276 MANUAL OF PHYSIOLOGY. 



it depends upon some physical change occurring during the car- 

 diac contraction. 



Against the view that the muscular tone is the cause of the 

 first sound, it has been urged that only tetanus (i.e., a rap- 

 idly repeated series of contractions fused into a continued 

 state of shortening which allows variations of tension) can cause 

 a muscle sound. And though in many ways it differs from the 

 single contraction of other muscles, yet the heart beat is, no doubt, 

 a single contraction. But the tone which may be heard during 

 the normal contraction of skeletal muscle has not been proved to 

 depend on recurrent contractions, such as occur in the tetanus 

 produced by an interrupted current. 



The auriculo-ventricular valves are made tense at the beginning 

 of the sound, and injury or disease of these valves is said to be 

 associated with a weak or altered first sound ; this is often ob- 

 served in disease of the mitral valve. The blood is said, by some, 

 to be necessary for the production of the sound, so that the gentle 

 closure and immediately subsequent tension of these valves might 

 have a share in causing the sound. 



As before remarked, the valvular tension would not account 

 for the occasionally heard presystolic sound, and the first sound 

 can be heard in an empty heart removed from the animal, in which 

 the valves cannot become tense (Ludwig). 



The sound has been analyzed with suitable resonators, and two 

 distinct tones made out : one, high and short, corresponding to 

 the closure of the valves ; the other, long and low, correspond- 

 ing in duration with the muscle contraction. 



The reasons given for thinking that the heart muscle cannot 

 produce a tone, suggest that the sudden state of tension of the 

 ventricular wall when tightened over the blood may give rise to 

 vibrations, and be an important item in causing the first sound. 

 This would explain the faintness of the sound, both when the 

 valves were injured and the muscle weak, or when the blood 

 is prevented from entering. It would also explain the pre- 

 systolic sound, which requires a certain auricular tension for 

 its production; but a sound can be heard to accompany the 

 contraction of the ventricles alone when they have been rapidly 



