CH. XX.] THE VALVES OF THE HEAKT 233 



ventricle has only to overcome peripheral resistance in the pulmonary 

 district. 



The shape of both ventricles during systole has been described as under- 

 going an alteration, the diameters in the plane of the base being diminished, and 

 the length of the ventricles slightly lessened. The whole heart, moreover, moves 

 towards the right and forwards, twisting on its long axis and exposing more of the 

 left ventricle anteriorly than when it is at rest. These movements, which were 

 first described by Harvey, have been since Harvey's time believed to be the cause 

 of the cardiac impulse or apex beat which is to be felt in the fifth intercostal 

 space about three inches from the irfiddle line. It has, however, been shown by 

 Haycraft that these changes only occur when the chest walls are open. When the 

 heart contracts in a closed thorax it undergoes no rotation, and the contraction is 

 concentric, that is, equal in all directions. The diminution of the heart's volume 

 which occurs in systole cannot be the cause of the apex beat ; it would rather tend 

 to draw the chest wall inwards than push it outwards. 



The apex beat is caused by two changes in the physical condition of the heart. 

 In the first place, on systole the heart becomes hard and tense, and secondly, its 

 attachment to the aorta becomes rigid instead of being flexible as it is in diastole. 

 Thus, in systole, the heart becomes rigidly fixed to the aorta, and, as this vessel is 

 curved, it tends to open out into a straight line, but is prevented by the counter- 

 resistance at the two ends of the arch. These are (a) the resistance of the chest 

 wall against the heart, and (/>) that of the vertebrae and ribs against the thoracic 

 aorta. The pressure of the heart against the chest wall is confined to a small area, 

 situated in the fifth intercostal space, because the heart surface is much more curved 

 than the internal thoracic wall. The forward movement this pressure causes is the 

 apex beat. It must be noted that this movement is not over the actual apex of the 

 heart, but is communicated from an area on the anterior cardiac surface. 



Action of the Valves of the Heart. 



1. The Auriculo- Ventricular. The distension of the ventricles 

 with blood continues throughout the whole period of their diastole. 

 The auriculo-ventricular valves are gradually brought into place by 

 some of the blood getting behind the cusps and floating them up ; 

 by the time that the diastole is complete, the valves are in appo- 

 sition, and they are firmly closed by the reflux current caused 

 by the systole of the ventricles. The diminution in the size of the 

 auriculo-ventricular rings which occur during systole, renders the 

 auriculo-ventricular valves competent to close these openings. The 

 margins of the cusps of the valves are still more secured in apposition 

 with one another, by the simultaneous contraction of the musculi 

 papillares, whose chordae tendineae have a special mode of attachment 

 for this object. The cusps of the auriculo-ventricular valves meet 

 not by their edges only, but by the opposed surfaces of their thin 

 outer borders. 



The musculi papillares prevent the auriculo-ventricular valves 

 from being everted into the auricle. For the chordae tendineae might 

 allow the valves to be pressed back into the auricle, were it not that 

 when the wall of the ventricle is brought by its contraction nearer 

 the auriculo-ventricular orifice, the musculi papillares more than 

 compensate for this by their own contraction ; they hold the cords 



