January 2001
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The ear is not a single organ, but two, being the peripheral receptor site both for stimuli derived from sound waves and for changes of posture. The structures subserving both of those functions are developed from an invagination of ectoderm early in embryonic life — the otocyst — to produce the epithelia of the membranous labyrinth of the inner ear. Superimposed upon, and developing slightly later, the first and second branchial arch systems provide structures which augment the hearing function. The endodermal component of the first branchial system, the branchial pouch, gives rise to the Eustachian tube and middle ear epithelia and the corresponding ectodermal outgrowth, the first branchial cleft, to the external ear epithelia. The connective tissue part of the local branchial system, the first and second branchial arches, produces the ossicles. The eighth cranial nerve outflow from the central nervous system grows to link up with the sensory epithelia lining the otocyst-derived cochlear and vestibular labyrinths and the cartilaginous, bony and muscular conformations of the ear are developed from the mesenchyme surrounding these early epithelia.