THE EAR 393 



and, behind this, a ridge, the crista vestibuli. Behind this 

 ridge is the opening of the aquceductus vestibuli. In the roof 

 is a depression, the fovea hemielliptica. 



Behind, the vestibule presents five foramina leading into 

 the semicircular canals, and in front a larger foramen leading 

 into the scala vestibuli of the cochlea. 



The semicircular canals are three bony tubes of unequal 

 length lying above and behind the vestibule, each forming 

 about two-thirds of a circle. Their general diameter is -$ 

 inch, but at one end is a dilatation, the ampulla, T V inch in 

 diameter. They empty into the vestibule by five apertures, 

 in one of which two tubes join. 



The superior is vertical and is set transversely, forming 

 an eminence seen on the upper surface of the petrous bone. 

 The ampulla of this tube opens into the upper part of the 

 vestibule, the other end opening by a foramen into the back 

 part, in common with the posterior canal. 



The posterior is also vertical, but is set anteroposteriorly 

 and is longer than the others, its ampulla being at the postero- 

 inferior part of the vestibule, the other extremity joining with 

 the preceding canal, as described. 



The external is horizontal and the shortest, its ampulla 

 being at the outer part, above the fenestra ovalis, and the 

 other end at the upper and back part of the vestibule. 



The cochlea resembles a snail shell. Its apex looks forward 

 and outward, and its base toward the internal auditory meatus. 

 Within is a centre piece, the modiolus, or columella, around 

 which the canal runs spirally for two and a half turns. 



Within the canal, and attached to the modiolus, is the 

 lamina spiralis. This plate of bone partially divides the spiral 

 canal into two compartments, or scalse, the division being 

 completed by a membrane (see below) which reaches the 

 outer wall of the cochlea. The upper scala is known as the 

 scala vestibuli; the lower is the scala tympani. 



The modiolus, or columella, the centre piece of the cochlea, 

 runs from base to apex. It is conical in form, the base corre- 

 sponding to that of the cochlea, and is pierced by foramina 

 for the cochlea branches of the auditory nerve and for the 

 vessels which pass to the lamina and spiral canal. One of 

 these, larger than the rest, is the opening of the canalis modioli 

 centralis. Diminishing gradually in size, the modiolus termi- 



