THE FROG — A REPRESENTATIVE VERTEBRATE 



405 



epithelium supported by fibrous connective tissue; (2) two layers of 

 smooth muscle— a much reduced (in the stomach) outer, longitudinal 

 layer in which the fibers more or less parallel the long axis of the gut, 

 and a thick, inner circular layer with fibers nearly at right angles to the 

 preceding; (3) a layer of highly vascular, fibrous connective tissue known 

 as the submucosa; and finally (4) the mucosa, or mucous membrane. 

 Movement of the food within the stomach and along the intestine is 

 accomplished by rhythmic waves of contraction of the muscle layers, 

 which are known as peristalsis. 



The mucosa consists of thin layers of longitudinal and circular 

 muscles (muscularis mucosae) next to the submucosa, plus connective 

 tissue and the simple columnar epithelium lining the lumen. The epi- 

 thelium contains numerous mucus-secreting goblet cells, and is invagi- 

 nated to form many gastric pits. From the base of each pit one or two 



Vomerine teeth 



Small intestine 

 Mesentery 



Al/ 



Laryngotrdch^dl chaabtT 

 Eustachidn , 



Esophagus 



Hepatic duct 

 Pancreatic duct 



Stomach 

 Common bile duct 



Duodenum 

 Pyloric sphincter 



Urinary bladder 



aperture (dorsal) 



Figure 21.9. A \ential view of the frog's digestive system. Ihe liver lobes have 

 been turned forward to show the gallbladder. Tongue action is shown in the insert. 

 (Insert after Gadow.) 



