brain and cord, apparently, is made
up of such connecting fibres, thus bringing the different ganglion cells
everywhere into communication one with another.

In the attempt to trace the connecting nerve tracts through this
white substance by either macroscopical or microscopical methods, most
important aid is given by a method originated by Waller in 1852. Earlier
than that, in 1839, Nasse had discovered that a severed nerve cord
degenerates in its peripheral portions. Waller discovered that every
nerve fibre, sensory or motor, has a nerve cell to or from which it
leads, which dominates its nutrition, so that it can only retain its
vitality while its connection with that cell is intact. Such cells he
named trophic centres. Certain cells of the anterior part of the spinal
cord, for example, are the trophic centres of the spinal motor nerves.
Other trophic centres, governing nerve tracts in the spinal cord itself,
are in the various regions of the brain. It occurred to Waller that
by destroying such centres, or by severing the connection at various
regions between a nervous tract and its trophic centre, sharply
defined tracts could be made to degenerate, and their location could
subsequently be accurately defined, as the degenerated tissues take on
a changed aspect, both to macroscopical and microscopical observation.
Recognition of this principle thus gave the experimenter a new weapon
of great efficiency in tracing nervous connections. Moreover, the same
principle has wide application in case of the human subject in disease,
such as the lesion of nerve tracts or the destruction of centres by
localized tumors, by embolisms, or by traumatisms.

All these various methods of anatomical examination combine to make the
conclusion almost unavoidable that the central ganglion cells are the
veritable "centres" of nervous activity to which so many other lines of
research have pointed. The conclusion was strengthened by experiments
of the students of motor localization, which showed that the veritable
centres of their discovery lie, demonstrably, in the gray cortex of the
brain, not in the white matter. But the full proof came from pathology.
At the hands of a multitude of observers it was shown that in certain
well-known diseases of the spinal cord, with resulting paralysis, it is
the ganglion cells themselves that are found to be destroyed. Similarly,
in the case of sufferers from chronic insanities, with marked dementia,
the