478 MOVEMENTS VOICE AND SPEECH. 



muscles to contract. The number of vibrations noted by Helmholtz was 

 per second ; but the sound heard was the first overtone, or the octave, the 

 fundamental tone being too low to be appreciated by the ear. 



Some physiologists have denied the supposed identity between the tetanic 

 contraction produced by a rapid succession of stimuli applied to a motor 

 nerve and voluntary muscular contraction. Complete fusion of contraction 

 occurs with twenty-seven or more stimuli per second applied to a nerve ; 

 but it is stated that stimuli applied to the motor cerebral centres, even when 

 very rapid, do not produce more than eight to thirteen muscular contrac- 

 tions, the average being ten per second (Horsley and Schafer, 1887). The 

 average in voluntary muscular contraction is about the same. From these 

 observations it is argued that the rate of so-called vibration in voluntary 

 muscular contraction has an average of about ten per second. This conclu- 

 sion is based upon actual myographic tracings. It is difficult, however, to rec- 

 oncile these results with those obtained by Marey, Helmholtz and others. 

 It is a fact, also, that distinct muscular contractions may be produced very 

 rapidly by an effort of the will. It is not difficult for any one to make five 

 taps of the finger per second for a few seconds, and skillful performers on 

 musical instruments are able, by using the same muscle or set of muscles, to 

 make movements that are very much more rapid, each movement presumably 

 requiring a distinct nervous impulse. It may be that in an unweighted mus- 

 cle, the contractions are discontinuous, and that the average number of 

 waves is about ten per second ; but it is probable that the estimate of Helm- 

 holtz 19 waves per second is nearly correct for muscles in a condition of 

 powerful contraction. In a series of observations by Griffiths (1888), it was 

 found that voluntary contraction of the biceps weighted with a little more 

 than eleven pounds (5,000 grammes), for one hundred seconds, gave an aver- 

 age of eighteen waves per second, the average for the unweighted muscle 

 being fourteen waves per second for thirty-three seconds. 



The nerves are not capable of conducting an artificial stimulus for an in- 

 definite period, nor are the muscles able to contract for more than a limited 

 time upon the reception of such an excitation. The electric current may be 

 made to destroy for a time both the nervous and muscular excitability ; and 

 these properties become gradually extinguished, the parts becoming fatigued 

 before they are completely exhausted. Precisely the same phenomena are 

 observed in the physiological action oi muscles. When a muscle is fatigued 

 artificially, a tetanic condition is excited more and more easily, but the power 

 of the contraction is proportionally diminished. Muscles contracting in 

 obedience to an effort of the will pass through the same stages of action. It 

 is probable that constant contraction is excited more and more easily as the 

 muscles become fatigued, because the nervous force gradually diminishes in in- 

 tensity ; but it is certain that the vigor of contraction at the same time pro- 

 gressively diminishes. 



The phenomena of muscular contraction thus far considered are those 

 produced by voluntary effort or by stimulation of motor nerves ; but many 

 important phenomena have been observed in muscles detached from the body 



