THE CARDIAC IMPULSE 161 



the period which precedes the ventricular systole the quiet heart rests with its 

 apex against the wall of the chest. When the ventricles contract, their walls 

 suddenly become firm and tense. Being firmly attached at the base the effect 

 of the movement is to press the hardened ventricle against the chest wall. 

 The discharge of the contents of the ventricle into the curved aorta intensi- 

 fies this pressure by its mechanical effect in tending to straighten the curve 

 of that vessel and thus holds the ventricle in firm contact with the chest. 

 It is this sudden pressure of the contracting heart against the chest wall that 

 is felt on the outside. The impact or shock is possibly more distinct because 

 of the partial rotation of the whole heart toward the right and front along 

 its long axis. The movement of the chest wall produced by the ventricular 

 contraction against it may be registered by means of an instrument called 

 the cardiograph; and the record or tracing, called a cardiogram, corresponds 



FIG. 157. Tvpical Cardiogram (upper trace) from the Dog. Taken simultaneously with the 

 aortic pressure (middle) and intra ventricular pressure (lower) tracings. Time in o.oi of a second. 

 (HUrthle.) 



almost exactly with a tracing obtained by an instrument applied over the 

 contracting ventricle itself. 



The cardiograph, figure 156, 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 tambour supplied 

 with a long and light writing lever. The shock of the heart's impulse being 

 communicated 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 

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