112 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



non-elastic kind, which mingle with the adjacent are- 

 olar tissue with which arteries are nearly always as- 

 sociated. These bundles often serve as support to 

 organs, by which the latter are more firmly an- 

 chored. From such a union between the vena cava 

 and abdominal aorta to the liver this organ receives 

 a substantial support. The kidneys and ovaries 

 are organs that may be cited as benefiting greatly 

 by such a connection. A considerable amount of 

 fat is often present in the adventitia, also connective- 

 tissue cells, nerves, a few smooth muscle fibers, 

 lymphatics, and blood-vessels. The latter are 

 called vasa vasorum, and play an important part in 

 the nourishment of the arterial wall. The vasa 

 vasorum are sub-branches derived usually from 

 some small branch of an adjacent artery, but may 

 come directly from a small branch of the same artery 

 which is given off at a higher point. 



As stated before, large arteries have relatively a 

 large amount of elastic fibers and a small amount of 

 smooth muscle. The aorta has scarcely any muscle. 

 In the small arteries the reverse is true. The wall 

 of large arteries is relatively thinner than that of 

 small ones. The reverse is true of the intima. In 

 large arteries the adventitia is also relatively scant, 

 while in the smaller ones the adventitia may be one- 

 half to two-thirds the thickness of the media. On 

 account of the rigid and elastic arterial wall these 

 vessels are usually empty after death, contracted 

 but retain their normal shape; while veins, on the 

 other hand, collapse and usually contain a certain 

 amount of blood. 



