CHAP, iv.] THE VASCULAR MECHANISM. 195 



The middle coat, which is generally many times thicker than 

 the inner coat, consists of elastic layers and muscular layers 

 placed in more or less regular alternation. The muscular layers 

 consist of bands of plain muscular fibres placed transversely and 

 united together by a very small amount of white connective tissue. 

 The elastic layers consist of somewhat thick fenestrated membranes 

 or of feltworks of elastic fibres running on the whole longitudinally, 

 but not unfrequently more or less obliquely ; these are also bound 

 together by a small quantity of white connective tissue. 



The outer coat consists of feltworks of elastic fibres, or in 

 some instances of fenestrated membranes, disposed chiefly longi- 

 tudinally, and separated by bundles of ordinary white connective 

 tissue, which become more and more predominant in the outer 

 portions of the coat. In many arteries bands of plain muscular 

 fibres are present in this coat also, and then run for the most part 

 but not exclusively in a longitudinal direction. 



Blood vessels for the nourishment of the tissue of the walls 

 (vasa vasorum) are present in the larger arteries, being most 

 abundant in the outer coat, but penetrating for some distance into 

 the middle coat. Nerves, consisting chiefly of non-medullated 

 fibres, may be traced through the outer coat into the middle coat 

 where they appear to end in connection with the muscular fibres. 



Lastly, in the case of most large arteries, the bed of connective 

 tissue in which the artery runs, is formed into a more or less 

 distinct sheath. In this sheath, the white connective tissue is 

 much more abundant than are the yellow elastic elements, so that 

 the sheath is far less elastic than the artery. Hence, when an 

 artery and its sheath are completely cut across, the artery is, by 

 elastic shrinking, retracted within its sheath. 



The most important structural features of a large artery may 

 then be summed up, by saying that the artery consists of a thin 

 inner coat consisting of an epithelioid lining resting on an elastic 

 basis of no conspicuous thickness, of a thick middle coat consisting 

 partly of muscular fibres disposed for the most part transversely, 

 and partly of stout elastic elements, this coat being the thickest 

 and most important of all three coats, and of an outer coat of 

 variable thickness consisting chiefly of elastic elements intermixed 

 with an increasing amount of white connective tissue. 



All arteries possess the above features. It may further be 

 said, that as a general rule the muscular element bears a larger 

 proportion to the elastic element in the smaller than in the larger 

 arteries, that is to say, the smaller arteries are more conspicuously 

 muscular, and the larger arteries more conspicuously elastic. It 

 must be remembered however that the several arteries of the body 

 differ considerably in minor features, such as the relative disposition 

 and amount of muscular and elastic elements in the middle coat, 

 the amount of muscular tissue in the outer coat, and the proportion 

 of white connective tissue present and the like ; in the aorta for 



132 



