ENDOTHELIUM OF SEKOUS MEMBRANES. 267 



the centrum tendineum, upon the hereafter to be described 

 fissures, and partially upon the inner surface of the external 

 pericardial lamina of Rabbits, the contours of the endothelial 

 cells present a remarkably well-defined sinuous character; and 

 this occurs also, though only partially, on the pleura and the 

 thoracic surface of the diaphragm. It is remarkable that in 

 silvered preparations the cell contours over the subsequently to 

 be mentioned broad fissures on the abdominal surface of the cen- 

 trum tendineum, as well as upon the lymphatics of the pleura (5), 

 are much more delicate than in the intervening spaces. 



The arrangement of the endothelial cells is that polyhedric 

 cells are simply juxtaposed, or where they are elongated their 

 extremities are intercalated between those of the next row, 

 or they are disposed radially, in groups of from four to ten, 

 around a common centre. They are arranged in the last-named 

 manner both upon the abdominal (6) and upon the thoracic 

 surface of the centrum tendineum of the diaphragm, upon the 

 mesentery, upon the abdominal surface (7) of the wall of the 

 cisterna lymphatica of the Frog, upon the pleura (8) and upon 

 the internal and external surface of the pericardium of the 

 Rabbit, Cat, and of Man, as well as upon the dura mater of 

 the Rabbit (9). 



In the centre of these endothelial groups we find one or 

 several (two or three) sharply defined, rounded, or triangular 

 spaces of various size, which, since the time of Recklinghausen 

 and Oedmanson, have been regarded as apertures between 

 the endothelial cells, or as the so-called stomata. 



On the other hand we meet also in these places with 

 radially arranged groups of endothelial cells, in the centre of 

 which none of the above-mentioned stomata can be found. 

 Considering that in silvered preparations minute but variously 

 sized areas, either isolated or in small groups, and always 

 sharply defined, occur between two or several endothelial cells, 

 and that in certain spots, on account of the proximity of figures 

 of very various size, and of the absence of nuclei, it appears 

 to be quite impossible to decide what is a stoma and what a 

 small endothelial cell, all inquirers have held the demonstration 

 of the existence of spaces between the endothelia in such 

 preparation to be incomplete. The above-described images, 



