HEARING. 383 



but not extending into the cochlea. These mem- 

 branes, which compose what has been termed, for 

 the sake of distinction, the membranous labyrinth^ 

 form one continuous, but closed sac, containing a 

 fluid,* perfectly similar in appearance to the peri- 

 lymph, which surrounds it on the outer side, and 

 intervenes between it and the sides of the osseous 

 labyrinth, preventing any contact with those sides. 

 In Fig. 395, which is on a still larger scale than 

 the preceding figure, the osseous labyrinth is laid 

 open, so as to show the parts it encloses, and more 

 especially the membranous labyrinth, floating in 

 the perilymph (p). The form of this latter part is 

 still more distinctly seen in Fig. 396, w here it is 

 represented in a position exactly corresponding to 

 the former figure, but wholly detached from the 

 bony labyrinth, and connected only with the nervous 

 filaments which are proceeding to be distributed to 

 its different parts. 



A simple inspection of these figures, in both of 

 which the corresponding parts are marked by the 

 same letters, will show at once the form and the 

 connexions of the three semicircular canals, (x, y, 

 z), each of which present, at their origin from the 

 vestibule, a considerable dilatation, termed an am- 

 pulla (a, a, a), while, at their other extremities, 

 where they terminate in the vestibule, there is no 

 enlargement of their diameter ; and it will also be 

 seen that two of these canals (x and v) unite into 

 one before their termination. The same descrip- 

 tion applies in all respects both to the osseous and 



* De Blainville has termed this fluid " la vitrine auditive," from 

 its supposed analogy to the vitreous humour of the eye. 



