The psychoses which are brought together under this classification include mental disorders which at first glance would appear to be of very wide variation, namely, conditions of maniacal excitement and those of depression. Further consideration, however, reveals the very evident reasons why they should be united as sub groups under the one head. The fact that these two mental states of seemingly opposite characteristics often appear alternately in the same individual, that in certain cases of each type there is a wave like feature in the nature of the attack and the frequency with which they tend to recur, together with other points of similarity in respect to duration, prognosis, etc., tend to point to their very close relationship. Kraepelin was the first to draw attention to these facts and advocate the present convenient and widely accepted classification of these disorders. The outstanding feature is the disturbed emotional state which dominates and overshadows all other symptoms and is fundamentally the same whether expressed through the excitement of mania or the depression of melancholia. =Etiology.=—Heredity is considered an important factor. Various authorities claim to have demonstrated direct hereditary influences in as many as eighty per cent and more of cases. Individual predisposition resolves itself into a matter of constitution and temperament in which there seems to be a greater tendency among those who are subject to the emotional extremes. Early adult life is by far its most frequent period of onset, though it may arise also somewhat later. In certain cases the beginning of the disorder dates from some psychic or emotional shock. Just what importance these factors have as causes is little known since other cases develop in which the constitutional element alone seems responsible and no immediate exciting cause can be demonstrated. The osteopathic viewpoint emphasizes the all important influence of spinal lesions as exciting factors. In individuals who have a tendency to this reaction their presence disturbing the cerebral circulation and nutrition may act as the direct causative factor. =Manic Phase.=—The manifestation of this condition is brought about by the release of the inhibiting influences which normally govern all psychic function. Various terms as hypomania, acute mania, delirious mania, etc., have been used to differentiate the different degrees in which the symptoms appear.