NERVE 167 



The importance of such a mechanism in the anterior part 

 of the animal in warning it of danger or making it aware of 

 the presence of its prey is manifest. 



In mammals the organ of hearing consists of the external, 

 the oniddle, and part of the internal ear. The purpose of 

 the first is to conduct the vibrations of the air to the second, 

 in which these vibrations produce to-and-fro movements of a 

 bony lever, by which the fluid in the third is alternately 

 compressed and relaxed. 



2. External Ear. 



The structure of this presents no point of special physio- 

 logical interest. In lower animals the pinna is under the 

 control of muscles, and is of use in determining the direction 

 from which sound comes. 



3. Middle Ear. 



The object of the middle ear is to overcome the mechanical 

 difficulty of changing vibrations of air into vibrations of a fluid. 



It consists of a chamber, the tympanic cavity, in the 

 petrous part of the temporal bone (fig. 83). Its outer wall 

 is formed by a membrane, the membrana tympani (Ty.), which 

 is attached to a ring of bone. Its inner wall presents two 

 openings into the internal ear — the fenestra ovalis (/.o.), an 

 oval opening, situated anteriorly and above, and the fenestra 

 rotunda (f.r.), a round opening placed below and behind. 

 Throughout life these are closed, the former by the foot of 

 the stapes, which is attached to the margin of the hole by a 

 membrane, the latter by a membrane only. The posterior 

 wall shows openings into the mastoid cells, and presents a 

 small bony projection which transmits the stapedius muscle. 

 The anterior wall has (a) above, a bon}^ canal carrying the 

 tensor tympani muscle, and (6) below this, the canal of the 

 Eustachian tube which communicates with the posterior nares 

 (fig. 83, En.T.). 



In the tympanic cavity are three ossicles — the malleus (m.), 

 incus (i.), and stapes (s.), forming a chain between the mem- 



