But it appears from dissection, that the tympanum is not the immediate organ of hearing, but that like the humours and cornea of the eye, it is only of use to prepare the object for the immediate organ. For the portio mollis of the auditory nerve is not spread upon the tympanum, but upon the vestibulum, and cochlea, and semicircular canals of the ear; while between the tympanum and the expansion of the auditory nerve the cavity is said by Dr. Cotunnus and Dr. Meckel to be filled with water; as they had frequently observed by freezing the heads of dead animals before they dissected them; and water being a more dense fluid than air is much better adapted to the propagation of vibrations. We may add, that even the external opening of the ear is not absolutely necessary for the perception of sound: for some people, who from these defects would have been completely deaf, have distinguished acute or grave sounds by the tremours of a stick held between their teeth propagated along the bones of the head, (Haller. Phys. T. V. p. 295).
Hence it appears, that the immediate organ of hearing is not affected by the particles of the air themselves, but is stimulated into animal motion by the vibrations of them. And it is probable from the loose bones, which are found in the heads of some fishes, that the vibrations of water are sensible to the inhabitants of that element by a similar organ.
The motions of the atmosphere, which we become acquainted with by the sense of touch, are combined with its solidity, weight, or vis intertiæ; whereas those, that are perceived by this organ, depend alone on its elasticity. But though the vibration of the air is the immediate object of the sense of hearing, yet the ideas, we receive by this sense, like those received from light, are only as a language, which by acquired associations acquaints us with those motions of tangible bodies, which depend on their elasticity; and which we had before learned by our sense of touch.
The objects of smell are dissolved in the fluid atmosphere, and those of taste in the saliva, or other aqueous fluid, for the better diffusing them on their respective organs, which seem to be stimulated into animal motion perhaps by the chemical affinities of these particles, which constitute the sapidity and odorosity of bodies with the nerves of sense, which perceive them.