CHAP, iv.] THE VASCULAR MECHANISM. 249 



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 

 further be urged, in support of this view, that even when the 

 thorax is opened, so that the respiratory movements can no longer 

 act towards producing a negative pressure in the auricle and 

 great veins ( 131), a minimum manometer placed in the right 

 auricle shews 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 circum- 

 stances 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. 



136. 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, lasting to d in Figs. 39, 40, 41. (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 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 

 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 

 these give only approximate results. Naturally the most interest 

 is attached to the duration 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 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. 



