< 
FLOW OF 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. Now, 
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 ts 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 
uillity 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. 
