BULLFROG AS TYPICAL VERTEBKATE ANIMAL 69 



digestive enzymes. This secretion is taken from the pancreas by 

 pancreatic ducts which empty into the bile duct that runs 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 stom.ach. Here 

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

 which convert the proteins to peptones. The muscles of the stomach 

 cause a thorough mixing of the gastric juice with the food and then 

 pass the partly digested food (chyme) posteriorly into the small in- 

 testine. 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 duodenum its 

 highly alkaline secretion. In addition, this pancreatic juice contains 

 three digestive enzymes : a protease, trypsin, which continues the di- 

 gestion begun by pepsin in the stomach, converting proteins to amino 

 acids ; an amylase, amylopsin, which changes starches into sugars ; and 

 a lipase, steapsin, which, aided by the bile, causes a splitting of the 

 fats into glycerol and fatty acids. 



The process of digestion is completed in the intestine and the 

 food products are taken up by absorption in its mucosa layer. These 

 foods in solution are taken by the blood stream and lymph vessels 

 to various parts of the body where they are utilized for building 

 tissue or for supplying energy, leaving as by-products urea and 

 carbon dioxide. Sugars that are not used are stored as glycogen in 

 the liver and in voluntary muscles. The liver also serves to store 

 fats and to secrete urea and sugai* directly into the blood stream. 



Food that is not digested passes to the large intestine where it is 

 retained for a time and then passed to the outside through the anus 

 as feces. 



Other Glands. — Attached by a mesentery to the wall of the intes- 

 tine near the anterior end of the rectum is the spleen. It is a small, 

 reddish, spherical, lymphoid organ, the functions of which are but 

 incompletely known. The destroying of red blood corpuscles is an 



