As I have already observed, an excitation of a nerve at any other point than the peripheral termination, occurs but rarely in normal life.  One familiar instance is the stimulation of the nerve running to the hand and fingers, by a sharp blow on the elbow over which it passes.  As everybody knows, this gives rise to a sense of pain at the extremities of the nerve.  The most common illustration of such errors of localization is found in subjective sensations, such as the impression we sometimes have of something creeping over the skin, of a disagreeable taste in the mouth, of luminous spots floating across the field of vision, and so on.  The exact physiological seat of these is often a matter of conjecture only; yet it may safely be said that in many instances the nervous excitation originates at some point considerably short of its peripheral extremity:  in which case there occurs the illusion of referring the impressions to the peripheral sense-organ, and to an external force acting on this.

The most striking instances of these errors of localization are found in abnormal circumstances.  It is well known that a man who has lost a leg refers all sensations arising from a stimulation of the truncated fibres to his lost foot, and in some cases has even to convince himself of the non-existence of his lost member by sight or touch.  Patients often describe these experiences in very odd language.  “If,” says one of Dr. Weir Mitchell’s patients, “I should say I am more sure of the leg which ain’t than the one which air, I guess I should be about correct."[30]

There is good reason for supposing that this source of error plays a prominent part in the illusions of the insane.  Diseased centres may be accompanied by disordered peripheral structures, and so subjective sensation may frequently be the starting-point of the wildest illusions.  Thus, a patient’s horror of poison may have its first origin in some subjective gustatory sensation.  Similarly, subjective tactual sensations may give rise to gross illusions, as when a patient “feels” his body attacked by foul and destructive creatures.

It may be well to remark that this mistaken interpretation of the seat or origin of subjective sensation is closely related to hallucination.  In so far as the error involves the ascription of the sensation to a force external to the sense-organ, this part of the mental process must, when there is no such force present, be viewed as hallucinatory.  Thus, the feeling of something creeping over the skin is an hallucination in the sense that it implies the idea of an object external to the skin.  Similarly, the projection of an ocular impression due to retinal disturbance into the external field of vision, may rightly be named an hallucination.  But the case is not always so clear as this.  Thus, for example, when a gustatory sensation is the result of an altered condition of the saliva, it may be said that the error is as much an illusion as an hallucination.[31]