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; lg. lung; l. liver; pp. body cavity; md. open end of Müllerian duct; wd. Wolffian duct; vc. vena cava inferior; ao. aorta; ch. notochord; mc. medullary cord.
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 containing 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 pericardial 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.
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; lg. lung; al. alimentary tract; ao. dorsal aorta; ch. notochord; rp. rib; st. sternum; sp.c. spinal cord.
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 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, become isolated from the remainder of the body or peritoneal cavity. As shewn by their development the so-called pleuræ or pleural sacks are simply the peritoneal linings of the anterior divisions of the body cavity, shut off from the remainder of the body cavity by the diaphragm.