188 THE HUMAN BODY 



inner side the Eustachian tube (R) proceeds to the pharynx, and 

 the mucous membrane of that cavity is continued up the tube to 

 line the middle ear; the proper tympanic membrane composed 

 of connective tissue is therefore covered by mucous membrane on 

 its inner, as it is by very thin skin on its outer, side. In the bony 

 inner wall of the middle ear are two small apertures, the oval and 

 round foramens, o and r, which lead into the labyrinth. During 

 life the round aperture is closed by the lining mucous membrane, 

 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. 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 Auditory Ossicles. Three small bones lie in the middle 

 ear forming a chain from the drum membrane to the oval fora- 

 men. The external bone (Fig. 70) is the malleus or hammer; the 

 middle one, the incus or anvil; and the internal, the stapes or 

 stirrup. The malleus, M, has an upper enlargement or head, 

 which carries on its inner side an articular surface for the incus; 

 below the head is a constriction, the neck, and below this two 

 processes complete the bone; one, the long or slender process, is 

 imbedded in a ligament which reaches from it to the front wall of 

 the tympanic cavity; the other process, the handle, reaches down 



