rom
the olfactory membrane in the nose resembles cells in the skin of the
earthworm, in that its cell-body lies actually amid the epithelium of
the skin-surface and is not deeply buried near or in the central nervous
organ. Further, it has at its external end tiny hairlets such as occur
in specially receptive-cells but not usually in purely nervous cells.
Hence we must think that one and the same cell by its external end
receives the environmental stimulus and by its deep end excites the
central nervous organ. The cell under the stimulation of the
environmental agent will therefore generate in itself a nervous impulse.
This is the clearest instance we have of a neurone being actually
excited under natural circumstances by an agent of the environment
_directly_, not indirectly. The deep ends of these olfactory neurones
having entered the central nervous organ come into contact with the
dendrites of large neurones, called, from their shape, mitral. In the
dog, an animal with high olfactory sense, the axone of each olfactory
neurone is connected with five or six mitral cells. In man each
olfactory neurone is connected with a single mitral cell only. We may
suppose that the former arrangement conduces to intensification of the
central reaction by summation. At the same time it is an arrangement
which could tend to smother sharp differentiation of the central
reaction in respect to locality of stimulus at the receptive surface.
Considering the diffuse way in which olfactory stimuli are applied in
comparison, for instance, with visual, the exact localization of the
former can obviously yield little information of use for locating the
exact position of their source. On the other hand, in the case of visual
stimuli the locus of incidence, owing to the rectilinear propagation of
light, can serve with extraordinary exactitude for inferences as to the
position of their source. The adaptation of the neural connexions of the
two organs in this respect is therefore in accord with expectation.

The earliest cerebral cortex is formed in connexion with the
neurone-chains coming into the central nervous organ from the patch of
olfactory cells on the surface of the head. The region of cerebrum thus
developed is the so-called olfactory lobe and hippocampal formation. The
greater part of the cerebral hemisphere is often termed the _pallium_,
because as its development extends it folds cloak-wise over the older
structures at the base of the