SECKETIOK. 299 



merous delicate elastic fibrillae, together with blood-vessels, nerves, and 

 lymphatics. In relation to the process of secretion, the layer of connec- 

 tive tissue serves as a groundwork for the ramification of blood-vessels, 

 nerves, and lymphatics. But in its usual form it is absent in some in- 

 stances, as in the arachnoid covering the dura mater, and in the interior 

 of the ventricles of the brain. The primary membrane and epithelium 



Fig. 219. Section of synovial membrane, a, Endothelial covering of the elevations of the 

 membrane ; 6, subserous tissue containing fat and blood-vessels ; c, ligament covered by the sy- 

 novial membrane. (Cadiat.) 



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

 which the free surface of the membrane is moistened. 



Functions. The principal purpose of the serous and synovial mem- 

 branes is to furnish a smooth, moist surface, to facilitate the movements 

 of the invested organ, and to prevent the injurious effects of friction. 

 This purpose is especially manifested in joints, in which free and exten- 

 sive movements take place ; and in the stomach and intestines, which, 

 from the varying quantity and movements of their contents, are in al- 

 most constant motion upon one another and the walls of the abdomen. 



Fluid. The fluid secreted from the free surface of the serous mem- 

 branes is, in health, rarely more than sufficient to ensure the mainte- 

 nance of their moisture. The opposed surfaces of each serous sac are at 

 every point in contact with each other. After death, a larger quantity of 

 fluid is usually found in each serous sac ; but this, if not the product of 

 manifest disease, is probably such as has transuded after death, or in 

 the last hours of life. An excess of such fluid in any serous sac consti- 

 tutes dropsy of the sac. 



