CHAPTER V.
COMPARISON WITH THE OTHER SENSES.
Our subject rather invites a few words on the comparative character of the senses. Enjoyment appears universally to be the main end and rule, the ordinary and natural condition; while pain is but the casualty, the exception, the necessary remedy, which is ever tending to a remoter good, in due consideration to an ever higher law of nature. Here, as in every part of the physical economy, nature has endowed these organs with a direct and particular sensibility to those impressions which have a tendency to injure its structure; whereas they delight in those impressions which are not injurious. These external agents, which are capable of affecting the different parts of the nervous system, so as to produce sensation, are governed by laws peculiar to themselves. Their structure is adapted in each particular to receive the impressions made by their respective agents, and are modified in exact conformity with the physical laws they obey. The structure of that part of the nervous system which receives visual impressions, viz., the retina, is adapted to the action of light; and the eye, through which the rays pass, is constructed with strict reference to that object.
The ear is formed to receive delicate impressions from those vibrations of the air which realize sound, and acquires a susceptibility of influences by its own appropriate agents, and by no others. In almost every case the impression
