CHAP, iv.] THE VASCULAR MECHANISM. 197 



the inner coat is thrown into longitudinal wrinkles or folds, so 

 that in transverse sections of an artery in this condition the inner 

 coat has a characteristic puckered appearance. 



The inner coat is somewhat delicate, and easily torn, so that 

 in injuries to arteries, as when an artery is forcibly ligatured, it is 

 apt to be broken. 



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 felt works 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 ; the inner coat is probably nourished directly by the 

 blood in the artery itself. Nerves, consisting chiefly of non-medul- 

 lated 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 



