306 PHYSIOLOGY. 



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 Eustachian Tube. The middle ear communicates 

 with the pharnyx by means of a narrow tube called the 

 eustachian tube. It admits air to equalize the pressure on 

 the two sides of the tympanic membrane. This tube is 

 probably closed most of the time, but opens when we 

 swallow. 



The Internal Ear. 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 con- 

 nected with an exceedingly complicated apparatus. 



The Production of Sound. Sound waves set the drum 

 skin or membrane of the tympanum 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. 



Only that part of the auditory nerve which is distributed in the 

 " snail shell " of the ear is now supposed to have to do with hearing. 

 It is no longer believed that the semicircular canals are concerned with 

 the process of hearing. There seems to be good evidence that the 

 semicircular canals inform us as to changes of the position of the body, 

 and they are regarded as the seat of an " equilibrium sense." The fact 

 that one of these canals is horizontal, and that the two vertical canals 



