PRESSURE IN VEINS. 181 



In experiments to determine what proportion of the 

 force of the left ventricle remains to propel the blood in 

 the veins, Valentin found that the pressure of the blood in 

 the jugular vein of a dog, as estimated by the heemadyna- 

 mometer, did not amount to more than T T T or ^ of that 

 in the carotid artery of the same animal ; and this esti- 

 mate is confirmed, in the instances of several other arteries 

 and their corresponding veins, by Mogk. In the upper 

 part of the inferior vena cava, Valentin could scarcely 

 detect the existence of any pressure, nearly the whole force 

 received from the heart having been, apparently, consumed 

 during the passage of the blood through the capillaries. 

 But slight as this remanent force might be (and the 

 experiment in which it was estimated would reduce the 

 force of the heart below its natural standard), it would be 

 enough to complete the circulation of the blood ; for, as 

 already stated, the spontaneous dilatation of the auricles 

 and ventricles, though it may not be forcible enough to 

 assist the movement of blood into them, is adapted to offer 

 to that movement no obstacle. 



Very effectual assistance to the flow of blood in the veins 

 is afforded by the action of the muscles capable of pressing 

 on such veins as have valves. 



The effect of muscular pressure on such veins may be thus 

 explained. "When pressure is applied to any part of a 

 vein, and the current of blood in it is obstructed, the 

 portion behind the seat of pressure becomes swollen and 

 distended as far back as to the next pair of valves. These, 

 acting like the arterial valves, and being, like them, inex- 

 tensile both in themselves and at their margins of attach- 

 ment, do not follow the vein in its distension, but are 

 drawn out towards the axis of the canal. Then, if the 

 pressure continues on the vein, the compressed blood, 

 tending to move equally in all directions, presses the valves 

 down into contact at their free edges, and they close the 

 vein and prevent regurgitation of the blood. Thus, what- 



