THE INTERNAL EAR. 



1513 



surface of the petrous bone. The semicircular canals open into the posterior part of 

 the vestibule by five apertures (Fig. 1267), the undilated ends of the superior and 

 posterior canals joining to form a common limb (crus commune). The horizontal 

 canal (canalis lateralis) alone communicates with the vestibule by two distinct open- 

 ings. Its ampulla is at its outer end and lies at the upper part of the vestibule above 

 the oval window, from which it is separated by a groove corresponding to the facial 

 canal. Lying above and close to this opening is placed the ampullary end of the 

 superior canal. The ampullary end of the posterior canal lies on the floor of the 

 vestibule, near the opening of the non-dilated end of the horizontal canal and of the 

 canalis communis. In the w'all of the ampulla of the posterior canal, a number of 

 small openings (macula cribrosa inferior) provide for the entrance of the special 

 branch of the vestibular nerve destined for this tube. 



The Cochlea. — The bony cochlea constitutes the anterior part of the labyrinth 

 and appears as a short blunt cone, about 5 mm. in height, whose base forms the an- 

 terior wall of the outer end of the internal auditory meatus. Its apex is directed hori- 



FiG. 1268. 



Scala vestibiili 



Scala lyiiipaui 



Modiolus 

 Area cochlearis 



Area vestibularis inferior 



Internal auditory canal 



Foramen singulare 



Hamulus, overlying 

 helicotrema 



Lamina spiralis ossea 



Canalis spiralis modioli 



Facial canal 



Crista falciformis 



Area vestibularis 

 superior 



Cochlea and bottom of internal auditory canal exposed by vertical section passing parallel with zygoma; prepara- 

 tion has been turned so that cochlea rests with its base downward and apex pointing upward. X 5. 



zontally outward, somewhat forward and downward, and reaches almost to the Eusta- 

 chian tube. Its large lower turn bulges into the tympanic cavity and produces the 

 conspicuous elevation of the promontory seen on the inner wall of the middle ear 

 (Fig. 1269). The bony cochlea consists essentially of a tapering central column, 

 the modiolus, around which the bony canal, about 30 mm. long, makes something 

 more than two and a half spiral turns, the basal, middle and apical. The conical 

 modiolus has a broad concave base which forms part of the base of the cochlea (basis 

 cochlea), and a small apex which extends nearly to the apex of the cochlea, or 

 cupola (cupula). It is much thicker within the lowest turn of the canal than above, 

 and is pierced by many small canals for the nerves and vessels to the spiral lamina 

 (Fig. 1268). The axis of the modiolus, from base to apex, is traversed by the 

 central canal, whilst a more peripherally situated channel, the canalis spiralis, 

 encircles the modiolus and contains the spiral ganglion and a spiral vein. Project- 

 ing at a right angle from the modiokis into the canal of the bony cochlea is a thin 

 shelf of bone, the lamina spiralis ossea, which is made up of two delicate bony 

 plates between which are fine canals containing the branches of the cochlear nerve. 

 The spiral lamina begins between the round window and the lower wall of the 



