THE DIGESTIVE SYSTEM 15 



part lining the inner surface of the body wall ; the mesen- 

 teries or ligaments, the double-walled continuations of the 

 peritoneum from the mid-dorsal or mid-ventral line run- 

 ning to the organs of the body cavity; the visceral peri- 

 toneum or serosa of the organs, formed from the two layers 

 of the mesenteries which separate and surround the organs. 



2. The Pleuroperitoneal Viscera. 



The viscera or organs are exposed by the opening of the 

 pleuroperitoneal cavity. The organs cannot be said to be 

 in the cavity since they are completely separated from it 

 by their peritoneum and its extensions the mesenteries. 

 They appear as if they had been let down into the cavity 

 from the dorsal wall, by the peritoneum, while still sur- 

 rounded by it. They are thus suspended in the cavity. 



Drawing 9. Make a line diagram of an imaginary section through 

 the anterior end of the pleuroperitoneal cavity showing body wall, 

 parietal peritoneum, visceral peritoneum of the intestine, and the 

 mesentery suspending it. 



3. The Digestive Tract. 

 A. External Aspects. 



The digestive tract consists of a thick-walled tube ex- 

 tending from the anterior end to the posterior end of the 

 body. It is differentiated into various regions having dis- 

 similar functions. Part of the digestive tract has already 

 been encountered, the mouth, the pharynx and the 

 esophagus. 



In the following study do not disturb the position of the 

 internal organs more than is absolutely necessary and do 

 not break them apart or separate them from the body wall. 

 Occasionally the stomach is accidentally everted into the 

 mouth cavity. In such cases it should be pulled back into 

 position. 



