Perhaps there is no system which in recent times has received more attention, from an anatomical as well as from a physiological point of view, than the central nervous system. The brain being built up of cells and fibres of a soft and friable material, imbedded in a still softer substance, and arranged so as to form a complicated network of fibres suspended between and connecting different systems of nuclei, it is not to be wondered at that its minute anatomy, previous to the employment of the various hardening methods now in use, was very imperfectly known. Even now, notwithstanding our present means of hardening nerve tissue, and improved appliances for preparing sections for microscopical examination, we often experience considerable difficulty in its investigation, not the smallest of which is encountered when we endeavour to follow the different nerve fibres and associate them with their proper nuclei. Sometimes this difficulty arises from several bundles of fibres connected with different nuclei running together, or occupying almost the same position and at other times from several nuclei connected with different groups of fibres being placed very close to each other, so that we may have in close proximity nuclei, or the fibres connected with nuclei, which differ greatly in function.