316 CIRCULATION. 



these are felt, there are many other causes which combine to 

 carry on the circulation, and many influences by which it is 

 retarded or obstructed. 



The great and uniform force which operates on the circu- 

 lation in these vessels is the vis a tefgo. We have repeatedly 

 referred to the entire adequacy of the arterial pressure, prop- 

 agated through the capillaries, to account for the movement of 

 blood in the veins, provided there be no very 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 circula- 

 tion ; and the only question which arises is whether there be 

 any force exerted in the capillaries themselves which is super- 

 added to the force of the heart. In discussing the capillary 

 circulation, there has been found no direct proof of the exist- 

 ence of a distinct " capillary power " influencing the move- 

 ment of blood in these vessels ; and consequently all the vis 

 a tergo operating on the circulation in the veins must be 

 attributed to the action of the left ventricle. 



The other forces which concur to produce movement of 

 blood in the veins are : 



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 in the contraction of the coats of 

 the vessels themselves. This is marked in the veins near the 

 heart, in some of the inferior animals. 



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

 which carry blood from above downward to the heart ; and 

 a little 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 obliterate the caliber of a vessel, when, from the 



