904 



PHYSIOLOGY 



ventricular wall pushes out the 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 



FIG. 401. 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 connect the drum of the cardiograph with a registering 

 tambour such as that shown in Fig. 395. 



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. 

 401. 



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 diameter) of the ventricles, 



FIG. 402. Cardiogram. (HURTHLE.) 



(2) the resistance to distortion (i.e. the tension) of the ventricular wall ; this 

 factor will increase in importance with increasing pressure of the cardiograph 

 button on the chest wall. Fig. 402 represents a cardiographic tracing or cardio- 

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

 we 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 endocardiac pressure curve ; in both there is an ascending 

 limb, a plateau, and a descending limb, 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 



