CHAPTEK LYI 



HEARING 



Anatomy of the Ear. 



THE Organ of Hearing (tig. 486) is divided into three parts, (1) the 

 external, (2) the middle, and (3) the internal ear. 



External Ear. The external ear consists of the pinna and the 

 external auditory meatus. The central hollow of the former 

 is named the concha. The auditory canal, with a slight 

 arch directed upwards, passes inwards and a little forwards to 

 the membrana tympani, to which it thus serves to convey the 

 vibrating air. 



Middle Ear or Tympanum. The middle ear, or tympanum or 

 drum (3, fig. 486), is separated by the membrana tympani from 

 the external auditory meatus. It is a cavity which communi- 

 cates posteriorly with the mastoid cells in the mastoid pro- 

 cess of the temporal bone; but its only opening to the external 

 air is through the Eustachian tube (4, fig. 486). The walls of the 

 tympanum are osseous, except where apertures in them are closed 

 with membrane, as at the fenestra rotunda, and fenestra ovalis, and 

 at the outer part where the bone is replaced by the membrana 

 tympani Its cavity is lined with mucous membrane, which is 

 continuous through the Eustachian tube with that of the pharynx. 

 A chain of small bones extends from the membrana tympani to 

 the fenestra ovalis. 



The membrana tympani is placed in a slanting direction at the 

 bottom of the external auditory canal, and consists of fibres, some 

 running radially, some circularly ; its margin is set in a bony groove ; 

 its outer surface is covered with a continuation of the cutaneous 

 lining of the auditory canal, its inner surface with the mucous 

 membrane of the tympanum. 



The ossicles are three in number; named malleus, incus, and 

 stapes. The malleus, or hammer-bone, has a long slightly-curved 

 process, called its handle, which is inserted vertically between the 



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