i8o THE CIRCULATION. 



of the bones, and the trunk and branches of the umbilical 

 vein are also destitute of valves. 



The principal obstacle to the circulation is already over- 

 come when the blood has traversed the capillaries ; and 

 the force of the heart- which is not yet consumed, is suffi- 

 cient to complete its passage through the veins, in which 

 the obstructions to its movement are very slight. For the 

 formidable obstacle supposed to be presented by the gravi- 

 tation of the blood, has no real existence, since the pres- 

 sure exercised by the column of blood in the arteries, will 

 be always sufficient to support a column of venous blood 

 of the same height as itself: the two columns mutually 

 balancing each other. Indeed, so long as both arteries 

 and veins contain continuous columns of blood, the force 

 of gravitation, whatever be the position of the body, can 

 have no power to move or resist the motion of any part of 

 the blood in any direction ; as if one had a circular tube 

 full of fluid at every part, the fluid might be made to 

 circulate with equal facility in either direction, or in any 

 position of the tube. The lowest blood-vessels have, of 

 course, to bear the greatest amount of pressure ; the 

 pressure on each part being directly proportionate to the 

 height of the column of blood above it : hence their 

 liability to distension. But this pressure bears equally on 

 both arteries and veins, and cannot either move, or resist 

 the motion of, the fluid they contain, so long as the columns 

 of fluid are of equal height in both, and continuous. Their 

 condition may, in this respect, be compared with that of a 

 double bent tube full of fluid held vertically ; whatever be 

 the height and gravitation of 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. 



