186 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
a milky appearance, but in the teleosts the lime is aggregated into one 
or more ‘ear stones’ of considerable size. 
With the appearance of cartilage the membranous labyrinth be- 
comes enclosed in a protecting otic capsule (p. 60), which usually 
follows pretty closely the divisions and canals of the epithelial parts, 
thus forming the skeletal labyrinth, separated from the membranous 
labyrinth by a slight gap filled with fluid (the perilymph). When 
ossification occurs the skeletal labyrinth is converted into the several 
otic bones. Sometimes the perilymph space is separated from the 
brain cavity by membrane alone, but usually firmer structures inter- 
vene, interrupted only by foramina for the passage of nerves and blood- 
vessels, for the endolymph duct and for a similar perilymph 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. 188), 
the fenestra tympani (f. rotunda), is closed by membrane. In the 
upper (fenestra ovale or vestibuli) the membrane supports a small 
bone, the stapes (p. 73). 
One part of this compound skeletal and membranous labyrinth of 
the mammals becomes very complicated. The lagena becomes greatly 
elongated and -in order to accommodate its length it is coiled in a 
spiral, its sides reaching the walls of the skeletal labyrinth on either 
side. In this way the perilymph space is divided into two spiral tubes 
(fig. 186), called scale, from their resemblance to spiral stairways. 
The upper of these is the scala vestibuli, the lower the scala tympani, 
while the scala media 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. 187). In general it may be said that the 
scala diminishes in width from base to apex of the cochlea, and is accom- 
panied 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, inclined to each other like the 
rafters of a roof, in an A-like manner (fig. 187). As the A’s diminish 
in width from base to apex of the cochlea, this part has been thought 
to play a part in the recognition of pitch. There is also a cuticular 
structure, the membrana tectoria, which extends from the medial wall out 
over the hair cells, and this may be the intermediate organ of stimulation 
