122 THE CIRCULATION. 



composing the walls of the heart to such an extent as are 

 the auricula -ventricular. It might be expected therefore 

 that their vibration would be more easily heard through a 

 stethoscope applied to the walls of the chest. 



The contraction of the auricles which takes place in the 

 end 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 con- 

 siderably in different individuals, and in the same indi- 

 viduals under different circumstances. 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 between 

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

 inflation of the portion of lung which overlaps the heart, 

 prevents 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 move- 

 ment is thought to be effected by the contraction of the 

 spiral muscular fibres of the ventricles, and especially of 

 certain of these fibres which, according to Dr. Reid, arise 

 from the base of the ventricular septum, pass downwards 



