CAUSES OF THE VENOUS CIRCULATION. 315 



capacity of the veins in a part, and force all the blood to 

 pass to the heart by a single vessel corresponding to the ar- 

 tery, the pressure in this vessel should be greatly increased. 

 Poiseuille has shown this to be the fact by the experiment of 

 ligating all the veins coming from a part, except one, which 

 had the volume of the artery by which the blood was sup- 

 plied, forcing all the blood to return by this single channel. 

 This being done, he found the pressure in the vein immensely 

 increased, becoming nearly equal to that in the artery. 1 



Rapidity of the Venous Circulation. It is impossible to 

 fix upon any definite rate as representing the rapidity of the 

 current of blood in the veins. It will be seen that various 

 circumstances are capable of increasing very considerably the 

 rapidity of the flow in certain veins, and that under certain con- 

 ditions the current in some parts of the venous system is very 

 much retarded. Undoubtedly the general movement of blood 

 in the veins is very much slower than in the arteries, from 

 the fact that the quantity of blood is greater. If it be as- 

 sumed that the quantity of blood in the veins is double that 

 contained in the arteries, the general average of the current 

 would be diminished one-half. As we near the heart, how- 

 ever, the flow becomes more uniform, and progressively in- 

 creases in rapidity. 



As the effect of -the heart's action upon the venous circu- 

 lation is subject to so many modifying influences through the 

 small arteries and capillaries, and as there are other forces 

 influencing the current, which are by no means uniform in 

 their action, with our present knowledge, estimates of the 

 general rapidity of the venous circulation, or the variations 

 in different vessels, would be founded on mere speculations. 



Causes of the Venous Circulation. 



In the veins, the blood is farthest removed from the influ- 

 ence of the contractions of the left ventricle ; and though 



1 BERARD, Cours de Physiologic, Paris, 1855, tome iv., p. 21. 



