THE .CIRCULATION OF THE BLOOD 285 



in the aortic pressure, corresponding to these two points. The curve of 

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

 somewhat 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 (see Fig. 122), however, whether completely 

 emptied or not, the ventricle suddenly relaxes, and its pressure soon falls 

 below that in the aorta. The immediate result is the sudden closure of 

 the semilunar valves by the backward movement of the blood under the 

 recoil of the elastic walls of the arteries. Regurgitation into the ventricle 

 is thereby prevented. An examination of the aortic pressure curve shows 

 a slight notch, the "dicrotic notch," just preceding a slight elevation, the 

 "dicrotic" wave. This notch occurs at the moment that the semilunar 

 valves close. The corresponding point on the ventricular pressure curve 

 has been placed just where the ordinate 4 cuts the descending portion. 

 At this moment, however, the pressure is still higher in the ventricle than 

 in the auricle, and continues so until it approaches the line of atmospheric 

 pressure. At this point the pressure in the auricle, due to the accumula- 

 tion 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. From 

 5, the opening of the auriculo-ventricular valve to the beginning of the 

 succeeding auricular contraction, a period of 0.32 sec., in man the entire 

 heart remains at rest, during which its cavities are filling with blood. 

 When this has been accomplished a new cycle is inaugurated by the con- 

 traction of the auricle. 



The ventricular pressure curve, as above determined, 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 a result of the auricular 

 systole. Immediately following this latter event the ventricular systole 

 begins, whereupon the mitral valve is suddenly and tightly closed. This 

 is marked on the curve where the ordinate cuts it, at o. With the closure 

 of the mitral valve the blood becomes imprisoned within a closed cavity, 

 closed at one orifice by the mitral valve and at the other orifice by the 

 semilunar valves. As the blood is incompressible the intra-ventricular 

 pressure under the force of the ventricular contraction rapidly rises and 

 continues so to do until the pressure in the ventricle exceeds that in the 

 aorta, at which moment the semilunar valves are suddenly opened and 

 the blood discharged. An examination of the aortic curve shows that for 

 a short time during the ventricular systole the pressure is falling, but at 

 one point the curve 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 pressure continues to rise, for the aortic pressure must not only be 

 exceeded, but a certain .velocity must be imparted to the blood, other- 

 wise it would not be possible for the ventricle to complete its discharge. 

 Between the ordinates i and 4, the semilunar valves remain open and the 



