DISSECTING MANUAL. 



tube into upper and lower passages, respectively the scala 

 vestibuli and scala tympani. The division is completed by a 

 membrane (membrana basilaris) which stretches from the 

 edge of the lamina to the outer wall of the tube. This shelf 

 begins in the vestibule near the foramen rotundum, and ends 

 near the apex in a sickle-shaped process (hamulus) which 

 helps form an aperture (helicotrema) connecting the seals. 

 The scala vestibuli begins in the vestibule. The scala tym- 

 pani begins at the fenestra rotunda, toward which a crest (crista 

 semilunaris) stretches from the attached margin of the lamina 

 spiralis; the aqueductus cochleae opens into the scala tympani 

 near the crest. [760] 



Internal Auditory Meatus. This canal runs in the petrous 

 bone, from the cranial cavity, toward the internal ear; a 

 transverse ridge (crista transversa) divides its bottom (fundus) 

 into upper and lower parts, the fossulae superior and inferior 

 respectively. Fossula superior: the anterior part (area 

 facialis) presents the orifice of the aqueductus Fallopii; the 

 posterior part (area vestibularis superior) presents foramina 

 for the nerves of the utricle and the ampullse of the superior 

 and external semicircular canals. Fossula inferior: the an- 

 terior part (area cochleae) presents the orifice of the canalis 

 centralis, surrounded by foramina (tractus spiralis foramino- 

 sus) for the nerves of the cochlea; behind this, and separated 

 by a ridge, is a group of foramina (area vestibularis inferior) 

 for the nerves to the saccule; the posterior part presents the 

 foramen singulare for the nerves to the ampulla of the posterior 

 semicircular canal. [761] 



Membranous Labyrinth. This structure has somewhat the 

 shape of the bony labyrinth and lies in it, filled with endo- 

 lymph and surrounded by perilymph. The semicircular and 

 cochlear canals lie throughout on the inner aspect of the outer 

 walls of their bony tubes. The bony vestibule contains two 

 communicating sacs, the utricle and saccule. The utricle is the 

 larger, lies postero-superiorly, and receives the semicircular 



[102] 



