VIII.] THE PERICARDIAL CAVITY. 267 



mesentery (Fig. 86, B). Posteriorly the horizontal sep- 

 tum grows in a slightly ventral direction along the 

 under surface of the liver (Fig. 87), till it meets the 

 abdominal wall of the body at the insertion of the 

 falciform ligament, and thus completely shuts off the 

 pericardial cavity from the body cavity. The horizontal 

 septum forms, as is obvious from the above description 

 the dorsal wall of the pericardial cavity. 



After the completion of this separation the right 

 and left sections of the body cavity, dorsal to the peri- 

 cardial cavity, rapidly become larger and receive the 

 lungs which soon sprout out from the throat. 



The diverticula which form the lungs grow out into 

 splanchnic mesoblast, in front of the body cavity, but 

 as they grow they extend into the two anterior com- 

 partments of the body cavity, each attached by its 

 mesentery to the mesentery of the gut (Fig. 87, lg.). 

 They soon moreover extend beyond the posterior limit of 

 the pericardium into the undivided body cavity behind. 



To understand the further changes in the peri- 

 cardial cavity it is necessary to bear in mind its rela- 

 tions to the adjoining parts. It lies at this period 

 completely ventral to the two anterior prolongations of 

 the body cavity containing the lungs. Its dorsal wall is 

 attached to the gut, and is continuous with the me- 

 sentery 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- 



