170 THE VASCULAR MECHANISM. 



change take 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. 57) we may 

 FIG. 58.^ ^ 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 

 carchogram from Man' ' diameter is somewhat lessened by a dim- 



inution or the volume of the ventricles 



due to the escape of blood into the great arteries ; and the cardiographic 

 tracing admits of a similar interpretation the apex relaxes its pressure on 

 the chest-wall. We may extend the same interpretation to the pressure 

 curve (Fig. 56). 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 pres- 

 sure in the ventricle is made continuous 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 canula 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 

 distention 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 that the point c' in the three figures under discussion, where the 

 descent of the lever changes in rate, becoming less rapid, corresponds to 

 the end of the outflow from the ventricle ; but this is not certain, and, 

 indeed, the exact interpretation of this part of the curve is especially 

 difficult. 



The escape from the ventricle is rapid and forcible ; the flow ceases sud- 

 denly. Hence, as we have already stated, 116, owing to the column of 

 blood tending to move on by virtue of its inertia after the propelling force 

 has ceased to act, a negative pressure makes its appearance behind the 

 column of blood discharged from the ventricle, and as soon as the column 

 is lodged in the aorta leads to a reflux toward the ventricle. This reflux 



