THE PHENOMENA NOTED DURING EACH CARDIAC CYCLE 301 



Thus, it has been suggested that the sudden cessation of the ventricu- 

 lar systole forces the column of blood onward with a certain momen- 

 tum, while in the wake of it is developed an area of very low pressure. 

 The second and more probable explanation takes into account the 

 fact that the sudden discharge of the ventricular contents gives rise 

 to an abrupt distention of the trunks of the aorta and pulmonary artery 

 which in turn leads to an expansion of the basal portion of the heart, 

 inclusive of the adjoining extent of the ventricle. This effect is espe- 

 cially evident in the left cavity, because its wall is more compact and 

 relatively unyielding. In an experimental way this condition may be 

 imitated by suddenly distending the roots of the aorta and pulmonary 

 artery with fluid, while the intraventricular pressure is being registered 

 by a mercury manometer which is connected with this cavity by means 



Fig. 154. — The Curv^e of Intraventricular Pressure. 

 AB, systole of ventricle; BC, plateau; CD, diastole; DA, pause. 



of a trocar inserted directly through its wall. Every distention then 

 gives rise to a negative pressure. 



The systole of the ventricle is indicated in Fig. 154 by the abrupt 

 rise occurring between A and B. The normally beating ventricle, how- 

 ever, does not relax immediately upon the completion of its contrac- 

 tion but remains in this condition for a brief period of time. We 

 observe, therefore, that the maximal value of the pressure is maintained 

 and that the summit of the curve is flat. The ''plateau" so formed^ 

 is indicated in the figure by the letters B and C. The subsequent re- 

 laxation of the ventricles occurring between C and D, is accompanied 

 by a rapid fall in pressure. During the pause the pressure rises very 

 slowly, owing to the gradual influx of blood through the just barely 

 opened auriculoventricular valves. As has been stated above, the 

 ventricles are filled for the most part before the succeeding auricular 

 contraction actually begins, so that the latter merely serves the purpose 

 of adding a certain extra amount of blood. 



1 The claim has recently been made by Straub that the summit is pointed ; 

 Hiirthle, however, has proved this view to be erroneous. 



