100 CIRCULATION OF THE BLOOD. 



the rest, are circumstances which involve great variations in pressure in different vessels. 

 It has been ascertained that, as a rule, the pressure is diminished as we pass from the 

 periphery toward the heart. In an observation on the calf, Volkmann found that, with a 

 pressure of about 6*5 inches of mercury in the carotid, the pressure in the metatarsal vein 

 was 1*1 inch, and but 0*36 in the jugular. Muscular effort has a marked 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 ligating 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 the pressure in the vein immensely increased, becom- 

 ing 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 

 circumstances are capable of increasing very considerably the rapidity of the flow in cer- 

 tain veins, and that, under certain conditions, 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 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. As we near 

 the heart, however, the flow becomes more uniform 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, 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 influence of the contractions of 

 the left ventricle ; and, although these are felt, there are many other causes which com- 

 bine to carry on the circulation, and many influences by which it is retarded or ob- 

 structed. 



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

 vis a tergo. We have repeatedly referred 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. There are no facts which lead us to 

 doubt the operation of this force as the prime cause of the venous circulation ; and the 

 only question which arises is whether there be any force exerted in the capillaries them- 

 selves which is superadded to the force of the heart. In discussing the capillary circu- 

 lation, we stated that there is no direct proof of the existence of a distinct " capillary 

 power " influencing the movement of blood in these vessels ; and consequently the 

 vis a tergo operating on the circulation in the veins must be attributed mainly to the ac- 

 tion of the left ventricle. 



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. 



