172 THE VASCULAR MECHANISM. 



auricular contents into the ventricle increases the front-to-back diameter of 

 the ventricle, and the momentum of the rapid stroke being considerable, the 

 lever is in each case carried too far forward, so that the rise is followed by a 

 fall, producing a notch. A similar though somewhat different shoulder is 

 also seen in the cardiogram, Fig. 58. In the curve of ventricular pressure 

 taken by means of the cardiac sound (Fig. 55) there is a similar temporary 

 increase b f in the ventricular pressure coincident with the auricular stroke b, 

 and in the " piston " pressure-curve of the rapidly beating heart (Fig. 56, A) 

 there is a similar shoulder b just preceding the rise of the ventricular systole. 

 The meaning of the last curve is, however, doubtful, for in the similar curve 

 of the more slowly beating heart (Fig. 56, B) it occurs immediately after the 

 relaxation of the ventricle, some time before the occurrence of the auricular 

 systole, and in many curves taken by the same method is absent altogether. 

 The exact meaning, therefore, of the shoulder b in the other curves must be 

 left at present undecided. 



124. We have still to consider the negative pressure shown by the 

 minimum manometer. This instrument, as we have said, merely shows that 

 the pressure in the ventricle (or auricle) becomes negative at 'some phase 

 or other of the cardiac cycle, but does not tell us in which phase it occurs. 



Now there are two ways in which such a negative pressure might originate. 

 In the first place, as we have just seen, a negative pressure makes its appear- 

 ance in the rear of the column of blood driven from the ventricle into the 

 aorta with great suddenness and rapidity. But this negative pressure, as we 

 have also seen, follows the column into the aorta past the semilunar valves, 

 and in part, at all events, determines the closure of the semilunar valves. 

 Hence if this is the negative pressure which the minimum manometer 

 records, it ought to be shown not only when the end of the tube connected 

 with the manometer is in the cavity of the ventricle, but also when the tube 

 is slipped out of the ventricle just past the semilunar valves. When the 

 tube, however, is in the latter situation the manometer does not show the 

 same marked negative pressure that it does when the tube is in the ventricle ; 

 the negative pressure which occurs in the aorta at each beat is sufficient to 

 produce such an effect on the minimum manometer as is produced when the 

 instrument is in the ventricle. Hence we infer that the negative pressure 

 shown by the minimum manometer is not produced in this way. We may, 

 moreover, conclude that the semilunar valves are closed before this negative 

 pressure makes its appearance in the ventricle ; otherwise, however produced, 

 it would be transmitted from the interior of the ventricle through the open 

 valves to the root of the aorta beyond. 



But there is another event which might give rise to a negative pressure. 

 The relaxation of the ventricular walls is, as the curves (Figs. 56, 57, 58) 

 show, a rapid process, something quite distinct from the mere filling of the 

 ventricular cavities with blood from the auricles ; and, though some have 

 objected to the view, it may be urged that this return of the ventricle from 

 its contracted (and emptied) condition to its normal form would develop a 

 negative pressure. This return is probably simply the total result of the 

 return of each fibre or fibre cell to its natural condition, though some have 

 urged that the extra quantity of blood thrown into the coronary arteries at 

 the systole helps to unfold the ventricles somewhat in the way that fluid 

 driven between the two walls of a double-walled collapsed ball or cup will 

 unfold it. 



Accepting the return of the ventricles to their normal form as the cause 

 of the negative pressure (and it may be remarked that the return of the 

 thick-walled left ventricle naturally exerts a greater negative pressure than 

 the thin-walled right ventricle), it is obvious that the negative pressure will 



