76 CIRCULATION OF THE BLOOD 



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 that combine to carry on the venous circulation, 

 and many influences by which it is retarded or obstructed. The prin- 

 cipal and uniform force operating on the circulation in these vessels is 

 the vis a tergo. Reference has been made to the adequacy of the 

 arterial pressure, extending 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 com- 

 pressed, 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 inspiration, 

 operating, however, only on 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 inferior animals. 



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

 blood from above downward to the heart, and a slight suction force 

 which may be exerted on 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 calibre of a vessel, when, from the free communications 

 with other vessels, the current is diverted into another channel ; ex- 

 piratory efforts; the contractions of the right side of the heart; and 

 the force of gravity, which operates, in the erect posture, on the cur- 

 rent in all excepting the veins of the head, neck and parts of the trunk 

 above the heart. 



Influence of Muscular Contraction. That the action of muscles has 

 considerable influence on the current of blood in the veins situated be- 

 tween them and in their substance, has long been recognized ; and this 

 action is so marked, that the vessels distributed to muscular tissue have 

 been compared by Chassaignac to a sponge full of liquid, vigorously 

 pressed by the hand. It must be remembered, however, that although 

 the muscles are capable of acting on the blood contained in veins in 

 their substance with great vigor, the heart is fully competent to carry on 

 the venous circulation without their aid ; a fact exemplified in the venous 

 circulation in paralyzed parts. 



