In the first part of the paper I adduce probable, or prima facie, evidence in favour of the theory that the power which produces voluntary motion is not derived solely, nor even chiefly, from the decomposition of the muscular tissues, as has been generally, if not universally, held by recent investigators, but is derived primarily and principally from the decomposition of the cerebral tissues; and second, that this decomposition of the brain is effected by a direct voluntary mental effort.