At a time like the present, when the claims of psychogenic treatment are attracting the attention not only of the medical, but also of the lay mind, it befits those of us who believe that beneath every mental process there lies a mechanical basis to take careful thought whether we are altogether blameless if comparatively easily understood psychological explanations of disease are substituted for the far more intricate and complex physiological, and to ask ourselves whether we have in fact always applied to the problems of nervous physics the knowledge we actually possess. We turn to text-books of mental diseases seeking enlightenment, and are put off by vague explanations, no more convincing and far less logical than those advanced by the psychologist, nor do we find any reference to much work on the physical nature of mind which has long been known, although, indeed, twenty years ago Crookshank made an effort to correlate physical signs with mental. conditions, and five years ago Dunlop Robertson drew attention to the importance of adrenalin in the production of depression. We do not know much about nerve action, but we do know something, and if we are not willing to apply to our own special work the simpler tests of the physiologist and the chemist we are in danger of seeing our teaching discredited; and therefore I have decided to address you to-day upon one class of mental illness which comprises two distinct processes, and to endeavour, by tracing the conditions present in both, to offer some explanations, not indeed of the ultimate, but of the proximate causes which may underlie each in turn. The two processes are involution and depression, and my title is: “The Mechanism of Involutionary Melancholia.” None of my facts are new, none have been worked out by any special labours of mine, but as there may be some who, like myself, banished in the depths of the country and many hours distant from a medical library, find a difficulty in marshalling current knowledge into its ordered place, I give these deductions for what they are worth; for the psychology of melancholia is the depressive emotion, and the anatomy of the depressive emotion is a sympatheticotonus, and the chemistry of a sympatheticotonus is an endogenous anoamia.