THE CIRCULATION OF THE BLOOD 



In the tracing, figure 162, the intervals between the vertical lines rep- 

 resent periods of a tenth of a second. The parts on which any given vertical 

 line falls represent simultaneous events. It will be seen that the contraction 

 of the auricle, indicated by the marked curve at A in the first tracing, causes 

 a slight increase of pressure in the ventricle which is shown at A ' in the second 

 tracing, and produces also a slight impulse, which is indicated by A" in the 

 third tracing. The closure of the semilunar valves causes a momentarily 

 increased pressure in the ventricle at D', affects the pressure in the auricle D, 

 and is also shown in the tracing of the cardiac impulse D". 



The large curve of the ventricular and the cardiac impulse tracings, 

 between A' and D', and A" and D", are caused by the ventricular contrac- 

 tion, while the smaller undulations, between B and C, B f and C', B" and C", 

 are caused by the vibrations consequent on the tightening and closure of 

 the tricuspid and mitral valves. 



FlG. 163. Apparatus for Recording the Endocardiac Pressure. (Rolleston.) 



It seems by no means certain that Marey's curves properly represent 

 the variations in intraventricular pressure. Objection has been taken to 

 his method of investigation : First, because his tambour arrangement does 

 not admit of both positive and negative pressure being simultaneously re- 

 corded; second, because the method is applicable only to large animals, 

 such as a horse; third, because the intraventricular changes of pressure 



