48 NORMAL HISTOLOGY. 



membranes are formed by layers of fibrillar connective- 

 tissue fibres mingled with a varying number of elastic 

 fibres, and containing ordinary flattened connective-tissue 

 cells. They are more or less abundantly furnished with 

 blood- and lymphatic-vessels. On the free surface of 

 these membranes rests a continuous layer of flattened 

 cells, differing in many respects from the ordinary con- 

 nective-tissue cells, and called endothelial cells or endo- 

 thelium. These cells are usually transparent, irregularly 

 polygonal in form, and frequently much elongated ; they 

 possess one or more ovoidal nuclei, which often project 

 above the general level of the free surface of the cell- 

 body ; they are placed edge to edge, like the stones in a 

 mosaic, and seem to be joined together by a minimal 

 amount of an albuminoid cement substance.* 



JSndothelium Covering the Mesentery. In order to bring 

 the outlines of the cells clearly into view, the membrane 

 should be first treated with a solution of nitrate of silver. 

 This substance in dilute solutions forms with the cement 

 substance between the endothelium an albuminate of 

 silver, which, on exposure to light, becomes brown or 

 black, thus clearly defining the outline of the cells. 



The mode of proceeding is as follows : A portion of 

 the mesentery of the recently killed dog or rabbit should 

 be carefully removed and laid over the rim of a shallow 

 dish so that it rests loosely over the opening. All stretch- 



* There is reason for believing that the cement substance between 

 the endothelium, as well as between many kinds of epithelial cells, is 

 permeable to fluids, and, under certain conditions, to solid particles 

 also, and thus forms an avenue of communication between adjacent 

 but separated cavities or lymph spaces. 



