NATURE OF THE NERVE IMPULSE. 119 



atmosphere of nitrogen, loses its irritability after a certain time, 

 this event occurs much more rapidly if the nerve is stimulated con- 

 stantly.* This fact would suggest that some oxygen is consumed 

 during functional activity, and that the ability of the nerve under 

 normal circumstances to escape the results of fatigue may be due 

 possibly to the fact that the consumption of material is small and 

 the processes of restoration are rapid and complete. 



Does the Nerve Fiber Show Any Evidence of Metabolism 

 During Functional Activity? — The functional part of a nerve 

 fiber in conduction is the axis cyhnder. 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. Rollestonf investigated the 

 question of heat production with the aid of a deHcate bolometer 

 capable of indicating a difference of temperature of 5-^00° C. The 

 frog's sciatic was used, but no increase in temperature during 

 stimulation could be demonstrated. Making use of a more sensi- 

 tive 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 of a degree centigrade. | On the other hand, Tashiro§ 

 reports that, by means of a new method which is capable of de- 

 tecting as little as 0.0000001 gm. of carbon dioxid, he has been able 

 to show that the resting nerve produces carbon dioxid and that 

 this production is increased about two and a half times when the 

 nerve is stimulated. Additional evidence for the occurrence of a 

 nerve metabohsm during activity is found in the fact, already 

 alluded to, that oxygen plays a part in maintaining the irritability 

 of nerves. An excised frog's nerve loses its irritabihty in an atmos- 

 phere deprived of oxygen, and regains it promptly when oxygen 

 is again supphed. When stimulated in an atmosphere free ifrom 

 oxygen the nerve shows signs of fatigue, while in the presence of 

 oxygen activity is maintained, one may say indefinitely, under 

 continuous stimulation. These facts warrant the belief that 

 in the normal activity of the nerve chemical changes of some kind 



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

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

 t Hill, "Journal of Physiology," 43, 433, 1911-12. 



§ Tashiro, "American Journal of Physiology," "Proc. of Am. Physiolog- 

 ical Soc," 31, 22, 1913. 



