1 80 THE EAR. 



the auditory nerve. The aqueduct of the vestibule also penetrates 

 it, from the posterior surface of the petrous bone. It also contains 

 two depressions called ^55a hemispherical and fossa semi-elliptica.° 



The semicircular canals^ are three curved cylinders of half a 

 line in diameter, whose extremities open into the vestibule by five 

 orifices, each extremity being expanded like a flask and called 

 ampulla.'' Two of these canals' p are vertical, and one is horizontal.* 

 The cochlea resembles a snail-shell, and consists of a conical canal, 

 which makes two turns and a half spirally around an axis called the 

 modiolus, which is a porous mass of bone, whose base is perforated 

 by the filaments of the cochlear nerve. The tube of the cochlea is 

 divided into two passages by the lamina spiralis, which terminates 

 at the apex with a hook, called hamulus ; which is covered by the 

 cupola. The upper passage of the canal is called scala vestibuli, 

 and the lower is the scala tympani. These passages communicate 

 by an opening at the apex called helicotrema. The scala vestibuli 

 opens into the vestibule, and the scala tympani through the fenestra 

 rotunda into the tympanum. The aqueduct of the cochlea opens by 

 one extremity into the scala tympani, and by the other upon the 

 inferior surface of the petrous bone. 



The membranous labyrinth has the same shape as the bony 

 cavities which it lines, and consists of a delicate membrane composed 

 of several layers, containing the limpid fluid of Cotunnius, and the two 

 small calcareous masses called otoconites. 



The auditory nerve divides in the meatus auditorius internus 

 into a vestibular and cochlear branch. 



the end. 



