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HANDBOOK OF PHYSIOLOGY. 



under surface of the diaphragm, at a point corresponding to the under 

 surface of the ventricle. The shock is felt, and possibly seen more dis- 

 tinctly because of the partial rotation of the heart, already spoken of, 

 along its long axis toward the right. The movement produced by the 

 ventricular contraction against the chest- wall may be registered by means 

 of an instrument called the cardiograph, and it will be found to corre- 

 spond almost exactly with a tracing obtained by the same instrument 

 applied over the contracting ventricle itself. 



The Cardiograph (fig. 166) consists of a cup-shaped metal box over the open 

 front of which is stretched an elastic India-rubber membrane, upon which is 

 fixed a small knob of hard wood or ivory. This knob, however, may be at- 

 tached, as in the figure, to the side of the box by means of a spring, and may 

 be made to act upon a metal disc attached to the elastic membrane. 



The knob is for application to the chest-wall over the place of the great- 

 est impulse of the heart. The box or tympanum communicates by means of 

 an air-tight tube with the interior of a second tympanum, in connection with 

 which is a long and light lever. The shock of the heart's impulse being 

 communicated to the ivory knob, and through it to the first tympanum, the 



Fig. 16? A. Cardiogram of Frog's Heart. 



c, Tracing of auricular and ventricular systole; T, time 

 n half seconds. 



effect is, of course, at once transmitted by the column of air in the elastic tube 

 to the interior of the second tympanum, also closed, and through the elastic 

 and movable lid of the latter to the lever, which is placed in connection with 

 a registering apparatus. This generally consists of a cylinder or drum covered 

 with smoked paper, revolving by clock-work with a definite velocity. The 

 point of the lever writes upon the paper, and a tracing of the heart's impulse 

 or cardiogram is thus obtained. 



Endocardiac Pressure. 



It cannot be considered, however, that the cardiogram represents 

 what is actually occurring within the heart itself. For determining 

 this, communication must be established with the cavities of the heart. 



