THE MECHANISM OF THE HEART PUMP 903 



shortening in the lateral and vertical directions and a lengthening in the 

 sagittal direction. During systole, when the heart becomes tense and all its 

 fibres are firmly contracted, the heart, whatever its previous condition, 

 takes the form of a truncated cone. Under normal circumstances the heart 

 in the unopened chest lies in the pericardium, which is attached above to the 

 great vessels and below to the central tendon of the diaphragm. It is 

 supported laterally by the lungs, which, however, owing to their elasticity, 

 have very little influence on its shape during diastole. 



When the heart is freed from the pericardium, the obliquity of its fibres 

 causes the apex to move forwards and to the right during systole; this 

 movement is normally prevented by the attachment of the pericardium to the 

 central tendon of the diaphragm, so that the most movable part of the heart 

 comes to be the base. If three needles be passed through the chest wall so that 

 their points lie, one in the base, one about the centre of the ventricles, and 

 one in the apex of the ventricles, each ventricular systole is found to be 

 accompanied by a movement of the needle in the base of the heart down- 

 wards, a slighter movement in the same direction of the needle in the middle 

 of the ventricles, and practically no movement at all of the needle which is 

 thrust into the apex. During systole the base of the heart moves downwards 

 towards the apex. This movement is determined partly by the shortening 

 of the fibres of which the ventricular wall is composed, partly by the lengthen- 

 ing of the great arteries as blood is forced into them under pressure fronTthe 

 ventricles. 



The changes in the shape of the cavities of the heart during contraction 

 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-ventricular 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 rarely proceeds to so great an extent. 



THE APEX BEAT 



The movement of the heart at each contraction is communicated to the 

 chest wall f 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. D.uring 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 contained fluid and become hard and rigid, assuming the form of 

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



