The grey matter covers the surface of the cerebrum
and cerebellum, the white matter being internal. The stem of the brain, the
medulla oblongata, and the spinal cord, consists externally of white
matter, the grey matter being internal. The grey matter consists for the
most part of nerve cells (ganglion cells), and the white matter consists of
nerve fibres; it is white on account of the phosphoretted fatty
sheath--myelin--that covers the essential axial conducting portion of the
nerve fibres. If, however, the nervous system be examined microscopically
by suitable staining methods, it will be found that the grey and white
matters are inseparably connected, for the axial fibres of the nerves in
the white matter are really prolongations of the ganglion cells of the grey
matter; in fact the nervous system consists of countless myriads of nervous
units or neurones; and although there are structural differences in the
nervous units or neurones, they are all constructed on the same general
architectural plan (_vide_ fig. 15). They may be divided into groups,
systems, and communities; but there are structural differences of the
separate systems, groups, and communities which may be correlated with
differences of function.
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