e added the sensation of pain, tickling, itching, and so on,
the needs of which arise from the complicated structure of the human body.
The great majority of sensations result from some stimulus or
outward agency; and yet some sensations, such as those of faintness,
restlessness, and fatigue seem to spring up within us in some mysterious
way, without any obvious cause.

310. Essentials of a Sense Organ. Certain essentials are necessary
for a sensation. First, there is a special structure adapted to a
particular kind of influence. Thus the ear is formed specially for being
stimulated by the waves of sound, while the eye is not influenced by
sound, but responds to the action of light. These special structures are
called terminal organs.

Again, a nerve proceeds from the special structure, which is in direct
communication with nerve cells in the brain at the region of
consciousness. This last point is important to remember, for if on
some account the impression is arrested in the connecting nerve, no
sensation will result. Thus a man whose spine has been injured may not
feel a severe pinch on either leg. The impression may be quite sufficient
to stimulate a nerve center in a healthy cord, so as to produce a marked
reflex act, but he has no sensation, because the injury has prevented the
impression from being carried up the cord to the higher centers in the
brain.

311. The Condition of Sensation. It is thus evident that while an
impression may be made upon a terminal organ, it cannot strictly be called
a sensation until the person becomes conscious of it. The consciousness
of an impression is, therefore, the essential element of a sensation.

It follows that sensation may be prevented in various ways. In the sense
of sight, for example, one person may be blind because the terminal organ,
or eye, is defective or diseased. Another may have perfect eyes and yet
have no sight, because a tumor presses on the nerve between the eye and
the brain. In this case, the impression fails because of the break in the
communication. Once more, the eye may be perfect and the nerve connection
unbroken, and yet the person cannot see, because the center in the brain
itself is injured from disease or accident, and cannot receive the
impression.

312. The Functions of the Brain Center in the Perception of an
Impression. Sensation is really the result of a change which occurs in
a nerve center in the brain, and yet we refer impressions to