The =osseous labyrinth= (Labyrinthus osseus) (Fig. 565) is excavated in
the petrous temporal bone to the inner side of the tympanic cavity. It
consists of three divisions: (1) a middle part, the =vestibule=; (2) an
anterior one, the =cochlea=; and (3) a posterior one, the =semicircular
canals=.
1. The =vestibule= (Vestibulum) is the central part of the osseous
labyrinth, and communicates in front with the cochlea, behind with the
semicircular canals. It is a small, irregularly ovoid cavity, which is
about 5 to 6 mm. in length. Its outer wall separates it from the
tympanic cavity, and in it is the =fenestra vestibuli=, which is
occupied by the base of the stapes. The inner wall corresponds to the
fundus of the internal auditory meatus. It is crossed by an oblique
ridge, the =crista vestibuli=, which separates two recesses. The
anterior and smaller of these is the =recessus sphæricus=, which lodges
the saccule of the membranous labyrinth. In its lower part there are
about a dozen minute foramina which transmit filaments of the vestibular
nerve to the saccule. The posterior and larger depression is the
=recessus ellipticus=, which lodges the utricle of the membranous
labyrinth. The crista vestibuli divides below into two divergent
branches, which include between them the small =recessus cochlearis=;
this is perforated by small foramina, through which nerve-bundles reach
the ductus cochlearis. Similar foramina in the recessus ellipticus and
the crista vestibuli transmit nerve filaments to the utricle and the
ampullæ of the superior and external semicircular ducts. The anterior
wall is pierced by an opening which leads into the scala vestibuli of
the cochlea. The posterior part of the vestibule presents the four
openings of the semicircular canals. The inner opening of the
=aquæductus vestibuli= is a small slit behind the lower part of the
crista vestibuli. The aquæductus passes backward in the petrous temporal
bone, and opens on the inner surface of the latter near the middle of
its posterior border; it contains the ductus endolymphaticus.
2. The =osseous semicircular canals= (Canales semicirculares ossei),
three in number, are situated behind and above the vestibule. They are
at right angles to each other, and are designated according to their
positions as superior, posterior, and external. They communicate with
the vestibule by four openings only, since the inner end of the superior
and the upper end of the posterior canal unite to form a common canal
(Crus commune), and the ampullate ends of the superior and external
canals have a common orifice. Each canal forms about two-thirds of a
circle, one end of which is enlarged and termed the =ampulla=. The
=superior canal= (Canalis semicircularis superior) is nearly vertical
and is placed obliquely with regard to a sagittal plane, so that its
outer limb is further forward than the inner one. The antero-external
end is the ampulla and opens into the vestibule with that of the
external canal. The opposite non-dilated end joins the adjacent end of
the posterior canal to form the crus commune, which opens into the
supero-internal part of the vestibule. The =posterior canal= (Canalis
semicircularis posterior) is also nearly vertical. Its ampulla is below,
and opens into the vestibule directly, while the non-dilated end joins
that of the superior canal. The =external canal= (Canalis semicircularis
lateralis) is nearly horizontal. Its ampulla is external and opens into
the vestibule with that of the superior canal.
3. The =cochlea= is the anterior part of the bony labyrinth. It has the
form of a short blunt cone, the =base= of which (Basis cochleæ)
corresponds to the anterior part of the fundus of the internal auditory
meatus, while the =apex= or =cupola= (Cupula) is directed outward,
forward, and downward. It measures about 7 mm. across the base and about
4 mm. from base to apex. It consists of a =spiral canal= (Canalis
spiralis cochleæ), which forms two and a half turns around a central
column termed the =modiolus=. The modiolus diminishes rapidly in
diameter from base to apex. Its base (Basis modioli) corresponds to the
area cochleæ of the fundus of the internal auditory meatus, and its apex
extends nearly to the cupola. Projecting from the modiolus like the
thread of a screw is a thin plate of bone, the =lamina spiralis ossea=.
This begins between the two fenestræ and ends near the cupola as a
hook-like process (Hamulus laminæ spiralis). The lamina extends about
half-way to the periphery of the cochlea and partially divides the
cavity into two passages; of these, the upper one is termed the =scala
vestibuli=, and the lower the =scala tympani=. The membrana basilaris
extends from the free margin of the lamina to the outer wall of the
cochlea and completes the septum between the two scalæ, but they
communicate through the opening (Helicotrema) at the cupola. The
modiolus is traversed by an axial canal which transmits the nerves to
the apical coil, and by a spiral canal (canalis spiralis modioli), which
follows the attached border of the lamina spiralis, and contains the
spiral ganglion and vein. Close to the beginning of the scala tympani is
the inner orifice of the =aquæductus cochleæ=, a small canal which opens
behind the internal auditory meatus, and establishes a communication
between the scala tympani and the subarachnoid space.
The =internal auditory meatus= has been described in part (_vide_
Osteology). The fundus of the meatus is divided by a ridge (Crista
transversa) into upper and lower parts. The anterior part of the upper
depression (Area n. facialis) presents the cranial opening of the facial
canal; and the posterior part (Area vestibularis superior) is perforated
by foramina for the passage of nerves to the utricle and the ampullæ of
the superior and external membranous semicircular canals. The anterior
part of the inferior depression (Area cochleæ) presents a central
foramen and a spiral tract of minute foramina (Tractus spiralis
foraminosus) for the passage of nerves to the cochlea. Behind these is
an area of small openings which transmit nerves to the saccule (Area
vestibularis inferior), and a single foramen (F. singulare) for the
passage of a nerve to the ampulla of the posterior semicircular canal.