THE CIRCULATION. 279 



TUNICA INTIMA. The tunica intima of the arteries is the thin- 

 nest coat, the most transparent and elastic. These properties per- 

 mit the caliber of the artery to be enlarged without any great danger of 

 rupturing its walls. 



TUNICA MEDIA. It is composed of two varieties of tissue: (1) 

 muscular and (2) elastic. 



The unstriped muscular fibers run in a circular direction around 

 the vessels. In the large arteries there is a predominance of elastic 

 tissue; in the arterioles there is no elastic, but muscular tissue. The 

 contractility of the arteries depends upon the muscular tissue. Where 

 there is an excess of elastic tissue there is very little muscular tissue 

 in the blood-vessel, and where the elastic tissue is at a minimum 

 there is a maximum of muscular tissue. 



TUNICA ADVENTITIA. This coat is composed of bundles of con- 

 nective tissue with some elastic tissue. 



VASA VASOHUM. Like every other tissue, the wall of the ves- 

 sels needs nutritive supplies. This is supplied by -small capillaries 

 which run only in the tunica adventitia of the blood-vessel. To these 

 vessels has been given the name of vasa vasorum. 



VEINS. 



Like the arteries, veins are branching tubes ; but they are larger, 

 more numerous, and as a consequence have more capacity to hold 

 blood. Veins have their beginnings in the capillary vessels, which 

 by gradually uniting form the small veins. These small veins unite 

 to form larger ones, the vena3 cava3, which empty into the right 

 auricle. The veins have about three times the capacity of the arteries. 

 The veins consist of a superficial and deep set, the former not 

 associated with the arteries and being subcutaneous, the deep set 

 usually running along the side of the artery and hence called venae 

 comites. Anastomoses between the veins of large size are more fre- 

 quent than in the corresponding arteries. The veins, like the arteries, 

 have an external, a middle, and an internal coat. The coats of the 

 veins are much thinner than the coats of the arteries, and when 

 divided the veins collapse, while the arteries, divided, stand open. 

 The walls of the veins are inelastic. 



Valves. The chief feature of the veins are the valves, which are 

 so arranged as to prevent the blood from flowing backward. The 

 valves ordinarily are in pairs opposite each other and are formed of 

 crescent-shaped doublings of the lining membrane of the veins, with 

 some interposed fibre-elastic tissue. The valves are directed toward 



