1 86 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 scalae, 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 maybe the intermediate organ of stimulation 



