SEMICIRCULAR CANALS. 681 



Lining membrane of the osseous labyrinth. A thin fibrous periosteal 

 membrane lines the vestibule and the semicircular canals, and is continuous 

 with the fibrous process in the aqueduct of the vestibule. On the outer 

 wall of the cavity it stretches over the fenestra ovalis; and in front it is 

 prolonged into the cochlea through the aperture of the scala vestibuli (fig. 

 244, g). 



Fig. 245. 



a. Upper vertical ; 6, posterior vertical ; and c, 

 horizontal canal. 



d. Common opening of the two vertical canals. 



e. Part of the vestibular cavity. 



/. Opening of the aqueduct of the vestibule. 



REPRESENTATION op THE SEMICIRCULAR CANALS ENLARGED 

 (from a model in University College Museum). 



The outer surface of the membrane is adherent to the bone; but the 

 inner is covered by a single layer of flattened epithelium. It is supplied 

 by the vestibular vessels; and it secretes a thin serous fluid, liquor Cotun- 

 nii, or perilymph, which surrounds the membranous labyrinth, and occu- 

 pies for a short distance the aqueduct : this fluid also fills the scalae of the 

 cochlea. 



COCHLEA. This part of the osseous labyrinth has a position anterior 

 to the vestibule, and has received its name from its resemblance to a spiral 

 shell. 



Dissection. To obtain a view of the cochlea it will be needful to cut 

 or file away the bone between the promontory of the tympanum and the 

 meatus auditorius internus, on the preparation before used for displaying 

 the vestibule; or this section may be made on another temporal bone in 

 which the semicircular canals are not laid bare. For the same dissection 

 in the recent state, a softened bone should be used. 



The cochlea (fig. 246) is conical in form, and is placed almost horizon- 

 tally in front of the vestibular space. The base of this body is turned to 

 the meatus auditorius internus, and is perforated by small apertures ; whilst 

 the apex is directed to the inner wall of the tympanum, opposite the canal 

 for the tensor tympani muscle. Its length is about a quarter of an inch, 

 and its width at the base is about the same. Resembling a spiral shell in 

 construction, the cochlea consists of a tube wound round a central part or 

 axis ; but it differs from the shell in having its tube subdivided by a par- 

 tition. 



The axis or modiolus (fig. 246, a) is the central stem which supports 



