THE SEROUS CAVITIES. 99 
mammal four large serous spaces, namely, the pericardial peri- 
toneal, and paired pleural cavities. 
The pericardial cavity, the smallest of these spaces, is situated 
between the paired pleural cavities. Its enclosing membrane, the 
pericardium, forms a capacious sac for the heart, and is reflected 
directly over the surface of the latter as a thin membrane, the 
epicardium. 
The pleural cavities are those lodging the lungs, the latter 
being projected into them from a medial position. The lining 
membrane or pleura is divided into three chief portions—the 
pulmonary pleura, investing the greater part of each organ, the 
costal pleura, lining the internal surface of the thorax, and the 
diaphragmatic pleura, covering the anterior surface of the dia- 
phragm. The latter is broadly connected with the pulmonary 
pleura through the pulmonary ligament. 
The peritoneal cavity, the largest of the serous spaces, com- 
prises in a mammal a general portion, the abdominal cavity, and 
its posterior extension into the pelvis—in the male also into the 
sac of the testis. The general relation of the cavity to the 
abdominal organs is indicated diagrammatically in Fig. 21. Its 
lining membrane, the peritoneum, is divisible into two portions, 
the parietal peritoneum, lining the abdominal wall, and the 
visceral peritoneum, investing the visceral organs. Of the latter 
the kidneys encroach only to a minor extent on the serous lining, 
so that they are covered by peritoneum only on their ventral sur- 
faces. The digestive tube, on the other hand, is removed to such 
an extent from the abdominal wall that the peritoneum forms a 
complete serous coat, and is connected with the parietal peritoneum 
of the wall through a thin transparent membrane, the mesentery. 
The latter consists of two plates of peritoneum, enclosing between 
them a thin layer of connective tissue, the lamina mesenterii 
propria, for the transmission of nerves, bloodvessels and lymph 
canals. 
As indicated above, the relations of the abdominal portion of the 
digestive tube are greatly modified by its elongation and displace- 
ment from a median position. Thus, while in the embryo the 
common mesentery (Fig. 40) is recognizable as a continuous 
median vertical fold, in the adult it follows the convolutions of the 
