326 THE ANIMALS AND MAN 



system. This is the pulse wave. It may be detected by the 

 tip of the finger where the artery lies near the surface, as in 

 the wrist, and the rate of the heart beat counted. This we 

 call "feeling the pulse." It is used by physicians to deter- 

 mine the condition of the heart; for conditions affecting the 

 heart will change the rate of the pulse beat. 



Sounds of the heart- -The heart in action makes two 

 sounds, one a long sound (lub), caused by contraction, 

 and another a short sound (dub), caused by the closing 

 of the valves. These strokes are repeated about seventy- 

 two times a minute, day and night. 



Nervous control of the circulatory apparatus. When 

 an organ is at work it needs a greater supply of blood than 

 when at rest. After a meal the stomach and intestines need 

 a large amount of blood. During exercise, the muscles 

 need a large supply. 



The caliber of the arteries of any organ is increased or 

 decreased according to the needs of the organ. During 

 activity of the organ, the caliber of its arteries is increased, 

 and hence the organ receives an increased supply of blood. 

 During rest, the caliber of the arteries is reduced and hence 

 part of the blood supply is shut off. 



This increase and decrease of the caliber of the blood- 

 vessels is effected by the nervous system. There are aug- 

 menting (hastening) and inhibiting (checking) nerve fibers, 

 passing from central nervous ganglia to each blood-vessel. 



Effects of exercise upon circulation. During muscular 

 activity, or exercise, the muscles need a large supply of blood. 

 The arteries of the muscles dilate and the heart increases 

 the rapidity of its beat. This results in a rapid supply of 

 blood to the lungs, a quick exchange of carbon dioxide and 

 oxygen, and a large supply of oxygen to the muscle. In 

 the muscle rapid metabolism is in progress which, if not 

 overdone, invigorates the whole system. The heart may 

 be weakened by over exercise or by vigorous exercise at 



