Chap. XIX.] ORGANS OF SPECIAL SENSE. 231 



the axones form the trunk of the auditory nerve and pass back 

 to the meduUa oblongata. 



The sense of hearing. — The cochlea consists essentially of a 

 spirally wound canal containing a long series of fibres stretched 

 across it like strings. These fibres increase in length from the 

 base of the cochlea upward, and in their action resemble the 

 wires in a piano, for vibrations of the endo-lymph of a certain 

 rate set up vibrations in the fibres of a certain length. 



All bodies which produce sound are in a state of vibration 

 and communicate their vibrations to the air with which they 

 are in contact, and thus the air is thrown into waves, just as a 

 stick waved backwards and forwards in water throws the water 

 into waves. 



When air-waves, set in motion by sonorous bodies, enter the 

 external auditory canal, they set the drum-membrane vibrating, 

 stretched membranes taking up vibrations from the air with 

 great readiness. These vibrations are communicated to the 

 chain of tiny bones stretching across the middle ear, and their 

 oscillations cause the membrane leading into the internal ear to 

 be alternately pushed in and drawn out, and vibrations are in 

 this w^ay transmitted to the peri-lymph. Each vibration com- 

 municated to the peri-lymph travels as a wave over the ves- 

 tibule, semicircular canals, and cochlea, and is transmitted 

 through the membranous walls to the endo-lymph. The vibra- 

 tions of the endo-lymph stimulate the cochlear nerve endings, 

 and nervous impulses are conveyed by the auditory nerve to 

 those parts of the brain, stimulation of which gives rise to the 

 sensation of sound. 



The effect produced by a sonorous vibration continues for a 

 short time after the cessation of its cause. Usually the interval 

 between two different impulses is sufficient to allow the first 

 impression to disappear before the second is received, and the 

 ear distinguishes them in succession. But if they follow each 

 other at equal intervals, with a certain rapidity, they produce 

 the impression of a continuous sound ; and this sound has a 

 higher or lower pitch according to the rapidity of its vibra- 

 tions. It has been discovered that sound-waves following each 

 other with a rapidit}^ of less than sixteen times per second, are 

 separately distinguishable ; but above that frequency they are 

 mergfed into a continuous sensation. When the sound-waves 



