74 THE FROG CHAP. 



of being absorbed, in which process the living tissues 

 appear to exercise some degree of selection. 



Action of Digestive Juices. This is exactly what is 

 done by the digestive juices. If white of egg or any 

 other proteid is mixed with gastric juice and kept at a 

 suitable temperature, it is converted into a form of 

 proteid called peptone, which is capable of diffusing 

 through an animal membrane. The change is effected 

 by means of a substance called pepsin, contained in the 

 gastric juice in which there is also a certain proportion 

 of hydrochloric acid. To this the acid reaction of the 

 gastric juice already alluded to (p. 68) is due. 



By means of the gastric juice the bodies of the animals 

 swallowed by the frog have their proteids largely con- 

 verted into peptones, which, being diffusible, pass through 

 the mucous membrane as readily as sugar or salt. Hence 

 the great diminution in the bulk of the food during its 

 sojourn in the stomach : a large proportion of it is ab- 

 sorbed there and then, and only a comparatively small 

 quantity is passed through the pyloric valve into the 

 intestine, where it becomes alkaline, owing to the 

 action of the fluid which enters the duodenum through 

 the bile-duct, and which, as we have seen (p. 70), consists 

 of bile and pancreatic juice. 



Pancreatic juice has a similar effect on proteids, the 

 change being effected by a substance called trypsin, 

 which, however, acts in an alkaline solution. It also 

 has the property of converting starch into sugar, and of 

 splitting up fats into fatty acids and glycerine, both of 

 which are diffusible. The substances by which these 

 changes are effected are called by the general name of 

 ferments : pepsin and trypsin are proteolytic or protein- 

 converting ferments, and the pancreatic juice also 

 contains an amylolytic ferment wliich converts starch 

 into sugar, and a fat-decomposing ferment. 



