CH. xx.] FREQUENCY AND FORCE OF THE HEART. 245 



Another valuable instrument introduced by Hiirthle is called the 

 differential manometer. In this instrument, two cannulae are brought 

 into connection with tamlxiurs (A and B) placed on opposite sides of the 

 fulcrum (F) of a lever. The lever works against a spring, and this sets in 

 motion a writinir style (s). This instrument enables us to determine the 

 relations of the pvessure changes in any two cavities. For instance, suppose 



B A 



Fig. 246. Diagram of Hiirthle's differential manometer. 



A is connected to the left ventricle, and B to the aorta ; when the pressure 

 in the ventricle is greater than that in the aorta, the writing style will be 

 raised : when the pressure in the aorta is greater than that in the ventricle, 

 the style will fall ; when the two pressures are equal, it will be in the zero 



position. 



Frequency and Force 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 corresponds 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. 

 Some figures in reference to the influence of age are appended. 



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 number of About the seventh 



pulsations per minute is 150 year . . . from 90 to 85 



Just after birth . from 140 to 130 About the fourteenth 



During the first year 13010115 year . . . 851080 



During the second In adult age . . 80 to 70 



year . . . 11510100 In old age . . . 701060 



During the third year 100 to 90 In decrepitude . . 75 to 65 



In health there is observed a nearly uniform relation between 

 the frequency of the beats of the heart and of the respirations ; 

 the proportion being, on an average, i respiration to 3 or 4 bents. 

 The same relation is generally maintained in the cases in which 

 the action of the heart is naturally accelerated, as after food or 

 exercise ; but in disease this relation may cease. 



In estimating the work done by any machine it is usual to 

 express it in terms of the unit of work. In England, the unit of 

 work is the foot-pound, and is defined to be the energy expended 



