248 NEGATIVE PRESSURE IN VENTRICLES. [BOOK i. 



merely shews 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 appearance 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 shewn 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 shew 

 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 insufficient 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 shewn 

 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 ven- 

 tricle ; 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. 39, 40, 41) shew, 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 develope a nega- 

 tive 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 some- 

 what 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 assist the circulation by 

 sucking the blood which has meanwhile been accumulated in 

 the auricle from that cavity into the ventricle, the auriculo-ven- 



