The external meatus is a curved tube, about an inch long. Frequently a tuft of hairs guards its entrance. The wax secreted by its wall serves to attach particles of dust, and to deter insects from entering the tube. The air at the end of it is at a uniform temperature. It is closed by the membrana tympani, or drum. This membrane receives the vibrations of sound; and, in order that it may collect them with absolute impartiality, it is in every respect the opposite in shape and structure to the top of a drum. The stretched parchment which covers a drum is flat. Its tension is uniform in all its parts. Movements have the greatest amplitude at the centre. Every precaution is taken to insure its emitting, with as little confusion as may be, the particular note to which it is tuned. The drum of the ear is shaped like the mouth of a trumpet, depressed to a point, but convex from this point outwards. Its elastic fibres, which are partly radial, partly circular, are at many different tensions. Its deepest part, to which the long arm of the hammer-bone is attached, is not its centre.
The “middle ear” is an irregular cavity communicating with the pharynx by the Eustachian tube. It is filled with air at the same pressure as the atmosphere. Except during the act of swallowing, when it is at first shut tightly and then opened, the pharyngeal end of the Eustachian tube is gently closed. When one is dropped in a lift rapidly down the shaft of a mine, the difference in pressure between the external air and the air in the middle ear stretches the drum to such an extent that deafness to low tones is produced. Conversation becomes inaudible. The deafness is remedied by swallowing saliva, and thus opening the end of the Eustachian tube. The commonest cause of permanent deafness is inflammation followed by thickening of the mucous membrane of the lower end of the Eustachian tube, with its consequent closure, due to frequent sore throats. The air in the middle ear is slowly absorbed. It needs to be constantly renewed through the Eustachian tube.
On the inner wall of the middle ear are two small apertures—the oval window and the round window. Both are closed with membrane. Into the oval window is fitted the sole-plate of the stirrup-bone. Three bones—hammer, anvil, and stirrup—combine in transferring the movements of the membrana tympani to the oval window. They constitute a jointed lever, which swings about an axis passing through the ligament of the anvil ([Fig. 38]), the excursions of the long arm of the hammer being reduced in amplitude by one-third at the stirrup-plate. As the oval window has only one-twentieth of the area of the drum, the movements of the latter are transmitted with concentrated force. Two points in the mechanism of these bones may be specially noticed: (1) The head of the hammer is free to rotate in the cavity of the anvil, checked by a cog. Every inward movement of the drum is faithfully transmitted to the oval window; but when the drum moves outwards, the hammer does not necessarily carry the anvil with it. (2) A muscle—tensor tympani—is inserted near the elbow of the long arm of the hammer. When high notes are listened to its contraction tightens the drum, rendering it more responsive to rapid vibrations. It has a tonic action, but it does not make any special contraction for low notes.
Behind the two windows, within the solid bone, is the inner ear, which our ancestors very aptly termed a “labyrinth.” It is filled with fluid—perilymph—which is shaken by every movement of the stirrup-plate. Since water is incompressible, no waves could be raised in the perilymph were there no second aperture. Every vibration conveyed by the stirrup-plate after passing through the labyrinth ends as a vibration of the membrane which closes the round window.
Nowhere does perilymph come in contact with auditory cells. All the endings of the nerve of hearing are contained within a membranous labyrinth which lies within the bony cavities. The way in which the waves of the perilymph are dispersed over the surface of this closed sac can be inferred from the diagram ([Fig. 38]). They sweep round the utricle and saccule, are lost in the narrow spaces which surround the semicircular canals, run up the scala vestibuli of the cochlea. The course of the waves which traverse the cochlea is of especial interest in connection with the physiology of hearing.
The cochlea—snail-shell—is a spiral tunnel of three turns, in hard bone, about an inch in length. A shelf of bone—lamina spiralis—projects into the tunnel on its convex side. From the free margin of this spiral lamina two membranes extend to the outer wall of the tunnel—one firm, containing straight, stiff, and probably elastic fibres which radiate outwards (the basilar membrane); the other an extremely delicate film of connective tissue. The tunnel is thus divided into three compartments, known as the scala vestibuli, scala media, scala tympani. The scala media belongs to the membranous labyrinth. Waves transmitted through perilymph pass, as we have already explained, up the scala vestibuli. At the apex of the cochlea the two scalæ are in communication; but the aperture is small, and it is unlikely that waves reach the lower passage from the upper through this opening. They pass through the thin membrane which roofs the scala media, shake its endolymph, and reach the lower passage through the basilar membrane. It is noteworthy that, since the round window at the lower end of the scala tympani is, with the exception of the oval window, the only opening of the bony labyrinth, all waves transmitted through the oval window must travel part of the way or all the way up and down the cochlea.
Fig. 39.—A Section through the Axis of the Column of the Cochlea.
The spiral sheet of nerve-fibres which supplies the organ of Corti is cut in eight places. If the bundle to the lowest coil of the shell (on the left side of the diagram) is followed, it will be seen to bear ganglion-cells where it enters the bony spiral lamina. This lamina divides the tube into two canals—scala vestibuli above, scala tympani below. From the edge of the lamina the membrane of Corti stretches to the outer wall. Above the organ of Corti is the membrana tectoria, and above this a very thin membrane which cuts off the ductus cochleæ from the scala vestibuli.