THE PERIOD OF S YSTOLIC O UTPUT. 2 1 



deduction of the observer's reaction time, has been taken as the point of 

 closure. 



The second sound, however, is not produced by the closure itself, but 

 by the tension of the valves which follows the closure, some fraction of 

 a second later. The closure takes place at the moment when the 

 ventricular pressure falls to the slightest degree below the aortic 

 pressure. 



By Hiirthle, the differential manometer l was employed to settle the 

 moment. A byway cannula was passed into the aorta and ventricle, and 

 the time marked on the intraventricular pressure curve when the 

 aortic pressure surpassed the ventricular pressure. 2 The aortic and 

 ventricular pressure curves are taken simultaneously with the differ- 

 ential curve by means of Y tubes. The point found corresponds in time 



FIG. 15. A, aortic; D, differential; V, ventricular pressure. Hiirthle. 



with the bottom of the dicrotic notch on the aortic curve, and is at the 

 end of the plateau (Fig. 15, 3). Fredericq, Porter, and Starling and 

 Bayliss agree with this conclusion. 



Chauveau and Marey placed the closure much lower down on the 

 descending limb of the curve, at a point where a notch occurs in the descent. 3 

 Chauveau has recently attempted to confirm this conclusion by an ingenious 

 experiment. He passed a byway sound down the carotid of the horse, until 

 one ampulla lay in the aorta, and the other in the ventricle. At the seat of 

 the semilunar valves, an electric contact was fixed to the sound ; the contact 

 was made by the closure of the valve, and a signal wrote the moment of 

 contact on the tracing. 4 It is most probable, however, that the contact 

 would be made, not by the closure, but by the tension of the valves, 

 for a certain amount of pressure would probably be required to effect the 

 contact. 



The differential manometer also shows the moment when the intra- 

 ventricular pressure overtops the aortic pressure, and the semilunar 

 valves open (Fig. 15, 1). This moment is found to be a little below the 



1 See p. 18 and Fig. 11, p. 17. 



a Arch.f. d. ges. Physiol., Bonn, Bd. xlix. S. 54. 



3 Mem. Acad. de med., Paris, 1863, tome xxvi. p. 298. 



4 Compt. rend. Acad. d. sc., Paris, tome cxviii. pp. 686, 690. 



