I 4 2 VETERINARY PHYSIOLOGY 



anterior part of this, a canal makes two and a half turns round 

 a central pillar, and then, turning sharply on itself, makes the 

 same number of turns down again, and ends at the fenestra 

 rotunda. This is the osseous cochlea (fig. 68, Coch.}. The 

 ascending and descending canals are separated from one an- 

 other, partly by a bony plate, partly by a membranous partition 

 the basilar membrane. At the base, the bony lamella is 

 broad, but at the apex its place is chiefly taken by the 

 membrane, which measures at the apex more than ten times 

 its width at the base. 



From the posterior and superior aspect of the vestibule a 

 mechanism unconnected with hearing has been evolved. Three 



semicircular canals (fig. 70), each 

 with a swelling at one end, open 

 ' c ' into the vestibule. One runs in the 

 horizontal plane, and has the swell- 

 ing or ampulla anteriorly (fig. 70, 

 h.c.). The other two run in verti- 

 cal planes placed obliquely to the 

 FIG. 70. -The Relationship of middle p lan6} ag ig indicated in fig. 

 the Semicircular Canals to one _-. m . 



another, h.c., horizontal canal ; 70 - The anterior or supenor canal 



s.c., superior canal ; p.c., pos- (s.c.), has its ampulla in front and 



terior canal. , the posterior (p.c.) has its ampulla 



behind. They join together, and enter the vestibule by a 



common orifice. 



In the perilymph of the bony labyrinth lies a complex 

 membranous bag, the membranous labyrinth. 



In the vestibule this is divided into two little sacs, the 

 utricle, related to the semicircular canals, and the saeeule, 

 related to the cochlea. They are joined together by a slender 

 canal. From the saccule comes off a canal which runs into 

 the cochlea upon the basilar membrane, forming a middle 

 channel between the other two, the scala media or membranous 

 cochlea. This terminates blindly at the apex. From the 

 utricle a membranous canal extends into each of the bony 

 semicircular canals, being provided with an ampulla, which 

 nearly fills up the bony ampulla, while the canal portion is 

 small, and occupies only a small part of the bony canal (fig. 68, 

 p. 139). 



In the membranous cochlea the lining cells form the organ 



