The Special Senses 267 



These little bones are connected with each other and the 

 tympanum by ligaments and moved by three tiny muscles. 

 Two are attached to the hammer and tighten and relax the 

 drum ; the other is attached to the stirrup and prevents it 

 from being pushed too deeply into the oval window. 



Experiment 121. To 'vibrate the tympanic membrane and the 

 little ear bones. Shut the mouth and pinch the nose tightly. Try to 

 force air through the nose. The air dilates the Eustachian tube and 

 is forced into the ear drum. The distinct crackle, or clicking sound, 

 is due to the movement of the ear bones and the tympanic membrane. 



419. The Internal Ear. The internal ear, one of the 

 most delicate and complex pieces of mechanism in the 

 whole body, receives the impression of sound and carries 

 it directly to the seat of consciousness in the brain. 



The internal ear, or labyrinth, consists of three distinct 

 parts, or variously shaped chambers, hollowed out in the 

 temporal bone, the vestibule, the semicir- 

 cular canals, and the cochlea, or snail's shell 

 (Figs. 144 and 145). 



420. The Vestibule. The vestibule is an 

 oval-shaped chamber, about one-fifth of an 

 inch in diameter, occupy- 

 ing the middle part of the 



internal ear. It is on the 

 inner side of the oval win- 

 dow, which is closed, as 

 we have seen, by the stir- FlG - I 4 2 - A Cast of 



, T^, . , the External Audi- -^sm^^ 



rupbone. From one side tory Canal . (Posterior view .) 

 of this vestibule, or cen- 

 tral hall, the three semicircular canals pass off, and from 

 the other side the cochlea. 



_, 421. The Semicircular Canals. The three semicircular 

 canals, so called from their shape, are simply bony tubes 



