MUSCLE AND NERVE 185 



or later, by the phenomena of fatigue, so it was hoped that 

 continuous functional activity of nerve fibers might exhibit 

 similar results. The obvious conclusion, then, would have been 

 that the conduction of the nerve impulse is associated with a 

 chemical change. Most of the effort expended in this direction 

 has been negative in results except that it proves the practical 

 unfatiguableness of nerve fibers under ordinary conditions of 

 stimulation. Under special conditions, when the irritability 

 of nerve fibers has been depressed by narcotics or by lack of 

 oxygen the fatiguing effect of stimulation can be demonstrated. 

 It shows itself by a lengthening of the refractory period fol- 

 lowing stimulation. Thus, when two stimuli follow each other 

 at an interval of time less than 0.006 second, then the second 

 stimulus is ineffective in arousing a nerve impulse. This is 

 normally due to a short-lasting fatigue following the first 

 stimulus. This period necessary for recovery is increased by 

 functional activity of nerve fibers. 



Further but not conclusive evidence for metabolism in nerve 

 fibers has been furnished by Waller, who showed that the 

 action current of a nerve is increased in intensity by a short 

 period of functional activity, i. e., stimulation with interrupted 

 induced current, just as it is by passing over it for a brief interval 

 a stream of carbon-dioxide gas. Many physiologists, therefore, 

 conceive the nerve impulse as a wave of chemical change which, 

 once started, is self-propagated along the fiber. The electrical 

 change that accompanies it is considered as due to the forma- 

 tion of electrolytes in the reaction and the accumulation of 

 negatively charged ions. Those who believe in the physical 

 character of the nerve impulse base their conception upon the 

 fact that no consumption of material can be demonstrated 

 under ordinary conditions. It is assumed that an electrical 

 charge constitutes the nerve impulse which passes over the 

 nerve because it is essentially a "core conductor." Such a 

 " core conductor" may be constructed out of a glass tube filled 

 with 0.6 per cent, sodium chloride, and in the axis of which there 

 is stretched a platinum wire. The neurofibrils are likened to 

 the platinum wire; the perifibrillar substance to the sodium 

 chloiide solution. In such a model many of the electrical 

 phenomena are similar to those shown by nerve. 



