CHAP, iv.] THE VASCULAR MECHANISM. 213 



of the auricles is always very brief, that the systole of the ven- 

 tricles is always very prolonged, always occupying a consider- 

 able portion of the whole cycle, and that the diastole of the 

 whole heart, reckoned from the end either of the systole, or 

 of the relaxation of the ventricle, is very various, being in quickly 

 beating hearts very short and in slowly beating hearts decidedly 

 longer. 



When we desire to arrive at more complete measurements, 

 we are obliged to make use of calculations based on various data ; 

 and the value of some of these has been debated. Naturally the 

 most interest is attached to the duration of events in the human 

 heart. 



A datum which has been very largely used is the interval 

 between the beginning of the first and the occurrence of 

 the second sound. This may be determined with approximative 

 correctness, and is found to vary from -301 to '327 sec., occupying 

 from 40 to 46 p.c. of the whole period, and being fairly constant 

 for different rates of heart beat. That is to say in a rapidly beating 

 heart it is the pauses which are shortened and not the duration 

 of the actual beats. 



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 undoubt3dly 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 con- 

 tractions. Similarly the occurrence of the second sound, which, 

 as we have seen, is certainly due to the closure of the semilunar 

 valves, may in accordance with the view expounded a little 

 while back, be taken to mark the close of the ventricular systole. 

 And on this view the interval between the beginning of the 

 first and the occurrence of the second sound may be regarded 

 as indicating approximatively the duration of the ventricular 

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

 contracting. 



By an ingenious arrangement a microphone attached to a 

 stethoscope may be made to record the heart sounds through the 

 stimulation of a muscle-nerve preparation ; and the record so 

 obtained may be compared with the various cardiac curves. When 

 this is done, the first sound is found to begin somewhere on the 

 systolic ascent of the ventricular curve, the exact point varying, 

 and the second sound to occur just as the ventricular curve begins 

 its diastolic descent. 



There has been however as we stated above great divergence of 



