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ANATOMY OF THE HORSE 
VESTIBULE.—This is a small roundish cavity, hardly so 
much as a quarter of an inch in diameter, situated between the 
cochlea and the semicircular canals ; to the outer side of it is 
the tympanum, with which it communicates through the fenestra 
ovalis. In its roof we find five openings, leading into the semi¬ 
circular canals; besides wdiich we notice two peculiar pits or 
fovea, containing membranous sacs, the sacculi vestibuli, filled 
with fluid, and furnished with expansions of nerve. Anatomists 
have been misled, by their examinations of these depressions in 
the dried bone, in supposing that they reverberated the sound; 
this shews the danger of forming conclusions from such artificial 
inquiries. 
Semicircular Canals. 
These canals are three in number, placed side by side, behind 
the vestibule, opposite to the cochlea; but they are entered by 
five openings through the upper part of the vestibule. They 
are distinguished as the superior or vertical canal, the posterior 
or oblique, and the exterior or horizontal. The superior and ex¬ 
terior canals possess one opening common to both, and one pe¬ 
culiar to each, besides; while the posterior canal opens into 
the vestibule by two distinct orifices; thus making altogether 
five apertures. The separate orifice of the superior canal opens 
nearly perpendicularly upon the fenestra ovalis. 
From the sacculi vestibuli branches of nerves are sent into 
the semicircular canals, in which they float loose and unattached 
in the fluid surrounding them. It was once supposed that the 
auditory nerve was spread over the periosteum by which the 
labyrinth is lined ; later researches, however, have shewn that it 
is within the sacculi it expands, which do not even come in con¬ 
tact with the periosteum, but are simply connected to it by a 
pellucid, cellular, mucus-like matter. 
The semicircular canals are formed out of a peculiarly hard 
brittle bone; and their diameter is so small, as scarcely to ad¬ 
mit of the introduction of the head of a common pin. 
Cochlea. 
This is the last division of the labyrinth, and by far the 
most complex one. It receives its name from its resemblance to 
the convolutions of the shell of the snail: it possesses a spiral 
or pyramidal form, and has (by no means inaptly) been com¬ 
pared to a spiral staircase, running round a column placed in 
the centre. It is situated below the vestibule ; its base resting 
against the meatus auditorius interims; its apex extending to 
the Eustachian tube. At its base it describes a large circle 
