CIRCULATION. 423 



or the opening of the semilunar valve, as the case may be. By a converse 

 process, the moment of opening of the cuspid valve, or of closing of the semi- 

 lunar, may also be ascertained. The practical difficulties in the way of 

 applying this method to the ventricle and auricle are much greater than to the 

 ventricle and artery. By another application of the principle just described, a 

 " differential manometer " has been devised for the purpose of registering as a 

 single curve the successive differences, from moment to moment, between the 

 ventricular and auricular pressures, or the ventricular and arterial pressures 

 (see Fig. 110). To this end, two elastic manometers are fastened immovably 

 together, and their two elastic disks, instead of bearing upon separate levers, 

 are made to bear upon a single one, which has its fulcrum between the disks, 

 and is a lever not of the third order, but of the first, like a common balance. 

 As the lever or beam of the balance turns from the horizontal as soon as the 

 scales are pressed upon by unequal weights, so the lever of the differential 

 manometer turns as soon as the disks are unequally affected by the pressures 

 within the ventricle and the auricle, or the ventricle and the artery. As, how- 

 ever, the pressures upon the scales are from above, while those upon the disks 

 are from below, the disk which tends to " kick the beam " is the one acted 

 upon by the greater pressure, instead of by the less, as in the case of the scales. 

 The manometric lever marks its oscillations as a curve upon the kymograph 

 by the help of a second or " writing lever " connected with it. The persistence 

 of exactly equal pressures, no matter what their absolute value, in the two 

 manometers would cause a horizontal line to be drawn by the writing lever. 

 This would serve as a base-line. The differential manometer is a valuable 

 instrument, although it is evident that where such minute differences of space 

 and time are recorded as a curve by such complicated mechanisms, the sources 

 of error must be numerous and difficult to avoid. 1 



The methods which proceed by the measurement of differences of pressure 

 may sometimes be controlled, or even replaced, by an easier method, as follows : 

 If two manometers simultaneously record on the same kymograph the pressure- 

 curves of the ventricle and the auricle, or of the ventricle and the artery, any 

 very sudden change of pressure, produced in auricle or artery at the opening or 

 shutting of a cardiac valve, will produce a peak or angle in the curve of pres- 

 sure of the auricle or artery. By the rules of the graphic method the point in 

 the pressure-curve of the ventricle can easily be found which was written at 

 the same instant with the peak or angle in the auricular or arterial curve. 

 That point upon the ventricular curve, when marked, will indicate the instant 

 of opening or shutting of the valve in question. In the pressure-curve ob- 

 tained from the aorta close to the heart, there is a sudden angle which clearly 

 marks the instant when the opening of the semilunar valve leads to the sudden 

 rise of pressure which causes the up-stroke of the pulse (see Fig. 108). Again, 

 the fluctuation of aortic pressure which we shall learn to know as the " dicrotic 

 wave " begins at a moment which many believe to follow closely upon the clos- 

 ure of the semilunar valve. That moment may be indicated by a notch in the 

 1 K. Hiirthle: Pftuger's Archivfur die gesammte Physiologie, 1891, vol. 49, p. 45. 



