IMPULSE OF THE HEART. 107 



of the pause is inaudible outside the chest, but may be heard, 

 when the heart is exposed and the stethoscope placed on it, as 

 a slight sound preceding and continued into the louder sound 

 of the ventricular contraction. 



The Impulse of the Heart. At the commencement of each 

 ventricular contraction, the heart may be felt to beat with a 

 slight shock or impulse against the walls of the chest. This 

 impulse is most evident in the space between the fifth and sixth 

 ribs, between one and two inches to the left of the sternum. 

 The force of the impulse, and the extent to which it may be 

 perceived beyond this point, vary considerably in different in- 

 dividuals, and in the same individuals under different circum- 

 stances. It is felt more distinctly, and over a larger extent of 

 surface, in emaciated than in fat and robust persons, and more 

 during a forced expiration than in a deep inspiration ; for, in the 

 one case, the intervention of a thick layer of fat or muscle be- 

 tween the heart and the surface of the chest, and in the other the 

 inflation of the portion of lung which overlaps the heart, pre- 

 vents the impulse from being fully transmitted to the surface. 

 An excited action of the heart, 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. 



The impulse of the heart is probably the result, in part, of a 

 tilting forwards of the apex, so that it is made to strike against 

 the walls of the chest. This tilting movement is thought to be 

 effected by the contraction of the spiral muscular fibres of the 

 ventricles, and especially of certain of these fibres which, ac- 

 cording to Dr. Reid, arise from the base of the ventricular 

 septum, pass downwards and forwards, forming part of the 

 septum, then emerge and curve spirally around the apex and 

 adjacent portion of the heart. The whole extent of the move- 

 ment thus produced is, however, but slight. The condition, 

 which, no doubt, contributes most to the occurrence and char- 

 acter of the impulse of the heart, is its change of shape ; for, 

 during the contraction of the ventricles, and the consequent 

 approximation of the base towards the apex, the heart becomes 

 more globular, and bulges so much, that a distinct impulse is 

 felt when the finger is placed over the bulging portion, either 

 at the front of the chest, or under the diaphragm. The pro- 

 duction of the impulse is, perhaps, further assisted by the ten- 

 dency of the aorta to straighten itself and diminish its curva- 

 ture when distended with the blood impelled by the ventricle ; 

 and by the elastic recoil of all the parts about the base of the 

 heart/ which according to the experiments of Kurschner, are 

 stretched downward and backward by the blood flowing into 



