CLASS AMPHIBIA 501 



it is present. The mucosa of the intestines has a number of longi- 

 tudinal and transverse folds which produce a great absorptive sur- 

 face through which the digested food can be taken up by the blood 

 stream and transported to different parts of the body. 



The liver lies on each side of and behind the heart. It is three- 

 lobed, two lobes being on the left and one on the right, connected 

 by narrow bridges of liver tissue. Between the right and left lobes 

 is the gall Madder, which receives an alkaline secretion known as bile 

 from the liver and stores it until needed in the process of digestion. 

 Bile is carried from the gall bladder to the duodenum by the hile 

 duct, which passes through the pancreas on its way. The liver is 

 not primarily a digestive gland, for, while the bile it secretes per- 

 mits the fats to be more easily digested by a lipase from the pan- 

 creas, the bile itself contains no digestive enzymes. Although its 

 function in altering fatty substances is important, of prime impor- 

 tance is its ability to store glycogen and the fat upon which a hiber- 

 nating frog lives. It is also concerned in the formation of urea 

 and in the destruction of red blood corpuscles. 



The pancreas lies in the loop between the stomach and duodenum. 

 It is a long, whitish, irregularly-lobed gland whose alkaline secre- 

 tion is of considerable importance in digestion, for it contains three 

 digestive enzymes. This secretion is taken from the pancreas by 

 pancreatic ducts which empty into the bile duct that passes through 

 the pancreas before entering the duodenum near its beginning. 



Intestines, liver, and pancreas are covered with peritoneum. The 

 mesenteries which hold the body organs in position and the internal 

 surface of the body wall likewise are made up of this peritoneal 

 membrane. 



Digestion. — Since frogs live primarily on insects, crayfish, and 

 other small invertebrate animals, their food is very rich in proteins. 

 Their vomerine and maxillary teeth are too feeble to do more than 

 slightly crush their prey, so digestion begins in the stomach. Here 

 the gastric glands secrete hydrochloric acid and an enzyme, pepsin, 

 which converts the proteins to peptones. Peristaltic contractions of 

 the stomach cause a thorough mixing of the gastric juice with the food 

 and then this partly digested food (chyme) is passed posteriorly into 

 the small intestine. Here, activated by the acid nature of the food, 

 the intestinal glands release into the blood stream a substance, secretin, 

 which on reaching the pancreas causes it to pour forth into the duo- 



