THE EAR 881 



the helicotrema. The osseous spiral canal is nearly an inch and a half (about 

 3"5 cm.) in length. The lamina spiralis is thin and dense in structure at its edge, 

 hut it is spongy and channelled ^vith canals for nerves and blood-vessels where it 

 starts shelf-like from the modiolus. 



The spiral canal of the modiolus coils round the pillar as it tunnels the base 

 of the spiral lamina. 



In the recent state, membranous structures complete the bony partition into two 

 separate chambers, or scalse: the scala vestibuli and the scala tympani, which 

 communicate through the helicotrema at the summit of the cochlea. 



The scala vestibuli commences from the cavity of the vestibule; in its first turn 

 it is somewhat narrower than the scala tympani. 



The scala tympani commences at the fenestra rotunda, which is filled with the 

 secondary membrane of Scarpa, and forms a kind of window which shuts it off 

 from the cavity of the tympanum. Near its commencement is the small orifice of 

 the aqueductus cochlese, which lodges a minute vein, and forms a communication 

 with the subarachnoid sjiace. 



The central axis, or modiolus, extends from the base to the apex of the cochlea. 

 It starts with a Ijroad base where it corresponds Avith the first turn of the cochlea, 

 and it is pierced by minute canals to receive the filaments of the cochlear division 

 of the auditory nerve. In the second coil the axis is diminished by more than 

 half, and it terminates in the remaining half coil or cupula in a bony plate, the 

 infundibulum, which expands towards the summit of the cochlea, with which it 



Fict. 503. — Section uf the Osseous Cochlea. (Enlarged.) 



Lamina spiralis 



Scala tympani 



Termination of internal auditory meatus 



becomes continuous. The central canal of the modiolus runs through its 

 centre. 



The Membranous Labyrinth, which lies within the bony labyrinth just de- 

 scribed, receives the terminations of the auditory nerve. It is for the most part 

 separated by the perilymph from the membrane Avhich lines the bony chambers, 

 and it contains within its own cavity the fluid endolymph. In the vestiljule and 

 semicircular canals it bears a near resemblance (tliough it is much smaller) to the 

 enclosing structures. V)Ut in the cochlea it not only divides the bony canal into the 

 two scalar, but forms between them a third space, the canal of the cochlea. 



Within the vestibule the membranous labyrinth consists of two sacs, which 

 do not directly communicate: the utricle, connected with the semicircular canals; 

 and the saccule, with the cochlea. 



The Utricle is the larger. It is oblong, laterally compressed; it occupies the 

 upper and back ]5art of the cavity, and is in contact with the wall of the fovea 

 hemielliptica. Filaments of the auditory nerve are distributed to the wall of the 

 sac opposite the crista vestibuli, passing through the bony foramina already de- 

 scribed. Its wall is here thicker than elsewhere, and calcareous particles (otoliths) 

 are attached to its inner surface. Behind, the cavity communicates by five orifices 

 with the semicircular canals. 



The Saccule, which is slightly flattened, occupies the front and lower part of 

 the cavity at the opening of the scala vestibuli of the cochlea. From the hollow of 

 the fovea hemispherica it receives numerous nerve filaments, and presents a thick- 

 ened area with attached otoliths. 



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