CHAP, iv.] THE VASCULAR MECHANISM. 263 



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

 tricles is always very prolonged, always occupying a considerable 

 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 

 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, 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 ocpur just as the ventricular curve begins 

 its diastolic descent. 



There has been however as we stated above great divergence of 

 opinion and much discussion as to the exact time of the closure of 

 the semilunar valves ; the view given in the text above, though it 

 seems to be supported by adequate arguments, is not the only one 



