HEARING 



411 



secreted by its wall serves to attach particles of dust, and to 

 deter insects from entering the tube. The air at the end of it 

 is at a uniform temperature. It is closed by the membrana 

 tympani, or drum. This membrane receives the vibrations of 



Fia. 38. THE EXTERNAL, MIDDLE, AND INTERNAL EAR OP THE LEFT SIDE. 



From right to left, the figure shows the concha and^lobule of the ear in profile ; the external 

 meatus (abbreviated) ; the drum, divided vertically, its posterior half visible ; the hammer- 

 bone, with the tip of its long arm attached to the drum, an arrow indicating the point of 

 attachment and line of action of the tensor tympani muscle ; the anvil attached by a 

 ligament to the bony wall of the middle ear ; the stirrup, with its foot-plate almost filling 

 the oval window ; the labyrinth, with the three semicircular canals above, and the scala 

 vestibuli below. The curled black line shows the situation of the scala media, or ductus 

 cochleae (which contains the organ of Corti). Pulsations of sound which move the mem- 

 brana tympani are transmitted by the three bones to the oval window. They shake the 

 perilymph, producing waves which travel along the scala vestibuli to the apex of the 

 cochlea, whence they return by the scala tympani to the round window (if they do not 

 take a shorter course through the ductus cochleae). The Eustachian tube opens out of the 

 lower part of the middle ear. 



sound ; and, in order that it may collect them with absolute 

 impartiality, it is in every respect the opposite in shape and 

 structure to the top of a drum. The stretched parchment 

 which covers a drum is flat. Its tension is uniform in all its 



