CIRCULATION OF THE BLOOD. 119 



the auriculo-vcntricidar. It might be expected therefore that their vibra- 

 tion 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 ven- 

 tricular contraction, the heart may be felt to beat with a slight shock or 

 impulse against the walls of the chest. The force of the impulse, and the 

 extent to which it may be perceived beyond this point, vary considerably 

 in different individuals, and in the same individual under different cir- 

 cumstances. 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 ex- 

 piration 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 sur- 

 face. An excited action of the heart, and especially a hypertrophied con- 

 dition of the ventricles, will increase the impulse; while a depressed con- 

 dition, or an atrophied state of the ventricular walls, will diminish it. 



Cause of the Impulse. During the period which precedes the 

 ventricular systole, the apex of the heart is situated upon the diaphragm 

 and against the chest-wall in the fifth intercostal space. When the ven- 

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

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

 by the tension within 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 under 

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

 of the ventricle. The shock is felt, and possibly seen more distinctly, 

 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 may be registered by means of an instrument called the cardio- 

 graph, and it will be found to correspond almost exactly with a tracing 

 obtained by the same instrument applied over the contracting ventricle 

 itself. 



The Cardiograph (Fig. 101) consists of a cup-shaped metal box, over 

 the open front of which is stretched an elastic membrane, upon which is 

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

 attached instead, 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. 



