CH. XXI.] 



WORK OF THE HEART 



247 



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 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, thus : 



Before birth the average number 



of pulsations per minute is . 150 

 Just after birth . . from 140 to 130 

 During the first year . ,, 130 to 115 

 During the second year ,, 115 to 100 



About the seventh year . from 90 to 85 

 About the fourteenth 



year . . . . ,, 85 to 80 

 In adult age . . . ,, 80 to 70 

 In old age 70 to 60 



In health there is a uniform relation between the frequency of 

 the heart -beats and of the respirations; the proportion being 1 

 respiration to 3 or 4 beats. The same relation is generally main- 

 tained in cases in which the action of the heart is naturally 

 accelerated, as after food or exercise; but in disease this relation 

 may cease. 



Work of the Heart. 



Waller compares the work performed by the heart in a day to 

 that done by an able-bodied labourer working hard for two hours. 

 The heart's work consists in discharging blood against pressure, and 

 in imparting velocity to it. It is therefore necessary to know how 

 much blood is expelled from the heart at each beat, the time 

 occupied in such expulsion, the velocity with which the blood is 

 expelled, and the pressure against which the heart has to act. 

 Without going into the somewhat elaborate calculations obtained 

 from these and other data, it will be sufficient to say that about 

 T V of the total energy of the heart is used in imparting velocity to 

 the blood, but when the blood reaches the aorta the velocity is checked, 

 and that the kinetic energy of the blood in the aorta is only about 

 -g-Jrj- of the total energy imparted to the blood by the heart. 



The Output of the Heart. Direct measurements of the heart's output have 

 been made by Stolnikow and Tigerstedt. The former cut off the whole of the 

 systemic circulation in the dog, and then measured the amount of blood passing 

 through the simplified circulation, which consisted only of the pulmonary and 

 coronary vessels by means of a cylinder interposed on the course of the vessels, 

 (see fig. 228). Tigerstedt made his observations by means of a Stromuhr (see next 

 chapter) inserted into the aorta. Severe operative measures of this kind, however, 

 interfere with the circulation a good deal. 



G. N. Stewart has introduced an ingenious method, the principle of which is 

 the following : A solution of an innocuous substance, which can be easily recog- 

 nised and estimated, is allowed to flow for a definite time and at a uniform rate into 

 the heart ; the substance selected was sodium chloride. This mingles with the 



