144 



MANUAL OP HISTOLOGY. 



An isolated endotlielial cell, when tilted up on its edge, pre- 

 sents the appearance of a straight or curved double contour, 

 with a central thickening corresponding to its nucleus. Viewed 

 en face, we observe the sinuous outline and the central or ec- 

 centric nucleus, with its surrounding granules of protoplasm. 

 The shape and contour of endothelial cells are subject to con- 



Fio. 58. Endothelium of the carotid artery of man, after treatment with nitrate of silver: a, cells; 

 6, clearer, c, darker intermediate spaces ; d, intra-cellular circular and spotted markings. Eberth. 



siderable variations in the different vascular districts. Such 

 differences also occur in the same district, with the varying de- 

 gree of expansion or contraction of the particular vessel under 

 observation. 



The capillaries proper. In point of wideness of distribu- 

 tion, this variety of blood-vessels greatly exceeds all others. 

 Indeed, the capillaries occupy a rank, in this respect, second 

 only to the connective- tissue group of histological struc- 

 tures. As regards importance to the economy, it will only be 

 necessary to advert to the vital processes of nutrition, secre- 

 tion, respiration, and excretion, to recall the quality and 

 extent of their physiological usefulness. Throughout the 



