i is purely mechanical, or non-mental, and is
ordinarily termed reflex action. The whole of the spinal cord and lower
part of the brain are made up of nerve-centres of reflex action; and, in
the result, we have a wonderfully perfect machine in the animal body
considered as a whole. For while the various sensory surfaces are
severally adapted to respond to different kinds of external
movement--the eye to light, the ear to sound, and so on--any of these
surfaces may be brought into suitable relation with any of the muscles
of the body by means of the cerebro-spinal nerve-centres and their
intercommunications.

So much, then, for the machinery of the body. We must now turn to
consider the corporeal seat of the mind, or the only part of the nervous
system wherein the agitation of nervous matter is accompanied with
consciousness. This is composed of a double nerve-centre, which occurs
in all vertebrated animals, and the two parts of which are called the
cerebral hemispheres. In man this double nerve-centre is so large that
it completely fills the arch of the skull, as far down as the level of
the eyebrows. The two hemispheres of which it consists meet face to face
in the middle line of the skull, from the top of the nose backwards.
Each hemisphere is composed of two conspicuously distinct parts, called
respectively the grey matter and the white matter. The grey matter is
external, enveloping the white matter like a skull-cap, and is composed
of an inconceivable number of nerve-cells connected together by
nerve-fibres. It is computed that in a human brain there cannot be less
than a thousand millions of cells, and five thousand millions of fibres.
The white matter is composed only of nerve-fibres, which pass downwards
in great strands of conducting tissue to the lower centres of the brain
and spinal cord. So that the whole constitutes one system, with the grey
matter of the cerebral hemispheres at the apex or crown.

That the grey matter of the cerebral hemispheres is the exclusive seat
of mind is proved in two ways. In the first place, if we look to the
animal kingdom as a whole, we find that, speaking generally, the
intelligence of species varies with the mass of this grey matter. Or, in
other words, we find that the process of mental evolution, on its
physical side, has consisted in the progressive development of this
grey matter superimposed upon the pre-existing nervous machinery, until
it has attained its latest and ma