TOUGH. 331 



labyrinth. The outmost bone is the malleus ; the inmost, 

 the stapes j and the middle bone, the incus. 



The Internal Ear, or Labyrinth, consists primarily of 

 chambers and tubes hollowed out in the temporal bone. 

 The middle chamber, called the vestibule ( V, Fig. 95), has 

 an opening, the oval foramen, o, in its outer side, into which 

 the inner end of the stapes, or stirrup-bone, fits. Behind, 

 the vestibule opens into three semicircular canals, one of 

 which is shown at B, Fig. 95; and in front into a spirally 

 coiled tube, S, the cochlea. In these bony chambers and 

 tubes lie membranous chambers and tubes, in which the 

 fibres @f the auditory nerve (A, Fig. 95) end. All the 

 labyrinth chamber outside these membranous parts is 

 occupied by a watery liquid, known as perilymph; the 

 membranous chambers are filled with a similar liquid, the 

 endolymph. 



When sound-waves of the air make the tympanic mem- 

 brane vibrate, it shakes the tympanic bones; the stapes 

 then shakes the liquids in the labyrinth, and sets up vibra- 

 tions in them, which excite the endings of the auditory 

 nerve. The stimulated auditory nerve then conveys a 

 nervous impulse to the brain-centre of hearing and excites 

 it, and a sensation of sound results-. 



Touch, or the Pressure Sense. Many sensory nerves end 

 in the skin, and through it we get several kinds of sensa- 



Of what does the internal ear primarily consist? What is the 

 vestibule? 



What is found on the outer side of the vestibule? 



Into what does the vestibule open behind? In front? What lie 

 in the bony cavities of the labyrinth? What is the perilymph? The 

 endolymph? 



What happens when sound-waves set the tympanic membrane in 

 vibration? 



