STRUCTURE OF ARTERIES. 115 



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 tessellated 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 for the most part trans- 

 versely to the long axis of the artery; while the elastic ele- 

 ment, taking also a transverse direction, is disposed in the 

 form of closely interwoven and branching fibres, which inter- 

 sect in all parts the layers of muscular fibre. In arteries of 

 various size there is a difference in the proportion of the mus- 

 cular 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 tis- 

 sue, consisting in part of coarse longitudinal branching fibres, 

 and in part of a very thin and brittle membrane which pos- 

 sesses little elasticity, and is thrown into folds or wrinkles 

 when the artery contracts. This latter membrane, the striated 

 or fenestrated coat of Henle, is peculiar in its tendency to curl 

 up, when peeled off from the artery, and in the perforated and 

 streaked appearance which it presents under the microscope. 

 Its inner surface is lined with a delicate layer of epithelium, 

 composed of thin squamous elongated cells, which make it 

 smooth and polished, and furnish a nearly impermeable sur- 

 face, along which the blood may flow with the smallest possible 

 amount of resistance from friction. 



The walls of the arteries, with the possible exception of the 

 epithelial lining and the layers of the internal coat immedi- 

 ately outside it, are not nourished by the blood which they 

 convey, but are, like other parts of the body, supplied with 

 little arteries, ending in capillaries and veins, which, branching 

 throughout the external coat, extend for some distance into 

 the middle, but do not reach the internal coat. These nutrient 

 vessels are called vasa vasorum. Nerve-fibres are also supplied 

 to the walls of the arteries. 



