760 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



Voluntary Contraction. There is evidence that the voluntary 

 contraction is a tetanus. 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 contracting the masseter and the other muscles 

 concerned in closing the jaws.* Discovered about ninety 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 contraction, 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. 310). 



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 struc- 

 tures in the middle ear, the rate they will most easily fall into if set 

 moving by an irregular mixture of faint, low-pitched tones and noises, 

 and not compelled to vibrate at some other rate by a distinct sound 

 of definite pitch. 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 ap- 

 peared, 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 due to irregularities in the contraction, con- 

 nected with a want of co-ordination of all the fibres (Haycraft), the ear 

 might from time to time pick out of the turmoil of feeble aerial waves 

 those corresponding to its resonance tone, just as a tuning-fork or a 

 piano-string attuned to a particular note would catch it up amid a 

 thousand other sounds and strengthen it. 



But while this renders it highly probable that the resonance of the 

 ear contributes to the production of the muscle-sound, and shows that 

 we cannot from the pitch of the muscle-sound alone deduce the rate 

 at which the muscle-substance is vibrating, it does not invalidate 

 Helmholtz's objective observations with the oscillating reeds. 



And several observers (Schafer, Horsley, v. Kries) have noticed 

 periodic oscillations, at the rate of 10 or 12 per second, in the 

 curves taken from muscles (Fig. 268), contracted voluntarily 

 against a small resistance. When the resistance is greater, the 

 rate may be as much as 18 or 20 a second, and in quick and rapidly 



* 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. 



