110 ZOOLOGY. 



11s; and inferiorly may be seen the opening of the Eusta- 

 chian tube, leading to the nasal i'uss;!-, and thus admitting the 

 external air into the cavities of the middle ear. Finally, this 

 cavity is traversed by a chain of small bones, extending from 

 the-drum of the ear to the bottom of the truest ra ovalis. 

 where it touches by the base of the stapes the membrandus 

 vestibule. 



The bones are four in number, and are called the malleus, 

 incus, lenticular bone, and stapes. A small stalk belonging to 

 the malleus rests on the drum of the ear. Finally, small 

 muscles attached to these bones augment or dimmish the 

 ten>ion of the chain. 



The internal ear is also enclosed within the rocky part of the 



temporal bone. Itiscompoeedaftoreepart*: 



. a the vestibule, the semicircular canals, and 



A cochlea. The vestibule is in the middle, 



..^A* ft\^ and communicates with the tympanum 



II X P* by the fenestra ovalis. The semicircular 



V fjmjila (in,\ arp t.Virpp in nnmVipr. ami avp 



(m) are three in number, and 

 rounded osseous and membranous tubes. 

 ^s> Finally, the cochlea () is a single organ, 

 resembling the shell of the whelk ; its 

 ' cavity is divided into two parts by a lon- 



gitudinal partition, half osseous, half mem- 

 F ' g ^fTh^Ear" 01168 branous > and communicates with the inte- 

 rior of the vestibule and with the tym- 

 panum by the fenestra rotunda. The internal ear is . 

 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 rocky 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. 



228. McrJtanism 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 

 I..- audible, they must reach the tluid which immediately 

 bathes the acoustic nerve. 



224. The sonorous vibrations first strike the external 



* a, the malleus ; ft, the incus ; c, the lenticular bone ; d, the stapes. 



