270 MANUAL OF PHYSIOLOGY. 



tinuous (tetanic) contraction of the skeletal muscles. It corre- 

 sponds in time and Duration with the contraction of the cardiac 

 muscle. In disease where the heart muscle is weak, the sound 

 becomes faint or inaudible, although the valves are made tense 

 by an intraventricular force sufficient to overcome the pressure in 

 the arteries ; for otherwise the circulation would cease. A prse- 

 systolic sound, like in character to the systolic sound, is now rec- 

 ognized by physicians as being produced by the auricular systole, 

 but this cannot depend on the vibrations of valves. According 

 to the most recent and careful observers, the first sound may be 

 heard when the heart is empty. 



All this evidence tends to show that the sound is produced by 

 the contraction of the muscle tissue of the heart, or, in short, that 

 it is the cardiac muscle tone. 



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

 sound, the undoubted fact is properly urged that only tetanus 

 (i.e., a rapidly repeated series of contractions fused into a con- 

 tinued state of shortening allowing variations of tension) can 

 cause a muscle sound, and a single contraction is not accom- 

 panied by any tone. Though in many ways it differs from the 

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

 a single contraction. And no good reason exists for believing 

 it can cause vibrations comparable with that of the tetanus of 

 skeletal muscles, or that it is capable of giving rise to a definite 

 sound. 



The closure of the auriculo-ventricular valve is synchronous 

 with the beginning of the sound, and injury or disease of these 

 valves is associated with a weak or altered first sound ; this is 

 often observed in disease of the mitral valve. The blood is said, 

 by some, to be necessary for the production of the sound, so that 

 the act of closure and subsequent tension of these valves would 

 seem to have a share in causing the sound ; but, on the other 

 hand, the character of the sound is not like that which would be 

 caused by the sudden closure or tension of the membranes, or of 

 the delicate tendinous cords. 



As before remarked, this would not account for prsesystolic 

 sounds, and the first sound can be heard in an empty heart re- 



