FUNCTIONS OF THE NERVE CELL 571 



Birge stimulated the spinal cord of the frog by plunging a very fine needle 

 into it and immediately withdrawing the same. He recorded the muscular 

 responses discharged by the stimulus and subsequently determined very accu- 

 rately the portion of the cord invaded by the needle. The result was that stimu- 

 lation of the white substance was found to produce only a single contraction, 

 but when the needle struck nerve cells in the anterior horn an actual tetanus 

 always appeared i. e., a stimulus occurring but once was transformed by the 

 nerve cells into a stimulus lasting for a much longer time. 



(2) Another peculiarity of nerve cells is that they respond especially well 

 to frequent stimulation, even though the strength of the stimulus is relatively 

 very weak. This means that nerve cells possess in a high degree the property 

 of summation. 



Thus Kronecker and Nikolaides found on stimulating the vasomotor center 

 that single induction shocks of great strength produced but slight effect, and 



FIG. 256. Reflex contractions of a frog's leg to electrical stimulation, after Stirling. To be read 

 from left to right. The middle line shows the time of stimulation; the lower line is a time 

 record in seconds. 



that repeated shocks of moderate strength and high frequency (optimum twenty 

 to thirty per second) were more efficacious than stronger shocks at a lower 

 frequency. 



Exactly the same thing is observed in reflex stimulation. It is extremely 

 difficult to get any response from a normal spinal cord with single induction 

 shocks (Setschenow). (Biedermann observed, however, that the responses are 

 easily obtained, if the spinal cord first be cooled.) But if the afferent nerve be 

 stimulated with rapidly repeated shocks, no difficulty is experienced, and with 

 a given strength of current the muscular responses appear more promptly the 

 more frequent the stimuli. This is not because a larger number of stimuli fall 

 within the latent period with the higher frequency; for the absolute number of 

 stimuli received before the end of the latent period may be even greater with 

 a low than with a high frequency. Once an adequate frequency has been 

 reached, the length of the latent period is, within wide limits, independent of 

 the strength of the stimulus (Stirling). 



The power of the nerve cells to store up stimuli is demonstrated in the most 

 striking way by the preliminary reflexes observed by Sanders-Ezn after chemi- 

 cal, and by Stirling (Fig. 256) after electrical stimulation. At first after a 

 short latent period several small twitches appear, then suddenly, after a long 

 latent period, a very powerful contraction is made. The reflex mechanism is 

 now exhausted; the preparation remains at rest notwithstanding the continu- 



