The physiological activity of the brain is not, however, altogether co-extensive with its psychological functions. The brain as an organ of motion and sensation, or presentative consciousness, is a single organ composed of two halves; the brain as an organ of ideation, or re-presentative consciousness, is a dual organ, each hemisphere complete in itself. When one hemisphere is removed or destroyed by disease, motion and sensation are abolished unilaterally, but mental operations are still capable of being carried on in their completeness through the agency of the one hemisphere. The individual who is paralysed as to sensation and motion by disease of the opposite side of the brain (say the right), is not paralysed mentally, for he can still feel and will and think, and intelligently comprehend with the one hemisphere. If these functions are not carried on with the same vigour as before, they at least do not appear to suffer in respect of completeness. As the object of this treatise is not anatomy but psychology, it will be unnecessary to describe minutely the entire of the brain structure. It will suffice for the present purpose to view the brain, above roughly sketched, as having three well marked divisions, each with definite and distinct functions. The ganglia at the base of the brain govern the actions of the body. The ganglia in the centre of the brain are the recipients of the impressions made upon the senses and thus connect us with the external world. The two hemispheres at the summit of the brain are the organs of the Intelligence. Professor FERRIER’S experiments were made with a view to ascertain whether the theory of Dr. CARPENTER is true, that the whole brain works in each mental action, or if the phrenological doctrine be the true one, that the several parts of the brain have several and distinct functions. Dr. CARPENTER had prematurely boasted that he had killed Phrenology. The boast would have been justified if his assertion (for it was merely a dogma, not a proved fact) had been found to be true. But Professor FERRIER’S experiments have decisively _disproved_ the boast of Dr. CARPENTER and killed his theory of mental unity. The experiments were conducted chiefly with monkeys and dogs. The former were the most valuable, because the brain structure of the monkey is almost identical with that of man. The experiments were certainly cruel and I should object to procure even such valuable knowledge at such a price. But, as it is obtained, we may use it. The experiments were performed by making the animal insensible by chloroform and then extracting in mass certain portions of the brain, or destroying parts of the brain by the actual cautery. Electrodes were applied to the various parts of the brain to which access had been thus obtained and their effects upon the actions of the animal were carefully observed.