i 9 2 TEXT-BOOK OF PHYSIOLOGY 



When the proteins which have escaped digestion in the stomach pass 

 into the small intestine and mingle with the pancreatic juice, they are 

 doubtless digested in the course of the intestinal canal, passing through the 

 stages just described. When proteins are artificially digested for a long 

 time there appears a number of compounds such as leucin, tyrosin, aspartic 

 acid, arginin, etc., which are representatives of a group of simple chemical 

 substances known as amino-acids. It would appear that the pancreatic 

 juice had the power under such circumstances of reducing the proteoses or 

 the peptones to their ultimate constituents. Whether this is due to the action 

 of trypsin or to the action of another enzyme erepsin is not very clear. 



The action of trypsin on proteins in an alkaline medium may be illustrated 

 by the following scheme: 



Protein 



Alkali-protein 



Proteoses secondary 



Peptone 



Leucin Tyrosin Aspartic acid Arginin Ammonia 



The view that the final stage in the digestion of proteins is the formation 

 of peptones, which in due time are absorbed and synthesized into blood 

 albumin, has been generally abandoned for there is an ever increasing 

 evidence that the final stage is the formation of the nitrogen-holding com- 

 pounds above mentioned; in other words, that the cleavage of the proteins 

 is far more complete than has heretofore been assumed. Indeed it is now 

 believed that they are reduced, if not to their ultimate constituents, the 

 amino- and diamino-acids, at least to one or more of the different polypeptid 

 stages. Ever since the discovery by Cohnheim of the existence in the 

 intestinal juice of a substance termed by him erepsin, which is capable of 

 splitting proteoses and peptones into simple nitrogen-holding compounds, 

 there has been slowly developing the idea that normally during intestinal 

 digestion the proteoses and peptones are reduced by this agent to leucin, 

 tyrosin, histidin, arginin, aspartic acid, ammonia, etc., which in turn are 

 absorbed and transported by the blood direct to the tissues. The discovery 

 by Vernon of erepsin in pancreatic juice lends further support to this view. 



3. On fat. If pancreatic juice be added to a perfectly neutral fat 

 olein, palmitin, or stearin and kept at a temperature of about iooF. 

 (38C.), it will at the end of an hour or two be partially decomposed into 

 glycerin and the particular fat acid indicated by the name of the fat used 

 e.g., oleic, palmitic, stearic. The oil will then exhibit an acid reaction. 

 The reaction is represented in the following formula: 



C,H ft (C t ,H M O a ), + 3H a O - 3 C 18 H 84 O a + C,H fl (OH), 

 Triolem. Water. Oleic Acid. Glycerin. 



If to this acidified oil there be added an alkali, e.g., potassium or sodium 

 carbonate, the latter will at once combine with the fat acid to form a 

 salt known as a soap. The reaction is expressed in the following equation : 



Sodium Carbonate. Oleic Acid. Sodium Oleate. Carbonic Acid. 



Na,CO, + C 18 H 84 O, - 2NaC 18 H 33 O a + H a CO, 



Coincident with the formation of the soap, the remaining portion of the 



