THE CTRCn.ATORY SYSTEM 



157 



Arteries 



The wall of an artery consists of three coats: 

 (i) An inner coat, the intima. 



(2) The middle coat, the media. 



(3) An outer coat, the adventitia. 



The intima consists of a single layer of endothelial cells, continu- 

 ous with and similar to that forming the walls of the capillaries, or, 



>c 



fed 



Fig. 93. — Diagram of Capillaries and Small Artery showing their structure and 

 relations, a, Capillaries; h, nuclei of capillary endothelium; c, precapillary arteries; d, 

 arteriole; e, large capillary; /, small artery. 



in arteries of considerable size, of this layer plus more or less connec- 

 tive tissue. The middle coat consists mainly of smooth muscle, the 

 outer of connective tissue. 



The structure of these three coats varies according to the size of 

 the artery, and while the transition between them is never abrupt, it 

 is convenient, for purposes of description, to distinguish {a) small 

 arteries, (&) medium sized arteries, and (c) large arteries. 



Small Arteries. — Passing from a capillary to an artery, the first 

 change is the addition of a thin sheath of connective tissue, the fibres 



