THE CARDIAC IMPULSE l8l 



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 



Screw to adjust the lever 



i 



Writing lever Tambour Tube to the cardiograph 



FIG. 156. Marey's Tambour, to which the Movement of the Column of Air in the 

 Cardiograph is Conducted by a Tube, and from which it is Communicated by the Lever 

 to a Revolving Cylinder so that the tracing of the movement of the cardiac impulse is 

 obtained. 



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. Typical Cardiogram (upper trace) from the Dog. Taken simultaneously 

 with the aortic pressure (middle) and intraventricular pressure (lower) tracings. Time 

 in o. 01 of a second. (Hiirthle.) 



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 



