MUSCLE 283 



It is premature to do more than outline the " physical " theory 

 which seems destined to take its place ; and the reader will per- 

 haps forgive if, for the sake of clearness, the case is put with 

 unjustifiable definiteness and simplicity. Proteid substances 

 are constituted of clusters of molecules. The form of the 

 clusters depends upon the salts (or, more precisely, the ions) 

 with which they are associated, and the associations depend 

 upon the electric charges which the ions carry. In resting 

 nerve-protoplasm the clusters are small, and, since the total 

 surface-area of a number of small spheres is greater than the 

 surface-area of the same weight of matter when condensed into 

 large spheres, there is, so to speak, more surface for the ions to 

 cling to. Conversely, when the ions leave the small clusters, the 

 latter are not protected from the influence of mutual attraction. 

 They fuse into larger clusters. Fusion is carried to its extreme 

 limits when a protein coagulates. A nerve-impulse is a 

 " wave " of partial coagulation. The positive electricity 

 generated in a cell-body by the metabolism of its tigroids 

 repels the positively charged ions which cling to the nearest 

 protoplasm-clusters in the axon. Like acrobats swinging from 

 trapeze to trapeze, each flight of ions dispossesses the ions from 

 the clusters in front of it ; and in this way the disturbance pro- 

 gresses down the axon as an electric wave. 



Thus we interpret the shadow cast by a theory of which 

 either of several pioneers who are diligently climbing may at 

 any time obtain a view. The conductivity of protoplasm (and 

 what is true of its conductivity will be found to hold good 

 equally for its irritability and changeableness of form) is due 

 to the readiness with which its molecules enter into unstable 

 associations with electrolytes. The instability of these associa- 

 tions is related to the tendency of the molecules to cluster. 

 An impulse is passed along a nerve as a displacement of ions ; 

 the ions being transferred from one molecule, or group of 

 molecules, to the next. Such an explanation of an impulse 

 involves no chemical breakdown of nerve-substance during its 

 passage along a nerve. It transfers the metabolism which 

 liberates energy (reinforcing tho impulses which have originated 

 in sense-organs) to the nerve-cell bodies. It is based upon 

 certain experimental data which appear to have been estab- 

 lished ; but, like all other hypotheses which are intended to 



