CHAP, iv.] THE VASCULAR MECHANISM. 245 



fibres, the pressure becomes greater and greater. At the point c 

 a change takes place in all three curves, the rise is converted into 

 a fall, which however is very gradual as far as d. In the case of 

 the front-to-back diameter curve (Fig. 40) we may interpret this 

 as meaning that while the continued contraction of the muscular 

 fibres still maintains that change in the form of the ventricle by 

 which the front-to-back diameter is increased, that same diameter 

 is somewhat lessened by a diminution of the volume of the ventricles 

 due to the escape of blood into the great arteries, and the cardio- 

 graphic tracing admits of a similar interpretation, the apex relaxes 

 its pressure on the chest-wall. We may extend the same inter- 

 pretation to the pressure curve (Fig. 39). Somewhere about c 

 the pressure in the (left) ventricle has become higher than the 

 pressure in the aorta, and in consequence blood escapes from 

 the former into the latter. Whether the exact moment of 

 the opening of the valves is absolutely identical with the turn 

 of the curve at c, the curve beginning to fall at the moment 

 when the area of high pressure in the ventricle is made con- 

 tinuous with the area of lower pressure in the aorta, or whether 

 it occurs a little before c, the still increasing contraction of the 

 ventricular fibres still increasing the pressure on the column of 

 blood as it begins to move from the cavity of the ventricle into 

 the aorta, may be left for the present undecided. The sudden fall 

 from d to a admits of only one interpretation and that in all the 

 curves ; this can only be due to the sudden relaxation of the 

 muscular fibres of the ventricle, whereby the front- to-back diameter 

 suddenly diminishes, the apex suddenly ceases to press on the 

 chest-wall, and the pressure which the ventricular walls were 

 previously exerting on the fluid in the cannula introduced into its 

 cavity also suddenly ceases. From b' to d then the ventricular 

 walls are still contracting ; during the whole of this time the real 

 systole is being continued, but gives place at d to a rapid 

 relaxation which ushers in or forms the first part of the 

 sequent diastole. Some little time after the beginning of this 

 systole, somewhere about c, as we have seen, blood begins to 

 escape from the ventricle into the aorta ; this escape is certainly 

 completed by the time d is reached and we have reason to think 

 that it is really completed some little time before. The entrance 

 into the aorta of the column of blood ejected by the ventricle 

 distends that vessel, and the distension passes on, as we have seen, 

 along the arterial track as the pulse. If now we measure the 

 time during which the aorta, even near the heart, is being 

 distended by the injection of the ventricular contents, we find 

 this to be appreciably less than the time from c to d, during 

 which the systole of the ventricle is still going on, though the 

 contents have already begun to escape at about c. This means 

 that the ventricle, though empty, remains contracted for some 

 little time after its contents have left the cavity. It is possible 



