EQUILIBRIUM SENSE. 317 



more than an inch, and is then completely shut off from the 

 cavities beyond by a thin membranous partition, the Tympanic 

 Membrane or Drum-skin. The skin of the ear dips into and 

 lines the external tube, and continues as a very thin layer over 

 the membrane of the tympanum. The Auditory Meatus, as 

 this passageway is called, is guarded by hairs, and is further 

 protected by wax secreted by glands of the lining. Beyond 

 the membrane of the tympanum is a cavity called the middle 

 ear. Extending across the cavity of the middle ear is a chain 

 of very small bones, the Hammer, Anvil, and Stirrup, the ham- 

 mer being attached to the inner surface of the membrane of 

 the tympanum, and the stirrup being fastened by its base 

 to the wall of the internal ear. The middle ear communi- 

 cates with the back part of the mouth (pharnyx) by means of 

 a narrow tube called the Eustachian Tube. This tube is prob- 

 ably closed most of the time, but opens when we swallow. 



The internal ear consists of several complicated cavities 

 and tubes which contain a liquid in which rest the nerves. 

 The principal cavity is the Cochlea, or snail-shell cavity, in 

 which the nerve endings are connected with an exceedingly 

 complicated apparatus. 



Sound-waves set the drum-skin or membrane of the tym- 

 panum in vibration ; the vibrations are conveyed by the chain 

 of bones across the middle ear to the liquid of the inner ear. 

 Through the complicated apparatus of the snail-shell the 

 vibrations of the liquid are made to start nerve impulses in 

 the fibers of the auditory nerve, and when these nerve impulses 

 are rightly received and interpreted by the brain, we have a 

 sensation called Sound. 



The Equilibrium Sense. Probably most of the senses 

 contribute to the maintaining of the equilibrium of the body 

 by giving information as to position, motion, etc., especially 

 sight and the muscular sense. 



