CAUSE OF THE CARDIAC IMPULSE. 



83 



The cause of the 'ventricular impulse has been much discussed. It 

 depends upon the following : 



(1.) The base of the heart (auriculo-ventricular groove) represents 

 during diastole a transversely-placed ellipse, while during contraction it 

 has a more circular figure. Thus, the long diameter of the ellipse is 

 diminished in the cat from 28 to 22*5 mm. (C. Ludwig) ; the small 

 diameter is increased (^ to -4-), while the base is brought nearer to 

 the chest- wall (Arnold, Ludwig) Fig. 26, 1. This alone does not cause 

 the impulse, but the basis of the heart, being hardened during the 

 systole and brought nearer to the chest-wall, allows the apex to 

 execute the movement which causes the impulse. 



(2.) During relaxation, the ventricle lies with its apex obliquely 

 downwards, and with its long axis in an oblique direction so that the 

 angles formed by the axis of the ventricles with the diameter of the 

 base are unequal represents a regular cone, with its axis at right angles 

 to its base. Hence, the apex must be erected from below and behind, 

 forwards and upwards (Harvey " cor sese erigere "), and when 

 hardened during systole presses itself into the intercostal space 

 (Ludwig) Fig. '20, II. 



Fig. 26. 



I, Schematic horizontal section through the heart and lungs, and the thoracic 

 walls, to show the change of shape which the base of the heart undergoes 

 during contraction of the ventricle 1, 2, transverse diameter of the ventricle 

 during diastole ; c, position of the thoracic wall during diastole ; a, b, trans- 

 verse diameter of the heart during systole, with e, the position of the anterior 

 thoracic wall during systole. II, Side-view of the heart s, apex during 

 diastole ; p, the same during systole (C. Ludwig). 



