VEINS AND HEART. 



lie against the sides of the vein and leave it 

 a free passage. The moment it tends to flow 

 downwards its own movement pulls out the 

 flaps, and its pressure forces them together, 

 thus closing the valve against its return. 



The presence of some of these valves can 

 easily be detected in the body by making down- 

 ward pressure along the large veins in the arm, 

 when the action of valves preventing the blood 

 from going in that direction may easily be 

 detected by knots or swellings caused in the 

 manner indicated at Y in the figure. 



The onward pressure force which, in metallic 

 pumps, is exerted by the piston, is in the veins 

 supplied in a variety of ways. 



When the muscles contract, either in severe 

 or gentle exercise, this contraction compresses 

 the veins between the muscles or between them 

 and the skin, and such compression, as we have 

 seen, tends to drive the blood always towards 

 the heart, since it is stopped in the other direc- 

 tion by the valves. 



Pressure from outside, as in massage or 

 pushing, compresses the veins, and moves on the 

 blood towards the heart. 



Change of position promotes movement of 

 the blood. Holding up the arm allows the blood 

 to run down the veins by gravity, the valves 

 being in the right direction for this movement. 



On the other hand, holding the limb down- 

 wards causes an accumulation of blood in the 



291 



