74 PHYSIOLOGY 



squeezed it all out, the ventricles contract again and 

 force in more blood, which again stretches them. Each 

 new impulse sends in new blood, causing a succession of 

 stretches in the artery as the column of blood passes 

 on. This succession of stretches known as the pulse 

 passes through the blood vessels much more rapidly 

 than the blood itself. A heart beat is indicated by 

 the beating of the artery on the wrist, for example, 

 long before the blood forced out of the heart reaches 

 there. As it is an accurate register of the heart beat 

 it is useful to physicians in determining the rapidity of 

 the heart beat. 



(5) Tone. — Tone is often confused with elasticity, 

 but in reality it is something quite different. It in- 

 volves a variation in the size of the arteries, but it de- 

 pends not on the mechanical stretching of their walls by 

 the entrance of blood, but upon the sustained activity* 

 of the muscles composing the walls through which the 

 tubes may become large or small at any given moment. 



The walls of the arteries, especially the small ones, 

 contain encircling muscle fibers ; if the fibers relax, the 

 opening becomes larger and more blood passes through ; 

 if they contract, the opening is smaller and less blood 

 passes through (Fig. 49). 



The tone of the blood vessels is extremely important, 

 because by means of it the system of tubes is kept com- 

 pletely filled with fluid. If the blood vessels in the abdo- 

 men should enlarge to their utmost, it would take all the 

 blood in the body to fill them and the other blood vessels 

 would be empty. This would mean a cessation of circu- 

 lation. But the tone is regulated in such a way that these 



