326 CIRCULATION. 



temporarily obliterated, aid in directing the current into an- 

 astomotic vessels. It is but rarely, however, that they act 

 thus in opposition to the force of gravity ; and it is only 

 when many of the veins of a part are simultaneously com- 

 pressed that they aid in diverting the current. When a sin- 

 gle vein is obstructed, it is not probable that the valves are 

 necessary to divert the current into other vessels, for this 

 would take place in obedience to the vis a tergo / but when 

 many veins are obstructed in a dependent part, and the 

 avenues to the heart become insufficient, the numerous 

 valves divide the columns of blood, so that the pressure is 

 equally distributed through the extent of the vessels. For it 

 must be remembered, the strength of the walls diminishes as 

 we pass from the larger veins to the periphery, and the small- 

 est vessels, which, were it not for the valves, would be sub- 

 jected to the greatest amount of pressure, are least calculated 

 to bear distention. This is but an occasional function which 

 the valves are called upon to perform ; and it is evident that 

 their influence is only to prevent the weight of the entire 

 column of blood, in vessels thus obstructed, from operating 

 on the smallest veins and the capillaries. It cannot make 

 the labor of the heart, when the blood is again put in mo- 

 tion, any less than if the column were undivided, as this 

 organ must have sufficient power to open successively each 

 set of valves, when, of course, they cease to have any influ- 

 ence whatsoever. 



It is in connection with the intermittent compression of 

 the veins that the valves have their principal and almost con- 

 stant function. Their situation alone would lead to this sup- 

 position. They are found in greatest numbers throughout 

 the muscular system, having been demonstrated by Sappey 

 in the smallest venules. They are also found in the upper 

 parts of the body, where they certainly do not operate against 

 the force of gravity, while they do not exist in the cavities, 

 where the venous trunks are not subject to compression. It 

 has already been made sufficiently evident that the action of 



