882 THE SENSE ORGANS AND SKIN OF THE HORSE 
sagittal plane, so that its outer limb is further forward then than the inner one. The 
antero-lateral end is the ampulla and opens into the vestibule with that of the lateral 
canal. The opposite non-dilated end joins the adjacent end of the posterior canal 
to form the crus commune, which opens into the dorso-medial part of the vestibule. 
The posterior canal (Canalis semicircularis posterior) is also nearly vertical. Its 
ampulla is ventral, and opens into the vestibule directly, while the non-dilated 
end joins that of the dorsal canal. The lateral canal (Canalis semicircularis lateralis) 
is nearly horizontal. Its ampulla is external and opens into the vestibule with that 
of the dorsal 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 cochleze) corresponds to the anterior 
part of the fundus of the internal acoustic meatus, while the apex or cupola (Cupola) 
is directed outward, forward, and downward. It measures about 7mm. across the 
base and about 4 mm. from base to apex. It consists of a spiral canal (Canalis 
spiralis cochleee), 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 cochle of the fundus of the internal 
acoustic 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 fenestrae and ends near the cupola as a hook-like 
process (Hamulus laminz spiralis). The lamina extends about half-way to the 
periphery of the cochlea and partly 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 lateral wall 
of the cochlea and completes the septum between the two scale, but they com- 
municate through the opening at the cupola (Helicotrema). The modiolus is 
traversed by an axial canal which transmits the nerves to the apical coil, and by a 
spiral canal (Canalis spiralis modiol), 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 aquaeductus cochlea, a small canal 
which opens behind the internal acoustic meatus, and establishes a communication 
between the scala tympani and the subarachnoid space. 
The internal acoustic 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 internal 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 ampulle of the dorsal and lateral semicircular ducts. The anterior part of 
the inferior depression (Area cochlea) 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 the foramen singulare for the passage of a 
nerve to the ampulla of the posterior semicircular duct. 
THE MEMBRANOUS LABYRINTH 
The membranous labyrinth (Labyrinthus membranaceus) lies within, but 
does not fill, the osseous labyrinth. It is attached to the latter by delicate tra- 
beculxe which traverse the perilymphatic space. It conforms more or less closely 
to the bony labyrinth, but consists of four divisions, since the vestibule contains 
two membranous sacs—the utricle and saccule. 
1. The utricle (Utriculus), the larger of the two sacs, lies in the postero- 
superior part of the vestibule, largely in the recessus ellipticus. It receives the 
openings of the semicircular ducts, and the small ductus utriculo-saccularis leads 
from its lower part to the ductus endolymphaticus. 
