Reflex Responses to Rhythmical Stimulation in the Frog. 329 



at this joint. The pelvis and femur of the frog was firmly fixed by pins to a 

 cork-plate so that the movement of other muscles of the body might not add 

 to that of the semitendinosus. The whole preparation was enclosed in a 

 moist chamber. 



The stimulating apparatus was an inductorium fed by two Daniell's cells. 

 To vary the rate of stimulation the " double wire torsion key " devised by 

 Sherrington* was used. The range of alteration of vibration-frequency of the 

 instrument was between 15 and 88 per second. Throughout the experiments 

 only break-shocks were employed. Care was taken that the cathode of the 

 electrodes was proximal to the anode on the afferent nerve for a reflex, and 

 distal on the motor nerve for a direct stimulation. The frogs were kept 

 over-night in the laboratory before being used for experiment ; and the time 

 which elapsed between the decapitation and the commencement of the 

 experiment was usually an hour. 



1. The Influence of Stirmdation-frequency upon the Refl,ex Beaction. 



The summation of stimuli in the reflex centre causes increase in the 

 resulting reflex with increase in the rapidity of the sequence of excitation. 

 Stirling (16) pointed out that when a reflex movement is produced in the 

 frog by stimulating a skin point by successive stimuli — the greater the 

 interval between them the higher is the intensity of the individual stimuli 

 necessary to produce the reflex result, and that increase in frequency is more 

 effective than increase in intensity. In the reflex excitation of automatic 

 centres — for example the vasomotor centre — the reflex effect increases with 

 increase in the frequency of stimulation up to the rate of 20-25 per 

 second (11). Matthaei (14) has recently found that the effective minimal 

 number of stimuli, which just produces a reflex reaction, increases in the 

 majority of cases with the frequency of stimulation, occasionally diminishes 

 at first, then reaches an optimum (frequency of 6-21 per second), and finally 

 increases with the stimulation-frequency. In recording my experimental 

 results I will first present the evidence for an optimal frequency of stimula- 

 tion, by which is meant the rate of stimulation at any given intensity, which 

 produces the most powerful reflex contraction. 



1. The Optimal Frequency-rate of Stiinulation. — In order to determine this, 

 an afferent nerve was stimulated at fixed intensity but at different frequency- 

 rates, and the series of reflex contractions of the semitendinosus thus evoked 

 was recorded isometrically. The intensity of stimulation selected was usually 

 the optimal for a moderately rapid frequency (see later). The lowest rate of 

 excitation was applied for a time sufficiently long to allow the production of 

 * A deacription of this key will appear shortly. 



2 c 2 



