94. CAUSES OF THE HEART-SOUNDS. 



lirst sound may be completely inaudible. In aortic insufficiency, in which, in con- 

 sequence of the reflux of blood from the aorta into the ventricle, the mitral valve 

 is gradually stretched, and sometimes even before the beginning of the ventricular 

 systole, the first sound may be absent. Both pathological cases show that for the 

 production of the iirst sound, muscle-sound and valve-sound must eventually 

 work together, and that the tone is altered, or may even disappear, when one of 

 these causes is absent. 



The Cause of the Second Sound is undoubtedly due to the prompt 

 closure, and therefore sudden stretching or tension, of the semi-lunar 

 valves of the aorta and pulmonary artery, so that it is purely a valvular 

 sound (Carswell and Rouanet, 1830). Perhaps it is augmented by the 

 sudden vibration of the fluid-particles in the large arterial trunks. As 

 already pointed out (p. 85), the aortic and pulmonary valves do not 

 close simultaneously. Usually, however, the difference in time is so 

 small that loth valves make one sound, but the second sound may be 

 double or divided when, through increase of the difference of pressure 

 in the aorta and pulmonary artery, the interval becomes longer. Even 

 in health this may be the case, as occurs at the end of inspiration or 

 the beginning of expiration (v. Dusch). 



[The second sound has all the characters of a valvular sound. That 

 the aortic valves are concerned in its production, is proved by intro- 

 ducing a curved wire through the left carotid artery and hooking up 

 one or more segments of the valve, when the sound is modified, and it 

 may be replaced by an abnormal sound or " murmur." Again, when 

 these valves are diseased, the sound is altered, and it may be accompanied 

 or even displaced by murmurs.] 



Where the Sounds are Heard Loudest. The sound produced by the 

 trinizpid ralce is heard loudest at the insertion of the fifth right rib into 

 the sternum, and from here somewhat inwards and obliquely upwards 

 along the sternum; as the mitral valve lies more to the left and deeper 

 in the chest, and is covered in front by the arterial orifice, the mitral 

 sound is best heard at the apex-beat, or immediately above it, where a 

 strip of the left ventricle lies next the chest-wall. [The sound is con- 

 ducted to the part nearest the ear of the listener by the muscular 

 substance of the heart.] The aortic and pulmonary orifices lie so 

 close together that it is convenient to listen for the second (aortic) 

 sound in the direction of the aorta and where it comes nearest to 

 the surface, i.e., over the first right costal cartilage close to its 

 junction with the sternum. The sound, although produced at the 

 semi-lunar valves, is carried upwards by the column of. blood and 

 by the walls of the aorta. 



The sound produced by the pulmonary artery is heard most distinctly 

 in the second left intercostal space, somewhat to the left and external 

 to the margin of the sternum (Fig. 31). 



