VOLUNTARY CONTRACTION 241 



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 muscle of the body is 

 caused by strong excitation of the cerebral cortex, as in epilepsy. On taking a record 

 of such contractions, Schafer 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 increasing 

 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, fiach 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 electrometer, he found that 

 each individual spasm could be compared only 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 electrometer. Piper 

 led off two points in the fore-arm, one electrode being placed about 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 by him to 

 give a typical diphasic variation in the muscles. When the muscles were 

 contracted voluntarily, well-marked oscillations of the galvanometer wire 

 were obtained, indicating the existence in the muscle of forty- eight to 

 fifty complete diphasic variations in the second (Fig. 93). Piper obtained 

 similar records on leading off other muscles of the body when these were 

 placed voluntarily in a state of contraction, and he concludes therefore 

 that each voluntary contraction, short or long, is a tetanus composed of 

 about fifty fused twitches per second. These results would indicate that 



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