1552 A MANUAL OF ANATOMY 



and a lower, or scala tympani, the commencement of which is at 

 the fenestra rotunda. Close to the cupola the lamina spiralis ter- 

 minates in a hook-like process, called the hamulus. The spiral 

 lamina consists of two plates of bone, between which there are 

 canals for nerve-filaments, these canals being offsets of the spiral 

 canal of the modiolus, which, as stated, contains the spiral ganglion 

 or ganglion of Corti. They extend to the free margin of the spiral 

 lamina. The free margin of the lamina spiralis is, in the recent 

 state, attached to the outer wall of the spiral canal of the cochlea 

 by means of the basilar membrane, and the scala vestibuli and 

 scala tympani are now completely separated, except in the region 

 of the hamulus, where they communicate through an opening, 

 called the helicotrema. 



The lamina cribrosa, at the deep end of the internal auditory 

 meatus, will be found described in connection with the Temporal 

 Bone. 



At the lower end of the scala tympani is the upper opening of the 

 aqueductus cochleae, which passes downwards and inwards to ter- 

 minate on the posterior border of the petrous part of the temporal 

 bone. It transmits a small vein to the inferior petrosal sinus, and 

 establishes a communication between the scala tympani and the 

 subarachnoid space. 



Membranous Labyrinth. 



The membranous labyrinth is situated within the osseous laby- 

 rinth, and its constituent parts receive the terminal branches of 

 the auditory nerve. It is separated from the periosteal lining of 

 the osseous labyrinth by the perilymph, and it contains the fluid 

 known as the endolymph. In the case of the vestibule and the 

 osseous semicircular canals the membranous labyrinth corresponds, 

 more or less, with them; but in the case of the osseous cochlea it 

 forms part of the septum between the scala tympani and scala 

 vestibuli, and contains a passage called the membranous canal of 

 the cochlea. 



Vestibular Part of the Membranous Labyrinth. — The vestibule 

 contains two membranous sacs, namely, the utricle and the saccule, 

 which are in close contact, but do not communicate with each other 

 directly. These sacs contain endolymph. 



The utricle is the larger of the two sacs, and into it the mem- 

 branous semicircular canals open. It occupies the upper and back 

 part of the vestibule, a portion of it, known as the recessus 

 utriculi, lying in the fovea elliptica. In the vicinity of the crista 

 vestibuli the wall of this recess receives fibres of the auditory 

 nerve and is thickened, this portion of it being called the macula 

 acustica utriculi. From the anterior and inner part of the utricle 

 a minute canal, called the ductus utriculi {ductus ulriculo-saccularis), 

 passes to join the ductus sacculi, and so form the ductus endo- 

 lymphaticus. 



