278 THE ANATOMY OF THE HORSE. 



Contained immediately within the osseous vestibule there is a quantity 

 of limpid, serous fluid — the perilymph, which surrounds the parts of the 

 membranous labyrinth here found. These are two delicate sacs — the 

 saccule and the utricle. 



The saccule is the anterior and smaller of the two sacs, and is lodged 

 in the fovea hemispherica. It contains a fluid termed the endolymph. 

 It communicates with the membranous canal of the cochlea by a 

 minute tube — the canalis reuniens, and with the utricle by a Y shaped 

 tube — the ductus vestibuli, the stem of which ends blindly in the aque- 

 duct of the vestibule. 



The Utricle, placed above and behind the saccule, is lodged in the 

 fovea hemi-elliptica. Like the saccule, it contains endolymph. It com- 

 municates, as aforesaid, with the saccule ; and the five openings of the 

 membranous semicircular canals open directly into it. 



The interior of both saccule and utricle is elevated into a ridge — the 

 crista acoustica, in which are distributed the terminal filaments of the 

 vestibular division of the auditory nerve. On this crest are certain 

 peculiar cells, each having a peripheral hair-like process which projects 

 into the endolymph, and a central process which is probably continuous 

 with a filament of the auditory nerve. Here are also found the otoliths, 

 which are minute calcareous particles embedded in a jelly-like material. 



The Semicircular Canals are placed behind the vestibule. They are 

 three in number, and are distinguished as su])erior, %)osterior, and external. 

 The two first have a vertical direction, while the latter is nearly hori- 

 zontal. Each canal opens into the vestibule by a dilated extremity, 

 termed the ampulla. The non-ampullated end of the external canal 

 opens by an independent orifice into the vestibule, while the non-ampull- 

 ated ends of the other two canals have a common opening into the same 

 cavity. The three canals have thus five openings into the vestibule, 

 and three of these openings are ampullated. 



The Membranous Semicircular Canals. — Contained immediately within 

 the osseous canals is a quantity of perilymph, which surrounds the 

 membranous canals. Each of these repeats the form of the osseous 

 canal in which it is lodged ; and they communicate with the utricle by 

 five openings, three of which are ampullated. The membranous canals 

 contain endolymph, and the ampullated end of each is raised inwardly 

 into a ridge, or acoustic crest, having hair cells, otoliths, and nerve 

 terminations similar to those of the saccule and utricle. 



The Cochlea is named from its resemblance to a snail's shell. It has 

 the form of a slightly tapering tube wound spirally two and a half 

 times around a central axis — the modiolus. It is thus somewhat 

 conical in form, the base lying inwards near the internal auditory 

 meatus, from which point the axis of the cone is directed outwards, for- 

 wards, and downwards to the apex. Projecting half way into the tube 



