I : 4/ Sound and the Ear 



21 



or for the displacement £ 



27TV 



= 10" 9 



cm 



o.i A 



This displacement is smaller than an atomic radius! 



The tympanic membrane forms the outer boundary of the middle ear. 

 The latter is an air-filled space in the temporal bone ; this space is referred 

 to as the tympanic cavity. It has a volume of about 1 ml and an irregular 

 shape. Within this cavity are three small bones or ossicles, which are 



Vestibular Portion 

 of Inner Ear , 



Tensor 

 Tympani 



Malleus 



Tympanic 

 Membrane 



'Eustachian 

 to Pharynx' 

 Stapes Presses on 

 Oval Window to Inner Ear 



Figure 9. The middle ear which is filled with air is connected 

 by two membranes called windows to the fluid-filled canals of 

 the inner ear. The eustachian tube connecting it with the 

 pharynx is even smaller in diameter than is indicated here. 

 Modified from Life : An Introduction to Biology by G. G. Simpson, 

 G. S. P. Hendrigh, and L. H. Tiffany, © 1957, by Harcourt, 

 Brace & World, Inc. 



named according to their shapes. These are the malleus (hammer), 

 the incus (anvil), and the stapes (stirrup). They are illustrated in Figure 

 9. The general purpose of these bones seems to be to help match 

 acoustic properties of the air and the inner ear. The ossicles act as a 

 mechanical transformer and increase the fraction of the incident energy 

 available to excite the mechanisms of the inner ear. 



The bones of the middle ear are so pivoted that they are particularly 

 insensitive to vibrations of the head and to bone-conducted sound waves. 

 One action of the ossicles is to amplify the acoustic pressure of vibrations 

 transmitted from the air via the tympanum, while at the same time 



