88 



THE VASCULAE SYSTEM 



arteries 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. 



The Large Arteries. The largest arteries differ from the me- 

 dium-sized type in the excess of elastic tissue and relative defi- 

 ciency 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 exceeds in volume the mus- 

 cular tissue, in the adventitia it forms a dense network of elastic 

 fibres. 



The adventitia of the largest arteries is extremely thin, that of 

 the thoracic aorta being not much thicker than its fibrous tunica 

 intima ; this coat, therefore, forms but a small portion of the vas- 

 cular wall in vessels of this type. In the medium-sized vessels, 



e. g., the iliac arteries, the tunica 

 adventitia more nearly approaches 

 the media in thickness. 



In the small arteries the elas- 

 tic tissue is relatively decreased 

 and the smooth muscle notice- 

 ably increased. The tunica in- 

 tima of these vessels is thin, and 

 is limited externally by an in- 

 ternal elastic membrane, which 

 stands out prominently because 

 of the relative deficiency of elas- 

 tic 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. 



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 



FIG. 89. TKANSECTION OF THE CCELIAC 

 AXIS OF MAN. 



a, tunica intima with a prominent in- 

 ternal elastic membrane ; 6, tunica media, 

 consisting chiefly of smooth muscle ; c, 

 external elastic membrane in the inner 

 portion of the tunica adventitia. Photo. 

 (After Magrath.) 



