112 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



It must be remembered, however, that, although the above experi- 

 ments demonstrate the practical " unf atigueableness " of nerve 

 fibers under ordinary conditions of stimulation, there are some 

 reasons to make us hesitate in supposing that these structures 

 function absolutely without fatigue. In all the experiments re- 

 ferred to the nerve was stimulated by induction shock, and although 

 these stimuli followed very rapidly there was a short period of 

 rest after each stimulus, and possibly this interval of rest is 

 quite sufficient in the normal nerve for recovery from the effect 

 of the previous stimulus. It has been shown,* for instance, that, if 

 two stimuli be applied to a nerve with a very brief interval between 

 (0.006 sec. or less, according to the temperature), the second stimu- 

 lus is ineffective so far as can be determined by the response of an 

 attached muscle or by means of a capillary electrometer. And it 

 may very well be that in this case the lack of response to the second 

 stimulus is due to a short-lasting fatigue from the first stimulus. 

 This point of view is strengthened by the fact that, when the irrita- 

 bility of the nerve is greatly depressed by narcotics, f this critical 

 interval is much lengthened; so that stimuli with a rate of more 

 than 10 per second may give an effect only for the first stimulus. 

 Garten has shown also that one nerve, the olfactory of the pike, 

 when stimulated by induction shocks, with an interval between 

 the stimuli of as much as 0.27 sec, gives evidence of fatigue, since 

 its action current as measured by the capillary electrometer dimin- 

 ishes in extent quite rapidly, and recovers after a short rest. J 



Does the Nerve Fiber Show any Evidence of Metabolism 

 during Functional Activity? The functional part of a nerve 

 fiber in conduction is the axis cylinder, and, indeed, probably the 

 neurofibrils in the axis cylinder. The mass of this material even 

 in a large nerve trunk is small, and its chemistry is but little known. 

 The efforts that have been made to prove a metabolism in the 

 nerve fiber during activity have been directed along the lines 

 indicated by what is known of muscle metabolism. In a muscle 

 during contraction heat is produced, the substance of the muscle 

 shows an acid reaction, and among the products formed carbon 

 dioxid gas is perhaps the most prominent. Efforts to show similar 

 reactions in stimulated nerves have been unsuccessful. Rolleston 

 investigated the question of heat production with the aid of a 

 delicate bolometer capable of indicating a difference of temperature 

 of -^^-qC. The frog's sciatic was used, but no increase in tem- 

 perature during stimulation could be demonstrated. No change 



* Gotch and Burch, "Journal of Physiology/' 24, 410, 1899. 

 t Frohlich, " Zeitschrift f. allgemeine Physiol.," 3, 468, 1904. 

 j Quoted from Biedermann, " Ergebnisse der Physiologie," vol. ii, part u, 

 p. 129. 



Rolleston, "Journal of Physiology," 11, 208, 1890. 



