296 FUNCTIONS OF NUTRITION. 



blood through the veins is less powerful and regular than that through 

 the arteries. It depends on the action of three different forces. 



I. The most important of these forces is the pressure from the capil- 

 lary circulation. The blood moves from the arteries into and through 

 the capillary vessels, under an impulse derived originally from the heart, 

 and afterward replaced by the comparatively uniform arterial pressure. 

 This pressure is not entirely exhausted in the capillaries ; and the blood 

 accordingly emerges from these vessels and enters the venous system 

 with a force sufficient to fill its rootlets, and to pass thence into its larger 

 branches and trunks. As the veins converge from the periphery toward 

 the centre, and unite into trunks of larger calibre, their extent of contact 

 with the circulating fluid, and their resistance to its movement, constantly 

 diminishes; while the contractions of the right ventricle relieve the 



FIG. 74. FIG. 75. 



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



ing off by a lateral channel. 



returning current from the obstacle of its accumulation. The contin- 

 uous pressure of the blood from the capillaries thus supplies an 

 effective cause for its movement through the veins. 



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

 the contraction of the voluntary muscles. The veins in the limbs, and 

 in the parietes of the head and trunk, lie among voluntary muscles which 

 are often in a state of alternate contraction and relaxation. At each 

 contraction the muscles become swollen laterally, thus compressing the 

 veins between them. As the blood, expelled by this pressure, cannot 

 regurgitate toward the capillaries, owing to the closure of the venous 

 valves, it is 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 circulation goes on as before. 



The muscular system acts in this way by communicating to the venous 

 current indirect impulses of frequent repetition, which, combined with 



