CHAP, iv.] THE VASCULAR MECHANISM. 141 



by the auricular and ventricular levers really represent variations 

 in the pressure exerted on the respective ampullse, and so far are 

 instructive; but they must not be taken as representing variations 

 in the pressure exerted on the blood in' the several cavities. For 

 we can easily conceive that, in the systole of the ventricle for 

 instance, the contraction of the muscular walls might continue 

 after all the blood contained in the ventricle had been driven out. 

 In such a case the ventricle would continue to press upon the 

 ampulla, and this continued pressure would be transmitted to the 

 lever, and indicated on the curve; but we should be in error in 

 interpreting this part of the curve as meaning that the ventricle 

 was still continuing to exert pressure on the blood as yet remain- 

 ing in its cavity. With this caution, and with the remark that the 

 tracing of the cardiac impulse is very unlike the usual cardio- 

 graphic tracings taken from man, we may use the curves to deduce 

 the following conclusions. 



A complete cardiac cycle is comprised between the vertical 

 lines I and II. The recording surface was travelling at such a 

 rate that the intervals between any two of the thin vertical lines 

 corresponds to one-tenth of a second. Hence in this case (the 

 heart being that of a horse) the whole cardiac cycle occupied 

 about fths of a second. Any point in the cycle might of course 

 be taken as its commencement. In the figure, the cycle is 

 supposed to begin shortly after the end of the ventricular systole, 

 and the beginning of the diastole. 



On examining the three curves we see, at a, a steady rise of the 

 auricular, accompanied by similar gradual ascents of the ventricular 

 and also of the cardiograph lever. These may be interpreted as 

 indicating that the blood is pouring from the great veins into the 

 auricle, increasing the pressure there, and at the same time 

 passing on into the ventricle, increasing the internal pressure 

 there as well, a, and also by distending the ventricle, causing 

 it to press somewhat on the chest-wall and thus to raise the 

 cardiograph lever, a . This continues for about -j^ths of a second, 

 and is then followed by the sudden rise of auricular pressure b due 

 to the auricular systole, followed by a sudden fall as the blood 

 escapes into the ventricle and the systole ceases. The sudden 

 entrance of blood into the ventricle causes a sudden increase of the 

 pressure in the ventricle as indicated by the ventricular lever b', and 

 a sudden increase in the pressure on the chest-wall b". The 

 auricular systole is followed immediately by the sudden strong 

 ventricular systole c', the lever rising very abruptly. Owing to 

 the presence of the tricuspid valves, the pressure exerted by the 

 ventricular systole is kept off the auricle almost altogether; but 

 the chest-wall, as shewn by the tracing at c", feels the sudden 

 increase of the pressure of the ventricle against it. The most 

 important points concerning this rise of ventricular pressure are 

 that it is sudden in its onset and also rapid in its decline, and 



