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 m regard to the distance which they extend backward are not uncommon. In one 

 case, tor instance a smaU aged horse, the right pouch extended along the cesophagus about five 

 mches (ca. 12 cm.) behind the ventral tubercle of the atlas, and the left one a Uttle more than two 

 mcnes (ca. b cm.). JNo pathological changes were apparent, and the condition was not recognizable 

 externa,lly Cases of extreme si2<^-so-called tympanites— of the pouches occur, and are apparently 

 congemtal 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 

 ot SIX quarts ihe anterior end formed a cul-de-sac about two inches (ca. 5 cm.) long between the 

 il^ustachian 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. 

 It 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 sphaericus, 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 eUipticus, which lodges the utricle of the membranous labyrinth. The 

 crista vestibuL' 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 

 ampullae 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 inner 

 opening of the aquaeductus vestibuli is a small slit behind the lower part of the 

 crista vestibuli. The aquseductus 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 

 ntunber, 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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