CIRCULATION OF THE BLOOD. 



507 



Sanderson's cardiograph consists of a hollow disk, the rim and back of 

 which are of brass, while the front is of thin rubber. To the back is fastened a 

 flat steel spring, bent twice at right angles in the same 

 direction, so that the free end, which is provided with 

 an ivory button, hangs directly over the centre of the 

 rubber membrane. The ivory button is on one extrem- 

 ity of a small screw which perforates the free end of the 

 lever, while the other end of the screw rests on the 

 rubber membrane, the rubber being protected from the 

 point of the screw by a light metal plate. The instru- 

 ment is further provided with three adjusting screws, by 

 which it rests on the chest-wall. The cavity of the 

 tympanum communicates by a rubber tube with the 

 interior of a somewhat similar drum, the rubber surface 

 of which is in communication with a long, light lever 'of 

 the second order (Fig. 200). 



On placing the ivory button of the cardiograph 

 over the point of cardiac impulse, and regulating the 

 adjusting screws so that the instrument is parallel to 

 the chest-walls and the screw-point of the button in 

 contact with rubber membrane, each movement of ascent 

 of the button creates pressure on the rubber membrane, 

 with a consequent diminution of the capacity of the 

 tympanum. Then, since the drum is in air-tight com- 

 munication with a second similar one, each diminution 

 in the capacity of the first causes a proportionate increase 

 in the contents of the second, a bulging of its rubber 

 face, and a consequent elevation of the lever with which 

 it is connected. Then, on causing this lever to record 

 its movements on the smoked surface of a revolving 

 drum, an exact record of the movement of the surface is 

 obtained with which the button is in contact. 



This instrument, applied to the study of the im- 

 pulse of a healthy human heart, shows that each systole 

 of the ventricle, when the button is precisely over the 

 apex, is marked by a sudden ascent of the lever, and the 

 end of the systole by a marked but more gradual descent 

 (Fig. 201). 



By shifting the cardiograph toward the sternum, ^ m 



so that the button is no longer over the point of im- 

 pulse, a tracing of an entirely different character is 

 obtained (Fig. 202). 



Although the ventricular systole is indicated by an 

 elevation of the lever, this ascent is immediately followed 

 by a sudden fall below the position of rest of the lever, < ( 



FIG. 199. SANDERSON'S CARDIOGRAPH. 



FIG. 200. M AREY'S TYM- 

 PANUM AND LEVER. 

 (Sanderson.) 



A, bearings in which the steel 

 axis of the lever works. It may be 

 raised or depressed by the adjusting 

 lever, the long arm of which extend* 

 downward and backward from A; 

 B. tympanum : F, tube by which the 

 cavity of the tympanum communi- 

 cates with the cardiograph. 



