USES OF THE VALVES OF THE VEINS 



79 



which the blood passes to the heart, while they are distended when the 

 hand is allowed to hang by the side and the blood has to rise against 

 the force of gravity. 



Some physiologists are of the opinion that the right ventricle exerts 

 an active suction force during its diastole ; but experiments on animals 

 do not sustain this view, and if such a force be exerted, its effect on the 

 circulation, even in the veins near the heart, must be very slight. In the 

 great irregularity in the rapidity of the circulation in different veins, it 

 must frequently happen that a vessel 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 large vein exerts a 

 certain suction force on the blood in the smaller vessel. 



USES OF THE VALVES OF THE VEINS 



There are two distinct conditions under which the valves of the veins 

 may be closed. One is the arrest of circulation, from any cause, in veins 

 in which the blood has to rise against the force of gravity ; and the 

 other, compression of veins, from any cause usually from muscular 

 contraction which tends to force the blood from the vessels com- 

 pressed, 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 a vessel is temporarily obstructed, they aid in directing 

 the current into anastomosing 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 compressed that they aid 

 in diverting the current. When a single vein is obstructed, it is not prob- 

 able 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 valves divide the columns of blood, so that the 

 pressure is equally distributed throughout the extent of the vessels. 

 This is, however, but an occasional action of the valves ; and it is evi- 

 dent that their influence is only to prevent the weight of the entire col- 

 umn of blood from operating on the smallest veins and the capillaries. 



It is in connection with the intermittent compression of the veins 

 that the valves have their principal use. Their situation alone leads to 

 this supposition. They are found in greatest numbers throughout the 

 muscular system, having been demonstrated in vessels one line (2.1 milli- 

 meters) in diameter. They are also found in the upper parts of the body, 

 where they certainly do not operate against the force of gravity, while 



