THE CIRCULATION 



55 



cup is full, it cannot become more full; not so with an artery. The 

 elastic connective tissue allows it to expand as a rubber hose does 

 under pressure. The first part of the aorta having expanded to receive 

 the incoming blood, the stretched walls contract because of the elas- 

 ticity of the outer connective tissue coat and force blood into the por- 

 tion of the aorta just ahead, forcing it to expand in turn. Thus a wave 

 of expansion travels along the arteries. This wave is called the pulse. 



The Pulse may be most easily felt in the wrists and neck. As the 

 artery stretches and springs back, one beat of the pulse is felt. In men 

 there are about seventy heart beats or 

 pulse beats a minute. In women the 

 rate is about eighty a minute. It is 

 slowest when one is lying down, faster 

 while sitting, still faster when stand- 

 ing, and fastest of all during running or 

 violent exercise. (Exp. 5.) It should 

 not be thought that the muscular or 

 middle layer of the artery actively con- 

 tracts and helps to send along the pulse 

 wave ; for this wave is simply the pas- 

 sive stretching and contracting of the 

 outer connective coat, and travels like 

 a wave crossing a pond when a stone 

 is dropped into the water. The force 

 of the pulse is furnished, not by the 

 muscle fibers in the artery, but by the 

 beat of the heart; the outer, or con- 

 nective tissue, coat enables the pulse 

 to travel. Why must there be a mid- 

 dle, or muscular, coat for variation in 

 size? 



Use of the Middle Coat ; Quantity of 

 Blood and its Distribution. The body 

 of an adult contains about five quarts 

 of blood. The blood furnishes the nourishment needed for the activity 

 of each organ. The more vigorous the work of any organ, the greater 

 is the amount of blood needed. The whole amount of blood in the body 

 cannot be suddenly increased, but the muscular coat of the arteries going 

 to the working organ relaxes, and allows the arteries to become enlarged 

 by the pressure from the heart. Consequently, more blood goes to the 

 active organ, and the other organs get along with less blood for the time. 

 When we are studying, our brains get more blood ; when running, the 



FIG. 56. SECTION OF ARTERY, 

 A, AND VEIN, V, showing inner 

 coat, e (endothelial) ; middle 

 coat, m (muscular) ; and third 

 coat, a (connective tissue). 



