180 



SENSE OF HEARING. 



consequently, here that the physical part of 

 audition terminates, and the nervous begins. 

 The labyrinth comprises the vestibule, semi- 

 circular canals, and cochlea. The vestibule 

 as its name imports is the hall, that com- 

 municates with all the other cavities of the 

 labyrinth. It would appear to be the most 

 essential part of the organ, as it often exists 

 alone. At its inner surface are numerous 

 small foramina, which communicate with the 

 bottom of the meatus auditorius internus, and 

 through which the filaments of the auditory 

 nerve reach the labyrinth. Externally, it 

 communicates with the cavity of the tympa- 



Labyrinth^tedfrom the solid num b J the foramen ovale. Posteriorly, it 



bone in which it lies embedded, opens into the semicircular canals by five fora- 



v. vestibule, x, Y, z. Semicir- mina; and anteriorly, by a single foramen, 



R. ta&totaSC into the internal scala of the cochlea. There 



Fig. 69. 



is, also, posteriorly and inferiorly, near the 



common orifice of the two vertical semicircular canals, the opening of a 

 small, bony duct, which terminates internally at the posterior surface of 



the petrous portion of the temporal bone. 

 This duct is called aquseductus seu diver- 

 ticulum vestibuli. The semicircular ca- 

 nals are three in number, and occupy the 

 hinder part of the labyrinth. They are 

 called superior vertical, posterior verti- 

 cal, and horizontal. They are cylindrical 

 cavities, curved semicircularly, and are 

 more expanded at their vestibular origin, 

 which has been, therefore, called ampulla. 

 They are constituted of a plate of bone, 



Cochlea divided parallel with its axis, situate in th f Spongy tissue of the pars 



through the centre of the modioius. petrosa, and all of them communicate 



w tn t ^ e vestibule. The cochlea is the 

 most anterior portion of the labyrinth. 



nerve, sending its filaments through centre Jfc jg go called in COnSCQUCnCC of its YQ- 

 of modioius. 4, 4. Scala tympani of first . . . ,* ,. 



turn of cochlea. 5, 5. Scala vestibuli of semblance in man and mammalia to a 



first turn. 6. Section of lamina spiralis, ___!_ cl^ll . V, AT1 A n lA if e "FVonnli anrl 



its zonula ossea ; one of the filaments of the Snail S SHCll , nenCC, aiSO, US 



cochlear nerve is seen passing between the GermannameS, UmaCOH. and S chn 6 eke. 



two layers of the lamina spiralis to be dis- JT . .' . y 



tributed upon the membrane which invests J.t IS tilC mOSt intricate part 01 the Organ 



the lamina. 7. Membranous portion of the /,-. . vj j*j.r> 



lamina spiralis. 8. Loops formed by fila- 01 hearing, and (IOCS not admit 01 easy 



ments of cochlear nerve. 9, 9. Scala tym- ^oc,,,.:^/^ Tf i<a n nnnmrlnl r>Qral cm 



pani of second turn of cochlea. 10, 10. description, it is a conoidai canal, spi- 



Scala vestibuli of second turn ; the septum ra lly convoluted, making tWO tumS UDOn 



between the two is the lamina spiralis. 11. . ^ , . ' 



Scala tympani of remaining half turn. 12. itselt, and resting On a DOny nUClCUS Or 



cu! pillar, called modiolui. The base of the 

 nucleus is concav ; corresponds to the 



rally around to constitute the infundibu- bottom of the meatllS auditorius 

 lum (2). 14. The helicotrema through . . . 1 . -1-1 



which a bristle is passed; its lower extre- nUS, and IS pierced by Small 



through which the filaments of the an- 



(Breschet.) 



