are perfected during the first post-natal
month, and those whose fibres are perfected after the first but before
the end of the fourth post-natal month. The regions thus marked out by
completion before birth are five in number, and are each connected, as
also shown by collateral evidence, with one or other particular species
of sense-organ. And these regions have another character in common
recognizable in the nerve-fibres entering and leaving them, namely, they
possess fibres projected to or from parts of the nervous system
altogether outside the cortex itself. These fibres are termed
"projection" fibres. Other regions of the cortex possess fibres coming
from or going to various regions of the cortex itself, but do not
possess in addition, as do the five primitive cortical fields, the
fibres of projection. So that the facts established by Flechsig for the
regions of pallium, which other evidence already indicated as connected
with the sense-organ of smell, support that evidence and bring the
olfactory region of cortex into line with certain other regions of
cortex similarly primarily connected with organs of sense.

It will be noted that what has been achieved by these various means of
study in regard to the region of the cortex to which olfactory functions
are attributed amounts at present to little more than the bare
ascertainment of the existence there of nervous mechanisms connected
with olfaction, and to the delimiting roughly of their extent and of
their ability to influence certain movements, and in man sensations,
habitually associated with exercise of the olfactory organ. As to what
part the cortical mechanism has in the elaboration or association of
mental processes to which olfaction contributes, no evidence worth the
name seems as yet forthcoming. In this respect our knowledge, or rather
our want of knowledge, of the functions of the olfactory region of the
cortex, is fairly typical of that to which we have to confess in regard
to the other regions of the cortex, even the best known.

_Visual Region of the Cortex._--There is a region of the cortex
especially connected with vision. The _optic nerve_ and _tract_
constitute the second link in the chain of neurones joining the retina
to the brain. They may therefore be regarded as the equivalent of an
intraspinal tract connecting the deep ends of the afferent neurones from
the skin with higher nervous centres. In the bony fishes the optic tract
reaches the gr