THE SENSES. 201 



The external and middle parts do not communicate directly, there 

 being interposed between the two a thin membrane (d), attached to 

 the bony sides of the canal, exactly like the parchment on a real 

 drum. On this membrane the vibrations of the air strike, and to it 

 there is attached a chain of small bones (Z>), which are also con- 

 nected with the internal ear, in which last is placed the nerve of 

 hearing. The vibrations, therefore, first strike the membrane of the 

 drum, and then pass along these bones to the auditory nerve, seen 

 in the figure, n. The cavity of the drum (), though it does not 

 communicate with the external ear, yet has air admitted to it. This 

 passes through a canal (e), called the Eustachian tube, which opens 

 into the back part of the throat or pharynx. Most persons have 

 felt their hearing become dull when inflammation of the throat 

 closes this tube, and prevents the passage of the air. The internal 

 ear is very intricate, and the uses of its different parts are not well 

 known. In the figure are seen parts of it called semicircular canals 

 (s), the cochlea (A:), the vestibule (t>), which are all filled with a fluid, 

 and the auditory nerve (n), going to these parts. 



The Ear, 



c, corcha or external gristle, m, csnal leading to t, the tympanum or drum, d, membrane of 

 the drum. 6, small bone of the drum, r, vestibule. 3, semicircular canals, ft, cochlea, n audi- 

 tory nerve, e, Eustachian tutxj. 



Of the parts described, it would seem that the internal ear is the 

 only one that is essential, for cases have occurred in which disease 

 has destroyed both the membrane of the drum and the small bones, 

 and yet hearing has remained. It is a curious observation, made 

 by Dr. Wollaston, that there are persons, of whom he himself was 

 one, who are insensible to very acute sounds, though all others are 

 perfectly heard. Some cannot hear the note of the bat or the chirp 

 of the grasshopper, while others are insensible even to the chirping 

 of the sparrow. 



