ENDOCARDIAC PRESSURE 



I8 7 



and the record or tracing, called a cardiogram, corresponds almost ex- 

 actly with a tracing obtained by an instrument applied over the con- 

 tracting ventricle itself. 



The cardiograph, figure 155, consists of a cup-shaped metal box over 

 the open front of which is stretched an elastic india-rubber membrane upon 

 which is fixed a small knob of hard wood or ivory. This knob, however, 

 may be attached, as in the figure, to the side of the box by means of a 

 spring, and may be made to act upon a metal disc attached to the elastic 

 membrane. 



The knob is for application to the chest wall over the place of the 

 greatest impulse of the heart. The box or tambour communicates by 

 means of an air-tight tube with the 

 interior of a second or recording tam- 

 bour supplied with- a long and light 

 writing lever figure 156. The shock 

 of the heart's impulse being communi- 

 cated to the ivory knob, and through 

 it to the first tambour, the effect is, 

 of course, at once transmitted by the 

 column of air in the elastic tube to 

 the interior of the second recording 

 tambour, also closed, and through 

 the elastic and movable disc of the 

 latter to the writing lever which is 

 adjusted to a registering apparatus. 

 This latter generally consists of a 

 cylinder or drum covered with smoked 

 paper and revolves by clock-work 

 with a definite velocity. The point 

 of the lever writing upon the paper 

 produces a tracing of the heart's im- 

 pulse, a cardiogram. 



Endocardiac Pressure. The effect 

 of the muscular contractions and 

 relaxations of the walls of the heart 

 during its systole and diastole is to 

 produce changes of pressure on its 

 content of blood. When this pressure 

 is measured by the proper instrument 

 it is found that the pressure in the 

 left ventricle varies between wide ranges. With the beginning of the 

 muscular contraction, the pressure rises till it slightly exceeds that of the 

 pressure of the aorta, remains high for a brief interval of time, then slowly 



FlG. 158. Double Cardiac Sound 

 for Simultaneous Registration of the 

 Blood Pressure in the Right Auricle 

 and Ventricle, or in the Aorta and 

 Left Ventricle. (Hurthle.) 



