232 THE LYMPHATIC SYSTEM 



THE SEROUS MEMBRANES 



The serous membranes form closed sacs which line the great cavities, 

 of the body and are reflected over the viscera to form a double covering, 

 the two layers of which are freely movable over one another. Of these 

 two layers the one, the parietal layer, is attached to the wall of the body 

 cavity, the other, the visceral layer, covers the surface of the inclosed 

 organ. 



The serous membranes consist of a mesothelial lining and a support- 

 ing membrane of areolar connective tissue which is richly supplied with 



capillary blood-vessels and lym- 

 phatics. The mesothelium con- 

 sists of large flat cells, pave- 

 ment epithelium, whose ser- 

 rated margins are firmly united 

 by an intercellular cement sub- 

 stance. Here and there mi- 

 nute openings are seen which 

 FIG. 239. TRANSECTION OP THE PERICAR- are surrounded by very small 

 DITJM OF A CHILD. mesothelial cells; these stoma ta 



a~a, mesothelium; b-b, submesothelial con- have been found to be in cer . 

 nective tissue. Hematein and eosm. Photo. 



x 500. tain instances directly connect- 



ed with the lymph vessels. 

 Some regard them as transient fenestra, others as artifacts. 



Tunica Propria. The mesothelium rests upon a layer of areolar 

 tissue which is richly supplied with small blood-vessels and lymphatics, 

 forming an abundant vascular plexus beneath the mesothelium. The 

 serous membrane is either directly united to the wall of the cavity and 

 the surface of the organ which it envelops, or it may be attached by a 

 loose layer of submesothelial connective tissue. 



The thickness of the mesothelial cells varies in different portions 

 of the serous membranes and is somewhat dependent upon the age of the 

 individual. In most portions it is no more than a pavement epithelium, 

 but over the surface of the functionally active ovary these cells are 

 much thickened and acquire a cuboidal shape ; thus it forms the 'germinal 

 epithelium' of the ovary. In young individuals, viz., in fetal life and 

 early childhood, the cuboidal cell type is found in many portions of 

 the peritoneum, pleura, and pericardium. 



The synovial membranes resemble the serous in their structure. They 



