THE HEART. 173 



assist the circulation by sucking the blood which meanwhile has been accu- 

 mulated in the auricle, from that cavity into the ventricle, the auriculo-ven- 

 tricular valves easily giving way. At the same time this very flow from the 

 auricle will at once put an end to the negative pressure, which obviously can 

 be of brief duration only. It may be further urged in support of this view, 

 that even when the thorax is opened, so that the respiratory movements can 

 no longer act toward producing a negative pressure in the auricle and great 

 veins, a minimum manometer placed in the right auricle shows frequently 

 no pressure at all (that is, a pressure equal to that of the atmosphere) 

 and sometimes a decidedly negative pressure. Seeing that the blood under 

 these circumstances is being driven along the great veins by a pressure which 

 though low is always above that of the atmosphere, we may conclude that 

 the negative pressure produced in the ventricle is the cause of this lowering 

 of the pressure in the auricle, though it is unable to make itself felt along 

 the great veins. 



125. The duration of the several phases. We may, first of all, distin- 

 guish certain main phases: (1) The systole of the auricles. (2) The systole, 

 proper, of the ventricles, during which their fibres are in a state of contrac- 

 tion, lasting to d in Figs. 56, 57, 58. (3) The diastole of the ventricles 

 that is to say, the time intervening between their fibres ceasing to contract 

 and commencing to contract again. To these we may perhaps add (4) The 

 pause or rest of the whole heart, comprising the period from the end of the 

 relaxation of the ventricles to the beginning of the systole of the auricles ; 

 during this time the walls are undergoing no active changes, neither con- 

 tracting nor relaxing, their cavities being simply passively filled by the 

 influx of blood. 



The mere inspection of almost any series of cardiac curves, however 

 taken those, for instance, which we have just discussed will show, apart 

 from any accurate measurements, that the systole of the auricles is always 

 very brief, that the systole of the ventricles 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 relaxa- 

 tion 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 these give only 

 approximate results. Naturally, the most interest is attached to the dura- 

 tion of events in the human heart. 



The datum which perhaps has been most largely used is the interval 

 between the beginning of the first and the occurrence of the second sound. 

 This may be determined with approximate correctness, and is found to vary 

 from 0.301 to 0.327 second, occupying from 40 to 46 per cent, 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 com- 

 mencement of its becoming audible may be slightly behind the actual begin- 

 ning of the muscular contractions. Similarly the occurrence of the second 



