PROPAGATION OF PHYSIOLOGICAL CHANGE 499 



the openings thus discredited? This is a claim which I have 

 ventured to satisf}'. 



Let the strip of protoplasm forming the core of the nerve- 

 fibre be thought of, not as a homogeneous mass of similar 

 molecules, but as a solution in which co-exist molecules of very 

 different sizes and kinds. For simplicity let us fix our attention 

 upon two kinds of molecules exhibiting these differences in 

 their utmost extreme — one kind large colloid molecules tending 

 to form aggregates, the other kind minute inorganic salt mole- 

 cules and ions mutually avoiding one another but tending to 

 be drawn into association with colloid aggregates. Imagine 

 also that a certain average size of aggregate represents the 

 normal condition of colloid solution in the nerve-fibre, but 

 that departures from this average occur as a consequence of 

 physical change or stimulation. The observation of precipitates 

 at excited points justifies the hypothesis that an increase in 

 this average size is a characteristic of excitation. Now let us 

 suppose that these aggregates or solid phases, or — less correctly, 

 but offering a readier picture — solid particles, attract the salt 

 molecules to their surfaces, so that layers of a condensed salt 

 solution are found thereon. It follows that, when excitation 

 takes place and the number of solid particles is diminished, they 

 in this new condition must offer a smaller total surface for the 

 retention of such a condensed solution of inorganic salts. As a 

 consequence a new mass of inorganic salt will have found its 

 way into the general bulk of aqueous solution surrounding the 

 colloid masses. We thus obtain a picture of the excited state 

 in a form which satisfies all the experimental evidence, and the 

 picture at the same time lends itself to a clear representation 

 of the mode of propagation of the excited state — i.e. nerve 

 impulse. To ensure the propagation of such a state in the 

 manner observed in nerve, all that is necessary is to postulate 

 that the exciting negative anion of the inorganic salt present 

 should have a greater velocity than the positive kation. The 

 salt is unmasked, it diff'uses, the next segment is rendered 

 negative, desolution accompanied by the unmasking of a further 

 quantity of salt in this new segment, further diffusion, etc., etc. 



Now if I am asked at what rate this change would be 

 propagated — and this is a question of great importance — 

 unprovided myself with more than vague uncertain mur- 

 murings I deliberately steal another's thunder. W. Sutherland 



