558 THE HUMAN BODY. 



From its inner side the EustacMan tube (R) proceeds to the 

 pharynx, and the mucous membrane of that cavity is con- 

 tinued up the tube to line the tympanum; the proper tym- 

 panic 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 

 tympanum are two small apertures, the oval and round fora- 

 mens, 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 tym- 

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

 tympanum, forms a shallow funnel with its concavity out- 

 wards. It is pressed by the external air on its exterior, and 

 by air entering the tympanic cavity through the Eustachian 

 tube on its inner side. If the tympanum 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 disconcerting, 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 

 under pressure into the tympanum, and will be held in part 

 imprisoned 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 fre- 

 quently 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 

 tympanum forming a chain from the drum membrane to the 

 oval foramen. The external bone (Fig. 163) 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 com- 

 plete the bone; one, the long or slender process, is im- 

 bedded in a ligament which reaches from it to the front 

 wall of the tympanum; the other process, the handle, 



