The eye and the ear present many analogies, both in regard to their functions and their anatomy. The pavilion of the ear has been compared to the eyelids, the auditory canal to the anterior chamber of the eye, the tympanum to the iris, the cavity of the tympanum to the posterior chamber, the small bones to the crystalline, and the liquor Cotunnii to the vitreous body. These organs differ in their nature, like the exciting agents which pass through them. Sound and light both originate in vibrations, but transparency is the essential condition of the organ through which light passes, while sounds are propagated through all bodies, solid, fluid, or gaseous.
The sense of light enables man to contemplate the admirable spectacle of the universe, but for the eye nature is mute; to it motion alone denotes life; hearing completes our impressions, everything is animated by it, and man takes part in the life of the external world, and shares the thoughts of his fellows. The perfection of these two senses enables us the better to appreciate the connection between the functions, and the unity of our organs. The sight speaks more directly to the intelligence, it enlarges the field of thought, it gives birth to precise notions of light, of form, of extent; and it permits the communication of thought by conventional signs. Hearing is a necessary condition of articulate language; without it man lives alone, affection and confidence lose their most precious forms of expression, and friendship cannot exist.
Auditory sensations act upon the nervous system with more force than visual sensations. We are carried away by rhythm, or it adapts itself to our ideas and our passions; music plunges us into an ideal world, and holds us by an indefinable charm; in a word, if sight speaks more especially to the intellect, hearing addresses itself to the affections.
Sight is certainly more necessary to man than hearing, but still the blind are generally gay and communicative, while the deaf seem inclined to melancholy. As to the relative influence of these two senses on the development of the intellect, we know that the education of the deaf is slow but may be complete, while that of the blind is, on the contrary, rather rapid, but is almost always very limited; many ideas cannot be acquired by them, and, as has been remarked by M. Longet, their minds rarely attain maturity.