372 OF THE CIRCULATION OF BLOOD. 



pears, from the experiments of Poisseuille, to be very nearly equal for equal 

 surfaces, throughout the larger arterial branches, since it diminishes regularly 

 in proportion to their calibre ; in the radial artery at the wrist, it was estimated 

 by him at 4 drachms. 



493. The number of contractions of the heart in a given time, is liable to 

 great variation, within the limits of ordinary health, from several causes ; the 

 chief of these are, diversities of age, of sex, of muscular exertion, of the con- 

 dition of the mind, of the state of the digestive system, and of the period of 

 the day. Putting aside the other causes of uncertainty, the following table 

 may be regarded as an approximation to the average frequency of the pulse, 

 at the several ages specified in it. 



Beats per minute. 



In the foetus in utero . . 140 150 



Newly-born infant . . 130 140 



During the first year . . 115 130 



During the second year . . 100 115 



During the third year . . 90 100 



About the seventh year . . 85 90 . 



Age of puberty . . 8085 



Manhood . . 70 80 



Old age . . 5065 



The difference caused by sex is very considerable, especially in adult age ; it 

 appears from the inquiries of Dr. Guy,* that the pulse of the adult female ex- 

 ceeds in frequency the pulse of the adult male, at the same mean age, by from 

 10 to 14 beats in a minute. The effect of muscular exertion in raising the 

 pulse is well known; as is also the fact, which is one exemplification of it, that 

 the pulse varies considerably with the posture of the body. The amount of 

 this variation has been made the subject of extensive inquiry by Dr. Guy, and 

 the following are his results. In 100 healthy males, of the mean age of 27 

 years, in a state of rest, the average frequency of the pulse was, when stand- 

 ing, 79 ; when sitting, 70 ; and when lying, 07 per minute. Several excep- 

 tions occurred, however, to the general law ; and when these were excluded, 

 the average numbers were, standing, 81 ; sitting, 71 ; and lying, 66 ; so that 

 the difference between standing and sitting was 10 beats, or jth of the whole ; 

 the difference between sitting and lying was 5 beats, or T ^th of the whole ; and 

 the difference between standing and lying was 15 beats, or -}th of the whole. In 

 50 healthy females, of the same mean age, the average pulse when standing 

 was 89 ; when sitting, 81 ; and when lying, 80. When the exceptions (which 

 were more numerous in proportion than in males) were excluded, the averages 

 were, standing, 91 ; sitting, 84 ; lying, 79 ; the difference between standing 

 and sitting was thus 7 beats, or T L th of the whole ; that between sitting and lying 

 was 4, or ^ r st f tne whole ; and that between standing and lying was 11, or 

 |th of the whole. In both sexes, the effect produced by change of posture in- 

 creases with the usual frequency of the pulse ; whilst the exceptions to the 

 general rule are more numerous as the pulse is less frequent. The variation 

 is temporarily increased by the muscular effort involved in the absolute change 

 of the posture ; and it is only by the use of a revolving board, by which the 

 position of the body can be altered without any exertion on the part of the 

 subject of the observation, that correct results can be obtained. That the differ- 

 ence between standing and sitting should be greater than that between sitting 

 and lying, is just what we should expect, when we compare the amount of 

 muscular effort required in the maintenance of the two former positions 

 respectively. 



* Guy's Hospital Reports, vol. iii., p. 312. 



