374 



STRUCTURE OF THE SEMICIRCULAR CANALS. 



3d. On the determination of the quality of sounds, the structure of the 

 semicircular canals, and their function. 



The semicircular canals are cylindroid tubes, developed, as it were, 

 ureofthe ^ rom the vestibule, and of a figure which has suggested their 

 semicircular name. They are three in number, and placed at right angles 

 to one another : two of them are vertical, the third horizon- 

 tal ; they all open into the vestibule, the adjacent branches of two of 

 them coalescing first. On one of the branches of each of them there is 

 a dilatation just before it joins the vestibule ; to this dilatation the des- 

 ignation of ampulla is given. The vestibule of the labyrinth may there- 

 fore be regarded as the common mouth of the semicircular canals, and of 

 the scala vestibuli of the cochlea ; or, considering its order of develop- 

 ment, the vestibule may be regarded as the essential portion of the lab- 

 yrinth, and the semicircular canals and cochlea appendices that have 

 branched forth from it. 



The vestibule and semicircular canals are lined with a membrane 

 which, of course, copies their shape, yet it is not in contact with their 

 bony walls, but is parted therefrom by a stratum of water, to which, as 

 has been said, the name of perilymph is given ; their interior is also 

 filled with a liquid the endolymph, it is called. The bony structure is 

 called the bony labyrinth ; this structure is the membranous labyrinth. 

 A portion of the auditory nerve divides into three main branches, which 

 present themselves for the supply of the ampullae : the brush-like termi- 

 nations of these are accommodated with an otolith. 



ILLUSTRATIONS OF LABYRINTH OF LEFT SIDE. 



EXTERNAL, OB TYMPANIC FACE OF LABYRINTH. 



Fig. 192. 



Tympanic face of the labyrinth. 



Fig. 192 : #, external wall of vestibule, on which 

 is seen, &, fenestra ovalis ; , fenestra rotunda, 

 and, d, tract of the facial nerve ; 0, superior semi- 

 circular canal ; /*, posterior semicircular canal ; <?, 

 horizontal semicircular canal ; i, i, i, wall of coch- 

 lea ; j, aqueduct of cochlea ; &, portion of petrous 



Fig. 193. 



INTERNAL, OR CRANIAL FACE OF LABYRINTH. 



Fig. 193 : a, internal wall of vestibule ; b, aque- 

 duct of vestibule ; c, base of cochlea ; d , aqueduct 

 of cochlea ; e, /*, conduit, at the bottom of which 

 are several holes for the passage of facia) and au- 

 ditory nerves; g, superior semicircular canal; A, 

 cranial face of the labyrinth, posterior semicircular canal ; i, horizontal semicir- 

 cular canal. 



