AETERIES 



181 



after death contain but little blood, and because of the density of the 

 tissues which compose their wall, these vessels retain, as a rule, their 

 cylindrical form. 



Large Arteries. The largest arteries differ from the medium-sized 

 type in the excess of elastic tissue and relative deficiency of muscle in 

 their media, the extreme thinness of their adventitia, and the relative 

 thinness of their wall, as a whole, when compared with their lumen. 

 Elastic tissue is especially abundant in all of these vessels; in the media 

 it equals in volume the muscular tis- 

 sue, in the adventitia it forms a dense 

 network of clastic fibers. In the 

 aorta and the pulmonary artery the 

 elastic tissue surpasses the muscular 

 in the media. These vessels lack a 

 distinct internal and external elastic 

 membrane. 



The adventitia of the largest ar- 

 teries is extremely thin, that of the 

 thoracic aorta being not much thicker 

 than its fibrous tunica intima; this 

 coat, therefore, forms but a small por- 

 tion of the vascular wall in vessels of 

 this type. 



Small Arteries. In the small ar- 

 teries the elastic tissue is relatively 

 decreased and the smooth muscle no- 

 ticeably increased. The tunica intima 

 of these vessels is thin, and is limited 



externally by an internal elastic membrane, which stands out promi- 

 nently because of the relative deficiency of elastic tissue in the tunica 

 media. 



In the tunica media of these vessels the plates of elastic tissue which 

 characterize the larger arteries are scarcely to be found. This coat in 

 the small arteries contains very little tissue other than smooth muscle. 



The external elastic membrane is indistinct, and the adventitia is 

 not more than one-half to two-thirds as thick as the tunica media. 



Arterioles. The arterioles possess a relatively thicker wall than any 

 other vessel of the arterial system. Their tunica intima is thin, but little 

 fibrous tissue being contained within it, and the internal elastic mem- 

 brane is represented only by a very incomplete layer of elastic libers. 



FIG. 



200. TRANSECTION OP 

 CELIAC Axis OF MAN. 



a, tunica intima with a prominent 

 internal elastic membrane; b, tunica 

 media, consisting chiefly of smooth 

 muscle; c, external elastic membrane 

 in the inner portion of the tunica ad- 

 ventitia. Photo. (After Magrath.) 



