98 ANATOMY OF THE HEAD AND NECK. 



The semicircular canals, above and behind the vestibule, may 

 followed out from their orifices in the vestibule by a file and strong 

 knife ; some of the instruments used by dentists are applicable to this 

 and other parts of the dissection of the internal ear. 



The SEMICIRCULAR CANALS, three in number, are of un- 

 equal length, though each forms more than half a circle ; 

 the}^ communicate by both ends with the vestibule ; as two 

 of them, however, blend together, they have but five open- 

 ings into that cavity. Each canal has a dilated extremity, 

 called its ampulla. From their different directions the 

 canals have been named superior, oblique, and horizontal. 

 The superior canal crosses the upper part of the petrous 

 bone transversely, on the anterior surface of which, within 

 the cranium, it makes a marked projection ; its ampulla is 

 at the outer end of the canal ; its inner end joins with the 

 oblique canal. The oblique canal is directed backward 

 toward the posterior surface of the temporal bone ; its 

 inner end is in common with that of the superior canal, 

 and its outer end is furnished with an ampulla. The hori- 

 zontal canal is the shortest of the three, and has separate 

 orifices ; it lies in the substance of the bone, nearly on a 

 level with the fenestra ovalis ; its ampulla is on the outer 

 side, close above that aperture. 



The cochlea is anterior to the vestibule. To expose it, the surface 

 of bone forming the promontory of the tympanum must be filed or 

 cut away ; the surface of the petrous bone above it should also be 

 removed. 



The COCHLEA is conical in form, with its base turned to- 

 ward the meatus auditorius internus. It resembles a snail- 

 shell in construction, consisting of a tube wound spiralb 

 round a central part or axis ; the tube makes two turns 

 and a half round the axis, and terminates in a closed ex- 

 tremity, called the cupola ; the axis, or modiolus, is th< 

 bony centre included within the coils of the spiral tube ; its 

 shape is conical, and its size diminishes as it reaches the 

 apex of the cochlea. Winding round the axis is a thii 

 osseous plate, called the lamina spiralis, which is project* 

 into the spiral tube, and forms part of a septum, completec 

 by a membrane, which divides the tube into two passages ; 

 these, however, communicate at the apex of the cochlea by a 

 foramen, called the helicotrema. The lamina spiralis termi- 

 nates at the apex of the cochlea, in a hook-shaped process 

 called the hamulus. The two divisions of the spiral tul 



