ARTERIES 



87 



this layer might well be considered as belonging to the tunica 

 media, of which coat it would then form the outermost stratum. 



The white fibres of the tunica adventitia are disposed in dense 

 interlacing bundles, to form a firm, unyielding coat. At the 

 periphery of the artery the connective- 

 tissue bundles of the adventitia inter- 

 mingle with those of the adjacent 

 areolar connective tissue, in which the 

 blood vessels are nearly always embed- 

 ded, hence the outer boundary of this 

 coat is usually more or less ill-defined. 



The fibrous bundles of the adven- 

 titia are disposed somewhat obliquely 

 or diagonally about the artery, thus 

 forming a closely felted connective- 

 tissue network. Small blood vessels, 

 both arteries and veins (vasa vasorum), 

 and minute nerve trunks with occa- 

 sional ganglia, occur in this coat. 

 From these vasa et nervi vasorum 

 capillaries and fine nerve fibres are 

 distributed to the muscular coat. No 

 blood vessels are found in the tunica 

 intima. 



General Characteristics of the Arte- 

 rial Wall. The tunica media is almost 

 invariably the thickest of the arterial 

 coats. In the larger vessels the adven- 

 titia is often of nearly equal thickness, 

 but in the medium sized and small vessels it is much thinner. 

 The arterial wall, as a whole, also, is very thick as compared with 

 the lumen of the vessel, and is much thicker than that of a vein 

 of corresponding size. 



The wall of the larger arteries is relatively thinner as compared 

 with the lumen than is the case with the smaller arteries ; in these 

 latter vessels the thickness of the arterial wall often exceeds the 

 diameter of their lumen. In certain small arteries, e. g., those of 

 the liver, even this ratio is exaggerated, the excess of muscular 

 tissue in these vessels resulting in a breadth of wall which may be 

 as much as two or three times that of the lumen. 



The arterial wall contracts firmly in rigor mortis, hence the 



FIG. 88. TBANSECTION OF THE 

 WALL OF THE AORTA OF A 



CHILD. 



The elastic tissue is deeply 

 stained. 1, tunica intima; #, 

 tunica media; 3, tunica adven- 

 titia. Weigert's elastic stain and 

 picro-fuchsin. Photo, x 64. 



