CAUSES OF THE VENOUS CIRCULATION. 95 



ure in the vein very much increased, becoming nearly equal to that in the 

 artery. 



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 conditions are capable of increasing very con- 

 siderably the rapidity of the flow in certain veins, and that under certain 

 conditions, the current in some parts of the venous system is very much re- 

 tarded. 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 assumed 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. Near the heart, however, the flow becomes more uni- 

 form and progressively increases in rapidity. 



As the effect of the heart's action upon the venous circulation 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, estimates of the general rapidity of the venous cir- 

 culation or of the variations in different vessels must necessarily be very 

 indefinite. 



CAUSES OF THE VENOUS CIKCULATION. 



In the veins the blood is farthest removed from the influence of the con- 

 tractions of the left ventricle ; and although these are felt, there are many 

 other causes which combine to carry on the venous circulation, and many 

 influences by which it is retarded or obstructed. 



The great and uniform force which operates on the circulation in these 

 vessels is the vis a tergo. Reference has been made to the entire adequacy 

 of the arterial pressure, propagated through the capillaries, to account for 

 the movement of blood in the veins, provided there be no great obstacles 

 to the current. The other forces which concur to produce movement of 

 blood in the veins are the following : 



1. Muscular action, by which many of the veins are at times compressed, 

 thus forcing the blood toward the heart, regurgitation being prevented by 

 the action of the valves. 



2. A suction force exerted by the action of the thorax in respiration, 

 operating, however, only on the veins in the immediate neighborhood of the 

 chest. 



3. A possible influence from contraction of the coats of the vessels 

 themselves. This is marked in the veins near the heart, in some of the in- 

 ferior animals. 



4. The force of gravity, which operates only on vessels which carry blood 

 from above downward to the heart, and a slight suction force which may be 

 exerted upon the blood in a small vein as it passes into a larger vessel in 

 which the current is more rapid. 



The obstacles to the venous circulation are : pressure sufficient to oblit- 

 erate the caliber of a vessel, when, from the free communications with other 



