TASTE, SMELL, AND HEARING 327 



595. The middle ear. To make hearing distinct, a 

 special mechanism is provided for transmitting the air 

 vibrations to the inner ear through two outer cavities. 



A small aperture connects the inner ear with a middle 

 cavity called the middle ear, or tympanum. The middle 

 ear is half an inch long and a quarter of an inch broad. 

 It is lined with mucous membrane and is filled with air. Its 

 outer end is closed like a drum, by a thin leaf called the 

 drum membrane or membrana tympani, while a similar mem- 

 brane closes the aperture to the inner ear. The cavity of 

 the middle ear is greatly increased by its extending back- 

 ward into a bony projection called the mastoid process, 

 which can be felt just behind the outer ear. It connects 

 with the pharynx by means of a tube which is about the 

 size of a knitting needle and is called the Eustachian 

 tube. The act of swallowing opens the tube. 



596. Bones of the middle ear. The essential part of the 

 middle ear is a chain of small bones called the malleus, 

 incus, and stapes, which extend across its cavity from one 

 membrane to the other. 



Air waves, striking the ear drum, throw it into vibra- 

 tions, which the chain of bones transmits to the inner ear. 

 The tympanum and its extension into the mastoid cells 

 act like the sounding box of a violin to increase the 

 vibrations. 



597. Deafness. The Eustachian tube permits air to pass in and 

 out of the middle ear so as to keep the air pressure within the same as 

 it is outside. When it is closed, the air pressure outside may change, 

 and thus the drum membrane will be pressed upon and prevented from 

 vibrating freely. This results in partial deafness. Enlarged tonsils 

 and adenoid vegetations are liable to cause a stoppage of the tube and 

 to produce deafness, and for this reason they should always be removed. 

 When the tube is stopped, there is a feeling of fullness in the ear, and 

 roaring or singing noises will be heard. 



