OTHER INFLUENCES WHICH AFFECT THE HEART 185 



center, produce reflex slowing of the heart, as well as reflex vaso-dilatation, 

 both of which relieve the high tension. This nerve reaction takes place with 

 a tension which still mechanically stimulates the cardiac-muscle substance, 

 and the inhibitory effects must therefore first overcome 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 115 to 130 



During the second year 100 to 1 15 



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, etc. 



FIG. 182. The Effect of an Intravenous Injection of Atropine on the Dog's Heart Rate Meas- 

 ured by Means of a Blood-Pressure Curve. (New figure by Doolev.) 



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 



