ALTERATION OF TEMPERATURE 217 



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 1 50 



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. When these endings are poisoned stimuli arising in the inhibi- 

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

 vagus, cannot reach the heart muscle, and inhibition is impossible. 



Muscarine, which is obtained from various species of poisonous fungi, 

 produces marked slowing of the heart-beats, and, in larger doses, stoppage 

 of the heart. It produces an effect similar to that of prolonged vagus stimu- 

 lation. The effect can be removed by the action of atropine, hence is 

 supposed to stimulate the nerve endings of the vagus. 



Digitalis slows the heart by stimulating the vagi at their origin in the 

 inhibitory center in the medulla. The heart muscle itself is also rendered 

 more excitable. 



Veratrine and aconitine have a somewhat similar effect. 



Nicotine and caffeine are both very powerful cardiac stimulants. The 

 great injurious effects of nicotine on the heart are due to two causes, first, 



