316 SECRETION. 



joints, in which free and extensive movements take place ; and 

 in the stomach and intestines, which, from the varying quan- 

 tity and movements of their contents, are in almost constant 

 motion upon one another and the walls of the abdomen. 



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, and 

 leave no space in which fluid can collect. 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 of the serous sacs constitutes dropsy of the 

 sac. 



The fluid naturally secreted by the serous membranes appears 

 to be identical, in general and chemical characters, with the 

 serum of the blood, or with very dilute liquor sanguinis. It 

 is of a pale yellow or straw-color, slightly viscid, alkaline, and, 

 because of the presence of albumen, coagulable by heat. The 

 presence of a minute quantity of fibrin, at least in the dropsical 

 fluids effused into the serous cavities, is shown by their partial 

 coagulation into a jelly-like mass, on the addition of certain 

 animal substances, or on mixture with certain fluids, especially 

 such as contain cells (p. 70 et seq.}. 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 bloodvessels. 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 exceptions to this rule ; for they 

 differ from the fluids of the other serous sacs not only in being 

 pellucid, colorless, and of much less specific gravity, but in 

 that they seldom receive the tinge of bile in the blood, and are 

 not colored by madder, or other similar substances introduced 

 abundantly into the blood. 



It is also probable that the formation of synovial fluid is a 

 process of more genuine and elaborate secretion, by means of 

 the epithelial cells on the surface of the membrane, and espe- 

 cially of those which are accumulated on the edges and pro- 

 cesses of the synovial fringes ; for, in its peculiar density, vis- 

 cidity, and abundance of albumen, synovia differs alike from 

 the serum of blood and from the fluid of any of the serous 

 cavities. 



The mucous membranes line all those passages by which in- 



