CHAP, iv.] THE VASCULAR MECHANISM. 



201 



to objection, we may obtain what is very nearly the same thing, 

 viz. a cardiographic tracing (Fig. 46) or cardiogram, that is to say, 

 a tracing of the cardiac impulse, a curve of the changes in the 

 pressure exerted by the apex of the heart on the chest-wall. 



Various forms of cardiograph have been used to record the cardiac 

 impulse. In some the pressure of the impulse is transmitted directly 

 to a lever which writes upon a travelling surface. In others the 

 impulse is, by means of an ivory button, brought to bear on an air- 

 chamber, connected by a tube with a tambour like that in Fig. 37 ; the 

 pressure of the cardiac impulse compresses the air in the air-chamber, 

 and through this the air. in the chamber of the tambour, whereupon the 

 lever is raised. In others the impulse, being received by a small, 

 elastic bag rilled with fluid and introduced through an opening made 

 in the chest-wall, the pleura being left intact, is transmitted through 

 fluid along a tube to a membrane-manometer. Or, to avoid opening 

 the chest-wall, the tube may be made to begin in a small, trumpet- 

 shaped opening or " receiver " covered with an elastic membrane, bearing 

 a central button of cork or other material ; the button being lightly 

 pressed on the spot where the impulse is felt, the impulse is transmitted 

 along the fluid of the tube from the elastic membrane of the receiver 

 to that of the manometer. 



In Fig. 46 we give two such cardiograms obtained by different 

 methods, in Fig. 54 a more diagrammatic curve. 



FIG. 46. CARDIOGRAMS. 

 The left-hand figure is from Roy and Adami. 



Since it is the contraction of the ventricular fibres which is the 

 actual propelling force, an exact record of this contraction, after 

 the manner of a muscle-curve, would serve, could it be obtained, 

 as the basis of discussion. Owing to the intricate arrangement of 

 the cardiac muscular fibres, such a simple record cannot be 

 obtained ; the nearest approach to it is the record of the changes 

 in the distance between two points on the surface of the heart 

 brought about during a beat. 



