In common with those of various other types of neurone, the dendrites of spinal motor cells are beset with “thorns.” These projections are not rugosities or serrations, but short, delicate threads which stand out at right angles from the dendrites (_cf._ Fig. 1). About a dozen years ago, the writer made a careful investigation of these structures; at a time when most anatomists regarded them as artifacts. He found that their claim to be regarded as parts of the neurone is as good as that of its axon or its dendrites; although never seen on certain types of cell, the thorns, of cells which carry them, are perfectly definite in arrangement and spacing. In some kinds of cell they are more numerous, in others less. Neuro-fibrillæ, as we now know them, had not been discovered at the date when this investigation was undertaken; but on various grounds the conclusion was arrived at that thorns are the cell-ends of fibrils which pass from the end-twigs of arborizing axons into dendrites. Upon this conclusion was based an hypothesis of conduction which is here submitted, not because there is not much to be said against it—or, at any rate, many a hiatus in knowledge to be filled—but because it happens to be the writer’s own. The chrome-silver and methylene-blue methods which reveal the existence of thorns do not stain neuro-fibrillæ. They colour the soft protoplasm in which fibrils are embedded. By modifying the chrome-silver method in every way which still allows a result to be obtained, it was found that thorns sometimes appear as comparatively long slender filaments, at others as shorter filaments ending in minute knobs, or as filaments bearing two or three dots; or finally no filaments are visible, but the dots are in the position which they would occupy if fibrils were present, but not stained. From this it was argued that the soft protoplasm which during life surrounds the filament as a continuous film, either falls back towards the cell after death or is made to shrink into the cell by reagents. This accounts for the appearance of rod and knob. What is supposed to happen may be illustrated by dipping a wire in treacle. At first, when the wire is withdrawn, it is surrounded with a film. Then the film gathers into droplets. It was suggested that the entrance of impulses into dendrites, their conduction across the space which separates the end-twigs of axons from the dendrites into which their impulses pass, is by means of the thorns, although the thorns are not in themselves conductors. Conduction occurs only when films of cytoplasm surround the thorns. The first effect of impulses is to call out the films, in the same kind of way that a current of electricity converts a row of falling drops into a continuous stream. A succession of impulses, by adding to the number of the filaments which are enveloped in cytoplasm, or by increasing the amount of cytoplasm investing certain groups of filaments, increases the openness of the path.