518 



THE PERICARDIAL CAVITY. 



m^ 



by its mesentery to the mesentery of the gut (fig. 354 Ig). They soon 



moreover extend beyond the region 

 of the pericardium into the undivided 

 body cavity behind. This holds not 

 only for the embryos of the Amphibia 

 and Sauropsida, but also for those of 

 Mammalia. 



To understand the further changes 

 in the pericardial cavity it is necessary 

 to bear in mind its relations to the 

 adjoining parts. It lies at this period 

 completely ventral to the two anterior 

 prolongations of the body cavity con- 

 taining the lungs (fig. 354). Its dorsal 

 wall is attached to the gut, and is con- 

 tinuous with the mesentery of the gut 

 passing to the dorsal abdominal wall, 

 forming the posterior mediastinum of 

 human anatomy. 



The changes which next ensue 

 consist essentially in the enlargement 

 of the sections of the body cavity 

 dorsal to the pericardial cavity. This 

 enlargement takes place partly by the 

 elongation of the posterior medias- 

 tinum, but still more by the two di- 

 visions of the body cavity which con- 

 tain the lungs extending themselves 

 ventially round the outside of the 

 pericardial cavity. This process is illustrated by fig. 355, taken from 

 an embryo Rabbit. The tw(j dorsal sections of the body cavity i^'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 ot 

 the body cavity, which were primitively placed 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 forma- 

 tion of a vertical partition across the body cavity, known as the 

 diaphragm, the pleural cavities, containing the lungs, become iso- 

 lated from the remainder of the body or peritoneal cavity. As shewn 

 by their development the so-called pleura or pleural sacks are simply 



Fig. 354. Section through the 

 <;ardiac region of an embryo op 

 Lacerta Muralis of 9 mm. to shew 

 the mode of formation of the 

 pericardial cavity. 



ht. heart ; jjc. pericardial cavity ; 

 al. alimentary tract ; Ig. lung ; I. 

 liver ; pp. body cavity ; md. open 

 end of MuUerian duct ; wd. Wolffian 

 duct ; vc. vena cava inferior ; ao. 

 aorta; eft. notochord; mc. medullary 

 cord. 



