SENSE OF HEARING. 413 



vestibule (scala vestibuli), and the other with the middle ear 

 or tympanum (by i\\Q fenestra ovalis). 



This collection of membranous pouches (utricule and sac- 

 cule), the semicircular canals and the cochlea, forms the 

 internal ear of the higher vertebrate animals. The auditory 

 nerve, or nerve of the eighth pair, terminates here in organs 

 differing apparently in form, but belonging all to the same 

 type, viz., that of organs capable of being set in motion by 

 the vibrations of the fluid in which they are immersed. In 

 the membranous pouches (utricule and saccule) these organs 

 consist of epithelial cells in contact with crystals of carbonate 

 of lime (otoliths), which strike against them at every oscilla- 

 tion of the fluid ; in the semicircular canals (in the am- 

 pullce), they consist of epithelial cells furnished with long 

 rigid cilia, which are immediately set in motion. The dis- 

 position of the cochlea is more complicated ; the cochlear 

 branch of the auditory nerve spreads through the spiral 

 membrane (membrana spiralis) in 3000 or 4000 small articu- 

 lated organs (organs of Corti), which cannot here be de- 

 scribed, 1 but which, in brief, resemble fragments joined to- 

 gether like the beams of a roof, and swing to and fro with 

 the oscillations of the ambient fluid. The whole of this 

 internal ear or labyrinth arises from a deep vegetation of 

 the integuments of the lateral portion of the head of the 

 embryo; this growth is afterwards more or less separated 

 from the surface from which it sprang. The organ of Corti 

 itself is, therefore, an epidermic production. 



The animals which live "in the air possess, besides the 

 internal ear, an additional organ, consisting of the middle 

 chamber of the ear or drum (tympanum). This part, which 

 would be useless in water animals, for whom the waves of 

 sound are communicated readily from the ambient fluid to 

 the fluid of the labyrinth, is necessary to facilitate the com- 

 munication of the waves of a gaseous medium to the fluid 

 medium of the organ ; thus we know that sounds do not 

 pass readily from the air into water. The middle chamber of 

 the ear resembles a drum hollowed out in the petrosal substance 

 and contains an organ of conduction intended to facilitate 

 the transmission of waves of sound (Fig. 110) ; it consists of 

 a more or less regular bony chain, leading from the internal 

 ear (fenestra ovalis) to the membrana tympani ; this latter 



1 See Lcewenberg, " La Lame spirale du Limagon." (** Journ. 

 de 1'Anat. et de la Physiol." 186ti.) 



