My object in this paper, is to endeavour to draw a strict line of demarcation between those forms of insanity which may be comprehended under the terms of mania and melancholia, and the delirium, which we witness in the various inflammatory affections of the brain fever, or delirium-tremens. If we were aware of the absolute pathological changes which exist in insanity, the correctness of a theory would be easily proved or disproved. In the absence, however, of evidence of this nature, which may be regarded as of a positive kind, I am compelled to draw my inferences from symptoms of disease during life, which may be regarded as evidence of a circumstantial nature. It is now some years since Dr. Wigan published a work on the duality of the mind, or, in other words, that as the brain consists of two sides or hemispheres, so these may have a distinct individuality of their own, just as occurs in the various senses, the eye, the ear, &c.