24 



Sound and the Ear /I : 4 



these are essential for hearing, although all are desired effects. It is 

 possible to hear without a tympanic membrane and without ossicles. 

 There is a hearing loss under these conditions, but this loss is com- 

 parable to the variations in the normal range of hearing thresholds. 

 However, the two windows to the inner ear, one of which is driven much 

 more than the other by the incident wave, are necessary for hearing. 



The inner ear consists of several portions all having two common fluids, 

 and all served by the eighth cranial nerve. Only the cochlear portion 

 of the inner ear is associated with hearing. Grossly, the cochlea is a 



Tectorial Membrane 



(a) 



r wmnnnir Membrane - : -' ; ^wfi& 

 tympanic . ■.--,: z-yMf Auditory 



Nerve 



(b) 



Figure 1 0. (a) The cochlea or inner ear removed from the bone. 

 (b) Cross section through one turn of the cochlea. The tym- 

 panic and vestibular canals are filled with perilymph and the 

 cochlear canal with endolymph. After A. J. Carlson and V. 

 Johnson, The Machinery of the Body (Chicago: The University 

 of Chicago Press, 1941). 



spiral; in the human there are two and a half complete turns. Around 

 this spiral run three parallel, fluid-filled ducts. These are illustrated in 

 Figures 10 and 11. The fluid in the tympanic and vestibular ducts is 

 called the perilymph. These two ducts (or scalae) are connected at the 

 apex of the spiral through a small duct called the helicotrema. Somewhat 

 sandwiched between these two ducts is the cochlear duct (or scala media). 

 It is filled with a fluid, similar to that of the other two, called the 

 endolymph. The endolymph and perilymph are anatomically and 

 electrically separated from each other. Between the cochlear duct and 

 the vestibular duct is a very thin fibrous membrane known as Reisner's 

 membrane. Between the cochlear duct and the tympanic duct is a thicker 

 membrane called the basilar membrane. The basilar membrane gets 



