660 A MANUAL OF PHYSIOLOGY 



rhythmical contraction of the heart is not a tetanus has already 

 been seen. It is a simple contraction, intermediate in its duration 

 and other characters between the twitch of voluntary muscle and 

 the contraction of smooth muscle. The contraction both of unstriped 

 and of cardiac muscle 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. There is evidence that the volun- 

 tary 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 stetho- 

 scope 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.* Dis- 

 covered 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 volun- 

 tary 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. 280). 



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 any- 

 where appeared, and the note corresponding to the real rate of vibra- 

 tion 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 con- 

 traction, connected 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 



* 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. 



