins, does 

 not take place in the small intestine of the higher animals, while 

 some of the invertebrates, such as the common house moth, are 

 manifestly capable of utilizing these also for purposes of nutrition. 



RESORPTION OF THE PRODUCTS OF PROTEOLYTIC 



DIGESTION. 



The resorption of the products of proteolytic digestion is essentially 

 the function of the intestinal mucosa. The gastric mucous mem- 

 brane probably plays a secondary role only, and, as I have already 

 pointed out, normal nutrition can be maintained in the absence of 

 the stomach. Of the manner in which resorption takes place we 

 know very little. That the process is not one of simple diffusion 

 seems to be definitely established, and there is good evidence to 

 show that the epithelial cells lining the mucous membrane are 

 actively concerned in the event. 



In the past much uncertainty existed in reference to the form 

 in which the albumins were absorbed, and it was generally taught 

 that digestion proceeded as far as the formation of albumoses and 

 " peptones," in the older sense of the term, and that the reconstruc- 

 tion of the albuminous molecule then occurred in the gastro-in- 

 testinal mucous membrane. This conclusion appeared to be well 

 supported by the observation of Hofmeister, Neumeister, and 

 Salvioli, that " peptones " will disappear from a solution in the 

 presence of particles of intestinal mucous membrane. More recent 

 experiments by Cohnheim have thrown a new lighten this observa- 

 tion and have materially contributed to our understanding of resorp- 

 tive processes. His researches centre in the discovery of a new 

 ferment, erepsin, in the intestinal mucosa, which is capable of 

 hydrolyzing acid albumin, albumoses, and peptones, but which is 

 without affect upon the native albumins. 



Erepsin. To isolate the ferment, the intestinal mucosa of 

 recently killed animals is scraped off with a piece of glass, triturated 

 with sand, extracted repeatedly from one-half to twelve hours with 

 alkaline normal salt solution, and finally pressed out in a tincture 



