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Plate 5: The Human Ear, and Tympanum of the Elephant

Fig. 1. The organ of hearing; a, the external ear; b, the meatus anditorius externus, or outward passage of the ear; leading to c, the membrana tympani, or drum; d, the ossicula auditus, or little bones of the ear; e, the semicircular canals; f, the cochlea; g, a section of the eustachian tube, which extends from the cavity of the tympanum, to the back of the mouth or fauces.

Fig. 2. The bones of the ear magnified. a, the malleus, or mallet, connected by a process to the tympanum; the round head is lodged in the body of, b, the incus, or anvil, and the incus is united to, c, the os orbiculare, or round bone, and this to, d, the stapes, or the stirrup. These bones are named from their shape, and the names assist in conveying an idea of their form. They are united by ligaments, and form an uninterrupted chain to transmit the vibrations of the atmosphere.

Fig. 3. The labyrinth, so named from the intricacy of its cavities; it is situated in the petrous part of the temporal bone, and consists of the vestibule, or central cavity, three semicircular canals, and cochlea, so named from its resemblance to the windings of a snail shell, and is best explained by the plate, Fig. 1, and 3.

The vibrations of sounds, striking against the membrana tympani, are propagated by the intervention of these four little bones, to the water contained with in the cavities of the labyrinth; and by means of this water the impression is conveyed to the extremities of the auditory nerve, and finally to the brain.

Fish require no tympanum, nor external opening to the ear; the fluid in which they live is the medium for conducting sounds through the bones of the head.

Fig. 4. The tympanum of the elephant, of its natural size, showing its radiated fibres, supposed to be muscular.