156 ADVANCED PHYSIOLOGY 



say that the pressure of blood in the artery is nine inches of mer- 

 cury. The pressure varies a little with each heart beat, being 

 greatest while the heart is contracting and least when the heart 

 is resting. The pressure decreases as the arteries become 

 smaller until it becomes very slight in the capillaries. This 

 should naturally be so, for, as seen on page 152, capillary walls 

 are extremely thin, and slight pressure would rupture them. 

 That they possess some elasticity, however, is evident from the 

 appearance of the face when blushing; it is also known that 

 the lymph passes through the walls of capillaries, sometimes 

 rapidly, sometimes slowly, and this seems to be due to a vary- 

 ing pressure within them as well as to a varying permeability 

 of capillary walls. 



With the veins, into which the blood flows from the capil- 

 laries, the conditions are very different. They are wide open 

 where they end at the heart, so that there is nothing to keep the 

 blood from flowing freely until it reaches that organ. There is 

 nothing corresponding to the nozzle of the hose. For these 

 reasons the blood in the veins is under much less pressure than 

 that in the arteries, and varies much with the location and posi- 

 tion of the organ. 



Bleeding from Arteries and Veins. If an artery is cut, the 

 blood will come out in forcible jets, and prompt action is 

 necessary to prevent the person from bleeding to death. The 

 bleeding must be stopped by compressing the artery between 

 the cut and the heart. Such accidents are most common in 

 the legs and arms where they can easily be treated. Figures 

 87, 88 and 89 show the course of the chief arteries in an arm 

 and leg. The easiest and most effectual way to stop bleeding 

 is to put a ligature above the wound. A doctor should be 

 summoned and the ligature kept in position till he arrives. 



Wounds in veins are generally less serious than in arteries, 

 but should a large vein be cut and the bleeding be so rapid 

 that clotting will not stop it, a ligature should be placed 

 beyond the wound. 



