FUNCTION OF THE VALVES. 325 



distended when the hand is allowed to hang by the side, and 

 the blood has to mount up against the force of gravity. 



In the extreme irregularity in the rapidity of the circula- 

 tion in different veins, it must frequently happen that a ves- 

 sel empties its blood into another of larger size, in which the 

 current is more rapid. In such an instance, as a physical 

 necessity, the more rapid current in the larger vessel exerts 

 a certain suction force on the fluid in the vessel which 

 joins with it. 



Function of the Valves. 



"With our present knowledge, it is difficult to compre- 

 hend how any anatomist could have accurately described 

 the valves of the veins, and yet be ignorant of their function ; 

 and the fact that their use was not understood before the 

 description of the circulation by Harvey, shows the greatness 

 of this as a discovery, and the shallow character of any pre- 

 tence that men of science had any idea of the motion of the 

 blood before his time. 



With our present knowledge of the course of the blood, 

 it is evident that the great function of the valves is in pre- 

 senting an obstacle to the reflux of blood toward the capil- 

 lary system ; and it only remains to study the conditions 

 under which they are brought into action. 



There are two distinct conditions under which the valves 

 of the veins may be closed. One of them is the arrest of cir- 

 culation, from any cause, in veins in which the blood has to 

 mount against the force of gravity ; and the other, compres- 

 sion of veins, from any cause (generally from muscular con- 

 traction) which tends to force the blood from the vessels 

 compressed into others, when the valves offer an obstruction 

 to a flow toward the capillaries, and necessitate a current in 

 the direction of the heart. 



In the first of these conditions, the valves are antagonistic 

 to the force of gravity, and, when the caliber of any vessel is 



