THE CIRCULATION - 311 



them may be lessened and the speed therefore as great as possible. They 

 are situated oftentimes superficially, because, from the teleological point 

 of view at least, injury to them from without is much less dangerous than 

 a similar wound of an artery would be, owing to the relatively small 

 pressure within them. The veins are furnished with valves lest local 

 conditions of muscular compression or pressure from other causes would 

 interfere with the round of the blood in a prompt and complete manner. 

 Whatever the source of external compression, when valves are present 

 the circulation is not hindered but rather helped. Another use of the 

 valves is seen especially in the legs : were they not present the veins of the 

 feet would have to support a column of blood a meter or more high. 

 This would not only fill them so full as to impede the movements of the 

 blood by friction, but it would compel an unnecessary resistance for the 

 heart and the other forces causing the circulation to overcome. As it is, 

 no portion of the great leg-veins supports more than a very short column 

 of blood even when the man is standing, for the support is divided among 

 many valves. 



