OF THE SENSE OF HEARING. 115 



poral bone, and separated from the auditory canal by the mem- 

 brane of the tympanum. Opposite to this membrane are two 



" f 



'' \ 



lob 



Fig. 74. Tympanum and Ossicula Auditus.* 



openings, called the fenestra ovalis and fenestra rotunda. In 

 the back wall of the cavity is an opening leading to the mastoid 

 cells ; and inferioiiy may be seen the opening of the Eusta- 

 chian tube, leading to the nasal fossaB, 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 fenestra ovalis, 

 where it touches by the base of the stapes the membranous 

 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 diminish the 

 tension of the chain. 



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

 temporal bone. It is composed of three parts : the vestibule, 



* Kepresents the cavity of the tympanum, the ossicula auditus, and their 

 muscles, magnified. a a, cavity of the tympanum; b, membrana tympani, or 

 rather the osseous circle to which it is attached; c, handle of the malleus, 

 resting on the middle of the membrana tympani ; d, head of the malleus 

 articulating with the incus ; e, long handle of the malleus, passing into the 

 glenoidal fissure ; the anterior muscle of the malleus is attached to it ; f, in- 

 ternal muscle of the malleus ; g, anvil ; h, lenticular bone ; i, stapes ; 

 k, musculus stapedius. 



i 2 



