Ear
the organ of hearing; is composed of three parts, the external ear, the middle ear, or tympanum, and the internal ear, or labyrinth. The external posed of three parts, the external ear, the middle ear, or tympanum, and the as the ear), and the auditory canal or ear consists of two portions, the auricle on its outer or more exposed surface, presents various eminences and depressions, resulting from the form of its EAGLE Top-Harpy Eagle EAGLE eign bodies, and prevents their farther particles of dust, and other small forpassage into the meatus. The middle ear, or cavity of the tympanum, is a space filled with air which is received from the pharynx through EAR the Eustachian tube and traversed by a chain of very small movable bones, which connect the membrane of the tympanum with the external ear. It lies, as its name implies, between the external meatus and the labyrinth or internal ear, and opens posteriorly into the cells contained in the mastoid portion of the temporal bone, and anteriorly into the Eustachian tube. The cavity is of an irregular shape, and is lined by a very delicate ciliated epithelium, which is a prolongation of that of the pharynx through the Eustachian tube. Its external wall is in great part formed by the membrane of the tympanum, which is nearly oval, and placed in a direction slanting inward, so as to form an angle of about 45° with the floor of the auditory canal.
The Eustachian tube, into which the tympanic cavity opens anteriorly, is about an inch and a half in length, and passes downward, forward, and into its opening in the pharynx. It is partly osseus but chiefly cartilaginous, and allows the free passage of air in and out of the tympanum.
The internal ear or labyrinth is the essential part of the organ of hearing, being the portion to which the ultimate filaments of the auditory nerve are distributed. It is composed of three parts: the vestibule, the semi-circular canals, and the cochlea, which form a series of cavities presenting a very complicated arrangement, and and lying imbedded in the hardest part of the petrous portion of the temporal bone. They communicate externally with the tympanum through the fenestra ovalis, and the fenestra rotunda; and internally with the internal auditory canal, which conveys the auditory nerve from the cranial cavity to the internal ear. The very dense bone immediately bounding these cavities is termed the osseous labyrinth, to distinguish it from the membranous labyrinth, which lies within a portion of it. The cochlea, so-called from its resemblance to a common snail-shell, consists of an osseous and gradually tapering canal, about an inch and a half in length, which makes two turns and a half spirally around a central axis, termed the modiolus, which is perforated at its base for the entrance of the filaments of nerve. the cochlear portion of the auditory This spiral canal gradually diminishes toward the apex of the cochlea. At its base it presents an opening into the vestibule, partially divided into two.
In the infant state, one of these openings (scala tympani) does not communicate with the vestibule, but is closed by the membrane of the fenestra rotunda.
Its interior is sub-divided into two pas- EARTH sages (scale) by an osseous lamina.
This is the lamina spiralis, which incompletely divides the cochlea into an upper passage, the scala vestibuli, and a lower one, the scala tympani-that is, the division is incomplete so far as the skeleton goes, but is completed during life by the lamina spiralis membranacea (or basilar membrane).