Ear. The ear is usually described as consisting of three parts: - (1) The external ear, which includes the pinna, auricle; or lobe of the ear and the external auditory meatus; (2) the middle ear or tympanic cavity; and (3) the internal ear.
1. The pinna or auricle consists of a framework of elastic cartilage, upon which the skin is moulded. Names are given by anatomists to the different parts of the auricle; thus, the central hollow is termed the concha, the prominence in front of this the tragus, the ridges situated posteriorly are the helix and anti-helix, and so on. The auditory meatus is directed inwards and slightly forwards, and is closed internally by a membrane called the tympanum. The total length of the external auditory meatus from the concha to the tympanum is about an inch and a quarter; its outer portion is composed of cartilage, its internal part of bone.
2. The middle ear occupies a cavity in the petrous portion of the temporal bone; the tympanic membrane forms the outer wall of this cavity. Internally, the middle ear is placed in direct communication with the pharynx by the Eustachian tube; bounding the cavity posteriorly are the mastoid cells. Within the middle ear are the minute bones, which are known as the auditory ossicles; these ossicles are three in number, and are called the malleus, incus, and stapes. The malleus or hammer bone is attached by one of its processes (the handle of the hammer) to the tympanic membrane; the head of the hammer articulates with the incus or anvil bone, and this latter is attached to the stapes or stirrup bone; the base of the stapes fits into a small opening on the inner wall of the cavity of the middle ear, called the fenestra ovalis. This arrangement ensures the transmission of vibrations of the tympanic membrane to the base of the stirrup, and, as the fenestra ovalis is situated between the middle and internal ears, any movement of the base of the stirrup is communicated to the fluid of the internal ear, which bathes the inner wall of the fenestra ovalis. Thus, every movement of the membrana tympani throws into vibration the fluid of the internal ear. Beneath the fenestra ovalis there is a second window connecting the cavities of the internal and middle ears; this window is round, not oval, in shape, and is called the fenestra rotunda.
3. The internal ear consists of a cavity surrounded by a dense bony framework, known as the osseous labyrinth. This framework encloses a sac, which is called the membranous labyrinth. In the space included between the osseous and membranous labyrinths is a fluid, called perilymph, while the fluid contained within the membranous labyrinth itself is called endolymph. The osseous labyrinth is made up of three parts - the vestibule, cochlea, and semicircular canals.
The vestibule is situated between the cochlea anteriorly and the semicircular canals behind; its inner wall is pierced by the auditory nerve, and on its outer wall are the two windows (the fenestra ovalis and the fenestra rotunda) already alluded to.
The semicircular canals are three in number (anterior, inferior, and horizontal); each canal presents a dilatation at one point, which is called the ampulla.
The cochlea is shaped like the shell of a snail, and is formed by a spiral canal which winds two and a half times round a central axis. If the cochlea be divided through the middle, it is found that the spiral canal throughout its whole length is divided into compartments, known as the scala tympani and scala vestibuli, which communicate at the apex of the cochlea by a small aperture, the helicotrema. In the fresh state a small portion of the scala vestibuli is separated off by a membranous partition forming the scala media, in which is found a very complex structure which runs spirally round the axis of the cochlea, supported upon the basilar membrane of the scala media, known as the organ of Corti. This last-named structure consists of a series of internal and external pillars known, as the rods of Corti, which form the walls of a tunnel running spirally round the axis of the cochlea; the tunnel supports a series of cells, furnished with minute hairlike processes, called the inner and outer hair cells; these hair cells are supposed to be in direct communication with the terminal fibrils of the auditory nerve.
Physiology of Hearing. The function of the external ear is to transmit and conduct the vibrations of the air to the membrana tympani; the vibrations of this last-named structure are, as has been seen, propagated by means of the auditory ossicles to the fluid of the internal car, and finally the disturbance reaches the organ of Corti. The exact manner in which the-physical disturbance leads to the production of impressions of sound is not understood; it is supposed that the part played by the cochlea is an essential one, and the air cells are usually credited with being the sound-perceiving organs. The semicircular canals seem to have a special and very peculiar function; it appears from experiments which have been made upon pigeons that injury to these structures is followed by a loss of the power of co-ordination of muscular movements, and it is supposed that variations of pressure of the fluid within the semicircular canals produce sensations which convey to the mind the means of forming a correct idea as to the position of the head with respect to external objects. When the semicircular canals are injured, the sense of position is on this hypothesis lost, and a disturbance of equilibrium is apt to result. The function of the Eustachian tube is probably that of regulating the pressure of air in the tympanic cavity.