SEROUS MEMBRANES. 391 



surface. In some folds of the serous membranes and especially in the great 

 omentum of many animals, including man, the meshes of the reticulation have 

 become open in many parts owing to the absorption of the intervening ground- 

 substance and the perforation of the cells covering it, so as to allow of a free com- 

 munication between the two sides of the fold of membrane. Where the membrane 

 is thicker, the ground-substance contains blood-vessels and lymphatics, with the 



Fig. 449. SMALL PORTION OF PERITONEAL 



SURFACE OF DIAPHRAGM OF RABBIT. 



(Klein.) MAGNIFIED. 



I, lymph-channel below the surface, lying 

 between tendon bundles, t, t, and over which 

 the surface-cells are seen to be relatively 

 smaller, and to exhibit five stomata, S, S, 

 leading into the lymphatic. The epithelium 

 of the lymphatic channel is not represented. 



Jymphoid and adipose tissue which 

 is so often found in the serous 

 membranes and especially in their 

 folds ; as well as connective tissue 

 corpuscles with their corresponding 

 cell-spaces (figs. 438, 439), which in 

 the serous membranes are very often 

 collected into endothelium-like 

 patches. In parts of the membrane 

 in which the corpuscles are more 



thinly scattered, they possess branching processes, some of which intercommunicate 

 with those of neighbouring cells, others may pass up to the surface of the mem- 

 brane as pseudostomata and others again become connected to the walls of the 

 lymphatics and blood-vessels. 



In the human subject, the serous membranes are bounded under the epithelium 

 by a distinct basement membrane (Bizzozero). 



The blood-vessels of the membrane end in a capillary network with compara- 

 tively wide meshes, which pervades the subserous tissue and the tissue of the serous 

 membrane. The vessels are much more numerous in the nodules and tracts of 

 lymphoid tissue (see below) as well as in the adipose tissue, which is found largely 

 developed in the serous membranes of fat animals. 



The lymphatics of the serous membranes are exceedingly abundant. Their 

 relation both to the cell-spaces of the tissue and to the surface of the membrane, as 

 well as their general arrangement, has been already noticed. They are sometimes 

 met with ensheathing the blood-vessels. 



Nodules of lymphoid tissue may occur, as before mentioned (p. 387), in the 

 substance of the serous membranes. More generally the lymph oid tissue of the 

 serous membranes takes the shape of elongated tracts which follow the course of the 

 small arteries and veins, receiving from the latter branches which divide to form a 

 capillary network. Lymphatic vessels run in these tracts alongside the blood- 

 vessels, and often partially enclose them. These lymphoid nodules and tracts are 

 more numerous in the young animal ; in the adult they are frequently found trans- 

 formed into lobules and tracts of adipose tissue. 



The nerves of the serous membranes are destined chiefly for the blood-vessels, 

 and for the most part accompany these in their course. A few pale fibres, however, 

 are distributed to the substance of the membrane, in which they form a plexus with 

 large meshes : from the branches of this plexus, fibrils may be traced which unite 

 into a somewhat finer plexus near the surface. 



