SEROUS MEMBRANES. 315 



The serous membranes are formed of fibro-cellular tissue, 

 interwoven so as to constitute a membrane, the free surface of 

 which is covered with a single layer of flattened cells, forming, 

 in most instances, a simple tessellated epithelium. Between the 

 epithelium and the subjacent layer of fibro-cellular tissue, is 

 situated the primary or basement-membrane (Bowman). 



FIG. 104. 



Plan of a secreting membrane : a, membrana propria, or basement-membrane ; 6, 

 epithelium composed of secreting nucleated cells ; c, layer of capillary bloodvessels 

 (after Sharpey). 



In relation to the process of secretion, the layer of fibro- 

 cellular tissue serves as a groundwork for the ramification of 

 bloodvessels, lymphatics, and nerves. But in its usual form 

 it is absent in some instances, as in the arachnoid covering the 

 dura mater, and in the interior of the ventricles of the brain. 

 The primary membrane and epithelium are probably always 

 present, and are concerned in the formation of the fluid by 

 which the free surface of the membrane is moistened. 



The serous membranes are of two principal kinds: 1st. 

 Those which line visceral cavities, the arachnoid, pericar- 

 dium, pleurae, peritoneum, and tunicse vaginales. 2d. The 

 synovial membranes lining the joints, and the sheaths of ten- 

 dons and ligaments, with which, also, are usually included the 

 synovial bursse, or bursce mucosce, whether these be subcuta- 

 neous, or situated beneath tendons that glide over bones. 



The serous membranes form closed sacs, and exist wherever 

 the free surfaces of viscera come into contact with each other, 

 or lie in cavities unattached to surrounding parts. The viscera, 

 which are invested by a serous membrane, are, as it were, 

 pressed into the shut sac which it forms, carrying before them 

 a portion of the membrane, which serves as their investment. 

 To the law that serous membranes form shut sacs, there is, in 

 the human subject, one exception, viz. : the opening of the 

 Fallopian tubes into the abdominal cavity, an arrangement 

 which exists in man and all Vertebrata, with the exception of 

 a few fishes. 



The principal purpose of the serous and synovial membranes 

 is to furnish a smooth, moist surface, to facilitate the move- 

 ments of the invested organ, and to prevent the injurious 

 effects of friction. This purpose is especially manifested in 



