(T/O THE CIRCULATION. 



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

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

 pressure 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, 

 Valentin found that the pressure of the blood in the jugular 

 vein of a dog, as estimated by the haemadynamometer, did 

 not amount to more than JL- or -^ of that in the carotid 

 artery of the same animal ; and this estimate is confirmed, 

 in the instances of several other arteries and their corre- 

 sponding 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 re- 

 maining 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 

 dt 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, inextensile both in 



