396 SECEETIOX. 



In relation to the process of secretion, the layer of fibro- 

 cellular tissues serves as a ground-work for the ramification 

 of blood-vessels, lymphatics, and nerves. But in its usual 

 form it is absent in some instances, as in the arachnoid 

 covering the dura mater, and in the interior of the ven- 

 tricles of the brain. The primary membrane and epithe- 

 lium are probably always present, and are concerned in 

 the formation of the fluid by which the free surface of the 

 membrane is moistened. 



The serous membranes are of two principal kinds : 

 1st. Those which line visceral cavities, the arachnoid, 

 pericardium, pleura?, peritoneum, and tunicse vaginales. 

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

 sheaths of tendons and ligaments, with which, also, are 

 usually included the synovial bursse, or burses mucosce, 

 whether these be subcutaneous, or situated beneath ten- 

 dons 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 surround- 

 ing parts. The viscera, which are invested by a serous 

 membrane, are, as it were, pressed into the shut sac which 

 it forms, carrying before them a portion of the membrane, 

 w r hich 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 Yertebrata, with the exception of a few 

 fishes. 



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 extensive move- 

 ments take place ; and in the stomach and intestines, which, 

 from the varying quantity and movements of their contents, 



