Thomas Willis[269] made some very significant references to the relation existing between mania and melancholia, in the seventeenth century:—"After melancholia we have to treat of mania, which has so many relations to the former, that the two disorders often follow each other, the former changing into the latter, and inversely. The melancholic diathesis, indeed, carried to its highest degree, causes frenzy, and frenzy subsiding changes frequently into melancholia (atrabiliar diathesis). These two disorders, like fire and smoke, often mask and replace each other, and if we may say that in melancholia the brain and the animal spirit are obscured by smoke and black darkness, mania may be compared to a great fire destined to disperse and to illuminate it." Morgagni,[270] "the father of pathology," also saw a close relation between these two conditions as is shown by the following quotation from his "De Sedibus et Causis Morborum;" etc., in 1761. "Melancholia," he says, "is so nearly allied to mania, that the diseases frequently alternate, and pass into one another; so that you frequently see physicians in doubt whether they should call a patient a melancholiac or a maniac, taciturnity and fear alternating with audacity in the same patient; on which account, when I have asked under what kind of delirium the insane persons have laboured whose heads I was about to dissect, I have had the more patience in receiving answers which were frequently ambiguous and sometimes antagonistic to each other, yet, which were, perhaps, true in the long course of the insanity." Flemming[271] in 1844 described a "dysthymia atra" (melancholia), a "dysthymia candida" (cheerful dysthymia) or "melancholia hilaris" characterized by elation with playfulness and a "tendency to see everything in the most pleasant and cheerful light" as well as a "dysthymia mutabilis," an alternating variety involving both of the above forms. He also spoke of a "dysthymia sparsa" (apathica) or "melancholia attonita," and a "vesania maniaca" or mania which he divided into the acute, delirious, alcoholic, affective, and puerperal types, together with an "occult amentia" embracing all of these forms. Griesinger[272] in 1845 called attention to the fact that "the transition of melancholia into mania, and the alternation of these two forms, are very common." In 1851 Falret, senior, first described circular insanity in his lectures at the Salpêtrière, quoted by Tuke[273] as follows:—"We have also to mention another case of intermittence observed between the periods of remission and excitement in the forme circulaire des maladies mentales." "It is a special form which we call 'circular' and which consists, not as has been frequently said, in a change of mania into melancholia separated by a more or less prolonged lucid interval, but in the change from maniacal excitement—simple overactivity of all the faculties—into mental torpor."