axone fibre does in the
vast majority of cases transgress the limits of the grey matter wherein
its perikaryon lies. The cortical neurone therefore collects impulses in
the region of cortex just about its perikaryon and discharges them to
other regions, some not cortical or even cerebral, but spinal, &c. One
question which naturally arises is, do these cells spontaneously
generate their impulses or are they stirred to activity by impulses
which reach them from without? The tendency of physiology is to regard
the actions of the cortex as reactions to impulses communicated to the
cortical cells by nerve-channels reaching them from the sense organs.
The neurone conductors in the cortex are in so far considered to
resemble those of reflex centres, though their reactions are more
variable and complex than in the use of the spinal. The chains of
neurones passing through the cortex are more complex and connected with
greater numbers of associate complex chains than are those of the spinal
centres. But just as the reflex centres of the cord are each attached to
afferent channels arriving from this or that receptive-organ, for
instance, tactile-organs of the skin, or spindles of muscle-sense, &c.,
so the regions of cortex whose function is to-day with some certainty
localized seem to be severally related each to some particular
sense-organ. The localization, so far as ascertained, is a localization
which attaches separate areas of cortex to the several species of sense,
namely the visual, the auditory, the olfactory, and so on. This being
so, we should expect to find the sensual representation in the cortex
especially marked for the organs of the great distance-receptors, the
organs which--considered as _sense_ organs--initiate sensations having
the quality of projicience into the sensible environment. The organs of
distance-receptors are the olfactory, the visual and the auditory. The
environmental agent which acts as stimulus in the case of the first
named is chemical, in the second is radiant, and in the last is
mechanical.

_Olfactory Region of Cortex._--There is phylogenetic evidence that the
development of the _cortex cerebri_ first occurred in connexion with the
distance-receptors for chemical stimuli--that is, expressed with
reference to psychosis, in connexion with olfaction. The olfactory
apparatus even in mammals still exhibits a neural architecture of
primitive pattern. The cell which conducts impulses to the brain f