THE CIRCULATION OP THE BLOOD. 



203 



Frequency of the Heart's Action. 



The heart of a healthy adult man contracts about 72 times in a 

 minute; but many circumstances cause this rate, which of course cor- 

 responds with that of the arterial pulse, to vary even in health. The 

 chief are age, temperament, sex, food and drink, exercise, time of day, 

 posture, atmospheric pressure, temperature; as follows: 



(1.) Age. The frequency of the heart's action gradually diminishes 

 from the commencement to near the end of life,, but is said to rise 

 again somewhat in extreme old age, thus : 



Before birth the average mini 



pulsations per minute is 150 

 Just after birth . f: 

 During the first year 

 During the second 



year 

 During the third year 



(2.) Temperament and Sex. In persons of sanguine temperament, 

 the heart acts somewhat more frequently than in those of the phleg- 

 matic; and in the female sex more frequently than in the male. 



(3 and 4.) Food and Drink.. Exercise. After a meal the heart's 

 action is accelerated, arid still more so during bodily exertion or mental 

 excitement; it is slower during sleep. 



(5.) Diurnal Variation. In health the pulse is most frequent in the 

 morning, and becomes gradually slower as the day advances : and this 

 diminution of frequency is both more regular and more rapid in the 

 evening than in the morning. 



(6.) Posture. The pulse, as a general rule, especially in the adult 

 male, is more frequent in the standing than in the sitting posture, and 

 in the latter than in the recumbent position; the difference being 

 greatest between the standing and the sitting postures. The effect of 

 change of posture is greater as the frequency of the pulse is greater, 

 and, accordingly, is more marked in the morning than in the evening. 

 By supporting the body in different positions, without the aid of mus- 

 cular effort of the individual, it has been proved that the increased fre- 

 quency of the pulse in the sitting and standing positions is dependent 

 upon the muscular exertion engaged in maintaining them; the usual 

 effect of these postures on the pulse being almost ent^ely prevented 

 when the usually attendant muscular exertion was rendered unnecessary. 



(7.) Atmospheric Pressure. The frequency of the pulse increases in 

 a corresponding ratio with the elevation above the sea. 



(8.) Temperature. The rapidity and force of the heart's contrac- 

 tions are largely influenced by variations of temperature. The frog's 

 heart, when excised, ceases to beat if the temperature be reduced to 



