576 A MANUAL OF PHYSIOLOGY 



is lengthened and made stronger by distension of the viscera in 

 whose walls they occur, just as a skeletal muscle contracts more 

 powerfully against resistance. 



Voluntary Contraction. It is often stated that the volun- 

 tary contraction is a tetanus, but in favour of this belief 

 there is little direct evidence. One of the strongest buttresses 

 of the theory of natural tetanus has been the muscle-sound, a 

 low rumbling note which can be heard by listening with a 

 stethoscope over the contracting biceps, or, when all is still, 

 by stopping the ears with the fingers and strongly contract- 

 ing the masseter and the other muscles concerned in closing 

 the jaws.* Discovered about eighty years ago, first by 

 Wollaston and then by Erman, half a century passed away 

 before it was investigated more fully by Helmholtz. The 

 latter observer, confirming the results of his predecessors, 

 put down the pitch of the sound at 36 to 40 vibrations per 

 second. He found, however, that little vibrating reeds with 

 a rate of oscillation of about 19*5 per second, were more 

 affected, when attached to muscle thrown into voluntary con- 

 traction, than those that vibrated at a smaller or a greater 

 rate. He therefore concluded that the fundamental tone of 

 the muscle corresponded to this frequency, although, since 

 such a low note is not easily appreciated, the sound actually 

 heard was really its octave or first harmonic (p. 263). The 

 objection has been brought forward that the resonance tone 

 of the ear also corresponds to a vibration frequency of 36 to 

 40 a second. In other words, this is the natural rate of 

 swing of the elastic structures in the middle ear, the rate 

 they will most easily fall into if set moving, and not com- 

 pelled to vibrate at some other rate. 



Now, this resonance tone might be elicited by a quivering muscle 

 if, among many diverse rates of oscillation of different portions of 

 its substance, the rate of 36 to 40 a second anywhere appeared, and 

 the note corresponding to the real rate of vibration of the muscle as 

 a whole might be overpowered. Or, even if there were no regular 

 rate of vibration of the whole muscle, but, instead, a series of 

 irregular tremors or pulls, waxing here, waning there, and flitting 



* In order that a muscular sound may be produced there must be a 

 certain abruptness in the contraction. Thus, the slowly-contracting 

 smooth muscles do not produce a sound, nor the slowly-contracting 

 heart-muscle of cold-blooded animals. 



