THE SEROUS MEMBRANES 145 



2. As dilated pouches having blind extremities, as in the villi 

 of the small intestine. 



3. By direct communication with the stomata of the serous 

 membranes. 



The lymph is derived from the tissue juices and by absorption 

 from the alimentary tract, and is conveyed by the lymphatic capil- 

 laries to larger and larger lymphatic vessels, which resemble the 

 small veins in their structure, and which finally empty into the 

 subclavian veins of the neck. 



THE SEROUS MEMBRANES. The serous membranes form 

 closed sacs which line the great cavities of the body and are re- 

 flected 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 in- 

 closed organ. 



The serous membranes consist of an endothelial lining and a 

 supporting membrane of areolar connective tissue which is richly 

 supplied with capillary blood vessels and lymphatics. The eudo- 

 thelium consists of large flat cells, pavement epithelium, whose 

 serrated margins are firmly united by an intercellular cement sub- 

 stance. Here and there minute openings are seen which are sur- 

 rounded by very small endothelial cells ; these stomata have been 

 found to be in certain instances directly connected with the lym- 

 phatic vessels. ___^ 



Tunica Propria. The 

 endothelium rests upon a 

 layer of areolar tissue which 

 is richly supplied with small 

 blood vessels and lymphat- 

 ics, forming an abundant 

 vascular plexus beneath the 



endothelium. The SerOUS p IG> 144. TRANSECTION OF THE PERICARDIUM 



membrane is either directly OF A CHILD. 



United to the Wall Of the -*i endothelium; 6-6, subendothelial con- 



., - nective tissue. Hematein and eosin. Photo. 



cavity and the surface of " 500 

 the organ which it envelops, 



or it may be attached by a looser layer of " su ^endothelial con- 

 nective tissue" 



The thickness of the endothelial cells varies in different por- 

 tions of the serous membranes and is somewhat dependent upon 

 11 



