268 PHYSIOLOGY 



corresponded to a vibration frequency of 36 to 40 per second. The 

 same observation was made by Helmholtz, and can be repeated by any 

 one who will place the end of a stethoscope on a muscle, e.g. the 

 biceps, and listen to the sound produced when it contracts. Helm- 

 holtz pointed out, however, that the tone heard corresponded to the 

 resonance tone of the external ear, and was the same as that noted 

 when listening to any irregular sound of low intensity. Thus the 

 roar of London that we hear in the middle of Hyde Park has the 

 same pitch as the muscle sound of the contracting biceps. The 

 muscle sound, therefore, teaches us nothing as to the pitch or number 

 of contractions per second making up the voluntary tetanus. It 

 merely points to an irregularity or discontinuity in this contraction. 

 By bringing vibrating reeds of different frequency in contact with 

 the contracting muscles of the frog, Helmholtz came to the conclusion 

 that the chief element in the muscle sound was the first over-tone 

 of a sound with a vibration frequency of 18 to 20 per second, which, 

 according to him, was to be taken as representing the number of 

 single contractions in every voluntary muscular contraction. 



Nearly all voluntary contractions present a certain degree of 

 irregularity, and the same irregularities are observed when a tetanic 

 spasm in the muscles of the body is caused by strong excitation 

 of the cerebral cortex, as in epilepsy. On taking a record of such 

 contractions, Schaefer and Horsley showed that in nearly all cases 

 the tracing presents superposed undulations repeated at the rate 

 of eight to twelve per second. These observers concluded that this 

 was the normal rate at which the impulses descend the nerve to 

 arouse a voluntary contraction. One difficulty in this conclusion is 

 that when human muscle is excited by eight to twelve stimuli per 

 second, we obtain, not a tetanic contraction with a few irregularities 

 superposed on it, but a series of single contractions, the so-called 

 clonus. In order to produce a nearly continuous contraction we 

 must employ a vibration, frequency of about 30 per second. It 

 has been suggested to get over this difficulty that under normal 

 circumstances the discharge does not travel along all the nerve fibres 

 at the same time, so that the different muscle fibres composing the 

 muscle will be in different phases of contraction, and there will be 

 never any large degree of relaxation between the individual con- 

 tractions of the whole muscle. Von Kries has found that the duration 

 of a muscle twitch may be lengthened by lengthening the duration 

 of the electrical change used to excite the nerve, and has suggested 

 that the normal excitatory process may resemble the prolonged 

 electrical change which can be produced electro-magnetically, rather 

 than the short sudden shock represented by the induced current 

 of an induction-coil. Attempts have been made to decide the 



