THE BONY LABYRINTH. 465 



known fact, that a small watch, when placed in the month, and not in coniact 

 with any part of it, is scarcely heard, if heard at all, renders this opinion very 

 doubtful. It ought, however, to be remembered, during the investigation of 

 this subject, that persons who hear with difficulty are almost invariably in 

 the habit of opening their mouths when they listen. 



Of the Lahyrinth. 



Those parts of the organ of hearing which have been ah-eady 

 described, seem calculated for concentrating the vibrations of 

 air; and for communicating, with some modification, the motion 

 they occasion, to the Lahyrinth. 



This important portion of the ear consists of three very dis- 

 similar parts, which communicate with each other, and form one 

 general cavity. 



The central part of the cavity is a chamber, of a form which 

 approaches to the oval, and has been compared to that of a 

 grain of barley. It is called the Vestibule. 



At one extremity of this chamber (the posterior) are three 

 tubes, each of which is curved so as to form a large portion of a 

 circle. These tubes communicate with the vestibule, by each 

 extremity ; but they form only five orifices, because two of their 

 extremities are united before they open into the vestibule. 

 These tubes are denominated the Semicircular Canals. 



At the other extremity of the vestibule is a conical tube con- 

 voluted like the shell of a snail. This is called the Cochlea. 

 It also conmnunicates with the vestibule. 



The Ldhyrinth, thus complicated in its form, is situated on 

 the inner side of the cavity of the tympanum. Its position is 

 such, that the Vestibule, and the Cochlea are opposite to the 

 Membrana Tympani and the Semicircular Canals are posterior 

 to it. , The apex of the cochlea is on the side of the labyrinth 

 which is next to the tympanum ; the basis of the cochlea is 

 next to the brain. 



The texture of the bone which immediately surrounds these 

 cavities is much harder than that of the other parts of the os 

 petrosum ; and if the bone of a foetus be used, these softer parts 

 may be cut away so as to leave behind the bony substance 



