The Electrical Response of Muscle 237 



Granting that the rhythm in voluntary contraction is, or is mainly, of 

 peripheral origin, how are we to account for the differences in fre- 

 (piency which prevail in my records of the responses of one and the same 

 muscle, and for those whicli both Piper and I find to exist in different 

 muscles '. To anyone who has taken records of the electrical response of 

 tlie same and of different muscles in a large number of strychnine frogs, 

 there is nothing surprising in the amount of variation which occurs in the 

 frequency of anything which may be compared to the wavelets. As I have 

 said elsewhere [(4), p. 148], the frequency of these in the frog's sartorius 

 in strychnine spasm maj^ vary in different preparations from 40 to 100 

 pjr second. Although I did not actually state for the strychnine reflex 

 i-esponses that the frequency is generallj^ greater in the gastrocnemius than 

 in the sartorius, the records reproduced [(4), pi. vii. ph. 36 (sartorius) and 

 38 (gastrocnemius)] showed it, and the}^ were, and are, typical. I did, as a 

 matter of fact, draw attention to the difference of behaviour of the two 

 muscles in this respect when the response was to a continuous stimulus, 

 or rather to one of very high frequency [(4), p. 139] ; or when it was to 

 the break of an ascending current through the nerve [(4), p. 142, and pi. vii. 

 ph. 33 (sartorius) and 34 (gastrocnemius)]. 



In the reflex responses of strychnine a wavelet f requeue}" of about 100 

 per second is most often met with in records taken with the frog's gastro- 

 cnemius, and the frequency is much more constant (whether there are waves 

 or not) than it is with any of the other muscles I have used. In records 

 taken with the sartorius a wavelet frequency of 50 to 60 per second more 

 usually presents itself, either by itself or side by side with a quicker rhythm. 

 Recent experiments have shown that with the triceps femoris as with the 

 gastrocnemius a quick rhythm is more usual, while with the biceps femoris 

 and the semitendinosus two or even three different frequencies are apt to 

 prevail in turn in one and the same response, so that the}* behave more like 

 the sartorius. 



With regard to the signiflcance of these differences of rhythm in the 

 different muscles of the frog, I would suggest that the}- are due to the 

 differences which Bonhoffer (12) has shown to exist in the kind of fibre 

 composing the different muscles, and in the proportion of one kind to 

 another in a single muscle. In view of the fact that it is the thick, '' quick " 

 ffbres (of Grlltzner), those which are poor in sarcoplasm, which are present 

 almost exclusively in the gastrocnemius and the triceps, it would be these 

 ffbres which exhibit a response frequency of about 100 per second to any 

 kind of stimulus which is not of such nature as to impress a rhythm of 

 its own upon them. In the sartorius and biceps there are as many, or 

 even more, thin, "slow" ffbres, rich in sarcoplasm, present. Bonhoffer 

 says nothing about the semitendinosus. My experiments suggest that in 

 it also the two kinds of fibres would be found. 



I have not been able to find any description of the histological structure 

 of the masseters of man compared with that of the flexores digitorum 



