oi2 THE CIRCULATION. 



and to pass thence into the larger branches and trunks of the venous 

 system. As the veins converge from the periphery toward the centre, 

 and unite into branches of larger calibre, the resistance afforded by 

 contact of the circulating fluid with their inner surfaces constantly 

 diminishes from without inward; and every contraction of the right 

 ventricle, accompanied by the closure of the tricuspid valve, expels a 

 certain quantity of venous blood, and thus relieves the returning current 

 from the obstacle of its accumulation. As the pressure of the blood 

 from the capillaries continues uniform, and as the resistance to it is 

 incessantly neutralized by the action of the right ventricle, it forms 

 the most simple and effective cause for the movement of the blood 

 through the venous channels. 



II. The flow of the blood through the veins is also aided in great 

 measure by the contraction of the voluntary muscles. The veins which 

 convey the blood through the limbs, and the parietes of the head and 

 trunk, lie among voluntary muscles which are more or less constantly 

 in a state of alternate contraction and relaxation. At every contraction 

 these muscles become swollen laterally, and thus compress the veins 

 situated between them. The blood, expelled from the vein by this pres- 

 sure, cannot regurgitate toward the capillaries, owing to the venous 

 valves, which shut back and prevent its reflux. It is accordingly forced 

 onward toward the heart ; and when the muscle relaxes and the vein is 

 liberated from pressure, it is again filled from behind, and the circula- 

 tion goes on as before. 



Fig. 119. Fig. 120. 



VEIN with valves open. VEIN with valves closed; stream of blood 



passing off by a lateral channel. 



This force is very efficient in maintaining the venous circulation ; 

 since the voluntary muscles are more or less active in every position of 

 the body, and the veins are thus alternately subjected to compression 

 and relaxation. The entire voluntary muscular system acts in this way 

 by communicating to the venous current indirect impulses of frequent 



