THE SPECIAL SENSES 311 



takes place, the orifice in the throat is stretched open, and the 

 air of the cavity of the tympanum may then be renewed. Air 

 may at will be made to enter through this tube, by closing the 

 mouth and nose, and then trying to force air through the latter. 



FIG. 72. SECTION OF THE RIGHT EAR 



A, The Concha E, Incus, or Anvil 



B, Auditory Canal M, Malleus, or Mallet 



C, Membrane of the Drum I, Eustachian Tube 



(the lower half) G, Semicircular Canals 



D, A Small Muscle H, Cochlea, or Snail's Shell 



When this is done, a distinct crackle or clicking sound is per- 

 ceived, due to the movement of the membranes, and of the 

 little bones of the ear. 



100. The Eustachian tube serves, also, as an escape-pipe for 

 the fluids which form within the middle ear ; and hence, when 

 its lining membrane becomes thickened, in consequence of a 

 cold or sore throat, and the passage is thus more or less choked 

 up, the fluids are unable to escape as usual, and therefore 

 accumulate within the ear. When this takes place, the vibra- 

 tions of the membrane are interfered with ; the sounds heard 



100. What other purpose does the Eustachian tube serve ? How is this shown ? 

 " Throat-deafness " ? Primary use of the Eustachian tube f 



