PRESSURE IN VEINS. 143 



of the columns of fluid, neither of them can move of its own 

 weight, each being supported by the other ; yet the least pres- 

 sure on the top of either column will lift up the other ; so, 

 when the body is erect, the least pressure on the column of 

 arterial blood may lift up 'the venous blood, and, were it not 

 for the valves, the least pressure on the venous might lift up 

 the arterial column. 



In experiments to determine what proportion of the force of 

 the left ventricle remains to propel the blood in the veins, Valen- 

 tin found that the pressure of the blood in the jugular vein of 

 a dog, as estimated by the hsemadynamometer, did not amount 

 to more than j\ or y 1 ^ of that in the carotid artery of the same 

 animal ; and this estimate 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, con- 

 sumed during the passage of the blood through the capillaries. 

 But slight as this remaining force might be (and the experi- 

 ment in which it was estimated would reduce the force of the 

 heart below its natural standard), it would be enough to com- 

 plete 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 ex- 

 plained. 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, inextensile both in themselves 

 and at their margins of attachment, 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, whatever 

 force is exercised by the pressure of the muscles on the veins, 

 is distributed partly in pressing the blood onwards in the 

 proper course of the circulation, and partly in pressing it back- 

 wards and closing the valves behind. 



The circulation might lose as much as it gains by such com- 

 pression of the veins, if it were not for the numerous anas- 



