212 DURATION OF CAEDIAC PHASES. [BOOK i. 



diastole. It is obvious, moreover, from what has gone before, that 

 the semilunar valves are closed before it occurs, and we may 

 dismiss the view which has been put forward that it is of the same 

 nature as the negative pressure which makes its appearance behind 

 a column of fluid moving rapidly and suddenly ceasing, as when a 

 rapid flow of water through a tube is suddenly stopped by turning 

 a tap. We may probably attribute it to the relaxation of the 

 ventricular walls. This, as all the curves shew, is a rapid process, 

 something quite distinct from the mere filling of the ventricular 

 cavities with blood by the venous inflow; and, though some 

 have objected to the view, it may be urged that this return 

 of the ventricle from its contracted condition to its normal form 

 would develop a negative pressure. This return we may probably 

 regard as simply the total result of the return of each fibre to 

 its natural condition, though some have urged that the extra 

 quantity of blood thrown into the coronary arteries at the systole 

 helps to unfold the ventricles somewhat in the way that fluid 

 driven between the two walls of a double-walled collapsed ball or 

 cup will unfold it. 



We may further conclude that such a negative pressure, when 

 it occurs, will assist the circulation (and it may be remarked that 

 the return of the thick-walled left ventricle naturally exerts a 

 greater negative pressure than the thin-walled right ventricle) by 

 sucking the blood which has meanwhile been accumulated in the 

 auricle from that cavity into the ventricle, the auriculo-ventricular 

 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 should, however, be added that many observers find the 

 development of a negative pressure to be by no means of such 

 constant occurrence, and not to reach such marked limits as might 

 be inferred from the numbers given above, at least in the unopened 

 chest. If so it cannot be an important factor in the work of the 

 circulation. 



118. The duration of the several phases. We may first of all 

 distinguish 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 contraction. (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 

 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 contracting 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 shew, apart from any accurate measurements, that the systole 



