194 STRUCTURE OF THE VERTEBRATES 



ment of the digestive tube. In most vertebrates a constrictor 

 muscle surrounds the posterior end of the esophagus; and, ex- 

 cept in the cyclostomes, the outlet of the stomach is guarded by 

 a strong pyloric sphincter. The intestine is the longest portion of 

 the tube. In this region the digestion of foods is continued and 

 the absorption of nutrient materials takes place. The intestine is 

 itself divided into regions. The stomach contents empty into the 

 small intestine. Its anterior portion is the short duodenum into 

 which the liver and pancreatic ducts empty, and is often classi- 

 fied as a separate part of the intestinal tract. In the higher ver- 

 tebrates the small intestine joins a heavy-walled large intestine 

 or colon which enters the rectum, an enlarged portion of the in- 

 testinal tract. In the fish, amphibia, reptiles, birds and mono- 

 tremes the anus empties into the cloaca, an enlarged sac-like 

 structure receiving the products of the digestive and urinogenital 

 tracts. In the higher mammals the cloaca is divided so that the 

 anus and urinogenital ducts empty separately. 



The digestive glands develop as outpocketings from the gut. 

 As in other glands, these vary from single secretory cells to large 

 compound glands supported by masses of connective tissue. Of 

 the latter, the pancreas and liver are the most important. 



1. The pancreas develops as one or several dorsal outpocket- 

 ings from the duodenal region of the intestine. In more primitive 

 animals it is fairly compact, but in the higher vertebrates the 

 pancreas is a large, diffuse glandular structure. The secretory 

 acini drain into numerous ducts which fuse to form one, two, or 

 three large ducts which carry the pancreatic juices to the in- 



Stomachy 



pUlslits^ ^Stomach 



intestine 



Phat-ynx 



Tancreas 



Fig. A Amphioxus Fig:. B Human Embryo 



Fig. 110. Development of the Digestive Glands. (A) The stomach-intes- 

 tine of Amphioxus with the hepatic caecum. (B) The human tract, show- 

 ing the endodermal outpocketings which give rise to the liver and pancreas. 



