46 NORMAL HISTOLOGY. 



thelial lining of the serous cavities (Fig. 24). These openings are 

 called stomata and furnish a direct communication between the se- 

 rous cavities and the lymphatic spaces in the tissues surrounding 

 them. These openings virtually convert the serous cavities into 

 enormous lymph-spaces forming a part of the general lymphatic 

 system. 



FIG. 23. 



Mesentery of frog treated with silver nitrate. The mesentery is covered on both surfaces 

 with a layer of endothelium. Between these is areolar connective tissue containing 

 bloodvessels, lymphatics, and nerves. In this figure only the two endothelial layers and 

 a capillary bloodvessel are represented : a, nucleus of endothelial cell belonging to upper- 

 most layer ; 6, nucleus of cell belonging to deep layer forming the lower surface of the 

 specimen; c, intercellular cement between cells of upper layer of endothelium ; d, d, 

 nuclei of endothelial cells, forming a capillary bloodvessel, seen in profile. The bodies of 

 these cells are not reproduced in the figure. The cement in the deep layer of endothe- 

 lium is represented by finer lines to distinguish it from that belonging to the upper layer. 



The edges of contiguous endothelial cells are not everywhere in 

 equally close approximation to each other (Fig. 25). The occasional 

 points where they are more widely separated than usual are occu- 

 pied either by an increased amount of the cement-substance, or pro- 

 cesses from cells in the underlying tissues are here intercalated 

 between the endothelial cells, reaching the surface of the serous 

 membrane. In either case these points of separation of the endo- 

 thelial cells are not openings through the tissue, though, as we shall 

 see in a subsequent chapter, they are spots where the tissue is rela- 



