882 THE SENSE ORGANS AND SKIN OF THE HORSE 



sagittal plane, so that its outer limb is further forward then than the inner one. The 

 antero-lateral end is the ampulla and opens into the vestibule with that of the lateral 

 canal. The opposite non-dilated end joins the adjacent end of the posterior canal 

 to form the crus commune, which opens into the dorso-medial part of the vestibule. 

 The posterior canal (Canalis semicircularis posterior) is also nearly vertical. Its 

 ampulla is ventral, and opens into the vestibule directly, while the non-dilated 

 end j oins that of the dorsal canal. The lateral canal (Canalis semicircularis lateralis) 

 is nearly horizontal. Its ampulla is external and opens into the vestibule with that 

 of the dorsal canal. 



3. The cochlea is the anterior part of the bony labyrinth. It has the form of a 

 short blunt cone, the base of which (Basis cochleae) corresponds to the anterior 

 part of the fundus of the internal acoustic meatus, while the apex or cupola (Cupola) 

 is directed outward, forward, and downward. It measures about 7 mm. across the 

 base and about 4 mm. from base to apex. It consists of a spiral canal (Canalis 

 spiralis cochleae), which forms two and a half turns around a central column termed 

 the modiolus. The modiolus diminishes rapidly in diameter from base to apex. 

 Its base (Basis modioh) corresponds to the area cochleae of the fundus of the internal 

 acoustic meatus, and its apex extends nearly to the cupola. Projecting from the 

 modiolus like the thread of a screw is a thin plate of bone, the lamina spiralis ossea. 

 This begins between the two fenestras and ends near the cupola as a hook-like 

 process (Hamulus laminae spiralis). The lamina extends about half-way to the 

 periphery of the cochlea and partly divides the cavity into two passages; of these, 

 the upper one is termed the scala vestibuli, and the lower the.scala tympani. The 

 membrana basilaris extends from the free margin of the lamina to the lateral wall 

 of the cochlea and completes the septum between the two scalae, but they com- 

 municate through the opening at the cupola (Helicotrema) . The modiolus is 

 traversed by an axial canal which transmits the nerves to the apical coil, and by a 

 spiral canal (Canalis spiralis modioli), which follows the attached border of the 

 laniina spiralis, and contains the spiral ganglion and vein. Close to the beginning 

 of the scala tympani is the inner orifice of the aquaeductus cochleae, a small canal 

 which opens behind the internal acoustic meatus, and establishes a communication 

 between the scala tympani and the subarachnoid space. 



The internal acoustic meatus has been described in part (vide Osteology). 

 The fundus of the meatus is divided by a ridge (Crista transversa) into upper and 

 lower parts. The anterior part of the upper depression (Area n. facialis) presents 

 the internal opening of the facial canal; and the posterior part (Area vestibularis 

 superior) is perforated by foramina for the passage of nerves to the utricle and 

 the ampullae of the dorsal and lateral semicircular ducts. The anterior part of 

 the inferior depression (Area cochleae) presents a central foramen and a spiral tract 

 of minute foramina (Tractus spiralis foraminosus) for the passage of nerves to the 

 cochlea. Behind these is an area of small openings which transmit nerves to the 

 saccule (Area vestibularis inferior) , and the foramen singulare for the passage of a 

 nerve to the ampulla of the posterior semicircular duct. 



The Membranous Labyrinth 



The membranous labyrinth (Labyrinthus membranaceus) lies within, but 

 does not fill, the osseous labyrinth. It is attached to the latter by delicate tra- 

 beculae which traverse the perilymphatic space. It conforms more or less closely 

 to the bony labyrinth, but consists of four divisions, since the vestibule contains 

 two membranous sacs — the utricle and saccule. 



1. The utricle (Utri cuius), the larger of the two sacs, lies in the postero- 

 superior part of the vestibule, largely in the recessus ellipticus. It receives the 

 openings of the semicircular ducts, and the small ductus utriculo-saccularis leads 

 from its lower part to the ductus endolymphaticus. 



