AETERIES 



179 



nective tissue. Relatively few clastic fibers 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 fibers form 

 an incomplete layer, which may be termed the external elastic membrane. 

 Like the internal elastic membrane, this layer might well be considered as 



FIG. 197. THE EXTERNAL CAROTID 

 ARTERY OP A CHILD. 



a, tunica intima, the internal elastic 

 membrane is prominent; b, tunica 

 media, containing smooth muscle and 

 several wavy layers of elastic tissue; 

 c, tunica adventitia, containing many 

 transversely and obliquely cut elastic 

 fibers and much wavy connective tissue. 

 Photo. (After Magrath.) 



JtH'Vr 



FIG. 198. TRANSECTION OF THE WALL 

 OP THE AORTA OF A CHILD. 



The elastic tissue is deeply stained. 

 1, tunica intima; 2, tunica media, 8, 

 tunica adventitia; 4, areolar connective 

 tissue. Weigert's elastic tissue stain and 

 picro-fuchsin. Photo. X 64 



belonging to the tunica media, of which coat it would then form the 

 outermost stratum. 



The collagenous fibers 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 intermingle 

 with those of the adjacent areolar connective tissue, in which the blood- 

 vessels are nearly always embedded, hence the outer boundary of this 

 coat is usually more or less ill defined. 



The fibrous bundles of the adventitia are disposed somewhat obliquely 

 or diagonally about the artery, thus forming a closely felted connective 



