158 



THE ORGANS 



of which are disposed longitudinally, around the outside of the endo- 

 thelial tube. A little farther back isolated smooth muscle cells, 

 circularly arranged, begin to appear between the endothelium and 

 the connective tissue. Such an artery is known as a precapillary 

 artery. The next transition is the completion of the muscular coat, 

 the muscle cells now forming a continuous layer. Such an artery, 

 consisting of three distinct coats, the middle coat composed of a single 

 continuous layer of smooth muscle cells, is known as an arteriole (Fig. 

 93, d\ Fig. 94, h). 



Mediimi-sized Arteries. — This group comprises all the named 

 arteries of the body with the exception of the aorta and the pulmo- 



FiG. 94. — Capillary Network from Human Pia Mater, showing also an arteriole in 

 " optical section" and a small vein. X3S0. (Technic i, p. 164.) a, Vein; h, arteriole; 

 c, large capillary; d, small capillaries. 



nary. Their walls are formed of the same three coats found in the 

 arteriole, but the structure of these coats is more elaborate. 



I. The INTIMA consists of three layers (Fig. 95). 



(a) An inner endothelial layer already described. 



{h) A middle layer, the intermediary layer of the intima. This 

 is composed of delicate white and elastic fibrils which run longi- 

 tudinally, and connective-tissue cells. 



(c) An outer layer, the elastic layer of the intima, or memhrana 

 elastica interna — a thin fenestrated membrane of elastic tissue. This 

 membrane is intimately connected with the media and marks the 

 boundary between the latter and the intima. In the smallest of 



