320 HAND-BOOK OF PHYSIOLOGY. 



in the formation of the fluid by which the free surface of tne membrane 

 48 moistened. 



Serous membranes are of two principal kinds: 1st. Those which line 

 visceral cavities, the arachnoid, pericardium, pleura?, peritoneum, and 

 tunicas vaginales. 2nd. The synovial membranes lining the joints, and 

 the sheaths of tendons and ligaments, with which, also, are usually in- 

 cluded the synovial bursce, or bursce mucosce, whether these be subcutane- 

 ous, 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 invested by a serous mem- 

 brane 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. 



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 almost 

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



Serous Fluid. The fluid secreted from the free surface of the serous 

 membranes is, in health, rarely more than sufficient to ensure the main- 

 tenance 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 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 saguinis. It is of a pale yellow or straw 

 color, slightly viscid, alkaline, and, on account of the presence of albu- 

 men, coagulable by heat. This similarity of the serous fluid to the liquid 

 part of blood, and to the fluid with which most animal tissues are moist- 

 ened, 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 exceptions to this rule; for they differ from the fluids 



