948 



PHYSIOLOGY 



wall and slightly deformed by the latter. In systole the ventricles contract 

 forcibly on the contained fluid and become hard and rigid, assuming the 

 form of a rounded cone. This sudden recovery of shape and hardening of 

 the ventricular wall pushes out that part of the chest wall in immediate 

 proximity to the ventricles and so gives rise to the ' apex beat.' 



The cardiac impulse may be registered by means of a cardiograph. In nearly all 

 forms of this instrument a button resting on the chest wall transmits the movement of 

 the latter to a tambour, which again is connected by a tube to a registering tambour. 

 One such instrument is shown in Fig. 411. 



Fio. 411. A cardiograph. This is strapped round the chest, the central button is 

 applied to the ' apex beat,' and its pressure on the chest wall regulated by 

 means of the three screws at the sides. The tube at the upper part of the 

 instrument serves to connfect the drum of the cardiograph with a registering 

 tambour such as thai; shown in Fig. 405. 



The curves so obtained, which are known as cardiograms, may vary considerably 

 in the same subject according to the pressure employed and the exact spot at which 

 the tambour is applied. Their interpretation often presents difficulties owing to the 

 fact that their form is conditioned by two factors, viz. (1) the actual size (antero-posterior 



FIG. 412. Cardiogram. (HTJRTHLE.) 



diameter) of the ventricles; (2) the resistance to distortion (t. &. the tension) of the 

 ventricular wall; this factor will increase in importance with increasing pressure of the 

 cardiograph button on the chest wall. Fig. 412 represents a cardiographic t raring 

 or cardiogram which may be spoken of as typical. In order to interpret this curve 

 ue must record at the same time either the intraventricular pressure in animals or the 

 heart sounds in man. This cardiogram presents considerable similarities to the cn<l<>- 

 canliac pressure curve ; in both there is an ascending limb, a plateau, and a descending 

 liml, and in many cases a small elevation occurs at the beginning of a curve during the 

 contraction of the auricle. The exact point at which the auricular passes into the 

 ventricular contraction varies from case to case and may be altered by altering the 

 degree of pressure put on the recording button. In the lirst figure given the auricular 

 systole tin is lies before the main rise of the lever occurs. In many cases however, the 

 elevation due to the auricular systole may take up the greater part of the ascending 

 limb of the curve, as in Fig. 



