Observation has enabled physiologists to distinguish in the spinal cord and spinal nerves, and even in the cranial nerves, the sensitive and motor portions; and we must admit from the results of experiment in comparative anatomy, that certain regions of the encephalon are endowed with sensibility, while others are insensible; but we have not yet been able to recognize in the encephalic mass the central organs, which preside over sensation and over motion. Nothing authorizes us to think that the insensible portions of the brain do not take a part in the motor and sensatory functions, and we are still less able to point out in the encephalon the seat of intelligence. We see the intellectual faculties develop themselves in the child, at the same time with, and in proportion to, the development of the brain; and we know that these faculties continue imperfect, or changed, when the normal development of the organ is arrested, and when it suffers from certain lesions; but these facts, incontestable in principle, have no absolute application. The brain may be wounded, and even a portion of it may be destroyed, without any sensible change in the intellectual faculties; a man of genius may have an ill-developed brain, as Bichat, for example, whose cerebral lobes were not of equal volume. On the other hand we see the intellect clouded under the influence of alcohol, of certain poisonous substances, or an attack of fever, and no trace is left in the encephalon of the temporary disturbance; sleep produces an analogous effect; dreams are only a succession of false ideas, a real delirium which ceases on awaking. And, indeed, in the insane, science can in many cases prove nothing but their misfortune, of which no part of the brain suggests in the slightest degree the organic cause. Physiology, therefore, is very reserved in regard to the cerebral functions, and most of its theories concerning them are disputed and uncertain.
The cerebral lobes do not seem to be essentially necessary to the perception of sensitive impressions, general or special. Thus, pathological observation has established the fact, that vision may be equally good in both eyes, although one hemisphere may be atrophied, or may have suffered, as from wounds, a great loss of substance. It is, on the contrary, exclusively in the cerebral lobes that the perception of sensations lies, and that the ideas are formed which these sensations create. It is also from the hemispheres that the impulse emanates which results in voluntary motion. Some physiologists have referred this impulsion to the white, and others to the gray substance of the brain. Wherever may be the seat of the motor principle, we know that the brain exercises a cross action on the muscles; that is, the left hemisphere induces the movements of the right side, and the right hemisphere those of the left. But in certain cases the action is direct notwithstanding; this has been explained by an exceptional incompleteness of the crossing of the cerebral fibres. Physiologists have sought in vain to localize the source of motion in the brain, and the difference of opinion on this point does not permit us to consider it a settled question.
The encephalon controls the intellectual phenomena, and most authors consider the cerebral lobes the seat of the soul. In the superior animals the most complete development of the brain proper coincides, in fact, with the greatest degree of intelligence, and the proportions of the brain of man unite with his intellect in placing an immense interval between him and animals the most gifted in this respect And lastly, the encephalon in idiots is specially characterized by atrophy of the cerebral lobes, of their convolutions, and of the gray or cortical substance. Several authors, from repeated observation of this latter fact, have considered the gray substance as the seat of the intellectual faculties.
We have already stated, in speaking of the skull, that Gall and his school have placed the intellectual faculties in the anterior lobes of the brain, the moral qualities or tendencies of the mind in the middle lobes, and the animal faculties or instinctive propensities in the posterior lobes. This doctrine seems to be the rational consequence of that which recognizes one portion of the encephalon as specially designed for the functions of the intellect; but if we admit the possible existence in the brain of distinct and multiplied apparatus in the explanation of psychological phenomena, it is simply a hypothesis of which it is out of our power to furnish a single proof. It is objected, and with reason, to the phrenological theory, that it groups all the faculties in those portions of the brain which correspond to the arch of the skull, to the exclusion of those resting on its base; and besides, pathological anatomy is not in accord with the theory of Gall, and comparative anatomy does not permit its admission.