630 



THE PERICARDIAL CAVITY. 



pericardial cavity become precisely those found in the embryos 

 of Elasmobranchii. The later changes are however very differ- 

 ent. Whereas in Fishes the right and left sections of the body 

 cavity dorsal to the pericardial cavity soon atrophy, in the 

 higher types, in correlation with the relatively backward situa- 

 tion of the heart, they rapidly become larger, and receive the 

 lungs which soon sprout out from the throat. 



The diverticula which form the lungs grow out into the 

 splanchnic mesoblast, in front of 

 the body cavity ; but as they 

 grow, they extend into the two 

 anterior compartments of the body 

 cavity, each attached by its me- 

 sentery to the mesentery of the 

 gut (fig. 354, Ig). They soon more- 

 over 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 pro- 

 longations of the body cavity con- 

 taining the lungs (fig. 354). Its 

 dorsal wall is attached to the gut, 

 and is continuous 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 mediastinum, but still more by the 

 two divisions of the body cavity which contain the lungs 

 extending themselves ventrally round the outside of the peri- 



--2)0 



FIG. 354. SECTION THROUGH 

 THE CARDIAC REGION OF AN EMBRYO 

 OF LACERTA MURALIS OF 9 MM. TO 

 SHEW THE MODE OF FORMATION OF 

 THE PERICARDIAL CAVITY. 



ht. heart ; pc. pericardial cavity ; 

 al. alimentary tract; Ig. lung; /. 

 liver; pp. body cavity; md. open 

 end of Mullerian duct ; wd. Wolrnan 

 duct ; vc. vena cava inferior ; ao. 

 aorta; ch. notochord; me. medullary 

 cord. 



