THE EPITHELIAL TISSUES. 



33 



ENDOTHEtHJM. 



Although endothelium is intimately related to the connective 

 tissues, being but modifications of the cells of this group, it is con- 

 venient to describe this tissue in the present place. Endothelium 

 forms a covering of the free surface of those spaces not directly 

 communicating with the external atmosphere, including, therefore, 

 the lining of the various serous cavities, as the pleura, pericardium, 

 and peritoneum (disregarding the communication established through 

 the oviduct), of the synovial surfaces of joints, of the heart and 

 blood-vessels, as well as of the 

 numerous lymphatic spaces 

 and vessels. These cells 

 occur normally as a single 

 layer of thin, irregularly poly- 

 hedral plates of variable size 

 and of great delicacy; they 

 possess an oval, sometimes 

 kidney-shaped, nucleus; they 

 never overlap, and usually 

 unite with neighboring cells 

 by serrated and tortuous lines 

 of cement-substance. The 

 endothelial plates covering 

 the serous membranes are, 

 in general, polyhedral, re- 

 sembling in outline the simple 



SCaly epithelium ; those lining Endothelium from peritoneal surface of diaphragm, 

 , , i j 1 i stained with silver : n, nucleus of endothelial plate ; s, 



the blood-VeSSelS are elOn- one of the intercellular clefts or stigmata. 



gated, irregular spindles, while 



those found in the lymphatic vessels are often still more unsym- 



metrical, being limited by very tortuous boundaries. 



For the satisfactory study of endothelium resource to silver 

 staining (see Appendix) must be had, by which method the inter- 

 cellular cement-substance is colored deeply brown or black, appear- 

 ing as dark, frequently-interrupted boundary-lines. In such prepara- 

 tions the points of union common to several cells are often marked 

 by small, deeply-stained areas the stigmata, or pseudo-stomata. 

 These figures are regarded by some as minute openings filled by 

 silver-stained albuminous substances ; according to Klein, however, 

 many of these stigmata are the protruding stained processes of con- 

 nective-tissue cells. In addition to these areas of questionable import 

 are true distinct openings, the stomata, which establish direct com- 

 munication with the adjacent lymphatic channels ; the diaphragm, 



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