206 THE CIRCULATION OF THE BLOOD 



reaction of these mechanical conditions on the heart itself. The isolated 

 cardiac muscle responds very quickly to variations in tension. Beginning 

 with a low tension the activity of heart muscle is increased up to a certain 

 optimum tension, after which further increase is unfavorable to the develop- 

 ment of automatic rhythm. A quite strong stretching will paralyze the 

 muscle. 



Tension on the whole heart influences its activity, not only through the 

 effects on the muscle, but indirectly through the nervous mechanism. High 

 tension, such as contracting against a high aortic pressure, stimulates sensory 

 nerves of the heart which, acting through the depressor nerve on the inhibi- 

 tory center, produce reflex slowing of the heart. It also produces reflex 

 vaso-dilatation. Both reflexes relieve the high tension on the heart. This 

 nerve reaction takes place with a tension which still mechanically stimulates 

 the cardiac- muscle substance, and the inhibitory effects must therefore over- 

 come the direct stimulating effect of the tension on the muscle fibers. 



Age, Sex, etc. The average heart rate for the normal adult man is 72 

 times a minute, but this rate will vary much in different individuals accord- 

 ing to the age, sex, size, and personal equation. The frequency of the heart's 

 action gradually diminishes from the commencement to near the end of life, 

 but is said to increase again somewhat in extreme old age, thus: 



Before birth the average number of pulsations 



per minute is 150 



Just after birth 130 to 140 



During the first year 1 1 5 to 130 



During the second year 100 to 1 1 5 



During the third year 90 to 100 



About the seventh year 85 to 90 



About the fourteenth year 80 to 85 



In adult age 70 to 80 



In old age 60 to 70 



In decrepitude 65 to 75 



The heart rate is greater in woman than in man. It is also greater in 

 small than in large individuals. The rate varies from the type in certain 

 individuals where no cause can be assigned other than personal equation. 



Poisons and Other Chemical Substances. A large number of chemical 

 substances have a distinct effect upon the cardiac contractions. Of these 

 the most important are atropine, muscarine, digitalis, barium, nicotine, 

 caffeine, etc. 



Atropine produces considerable augmentation of the heart-rate, and 

 when acting upon the heart prevents inhibition by vagus stimulation. Its 

 effects are produced by poisoning the nerve endings of the vagus within 

 the heart. With these endings poisoned, stimuli arising in the inhibitory 

 center of the medulla (tonic activity), or artificially applied to the vagus, 

 cannot reach the heart muscle, and inhibition is impossible. 



