98 PHYSIOLOGY FOR BEGINNERS CHAP. 



contraction of the heart which, by getting up the pressure in 

 the vessels, the blood pressure, drives the blood round the 

 whole vascular system back to the heart again. The arteries, 

 as we have seen, have muscular tissue in their walls, but this 

 does nothing to help the blood onward. We shall see that 

 the muscular tissue of the arteries has its uses, but so far as 

 conducting the blood onward is concerned, the arteries simply 

 act as elastic Jubes, merely pressing on the blood in them 

 because they are kept over-distended by the blood forced into 

 the aorta by the heart. 



The Pulse. Each fresh quantity of blood forced into the 

 aorta produces a sudden extra distension of the aorta. This 

 extra distension runs quickly over the walls of the whole 

 arterial system, so as to give all the- arteries a little extra dis- 

 tension. This extra distension, travelling rapidjy along the 

 walls of the artenes,"Ts called the pulse. The pulse may be 

 felt in any artery near the surface of the body ; it is very 

 distinct in the artery at the wrist. This travelling of the 

 distension is quite a different thing from the flow of the 

 blood itself. The pulse travels over the arterial walls at the 

 rate of 30 feet per second, going at about the same speed 

 in all the arteries, and hence takes only y^n of a second 

 to travel from the beginning of the aorta to the wrist, 

 while the blood itself travels along much more slowly, 

 requiring about five seconds to reach the wrist. As the pulse 

 runs over the arteries from the aorta the amount of extra 

 distension it causes diminishes continually until in the smallest 

 arteries the effect it produces is small, and, moreover, is dis- 

 tributed over an enormous number of vessels, and in the 

 capillaries the pulse is lost ; hence no pulse is perceptible in the 

 veins beyond them. When an artery is cut the blood is thrown 

 out in jerks corresponding to the pulse, and also flows out between 

 the jerks, because the arteries are kept over full, and the 

 blood is pressed out by their elastic walls. When a vein is 

 cut the blood flows out steadily and without jerks, being 

 merely pressed out by more blood coming on through the 

 capillaries. 



Velocity of the Blood. The same quantity of blood 

 goes along the aorta in a second as goes through all its 

 capillaries put together in a second, and as goes along the 



