207 



transmission of energy. One can therefore realise how important 

 it is for there to be some open communication between the tym- 

 panic cavity and the atmosphere. By means of the Eustachian 

 tube, communication is established between the middle ear and 

 the pharynx and through the latter with the exterior, and so both 

 sides of the tympanic membrane are kept at atmospheric pressure. 

 Normally, it is closed by an arch of cartilage which surrounds the 

 lower end. The tensor palati muscle is inserted in one side of this 

 arch, and when this muscle contracts during the act of swallowing 

 (Chap. XXVII.), it draws down and flattens the cartilage and so 

 opens the tube. Occlusion of the tube by mucus or by inflam- 

 mation of the throat, isolates the air in the middle ear. The air is 

 gradually absorbed by the tissues, pressure is thus reduced and the 

 drum is sucked inwards. This increased tension in the membrane 

 makes it less responsive to sound. Temporary deafness caused 

 by a rapid alteration in external air pressure (e.g. rising in an 

 aeroplane ; descending in a submarine, or diving in a bell or suit, 

 entering a caisson, etc.) is immediately relieved by movements of 

 swallowing. The Eustachian tube is also the drainage tube to the 

 middle ear, preventing the accumulation of mucus (Figs. 42 and 43). 



Mechanism of the middle ear. 



The function of the mechanism is to transform the alternate 

 condensations and rarefactions of air, which we call sound, into 

 a series of hydraulic movements of the fluid in the internal ear. 

 Direct observation has shown that the ear-bones form a chain 

 of levers which together conduct the vibrations incident on the 

 drum to the foot of the stapes. 



A. Let us look first at the mechanism of the levers. In Fig. 44 

 is given a schematic sketch of the ossicles illustrating their lever 

 action. The axis on which the malleus and incus together turn 

 is represented by the line A-B passing from the tip of the short 

 process of the malleus to the tip of the short process of the incus. 

 A movement inwards of the manubrium will cause the head of 

 the malleus to swing outwards, carrying with it the upper part 

 of the incus and so moving the long process of the incus in the same 

 direction as the manubrium. This movement is directly trans- 

 mitted to the stapes. The chain of bones, therefore, acts as a bent 

 lever whose fulcrum is at a,* the power arm being represented by 

 the dotted line and the load arm by the line i- a. According to 



* a = junction of dotted line with A - B ; i = tip of long process of incus ; p = point 

 of application of power, i.e. tip of manubrium mallei. 



