62 A COURSE ON ZOOLOGY. 



A curvilinear canal, called the Eustachian tube, extends 

 from the middle ear to the pharynx, putting the middle 

 ear in communication with the external air; in this 

 manner the internal face of the tympanum is exposed to 

 the same pressure as the external face to which the air 

 penetrates through the external auditory canal. 



FIG. 34. 



A VIEW OP THE LABYRINTH LAID OPEN (highly magnified). 1, 1, cochlea; 

 2, 3, two canals that wind two and a half turns around a hollow axis (5) ; 7, 

 vestibule; 8, fenestra rotunda; 9, fenestra ovalis; 11, 12, 13, 14, 15, 16, 17, 

 18, the semicircular canals. 



The internal ear, or labyrinth, is made up of the vesti- 

 bule, three semicircular canals, and a spiral tube, shaped 

 like a snail-shell, and called the cochlea. These com- 

 partments communicate with one another, and are filled 

 with a liquid in which terminate the delicate filaments 

 of the auditory nerve ; the latter enters the labyrinth 



