36 ELEMENTARY PHYSIOLOGY 



regularly towards the hand, so long as the current of the 

 blood, which is running in them, from the hand to the 

 upper arm, is uninterrupted. 



But if a linger be pressed upon the up{)er part of one 

 of these veins, and then passed downwai'ds along it, so as 

 to drive the blood which it contains backwards, sundry 

 swellings, like little knots, will suddenly make their ap- 

 pearance at several points in the length of the vein, where 

 nothing of the kind was visible before. These swellings 

 are simi)ly dilatations of the wall of the vein, caused by 

 the pressure of the blood on that wall, above a valve 

 which opposes its backward progress. The moment the 

 backward impulse ceases the blood flows on again ; the 



Pio. 10. — The Valvks ok Vkins. 



C, H, C, H, diagrammatic sections of veins with valves. In the upper 

 figure the blood is supposed to lie flowing in the direction of the arrow, 

 towards the heart ; in the lower, back towards the capillaries ; C, cai)illary 

 side ; H, heart side. A, a vein laid open to show a pair of pouch-shaped 

 valves. 



valve, swinging back towards the wall of the vein, affords 

 no obstacle to its progress, and the distention caused by 

 its pressure tlisappears (Fig. 10). 



These valves play an important part in determining the 

 flow of l)lood along the veins from the capillaries towards 

 the heart. This they do not in virtue of any propulsive 

 power of their own, but in response to pressure applied to 

 the veins from their exterior. Such pressure tends to 

 squeeze the blood out of that pare of the vein on which it 

 is brought to ber.r ; but since the valves only open 

 towards the heart the blood is thereby driven on in the 



