VOLUNTARY CONTRACTION 241 



voluntary tetanus. It merely points to an irregularity or discontinuity in 

 t.iis 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 muscle 

 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 contractions 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 question by 

 recording the electrical changes accompanying the natural contractions 

 of a muscle, i.e. those excited reflexly from the central nervous system. 

 It was long ago shown by Loven that a certain discontinuity could be seen 

 in records of the electrical changes obtained from a frog's muscle in the 

 tetanic spasms produced by an injection of strychnine, but according to 

 Burdon Sanderson this discontinuity represents a series of spasms discharged 

 from the central nervous system. Each discharge produces, not a twitch, 

 but a continued contraction of short duration. On photographing the 

 electrical changes of strychnine spasm as obtained by a capillary electro- 

 meter, he found that each individual spasm could only be compared to a 

 short tetanus. The most recent investigations of the question we owe to 

 Piper, who made use of the string galvanometer, an instrument much more 

 delicate in the reproduction of rapid changes than is the capillary electro- 

 meter. Piper led off two points in the fore-arm, one electrode being placed 

 akout two inches below the bend of the elbow, and the other about four 

 inches above the wrist. A single stimulus of the median nerve was found 



