A theory of the effects of fibre size in medullated nerve

Our knowledge of the mechanism of nerve action is still insufficient to predict what happens in terms of electrical and chemical structure. If our theoretical knowledge were as weak in every aspect as it is upon the behaviour of the excitable membrane, we should not be able to predict confidently any relation but this-that two nerves which were exactly the same in structure, composition and properties, must behave in exactly the same way. However, both Ringer's fluid and axoplasm appear to be homogeneous conductors, so the current distribution in those regions is exactly defined by electrical theory. If, then, two nerves are not identical but have membranes with the same specific properties, it might still be possible to compare them exactly. For, though the properties of the membrane are unknown, they are the same in the two nerves, and though the distribution of current in Ringer and axoplasm is different it is precisely understood. It turns out that there is only a very narrow range of conditions which will permit this argument being applied. It is when the two fibres are dimensionally similar, which is nearly the same as their being geometrically similar. Now the interesting thing is that the nerve fibres do, in fact, exhibit the structural similarity demanded by the theory, and hence it is worth while to examine how far the variation in properties also accords with theoretical predictions.