120 



THE HUMAN MECHANISM 



A 



B 



in the intestine it becomes at once more nearly neutral or 

 even alkaline. Since pepsin acts only in an acid medium, 

 the gastric juice now becomes inactive and is soon de- 

 stroyed by the pancreatic juice, so that it plays no further 



r61e in protein digestion. This 

 is henceforward carried on by 

 an enzyme of the pancreatic 

 juice, trypsin, which acts most 

 vigorously in a neutral or 

 slightly alkaline medium. It 

 forms from the proteins of the 

 food the same general class of 

 peptone-like substances pro- 

 duced by the action of the 

 gastric juice, but carries this 

 cleavage further into smaller 

 peptids and even to some ex- 

 tent to the constituent amino- 

 acids. Trypsin continues the 

 digestion of proteins begun by 

 FIG. 59. Longitudinal section of pepsin. Indeed, in some cases 

 the tip of a villas t j ie preliminary action of pep- 



Showing the columnar lining cells sin necessary, since trypsin 



B through which the products of J ' J ^ 



digestion must pass on their way to does not act SO readily Upon 



the " inal >tein as it does 



columnar cells and the vessels is in- upon the earlier products of 

 dicated diagrammatically and with- t : p r li option nnon thpsp 



out showing its structure. A, cell P 6 P t] 



which manufactures mucus ; C, cap- cleavage products, however, its 



illaries; D. lacteal, or lymphatic ^^ j g mog( . vigorous . 



In addition to trypsin the pancreatic juice contains at least 

 two other important enzymes. One of them, amylopsin, is 

 practically identical with the ptyalin of the saliva and 

 changes starch into sugar much as happens in salivary diges- 

 tion. The other enzyme, lipase, acts upon fats, changing 

 them into fatty acids and glycerin. We cannot go into the 



