496 THE TISSUES. 



to occur from the weaker vessels ; and hence ecchymoses are very 

 common. The polypi, so called, of the stomach, are merely groups 

 of peptic glands, rendered prominent by an exudation deposited 

 around them. 



3. The thinness of the corium of the mucous membrane accounts 

 for its tendency to losses of substance by ulceration; the bottoms 

 of which are covered by various products (exudation). The latter 

 sometimes rise like a plug above the level of the membrane (as in 

 typhoid fever). Vegetable parasites are also frequently developed 

 in exudations on mucous membranes. 



III. THE SEROUS MEMBRANES. 



The true serous membranes are the peritoneum, the pleura, the 

 tunica vaginalis testis, and the pericardium. All these have a sim- 

 ple scaly epithelium (p. 238, and Fig. 140), generally a thinner 

 corium than the mucous membrane, and which presents neither 

 papillae nor glands; and constitute closed cavities, moistened by the 

 secretion of their epithelial cells, and an accompanying transudation 

 (pp. 180 1). They will be described in connection with the parts 

 and organs into the structure of which they respectively enter. 

 The tunica vaginalis is originally an offset from the peritoneum. 



The synovial membranes are often called serous membranes; as 

 are also the bursae mucosae and the vaginal sheaths of tendons. It 

 has, however, been shown that histologically they are not such, 

 since they do not form closed cavities (p. 344, and Fig. 233). The 

 arachnoid also has but a single layer (p. 469) and the ependyma of 

 the ventricles has not everywhere a corium (p. 469). The arach- 

 noid, however, normally presents villi; which, becoming enlarged, 

 constitute the Pacchionian bodies. 



Function of Serous Membranes. 



The serous membranes proper, merely subserve the mechanical 

 purpose of facilitating motion of one part on another, and diminish- 

 ing friction; both by the smoothness of their surface and the secre- 

 tion they produce (p. 181). 



An abnormal accumulation of the natural secretion of the serous 

 membranes in their cavities, constitutes the various forms of dropsy; 

 e.g. in the pleural cavity, hydrothorax; in the peritoneal, ascites; in 

 the tunica vaginalis, hydrocele; and in the pericardium, hydrops pe- 

 ricardii. An exudation upon the pleura becoming degenerated into 

 pus, constitutes empyema. The conversion of exudations into new 

 formations is explained under the next head. 



