466 VESTIBULE. COCHLEA. 



which surrounds these cavities, corresponding exactly to their 

 form. 



The Lnhyrinth, when thus prepared, may be considered as 

 a solid body which has been enveloped in a softer substance, 

 and is brought into view by detaching the soft substance, 

 which surrounded it. 



In the foetus it is nearly as large as in the adult, so that the 

 structure of the ear can be investigated with great advantage 

 in such subjects. 



The Vestibule 



Is situated within the Foramen Ovale. There are two 

 remarkable depressions of its internal surface ; one, which is 

 in the superior part, is called Semi-eliptical ; the other, which 

 is below, has the name of Hemispherical. 



When the dried preparation is examined, there are several 

 foramina in this cavity, viz. the Foramen Ovale, already men- 

 tioned, a round aperture, by which it communicates with 

 one of the cavities of the cochlea, and the five openings of the 

 semicircular canals. Besides these, there are several small 

 perforations from the Meatus Auditorius Internus for the 

 transmission of nerves. 



The Cochlea 



Commences at the anterior part of the vestibule. It is a 

 conical tube, so convoluted that it has the form of the shell of 

 a snail making two circuits and a half round a centre. 



It may be considered as wound in a spiral direction round a 

 pillar of bone. To this central pillar the name of Modiolus is 

 applied. It commences at the Cavity called Meatus Auditorius 

 Internus ; and at its base is somewhat excavated. It gradually 

 diminishes in diameter as it proceeds towards the apex of the 

 cochlea, and is, therefore, conical in form ; but it does not pre- 

 serve this form to its termination, for near the apex it gradu- 

 ally becomes broader, and thus forms a second cone inverted. 



