68 



ESSENTIALS OF ZOOLOGY 



to the floor of the mouth just back of the tip of the lower jaw, and 

 its posterior end is free. Taste buds are present on the tongue and 

 palate. 



Esophagus, stomach, and intestine have an outer longitudinal and 

 an inner circular layer of smooth muscle. The peristaltic contrac- 

 tions of these muscles pass the food through the digestive tract and 

 aid in mixing it with the gastric juice in the stomach. They may 

 also be used to regurgitate a disagreeable substance sw^allowed by 

 the frog, in which case the stomach turns inside out and protrudes 

 into the mouth cavity. 



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 



Vomerine teeth 



Floor of orbit — 



Esophagus -||v* 



Vocal sac 



Maxillary t^cth 

 _ Internal na'-es 



_ SjIojs marqinaiis 



_ E.U5tachian tuoe 

 Glottis 



c Toncjue 



Figr. 19. — Mouth, or buccal cavitj^ of the bullfrog. 



is the gall bladder, 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 probably contains no digestive enzymes. Al- 

 though its function in altering fatty substances is important, of 

 prime importance is its ability to store glycogen and the fat upon 

 which a hibernating 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 



