CIU CULA TION. 1 23 



misleading has a real value, and will cease to confuse if its limitations be care- 

 fully noted. 



The Brevity and Variability of Each Cycle. — From the frequency with 

 which the cycles recur, it follows at once that each one, with its complex 

 changes in the walls, chambers, and valves, is very rapidly performed. If, for 

 instance, the heart beat 72 times in one minute, each cycle occupies onlv a 

 little more than 0.83 of a second. The brevity of each cycle is both an im- 

 portant physiological fact and a cause of difficulty in studying details. Each 

 cycle, however, necessarily is capable of completion in much less time if the 

 pulse-rate rise ; for instance, during exercise. If repeated 144 times a minute 

 instead of 72 times, each cycle would occupy only one-half of its previous 

 time of completion. With a pulse of less than 60, again, each cycle would 

 occupy over one second. 



Relative Lengths of the Ventricular Systole and Diastole. — An im- 

 portant question is whether or no there is any fixed relation between the time 

 required for a systole of the ventricles and the time required for a diastole. 

 When the length of the cycle changes from one second to one-half a second, 

 will the length of the systole be diminished by one-half, and that of the dias- 

 tole also by one-half? Or is a nearly invariable time required for the ventri- 

 cles to do their work of ejection, while the period of rest and of receiving blood 

 can be greatly shortened, for a while at least? The answer is that, while both 

 systole and diastole may vary in length, the length of the systole is much the 

 less variable, while the diastole is greatly shortened or lengthened according as 

 the heart beats often or seldom. 



These facts have been ascertained as follows: A trained observer 1 auscul- 

 tated the sounds of the human heart during a number of cycles, and, at the 

 instant when he heard the beginning either of the first or of the second sound, 

 made a mark upon the revolving drum of a kymograph by means of a sig- 

 nalling apparatus. Of course, careful account was taken of the time lost 

 between the occurrence of a sound and the recording of it. It was found 

 that the time between the beginning of the first and that of the second 

 sound did not vary to the same degree as the frequency of the beats. 

 Although the interval in question may not be an exact measure of the 

 period of ventricular systole, it is sufficiently near it for the purposes of this 

 observation. 



A second method 2 depended upon the interpretation of the curve inscribed 

 by a lever pressed upon the skin over a pulsating human artery. Such a curve 

 exhibits two sudden changes of direction, which were taken to indicate approxi- 

 mately the beginning and end of the injection of blood by the ventricle, and, 

 therefore, to afford a rough measure of the duration of its systole. \\ bile the 

 interpretation of the curve in question is not wholly settled, it seems, aeverthe- 



1 F. C. Donders: " De Rhytlmuis der Hartetoonen," Nederla/ndsch Archief - <n 



Natuurkunde, 1865, p. 141. 



2 E. Thurston : "The Length <>t' the Systole of the Heart as Intimated from Sphygrnogrnphic 

 Tracings," Journal of Anatomy and Physiology, ls7t>, vol. x. p. 494. 



