220 HUMAN PHYSIOLOGY 



sequence of events in the frog's heart might be schematically 

 represented us in Fig. 110. 



The events from the beginning of the contraction of any 

 cavity to the beginning of the next contraction of that cavity 

 constitute a cardiac cycle. Usually the cardiac cycle is 

 reckoned from the beginning of the systole of the sinus 

 (Practical Physiology, Chap. IX.). 



In the mammalian heart the rate of recurrence of the 

 cardiac contraction varies with the animal examined. In 

 man it is in adult life about 72 per minute. Many factors 

 modify the rate of the heart. 



1. Period of Life. The following table shows the average 

 rate of the heart at different ages : 



Foetus . . . 140 per minute. 



Under 1 year . . 120 



1 to 3 years . . 100 



7 to 14 years . . 85 



21 to 61 years . . 70 to 75 



Old age ... 75 to 80 



2. Period of the Day. The pulse is generally slowest in 

 the early morning, and quickest in the evening. 



3. Temperature of the Body. The pulse varies with the 

 body temperature, generally being increased about 10 beats 

 with each degree Fahr. of elevation of temperature. 



4. Muscular Exercise increases the rate of the heart, first 

 by driving the blood from the muscles into the great veins 

 (p. 274), and second, by developing toxic substances which 

 act directly upon the heart. 



5. Posture has also an important influence. Suddenly 

 assuming the erect position accelerates the heart by caus- 

 ing the blood to accumulate in the abdominal veins, 

 and thus checking its transference on into the arteries 

 (p. 276). 



6. The condition of the central nervous system may 

 modify the rate of the heart, any disturbance accompanied 

 by emotional changes either accelerating or retarding the 

 rate. 



7. Stimulation of certain nerves especially those of the 



