74 THE FROG chap. 



frog, they must, in some way, be rendered capable of 

 being absorbed. 



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 tem- 

 perature, 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 converted 

 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 absorbed 

 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 a alkaline solution. It also has the 

 property of converting starch into sugar, and of splitting 

 up fats into fatty acids and gl)Cerine, both of which are 

 diffusible. The substances by which these changes are 

 effected are called by the general name of ferments : 

 pe[)sm and trypsin are proteolytic or proteid-convertlng 

 ferments, and the pancreatic juice also contains an mnylolvtic 



