NATURE OF THE NERVE IMPULSE. 119 



a metabolism, and in this respect also the functional activity of 

 the nerve would be placed in contrast with that of other organs. 

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

 periments demonstrate the practical " unfatigueableness " of 

 nerve fibers under ordinary conditions of stimulation, there are 

 some reasons to make us hesitate in supposing that in these 

 structures functional activity is entirely without a depressing 

 effect upon irritability. Garten has shown 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, diminishes in extent quite rapidly, and 

 recovers after a short rest. * So also it has been found that while 

 a nerve deprived of oxygen, by keeping it in an atmosphere of 

 nitrogen, loses its irritability after a certain time, this event occurs 

 much more rapidly if the nerve is stimulated constantly.! This 

 fact would suggest that some oxygen is consumed during functional 

 activity, and that the ability of the nerve under normal circum- 

 stances to escape the results of fatigue may be due possibly to the 

 fact that the supply of oxygen is sufficiently abundant to oxidize 

 promptly the fatigue substances formed during activity. 



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 (about 9 per cent.), 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 carbon dioxid is formed. 

 Efforts to show similar reactions in stimulated nerves have been 

 only partially successful. RollestonJ investigated the question of 

 heat production with the aid of a delicate bolometer capable of in- 

 dicating a difference of temperature of -gunnr C. The frog's sciatic 

 was used, but no increase in temperature during stimulation could 

 be demonstrated. Making use of a more sensitive instrument, 

 Hill has obtained the same negative result. If any heat is 

 produced by the transmission of a nerve impulse it must be 

 less, according to his measurements, than a hundred-millionth 



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

 p. 129. 



t Thorner, "Zeitschrift f. allg. Physiologie," 8, 530, 1908. 

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



