1 5 14 HUMAN ANATOMY. 



vestibule (Fig. 1269), and after winding spirally around the modiolus to the 

 apex of the cochlea, ends in a hook-like process, the hamulus, which forms part 

 of the the boundary of the helicotrema (Fig. 1269). The partial division of the 

 canal of the bony cochlea effected by the osseous spiral lamina is completed by the 

 membranous spiral lamina, which stretches from the free edge of the osseous 

 lamina, to which it is attached, to the outer wall of the canal (Fig. 127 1). The 

 upper division of the canal is called the scala vestibuli and communicates with 

 the vestibule, whilst the lower division, the scala tympani, would open into the 

 tympanic cavity, were it not separated from that space by the secondary tympanic 

 membrane. These scalae communicate with each other through an opening, the 

 helicotrema, at the apex of the cochlea. Close to the beginning of the scala tym- 

 pani at the round window is the inner orifice of the aquaeductus cochleae (ductus 

 perilymphaticus), its outer opening being in a depression on the lower surface of 

 the pyramid near its posterior edge. It transmits a small vein and establishes a 

 communication between the subarachnoid space and the scala tympani. 



The internal auditory canal communicates with the cranial cavity by an oval 

 opening on the posterior surface of the pyramidal portion of the temporal bone, from 

 which it extends outward to the internal ear. Its outer or lateral end, the fundus, 

 is divided into a smaller superior and a larger inferior fossa by a transverse ridge, the 

 crista falciformis. In the anterior part of the superior fossa (area facialis) is the 

 opening of the facial canal (aquaeductus Fallopii) for the transmission of the facial 

 nerve. In its posterior part are the openings (area vestibularis superior) for the 

 branches of the vestibular nerves which supply the utricle and the ampullae of the 

 superior and horizontal semicircular canals. These openings appear in the macula 

 cribrosa superior on the inner surface of the bony labyrinth (page 15x2 ). The ante- 

 rior part of the inferior fossa is called the area cochlearis and is perforated about its 

 middle by the opening of the central canal of the modiolus. Surrounding this are 

 the numerous small apertures of the tractus spiralis foraminosus for the trans- 

 mission of branches of the cochlear nerve to the two lower turns of the cochlea. 

 Behind the area cochlearis and separated from it by a ridge, lies the inferior area of 

 the vestibule (^area vestibularis inferior) with its small openings for the passage of 

 nerves to the saccule. The macula cribrosa media, described above, is formed by 

 these openings. Behind the inferior fossa is a large opening, the foramen singu- 

 lare, which leads into a canal at the other end of which are the small openings 

 of the macula cribrosa inferior. It transmits the branch of the vestibular nerve des- 

 tined for the ampulla of the posterior semicircular canal. 



The Membranous Labyrinth. 



The membranous labyrinth (labyrinthus membranaceus) lies within the bony 

 labyrinth, which it resembles in general form. This agreement is least marked 

 within the vestibule, since here the single division of the bony capsule is occupied 

 by two compartments of the membranous sac, the utricle and the saccule. The 

 membranous labyrinth comprises: (i) the utricle and the saccule, which, with the 

 ductus endolymphaticiis, lie within the vestibule; (2) the three viembrmious se7ni- 

 circidar canals lodged within the bony semicircular canals; and (3) the vicin- 

 branous cochlea enclosed within the bony cochlea. The membranous labyrinth 

 is attached, especially in certain places, by connective tissue to the inner wall of the 

 bony capsule. The interval between the membranous and bony labyrinths, largest 

 in the scalae tympani and vestibuli of the cochlea and in the vestibule, constitutes the 

 perilymphatic space (spatium perilymphaticum) and contains a modified lymphatic 

 fluid, the perilymph. The fluid within the membranous labyrinth, appropriately 

 called the endolymph, can pass from one part of the labyrinth to another, although 

 the saccule and utricule are only indirectly connected through the ductus endo- 

 lymphaticus and a narrow channel, the canalis utriculo-saccularis. 



The Utricle. — The utricle (utriculus) occupies the recessus ellipticus in the 

 upper back part of the vestibule. It is larger than the saccule and communicates 

 with the three membranous semicircular canals. Attached to the upper and inner 

 walls of the vestibule by connective tissue, it extends from the roof of the vestibule 



