536 THE HUMAN BODY. 



The Tympanum. (P, Fig. 143) is an irregular cavity in: 

 the temporal bone, closed externally by the drum mem- 

 brane. From its inner side the Eustachian tube (E) pro- 

 ceeds and opens into the pharynx (g, Fig. 89, page 309), 

 and the mucous membrane of that cavity is continued ii]> 

 the tube to line the tympanum; between this inside, and 

 the skin outside, is the proper tympanic membrane com- 

 posed of connective tissue. The inner wall of the tym- 

 panum is bony except for 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 de- 

 scribed presently. The tympanic membrane, T, stretched 

 across the outer side of the tympanum, forms a shallow 

 funnel with its concavity outwards. It is pressed by the- 

 external air on its exterior, and by air entering the tym- 

 panic cavity through the Eustachian tube on its inner side.. 

 If the tympanum were closed these pressures would not 

 be always equal when barometric pressure varied, and the- 

 membrane would be bulged in or out according as the ex- 

 ternal 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, keeping 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 frequently causes painful tension of the 

 drum membrane, which may be greatly alleviated by fre- 

 quent swallowing. 



The Auditory Ossicles. Three small bones lie in the 

 tympanum forming a chain from the drum membrane to 

 the oval_foramen. The external bone (.b I ig7"i44) is the 

 malleus or hammer; the middle one, the incus or anvil; and 

 the internal, the stapes or stirrup. The malleus, M, has 



