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), by 'a 

 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 yery obvious in the 

 arm of a person not too fat, 



-____ ;.jw'j|^ M ' when it is encircled by a bandage 

 that causes distension of the 



Fig. 128,-VEiK LA ID OPEK, TO superficial veins. _ The structure 

 SHOW ITS VALVES. oi these valves is seen at o 0, 



