THE BLOOD-VESSELS. 143 



the terminal arteries, and, on the other, with the intima of the 

 veins, hence also with the lining membrane of the heart. They 

 are called endothelia, and since they constitute the only struc- 

 tural elements which enter into the composition of all blood- 

 vessels, we will first consider them and their relations to these 

 vessels. 



The vascular endothelium. Histologists understand by 

 the term endothelium a thin layer of flattened cells lining the 

 free surface of various membranes, canals, sheaths, and cavi- 

 ties, all belonging to the serous type. Epithelium, on the 

 other hand, is found covering the skin and mucous surfaces. 

 All endothelia, in common with the blood, the blood-vessels, 

 and connective tissues, are derived from the mesoblast, or mid- 

 dle of the three fundamental layers of the embryo. The epi- 

 thelia, it will be remembered, originate in the two other layers, 

 called epiblast and hypoblast, respectively the former being 

 the superior and the latter the inferior layer of the embryo. 



In adult human subjects the vascular endothelia are made 

 up of thin, polygonal, sometimes irregularly pentagonal, flat- 

 tened cell-plates. Most of the elements are furnished with a 

 rounded or ovoid nucleus, of central or more or less peripheral 

 location (Fig. 58). Some have two nuclei. In general, the cells 

 are somewhat elongated in the longitudinal direction of the ves- 

 sel to which they belong. They also grow slightly narrower as 

 the calibre of the vessel decreases. Their borders are serrated 

 or scalloped, and dove-tailed into one another. An albuminoid 

 substance, ordinarily invisible, cements their adjoining edges. 

 This substance has the peculiar property of effecting an ener- 

 getic reduction of silver nitrate. Hence, by proper manage- 

 ment, the outlines of each individual cell may be made visible 

 as a black zigzag surrounding a nucleus. Every cell represents 

 a plate-like expanse of modified protoplasm. Remnants of this 

 original substance may be seen to surround the nuclei of young 

 vessels, where they appear in the shape of varying quantities 

 of distinctly granular matter. Klein has described an intra- 

 cellular network, formed by plexuses of minute fibrils, and 

 associated with a second denser reticulum within the nucleus, 

 called the intranuclear network. Whatever interpretation we 

 choose to give these minute structures, the fact of their exist- 

 ence is indisputable. In man, however, their presence is not 

 as readily demonstrable as in animals. 



