236 PHYSIOLOGY. 



CARDIAC IMPULSE. 



The cardiac revolution is manifested by two signs, the cardiac 

 impulse and the sounds of the heart. There are two things to be 

 noted in the production of the cardiac impulse. The first thing is 

 the intimate contact of the heart with the wall of the thorax during 

 the increase of the antero-posterior diameter by the cardiac systole, 

 and the second is the simultaneous and sudden hardening of the 

 ventricles. It suffices simply to press the finger deeply into the 

 intercostal space to perceive the hardening of the ventricle at the 

 moment of its production. 



This area is located in the fifth left intercostal space between 

 the mammary and midsternal lines. The center of this area is 



Fig. 70. Sanderson Cardiograph. 



described as being two inches below the nipple and one inch to its 

 sternal side. 



The cause of the impulse of the heart is not the apex but the 

 change in form and consistency of the ventricles, when these pass 

 from the diastole to the systole and in the instantaneous transfor- 

 mation. It is the sudden hardening of the ventricle. 



The impulse takes place at the same time as the systole of the 

 ventricle, and is caused by the ventricle, which is pressed very 

 firmly against the chest. At the time of the contraction of the 

 ventricle the outline of the heart changes; instead of being an 

 oblique cone having an elliptical base, as at rest, it becomes a 

 regular cone with a regular base. 



For giving more accurate accounts of the heart's movements 

 recourse is had to the instruments called cardiograpliK. 



Cardiographs. These are instruments which give graphic rec- 

 ords of the heart's movements. They register at the same time the 



