226 ANATOMY FOK NUKSES. [CHAP. XIX. 



specialized epithelium, some of the cells of which have hair-like 

 processes projecting into the fluid, others have tiny grains of 

 calcareous sand 1 attached to their surfaces, and all of them are 

 connected with the thousands of nerve-fibres into which the 

 auditory nerve divides after entering the internal ear. The 

 construction of the whole ear and its position in the centre of a 

 bone is for the purpose of protecting this delicate and highly 

 organized membrane, by means of which alone sound-impres- 

 sions can be advantageously conveyed to the brain and give rise 

 to the sensation of hearing. 



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 way 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 are communicated to the specially modi- 

 fied cells connected with the endings of the auditory nerve, and 

 nervous impulses are conveyed by the auditory nerve to those 

 parts of the brain, stimulation of which gives rise to the sensa- 

 tion 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 



1 In the walls of the vestibule are two small masses, called otoliths, which are 

 composed of grains of calcareous sand. It is not known what special part these 

 otoliths play in the production of sound. 



