Responses to Rhythmical Stimulation in the Frog. 335 



(figs. 3 and 5). The critical value of stimulation-frequency often coincides 

 with the optimal rate, and is approximately constant in any preparation, as 

 long as this remains in good condition without serious change of reflex 

 excitability. 



(2) Intensity Fixed, Frequency Varied. — In this method the optimal rate of 

 stimulation in each series is found. If the ampUtude of any reflex reaction 

 in one series is compared with the corresponding one in a series obtained at 

 lower intensity, it is found that the reflex response at low frequency is less 

 powerful with low intensity than with higher, while the opposite is the case 

 with greater frequency of stimulation. There has been a shift of the 

 optimal frequency from a lower value at higher intensity to a higher one at 

 lower intensity. Therefore, if the optimal intensity of stimulation for the 

 highest frequency employed has been found, and a series of reflex reactions 

 is registered at this intensity and at progressively increasing rates, the 

 amplitude of reflex response is not infrequently found to increase to the 

 maximum with increase of stimulation-frequency. 



The presence of an optimal intensity of stimulation is not limited to very 

 rapid frequency-rates. With moderate frequency it is usually observed, 

 that by increase of intensity beyond a certain value, the reflex reaction thus 

 evoked is less powerful, though the decrease is not so obvious as at higher 

 frequencies (fig. 3). This phenomenon is more marked in some preparations, 

 particularly in those which are impaired by prolonged experiment or are 

 observed during the cold season. Even with a frequency as low as thirty per 

 second, an optimum value may sometimes be obtained if a wide range of 

 intensity is employed. Generally, the higher the stimulation-frequency, the 

 more evident is the existence of an optimal intensity. I have several times 

 observed that the optimal frequency is lower in winter frogs than in summer ; 

 in such cases the optimal intensity is more marked with comparatively lower 

 frequencies. 



In speaking of an optimal rate of stimulation, it might seem more 

 reasonable to limit the term to the value obtained at the optimal intensity. 

 The latter value is, however, dependent upon the frequency at which it is 

 elicited. Sometimes the reflex response obtained at the optimal intensity at 

 rapid frequencies is equal to, or even greater than, that obtained at the 

 optimal frequency (fig. 3). The contraction has in these cases a smooth and 

 regular course. This optimal intensity of stimulation is evidently of central 

 origin, since a series of direct neuromyal tetani, produced with progressively 

 increasing intensity of faradisation, presents no evidence of the existence of 

 optimal values, except at frequency-rates considerably more rapid than those 

 which exhibit it in the reflex observations (see later). The degree of 



