1550 A MANUAL OF ANATOMY 



of the superior and external semicircular canals. Below the fovea 

 elliptica is the opening of the aqueductus vestibuli, which leads to 

 the posterior surface of the petrous part of the temporal bone, 

 about \ inch external to the orifice of the meatus auditorius 

 internus. It transmits the ductus endol5miphaticus and a minute 

 vein. 



Anteriorly the vestibule communicates with the scala vestibuli 

 of the cochlea, and posteriorly it presents the five openings of the 

 semicircular canals. 



Semicircular Canals. — The osseous semicircular canals are situated 

 behind the vestibule. They are three in number — -superior, pos- 

 terior, and external — and they open into the vestibule by five 

 circular apertures, the contiguous ends of the superior and posterior 

 canals having a common orifice. Each canal forms about two- 

 thirds of a circle, and each presents at one end an enlargement, 

 called the ampulla. The superior semicircular canal occupies a 

 vertical position, and lies transversely as regards the long axis of 

 the petrous part of the temporal bone, giving rise to the eminentia 

 arciiata on its superior surface. Its ampullary end (antero-external) 

 opens independently into the upper part of the vestibule, above 

 the ampullary orifice of the external canal. Its non-ampullary 

 end unites with the non-ampullary end of the posterior canal, and 

 the two open by a common orifice into the vestibule. The posterior 

 semicircular canal arches backwards towards the posterior surface 

 of the pars petrosa, with which it is almost parallel, and, like the 

 superior, it occupies a vertical position. Its ampullary end (inferior) 

 opens independently into the lower and back part of the vestibule, 

 and its non-ampullary end, as just stated, joins that of the superior 

 canal. The external semicircular canal arches outwards, and 

 occupies a horizontal position. Its extremities are independent 

 of those of the other two canals, and they open by separate apertures 

 into the upper and back part of the vestibule. Its ampullary end 

 is external. 



Cochlea. — ^The osseous cochlea is situated in front of the vesti- 

 bule. It consists of a tube coiled spirally upon itself, like a snail's 

 shell, and is of conical outline. Its base is opposite the lamina 

 cribrosa at the deep end of the meatus auditorius internus ; and its 

 apex, known as the cupola, is directed outwards and slightly for- 

 wards towards the canal which contains the tensor tympani muscle. 

 Its length from base to apex is about \ inch. It consists of (i) a 

 winding tube, called the spiral canal of the cochlea; (2) a central 

 pillar, called the modiolus, round which the spiral canal turns; and 

 (3) a thin plate of bone, called the lamina spiralis, which winds 

 spirally round the modiolus and projects into the spiral canal of 

 the cochlea. 



The spiral canal of the cochlea (cochlear canal or tube) winds 

 round the modiolus, which forms its inner wall. It descriljes two 

 and three-quarter turns, and its basal turn or coil gives rise to the' 

 promontory on the inner wall of the tympanum proper. At the 



