THE RATE OF HEART BEAT 



197 



Heavy bed clothing sufficient to produce a growing sensation of warmth 

 quickens the pulse frequency considerably. Exposure of the naked body to 

 air at a low temperature reduces the rate; but if the temperature of the air 

 is high the rate rises. Hot and cold drinks have the same effect as external 

 temperature: drinking hot water accelerates, drinking cold water retards the 

 pulse. A burning sensation or 

 sensation of pressure, etc., in the 

 stomach or intestine quickens the 

 pulse. 



Under such circumstances it is 

 evident that a meal may exercise 

 an important influence on the pulse to 

 frequency, and this is confirmed 

 by experience. The pulse rate as 

 a rule is higher at meal times, 

 owing mainly to the addition of 

 heat to the body. 



Bodily movements exercise the 

 most profound influence on the 

 pulse rate, and we can almost say 

 that the rate increases in direct 

 proportion to the effort required 

 and the extent and vigor of the 

 movement. Detailed experiments 

 are at hand showing that the in- 

 crease is due in small part to a 

 direct effect of products formed in 

 the muscular activity on the heart 

 itself, but in by far the greater part to the fact that along with the vol- 

 untary impulses from the higher brain centers to the muscles there go also 

 involuntary impulses to the cardiac centers, whereby the tonus of the inhibi- 

 tory center is diminished or the accelerator center is excited (Johansson; 

 cf. page 195). 



Figure 71 shows how the pulse rate varies with age. In the first year it 

 is highest ; it then falls to a minimum of about seventy per minute near the 

 twentieth year (for males), where it remains until the approach of old age, 

 when it again rises somewhat. 



The higher pulse rate in children is due in part to the smaller size of the 

 body, just as we find in grown animals of different species that the large ones 

 have a slow pulse, the small ones a rapid pulse (e. g., horse and ox 36 to 50, 

 rabbit 200 per minute). This difference is without doubt connected with the 

 relatively more active metabolism of small and young individuals (cf. 

 page 117 et seq.). 



Figure 71 also furnishes information with regard to the influence of sex. 

 The pulse frequency for all ages above two years is higher in the female than 

 in the male. The smaller size of the female body is the most important factor 

 in this difference. One finds on comparison of the pulse rate in men and 

 women of the same height that it is somewhat higher in the latter, but the 



tt X.9 M <H * M 9 



FIG. 71. The pulse rate in man at different ages, 

 after Guy. , males ; , females. 



