THE LYMPHATIC SYSTEM. 



first formed. In the closed sacs constituted by the serous mem- 

 branes a parietal and a visceral layer are always distinguishable ; 

 the connection of these with the subjacent structures is slight or 

 intimate according to the character and amount of the subserous 

 tissue. 



Every organ which projects beyond the wall of the serous pouch 

 into its cavity must be enveloped by the serous membrane to a 

 greater or less degree. When the organ remains closely attached 

 to the wall of the body-cavity, as does the kidney, it obtains only 

 a partial serous investment; where, on the other hand, the organ 

 leaves the parietes and encroaches upon the cavity, the serous invest- 

 ment becomes almost complete, as in the case of the small intestines. 

 In all cases the viscera lie outside the serous sac, the membrane 

 which constitutes 'the lining of the space being pushed before the 

 encroaching organ to form a serous covering more or less complete. 

 The serous cavity of greatest extent that of the peritoneum in the 

 female presents an exceptional arrangement in possessing outlets at 

 the orifices of the oviducts ; in this connection, however, it must 

 be remembered that the oviduct is the persistent Miillerian duct, 

 which is only one of a number of tubes formed during early foetal life 

 by evagination of the primary serous membrane, thus establishing 

 communication with surfaces exterior to the 

 serous cavity. While such tubes in the FIG. 157. 



higher animals are only transient, in the 

 lower types they may remain as permanent 

 structures. 



The serous membranes are sufficiently 

 thin and transparent to permit the color of 

 the underlying parts to be seen readily 

 through them ; moderate strength, extensi- 

 bility, and elasticity are among their physical 

 properties. These membranes consist of the 

 endothelium covering their free surface and 

 resting upon the connective-tissue stroma, 



Which Constitutes the Chief Substance Of the Peritoneal endothelium of dog, 



membrane; external to this layer a variable Z^^J^Z 



amount of subserous tissue usually is pres- among the ceils. 



ent. The endothelium comprises a single 



layer of the large, thin, irregularly - polyhedral connective - tissue 



plates already described and figured in Chapter II. 



In addition to the minute deeply-stained intercellular areas, or 

 pseudo-stomata, true openings, or stomata, also exist occasion- 

 ally in serous membranes. These orifices are especially well seen in 

 silver preparations from the posterior wall of the frog's peritoneal 



9 



