CH. XX.] VALVES OF THE HEART. 233 



is greatest, and the pressure within the ventricles exceeds that in 

 the large arteries which originate from them, the semilunar 

 valves are opened, and the ventricles empty themselves, the left 

 into the aorta, the right into the pulmonary artery. Each 

 ventricle ejects about 3 ozs. of blood with each contraction ; the 

 left in virtue of its thicker walls acts about thrice as forcibly as 

 the right. The greater force of the left ventricle is necessary as 

 it has to overcome the resistance of the small vessels all over 

 the body ; whereas the right ventricle has only 10 overcome 

 peripheral resistance in the pulmonary district. 



The shape of both ventricles during systole is generally described as 

 undergoing 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 together with 

 the hardening that occurs when the ventricles contract, 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 

 middle line. It has, however, been recently shown by Haycraft that these 

 changes only occur when the chest walls are open. When the heart con- 

 tracts in a closed thorax it undergoes no change in shape, as 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. 



Doubtless the apex beat is produced by the increased pressure in the 

 aorta being transmitted backwards to the heart, and causing it to press 

 more closely than it does in diastole against the chest walls. 



Action of the Valves of the Heart. 



i. 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 ; and by the time that the diastole is complete, the valves are 

 in apposition, and they are firmly closed by the reflux current 

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

 size of the heart during ventricular systole is well marked in the 

 neighbourhood of the auriculo-ventricular rings, and this aids in 

 rendering the auriculo-ventricular valves competent to close the 

 openings, by greatly diminishing their diameter. 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 chordce tendinese 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. 



