402 SECRETION. 



are in almost constant motion upon one another and the 

 walls of the abdomen. 



The fluid secreted from the free surface of the serous 

 membranes is, in health, rarely more than sufficient to 

 ensure 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 6f 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 san- 

 guinis. It is of a pale yellow or straw-colour, 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. 73 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 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, coloured 

 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, colour- 

 less, and of much less specific gravity, but in that they 

 seldom receive the tinge of bile in the blood, and are not 

 coloured by madder, or other similar substances introduced 

 abundantly into the blood. 



