NATURE OF THE NERVE IMPULSE. 117 



pulse is associated with a chemical change of a catabolic nature in 

 the substance of the fiber. Experimental work, however, has 

 shown that under normal conditions the nerve fiber shows no 

 fatigue. The experiments made upon this point have been nu- 

 merous and varied. The general idea underlying all of them has 

 been to stimulate the nerve continuously, but to interpose a block 

 somewhere along the course of the nerve so that the impulses should 

 not reach the end-organ. This precaution is necessary because 

 the end-organ muscle, gland, etc. is subject to fatigue, and 

 must therefore be protected from constant activity. From time 

 to time or at the end of a long period of stimulation the block is 

 removed and it is noted whether or not the end-organ for in- 

 stance, the muscle gives signs of a stimulation. The removable 

 block has been obtained by the action of a polarizing current, by 

 cold, by narcotics, by curare, etc. Using curare, for instance, 

 Bowditch* found that the sciatic nerve might be stimulated continu- 

 ously by induction shocks for several (four to five) hours without 

 complete fatigue, since as the curare effect wore off the muscle 

 whose contractions were being recorded (M. tibialis ant.) began 

 to respond, at first with single and finally with tetanic contractions. 

 The curare in this case may be supposed to have blocked the nerve 

 impulse at the motor end-plate and thus protected the muscle 

 from responding until the lapse of several hours, although the 

 nerve was under stimulation during this entire time. This 

 experiment has since been repeated by Durig,f who has made use 

 cf the fact that the effects of curare can be removed within a few 

 minutes by the salicylate of physostigmin. Durig stimulated the 

 nerve for as much as ten hours and then upon removing the curare 

 block found from the contraction of the muscle that the nerve 

 was still conducting. EdesJ and others have shown that the 

 same result is obtained when the nerve is tested by a capillary 

 electrometer instead of by the response of an end-organ. Under 

 such conditions the nerve exhibits an undiminished action cur- 

 rent, although constantly stimulated by tetanizing shocks from an 

 induction apparatus. Brodie and Halliburton have found that 

 the non-medullated fibers in the splenic nerve can also be stimulated 

 for many hours without losing their power of conduction, that 

 is, without showing fatigue. Many other observers have obtained 

 similar results, which have confirmed physiologists in the belief 

 that the nerve fibers may conduct impulses indefinitely, or, in 

 other words, that their normal functional activity may be carried 



* Bowditch, "Journal of Physiology," 6, 133, 1885. 



t Durig, "Centralblatt f. Physiol.," 15, 751, 1902. 



t Edes, "Journal of Physiology," 13, 431, 1892. 



Brodie and Halliburton, "Journal of Physiology," 28, 181, 1902. 



