Responses to Rhythmical Stimulation in the Frog. 333 



of the reflex tracing. The spinal centre excited reflexly at this rate does 

 not appear to be easily fatigued, especially since the contraction is not 

 maximal and time is allowed for recovery during the intervals between 

 stimuli. 



At a moderate rate of stimulation (40-60 per second), the typical myogram 

 has a smoother and less undulating form (figs. 1 and 4, D). It attains its 

 maximal height rapidly, and maintains it as a . smooth plateau so long as 

 the stimulation continues. If the excitation is prolonged beyond a certain 

 duration the height of the reflex contraction gradually decreases. 



At rapid frequency rates the reflex myogram is again irregular. The 

 initial contraction subsides rapidly, especially if the hitensity of the stimuli 

 is high (fig. 1). This phenomenon recalls the so-called " initial twitch," 

 which was first described by Bernstein (2) in direct tetanus produced by 

 induction shocks of rapid frequency. The similar " initial reflex tetanus " 

 was noted by Frohlich (8). At the optimal intensity of stimulation for 

 this high rate the myogram is often quite regular, though fatigue easily 

 occurs. In general, a smooth and regular course of reflex response is 

 associated with a powerful contraction. 



The results of my experiments are concordant with those obtained by the 

 study of the action current waves of muscles in reflex reaction, in that the 

 central reflex discharge is optimal, when the spinal reflex centre receives 

 impulses from an afferent nerve at the optimal rate. Though the muscular 

 electrical waves in the reflex tetanus are synchronous with the stimulation 

 rhythm up to 100 per second, in summer frogs (Hoffmann (10), BeritojT (1)), 

 the rhythm at which the reflex centre tends to react may be about fifty 

 per second, and the reflex response at this rate is fairly regular and 

 powerful, as judged by the amplitude of deflection of the galvanometer 

 string (Hoffmann (10)). In my experiments, the optimal rate of stimulation, 

 which averages about fifty per second, gives a smooth and powerful reflex' 

 response. The irregularity and diminished height of contraction at frequency 

 rates of stimulation higher than the optimal are in accordance wirh the 

 observation that the muscular action current waves in reflex responses at 

 rapid frequency present irregularity and rapidly diminishing rhythm. 



II. The Influence of Strength of Stimulation upon the Reflex Reaction. 



The reflex contraction elicited by faradisation at or little above the 

 threshold value is inconstant in regard to the height and progress of con- 

 traction. The threshold value itself is continually altering, even in good 

 preparations, though within small limits. When, however, the stimulation is 

 strengthened to a certain intensity, the reflex reaction becomes constant in 



