292 HUMAN PHYSIOLOGY 



while the space between the membranous and the bony laby- 

 rinth is filled with the perilymph. 



The auditory nerve divides into two branches : 



1. The cochlcar nerve, the real nerve of hearing, enters 

 at the axis of the cochlea and in the lamina spiralis ossea 

 spreads out its fibres like a fan. Its fibres finally unite with 

 the hair cells of the organ of Corti (see Fig. 42, A r , ;/ ;/). 



2. The vestibular nerve (see page 295). 



(b) The auditory sensation. The membrana basilaris is 

 set in vibration by the perilymph. By this the cells of Corti 

 are probably mechanically stimulated and thus the auditory 

 sensation is produced. 



Auditory sensations may be classified as tones and noises. 

 The tones (musical) are produced by regular vibrations and 

 may be distinguished by pitch and timbre. The pitch of a 

 musical tone depends upon its number of vibrations. The 

 greater the number of vibrations per second the higher the 

 pitch. The audible tones lie between those having 19 and 

 40,000 vibrations per second (i i^ octaves). The tones used 

 in music lie between those having 33 (contra C) and 4000 

 (a""] vibrations. 



The time which the tone must act in order to be heard 

 depends upon the pitch of the tone. Those of higher pitch 

 need less time than the lower tones. In order to judge of 

 the pitch of a tone, at least 16 single vibrations must strike 

 the ear. If less than 16 vibrations strike the ear. we cannot 

 accurately judge of the pitch. Auditory sensations, how- 

 ever, are still produced if but two single vibrations reach the 

 ear. 



The accuracy of determining the pitch of a tone varies 

 much in different individuals. It depends upon ability and 

 practice. Trained musicians can still discriminate between 

 the pitch of two tones having 1000 and 1001 vibrations 

 (musicians call this T ^ of a whole note). 



The perception of tones of different pitch has been explained by 

 Helmholtz by the resonance theory as follows : 



The membrana basilaris decreases in width as we proceed from 



