412 THE BODY AT WOEK 



parts. Movements have the greatest amplitude at the centre. 

 Every precaution is taken to insure its emitting, with as little 

 confusion as may be, the particular note to which it is tuned. 

 The drum of the ear is shaped like the mouth of a trumpet, 

 depressed to a point, but convex from this point outwards. 

 Its elastic fibres, which are partly radial, partly circular, are 

 at many different tensions. Its deepest part, to which the 

 long arm of the hammer-bone is attached, is not its centre. 



The " middle ear " is an irregular cavity communicating 

 with the pharynx by the Eustachian tube. It is filled with 

 air at the same pressure as the atmosphere. Except during 

 the act of swallowing, when it is at first shut tightly and then 

 opened, the pharyngeal end of the Eustachian tube is gently 

 closed. When one is dropped in a lift rapidly down the shaft 

 of a mine, the difference in pressure between the external air 

 and the air in the middle ear stretches the drum to such an 

 extent that deafness to low tones is produced. Conversation 

 becomes inaudible. The deafness is remedied by swallowing 

 saliva, and thus opening the end of the Eustachian tube. The 

 commonest cause of permanent deaf ness is inflammation followed 

 by thickening of the mucous membrane of the lower end of the 

 Eustachian tube, with its consequent closure, due to frequent 

 sore- throats. The air in the middle ear is slowly absorbed. 

 It needs to be constantly renewed through the Eustachian 

 tube. 



On the inner wall of the middle ear are two small apertures 

 the oval window and the round window. Both are closed 

 with membrane. Into the oval window is fitted the sole-plate 

 of the stirrup-bone. Three bones hammer, anvil, and stirrup 

 combine in transferring the movements of the membrana 

 tympani to the oval window. They constitute a jointed lever, 

 which swings about an axis passing through the ligament of 

 the anvil (Fig. 38), the excursions of the long arm of the 

 hammer being reduced in amplitude by one-third at the 

 stirrup- plate. As the oval window has only one- twentieth of 

 the area of the drum, the movements of the latter are trans- 

 mitted with concentrated force. Two points in the mechanism 

 of these bones may be specially noticed : (1) The head of the 

 hammer is free to rotate in the cavity of the anvil, checked by a 

 cog. Every inward movement of the drum is faithfully trans- 



