The middle ear or tympanum is an irregular cavity in the petrous portion of the temporal bone. Its outer wall is formed by the membrana tympani or drum, an oval translucent membrane placed obliquely at the bottom of the external auditory canal. The middle ear communicates with the inner ear through the fenestra ovalis or oval window and contains the ossicles, the [malleus] or hammer, the [incus] or anvil, and the [stapes] or stirrup, which are arranged in a movable chain from the drum to the oval window. The malleus, which is connected with the membrana tympani, articulates by its head with the body of the incus, while the stapes articulates with the incus by its head and is connected by its base with the margin of the oval window. Connection is made between the middle ear and the pharynx and the pressure of the air upon the drum made equal on either side by means of the [Eustachian tubes]. These tubes are about an inch and a half long, have cilia, and convey wax and other matter from the ear to the pharynx. Occasionally in a cold or for some other reason they become stopped up and trouble results in the middle ear. Some of the mastoid cells also connect with the middle ear and may become infected, causing mastoid disease.
Fig. 22.—Interior view of left bony labyrinth after removal of the superior and external walls: 1, 2, 3, the superior, posterior, and external or horizontal semicircular canals; 4, fovea hemi-elliptica; 5, fovea hemispherica; 6, common opening of the superior and posterior semicircular canals; 7, opening of the aqueduct of the vestibule; 8, opening of the aqueduct of the cochlea; 9, the scala vestibuli; 10, scala tympani; the lamina spiralis separating 9 and 10. (From Quain, after Sömmerring.)
The internal ear consists of various chambers hollowed out in the petrous portion of the temporal bone. There is an osseous labyrinth, consisting of a central cavity known as the vestibule, three [semicircular canals], and the [cochlea] and within the osseous labyrinth, surrounded by perilymph, is the membranous labyrinth, of like form, filled with the endolymph. Communication exists externally with the middle ear by the round and oval windows and internally with the [internal auditory canal], through which passes the eighth cranial or auditory nerve, the special nerve of hearing, which is distributed to the inner ear only. When the auditory nerve enters the ear through this internal auditory meatus it divides into two branches, of which one goes to the vestibule and the other to the organ of Corti, a group of specially modified epithelial cells in the cochlea of the membranous labyrinth, which is very important in transmitting the impulses to the brain. The nerve also breaks up into very small branches and is distributed practically throughout the wall of the labyrinth.
The sensation of hearing is the result of impulses transmitted to the auditory nerve and so conveyed to the auditory center in the brain. It is caused by sound waves which travel through the air from their point of origin and enter the external ear. This collects and selects the waves of sound and helps one to a certain extent to determine the direction from which the sound comes. As they pass through the external meatus the sound waves are collected into a comparatively small area for transmission to the middle ear, where, by means of the drum, they set in vibration the chain of ossicles. Through these the vibrations are in turn transmitted to the oval window, being intensified in the process. Here again they are taken up by the perilymph, from which they pass through the wall of the membranous labyrinth to the endolymph, affecting the epithelial lining of the labyrinth in such a way that the impulses are transmitted to the auditory nerve, more particularly in the vestibule, from which the vibrations enter the cochlea. They also affect the cells of the organ of Corti in like manner as they pass from the perilymph to the endolymph. The membrane that covers the fenestra rotunda or round window relaxes and expands as the vibrations strike it, thus serving to eliminate the shock of impact.
Musical sounds are caused by rhythmical or regularly repeated vibrations, while irregular vibrations give rise to noises. In musical sounds loudness is determined by the height or amplitude of the vibrations, pitch by the length of the wave, and quality by the number of so called partial tones. A sensation of sound cannot be produced by less than 30 vibrations a second and the ordinary person cannot hear more than 16,000 vibrations a second. Different sounds can be distinguished when they follow each other as closely as by one one-hundredth of a second.
All sound does not come through the canal of the ear. The bones of the head vibrate and carry sound. So there are instruments for the deaf which are put in the ear and others which are placed between the teeth.
The [semicircular canals] are not essential to hearing but have something to do with a person’s power of maintaining his equilibrium. Injury to them may cause dizziness and loss of equilibrium.
The Eye.—One more feature, perhaps the most expressive, remains to be described, the eye. The senses are all modifications of the original cutaneous sensibility and the nerve of sight is no more sensitive to light than any other nerve. It therefore needs an end organ that is sensitive to the motions of the ether in order to give impressions of light. This organ is provided in the eye, which is not only itself capable of being moved in every direction, but is placed in the most movable part of the body, the head, which can be turned in almost a complete circle. The eyeball is spherical and lies in the cavity of the orbit upon a cushion of fat, where it has a large range of sight but is securely protected from injury by its bony surroundings. The sunken eyes following protracted illness are due to the using by the system of the fat on which the eyeball ordinarily rests.
Each orbital cavity is formed by the juncture of some seven bones and communicates with the cavity of the brain through the optic foramen and through the sphenoidal fissure. Above the orbits are arched eminences of skin, the eye-brows, from which several rows of short hairs grow longitudinally and which serve to protect the eyes and to limit the amount of light to a certain extent, as in frowning.