The ossicles of the tympanum are three—viz., the malleus, the incus (or anvil), and the stapes. We have already explained how the malleus is connected with the mem brane of the tympanum by means of its handle. Through this connection, the tension of that membrane may be modified by the agency of one or two muscles which are attached to this ossicle. These muscles are the laxator tympani, which arises from the sinuous process of the sphenoid bone (q.v.), and is inserted into the processus gracilis; and the tensor tympani, which arises from the under surface of the petrous portion of the temporal bone, and is inserted into the handle of the malleus immediately below the commencement of the processes gracilis. The existence of the former of these muscles is doubtful, many anatomists regarding the structure in question as ligamentous rather than muscular. The incus much more closely resembles a molar tooth with two fangs, than the anvil from which it derives its name. Of the two processes which it gives off, the short one runs backwards, and projects into the mastoid cells behind the tympanic cavity; while the long one inclines downwards, and terminates in the lenticular or orbicu lar process, to which the head of the stapes is attached. It has a head, neck, two branches, and a base, which, as has been already mentioned, fits into the fenestra ovalis. A minute muscle, the stapedies,•takes its origin from a hollow conical eminence termed the pyramid, which lies behind the fenestra ozyai,v, and is inserted into the neck of the stapes; by pulling the neck backwards, it probably compresses the contents of the ves tibule.
. The Eustachian tube, into which the tympanic cavity opens anteriorly, is about an inch and a half in length, and passes downwards, forwards, and inwards to its opening in the pharynx. It is partly bony, but chiefly cartilaginous. Its use is to allow the free passage of air in and out of the tympanum, and to admit of the egress of the mucus secreted in that cavity.
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 (q.v.) are distributed. It is composed of three parts—viz., the vestibule, the semicircular canals, and the cochlea, which form a series of cavities presenting a very complicated arrangement, and lying imbedded in the hardest part of the petrous portion of the temporal bone. They com municate externally with the tympanum by the two openings already described—the fenestra avails, 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 dis tinguish it from the membranous labyrinth, which lies within a portion of it.
The vestibule is a common central cavity into which the semicircular canals and the cochlea open. It is about a fifth of an inch in height, and in length from before back wards its transverse diameter (from side to side) being somewhat less. On its posterior
wall are five orifices for the semicircular canals, one of the orifices being common to two of the canals. Anteriorly, the cochlea enters it by a single opening, the beginning of the scala vestibuli. On its outer wall is the fenestra ovalis, and on its inner are the fovea hemispherica, containing several minute orifices for the entrance of filaments of the auditory nerve, and the fovea semi-elliptica.
The semicircular canals are three in number, and open at both ends into the vestibule. They in length, and notwithstanding their name, each is considerably more than a semicircle, the superior vertical canal being the longest. Their average diameter is about a twentieth of an inch, the extremity of each canal exhibiting a dilatation or ampulla. Each canal lies in a different plane, very nearly at right angles to the planes of the other two, hence their names of the superior vertical, the ulterior vertical, and the horizontal canals.
The cochlea, which derives its name from its resemblance to a common snail-shell, forms the anterior portion of the labyrinth. It 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 the cochlear portion of the auditory nerve. This spiral canal gradually diminishes towards the apex of the cochlea. At its base, it presents two openings, one into the vestibule, and the other (closed by a membrane, and communi cating with the tympanum) being the fenestra rotunda already described. Its interior is subdivided into two passages (scaly) by an osseo-membranous lamina. This is the lamina spiralis, which divides the cochlea into an upper passage, the scala vestibuli, and a lower one, the scala tympani. At the apex, these two passages communicate by an opening to which the term helicotrema has been applied. Between the two scalce, there • is a third space termed the ductus cochelaris, or scala intermedia. In this space the fila ments of the auditory nerve terminate, by being connected with a complicated arrange ment of peculiarly formed epithelial cells, constituting the organ of corbi. For a notice of the membranous portion of the lamina spiralis, see AUDITORY NERVE.
We now return to the membranous labyrinth. The membranous and osseous laby rinths have the same shape, but the former is considerably smaller than the latter, a fluid, termed the perilymph, intervening in some quantity between them. At certain points, recent investigations have shown that the membranous is firmly adherent to the inner surface of the osseous labyrinth. The vestibular portion consists of two sacs, an upper and larger one, of an oval shape, termed the utriculus, or common sinus, and a lower and smaller oue of a more globulous shape, called the sacculus.