OUR FACE FROM FISH TO MAN 



with the sense of balance; (b) the cochlea, a 

 spirally-wound double tube filled with liquid and 

 containing between the upper and lower inner 

 tubes the spirally -wound organ of Corti, the true 

 organ of hearing. The sound waves in the air 

 cause the drum membrane to vibrate, the ossicles 

 magnify the movement and set up mechanical 

 waves in the liquid of the cochlea. It is these 

 mechanical waves and not the sound waves them- 

 selves that are picked up by the little rods of the 

 organ of Corti and transmitted to the nerves of 

 hearing. 



In the more primitive fishes at the lower end 

 of the vertebrate series there is no middle ear and 

 the inner ear consists chiefly of the semicircular 

 canals, which may be followed throughout the 

 series without a break from fish to man. 



The labyrinth arises in the embryo shark, as in 

 the embryo man, by the formation of a sac or 

 pocket in the ectoderm or outer cell layer on either 

 side of the tube that gives rise to the hind brain. 

 The sac later becomes surrounded by cartilage 

 which finally ossifies. The nerves of the semi- 

 circular canals appear to be part of the fore and 



aft series that innervates the "ampullae" of the 



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