CHAP. VI1L] THE VASCULAR SYSTEM. 



91 



Serous membranes. Serous membranes are thin and transparent, tol- 

 erably strong, extensile, and elastic. They are lined on the inner surface by 

 a simple epithelial layer of flattened cells (endothelium). The surfaces are 

 moistened by a fluid resembling serum, and from which the membranes 

 obtain their name of serous membranes. Here and there between the cells 



FIG. 70. PORTION OF ENDOTHELIUM OF PERITONEUM. (Klein.) a, larger cells; 

 6, smaller ones, with here and there a pseudo-stoma between. 



openings are seen, which are of two kinds. The smaller and more numerous 

 are false openings, and are termed pseudo-stomata ; the larger or true aper- 

 tures are termed stornata, and open into subjacent lymphatics. The sub- 

 stance of serous membranes underneath the endothelium is composed of 

 a network of connective tissue containing a variable amount of white 

 and elastic fibres. Where the membrane is thick, this ground substance 

 contains blood-vessels and lymphatics, the lymphatics being exceedingly 

 abundant. 



Serous membranes form closed sacs, one part of which is attached to the 

 walls of the cavity which it lines, the parietal portion, whilst the other 

 is reflected over the surface of the organ or organs contained in the cavity, 

 and is named the visceral portion of the membrane. In this way the viscera 

 are not contained within the sac, but are really placed outside of it, and 

 some of the organs may receive a complete, while others receive only a par- 

 tial or scanty investment. 



In passing from one part to another the membrane frequently forms 

 folds, some of which are designated by special names, such as the mesen- 

 tery, meso-colon, and omentum. 



The chief serous membranes are the peritoneum, the largest of all, lining 

 the cavity of the abdomen ; the two pleurae, lining the chest and covering 

 the lungs ; the pericardium, covering the heart. 



