758 THE SENSE ORGANS AND SKIN OF THE HORSE 



The anterior end formed a cul-de-sac about two inches (ca. 5 cm.) long between the Eustachian 

 tube and levator palati internally and the internal pterygoid muscle externally. 



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 auchtory cells and 

 the peripheral ramifications of the auditory nerve; (2) a series of cavities in the 

 petrous tem]ioral V)()ne. which inclose the membranous ]oart. 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. 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 laliyrinth, 

 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 obliciue ritlge, the 

 crista vestibuli, which separates two recesses. The anterior and smaller of these is 

 the recessus sphaericus, which lodges the saccule of the membranous laliyrinth. 

 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 l^elow into two divergent branches, which include l^etween 

 them the small recessus cochlearis ; this is perforated l;y 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 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 aquaeductus vestibuli is a small slit behind the lower part 

 of the crista vestibuli. The aquaeductus 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 (Canalcs seiricirculares 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 ex- 

 ternal canals have a common orifice. Each canal forms a))out two-thirds of a 

 circle, one end of which is (>nlarged and termed the ampulla. The superior canal 

 (Canalis semicircularis sui)erior) is nearly vertical and is placed ol)liciuely with 

 regard to a sagittal plane, so that its outer limb is fiu'ther forward than the inner 

 one. The antero-external end is the am]:)ulla and ojiens 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 conunune, which opens into the supero-internal 

 part of the vestibule. The posterior canal (Canalis semicircularis posterior) is 



