238 FLOW OF BLOOD THROUGH THE VEINS. 
278. The impulse of the heart, and the elasticity of the 
arteries, which together propel the blood through the capillary 
system, continue to act upon it after it is received into the 
veins ; and are in fact the chief causes of its movement in 
them. If we interrupt the current of blood through an artery 
by making pressure upon it, and open the corresponding vein, 
. the fluid will continue to flow from the latter, so long as the 
artery contains blood enough to be forced into the vein. by its 
own contraction ; but as soon as it is emptied, the flow from 
the orifice in the vein will cease, even though the vein itself 
remains nearly full. If the pressure be then taken off the 
artery, there is an immediate renewal of the stream from the 
vein, which may be again checked by pressure on the artery. 
In the ordinary operation of bleeding, we cause the superficial 
veins of the arm to be distended, by tying a bandage round 
them above the point at which we would make the incision ; 
and when an aperture is made, the blood spouts forth freely, 
being prevented by the bandage from returning to the heart. 
But if the bandage be too tight, so that the artery also is 
compressed, the blood will not flow freely from the vein ; and 
the loosening of the bandage will then produce the desired 
effect. When a sufficient quantity of blood has been with- 
drawn, the bandage is removed ; and the return-flow through 
the veins being now unobstructed, the stream from the orifice 
immediately diminishes so as to be very easily checked by 
pressure upon it, or may even cease altogether. 
a 279. The veins contain a great 
number of valves, which are 
formed, like the semilunar valves 
of the aorta (§ 273), bya 
doubling of their lining mem-. 
brane. Their situation may be 
known by the little dilatations 
which the veins exhibit at the 
points where they occur; and 
which are very obvious in the 
arm of a person not too fat, 
when it is encircled by a bandage 
} that causes eke ed at 
superficial veins. e structure 
gig UES ee ahaa of tha valves is seen at b 8, 
