DELIRIUM is a term which has been so variously applied that a brief definition of it is wellnigh impossible. Illogical or unreasoning and incoherent thoughts expressed in words and acts may suffice to give a general idea of the condition. Extreme applications of the term are, for example, to say that in a case of extreme dementia the rambling, disconnected talk is delirium, or that in certain forms of monomania the {25} expression of the patient's peculiar delusion is delirium. It seems to us that there should be a certain degree of activity in the production of morbid ideas, with confusion in their expression, to justify the use of the term delirium. Again, in some instances the delirious talking and acting are only the reflex of abundant hallucinations of various senses which beset the patient. In some other respects the term delirium is applied in several distinct ways: first, in a substantive form as a designation for the incoherent words and acts of a patient. Usually, it is then put in the plural form of deliria. Thus we have the more or less highly organized, fixed or changeable deliria of monomania, chronic mania, melancholia, paretic dementia, etc., and the confused and evanescent deliria of acute general diseases, intoxications, and many forms of insanity. In short, we may speak of a sick person's deliria as we would of a normal person's thoughts; or in still more elementary analysis deliria are abnormal or insane thoughts and corresponding action. Second, delirium is used adjectively as designating certain diseases—_e.g._ delirium tremens, delirium a potu, acute delirium, delirium of acute diseases, etc. The seat of the psychic processes which go to make delirium is undoubtedly the cerebral cortex. This view is supported partly by the clinical consideration that delirium bears a certain relation to the psychic development of the subject. Thus, we see in children and in the higher animals rudimentary or fragmentary deliria; in advanced age the delirium is feeble and wellnigh absent; while in ordinary adults with well-developed cerebration deliria are abundant and varied. From pathological anatomy we learn that deliria become simplified and subside in proportion as the cerebral cortex becomes more and more damaged by effusions, by pressure effects, or by degenerative changes. As to the relation between special histological pathological changes, our knowledge is small and to a certain extent paradoxical. Thus, it is universally admitted that delirium may be due either to hyperæmia or to anæmia of the brain. The delirium of alcoholic or cannabis intoxication may be fairly assumed to be of sthenic or hyperæmic origin, either by the nervous elements themselves being in an exalted state of irritability, or because an increase in the circulation of arterial blood in the brain leads to greater activity of the cellular elements. Again, delirium appears in conditions of general or cerebral