262 AUDITORY BIOPHYSICS 



mean volume of this cavity as measured by Bekesy [1936] was found to be 

 2.0 cc. The ossicles and their suspensory ligaments occupy from 0.5 to 

 0.8 cc. In a study by the author of the middle-ear mechanism it was 

 found that the bones themselves occupy only about 7 per cent of the 

 volume of the cavity. The air cavity is therefore small so that its reson- 

 ance frequency is high. Experimentally the natural frequency of the 

 middle-ear cavity has been found to lie between 800 and 1500 cycles. 



In order to change the air pressure in this cavity the nasal end of the 

 Eustachian tube may be opened. The effect of changes in external air 

 pressure experienced in moving elevators and airplanes, or on entering 

 a compressed-air lock, may be relieved by swallowing. This action 

 opens the Eustachian tube and allows the pressure on both sides of the 

 eardrum to equalize. 



TABLE VII-1 



Wrightson and Keith's Dimensions op the Tympanic Membrane 



Quadrant Area, mm 2 Mean Radius 



Vertical diameter 11.8 Mean area* 



Horizontal diameter 8.9 52-90 mm 2 



* • 



♦Wrightson and Keith [1918]; Frank [1923]; Troger [1930]; and Geffcken [1934]. 



Tympanic Membrane 



The so-called eardrum, or tympanic membrane, is a very thin, dense 

 partition forming the boundary between the external-ear and middle- 

 ear cavities. The area assigned to the tympanic membrane by various 

 investigators varies from 0.52 to 0.90 sq cm, Wrightson's value of 0.66 

 herewith being accepted as a mean. Values adopted by Wrightson and 

 Keith [1918] for the various detailed dimensions of the tympanic mem- 

 brane are shown in Table VII-1. It is about 0.1 mm thick and composed 

 of three layers of tissue. Its outer surface is a thin continuation of the 

 external layer of skin; its inner surface, a continuation of the mucous 

 membrane of the middle-ear cavity. Between these membranes is a thin 

 fibrous layer. 



The fibrous layer is built up of circular and radial fibers. The density 

 of distribution of the circular fibers is more pronounced at the center and 

 near the periphery. The radial fibers terminate in the fibrocartilaginous 



