ganglion cells of the cortex of the brain are found to have
undergone degeneration. The brains of paretics in particular show such
degeneration, in striking correspondence with their mental decadence.
The position of the ganglion cell as the ultimate centre of nervous
activities was thus placed beyond dispute.

Meantime, general acceptance being given the histological scheme of
Gerlach, according to which the mass of the white substance of the
brain is a mesh-work of intercellular fibrils, a proximal idea seemed
attainable of the way in which the ganglionic activities are correlated,
and, through association, built up, so to speak, into the higher mental
processes. Such a conception accorded beautifully with the ideas of
the associationists, who had now become dominant in psychology. But
one standing puzzle attended this otherwise satisfactory correlation
of anatomical observations and psychic analyses. It was this: Since,
according to the histologist, the intercellular fibres, along which
impulses are conveyed, connect each brain cell, directly or indirectly,
with every other brain cell in an endless mesh-work, how is it possible
that various sets of cells may at times be shut off from one another?
Such isolation must take place, for all normal ideation depends for
its integrity quite as much upon the shutting-out of the great mass of
associations as upon the inclusion of certain other associations. For
example, a student in solving a mathematical problem must for the moment
become quite oblivious to the special associations that have to do with
geography, natural history, and the like. But does histology give any
clew to the way in which such isolation may be effected?

Attempts were made to find an answer through consideration of the very
peculiar character of the blood-supply in the brain. Here, as nowhere
else, the terminal twigs of the arteries are arranged in closed systems,
not anastomosing freely with neighboring systems. Clearly, then, a
restricted area of the brain may, through the controlling influence of
the vasomotor nerves, be flushed with arterial blood while neighboring
parts remain relatively anaemic. And since vital activities
unquestionably depend in part upon the supply of arterial blood, this
peculiar arrangement of the vascular mechanism may very properly be
supposed to aid in the localized activities of the central nervous
ganglia. But this explanation left much to be desired--in particular
when