TWO SETS OF VEINS. 241 



tissue restores the artery nearly to its former dimensions, so 

 that, though empty, the arteries are not found actually closed 

 some time after death. 



This vital contractility in the arteries seems to have its seat 

 in the soft pale fibres of the middle coat. This property is 

 readily demonstrated in the small-sized arteries of the trans- 

 parent parts in cold-blooded animals. Some physiologists 

 deny the existence of this property in the large arteries, but, 

 as it would seem, without sufficient reason. 



The veins in most regions of the body are larger and more 

 numerous than the arteries. The capacity of the venous 

 system thus much exceeds that of the arterial system. The 

 capacity of the venous system of the lungs does not come 

 under this rule ; that is, the capacity of the venous system 

 of the lungs which, however, contains arterial blood does 

 not exceed that of the system of the pulmonary artery. 



There are two sets of veins the vena comites, which gene- 

 rally take a course corresponding to that of the arteries ; and 

 the superficial or subcutaneous veins, so conspicuous in the 

 skin of the horse when heated with exercise. The anasto- 

 moses between the veins are more numerous than between the 

 arteries. The veins have thinner coats than the arteries, and 

 collapse when cut across. The veins, nevertheless, are pos- 

 sessed of considerable strength, being, according to some 

 authorities, stronger than arteries of the same calibre. As to 

 the number of their coats there is little agreement ; three coats 

 are, however, commonly admitted. 



Some traces of a vital contractility analogous to that of the 

 arteries have been detected, particularly in the great veins near' 

 the auricles of the heart ; but little has been ascertained of cer- 

 tain on this point. 



The most remarkable peculiarity in the veins is the presence 

 of valves adapted to prevent the reflux of the blood. The veins 



Q 



