IXTERXAL EAR OR LABYRINTH. 



371 



The meatus internus has elsewhere been stated to be sit- 

 uated on the posterior cerebral surface of the petrous bone. 

 It is lined by dura mater, and Fio. 108. A 



has a depth of about one quarter 

 of an inch. Its bottom pre- 

 sents a cul de sac or cribriform 

 plate, divided by a bony crest 

 into two portions, the superior 

 has a large opening, the aqueduct 

 of FallopiuSj for transmitting' 

 the facial nerve, and some small 

 foramina which open into the 

 vestibule. The lower portion 

 corresponds to the base of the 

 cochlea, and is perforated by 

 many foramina ; a part of this 

 inferior depression has apertures 

 also leading into the vestibule. It is through the cribri- 

 form base of the meatus internus that the labyrinth is 

 supplied with its nerves and most of its vessels. The an- 

 terior corner of the vestibule has a depression called fovea 

 hemispherica; and another above this, the fovea elliptica. 



The Cochlea, so called from its resemblance to a snail 

 shell, is situated in the anterior part of the petrous bone. 

 Its form is conical, its apex being directed forward, while 

 its base corresponds with the bottom of the internal audi- 

 tory meatus. It consists of a narrowing tube, divided by a 

 partition extending from the base to the apex, into two 

 tubes, and coiled round a central pillar. The tube of the 

 cochlea is described as being about one inch and a half 

 in length, one tenth of an inch in diameter at its base, and 



FIG. 108, A represents the Labyrinth of the Ear laid open, a Fovea 

 elliptica. b Fovea hemispherica. c Common entrance of the posterior and 

 superior semi-circular canals, d Opening of the aqueduct of the vestibule. 

 e Superior semi-circular canal. / Posterior, g Inferior semi-circular canal. 

 h Spiral canal of the cochlea, i Opening of t the aqueduct of the cochlea, 

 j Lamina spiralis. 



Fio. 108, B Cochlea laid open, a Modiolus. 6 Lamina spiralis. c Scala 

 tympani. d Scala vestibuli. 



