SEROUS MEMBRANES. 273 



genital eanal in the vagina in the female, in the urethra in the 

 male; the membrane so formed is termed the urino-genital 

 mucous membrane, and it becomes continuous with the skin at 

 the orifices of these organs. Mucous membrane also lines the 

 ducts of the mammary glands; and in fact the ducts of all 

 glands which open on the skin or mucous membranes. 



SEROUS MEMBRANES. 



The closed cavities of the body are lined by membranous 

 structures of this class. By a closed cavity we understand a 

 receptacle impervious, under normal circumstances, to the atmo- 

 spheric air. A serous membrane consists of an endothelial and a 

 subendothelial portion, the cells of the former being arranged in 

 a single layer. Synovial membranes may-be regarded as a variety 

 of serous membranes, differing chiefly in the nature of their secre- 

 tion ; they have already been described (see p 125). Since a 

 serous raembrane is so arranged as to line a closed cavity, and at 

 the same time to cover its contents, it follows that the entire 

 membrane must form a closed sac, the endothelial layer being on 

 the inside ; such a sac is called a serous sac or cavity. The fold 

 of the membrane which lines a cavity is called the parietal, that 

 which covers its contents the visceral portion ; the two surfacea 

 contacting, and gliding readily upon each other, are lubricated hy 

 a fluid secretion contained in the sac ; hence one use of these 

 membranes is to prevent friction between the walls of cavities 

 and the organs contained therein. The following are the serous 

 membranes and their position in the body : — The arachnoid 

 membranes form closed sacs, situated, one between the two 

 cbverings of the brain, the other between those of the spinal 

 cord. The peritoneum lines the abdominal cavity and covers its 

 viscera; the pleurae, right and left, each covers the lung, and 

 lines the half of the thoracic cavity of its own side. The peri- 

 cardium lines the fibrous sac which contains and supports the 

 heart, and also covers the latter. The cavities of the heart are 

 also lined by a membrane of a serous nature — the endocardium 

 — which is continued through the veins and arteries; as the 

 chief lymphatic vessels communicate with the venous system, the 

 endothelial lining of the former becomes continuous with that of 

 the latter. Lymphatic vessels open into the serous sacs by very 

 small apertures, or stomata, so that these sacs are now con- 



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