Two plans have been adopted to ascertain the seat of this supposed centre of our sensations : the first has con sisted in examining the brain after its different parts have been injured by accident or disease, and noticing what effects were produced on the faculties ; the other pro ceeded upon the principle of actually up the nerves to some one spot within the cranium, where, it was supposed, their actions might terminate. With re spect to the first of these methods, the correspondence be tween the injury of the brain and the loss of any par ticular faculties, we have not yet been able to obtain any very decisive results. It seems, indeed, to be establish ed, that, the external cineritious part of the brain is less sensible than the medullary part ; and we have many ex amples, in which large portions of the former have been removed or destroyed without any material injury to the powers or functions either of the body or the mind. There are also many curious and well authenticated cases on record, where different parts of the medulla of the brain have been destroyed ; and even such as, from their for ni and texture, or from their situation with respect to the organs of sense, might have been supposed the most essential, and yet the faculties have remained. It occa sionally happens that water collects in the ventricles at the base of the brain, and, if the effusion be not too rapid, or if the skull yields to the distending force, so that the pressure be not too suddenly applied, life may be pre served for a long time. When such cases are examined after death, the form and texture of the brain is found to he completely altered ; and it has been even supposed, that a considerable portion of the cerebral matter has been removed by the action of the absorbents. An alteration of this kind must be extremely slow, so that we may ven ture to assert, that for some time before death, the head must have been nearly in the same state as it is found 'upon dissection, yet the faculties, both physical and intel lectual, have remained in tolerable perfection, and the pa tient has suffered rather from general indisposition, and from the inconvenience of an unwieldy head, dispro portioned to the rest of the body, than from any direct in jury or diminution of the nervous powers.
Nor has any great degree of success attended the at tempts which have been made to discover the sensorium commune, by tracing the nerves of the different organs of sense up to one spot within the brain, although this has been attempted by some very learned and skilful anato mists. Indeed the general result of our inquiry respect ing the sensorium commune is, that it is rather the brain in its whole extent which is to be regarded as the com mon centre of sensation, than any particular part of it ; or. the contrary, it is probable that different portions of the brain have different functions, or are more immediately subservient to different faculties, and therefore that the integrity of the whole is, at least to a certain extent, ne cessary for the due performance of the nervous functions.
This doctrine, of the different parts of the brain exer cising different functions, was first explicitly advanced by Willis, who thought that he had proved this to be the case with respect to the cerebrum and the cerebellum, the one belonging more immediately to the organs of sense and the mental faculties, the other to the muscles.
Willis's particular hypothesis is not now conceived to be correct, but the principle upon which it was founded is ge nerally admitted, that the cerebrum and cerebellum serve different purposes in the animal economy, although it may not be in our power to assign the nature and extent of their respective powers. It is well known that this idea has of late been carried to a much greater length than it was by Willis, even so far as to assign separate organs, not only for the powers which are more particularly de pendent upon sensation and motion, but even for all the different mental faculties ; this suhjcct, however, will be more fully considered in a subsequent part of the ar ticle.
The metaphysicians of the seventeenth and eighteenth centuries were very anxious to discover the seat of the soul, an inquiry which every one is now well aware must be entirely fruitless. It would be an absolute waste of timeto relate all the different discussions that took place on this subject, or to attempt to rescue them from the merited oblivion into which they have fallen ; but, as a matter of literary history, it may not be improper to al lude to an opinion which was maintained by Descartes, and which, from the celebrity attached to his name, was for some time favourably received, viz. that the seat of the soul is in the pineal gland. The situation of this gland, and the manner in which it is protected from ex ternal injury, appeared to point it out as serving some use ful purpose ; and upon examining the brain of certain idiots, it was discovered in these cases to contain a quan tity of silex. This earthy matter was conceived to be a morbid deposition, and it was hastily concluded, that it must have the effect of interrupting the functions of the part, and that the idiotism of the subjects in question depended upon this cause, hence affording us a proof of its being the proper seat of the intellect. \Ve have, however, learned since the time of Descartes, that earthy matter is always found in the gland, so that the hypothesis of this philosopher is entirely without foundation. But although it may appear that we are not able to point out any one part of the cerebral mass, as the more immediate centre of sensation, there are certain points connected with this subject which it is important to attend to. Dr. Philip, whose researches on the nervous system are pe culiarly valuable, has shown that the action of the nerves is different from, and independent of that of the brain ; and he designates them respectively by the names of the nervous and the sensorial power. It would appear that the former of these consists simply in the transmission of certain effects from one part of the system to another, which takes place without the intervention of the brain, while the latter consists in the functions of perception and volition, in which the nerves and the brain are both concerned. It is obviously to the latter of these powers alone, that the inquiry respecting the common centre of sensation can refer ; and there are certain circumstances which seem to render it probable that different parts of the encephalon are respectively concerned in the functions of perception and volition.