48 NORMAL HISTOLOGY. 



tissue with a 1 per cent, solution of nitrate of silver for a few mo- 

 ments, washing with distilled water, and then exposing it to the 

 rays of the sun. During this treatment the intercellular substance 

 enters into combination with the silver. Upon exposure to strong 

 light this compound is destroyed, leaving an insoluble black precipi- 

 tate of silver oxide. When the specimen is examined under the 

 microscope, the site of the cement-substance is marked by the 

 presence of this precipitate. Endothelium so treated shows a net- 

 work of fine dark lines, the meshes of which are occupied by the 

 cells of the tissue. When no such method has been employed to 

 render the intercellular substance conspicuous, the outlines of the 

 cells cannot be distinguished, and the tissue appears as a continuous, 

 nearly homogeneous membrane containing nuclei at more or less 

 regular intervals. When seen in profile or vertical section, endo- 

 thelium appears as a delicate line, expanded at intervals to enclose a 

 nucleus (Fig. 26). The nuclei of the endothelial cells are round or 



Diagram of vertical section through a serous membrane : a, nucleus of endothelial cell : b t 

 body of cell ; c, line of junction between two cells occupied by cement-substance ; d, pro- 

 cess of connective-tissue cell occupying a portion of the intercellular space between two 

 endothelial cells, one variety of pseudostoma ; e, areolar tissue with fusiform and stel- 

 late cells. The vessels and nerves in the areolar tissue have been omitted. 



oval, and each cell usually possesses but a single nucleus situated 

 near its centre, but occasionally cells with two nuclei are observed. 

 Functionally, endothelium appears to play only a passive role in 

 most situations in which it is found. It furnishes a smooth cover- 

 ing for those internal surfaces of the body which are exposed to 

 friction, as, for example, in the serous cavities and the inner sur- 

 faces of the vascular systems. In the capillary bloodvessels and 

 lymphatics endothelium forms the entire wall of the vessels, and 

 its thinness permits the passage of fluids through those walls. The 

 fact that the lymph in different parts of the body varies somewhat 



