ANGIOLOGY 211 



The adventitia is composed of loose connective tissue with some 

 elastic fibers. A few smooth muscle cells are present, which, as are 

 also the elastic fibers, are arranged longitudinally. The adveniitia, 

 blending with the connective tissue surrounding the arteries, serves 

 to anchor the vessels to the surrounding structure. 



Structure of the Veins. In many respects the walls of the veins 

 resemble those of the arteries. The same three coats exist and the 

 same elements enter into their structure. The transition from capil- 

 lary to small veins, and from those to larger veins, is similar to the 

 transition from the arteries to capillaries, in inverse order. The 

 walls are not so thick as those of arteries. The elastic tissue is 

 much less in quantity and in smaller veins disappears. There is 

 not a clear line of demarcation between the intima and the media. 



The veins of birds differ from those of mammals in that they have 

 fewer valves. The valves are also less perfect, and often permit a 

 backward flow of blood. 



The walls of the arteries and of the veins are supplied with nutri- 

 ent blood-vessels. These are called the vasa vasorum, or blood-ves- 

 sels of the blood-vessel wall. They are mostly in the adventitia. 

 They may arise from the vessel to which they are distributed or take 

 origin from an adjacent vessel. These small arteries supplying the 

 vessel coat after terminating into capillaries form small veins through 

 which the blood, from the structure of the vessel wall, is returned. 



The walls of the blood-vessels are supplied with both medullated 

 and non-medullated nerves. The non-medullated nerve fibers are 

 axones of the sympathetic neurones and control the caliber of the 

 vessels. These fibers, are called the vasomotor nerves. They form 

 plexuses in the adventitia, from which are given off branches which 

 penetrate the media and terminate on the muscle cells. The medul- 

 lated nerves are the axones of the spinal nerves. The larger fibers 

 are found in the connective tissue outside the adventitia and give 

 off branches to the media where they divide repeatedly, lose their 

 sheath, and terminate in the media and at times in the intima. 



TKE ARTIHIAL TIUNKS 



The common aorta is short; it originates from the left ventricle 

 (Fig. 61.6, No. K, 14), and is guarded by three semi-lunar valves. The 

 aorta breaks through the pericardium just to the right of the pul- 

 monary arteries in a ventral direction; it then turns upward dorsally 

 and to the right of the inferior bodies of the vertebrae. It is then 



