ACUSTIC NERVE 565 



membrana tympani. The bony tunnel which it occupies can 

 easily be opened up in a decalcified bone, but is somewhat 

 difficult to expose in the hard bone. After entering the 

 tympanum the chorda tympani runs anteriorly upon the upper 

 part of the membrana tympani under cover of the mucous 

 layer. It crosses the handle of the malleus on the medial 

 aspect near its root. Finally, reaching the anterior end of 

 the tympanic cavity it crosses the anterior process (O.T. 

 gracilis) of the malleus, passes above the tensor tympani, and 

 traverses the medial end of the petro-tympanic fissure, which 

 conducts it to the exterior of the skull. From this point to 

 its junction with the lingual nerve the chorda tympani has 

 already been traced (p. 278). 



Acustic Nerve. In the internal acustic meatus the 

 acustic nerve lies at a lower level than the facial, and at the 

 bottom of the passage it splits into two parts, termed the 

 cochlear and vestibular divisions. These trunks again sub- 

 divide and supply the different parts of the labyrinth of the 

 ear through the foramina of the lamina cribrosa. 



After the examination of the intrapetrous part of the facial nerve and 

 the acustic 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, the osseous labyrinth^ and a series of hollow membranous 

 structures, connected with the filaments of the acustic nerve, 

 which lie in the osseous labyrinth and constitute the mem- 

 branous labyrinth. 



The osseous labyrinth is composed of an intermediate 

 chamber termed the vestibule, posterior to which are placed 

 the three semicircular canals, whilst anteriorly is the cochlea. 

 All these cavities communicate with one another. The corre- 

 sponding membranous parts do not completely occupy the 



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