THE CIRCULATION , 287 



various movements of the heart occupy periods of time which may be 

 stated exactly in whole tenths of a second. Personal variations of con- 

 siderable degree from the average are common. Some individuals in 

 perfect health exhibit a normal rate of 60, or even less, while the hearts 

 of others contract year after year 90 times per minute. A rate of 

 20 has once or twice been recorded, and several times rates of much over 

 100. Napoleon exhibited his variation from the average of humanity 

 by a pulse-rate of 40 not less than by so many other deviations. 



Many sorts of influences effect the pulse-rate, most of which, at least, 

 are dependent on the activity of tissue-metabolism or on the degree of 

 excitement in the nervous system, or on both of these at once, although, 

 in a sense, nervous excitement is reducible to the terms of tissue-metabo- 

 lism. It is, then, not far wrong to say that any influence or condition 

 which increases metabolism to a considerable degree increases the heart- 

 rate. There are sixteen or seventeen of these conditions in man which 

 may be noted. Most conspicuous, perhaps, of these is age, for in the 

 embryo the heart beats 150 times or so per minute. The rate falls 

 progressively to the normal adult figures given above and rises again 

 slightly in old age. Sex is practically the next most important variant 

 of the heart-rate. Females have higher rates than males of like age and 

 temperament. One sees this difference not only as regards the number 

 but as regards the variability of the female rate. Young girls especially 

 are subject to very wide changes in the pulse-rate in a purely physio- 

 logical way. Size influences pulse-rate, for it is higher in small than in 

 large persons. Temperament is very effective, slow phlegmatic persons 

 having a lower heart-rate than those with short reaction-times. Persons 

 who are normally nervous have a wide range in their heart-rates. Bodily 

 or atmospheric temperature causes a variation, a rise of temperature 

 within or without the body producing an increase in the rapidity of the 

 heart. Eating increases the rate for two hours or so, until the activity 

 of metabolism has dropped back to its average. Being above the sea- 

 level increases the pulse-rate because the necessary increase in respira- 

 tion demands greater activity in the circulation. Increase of respira- 

 tions from whatever cause tends to increase the activity of the heart, 

 there being a tendency to keep a ratio of one to four. Sleep lessens the 

 heart-rate. Muscular exercise increases the pulse-rate markedly, even 

 up to 200 or more. Posture has considerable influence, the rate being 

 higher when a person stands than when he sits and lowest when he is 

 reclining. Mental excitement increases the frequency of the heart-rate, 

 especially in females. Pain increases the pulse-rate in man, although 

 in the rabbit, for example, it decreases it. Extreme bodily weakness 

 is liable to show increase in the heart-rate. An increase in arterial blood- 

 pressure raises it, the more rapid rate being necessary to compensate 

 for the increased friction. And many drugs change the frequency of the 

 heart-beat in one or the other direction. 



These variants of the pulse-rate are of much practical importance. 

 The more of them the physician takes into consideration in estimating 



