THE EAR. 425 



with the middle ear by the fenestra cochleae and the fenestra 

 vestibuli, which have been described in the account of the 

 petrous bone (page 33). In the recent state the fenestra 

 vestibuli is closed by the base of the stapes, while the fenestra 

 cochleae is covered by a delicate membrane, so that the in- 

 ternal ear forms a closed cavity. It is possible to distinguish 

 a bony labyrinth and a membranous labyrinth (Fig. 173), the 

 former being the cavity in the petrous bone, while the latter is 

 the real auditory organ, situated within the cavity. The mem- 

 branous labyrinth repeats in general the form of the bony laby- 

 rinth, so that it will not be necessary to distinguish the two in 

 our account of the main features of the structure. The auditory 

 nerve is distributed to the membranous labyrinth. For an 

 account of the minute structure of this organ reference should 

 be made to a text-book of general histology or physiology. 



The labyrinth (Fig. 173) consists of a cavity, the vesti- 

 bule (a], and connected with this a coiled 

 tube, the cochlea (b], and the three semi- 

 circular canals (c, d, and e). 



The cochlea (b) is situated in the pro- 

 montory, the rounded elevation of the 



petrous bone extending craniad from the 



- , , T r i FIG. 173. MEMBRANOUS 



ienestra cochleae. It consists of a bony LABYRINTH. 



tube coiled spirally about a central column a, vestibule; 6, cochlea; 



i ~j;~i , "-ru -1 i- <" external semicircular 



known as the modiolus. The coil has cana]; ^ superior semi . 

 nearly the form of a snail's shell, and is circular canal; ^posterior 



. . , semicircular canal; /", scala 



somewhat more lengthened in the cat than tympani;^. scala vestibuli; 

 in man. A plate of bone, the lamina ^ fenestra cochleae; ,' 



fenestra vestibuli. 



spiralis, incompletely divides the cavity of 

 the canal lengthwise into two parts, known as scalae (/and^-). 

 One of these opens at the fenestra cochleae (//) into the tympanic 

 cavity (or would so open but for the thin membrane covering 

 the fenestra) ; this is known as the scala tympani (/). The 

 other scala is connected with the vestibule (a) and is known 

 as the scala vestibuli (g). 



The vestibule (a) is a pyramidal cavity situated mediad of 

 the caudal end of the cochlea. It is connected with the scala 

 vestibuli of the cochlea, and with the three semicircular canals. 



