THE BODY CAVITY. 



631 



cardial cavity. This process is illustrated by fig. 355, taken 

 from an embryo Rabbit. The two dorsal sections of the body 

 cavity (pl.p] finally extend so as completely to envelope the 

 pericardial cavity (pc), remaining however separated from each 

 other below by a lamina extending from the ventral wall of the 

 pericardial cavity to the body wall, which forms the anterior 

 mediastinum of human anatomy. 



By these changes the pericardial cavity is converted into a 

 closed bag, completely surrounded at its sides by the two lateral 

 halves of the body cavity, which were primitively placed 



. C. 



FIG. 355. SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO SHEW 

 HOW THE PERICARDIAL CAVITY BECOMES SURROUNDED BY THE PLEURAL 

 CAVITIES. 



ht. heart; pc. pericardial cavity; pl.p pleural cavity; Is;, lung; a/, alimentary 

 tract; ao. dorsal aorta; ch, notochord ; ;/. rib; st. sternum; sp.c, spinal cord. 



dorsally to it. These two sections of the body cavity, which in 

 Amphibia and Sauropsida remain in free communication with 

 the undivided peritoneal cavity behind, may, from the fact of 

 their containing the lungs, be called the pleural cavities. 



In Mammalia a further change takes place, in that, by the 

 formation of a vertical partition across the body cavity, known 

 as the diaphragm, the pleural cavities, containing the lungs, 



