CHAP, iv.] THE VASCULAR MECHANISM. 211 



have said, is unsuitable for following the rapid changes constituting a 

 single beat, may be used as a maximum or minimum instrument 

 for determining the highest or lowest pressure reached in one or 

 other of the heart's cavities during a series of beats. 



The principle of one form of maximum manometer, Fig. 55, consists 

 in the introduction into the tuho loading from the heart to the mercury 

 column, of a (modified cup-and-hull) valve, opening, like the aortic 

 semilunar valves, easily from the heart, but closing firmly when fluid 

 att'-mpts to return to the heart. The highest pressure is that which 

 drives the longest column of fluid past the valve, raising the mercury 

 column to a corresponding height. Since this column, once past the 

 valve, cannot return, the mercury remains at the height to which it was 

 raided by it, and thus records the maximum pressure. By reversing 

 tlit- direction of the valve, the manometer is converted from a maximum 

 into a minimum instrument. 



A simpler form of maximum and minimum manometer is that of 

 Hurthle, which consists of a small chamber connected with two mano- 

 meters, the opening of each manometer into the chamber being armed 

 with a valve of thin membrane, so arranged that it permits in the case 

 of one manometer, the maximum one, the entrance only of the mercury, 

 in the case of the other, the minimum one, the exit only. 



By means of the maximum manometer the pressure in the 

 left ventricle in the dog has been observed to reach a maximum 

 of about 140 mm. (mercury), in the right ventricle of about 

 60 mm. and in the right auricle of about 20 mm. These figures, 

 however, are given as examples, and not as averages. Simi- 

 larly negative pressures of from 50 mm. to 20 in the left 

 ventricle of the dog, of about 15 mm. in the right ventricle, and 

 of from 12 mm. to 7 mm. in the right auricle, have been 

 observed by the minimum manometer. Part of this diminution of 

 pressure in the cardiac cavities is due, as will be explained in a 

 later part of this work, to the aspiration of the thorax in the 

 respiratory movements. But even when the thorax is opened, and 

 artificial respiration kept up, under which circumstances no such 

 aspiration takes place, a negative pressure may be still observed, 

 the pressure in the left ventricle sinking according to some obser- 

 vations as low as 24 mm. Now, what the instrument actually 

 shews is that at some time or other during the number of beats 

 which took place while the instrument was applied (and these \\r.\\ 

 have been very few), the pressure in the ventricle sank so inuny 

 mm. below that of the atmosphere. Since, however, the m^tivi- 

 pressure may be observed when the heart is beating quite regularly, 

 each beat being exactly like the others, we may infer that the nc^at ive 

 pressure is repeated at some period or other of each cardiac cycle. 

 The instrument itself gives us no information as to the exact phase 

 of the beat in which the negative pressure occurs ; but it is clear 

 from what we have already seen that when it occurs, it must 

 take place at the end of the systole, at the beginning of the 



