Delusional insanity or monomania has long been looked upon as one of the chief forms or types of insanity, and it is so described in the leading works on the subject. It is doubtful, however, whether it would be considered by all to be a pathological entity apart from other forms of insanity; indeed, many consider it as simply a variety of mania, and in the returns to the Commissioners of Lunacy mania and monomania are classified under one head. So far, however, from these two affections being allied, I venture to consider them as fundamentally distinct, and having each a pathology of its own. To put the matter tersely, I would say that mania begins from the top, monomania from the bottom. This proposition, doubtless, requires some explanation. I have elsewhere given reasons for my belief that mania is an affection of the highest controlling and co-ordinating plexuses of the brain; that what indeed we know as mania is the manifestation of activity on the part of certain lower centres (the larger portion) of the brain, this over-activity being permitted by under-activity, or abeyance of function, on the part of the highest centres (or smaller portion) of the brain. This is what I mean by insanity commencing from the top.