102 THE PHILOSOPHY OF BIOLOGY 



disintegrations rather than combustions). If we 

 imagine a long rod of dynamite, or picric acid, or a long 

 strand of loosely-packed gun-cotton to be exploded by 

 percussion at one end, then a transmission of the 

 chemical disintegration of any of these substances will 

 pass along the rod, etc., with a velocity which will 

 certainly vary with the physical condition of the 

 material. It would be a high velocity in a rod of 

 dynamite, or fused picric acid, but a lower velocity 

 in a loosely aggregated strand of gun-cotton, or a 

 trail of picric acid powder. Is this what happens in 

 the nerve when an impulse travels along it ? Ob- 

 viously not, since the substance of the nerve is not 

 altered appreciably, while that of the explosive sub- 

 stance passes into other chemical phases. We might 

 imagine, then, such a change in the nerve fibrils as that 

 of a reversible transformation of some chemical con- 

 stituent : — 



Let us imagine the substance of the fibril to be composed 

 of, or at least to contain, the substances a+b which 

 dissociate reversibly into the substances c+d. At any 

 moment, and in any particular physical state, as much 

 of a and b pass into c and d diS c and d pass into a and 

 b. There will be equilibrium. But now let a stimulus 

 alter the physical conditions : prior to the stimulus 

 the phase was a„, + ^» = Cp + d, — the suffixes m, n, p, r, 

 denoting the concentrations of a, b, c, and d — but after 

 the stimulus the phase may be rt„,i + 6„i=Cpi + ^,.i. Now 

 the element of the nerve substance (i) forms a system 



