THE EAR. 509 



THE MIDDLE EAR. 



The middle ear or tympanum consists of a small cavity 

 lying between the external auditory meatus and the inner 

 ear. Its size in man is from three-quarters of an inch in its 

 widest dimension to about a half inch in the direction at 

 right angles to this. It is, therefore, although a small 

 opening almost as large as the entire internal ear, or in 

 space equal to the external auditory meatus. 



1. Eustachian Tube. This cavity is filled with air 

 which is able to reach it through the Eustachian tube. 

 This tube runs from the middle ear and opens into the back 

 part of the mouth. By means of this tube the pressure of 

 air in the tympanum is kept the same as that of the sur- 

 rounding atmosphere, and so barometric changes in it do 

 not exert their effect upon the tympanic membrane as 

 would otherwise happen. Through this Eustachian tube 

 the mucous membrane of the middle ear is a continuation 

 of the mucous membrane of the mouth, a condition which 

 unfortunately makes possible the extension of an inflam : 

 mation into the middle ear from the mouth, such as occurs 

 in some forms of catarrh. An inflammation of the middle 

 ear and Eustachian tube soon causes the closing of the 

 Eustachian tube by its swelling, and thus the only exit 

 for the matter which may be produced is by rupturing the 

 tympanic membrane and escaping through the auditory 

 meatus. Such a result is usually designated as a gathering 

 in the head. The Eustachian tube is usually closed, the 

 two walls of it are pressed together and separate regularly 

 only during the process of swallowing. The explanation 

 of the regularly closed condition is found in the fact that 

 such a closing prevents the sound from entering the middle 

 ear by that avenue, and so to some extent interfering with 

 the sound reaching it regularly, and, secondly, that it pre- 

 vents the sounds produced in the mouth (the voice) , from 

 reaching the ear with the force that they would do if this 

 canal were open. 



