AUDITORY ORGANS 



20I 



ossification occurs the skeletal labyrinth is converted into the several 

 otic bones (p 75). Sometimes the perilymph space is separated 

 from the brain cavity by membrane alone, but usually firmer struc- 

 tures intervene, interruptedonly by foramina for the passage of nerves 

 and blood-vessels, for the endolymph duct and for a similar peri- 

 l3miph duct which extends downward. On the other hand, in all 

 vertebrates in which the middle ear is developed the lateral part of 

 the skeletal wall has two openings into the middle ear. The lower of 

 these (fig. 210), the fenestra tympani (f. rotunda), is closed by mem- 

 brane. In the upper (fenestra ovale or vestibuli) the membrane 

 supports a small cartilage or bone, the stapes (p. 81). 



One part of this compound skeletal and membranous labyrinth of 

 the mammals becomes very compUcated. The lagena becomes greatly 



Fig. 20Q. — Organ of Corti of guinea pig, after Schneider, d, Deiter's cells; he, 

 Henson's cells; ih, inner hair cells; ip, inner pillar cells; Is, limbus spiralis; mt, membrana 

 tectoria; n, nerve fibres; oh, outer hair cells; op, outer pillar cells; si, inner sulcus; s<, 

 scala tympani; t, tunnel; tn, tunnel nerve. 



elongated and in order to accommodate its length it is coiled in a 

 spiral (fig. 207, c), its sides reaching the walls of the skeletal laby- 

 rinth on either side. In this way the perilymph space is divided into 

 two spiral tubes (fig. 208), called scalae, from their resemblance to 

 spiral stairways. The upper of these is the scala vestibuli, the 

 lower the scala tympani, while the scala media or cochlear duct is 

 formed by the lagena. This whole part of the inner ear is the 

 cochlea, so-called from its resemblance to a spiral shell. 



The sense organ of the scala media is very specialized and is 

 known as the organ of Corti (fig. 209). In general it may be said 

 that the scala diminishes in width from apex to base of the cochlea, 

 and is accompanied in its coils by a branch (cochlear) of the acustic 

 nerve. The sensory structures consist of hair cells and Deiter*s 

 cells, regularly arranged, and a series of pillar cells, incUned to each 



