The white substance of the brain is composed of a series of tubules in exact juxtaposition, and serving as it were as isolated conductors for each group of cells with which they are in connection, like the electric wire that carries the imponderable matter secreted by the pile with which it is united. Originating in the midst of the plexus of cells of the cortical substance, after having travelled with their partners for a while they separate from them one by one, abandon their primitive direction, pass across the mesial line, and are finally lost in the homologous regions of the opposite hemisphere. The white nerve fibres, like true bonds of union, serve then merely to connect two regions of associated cells, and thus to establish between them a natural channel for the propagation of nervous activity. The fibres themselves are thus, by reason of their relations with the grey elements, true commissures distributed everywhere in infinite numbers.