HALLUCINATIONS.—By this term is designated the result of the projection into the external world, through nerves of common or special sensations, of formed sensations which arise in a disordered sense-apparatus or nerve-centre; or, in more popular language, it may be said to mean the perception of non-existent objects or impressions, creations of the imagination. Examples: Disease (sclerosis) of the posterior columns of the spinal cord irritates the roots of the sensory nerves, the result being pain at the periphery in the parts connected with the affected segment of the cord. So objective and real do these peripheral pains seem that if the patient's mind be weakened he may assert that they are due to his being beaten, stabbed, or bitten by some one or by an animal. After amputation, the absent member is long perceived by the subject, often with startling distinctness, and even after the sensation has passed away it may be brought back by faradizing the nerve-trunks above the stump. The patient may hear voices, music, or simple sounds when in reality there is silence, or he may be surrounded by imaginary images or plagued by hallucinatory smells and tastes. Hallucinations may also arise in the distribution of optic nerves. Besides common hallucinations with their seeming reality and objectivity, we admit others which are less vivid, which do not startle or frighten the subject, and which are simply the outward projections of the patient's own thoughts (delusions). The subject of persecution by imaginary enemies may see around him the faces of his pursuers with appropriate expressions, or hears their insulting or threatening remarks, as outward plastic reproductions of his thoughts; but the patient himself recognizes the want of actual objectivity and clearness in these images. These we call, after Baillarger, psychic hallucinations or pseudo-hallucinations. Similar phenomena are observed in some sane persons under excitement and betwixt sleep and waking.