THE SEROUS CAVITIES 135 



The Serous Cavities 



The organs collectively described as visceral are those associated 

 with the serous cavities. They belong to several systems, but 

 present the common feature of being projected into the membra- 

 nous linings of these cavities so that they are more or less completely 

 invested by them without interrupting them at any point. 



The serous sacs are extensive body-spaces, derivatives of a 

 primary body cavity or coelom. They are usually considered 

 loosely as containing the visceral organs, but the condition is more 

 accurately described as one in which the visceral organs encroach, 

 chiefly from a dorsal position, on the enclosing membranes. The 

 latter are thus divided into two portions, one of which is distributed 

 as a parietal or peripheral layer, forming the enclosure of the sac, 

 while the other is disposed as a visceral layer on the surface of the 

 visceral organs (Fig. 24). The serous sacs are enclosed by thin, 

 moist, serous membranes, consisting chiefly of mesothelium, which 

 give to the visceral organs their characteristic appearance. 



In lower vertebrates, where the diaphragm is absent or im- 

 perfectly developed, the coelom is divided into two chief portions — 

 the pericardial cavity, enclosing the heart, and the pleuroperitoneal 

 cavity, lodging the remaining visceral organs, including the lungs 

 in terrestrial vertebrates. In the mammalia, the pleuroperitoneal 

 cavity is completely divided into two portions by the diaphragm, 

 the smaller pleural portion being again divided into right and left 

 pleural cavities through the presence of certain structures filling 

 the median portion of the thorax. There are thus recognizable in 

 a mammal four large serous spaces, namely, the pericardial, the 

 peritoneal, 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 pul- 

 monary pleura, investing the greater part of each organ, the costal 

 pleura, lining the internal surface of the thorax, and the dia- 

 phragmatic pleura, covering the anterior surface of the diaphragm, 



