BOUNDARIES OF VESTIBULE. 



679 



The chorda tympani (fig. 243, 8 ) is a branch of the facial nerve (p. 144). 

 Entering the cavity behind, it crosses the membrana tympani, and issues 

 from the space by an aperture in, or internal to the Glaserian fissure ; it 

 joins the gustatory nerve, and its farther course to the tongue is described 

 at page 97. 



The auricular branch of the vagus nerve, though not a nerve of the 

 tympanum, is an offset to the outer ear, and may be now traced in the 

 softened bone. Arising in the jugular fossa (p. 113), the nerve enters a 

 special canal, and crosses through the substance of the temporal bone to 

 the back of the ear. 



THE LABYRINTH. The inner portion of the organ of hearing is so 

 named from its complexness. It consists of three spaces surrqunded by 

 dense bone ; and of sacs containing fluid for the expansion of the ^auditory 

 nerve, which are contained within the former. 



The OSSEOUS LABYRINTH includes the vestibule, the semicircular canals, 

 and the cochlea : these communicate externally with the tympanum, and 

 internally, through the meatus auditorius internus, with the cranial cavity. 



The VESTIBULE (fig. 244), or the central cavity of the osseous labyrinth, 

 is placed behind the cochlea, but in front of the semicircular canals. 



Fig. 244. 



VIKW OP THE VESTIBULE OBTAINED BY CUTTING AWAY THE OTTER BOUNDARY IN A FOZTUS, 

 enlarged three times. 



a. Fovea hemispherica. e. Upper vertical semicircular canal partly 



b. Crista vestibuli. laid open. 



c. Aperture of aqueduct of the vestibule. /. Horizontal semicircular canal, partly opened. 



d. Common opening of two semicircular canals, g. Opening of the scala vestibuli. 



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 ves- 

 tibular space will appear ; and the end of the superior semicircular canal 

 will be opened. 



