CIRCULATION. 



421 



are due, partly at least, to the inertia of the transmitting liquid ; but, with due 

 allowance made for these, the cardiac pressure is seen to maintain itself at a 

 high point throughout most of the systole until the rapid fall begins. During 

 this period of high pressure, the height about which the fluctuations occur may 

 remain nearly the same; or this height may gradually increase, or gradually 

 decrease, up to the beginning of the rapid fall. As is shown by Figure 108, 

 this course of the systolic pressure causes its curve to bend alternately down- 

 ward and upward between the end of its greatest rise and the beginning of its 

 greatest fall ; but between these two points the general direction of the curve 

 approaches the horizontal, and therefore entitles this portion of it to the name 

 of the "systolic plateau." The best of the manometers with air-transmis- 

 sion yields a curve of the" pressure within the ventricle which presents a 

 different pic\ure (Figs. 107 and 109). The steeply rising line may diminish 



i 2 



Millimeters of 

 mercury. 



Line of atmospheric 

 pressure. 



Tenths of a second. 



FIG. 109. Magnified curve of the course of pressure within the left ventricle of the dog, the chest 

 being open ; to be read from left to right. Recorded by the elastic manometer with transmission by air. 

 The ordinates have the following meaning : 1, the closure of the mitral valve ; 2, the opening of the semi- 

 lunar valve ; 3, the closure of the semilunar valve ; 4, the opening of the mitral valve (von Frey). 



its steepness somewhat as it ascends, but its rapid turn at the highest point of 

 the curve is succeeded by no plateau. The line simply describes a single peak, 

 and begins the descent which marks the rapid fall of pressure recognized by 

 all observers. In these peaked curves this descent is often steepest in its 

 middle part. Such a peaked curve would indicate, of course, that there is no 

 such thing as the maintenance, during any large part of the systole of the 

 ventricles, of a varying but high pressure. The experienced observer who is 

 the chief defender of the peaked curve holds the plateau to be a product either 

 of too much friction within the manometer tubes, or of a faulty position of the 

 cannula within the heart, whereby communication with the manometer is, for 

 a time, cut off. The able and more numerous adherents of the plateau, on the 

 other hand, attribute the failure to obtain it to the sluggishness of the instru- 

 ment employed. Recent comparative tests of elastic manometers, and other 

 studies, would seem to show that the curves obtained by liquid transmission, 

 and which exhibit the plateau, afford a truer picture of the general course of the 

 pressure within the ventricles than the peaked curves written by means of air. 



