The mechanism of hallucinations is partly understood, and may be stated as follows: In some few cases a real disorder or defect in the {22} peripheral sense-organ may give rise to false projections; for example, a tinnitus may become transformed into a distinct voice, a scotoma may be the starting-point of false pictures of a man or animal. The simpler hallucinations of pain, cutaneous, muscular, and visceral sensations may originate in irritation of the nerve-trunks (as where the nerves of an arm-stump are faradized and the patient feels his hand with fingers in motion). But the general or common genesis of hallucinations is in disordered states of nerve-centres, those for common sensations and the special centres or cortical areas in the brain. Thus, a morbid irritation of the cortical visual area or sphere will give rise to abundant hallucinations of sight; irritation of the auditory sphere to hallucinations of hearing, as sounds and voices, etc. It must be borne in mind that, however pathological hallucinations may be, they arise from the operation of a fundamental physiological law. In health we constantly refer our sensations or transfer them into the external world, thus creating for ourselves the non-Ego. All terminal sensory nerve-endings receive only elementary impressions or impulses from external agencies, and these are perceived and conceived as images, formed sounds, etc. in the appropriate cortical centres; then by the law of reference of sensations these elaborated, idealized conceptions or pictures are thrown outward again and contemplated as objective. In this physiological mechanism lies the kernel of truth which is included in idealism. Hallucinations may occur without derangement of mind or impairment of judgment. Many instances are on record of transient or permanent hallucinations of various senses in perfectly healthy persons who were fully aware of the unreal character of what they saw or heard. Being of sound mind, they were able to make the necessary correction by reasoning or by the use of other senses. In very many forms of insanity hallucinations are prominent, though they also occur in quasi-sane conditions, as in hypochondriasis, hasheesh, belladonna, and opium intoxication, the stage between sleeping and waking, etc. As long as the subject is able to correct the false projections by reason or by the use of other senses he is considered sane. Hallucinations are sometimes the cause of acts by the insane, some of them violent and even murderous actions. Hallucinations of sight and hearing are especially prone to lead to assaults, murders, etc. The occurrence for any length of time of acoustic hallucinations in insanity is accounted of bad prognosis.