MIDDLE-EAR CAVITY 261 



meatus, a horizontal canal containing two slight bends, the first of 

 which is directed upward and forward and the more internal portion is 

 directed backward and downward. Its lower wall forms with the tym- 

 panic membrane an acute-angled depression. (See Fig. VII-8.) The 

 canal is lined by a prolongation of the external skin, which also forms 

 with a greatly diminished thickness the outer surface of the tympanic 

 membrane. The tympanic membrane closes the canal and acts as a 

 partition separating the outer from the middle ear. 



II. The middle ear is essentially an air cavity lying in the temporal 

 bone, lined with very thin mucous membrane. It contains the auditory 

 ossicles and can be connected with the external air by way of the Eus- 

 tachian tube, which communicates with the cavity of the nasopharynx. 



III. The internal ear is contained in a bony cavity lying in the petrous 

 portion of the temporal bone (Fig. VII-9). This cavity contains two 

 chambers: one, called the vestibule, contains a series of organs which 

 concern equilibrium (semicircular canals), but which have no connection 

 with the hearing process; and the adjoining cavity called the cochlea. 



Pinna 



The surface of the pinna is not a reflector of sound, for reflection can 

 take place only from a surface which is large in comparison with the 

 incident wavelength. The pinna can scatter acoustic waves so that some 

 of the scattered energy enters the ear canal. When a cupped hand is 

 held near the pinna, passing acoustic waves are scattered by the hand 

 and enter the ear canal, making the sound louder. Bringing the cupped 

 hand in contact with the ear creates a resonance cavity composed of the 

 palm of the hand, pinna, and ear canal. The response of this composite 

 resonator may be heard. 



External Auditory Canal 



The external opening of the auditory canal has an area of 0.3 to 0.5 sq 

 cm, and according to Bekesy [1932] it has an average volume of 1.04 cc, 

 an average depth of 2.7 cm, and an average diameter of 0.7 cm. 



If the resonance frequency of this cavity is calculated on the assump- 

 tion that it is a Helmholtz resonator without neck, it is found to pos- 

 sess a fundamental frequency of 430 cycles. Whether the resonance 

 frequency of the meatus contributes anything to the hearing process is 

 not known. 



Middle-Ear Cavity 



The middle-ear cavity, also called the tympanic cavity, is separated 

 from the external ear by the eardrum, or tympanic membrane. The 



