The title of the present work is in itself a sufficiently clear indication of the contents. In it, the attempt is made to show the connexion between two sciences whose subject−matters are closely interrelated, but which have, for the most part, followed wholly divergent paths. Physiology and psychology cover, between them, the field of vital phenomena; they deal with the facts of life at large, and in particular with the facts of human life. Physiology is concerned with all those phenomena of life that present them selves to us in sense perception as bodily processes, and accordingly form part of that total environment which we name the external world. Psychology, on the other hand, seeks to give account of the interconnexion of processes which are evinced by our own consciousness, or which we infer from such manifestations of the bodily life in other creatures as indicate the presence of a consciousness similar to our own.