Responses to Rhythmical Stimulation in the Frog. 331 



since direct stimulation of the motor nerve gives an undiminished response, 

 and shifting the electrodes centralwards on the afferent nerve or elicitation 

 of the reflex by other afferent nerves does not notably affect the degree of 

 impairment. Local damage is more pronounced when a smaller afferent 

 nerve is used. Even when the diminution of reflex excitability is marked, 

 the optimal rate usually remains the same. In fig. 1 there is no sign of such 

 change of excitability throughout the experiment. 



(2) Mechanical Reflex Rhythm vnth varying Frequency of Stimulation. — 

 Various results have been obtained by earlier workers in regard to the 

 relation between the frequency of rhythmic stimulation of the afferent 

 nerve and that of the impulses sent out from the spinal centre to the 

 motor organ. Some investigators have shown that the central nervous 

 system tends to reproduce an intrinsic rhythm of innervation, i.e., about 

 20 per second (Kronecker and Stanley Hall and others), or about 10 per 

 second (Horsley and Schafer and others), irrespective of the rhythm of 

 excitation applied to an afferent nerve or to the central nervous system 

 itself. Von Limbeck (12) has, however, found that both in warm-blooded 

 and cold-blooded animals, on artificially stimulating the brain or spinal coid 

 by induction currents, the number of muscular vibrations follows the 

 rhythm of stimulation within wide limits. In the frog and toad the con- 

 traction of the gastrocnemius muscle, evoked directly by excitation of the 

 spinal cord, or reflexly by stimulation of the sciatic nerve at a frequency- 

 rate of 13 per second, shows the same rhythm as that of stimulation. He 

 notes besides that, at slower rates of excitation, the mechanical vibrations 

 are double the frequency-rate of stimulation, both make and break shocks 

 taking effect. By a more delicate " resonance method " it has been recently 

 shown by Dreyer and Sherrington (5) that the myograph of the reflex 

 contraction in cats exhibits a mechanical rhythm synchronous with the 

 stimulation at rates up to, and even beyond, 55 a second. 



In order to re-examine this point in frogs, a myograph recorder, whose 

 vibration-frequency was 30 per second, was employed. The torsion key 

 was placed upon another table, so that the direct mechanical influence of 

 the vibration of this instrument on the recording lever might be avoided. 

 By this resonance method, the reflex myograph of the semitendinosus 

 evoked by stimulating an afferent nerve shows in the majority of cases more 

 or less clear mechanical tremors synchronous with the frequency of stimu- 

 lation, whether the rate is 15, 20, or 30 per second (figs. 1 and 2). Further 

 increase of frequency of stimulation up to 40 per second is sometimes 

 followed by the same rhythm of reflex contraction. The reflex centre 

 discharge appears to follow the successive volleys of centripetal impulses 



