156 THE SENSES. 



What is the function of the external ear ? 



It serves to receive the sound-waves and to indicate the direction 

 from which they come in animals who possess the power of moving 

 the organ. Through the external auditory canal the sound-waves 

 are conducted to the middle ear. 



Describe the middle ear. 



It is a cavity in the temporal bone which is shut off from the 

 external auditory canal by the tympanum. The Eustachian tube 

 connects this cavity with the pharynx. The lining of the middle 

 ear is ciliated epithelium, continued from the mucous membrane of 

 the pharynx through the Eustachian tube. There are two openings 

 of importance the fenestra rotunda and the fenestra ovalis in the 

 bony wall, but they are covered, the former by membrane, the latter 

 by the stapes. There is a chain of small bones (ossicles) which 

 connect the tympanum and the fenestra ovalis. 



What is the tympanum? 



It is a tough, tense, fibrous membrane set in the bony opening 

 of the external auditory canal. The degree of tension of the tym- 

 panum is regulated by the tensor tympani muscle. 



Fig. 43. 



Right Temporal Bone of the New-born Infant, seen from its inner side, showing the mem- 

 brana tympani and chain of bones in their natural position (Riidinger). 



What is the function of the ossicles ? 



They are three in number, and are so articulated as to commu- 



