STRUCTURE OF THE EAR 



549 



lagena originates ; it is little more than a small hollow knob 

 in Fishes and Amphibians, but becomes large and im- 

 portant in Sauropsida and Mammals. 



As this differentiation of the parts of the internal ear takes place, the 

 lining epithelium also becomes differentiated into flattened covering cells 

 and sensory auditory cells. The auditory cells are arranged in patches 

 to which branches of the auditory nerve are distributed. With these 

 sensory patches calcareous concretions (otoliths) are associated, except 

 in the cochlea of Mammals. 



Fig. 311. — Diagram showing the ear and related parts 



in a young cat. 



P., Pinna; Sq., squamosal; E.A.M., external auditory meatus; T., 

 tympanum ; M., malleus ; /., incus ; St., stapes abutting on foramen 

 ovale ; B., bulla of tympanic bone ; Se., a septum in the bulla ; E.T., 

 Eustachian tube leading from the tympanic cavity to the back of the 

 mouth ; B.O., basi-occipital ; C, cochlea ; S., sacculus ; U., utriculus ; 

 D.E., ductus endolymphaticus ; N., auditory nerve ; S.C., semi- 

 circular canal ; PE., periotic bone. 



The fact that lime salts are often deposited in the skin, and that the 

 ear-sac arises as an insinking of ectoderm, may perhaps shed some light 

 on the origin of otoliths. 



The parts which we have so far considered constitute together the 

 membranous labyrinth of the ear. Round about them the mesoderm 

 (mesenchyme) forms a tv/o-layered envelope. Its inner layer disin- 

 tegrates to produce a fluid, the perilymph, which bathes the whole 

 outer surface of the membranous labyrinth. Its outer layer forms a 

 firm case, the cartilaginous or bony labyrinth, surrounding the internal 

 ear. The membranous labyrinth itself contains another fluid, the 

 endolymph. 



