HEARING 413 



mitted to the oval window ; but when the drum moves out- 

 wards, the hammer does not necessarily carry the anvil with 

 it. (2) A muscle tensor tympani is inserted near the elbow 

 of the long arm of the hammer. When high notes are listened 

 to its contraction tightens the drum, rendering it more respon- 

 sive to rapid vibrations. It has a tonic action, but it does not 

 make any special contraction for low notes. 



Behind the two windows, within the solid bone, is the inner 

 ear, which our ancestors very aptly termed a " labyrinth." It 

 is filled with fluid perilymph which is shaken by every 

 movement of the stirrup-plate. Since water is incompressible, 

 no waves could be raised in the perilymph were there no 

 secdnd aperture. Every vibration conveyed by the stirrup- 

 plate after passing through the labyrinth ends as a vibration 

 of the membrane which closes the round window. 



Nowhere does perilymph come in contact with auditory cells. 

 All the endings of the nerve of hearing are contained within a 

 membranous labyrinth which lies within the bony cavities. 

 The way in which the waves of the perilymph are dispersed 

 over the surface of this closed sac can be inferred from the 

 diagram (Fig. 38). They sweep round the utricle and saccule, 

 are lost in the narrow spaces which surround the semicircular 

 canals, run up the scala vestibuli of the cochlea. The course 

 of the waves which traverse the cochlea is of especial interest 

 in connection with the physiology of hearing. 



The cochlea snail-shell is a spiral tunnel of three turns, in 

 hard bone, about an inch in length. A shelf of bone 

 lamina spiralis projects into the tunnel on its convex side. 

 From the free margin of this spiral lamina two membranes 

 extend to the outer wall of the tunnel one firm, containing 

 straight, stiff, and probably elastic fibres which radiate out- 

 wards (the basilar membrane) ; the other an extremely delicate 

 film of connective tissue. The tunnel is thus divided into 

 three compartments, known as the scala vestibuli, scala media, 

 scala tympani. The scala media belongs to the membranous 

 labyrinth. Waves transmitted through perilymph pass, as 

 we have already explained, up the scala vestibuli. At the 

 apex of the cochlea the two scalae are in communication ; but 

 the aperture is small, and it is unlikely that waves reach the 

 lower passage from the upper through this opening. They pass 



