HEAD AND NECK OF THE HORSE 



161 



joins the corresponding part of the posterior canal to form a cms 

 covimwne that opens into the medial part of the vestibule. The 

 posterior canal is also vertical. Its non-ampullated end joins the crus 

 commune ; while its lower and ampul lated end opens into the lower and 

 posterior part of the vestibule. The lateral canal is horizontal, with an 

 ampulla, lateral in position, opening into the vestibule close to that of 

 the superior canal. The non-ampullated end opens above the ampulla 

 of the posterior canal. 



The cochlea'^ forms the most anterior part of the bony labyrinth, 

 and consists essentially of a spiral canal (canalis spiralis cochlete) 

 wound round a central column of bone known as the modiohisr The 



Superior semicircular canal. 



Posterior 

 semicircular canal 



Lateral semicircular 

 canal. 



Superior semicircular duct. 



Ampullse. 



Aqueduct of the vestibule. / 

 Fenestra cochlese, 



Posterior 



semicircular 

 duct. 

 Vestibule. 



Aminillae. 



/ 

 ■^ Utricle. 



^Saccule. 



Cochlea. 



Fenestra vestibuli. 



Lateral 

 semicircular 

 duct. 



Cochlear duct. 



Ductus reuniens. 

 Utriculo-saccular duct. 

 Endolymphatic duct. 

 Saccus endolymphaticus. 



A B 



Fig. 73.— The osseous {A) and membranous (fi) labyrinths of the internal ear. 



canal ends blindly at the apex of the cochlea, where it forms the 

 cupula. Into the canal a spiral lamina of bone (lamina spiralis ossea) 

 projects for some distance from the inner (modiolar) wall, and thus 

 partially divides the canal into two compartments. One of these, the 

 scala^ vestibuli, communicates with the vestibule; while the other, the 

 scala tympani, in the macerated bone, opens by the fenestra cochleae 

 into the cavity of the tympanum. In the fresh condition the fenestra is 

 closed by the membrana tympani secundaria. The spiral lamina begins 

 at the floor of the vestibule, and ends close to the cupula in a hook-like 

 process, the hamulus,^ which assists in the formation of an opening, the 



' Cochlea [L.], /coxX'as (cochlias) [Gr.], a snail ; in a borrowed sense, anything 

 twisted spirally. 



2 Modiolus [L.], the nave or hub. 

 ^ Scala [L.], a staircase. 

 ^ Hamulus (dim. of hamus) [L.], a hook. 

 11 



