182 



THE CIRCULATION OF THE BLOOD 



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 tambour supplied 



with a long and light writing lever. The 

 shock of the heart's impulse being com- 

 municated 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 mov- 

 able disc of the latter to the writing lever 

 which is adjusted to a registering appa- 

 ratus. This latter generally consists of a 

 cylinder or drum covered with smoked 

 paper and revolving by clock-work with 

 a definite velocity. The point of the 

 lever writing upon the paper produces 

 a tracing of the heart's impulse, a 

 cardiogram. 



Endocardiac Pressure. The effect 

 of the muscular contractions and relaxa- 

 tions of the walls of the heart during its 

 systole and diastole is to produce varying 

 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 begin- 

 ning 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 



and quietly decreases to less than that of atmospheric pressure and remains 

 low until the beginning of the next systole. For the right ventricle the events 

 and variations are relatively the same. 



In order to determine the endocardiac pressure communication must 

 be established with the cavities of the heart. This is accomplished by a 

 tube known as a sound, which is introduced into the left ventricle by passing 

 it down the common carotid artery or into the right auricle and ventricle 



FIG. 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.) 



