1018 PHYSIOLOGY 



lengthening of the great arteries as blood is forced into them under 

 pressure from the ventricles. 



The changes in the shape of the cavities of the heart during contrac- 

 tion have been studied in the stage of extreme contraction produced 

 by heat rigour. In such hearts it is found that the cavities are never 

 entirely obliterated, though the right ventricle is reduced to a narrow 

 slit widening out slightly in the neighbourhood of the auriculo-ventri- 

 cular orifices, while in the left ventricle a distinct cavity is left between 

 the mitral valves and the free ends of the papillary muscles. During 

 normal activity it is probable that the emptying of the cavities never 

 proceeds to so great an extent. 



THE APEX BEAT 



The movement of the heart at each contraction is communicated 

 to the chest wall, over a limited area of which it may be felt and 

 seen, except in fat individuals. The region where the pulsation of the 

 chest wall is most marked lies in the fifth intercostal space, a little 

 to the median side of the left nipple. The pulsation is spoken of as the 

 ' apex beat/ and was formerly thought to be due to the twisting forward 

 of the apex at each systole. The apex of the heart is really situated 

 lower down, and, as we have already seen, so long as the pericardium 

 is intact is relatively motionless. During diastole the ventricles form 

 a flabby flattened cone lying against the chest wall and slightly deformed 

 by the latter. In systole the ventricles contract forcibly on the con- 

 tained fluid and become hard and rigid, assuming the form of a rounded 

 cone. This sudden recovery of shape and hardening of the ventricular 

 walls 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 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. 400. 



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, (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. 401 represents a cardiographic tracing 

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

 this curve we must record at the same time either the intra ventricular 



