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 — while 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 



