178 THE BLOOD VASCULAR SYSTEM 



due to loss of elasticity resulting from a transformation of elastin 

 into elacin. 



The internal elastic membrane is a layer of elastic tissue, consisting 

 of an intimately united fibrous mass, which completely encircles the 

 artery. In the smaller vessels the elastic fibers of this layer form only 

 a reticulated structure, hut in the larger arteries they are so abundant 

 and so closely interwoven as to form a complete membrane, which can 

 be readily stripped from the subjacent tissue. If the membrane thus 

 prepared is examined microscopically, it will be found to present numer- 

 ous small openings at points where the elastic tissue is deficient. It is 

 this appearance which led to its description as a 'fenestrated membrane/ 

 The internal elastic membrane is intimately united to the tunica media, 

 upon which it rests; in fact, it may perhaps be better considered as the 

 innermost layer of this tunic, for, in the larger arteries, e.g., the aorta, 

 it can only with difficulty be distinguished from the adjacent layers of 

 elastic tissue which form a large portion of the tunica media of these 



The tunica media, or middle coat, contains smooth muscle, sheets 

 of elastic tissue, and a very delicate fibrous connective tissue. The pro- 

 portion of these elements present in any given artery varies with the 

 size of the vessel. Muscular tissue usually predominates, but in the 

 larger arteries elastic tissue is so abundant as to appear quite in excess 

 of the muscular ; in the smaller arteries, however, the muscular tissue is 

 by far the more abundant. 



The smooth muscle fibers are circularly disposed in the wall of the 

 vessel; they are short, of irregularly serrated outline, and are intimately 

 united with one another. Quite frequently the muscle fibers possess short 

 branches which interdigitate with those of neighboring fibers. In the 

 larger vessels they are arranged in layers which alternate with the sheets 

 of elastic tissue. Small bundles of longitudinal smooth muscle fibers 

 are occasionally found in the outer portion of the tunica media. 



The elastic tissue of the middle coat is disposed in membranous 

 sheets which, in the larger vessels, are embedded in a fine fibrillar con- 

 nective tissue. In these vessels, also, the fibro-elastic membranes thus 

 formed alternate with the layers of smooth muscle, throughout the entire 

 thickness of the tunica media. In consequence of the relaxation of the 

 normal arterial tone and the contraction of the muscular wall in rigor 

 mortis, as seen' in the usual preparations, these elastic layers, as well as 

 the internal elastic membrane, are thrown into wavy folds. 



The external coat, tunica adventitia, consists chiefly of fibrous con- 



