682 SPECIAL PHYSIOLOGY. 



columns of water in a U-shaped tube. With regard to the vessels 

 above the heart, they also form a double closed system, and the ad- 

 vantage of gravity on the venous side is, so far as the heart's action is 

 concerned, counterbalanced by the disadvantage on the arterial side. 



Gravity, however, does actually affect the circulation, through its 

 influence on the circulatory organs, especially. on the capillaries and 

 veins ; for these vessels are not rigid, like a U-shaped tube, but yield- 

 ing. The weight of the entire column of venous blood, for example, 

 is supported by the coats of the veins, those of the lower limbs having 

 more weight to bear than the veins of the trunk, and these again 

 more than the veins of the upper limbs,. neck, and head. Hence, the 

 coats of the veins, in the lower limbs, especially those of the less sup- 

 ported subcutaneous veins, are proportionally thicker than those in 

 the upper parts of the body, the coats of the jugular vein being very 

 thin, and those of the saphenous vein very thick, in proportion to 

 their size. Hence, too, the valves are more numerous, and of greater 

 strength, in the veins of the lower limbs than in those of the upper 

 limbs ; whilst in the neck, they exist only in the neighborhood of the 

 chest. The mechanical effect of these valves is to save the entire 

 length of the vein from the total pressure of the venous column, and 

 to divide it into shorter subordinate columns, into which, however, 

 weight is still transmitted by the collateral veins. When the valves 

 of the veins of the lower limbs are weakened, and no longer close per- 

 fectly, those vessels become distended, arid varicose. If the tonicity 

 and elasticity of the smaller veins be impaired, or overcome, by pro- 

 longed over-distension, from obstructions to the return of blood from 

 them, or by general debility, the fluid part of the blood is liable to 

 escape through the coats of the capillaries and minute veins, so as to 

 cause dropsy. 



The rate of motion of the blood in the veins is much quicker than 

 that in the capillaries ; but not so quick as in the arteries. In the 

 jugular vein of a dog. the rate of motion has been estimated at fifth 

 inches per second. (Volkmann.) Considered generally, the average 

 velocity of the blood in the veins is said to be from J to J of that of 

 the blood in the corresponding arteries ; this estimate is founded on 

 the supposed relative capacity of the venous, as compared with the 

 arterial system, which is believed to be as 2 or 3 to 1. As the veloc- 

 ity of the arterial blood diminishes in the smaller arteries, partly in 

 consequence of friction, but also owing to the increased capacity of 

 the branches in comparison with the trunks, so inversely, as the veins 

 diminish in capacity from their branches to their trunks, the velocity 

 of the blood in them increases as it approaches nearer and nearer to 

 the heart, and, in the larger veins, becomes ,equal to about fths, or more, 

 of the velocity in the corresponding arteries. The form of the entire 

 vascular system has indeed been likened to two bent cones, joined at 

 their apices in the heart, and at their bases in the capillary system. 

 The quantity of blood received by the right auricle, closely agrees 

 with that thrown from the left ventricle ; hence, therefore, the veloc- 

 ity of the venous current, as it enters the right auricle, must be less 

 than that of the arterial blood passing through the aortic orifice ; for 



