THE MEMBRANOUS LABYRINTH 759 



also nearly vertical. Its ampulla is l)elow, and opens into the vestibule directly, 

 while the non-dilated end joins that of the superior canal. The external canal 

 (C'analis semicircularis lateralis) is nearly horizontal. Its ampulla is external and 

 opens into the vestil)ule with that of the superior canal. 



3. The cochlea is the anterior part of the bony labyrinth. It has the form of a 

 short blunt cone, the base of which (Basis cochleae) corresponds to the anterior 

 part of the fundus of the internal auditory meatus, while the apex or cupola (Cupula) 

 is direct(Ml outward, forward, and downward. It measures about 7 mm. across the 

 base and about 4 mm. from base to apex. It consists of a spiral canal (Canalis 

 spiralis cochlea?), which forms two and a half turns around a central column termed 

 the modiolus. The modiolus diminishes rapidly in diameter from base to apex. 

 Its base (Basis modioli) corresponds to the area cochleae of the fundus of the internal 

 auditory meatus, and its apex extends nearly to the cupola. Projecting from the 

 modiolus like the thread of a screw is a thin plate of bone, the lamina spiralis ossea. 

 This begins between the two fenestrse and ends near the cupola as a hook-like 

 process (Hanmlus lamina? spiralis). The lamina extends about half-way to the 

 perii)hery of the cochlea and partially divides the cavity into two passages; of 

 these, the upper one is termed the scala vestibuli, and the lower the scala tympani. 

 The membrana basilaris extends from the free margin of the lamina to the outer wall 

 of the cochlea and completes the septum between the two scalae, but they com- 

 municate through the opening (Helicotrema) at the cupola. The modiolus is 

 traversed by an axial canal which transmits the nerves to the apical coil, and by a 

 si^iral canal (canalis spiralis modioli), which follows the attached border of the 

 lamina spiralis, and contains the spiral ganglion and vein. Close to the beginning 

 of the scala tympani is the inner orifice of the aquaeductus cochleae, a small canal 

 which opens behind the internal auditory meatus, and establishes a communication 

 between the scala tympani and the subarachnoid space. 



The internal auditory meatus has been described in part {vide Osteology). 

 The fundus of the meatus is divided by a ridge (Crista transversa) into upper and 

 lower parts. The anterior part of the upper depression (Area n. facialis) presents 

 the cranial opening of the facial canal; and the posterior part (Area vestibularis 

 superior) is perforated by foramina for the passage of nerves to the utricle and 

 the ampullae of the superior and external membranous semicircular canals. The 

 anterior part of the inferior depression (Area cochleae) presents a central foramen 

 and a spiral tract of minute foramina (Tractus spiralis foraminosus) for the passage 

 of nerves to the cochlea. Behind these is an area of small openings which transmit 

 nerves to the saccule (Area vestibularis inferior), and a single foramen (F. singulare) 

 for the passage of a nerve to the ampulla of the posterior semicircular canal. 



The Membranous Labyrinth 

 The membranous labyrinth (Labyrinthus membranaceus) lies within, but 

 does not fill, the osseous labyrinth. It is attached to the latter by delicate tra- 

 beculse which traverse the perilymphatic space. It conforms more or less closely 

 to the bony labyrinth, but consists of four divisions, since the vestibule contains 

 two membranous sacs — the utricle and saccule. 



1. The utricle (Utriculus), the larger of the two sacs, lies in the postero- 

 superior part of the vestibule, largely in the recessus ellipticus. It receives the 

 openings of the membranous semicircular canals, and the small ductus utriculo- 

 saccularis leads from its lower part to the ductus endolymphaticus. 



2. The saccule (Sacculus) is situated in the recessus sphsericus of the vestibule. 

 From its lower part the ductus reuniens proceeds to open into the ductus cochlearis, 

 a little in front of the blind end of the latter. A second narrow tube, the ductus 

 endolymphaticus, passes from the posterior part of the saccule, and is joined by the 

 ductus utriculo-saccularis; it then traverses the aquaeductus vestibuli, and term- 



