906 PHYSIOLOGY 



as determined by the hardening of the apex of the heart, felt as the apex beat, 

 and can be shown to be synchronous with the closure of the aortic valves. It 

 is, in fact, caused by the sudden shutting and stretching of these valves that 

 occur directly the heart ceases to contract and to force the blood into the 

 aorta. If the valves be hooked back in an animal by means of a wire passed 

 down a carotid artery, the second sound disappears and is replaced by a 

 murmur caused by the blood rushing back into the ventricle at the end of the 

 systole. The same disappearance of the normal second sound is observed in 

 cases where the valves are prevented from closing by diseased conditions. 



The pulmonary and aortic valves generally close simultaneously. In 

 some cases, however, the aortic may close slightly before the pulmonary, 

 giving rise to a ' reduplicated second sound.' The pulmonary element 

 of this sound is best heard over the second left cartilage, or in the second left 

 intercostal space. 



The first sound has probably a twofold origin, viz. from the sudden closure 

 of the auriculo- ventricular valves and from the contraction of the thick 

 muscular wall of the ventricle. 



If the veins going to the heart be clamped so that the heart can no longer 

 be distended with blood nor the valves put on the stretch, the first sound 

 is altered in character, but not abolished. The first sound may indeed 

 be heard on listening with a stethoscope to the beat of an excised heart. It 

 is said that two notes may be detected in the first sound a high note of 

 short duration due to closure of the valves, and a low-pitched note due to the 

 muscular contraction. The muscular element of the first sound has the 

 same pitch as the sound produced by contracting voluntary muscle, and 

 therefore as the resonance- tone of the ear. This consideration prevents our 

 arguing from the tone that a cardiac contraction is a tetanus. As we shall 

 show later on, each ventricular contraction is analogous to a simple muscle- 

 twitch and not to a tetanus. 



THE THIRD HEART SOUND. A number of observers have described a third 

 heart sound as occurring in certain individuals during the diastole, a short time after 

 the second sound. It is softer and of a lower pitch than the second sound, and is heard 

 most distinctly over the apex beat. It is probably due to the vibrations set up in the fluid 

 itself or in the auriculo-ventricular valves by the sudden inrush of blood from auricles 

 to ventricles at the beginning of the diastole. The sound is shown objectively by the 

 vibrations on the endocardiac pressure curve given in Figi 400 (Straub). It has also 

 been registered electrically by Einthoven. 



CARDIAC MURMURS 



When a fluid escapes through a narrow orifice into a wider space vibrations 

 are set up in the fluid and may be transmitted by any elastic medium to the 

 ear, giving rise to the sensation of sound. Such a sound is produced when 

 water is allowed to run from a tap into a vessel of water, or when air is blown 

 out between the partially closed lips. The formation of such vibrations 

 forms, indeed, the basis for the construction of many musical instruments. 

 The same sort of vibration may be set up in the large vessels or in the heart, 

 whenever the blood passes rapidly through a narrow orifice into a wider space. 



