NATURE OF THE NERVE IMPULSE. 119 



was used, but no increase in temperature during stimulation could 

 be demontrated. 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 

 of a degree centigrade.* On the other hand, Tashirof reports 

 that, by means of a new method which is capable of detecting 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 pro- 

 duction is increased about two and a half times when the nerve is 

 stimulated. Additional evidence for the occurrence of a nerve 

 metabolism 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 irritability in an atmos- 

 phere deprived of oxygen, and regains it promptly when oxygen 

 is again supplied. When stimulated in an atmosphere free from 

 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 

 oxygen is used by the nerve during activity, and presumably it 

 is used in this as in the other tissues to produce physiological 

 oxidations. Another fact which points in the same direction 

 is the high value of the temperature coefficient for nerve conduc- 

 tion, which has been referred to above. Bearing these two 

 general considerations in mind, we can hardly escape the con- 

 viction that the functional activity of the nerve fiber is connected 

 with a chemical reaction of some kind, most probably a reaction 

 in which some material in the nerve undergoes oxidation. 



Views as to the Nature of the Nerve Impulse. The older con- 

 ceptions of the nerve principle, while they varied in detail, were 

 based upon the general idea that the nervous system contains a 

 matter of a finer sort than that visible to our senses. This matter 

 was pictured at first as a spirit (animal spirits) , and later as a mate- 

 rial comparable to the luminiferous ether or to electricity. Since 

 the discovery that the nerve impulse travels with a relatively slow 

 velocity and is accompanied by a demonstrable change in the 

 electrical condition of the nerve, many different views regarding 

 its nature have been proposed. In discussing the matter it is 

 evident that two perhaps different phenomena have to be consid- 

 ered, namely, the act of excitation by natural or artificial stimuli 

 and the act of propagation or conduction. Formerly, it was held 



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



t Tashiro, "American Journal of Physiology," "Proc. of Am. Physiologi- 

 cal Soc.," 31, 22, 1913. 



