THE CIRCUI.ATORY SYSTEM 



159 



the medium-sized arteries the intermediary layer is often wanting, 

 the endotheUal cells resting directly upon the elastic membrane. 

 Owing to the extensive amount of elastic tissue in their walls, there 

 is a postmortem contraction of arteries which results in the intima 

 being thrown up into folds. For tliis reason the elastic membrane 

 presents, in transverse sections of an artery, the appearance of a 

 wavy band (Fig. 95). 



Fig. 95. — From Cross-section through Walls of Medium-sized Artery and its 

 Accompanying Vein. XJS- (Technic3, p. 164.) .4, Intima of artery; d, its endothe- 

 lial layer; b, its intermediary layer; c, its elastic layer; B, media of artery; C, adventitia, 

 the upper part belonging to the artery, the lower to the vein; within the adventitia are 

 seen the vasa vasorum; D, media of vein; E, intima of vein; i, its intermediary layer; 

 j, its endothelial layer. 



2. The MEDIA is a thick coat of circularly disposed smooth muscle 

 cells (Fig. 95, ^). Its thickness depends largely upon the size of the 

 vessel, though varying somewhat for different vessels of the same size, 

 A small amount of fibrillar connective tissue supports the muscle cells. 

 Elastic tissue is present in the media, the amount being usually pro- 

 portionate to the size of the vessel. In the smaller of the medium- 

 sized arteries, the elastic tissue is disposed as dehcate fibrils among 

 the muscle cells. In larger arteries many coarse fibres are intermin- 

 gled with the fine fibrils. When much elastic tissue is present the 

 muscle cells are separated into more or less well-defined groups. In 

 such large arteries as the subclavian and the carotid, elastic tissue 



