126 DIGESTION 



by temperature; it is more active at ordinary body tem- 

 perature (98-102° F.), while it is completely destroyed at 

 a temperature of 172° to 176° F. Its action upon proteid 

 is very similar to that of pepsin in that it converts pro- 

 teid into soluble peptone. It differs from pepsin in that 

 there is no swelling of the proteid under its action. The 

 process is rather an eating away of the proteid part, and 

 in this action the indigestible matter is left in its original 

 shape to be broken into fragments later by the churning 

 action of the intestine. It also reduces some albuminoids 

 to peptone. Most of the proteid nutrient of our food 

 receives its final preparation for absorption in the small 

 intestine under the action of this enzyme. 



Amylopsin. (See Ex. XXXVIII., b.) Practically all 

 the starch of foods is digested by the action of this enzyme. 

 It is possible that some of the ptyalin of the saliva whose 

 action was stopped by the acidity of the stomach shares 

 in this digestive action upon returning to the alkaline 

 medium of the intestine. It would be impossible to 

 determine this definitely, since amylopsin and ptyalin are 

 apparently identical in structure and action, and in Ger- 

 man texts on physiology the name ptyalin is used for 

 both enzjmies. The use of the two names is, however, 

 convenient in indicating their origin. Both convert starch 

 to sugar under similar conditions and thus transform it 

 into a soluble substance. 



Steapsin. (See Ex. XXXVIII., c.) This enzyme (also 

 called lipase), which is extremely sensitive to temperature, 

 performs two distinct operations upon fats, saponification 

 and emulsification. 



In saponification the fat is first split into glycerine and 

 fatty acids. These latter unite with the alkalies and 



