=Dependence of Mind on Nerves and Brain.= We have seen how in a general
way the mind is dependent on the body. We have seen how in a more
intimate way it is dependent on the special sense organs. But the part
of the body to which the mind is most directly and intimately related is
the nervous system. The sense organs themselves are merely modifications
of the nerve ends together with certain mechanisms for enabling stimuli
to act on the nerve ends. The eye is merely the optic nerve spread out
to form the retina and modified in certain ways to make it sensitive to
ether vibrations. In addition to this, there is, of course, the focusing
mechanism of the eye. So for all the sense organs; they are, each of
them, some sort of modification of nerve-endings which makes them
sensitive to some particular force or substance.

Let us make the matter clear by an illustration. Suppose I see a picture
on the wall. My eyes are directed toward the picture. Light from the
picture is refracted within the eyes, forming an image on each retina.
The retina is sensitive to the light. The light produces chemical
changes on the retina. These changes set up an excitation in the optic
nerves, which is conducted to a certain place in the brain, causing an
excitation in the brain. Now the important point is that when this
excitation is going on in the brain, _we are conscious, we see the
picture_.

As far as science can determine, we do not see, nor hear, nor taste, nor
smell, nor have any other sensation unless a sense organ is excited and
produces the excitation in the brain. There can be no doubt about our
primary, sensory experience. By primary, sensory experience is meant our
immediate, direct knowledge of any aspect of the world. In this field of
our conscious life, we are entirely dependent upon sense organs and
nerves and brain. Injuries to the eyes destroying their power to perform