86 



THE VASCULAR SYSTEM 



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

 sheets of elastic tissue, and a very delicate fibrous connective tis- 

 sue. The proportion of these elements present in any given artery 

 varies with the size of the vessel. Muscular tissue usually pre- 

 dominates, but in the larger arteries elastic tissue is so abundant 

 as to appear quite in excess of the muscular ; in the smaller ar- 

 teries, however, the muscular tissue is by far the more abundant. 



The smooth muscle fibres 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 

 fibres possess short branches which interdigitate with those of 

 neighboring fibres. In the larger vessels they are arranged in 



layers which alternate with the 

 sheets of elastic tissue. Small 

 bundles of longitudinal smooth 

 muscle fibres are occasionally 

 found in the outer portion of the 

 tunica media. 



The elastic tissue of the mid- 

 dle coat is disposed in membra- 

 nous sheets which, in the larger 

 vessels, are embedded in a fine 

 fibrillar connective tissue. In 

 these vessels, also, the fibro-elas- 

 tic membranes thus formed alter- 

 nate with the layers of smooth 



a, tunica intinia, the internal elastic muscle, throughout the entire 

 membrane is prominent ; 6, tunica media, thickness of the tunica media, 

 containing smooth muscle and several T , , . . 



wavy layers of elastic tissue; c, tunica In consequence of the relaxation 



adventitia, containing many transversely of the normal arterial tone and 

 and obliquely cut elastic fibres and much the cont raction of the HlUSCUlar 

 wavy connective tissue. Photo. (Alter 



Magrath.) wall in rigor mortis, as seen in 



the usual preparations, these elas- 

 tic layers, as well as the internal elastic membrane, are thrown 

 into wavy folds. 



The external coat, tunica adventitia, consists chiefly of fibrous 

 connective tissue. Relatively few elastic fibres occur in this coat, 

 and these for the most part lie in its inner portion, adjoining the 

 tunica media. In the larger arteries, when especially abundant, 

 the elastic fibres form an incomplete layer, which may be termed 

 the external elastic membrane. Like the internal elastic membrane, 



FIG. 87. THE EXTERNAL CAROTID ARTERY 

 OF A CHILD. 



