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 relations 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 



