To these may be 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 the various terminal organs.  Thus, when the skin is pinched, the sensation is referred to the skin, although the perception is in the brain.  We may think it is the eyes that see objects; in reality, it is only the brain that takes note of them.

This is largely the result of education and habit.  From a blow on the head one sees flashes of light as vividly as if torches actually dance before the eyes.  Impressions have reached the seeing-center in the brain from irritation of the optic nerve, producing the same effect as real lights would cause.  In this case, however, knowing the cause of the colors, the person is able to correct the erroneous conclusion.