228 THE BODY AT WORK 



The " semilunar valves," which close the apertures into the 

 aorta and pulmonary artery, have each three flaps. The aortic 

 semilunar valve, which has the higher pressure to bear, shows 

 its characteristic features in a rather more marked degree 

 thari the other. Each of its three flaps is a half-cup. At the 

 centre of the margin of the half-cup is a small fibrous nodule. 

 The edge of the cup on either side of this is very thin. Fine 

 elastic fibres radiate from the nodule to all parts of the flap. 

 The wall of the aorta shows three bays, or " sinuses," one 

 behind each flap. Hence, when the valve is forced by the rise 

 of pressure in the ventricle, the flap is not flattened back 

 against the wall of the aorta. There is always a certain 

 amount of backwash in the pocket behind it. The instant the 

 pressure in the ventricle begins to fall, the three flaps come 

 together with a click, so smart as to be plainly audible over 

 most of the front of the chest. The click is the " second sound " 

 of the heart. The auriculo- ventricular valves also make a 

 sound when they close ; but this " first sound of the heart " 

 has a different character. It is prolonged, soft, low-pitched. 

 It is customary to represent the sounds by the syllables " lubb 

 dup lubb dup," the pause during diastole being of about 

 the same length as the sounds when the heart is beating with 

 its normal rhythm. The duration of systole is little affected by 

 variations in the rate of beat. It is diastole that is shortened 

 or prolonged. The second sound is due entirely to the closure 

 of the semilunar valves. It is heard most clearly when the 

 stethoscope is placed over the region where the aorta comes 

 nearest to the wall of the chest at the second rib cartilage 

 on the right side of the breast-bone. The first sound is loudest 

 near the apex of the heart. It is generally agreed that it is 

 not wholly due to the closure of the auriculo-ventricular 

 valves, but possesses a second constituent. Some persons 

 assert that they can with the ear distinguish the clearer 

 valvular sound at the commencement from the general rumble 

 which overtakes it. The main part of the sound, if it have 

 two constituents, or the whole sound, if there be no dis- 

 tinguishable valvular constituent observers differ is just 

 the noise of a distant cab (bruit du cab) or the waves on a far- 

 off beach ; it is the sound which the ear picks up from any 

 irregular mixture of tones which it cannot analyse. It is 



