172 IRRITABILITY 



ing each other at intervals of less than a second are without effect. 

 It is possible at this stage, as Tiedemann 1 did, to graphically record 

 the reactions. He severed the sciatic nerve on one side and 

 stimulated its central stump, at the same time connecting the 

 triceps with a writing lever. It is then found that when the 

 single induction shocks follow each other at intervals of a second 

 or more every stimulus produces a contraction, but that on the 

 contrary only the first stimulus of a rhythmical series is opera- 

 tive and all those succeeding ineffectual, if the stimuli follow each 

 other at shorter intervals. The refractory period becomes, how- 

 ever, more and more prolonged. The rhythm of the stimulus 



Fig. 33. 



Development of the refractory period in the spinal cord of a strychninized frog. Lower 

 line indicates seconds; upper line stimuli. Of a series of stimuli only the first ones 

 are operative with decreasing effect. 



must become continually slower if each individual stimulus is 

 to remain effective. If the rhythm is even slightly too rapid 

 only the first few stimuli of a rhythmical series are effective 

 and this with decreasing response and later no contraction at all 

 is observed. With a further continuance of the experiment, the 

 stimuli are only effective when following each other at long 

 intervals. It is necessary that a period of recovery lasting 

 several seconds must take place before the following stimulus 

 can meet with response. (Figure 33.) The refractory period 

 can gradually be prolonged for the space of a minute or longer, 



I Tiedemann: "Untersuchungen iiber das absolute Refractaerstadium und die 

 Hemmungsvorgaenge im Riickenmark des Strychninfrosches." Zeitschrift f. allgem. 

 Physiologic Bd. X, 1910. 



