DIGESTION. 189 



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

 stages just described. As leucin and tyrosin are found in the intestine during 

 digestion, it is probable that a portion of the peptone undergoes decompo- 

 sition into these bodies; but as to the extent to which this takes place or in 

 how far it is a necessary process under normal conditions is yet a subject of 

 investigation. It is certain that it takes place when there is an excess of 

 protein food or when for any reason digestion is prolonged or absorption is 

 delayed. 



Though 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 accepted, there is an ever increasing 

 evidence that it is not wholly true, and that the final stage may be the forma- 

 tion of the nitrogen-holding compounds above mentioned; in other words, 

 that the cleavage of the proteins is far more complete than has heretofore 

 been assumed. Indeed it has been asserted 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 Cohn- 

 heim 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, etc., which 

 in turn are absorbed and synthesized to blood or tissue albumin. 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 100 F. 

 (38 C.), 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 3 H 5 (C 18 H 33 2 ) 3 + 3 H 2 = 3 C 18 H 34 2 + C 3 H 6 (HO) 3 

 Triolein. 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 2 C0 3 + C 18 H 34 2 = 2 NaC 18 H, 3 2 + H 2 CO 8 



Coincident with the formation of the soap the remaining neutral oil under- 

 goes division into drops of microscopic size, which float in the soap solution, 

 forming what has been termed an emulsion, which is white and creamy 

 in appearance. The action of the pancreatic juice may then be said to 

 consist in the cleavage of the neutral fats into fatty acids and glycerin, after 

 which the formation of the soap and the division of the fat takes place spon- 

 taneously. The enzyme which produces the cleavage of the neutral fats 

 has been termed steapsin or lipase. The extent to which the cleavage of the 

 fat takes place in the intestine has not been definitely determined. There are 

 some who think the amount is relatively small, while others consider that 



