246 LECTURES TO SCIENCE TEACHERS. 



Here is another instrument, constructed on the same 

 principle as the others, the cardiograph its purpose being 

 to record those motions of the wall of the chest, which are 

 occasioned by what is called the impulse of the heart. Each 

 time that the heart contracts, it alters its shape, becoming 

 globular and much harder than it was immediately before. 

 The consequence of these facts is, that the moment of con- 

 traction is marked by an expansion of that part of the 

 wall of the chest nearest the heart an expansion which we 



FIG. 8. The Cardiograph. The tympanum rests on three legs, of which the lengths 

 may be varied. The ivory button is supported by a steel spring, so that the 

 movements imparted to it by the wall of the chest are communicated to the 

 membrane of the tympanum, the surface of which faces downwards. The 

 cavity of the tympanum communicates with a second (recording) tympanum, by 

 a flexible tube, not shown in the drawing. The instrument is secured in its 

 position by a strap, which is buckled round the chest. 



can all readily feel when we place the hand on the left side 

 immediately below the nipple. This motion is different 

 in different persons, and under different conditions of 

 health. An importance therefore attaches to it which is 

 similar to that of the better known phenomenon which we 

 have already studied, the pulse. I show you the tracings 

 obtained by it, but I will not dwell on their characters, for 

 their significance can hardly be appreciated by any one who 

 is not already familiar with the structure and mode of 

 action of the heart. It will be sufficient to say that by the 

 cardiograph we can measure not merely the rate, i.e., the 

 frequency of the contractions, and appreciate their vigour, 



