104 CIRCULATION OF THE BLOOD. 



In the extreme 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 larger vessel exerts a certain suction force on the fluid in the vessel 

 which joins with it. 



Function of the Valves of the Veins. 



It is difficult to comprehend, at the present day, how any anatomist could have accu- 

 rately described the valves of the veins and yet have been ignorant of their function ; and 

 the fact that their use was not understood before the description of the circulation by Har- 

 vey shows the greatness of this as a discovery and the shallow character of any pretence 

 that men of science had any definite 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 to present an obstacle to the reflux of blood toward the 

 capillary 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 circulation, from any cause, in veins in which the blood has 

 to mount against the force of gravity ; and the other, compression of veins, from any 

 cause (generally from muscular contraction) 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 temporarily obliterated, they aid in directing the current into anastomotic 

 vessels. It is but rarely, however, that they act thus in opposition to the force of grav- 

 ity ; 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 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 depend- 

 ent part, and the avenues to the heart become insufficient, the numerous valves divide 

 the columns of blood, so that the pressure is equally distributed throughout 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 smallest vessels, which, were 

 it not for the valves, would be subjected 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 motion, 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 influence whatsoever. 



It is in connection with the intermittent compression of the veins that the valves 

 have their principal and almost-constant function. Their situation alone would lead to 

 this supposition. 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 com- 

 pression. It has already been made sufficiently evident that the action of muscles sec- 

 onds most powerfully the contractions of the heart. The vis a tergo from the heart is, 

 doubtless, generally sufficient to turn this influence of muscular compression from the 

 capillary system, and the valves of the veins are open ; but they stand ready, neverthe- 

 less, to oppose any tendency to regurgitation. 



In the action of muscles, the skin is frequently stretched over the part, and the cuta- 



