418 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



to that within the artery which it supplies with blood ; for the arterial pressure, 

 although it fluctuates, is at all times far above that of the atmosphere, and is 

 able, as we have seen, to maintain the circulation while the semilunar valve is 

 closed and the ventricular muscle is at rest. On the other hand, the pressure 

 within the ventricle, when at its highest, rises decidedly above the highest 

 arterial pressure, and thus the ventricle can overcome this and other opposing 

 forces, open the valve, and expel the blood. These facts have been stated 

 already. In falling, however, the pressure within the ventricle not only sinks 

 below that in the artery, and so permits the semilunar valve to close, but 

 sweeps downward to a point, it may be, below the pressure of the atmosphere, 

 and, in so doing, falls below the pressure in the auricle, and permits the open- 

 ing of the auriculo-ventricular valve and the entrance of blood out of the 

 auricle and the veins. As such a great range of pressure occurs in either 

 ventricle of a heart which is repeating its cycles with entire regularity, it is 

 presumable that at every cycle the pressure not only rises above that in the 

 arteries but may sink below that of the atmosphere. 



Methods of Recording the Course of the Ventricular Pressure. It 

 now becomes of interest to ascertain, if possible, not only the range, but the 

 exact course, of these swift variations of pressure ; the causes of them, and the 

 effects which accompany them. It is hard to obtain, by the graphic method, a 

 correct curve of the pressure within either ventricle. We have seen that the 

 mercurial manometer is useless for this purpose ; and it is very difficult to 

 devise any self-registering manometer which shall truly keep pace with fluctu- 

 ations at once so great and so rapid. The true form of this pressure-curve, 



FIG. 106. Diagram of the elastic manometer : A, auricle ; V, ventricle ; D, drum of the kymograph, 

 revolving in the direction of the arrow, and covered with smoked paper; L, recording lever in contact 

 with the revolving drum. (The working details of the instrument are suppressed for the sake of clear- 

 ness.) 



therefore, still is partially in doubt, and is the subject of controversies which 

 largely resolve themselves into contests between rival instruments. We may 

 pass by without mention methods which are either antiquated or little used. 

 The following characters are common to the manometers with which the most 

 serious attempts have lately been made to obtain a true and minute record of 

 the fluctuations of pressure, even if great and rapid, within the heart or the 

 vessels (see Fig. 106). As in the case of the mercurial manometer, a cannula, 



