STRUCTURE OF ARTERIES. 



133 



THE ARTERIES. 



The walls of the arteries are composed of three principal 

 coats, termed the external or tunica adventitia, the middle, 

 and the internal, while the latter is lined within by a single 

 layer of tesselated epithelium. 



The external coat or tunica adventitia, the strongest and 

 toughest part of the wall of the artery, is formed of areolar 

 tissue, with which is mingled throughout a network of 

 elastic fibres. At the inner part of this outer coat the 

 elastic network forms in most arteries so distinct a layer 

 as to be sometimes called the external elastic coat. 



The middle coat is composed of both muscular and elastic 

 fibres. 



The former, which are of the pale or unstriped variety (see 

 Chapter on Motion), are arranged Fig. 40.* 



for the most part transversely to 

 the long axis of the artery; while 

 the elastic element, taking also 

 a transverse direction, is disposed 

 in the form of closely inter- 

 woven and branching fibres, 

 which intersect in all parts the 

 layers of muscular fibre. In 

 arteries of various size there is 

 a difference in the proportion of 

 the muscular and elastic element, 

 elastic tissue preponderating in 

 the largest arteries, while this 

 condition is reversed in those of medium and small size. 



The internal arterial coat is formed by layers of elastic 

 tissue, consisting in part of coarse longitudinal branching 

 fibres, and in part of a very thin and brittle membrane 

 which possesses little elasticity, and is thrown into folds 



* Fig. 40. Muscular fibre-cells from human arteries, magnified 350 

 diameters (Kolliker). a, natural state ; I, treated with acetic acid. 



