8.T7 



CHAPTER V. 



AIDITOHY APPARATUS. 



TIIK sense of hearing, destined for the perception of sounds produced by the 

 vibration of bodies, has for essential agents the auditory or eighth pair of 

 : !,;ili<- iH-rves, whose terminal fihrilhi; minify in the membranous walls 

 of a system of cavities forming the internal ear ; these cavities are excavated in 

 the substance of the petrous bone, and communicate, externally, by means of 

 two other systems of diverticuli, which constitute the middle and external ear. 



ABTICLE I. INTERNAL EAR, OB LABYRINTH. 



partition. The dissection of this part, from its minuteness and complexity, as 

 well ;i ;y of the bone Hiirrouinling it, cannot Ix- nuule with i.<lvai:ta#: by the 



it. He is, tin n-fdix-, recommended to study it in special preparations, and in the 

 following description.) 



The cavities which, together, compose this part of the auditory apparatus, 

 being entirely channeled within the petrous portion of the temporal bone, 

 have their walls, forming the osseous labyrinth, constituted by that bone. 

 They contain the soft parts, named* the membranous labyrinth, and fluids 

 (endolymph). 



THE OSSEOUS LABYRINTH. 



This is composed of three portions : the vestibule, semicircular canals, and 



1. Tlie Vestibule. 



This is a small, somewhat oval cavity, in the centre of the bone, and out- 

 side the perforated bony plate that forms the bottom of the internal auditory 

 hiatus. It is a real vestibule, with regard to the other parts of the labyrinth, 

 which all open into it. 



On its external watt is the fenestra otalis (fenestra vcslibuli), an opening 



closed by the stapes. The inner shows the foramina through which the 



filaments of the vestibular branch of the acoustic nerve pass. Belmr, and in 



'. is a large orifice, the commencement of the scala cochleae; above, are 



five little apertures, the openings of the semicircular canals. 



2. The Semicircular Canals. 



Three in number, and very narrow, these canals owe their name to their 

 form. They are placed above the vestibule, like three semicircular arches 

 united in a triangular manner at their base, and are distinguished as superior 



/-, posterior, and external. The first two open together, by 

 adjacent extremities, into the vestibule; con.-cfjui-ntly. tin-re are only five 

 orifices of the semicircular canals in thin cavity ; in addition, the adjoining 

 openings of the posterior and external canals are so close to each other, 

 that they appear to be sometimes united at the bottom of a short common 



<-;U.:il. 



3. The Cochlea. 



Situated external to, and below the vestibule, at the inner wall of tin- 



cavity of the tympanum, the snail-dull or <-./,/,./ is \\ 11 named, as it presents 



ly the form of certain molluscs' shells. It in a spiral conical canal. 



