238 COMPARATIVE PHYSIOLOGY. 



rhythm, show that of the whole period, the auricular systole 

 occupies ^ or T^ of a second ; the ventricular systole, | or ^ of 

 a second; and the diastole, -f or j\ of a second. 



With the more rapid beat in man (70 to 80 per minute), the 

 duration of the cardiac cycle may be estimated at about ^% of 

 a, second, and the probable proportions for each event are about 

 these: The auricular systole, yV of a second; the ventricular 

 systole, -^ of a second ; and the pause, y% of a second. 



It will be noted that the pause of the heart is equal in dura- 

 tion to the other events put together ; and even assuming that 

 there is some expenditure of energy in the return (relaxation) 

 of the heart to its passive form, there stUl remains a consider- 

 able interval for rest, so that this organ, the very type of cease- 

 less activity, has its periods of complete repose. 



THE WORK OP THE HEART. 



Since the pressure against which the heart works must, as 

 we shall see, vary from moment to inoment, and sometimes 

 very considerably, the work of the heart must also vary within 

 wide limits, even making allowance for large adaptability to 

 the burden to be lifted ; for it wUl be borne in mind that the de- 

 gree to which the heart empties its chambers is also variable. 



If one knew the quantity of blood ejected by the left ven. 

 tricle, and the rate of the beat, the calculation of the work done 

 would be an easy matter, since the former multiplied by the 

 latter would represent, as in the case of a skeletal muscle, the 

 work of the muscles of the left ventricle ; from which the work 

 of the other chambers might be approximately calculated. 



The work of the auricles must be slight. The right ven- 

 tricle, it is estimated, does from one fourth to one third the 

 work of the left. 



When we calculate the work done by the heart for certain in- 

 tervals, as the day, the week, month, year, and especially for a 

 moderate lifetime, and compare this with that of any machine it 

 is within the highest modern skill to construct, the great superi- 

 ority of the vital pump in endurance and working capacity will 

 be very apparent ; not to take into the accoimt at all its wonder- 

 ful adaptations to the countless vicissitudes of life, without which 

 it would be absolutely useless, even destructive to the organism. 



Some of these variations in the working of the heart we may 

 now to advantage consider. 



