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A MANUAL OF PHYSIOLOGY 



So far as we know, what takes place in the ear is essentially similar 

 that is to say, a mechanical stimulation of the ends of the auditory 

 nerve, but a stimulation which acts through, and is graduated and 

 controlled by, a special intermediate mechanism. 



As the visual apparatus consists of a sensitive surface, the 

 retina, which contains the end-organs of the optic nerve and of 

 dioptric arrangements which receive and focus the rays of light, 

 the auditory apparatus consists of the sensitive end-organs of 

 the cochlear division of the eighth nerve and of a mechanism 

 which receives the sound-waves and communicates them to these. 



Physiological Anatomy of the Ear. At the bottom of the external 

 auditory meatus lies the membrana tympani, a nearly circular 

 membrane set like a. drum-skin in a ring of bone, and separating 



the meatus from the tympa- 

 num or middle ear. Its ex- 

 ternal surface looks obliquely 

 downwards, and at the same 

 time somewhat forwards, so 

 that if prolonged the mem- 

 branes of the two ears would 

 cut each other in front of, 

 and also below, the hori- 

 zontal line passing through 

 the centre of each (Figs. 

 419, 420). 



The tympanum contains a 

 chain of little bones stretch- 

 ing right across it from outer 

 to inner wall. Of these the 

 malleus, or hammer, is the 

 most external. Its manu- 

 brium, or handle, is inserted 

 into the membrana tympani, 

 which is not stretched taut 

 within its bony ring, but 

 bulges inwards at the centre, 

 where the handle of the mal- 

 leus is attached. The stapes, 

 or stirrup, is the most internal 

 of the chain of ossicles, and 



is inserted by its foot-plate into a small oval opening the fora- 

 men ovale on the inner wall of the tympanic cavity. A mem- 

 branous ring the orbicular membrane surrounds the foot of the 

 stapes, helping to fill up the foramen and attaching the bone ta 

 its edges. The inner surface of the foot of the stapes is in contact 

 with the perilymph of the internal ear. The incus, or anvil, forms 

 a link between the malleus and the stapes. The auditory ossicles, 

 as well as the whole cavity of the tympanum, are covered by pave- 

 ment epithelium. 



The tympanum is not an absolutely closed chamber ; it has one 

 channel of communication with the external air the Eustachian 

 tube which opens into the pharynx. By the action of the cilia 

 lining this tube the scanty secretion of the middle ear is moved 

 towards its pharyngeal opening, which, usually closed, is opened 



c I 



FIG. 419. THE EAR. 



m, external meatus ; /, head of malleus ; 

 o, short process of malleus ; g, handle of 

 malleus 3 -; h, incus ; i, foot of stapes in oval 

 foramen ; e, tympanic membrane. 



