FLOW OP BLOOD THROUGH THE VEINS. 239 



fig. 128; they consist of pocket-like folds of the lining mem- 

 brane, which allow the blood free passage as it flows towards 

 the heart, but check its reflux into the arteries. Hence it 

 follows, that every time pressure is made upon the veins, it 

 will force towards the heart a portion of the blood they con- 

 tain, since this cannot be driven in a contrary direction. IsTow, 

 from the manner in which the veins are distributed, some of 

 them must be compressed by almost every muscular move- 

 ment; these will become refilled as soon as the muscles relax; 

 and they will be again pressed-on, when the movement is 

 repeated. Hence a succession of muscular movements will act 

 the part of a diffused heart, over the whole of the venous 

 system, and will very much aid the flow of blood through its 

 tubes. It is partly in this manner, that exercise increases the 

 rapidity of the circulation. If the blood is brought to the heart 

 by the great veins more rapidly than usual, the heart -must go 

 through its operations more rapidly, in order to dispose of the 

 fluid ; and if these actions be impeded, great danger of their 

 entire cessation may exist. Hence the importance of bodily 

 tranquillity to those affected with diseases of the heart or 

 lungs ( 271). 



280. Besides the aid thus afforded to the venous circulation, 

 it is probable that there is another cause of the motion of the 

 blood in them, which is independent of the action of the 

 heart and of the arteries. Many facts lead to the belief that 

 a new force is produced, while the blood is flowing through 

 the capillary vessels, a force which may, in some instances, 

 maintain the circulation by itself alone. Thus in many of 

 the lower animals, it seems as if the power of the heart were 

 so unequal to the maintenance of the circulation, that this 

 must partly depend upon some other influence ; and even in 

 the highest, there is evidence that the movement of blood in 

 the capillaries may continue for a time, after the action of the 

 heart and of the arteries has ceased, to affect it.* This 

 movement seems intimately connected with the changes 

 to which the blood, is subservient in the capillaries; for, 

 if these be checked, not even the heart's action can 

 propel the blood through them, although no mechanical 



* For a full consideration of this question, see the Author's Principles 

 of Comparative Physiology (4th edition), 247-251 ; and Principles 

 of Human Physiology (5th edition), 267-275. 



