THE INTERNAL EAR OR LABYRINTH. 837 



CHAPTEE V. 



AUDITORY APPAEATUS. 



The 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 

 encephalic nerves, whose terminal fibrillse ramify 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. 



Aeticlb I. — Internal Eae, oe Labyrinth. 



(Freparaiion. — The dissection of this part, fiom ita minuteness and complexity, aa 

 well as the density of the bone surrounding it, cannot be made with advantage by the 

 student. He is, tlierefore, 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). 



IKE OSSEOUS LABTEmTH. 



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



1. The 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 wall is the fenestra ovalis (fenestra vestibuli), an opening 

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

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

 front, 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 

 or anterior, posterior, and external. The first two open together, by their 

 adjacent extremities, into the vestibule ; consequently, there are only five 

 orifices of the semicircular canals in this 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 

 canal. 



3. The Cochlea. 



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

 cavity of the tympanum, the snail-shell or cochlea is well named, as it presents 

 exactly the form of certain molluscs' shells. It is a spiral conical canal, 



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