is 



NORMAL HISTOLOGY. 



ishing the friction resulting from their movements against each 

 • it her. It does not occur in any situation where it would be exposed 

 directly to the air. 



The eells of endothelium vary somewhat in size and shape. They 

 mav be polygonal, rhomboid, or stellate in form, and during life are 



soft and extensible so that their 

 sizes may be modified by stretch- 

 ing; or tension in one or more 

 directions. The cell-bodies, or 

 cytoplasm, are usually clear and 

 apparently structureless or only 

 slightly granular, but occasion- 

 ally some of the cells are smaller 

 and more granular than the 

 majority. This is especially 

 marked in the cells surroundino; 

 minute apertures that are found 

 here and there in the endothelial 

 lining of the serous cavities (Fig. 

 24). These stomata furnish a 

 direct communication between 

 the serous cavities and the lym- 

 phatic spaces in the tissues sur- 

 rounding them. These openings 

 virtually convert the serous 

 cavities into enormous lymph- 

 spaces forming a part of the 

 general lymphatic system. 



The edges of contiguous endo- 

 thelial cells are not everywhere 

 in equally close approximation 

 to each other (Fig. 25). The 

 points where they are more 

 widely separated than usual are 

 occupied by an increased amount 

 of the cement-substance, or 

 processes 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 endothelial cells are not openings through 

 the tissue, though they are spots where the tissue is relatively more 



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 contain- 

 ing bloodvessels, lymphatics, ami nerves. 

 In this figure only the two endothelial 

 layers and a capillary bloodvessel are 

 represented: ", nucleus of endothelial 

 cell belonging to uppermost layer; 6, nu- 

 - of cell belonging to deep layer form- 

 ing the lower surface of the specimen; 

 c, intracellular cement between cells of 

 upper layer of endothelium ; </. '/. nuclei 

 of endothelial cells, forming a capillary 

 bloodvessel, aeen in profile. The bodies 

 of these cells are not reproduced in the 

 figure. 'lie- cement in the deep layer of 

 endothelium is represented by finer lines 

 to distinguish it from that belonging to 

 tie- upper layer. 





