s. In them one neurone continues the line of conduction
where the immediately foregoing neurone left it. That is, the neurones
are laid in conductive series, the far end of one apposed to the near
end of its precursor. The place of juxtaposition of the end of one
neurone against the beginning of another is called the _synapse_. At it
the conduction which has so far been wholly intra-neuronic is replaced
by an inter-neuronic process, in which the nerve impulse passes from one
neurone to the next. The process there, it is natural to think, must be
physiologically different from that conductive process that serves for
transmission merely within the neurone itself. It may be that to this
inter-neuronic conduction are due the differences between conduction in
nerve-_arcs_ and nerve-_trunks_ (nerve-fibres) respectively. Significant
of the former are changes in rhythm, intensity, excitability and
modifications by summation and inhibition; in fact a number of the main
features of nervous reaction. These characters impressed upon conduction
in nerve arcs (neurone-chains) would therefore be traceable to the
intercalation of perikarya and synapses, for both these structures are
absent from nerve-trunks. It is therefore probably to perikarya and
synapses that the greater part of the co-ordination, elaboration and
differentiation of nervous reactions is due. Now, perikarya and synapses
are not present in the _white_ matter of the central nervous organ, any
more than they are in nerve-trunks. They are confined exclusively to
those portions of the central organ which consist of _grey_ matter (so
called from its naked-eye appearance). Hence it is to the great sheet of
grey matter which enfolds the cerebrum that the physiologist turns, as
to a field where he would expect to find evidences of the processes of
cerebral co-ordination at work. It is therefore to items regarding the
functions of the great sheet of cerebral cortex that we may now pass.

_The Cerebral Cortex and its Functions._--The main question which vexed
the study of the physiology of the cerebral hemispheres in the 19th
century was whether differences of function are detectible in the
different regions of the hemisphere and especially in those of its
cortex. One camp of experimenters and observers held that the cortex was
identical in function throughout its extent. These authorities taught
that the various faculties and senses suffer damage in proportion to the
amount of