THE ACTIVITIES OF THE ORGANISM 101 



Several facts of capital importance result from the 

 experimental work, (i) The impulse travels with a 

 velocity variable within certain limits, say from 8 

 to 30 metres per second ; (2) it travels faster if the 

 temperature is raised (up to a certain limit) ; (3) it is 

 difficult to demonstrate that the passage of this impulse 

 is accompanied by definite chemical changes in the nerve 

 substance : it is stated that carbon dioxide is produced, 

 but this is not certainly proved ; (4) an electric 

 current is produced in the nerve as the result of stimula- 

 tion ; (5) no heat is produced, or at least the rise of 

 temperature, if it occurs, is less than 0.0002° C. 



Thus it is quite certain that physical changes 

 accompany the propagation of the nerve-impulse, for 

 the latter has a certain velocity, which depends on the 

 temperature, and an electric change also occurs in the 

 substance of the nerve. Is this electric change the 

 actual nerve impulse ? It is hardly likely, since the 

 velocity of the impulse is very much less than that 

 of the propagation of an electric change through a 

 conductor ; besides, the passage of the impulse is not 

 accompanied by a measurable heat evolution, although 

 the flow of electricity along a poor conductor must 

 generate heat and dissipate energy. Is it a chemical 

 change ? Then we should be able to observe meta- 

 bolism in the nerve substance — that is if the energy- 

 change is a thermodynamic one — w^hile it is not at all 

 certain that metabolic changes do occur. Nevertheless 

 it seems probable that a physico-chemical change is 

 actually propagated when we consider the chemical 

 specialisation of the substance of the axis-cylinder of 

 the nerve. Now the velocity of propagation of the 

 nervous impulse is of the same order of magnitude 

 as that of an explosive change in chemical substances 

 {using the term " explosion " to connote chemical 



