THE EAR. 425 
with the middle ear by the fenestra cochlew 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 
cochlee 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 (4), and the three semi- 
circular canals (c, d, and e). 
The cochlea (4) is situated in the pro- 
montory,—the rounded elevation of the 
petrous bone extending craniad from the 
: Fic. 173.—MEMBRANOUS 
fenestra cochlee. It consists of a bony LANVRINEE: 
tube coiled spirally about a central column — ¢, vestibule; 4, cochlea; 
known as the modiolus. The coil has Gay) superior some 
nearly the form of a snail’s shell, and is circular canal; e, posterior 
7 semicircular canal; /, scala 
somewhat more lengthened in the cat than tympani; g, scala vestibuli; 
in man. A plate of bone, the lamina i fenestra cochlez; 7, 
3 ome ; enestra vestibuli. 
spiralis, incompletely divides the cavity of : 
the canal lengthwise into two parts, known as scala (fand ¢). 
One of these opens at the fenestra cochlez (/) 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 (). 
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. 
