BY DR. BURDON-SANDERSON. 265 



60. The Cardiograph. The cardiograph I use is shown 

 in fig. 230. Its most important part is a hollow disk, the rira 

 and back of which are of brass; the front is of. thin India- 

 rubber membrane. This disk is called a tympanum. To the 

 brass back a flat steel spring is screwed, which is bent twice at 

 right angles in the same direction, in such a way that it over- 

 hangs the India-rubber membrane. The extremity of this 

 spring, which is exactly opposite the centre of the face of the 

 tympanum, is perforated by a steel screw, the point of which 

 rests on the membrane, while its head is surmounted by an 

 ivory knob. The tympanum is further provided with three 

 adjusting screws, by which, when in use, it rests on the wall 

 of the chest, with its face parallel to the surface, and can be 

 approximated or withdrawn at will. It is evident that when 

 the screws are so adjusted that the spring presses on the chest, 

 whatever movements of expansion or retraction are made by 

 the surface to which it is applied are communicated to it, and 

 by it to the India-rubber membrane with which its point is in 

 contact. The cavity of the disk communicates by a vulcanized 

 India-rubber tube with a second tympanum, represented in fig. 

 231, in such a way that the two tympana and the tube inclose 

 an air-tight cavity. The result of this arrangement is, that 

 whatever movement is performed by the first is simultaneously 

 reproduced, but in the reverse direction, by the second. If 

 the tympana are of equal area, the extents of the primary and 

 secondary movements are equal. When, as is usually the 

 case, the areas are unequal, the extent of movement is approxi- 

 mately inversely proportional to tha areas. The movement of 

 the second tympanum is magnified and inscribed on the regis- 

 tering cylinder by a lever in the manner explained in a pre- 

 vious paragraph. By this apparatus a tracing is obtained, 

 which is an exact representation of the movements of the sur- 

 face against which the spring is applied, so that, if the instru- 

 ment is graduated, it may be used not only for the purpose of 

 estimating the relative duration of those movements, but for 

 measuring their extent. 



For the purpose of studying the cardiac impulse in the hu- 

 man chest, the subject should be allowed to rest supine on a 

 flat surface, with his head on a pillow. The impulse is sought 

 for in the normal position, i.e., in the space between the fifth 

 and sixth ribs, about half an inch nearer the sternum than the 

 mammary line (the line which passes vertically through the 

 nipple). On applying the cardiograph in this position, with 

 the ivory knob pressing against the seat of impulse, a 'tracing 

 is always obtained which has the general characters exhibited 

 in Fig. 232a, in which the moment of hardening is indicated 

 by a sudden ascent of the lever, and the end of the ventricular 

 systole by an equally marked, but not so sudden, descent. If 



