250 DURATION OF CAKDIAC PHASES. [BOOK i. 



The observer, listening to the sounds of the heart, makes a signal at 

 each event on a recording surface, the difference in time between the 

 marks being measured by means of the vibrations of a tuning-fork 

 recorded on the same surface. By practice it is found possible to 

 reduce the errors of observation within very small limits. 



Now whatever be the exact causation of the first sound, it is 

 undoubtedly coincident with the systole of the ventricles, though 

 possibly the actual commencement of its becoming audible may be 

 slightly behind the actual beginning of the muscular contractions. 

 Similarly the occurrence of the second sound, which, as we have 

 seen, is certainly due to the closure of the semilunar valves, has 

 been taken to mark the close of the ventricular systole. And 

 on this supposition the interval between the beginning of the 

 first and the occurrence of the second sound has been regarded 

 as indicating approximatively the duration of the ventricular 

 systole, i.e. the period during which the ventricular fibres are con- 

 tracting. We have however urged above that the ventricles still 

 remain contracted for a brief period after the valves are shut ; if 

 this view be correct then the second sound does not mark the 

 end of the systole, and the duration of the systole is rather longer 

 than the time given above. 



The determination of the separate duration of each of the 

 three periods of the ventricular systole, viz. the getting up the 

 pressure, the discharge of the contents, and the remaining emptied 

 but contracted, is subject to so much uncertainty that it need not 

 be insisted on here ; it may however be said that, roughly speaking, 

 each phase occupies probably about *1 sec. 



In a heart beating 72 times a minute, which may be taken as 

 the normal rate, each entire cardiac cycle would last about 0'8 sec., 

 and taking 0'3 sec. as the duration of the ventricular systole, the 

 deduction of this would leave 0'5 sec. for the whole diastole of the 

 ventricle including its relaxation, the latter occupying about or 

 somewhat less than '1 sec. In the latter part of this period there 

 occurs the systole of the auricles, the exact duration of which it is 

 difficult to determine, it being hard to say when it really begins, 

 but which, if the contraction of the great veins be included, 

 may perhaps be taken as lasting on an average 01 sec. The 

 'passive interval' therefore, during which neither auricle nor 

 ventricle are undergoing contractions, lasts about '4 sec. and the 

 absolute pause or rest during which neither auricle nor ventricle 

 are contracting or relaxing about *3 sec. ; if however a longer 

 period be allotted to the ventricular systole, these periods must be 

 proportionately shortened. The systole of the ventricle follows 

 so immediately upon that of the auricles, that practically no 

 interval exists between the two events. 



The duration of the several phases may for convenience sake 

 be arranged in a tabular form as follows ; but in reading the table 



