CHAPTER III 

 THE FROG RAN A TEMPO R ARIA (continued) 



Alimentary System Respiratory System Circulatory System Urogenital 



System. 



Alimentary or Digestive System. 



The alimentary canal commences at the mouth, which leads 

 directly into the buccal cavity. The structures in this, and the way 

 in which it narrows down at the back to form an indistinct pharynx 

 leading into the oesophagus, have already been described. The 

 oesophagus or gullet is a wide short tube situated in the pleuro- 

 peritoneal cavity, and attached to the dorsal wall by a mesentery. 

 It passes on without a sharp line of demarcation into the stomach, a 

 wide slightly curved sac lying to the left of the middle line. The 

 walls of the stomach are very well supplied with muscles, by means of 

 which a churning movement is maintained as long as it contains 

 food. At the lower end of the stomach a band of circular muscles, a 

 sphincter, is developed, by means of which the food can be retained. 

 The position of this muscle is marked externally by a slight con- 

 striction, and the slightly swollen part of the stomach in its imme- 

 diate neighbourhood is known as the pylorus. From this point the 

 first part of the intestine, the duodenum, runs forward nearly parallel 

 with the stomach, and united to it by a fold of peritoneum, the 

 gastro-duodenal omentum. At the posterior end of the liver, to 

 which it is bound by the duodeno-hepatic omentum, it turns sharply 

 backwards, and is known as the small intestine or ileum. This 

 pursues a complicated course, and is thrown into a number of con- 

 volutions all held together by mesentery, and finally it returns to 

 the level of the hinder end of the stomach, where it swells out to form 

 the large intestine or rectum. This runs straight backwards for 

 just over an inch, and opens by the cloaca to the exterior. At the 

 point where the large intestine joins the small it gives off a small 

 dorsal projection, which may perhaps be homologous with a large 

 sac-like structure, called the ccecum in the higher animals. The 

 cloaca is practically the end of the intestine, but as it has connected 

 with it the excretory and reproductive organs it is more conveniently 



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