CHAPTER XVII.
THE HEARING APPARATUS AND HEARING IN MUSIC.
So important are the ingoing sensory messages (impulses) that originate in the ear, as a guide not only in the appreciation of musical sounds but in those movements on which all musical execution, all vocal effects, whether of song or speech, depend, that we think the reader will welcome a chapter on the ear, even though it be no part of the vocal apparatus proper.
The essential mechanism used by Nature to give us the sensation of sound consists of (1) a complicated form of nerve-ending; (2) an auditory nerve leading from, and a continuation, in a certain sense, of, the latter; (3) nerve tracts and hearing centres in the brain. The whole constitutes a very complicated mechanism, but the principles on which it is constructed may be reduced to a few. Mechanical or physical principles, as well as physiological ones, are involved.
The entire apparatus has for its purpose the conversion of the vibrations of the air into the vibrations of a fluid, which thus stimulates the end-organ, and brings about those changes in the nerve which result in corresponding changes in the brain, that are associated, in some way we cannot explain, to that state of consciousness we term hearing. Complicated as is the auditory apparatus, it can be readily enough comprehended, if the reader accompany the perusal of the text by an examination of the figures introduced.
Fig. 60. (Beaunis). In this illustration parts are exposed to view by the removal of others. The whole of the inner ear lies within bone, which in this figure is cut away. The drum-head (membrana tympani); the Eustachian tube, extending from the back of the throat, and opening into the middle ear; the semicircular canals (which are not concerned with hearing, but with the maintenance of equilibrium); the cochlea, (snail-shell), which contains the various parts most essential to hearing, as the "hair-cells," the terminals of the auditory nerve, the latter nerve itself, and several other parts—are well shown. Should the Eustachian tube be closed owing to swelling of its lining mucous membrane, a certain amount of temporary deafness may result, because, the air within the middle ear (drum) being absorbed, and fresh air not being admitted, the outer air presses against the drum-head uncounteracted, and renders the conducting mechanism too rigid.
Anatomists speak of (1) an outer or external ear, (2) a middle ear, drum, or tympanum, and (3) an inner ear, or labyrinth.
