AUDITORY SENSATIONS. 607 



The basilar membrane is made up of fibrous bands held together 

 by a delicate membrane. The fibres pass transversely across the 

 spiral canal of the cochlea, so as to subtend the bases of the outer 

 and inner rods. The basilar membrane gradually becomes wider 

 as it passes from the base to the summit of the cochlea. The 

 length of the rods also increases towards the summit of the organ, 

 their bases being more widely separated from one another and 

 their point of junction nearer to the basilar membrane, so as to 

 form a lower and wider tunnel. The entire number of rods of 

 Corti has been estimated at 3000. 



STIMULATION OF THE AUDITORY NERVE. 



The stimulation of the nerve of hearing by sound-vibrations of 

 the air is less difficult to understand than the excitation of the 

 optic nerve by light-waves which are conveyed by an imponderable 

 medium. The motions of the membrane of the drum, being con- 

 veyed in the manner already indicated to the liquids within the 

 internal ear, pass first over and then under the cells connected 

 with the nerve terminals, which are placed on the elastic basilar 

 membrane. The transverse fibres are set in motion by the waves 

 in the fluid, and as they vibrate they communicate the motion to 

 the rods of Corti. The bases of the inner rods, being fixed at the 

 inner margin of the basilar membrane, can move but little, and 

 the bases of the outer rods being placed near the middle of the 

 fibres of the membrane, where the motion of the vibrations is most 

 extensive, a slight change in their relative positions, and a con- 

 sequent movement of the apex of the bow, must take place. This 

 movement at the apex of the bow, where the rods join, is commu- 

 nicated by the medium of the reticular membrane to the hairs in 

 the special auditory cells, thence to the nerves, where an excita- 

 tion is produced which gives rise to the transmission of an impulse 

 to the brain. 



But we can distinguish differences of (1) loudness, of (2) pitch, 

 and of (3) quality in sound. 



Since the loudness depends simply on the amplitude of the vi- 

 bration, we can have no difficulty in understanding how varieties 



