iv] MODES OF NUTRITION 95 



which act on proteids ; they are the pepsin of the 

 gastric juice, the trypsin from the pancreas, and the 

 erepsin and enterokinase secreted by the wall of the 

 intestine. They decompose the proteids first of all 

 to peptone and then to amino-acids, but what 

 proportion of the proteid taken into the intestine 

 is broken down so profoundly as the stage of amino- 

 acids is still uncertain. The lipolytic enzymes are 

 those which split up the fats into fatty acids and 

 glycerine : the principal one is the lipase from the 

 secretion of the pancreas. The amylolytic enzymes 

 are those which resolve the starches into sugar or 

 rearrange the sugars into forms which are capable 

 of assimilation. They are the ptyalin of the saliva, 

 and the amylase from the pancreatic juice. 



Thus the proteids are converted into amino-acids, 

 the fats to fatty acids and glycerine, and the starches 

 to sugars all this by the activity of the enzymes 

 of the alimentary canal. These products of digestion 

 the food-stuffs we may call them are soluble in 

 water and they are absorbed by the cells lining the 

 wall of the intestine. But these cells also contain 

 the enzymes and the action of the latter is reversible, 

 that is they can not only split up the proteids to 

 amino-acids but they can recombine these substances 

 back again into proteid. So in the process of 

 digestion the amino-acids are reconverted into proteid, 

 the fatty acids and glycerine are reconverted to fats, 



