THE INTERNAL EAR—THE OSSEOUS LABYRINTH 881 
This opening is only about an inch (ca. 2.5 cm.) long. The two pouches are often unequal in size, 
and variations in regard to the distance which they extend backward are not uncommon. In one 
case, for instance, a small aged horse, the right pouch extended along the cesophagus about five 
inches (ca. 12 cm.) behind the ventral tubercle of the atlas, and the left one a little more than two 
inches (ca.6em.). No pathological changes were apparent, and the condition was not recognizable 
externally. Cases of extreme size—so-called tympanites— of the pouches occur, and are apparently 
congenital defects. In a case in a yearling colt, the head of which was 24 inches long, the left 
pouch extended about 12 inches (ca. 30 cm.) behind the tubercle of the atlas, and had a capacity 
of six quarts. The anterior end formed a cul-de-sac about two inches (ca. 5 em.) long between the 
Eustachian tube and levator palati medially and the lateral pterygoid muscle laterally. 
THE INTERNAL EAR 
The internal ear or labyrinth (Auris interna s. Labyrinthus) consists of two 
parts, viz.: (1) A complex membranous sac, which supports the auditory cells and 
the peripheral ramifications of the auditory nerve; (2) a series of cavities in the 
petrous temporal bone, which encloses the membranous part. The first is called 
the membranous labyrinth, and contains a fluid, the endolymph. The second is 
the osseous labyrinth. The two are separated by the perilymphatic space, which is 
occupied by a fluid termed the perilymph. 
THE OssEouS LABYRINTH 
The osseous labyrinth (Labyrinthus osseus) (Fig. 702) is excavated in the 
petrous temporal bone medial to the tympanic cavity. It consists of three divisions: 
(1) a middle part, the vestibule; (2) an anterior one, the cochlea; and (3) a pos- 
terior 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. 
Tt is a small, irregularly ovoid cavity, which is about 5 to 6 mm. in length. Its 
lateral wall separates it from the tympanic cavity, and in it is the fenestra vestibuli, 
which is occupied by the base of the stapes. The medial wall corresponds to the 
fundus of the internal acoustic meatus. It is crossed by an oblique ridge, the 
crista vestibuli, which separates two recesses. The anterior and smaller of these is 
the recessus spkericus, 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 
ampullz of the dorsal and lateral 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 mner 
opening of the aqueductus vestibuli is a small slit behind the lower part of the 
crista vestibuli. The aqueductus passes backward in the petrous temporal bone, 
and opens on the medial 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 dorsal, posterior, 
and lateral. They communicate with the vestibule by four openings only, since 
the inner end of the dorsal and the upper end of the posterior canal unite to form a 
common canal (Crus commune), and the ampullate ends of the dorsal and lateral 
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 dorsal canal (Canalis 
semicircularis dorsalis) is nearly vertical and is placed obliquely with regard to a 
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