116 ZOOLOGY. 



the semicircular canals, and cochlea. The vestibule is in the 

 middle, and communicates with the tympanum by the fenestra 

 ovalis. The semicircular canals (m) are three in number, and 

 are rounded osseous and membranous tubes. Finally, the 

 cochlea (n) is a single organ, resembling the shell of the 

 whelk ; its cavity is divided into two parts 

 by a longitudinal partition, half osseous, 

 half membranous, and communicates with 

 the interior of the vestibule and with the 

 tympanum by the fenestra rotunda. The 

 internal ear is osseous and membranous, 

 containing a watery fluid, and, even in 

 man, some remains of a semi-solid body, 

 analogous to the vitrine of the eye. The 

 auditory nerve enters the petrous part of the 

 temporal by the internal auditory canal, and 

 terminates in the interior of the membranous 

 pouches of the vestibule, semicircular 

 canals, and cochlea. On its integrity depends the sense of 

 hearing. 



223. Mechanism of Hearing. Sound is the result of 

 a very rapid vibratory movement which the particles of 

 sonorous bodies experience when struck. The undulations of 

 the sonorous body are communicated to the air which is in 

 contact with it, and are thus propagated to a distance. To 

 be audible, they must reach the fluid which immediately 

 bathes the acoustic nerve. 



224. The sonorous vibrations first strike the external 

 ear ; by it they are reflected and strengthened, and directed 

 towards the middle ear by the auditory passage ; but in man, 

 probably from the smallness of the external ear, the loss of it 

 does not greatly affect the hearing. The vibrations excited 

 in the external ear pass internally by the walls of the canal, 

 but chiefly through the air which fills tEe canal, and so reach 

 the middle ear. 



225. The membrane of the tympanum is the chief agent 

 in facilitating the transmission of the sonorous vibrations of 

 the external air towards the acoustic nerve ; this has been 

 proved by many experiments. 



226. The vibrations are transmitted from the membrane 

 of the tympanum to the small bones of the ear, to the walls 



^ * a, the malleus ; 6, the incus; c, the lenticular bonej d } the stapes. 



