814 



DISSECTION OF THE EAR. 



to otic 

 ganglion. 



Nerves for 

 the muscles. 



Chorda 

 tympani 



cavity. 



Branch of 

 vagus to th 

 outer ear. 



Labyrinth 

 formed of 

 osseous and 

 mem- 

 branous 

 parts. 



Constitu- 

 ents of the 

 osseous 

 part. 



Vestibule ; 



dissection 

 to see it ; 



form and 



dimensions 



apertures 

 before and 

 behind ; 



in outer 

 wall; 



bone, to end in the otic ganglion, and eventually, in great part, to 

 enter the auriculo-temporal nerve and be distributed to the parotid 

 gland. 



NERVES TO MUSCLES. The tensor tympani muscle is supplied by. 

 a branch from the otic ganglion (fig. 300, 9 ) ; and the stapedius 

 receives an offshoot from the facial trunk. 



The CHORDA TYMPANI (fig. 300, 8 ) is a branch of the facial nerve. 

 Entering the cavity behind, it crosses the membrana tympani, lying 

 on the inner side of the handle of the malleus, and issues from the 

 space by an aperture internal to the Glaserian fissure ; it joins the 

 lingual nerve, and its farther course to the tongue is described 

 at p. 688. 



The AURICULAR BRANCH OF THE VAGUS, though not a nerve of the 

 tympanum, may now be traced in the softened bone. Arising in 

 the jugular fossa (p. 633), the nerve enters the special canal, and 

 crosses through the substance of the temporal bone to the back of 

 the ear. 



INTERNAL EAR OR LABYRINTH. 





The inner portion of the organ of hearing consists of a complex 

 chamber surrounded by dense bone, within which are included sacs 

 containing fluid, for the terminal expansion of the auditory nerve. 



THE OSSEOUS LABYRINTH comprises the vestibule, the semi- 

 circular canals, and the cochlea : in the macerated bone these 

 communicate externally with the tympanum, and internally through 

 the meatus auditorius internus with the cranial cavity. 



THE VESTIBULE (fig. 301), or the central cavity of the osseous 

 labyrinth, is placed behind the cochlea, but in front of the semi- 

 circular canals. 



Dissection. This space may be seen on the dry bone which has 

 been used for the preparation of the tympanum. The bone is to 

 be sawn through vertically close to the inner wall of the tympanum, 

 so as to lay bare the fenestra ovalis leading into the vestibule. By 

 enlarging the fenestra ovalis a very little in a direction upwards and 

 forwards, the vestibular space will appear ; and the end of the 

 superior semicircular canal will be opened. 



Other views of the cavity may be obtained by sections of the 

 temporal bone in different directions, according to the opportunities 

 and skill of the dissector. 



The vestibular space (fig. 301) is ovoidal in form, and the ex- 

 tremities are directed forwards and backwards. The larger end is 

 turned back, and the under-part or floor is more narrowed than the 

 upper part or roof. It measures about one-fifth of an inch in length ; 

 but it is narrower from without inwards. The following objects 

 are to be noted on the boundaries of the space. 



In front, close to the outer wall, is a large aperture (#) leading 

 into the cochlea ; and behind are five round openings of the three 

 semicircular canals (d, e t /). 



The outer wall corresponds with the tympanum, and in it is the 

 aperture of the fenestra ovalis. On the inner wall, nearer the front 



