IV. VERTEBRATA. 



543- 



appears even in the fishes as a small pocket, the lagena, containing 

 a part of the macula acustica; in the reptiles and birds the lagena 

 becomes much larger, and in the mammals is spirally coiled and is 

 known as the cochlea. A part of the macula acustica of the lagena 

 develops into a special nerve-end apparatus, the organ of Corti. 



The membranous labyrinth described above is partially or en- 

 tirely enclosed in the side wall of the skull in the otic capsule, 

 which may ossify to the otic or petrosal bones. In the birds and 

 mammals the enclosure is such that the structure is duplicated in 

 bone, so that the membranous labyrinth lies in a bony labyrinth,. 



FIG. 576. Diagram of human ear. (From Wiedersheim.) a, 7), vertical semicircular 

 canals ; c, their upper connexion ; Co, the connexion in bony labyrinth ; Cow, 

 ductus cochlearis; Con', cochlea; Cr, canalis reunions; Ct, tympanic cavity 

 (left), cupula terminates (right); d, perilymph; De ductus endolymphaticus ; Dp, 

 Dp', ductus perilymphaticus ; Kl, Kl\ bony labyrinth surrounding the mem- 

 branous labyrinth, the perilymph space black ; M, conch of ear (left), membrane 

 closing fenestra rotunda (right); Mae, external auditory meatus; Jffc, tympanic 

 membrane; S, sacculus; SAp, ear bones (represented as a rod) ; Se, sacculus en- 

 dolymphaticiis ; St, Sv, scalae tympani and vestibuli ; Tb, Tb\ Eustachian tube and 

 its entrance into pharynx ; *, connexion between scalae tympani and vestibuli; 

 t, insertion of ear bones in fenestra ovalis ; 2, utriculus. 



the two being separated by lymph spaces (fig. 576). These spaces- 

 are developed in the cochlea into two tubes, the scala tympani and 

 scala vestibuli, the two connecting only at the tip, being separated 

 elsewhere in part by the membranous cochlea (the ductus cochlearis 

 or scala media). The spaces of the bony labyrinth are filled by 

 two different fluids: inside the membranous labyrinth an en- 

 dolvmph, and between this and the walls of the bony labyrinth a 

 perilymph. 



