THE MECHANISM OF THE HEAKT PUMP 947 



and the direction of its support. Thus if the chest and the pericardium be 

 opened and the animal be in the supine position, the heart during diastole 

 will be flattened from before backwards as a result of the simple weight of 

 its contents. In this position therefore, systole will be accompanied by a 

 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 he, 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 downwards, 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 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 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, 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 



rpex 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 



iastole the ventricles form a flabby flattened cone lying against the chest 



