120 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



take place, and another fact which points in the same direction 

 is the high temperature coefficient (p. 115) exhibited by the nerve. 

 The evidence furnished (necessity of oxygen and formation of 

 carbon dioxid) indicates a reaction of oxidation which takes place 

 at some period in the cycle of activity extending from the moment 

 of stimulation to the time of return to the normal resting state. 

 From the quantitative standpoint the amount of material used up 

 in this reaction must be exceedingly small. 



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 

 in a general way that the nerve impulse depends upon the breaking 

 down of some unstable substance within the axis cyhnder. It was 

 assumed that this sensitive and unstable material is upset by the 

 energy of the stimulus at the point stimulated, and that the energy 

 thus liberated acts upon contiguous particles, and so the disturb- 

 ance is propagated along the nerve as a progressive chemical 

 change which in a very general way may be compared to the pas- 

 sage of a spark along a hne of gunpowder. A fundamental ob- 

 jection to such a view is the uncertainty of the proof regarding the 

 consumption of material in a nerve during activity, as has been ex- 

 plained in the preceding sections. Quite the opposite point of 

 view has also been held, namely, the idea that the nerve impulse 

 is a purely physical process, which involves no chemical change 

 and no using up of material. Various suggestions have been 

 offered as to the character of this physical change, but the one that 

 is perhaps most worthy of consideration identifies the nerve im- 

 pulse with the electrical change. 



Hypotheses of several kinds have been proposed to account for 

 the origin of this electrical change and its propagation along the 

 fiber. One of the most attractive and suggestive of these theories 

 we owe to Lillie.* He draws an analogy between the nerve and 



* R. S. Little, "Science," 48, 51, 1918, and "Scientific Monthly," 8, 456, 

 1919. 



