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.  The systems may be divided into:  (1) afferent sensory, including the special senses and general sensibility; (2) motor efferent; (3) association.

[Illustration:  Fig. 15]

[Description:  FIG. 15.—­Diagrammatic representation of a motor neurone magnified 300 diameters.  Whereas the nerve cell and its branching processes (the dendrons) form but a minute speck of protoplasm, the nerve fibre which arises from it, although microscopic in diameter, extends a very long distance; in some cases it is a yard long; consequently only a minute fraction of the nerve fibre is represented in the diagram.]

The great brain or cerebrum consists of two halves equal in weight, termed hemispheres, right and left; and the grey matter covering their surface is thrown into folds with fissures between, thus increasing enormously the superficial area of the grey matter and of the neurones of which it consists without increasing the size of the head.  The pattern of the folds or convolutions shows a general similarity in all human beings, certain fissures being always present; and around these fissures which are constantly present are situated fibre systems and communities of neurones having particular functions (vide fig. 16.) Thus there is a significance in the convolutional pattern of the brain.  But just as there are no two faces alike, so there are no two brains alike in their pattern; and just as it is rare to find the two halves of the face quite symmetrical, so the two halves of the brain are seldom exactly alike in their pattern.  Although each hemisphere is especially related to the opposite half of the body, the two are unified in function by a great bridge of nerve fibres, called the corpus callosum, which unites them.  The cortical centres or structures with specialised functions localised in particular regions of one hemisphere are associated by fibres passing to the same region in the opposite hemisphere by this bridge.

[Illustration:  Fig. 16]