RAPIDITY OF THE CURRENT OF BLOOD IN THE VEINS 75 



at different times. As a rule, it is in an inverse ratio to the arterial pres- 

 sure. Whatever favors the passage of blood from the arteries into the 

 capillaries has a tendency to diminish the arterial pressure, and as it 

 increases the quantity of blood passing into the veins, it must increase 

 the venous pressure. The great capacity of the venous system, its 

 frequent anastomoses and the presence of valves which may shut off a 

 portion from the rest, are conditions that involve considerable varia- 

 tions in pressure in different vessels. Muscular effort has a decided 

 influence on the force of the circulation in certain veins and produces 

 an elevation in the pressure. As the reduced pressure in the veins is 

 due in a measure to the great relative capacity of the venous system 

 and the free communications between the vessels, it would seem that 

 if it were possible to reduce the capacity of the veins in a part and 

 force all the blood to pass to the heart by a single vessel corresponding 

 to the artery, the pressure in this vessel would be greatly increased. 

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

 the veins coming from a part, except one which had the volume of the 

 artery by which the blood was supplied, forcing all the blood to return 

 by this single channel. This being done, he found that the pressure in 

 the vein was much increased, becoming nearly equal to the pressure in 

 the artery. 



RAPIDITY OF THE CURRENT OF BLOOD IN THE VEINS 



It is impossible to fix on any definite rate as representing the ra- 

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

 conditions are capable of increasing very considerably the rapidity of 

 the flow in certain veins, and that the current in some parts of the 

 venous system may be much retarded. Undoubtedly, the general 

 movement of blood in the veins is much slower than in the arteries, 

 from the fact that the quantity of blood is greater. If it is 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 uniform and 

 progressively increases in rapidity, as the quantity of blood received by 

 the heart is equal to the quantity discharged into the arteries. 



As the effect of the heart's action on the venous circulation is subject 

 to many modifying influences through the small arteries and capillaries, 

 and as there are other forces influencing the current, that are by no 

 means uniform in their operation, estimates of the general rapidity of 

 the venous circulation or of the variations in different vessels must 

 necessarily be indefinite. 



