THE SPECIAL SENSES 313 



are seen to lie scattered upon and among the delicate filaments 

 of the auditory nerve ; and it is probable that, as the tremulous 

 sound-wave traverses the fluid of the vestibule, the sand rises 

 and falls upon the nerve-filaments, and thus intensifies the 

 sonorous impression. 



103. In the cochlea, or snail's shell, which contains the 

 fluid, but no membrane, the nerve branches upon a spiral 

 shelf, which, like the cochlea itself, takes two and. a half 

 turns, growing continuously smaller as it winds upward. As 

 many as three thousand nerve-fibres of different lengths have 

 been counted therein; these, it has been thought, form the 

 grand, yet minutely small key-board, upon which strike all the 

 musical tones that are destined to be conveyed to the brain. 

 The vestibule, it is also supposed, takes notice of noise as 

 distinguished from musical sounds; while the office of the 

 semicircular canals is, in part at least, to prevent internal 

 echoes, or reverberations. 



104. The vestibule communicates with the chain of bones of 

 the middle ear by means of a small opening, called the " oval 

 window," or fenestra ovalis. Across this window is stretched 

 the membrane, which has already been alluded to as being 

 joined to the stirrup-bone of the middle ear. Through this 

 window, then, the sound-wave, which traverses the external 

 and middle ear, arrives at last at the labyrinth. The limpid 

 fluid which the latter contains, and which bathes the terminal 

 iibres of the nerve of hearing, is thus agitated, the nerve-fibres 

 are excited, and a sonorous impression is conducted to the 

 brain, or, as we say, a sound is heard. 



105. Protection of the Sense of Hearing. From what has 

 been seen of the complicated parts which compose the organ of 

 hearing, it is evident that while many of them possess an 

 exquisite delicacy of structure, Nature has well and amply 

 provided for their protection.- We have observed the concealed 



108. In the cochlea, or snail's shell? " Key -board " in the internal ear ? The vestibule f 

 Semicircular canals ? 



104. With what does the vestibule communicate f What is the method by which sound 

 It conducted to the brain ? 



1JC* TL formation of the organ of hearing with a view to its protection* 



