SECRETION. 



327 



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 insure the maintenance 

 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 mani- 

 fest disease, is probably such as has transuded after death, or in the last 

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

 tutes dropsy of the sac. 



The fluid naturally secreted by the serous membranes appears to be 



FIG. 226. Section of synovial membrane, a, endothelial covering of the elevations of the mem- 

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

 membrane. (Cadiat.) 



identical, in general and chemical characters, with very dilute liquor 

 sanguinis. It is of a pale yellow or straw color, slightly viscid, alkaline, 

 and on account of the presence of albumen, coaguable by heat. This 

 similarity of the serous fluid to the liquid part of blood, and to the 

 fluid with which most animal tissues are moistened, renders it probable 

 that it is, in great measure, separated by simple transudation, through 

 the walls of the blood-vessels. The probability is increased by the fact 

 that, in jaundice, the fluid in the serous sacs is, equally with the serum 

 of the blood, colored with the bile. But there is reason for supposing 

 that the fluid of the cerebral ventricles and of the arachnoid sac are ex- 

 ceptions to this rule; for they differ from the fluids of the other serous 



