Phonoreception 



481 



three outer rows. The neurones of the auditory nerve are bipolar, have 

 their cell bodies in the spiral ganglion which lies within the modiolus, and 

 are between 25,000 and 29,000 in number. Each inner hair cell is innervated 

 by one or two nerve fibers, and each of these nerve fibers is connected to one 

 or two hair cells. The external hair cells, however, have a multiple innerva- 

 tion. A nerve fiber may connect with many external hair cells, which are 

 arranged over as much as one-half a turn, and each hair cell may be connected 

 to several nerve fibers (Fig. 155). The inner hair cells, because of their 

 connections to only one or two dendrites, may be used for more exact dis- 

 crimination of frequency than the more numerous outer hair cells. 



The axons of the ganglion cells form the cochlear branch of the eighth 

 cranial nerve. The fibers of this nerve are twisted around each other in a 

 very orderly fashion, somewhat like the fibers of a rope. Those from the 



db below 2v 

 from oscillator 



10 

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 50 

 60 

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no 



100 



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1000 2000 4000 

 frequency 



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Fig. 159. Curves showing the frequency response of four different auditory nerve fibers 

 as a function of intensity. Note that the range of frequencies capable of exciting a given 

 fiber increases with increasing intensity'. After Galambos and Davis,^' from Fulton. ^^ 



region about one fourth of the length from the basilar membrane are not 

 twisted and form the core of the rope. Those coming from the apex are 

 twisted in one direction and those from the base are twisted in the other. 

 The genesis of this arrangement is readily understood from an embryological 

 viewpoint, for the fibers merely follow the organ of Corti as it grows into its 

 final spiral form. 



On entering the dorsolateral wall of the pons at its junction with the 

 medulla, the axons of the cochlear nerve pass to the cochlear nuclei. From 

 the dorsal and ventral cochlear nuclei impulses pass by way of the lateral 

 lemniscus to the inferior colliculus of the midbrain and through the medial 

 geniculate body to the temporal lobe of the cerebral cortex. Local exploration 

 of each of the stations of the auditory pathway indicates fairly precise pro- 

 jection of the cochlea. 



