The object of nature in establishing the organizations we have been reviewing is to produce certain modified impressions on the extremities of particular nervous filaments provided to receive them; but these impressions constitute only the commencement of the series of corporeal changes which terminate in sensation; for they have to be conveyed along the course of the nerves to the brain, or central organ of the nervous system, where, again, some physical change must take place, before the resulting affection of the mind can be produced. The particular part of the brain where this last physical change, immediately preceding the mental change, takes place, is termed the Sensorium. Abundant proofs exist that all the physical changes here referred to really occur, and also that they occur in this order of succession: for they are invariably found to be dependent on the healthy state, not only of the nerve, but also of the brain; thus, the destruction, or even compression of the nerve, in any part of its course between the external organ and the sensorium, totally prevents sensation; and the like result ensues from even the slightest pressure made on the sensorium itself.

Although the corporeal or physical change taking place in the sensorium, and the mental affection we term sensation, are linked together by some inscrutable bond of connexion, they are, in their nature, as perfectly distinct as the subjects in which they occur; that is, as mind is distinct from matter; and they cannot, therefore, be conceived by us as having the slightest resemblance the one to the other.