THE CIRCULATION OF THE BLOOD. 



197 



and especially a hypertrophied condition of the ventricles, will increase 

 the impulse; while a depressed condition, or an atrophied state of 'the 

 ventricular walls, will diminish it. 



Cause of the Impulse. During the period which precedes the ven- 



Tube to communicate 

 with tambour. 



Ivory 

 knob. 



Tympanum. 

 Fig. 166. Cardiograph. (Sanderson's.) 



Tape to attach the instrument 

 to the chest. 



tricnlar systole the apex of the heart is situated upon the diaphragm and 

 against the chest-wall in the fifth intercostal space. When the ventri- 

 cles contract, their walls become hard and tense, since to expel their 

 contents into the arteries is a distinctly laborious action, as it is resisted 



Screw to regulate elevation of lever. 



Writing lever. 



Tambour. 



Tube to cardiograph. 



Fig. 167. Marey's Tambour, to which the movement of the column of air in the first tympanum 

 is conducted by a tube, and from which it is communicated by the lever to a revolving cylinder, so 

 that the tracing of the movement of the impulse beat is obtained. 



by the elasticity of the vessels. It is to this sudden hardening that the 

 impulse of the heart against the chest-wall is due, and the shock of the 

 sudden tension may be felt not only externally, but also internally, if 

 the abdomen of an animal be opened and the finger be placed upon the 



