MIDDLE EAR 



201 



insects. This is its main function, (ii) The sound waves are 

 conducted by reflection from the walls without loss of intensity, 

 and directed almost perpendicularly on to the drum which lies 

 at an angle of 150 to the axis of the canal. 



2. Middle ear. The mechanism found in the middle ear con- 

 verts vibrations in air into vibrations in fluid by means of mem- 

 branes and a series of levers. It consists of an air-filled cavity 

 hollowed out of the petrous part of the temporal bone. It is 

 separated from the external ear by the tympanic membrane, and 



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FIG. 43. Diagrammatic viewof auditory organ. (After Schafer.) 

 1, Acoustic nerve ; 2, internal acoustic meatus ; 6, canalis media of cochlea ; 

 9, vestibule containing lymph; 12, stapes; 13, fenestra cochleae (rotunda); lit, 

 incus; 18, malleus ; 17, membrana tympani ; 16, external acoustic meatus ; 14, 

 auricle or pinna ; 23, Eustachian or auditory tube. 



from the internal ear by the membrane closing the round window 

 and by a disc of bone the foot of the stapes, which along with the 

 membranous collar surrounding this bone makes a fluid-tight 

 packing or gland filling the fenestra ovalis, the oval opening into 

 the internal ear. Between the drum and the stapes lie two bony 

 levers the malleus and the incus. 



(a) Membrana tympani. This structure is fixed in a frame 

 of bone which is almost circular (vertical diameter 10 mm. ; hori- 

 zontal diameter 8-5 mm.). Although it is not more than 0-1 mm. 



