156 THE BIOLOGY OF THE FROG chap. 



capillaries to the hepatic ducts, and thence into the gall 

 bladder, where it is stored until food passes out of the 

 stomach, when it is discharged through the common bile duct 

 into the intestine. Bile is an alkaline fluid of complex com- 

 position. Some of its constituents, such as the fatty sub- 

 stance, cholesterin, and the bile pigments, are simply waste 

 products, but others play a certain part in digestion. In 

 higher vertebrates it has been shown that the bile helps to 

 .emulsify fats and facilitates their absorption from the intes- 

 tine ; it also has a slight power of converting starch into sugar. 



Intestinal Digestion and Absorption. — The food when 

 it is passed from the stomach into the duodenum, possesses 

 an acid reaction due to the acidity of the gastric juice with 

 which it is mixed. In the duodenum it becomes mixed with 

 the pancreatic juice and bile, both of which are alkaline, and 

 its acidity is neutralized. The proteids which may have 

 escaped the fermentative action of the pepsin in the stomach 

 are acted upon by the trypsin of the pancreatic juice and 

 converted into peptones. The /starchy constituents of the 

 food are converted into sugar mainly by the secretion of the 

 pancreas, though perhaps also to a slight extent by the bile, 

 and the fats are partly split into fatty acid and glycerin, and 

 partly emulsified by the action of the pancreatic juice. The 

 role of the intestinal juice in the frog is uncertain ; but in 

 some of the higher vertebrates it has the property of con- 

 verting starch into sugar. 



When the various constituents of the food are digested or 

 rendered soluble by the action of the digestive juices, they 

 are absorbed through the walls of the intestine into the 

 blood and lymph. In the higher vertebrates most of the 

 fat is taken up by the lymph vessels of the intestine, and it 

 is generally held that a large part of the sugar and peptoties 

 is absorbed by the capillaries of the blood vessels. Almost 



