THE CIRCULATION OF THE BLOOD. 281 



with the pressure in the left ventricle (Fig. 123), and by comparing 

 these with the curve of the successive differences of pressure in these 

 two cavities as determined by the "differential manometer," it be- 

 comes possible to mark on the ventricular pressure curve the points 

 at which the foregoing events take place. 



During the systolic plateau the blood is passing from the ventricle 

 into the aorta. Independent of the slight elevations and depressions 

 there is an absolute fall of pressure between the beginning and the 

 end of the plateau. There is also a corresponding fall in the aortic 

 pressure, corresponding to these two points. The curve of the dif- 

 ference of pressure shows, however, that the ventricular pressure is 

 slightly higher than the aortic. This fall in both ventricular and 

 aortic pressures is due to the escape of blood from the arterial 

 into and through the capillary system. At 3, however, whether 

 completely emptied or not, the ventricle suddenly relaxes, and its 

 pressure soon falls below that in the aorta. As soon as this takes 

 place the semilunar valves must close, if regurgitation into the ven- 

 tricular cavity is to be prevented. A comparison of the aortic pres- 

 sure curve shows a slight notch, the "dicrotic notch," just preceding 

 a slight elevation, the "dicrotic" wave. This notch is taken as the 

 moment when the semilunar valves close. The corresponding point 

 on the ventricular pressure curve has been placed just where the 

 ordinate 4 cuts the descending portion. As yet, however, the pressure 

 is higher in the ventricle than in the auricle, and so continues until 

 near the line of atmospheric pressure. At this point the pressure in 

 the auricle, due to the accumulation of blood during the ventricular 

 systole, now forces open the mitral valve and the blood flows into the 

 ventricle. The opening of the mitral valve occurs about the point 

 where the ordinate 5 cuts the curve. 



The ventricular pressure curve affords but slight, if any, indication 

 of the auricular systole. It apparently does not give rise to any 

 noticeable increase in the ventricular pressure. The slight rise in 

 the pressure curve, which just precedes the abrupt rise due to the 

 ventricular systole, may be taken as an indication of an increasing 

 pressure due to the inflow of blood from the auricle. As soon as 

 the pressure in the ventricle exceeds that in the auricle the mitral 

 valve closes. This is marked on the curve where the ordinate cuts 

 it, at o. Coincident with this, the ventricular systole begins, and as 

 the blood is contained within a closed cavity the pressure abruptly 

 rises. A comparison of the aortic curve shows that for a short time 

 during the ventricular systole, the pressure is falling, but at one point 

 it turns at a sharp angle and rapidly rises. This is an indication that 

 the semilunar valves are suddenly thrown open and the blood begins 

 to pass into the aorta. This event occurs at a moment marked on 

 the ventricular curve by the ordinate i. Beyond this point the pres- 



