INTERNAL EAR 527 



INTERNAL EAR. 



Dissection. After the examination of the intrapetrous part 

 of the facial nerve and the acoustic nerve is completed the 

 dissector should display the labyrinth of the internal ear by 

 means of two saw cuts (i) an antero-posterior vertical cut, 

 carried from the upper surface of the bone downwards to the 

 floor of the tympanum, along the junction of its medial and 

 posterior boundaries ; (2) a horizontal cut. This cut should be 

 commenced at the apex of the petrous part of the temporal bone 

 and should be carried laterally till it joins the vertical cut, 

 posteriorly, and enters the tympanic cavity, anteriorly, at the 

 level of the mid-height of the promontory. When the upper 

 part of the petrous portion of the temporal bone, separated by 

 the two cuts, is removed, the vestibular and cochlear parts of 

 the labyrinth and portions of the semicircular canals will be 

 displayed. The dissector should demonstrate the positions and 

 curves of the semicircular canals and the canalis facialis by 

 passing bristles through them. 



Auris Interna. The internal ear or labyrinth consists of an 

 intricate system of cavities in the petrous part of the temporal 

 bone, called the osseous labyrinth, and a series of hollow 

 membranous structures, connected with the filaments of the 

 acoustic nerve, which lie in the osseous labyrinth and constitute 

 the membranous labyrinth. 



The osseous labyrinth is composed of a chamber termed 

 the vestibule, posterior to which are placed the three 

 semicircular canals, whilst anteriorly is the cochlea. All the 

 cavities communicate with one another. The corresponding 

 membranous parts do not completely occupy the osseous 

 chambers, and the intervening space is filled with a fluid 

 termed the perilymph. The membranous labyrinth also contains 

 a fluid which receives the name of endolymph. 



Vestibulum. The vestibule is a small bony chamber of 

 ovoid form, which possesses an antero-posterior diameter of 

 about 4 mm. (one-sixth of an incJi). It is situated between the 

 medial wall of the tympanum and the bottom of the internal 

 acoustic meatus. 



Into the posterior part of the vestibule the three semi- 

 circular canals open by five round apertures', whilst in its 

 lower and anterior part is the opening of the scala vestibuli 

 of the cochlea. 



On the lateral wall is the fenestra vestibuli, which is closed, 

 in the recent state, by the delicate periosteal lining of the 

 chamber and by the base of the stapes. When those parts 

 are removed, the vestibule communicates directly with the 



