VALVES OF VEINS. 169 



many they are absent altogether. They are most numerous 

 in the veins of the extremities, and more so in those of the 

 leg than the arm. They are commonly absent in veins of 

 less than a line in diameter, and, as a general rule, there 

 are few or none in those which are not subject to muscular 

 pressure. Among those veins which have no valves may be 

 mentioned the superior and inferior vena cava, the trunk 

 and branches of the portal vein, the hepatic and renal 

 veins, and the pulmonary veins ; those in the interior 

 of the cranium and vertebral column, those 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 sufficient 

 to complete its passage through the veins, in which the 

 obstructions to its movement are very slight. For the for- 

 midable obstacle supposed to be presented by the gravita- 

 tion of the blood, has no real existence, since the pressure 

 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 con- 

 tain 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. 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 



