DEVELOPMENT OF THE EAR. 263 



hence is called the u snail " {cochlea, b). On the thin wall 

 of this delicate membranous labyrinth, the auditory nerve, 

 which passes from the after-brain to the labyrinth, is dis- 

 tributed in a very complex manner. It divides into two 

 main branches, the nerve of the cochlea, and the nerve of 

 vestibule, for the remaining part of the labyrinth. The 

 former seems specially to determine the quality of the sound 

 heard, the latter its quantity. The nerve of the cochlea 



Fig. 245. — The bony labyrinth of the human ear 

 (left side) : a, vestibule j b, cochlea ; c, upper semi- 

 circular canal; d, posterior semi-circular canal; e, 

 outer semi-circular canal ; f, fenestra ovalis ; g, fenestra 

 rotunda. (From Meyer.) 



tells us the pitch and quality of sounds, the nerve of the 

 vestibule their strength. 



The first rudiment of this extremely complex organ of 

 hearing is very simple in the human embryo, as in those 

 of all other Skulled Animals (Craniota) ; it is a groove-like 

 depression of the outer skin (epidermis). At the back of the 

 head, near the after-brain, at the upper end of the second 

 gill-opening, a little wart-like thickening of the horn-plate 

 arises on each side (Figs. 246, A, fl ; 248, g). This deepens 

 into a small groove, and separates from the outer-skin, just 

 as does the lens of the eye. (Cf. p. 253.) A small vesicle 

 filled with fluid, the primitive ear-vesicle, is thus formed 

 ©n each side, immediately below the horn-plate of the back 

 part of the head ; this is also called the " primary laby- 

 rinth " (Plates VI. and VII.). As this separates from its 

 original site, the horn-plate, and grows inward and down- 

 ward in the skull, it changes from a globular to a pear- 

 shaped form (Figs. 246, B, iv ; 249, o). The outer part has 



