ARTERIES. 77 



Arteries : The arteries vary in structure somewhat that is, 

 in the relative proportions of their several constituents ac- 

 cording to their size and degree of subdivision (Fig. 37). The 

 coats of arteries are usually described as consisting of three 

 divisions, the tunica intima, tunica media, and tunica adven- 

 titia. The structure of medium-sized arteries, or all but the 

 largest and the smallest, is as follows : 



The tunica intima, or internal coat, consists of three layers. 

 It is lined internally by endothelium, a single layer of flat 

 sqtiamous cells uniting by somewhat sinuous edges, oval and 

 elongated in shape and with oval nuclei ; the cells and nuclei 

 lie with their long axes parallel with the course of the 

 artery. These cells present a smooth surface to the blood- 

 stream. 



Beneath the endothelium is the subendothelial connective 

 tissue, a thin delicate layer of white and elastic fibrous tissue 

 with stellate connective-tissue cells. In the smallest arteries 

 this layer becomes thinner and finally disappears. 



The lowest layer of the tunica intima, separating it from 

 the tunica media, is the internal elastic lamina, a thin but 

 prominent layer of interlacing yellow elastic fibres forming a 

 perforated or sometimes almost continuous membrane ; it is 

 sometimes called the a fenestrated membrane of Henle." In 

 empty and contracted arteries this elastic lamina is thrown 

 into longitudinal folds, and in cross-sections of arteries it con- 

 sequently presents a very conspicuous and characteristic ap- 

 pearance as a clear sinuous yellow line surrounding the lumen 

 of the vessel. 



The tunica media is a thick middle coat, between the intima 

 and adventitia, consisting chiefly of intermingled yellow elastic 

 fibres and involuntary-muscle cells, along with a small amount 

 of white fibres. The muscle-cells are for the most part ar- 

 ranged circularly or transversely, but in a few arteries longi- 

 tudinal cells also occur sparingly. The elastic tissue occurs 

 as sinuous fibres, networks, and plates. In relative quantity 

 the elastic and muscular elements vary inversely to each other 

 according to the size of the artery. 



* In the large arteries the elastic tissue greatly predominates 

 in amount over the muscle ; as the arteries decrease in size 

 these proportions are gradually reversed, until in the small 



