228 THE HUMAN BODY 



and the oval in another way, to be described presently. The 

 tympanic membrane, T, stretched across the outer side of the 

 middle ear, forms a shallow funnel with its concavity outwards. 

 It is pressed by the external air on its exterior, and by air enter- 

 ing the tympanic cavity through the Eustachian tube on its inner 

 side. If the middle ear were closed the pressures on the inner and 

 outer sides of the drum membrane would not be always equal 

 when barometric pressure varied, and the membrane would be 

 bulged in or out according as the external or internal pressure on 

 it were the greater. This unequal pressure would interfere se- 

 riously with the freedom of vibration of the membrane and so 

 impair hearing. On the other hand, were the Eustachian tube 

 always open the sounds of our own voices would be loud and dis- 

 concerting, so it is usually closed; but every time we swallow it is 

 opened, and thus the air-pressure in the cavity is kept equal to 

 that in the external auditory meatus. By holding the nose, keep- 

 ing the mouth shut, and forcibly expiring, air may be forced un- 

 der pressure into the middle ear, and will be held in part im- 

 prisoned there until the next act of swallowing. On making a 

 balloon ascent or going rapidly down a deep mine, the sudden 

 and great change of aerial pressure outside frequently causes 

 painful tension of the drum membrane, which may be greatly 

 alleviated by frequent swallowing movements. 



The great importance of the Eustachian tubes in hearing is 

 illustrated by the deafness that results from their continued closure. 

 This condition is frequently brought about in children by the 

 growth of adenoids (see p. 383) in the back of the throat, which 

 press upon and close the Eustachian tubes. 



Essential as these tubes are for good hearing they constitute a 

 frequent source of ear trouble. The congestion of the mucous 

 membranes of the throat and nose in a "cold in the head" is apt 

 to involve the Eustachian tubes and the lining of the middle ear. 

 Sometimes an exudate from the congested membranes fills the 

 middle ear completely, and by its pressure causes acute pain, as 

 well as deafness. Unless relief is obtained the tympanic membrane 

 may be ruptured. Earache resulting from a cold should therefore 

 not be neglected. Less commonly but more seriously actual in- 

 fection (p. 306) of the middle ear may occur, the infection invading 

 the region by way of the Eustachian tubes. The mastoid process 



