The ear is a three-chambered sensory organ that functions as an auditoiy system for sound perception and as a vestibular system for balance. Each of the three divisions of the ear, the external ear, middle ear, and internal ear, is an essential part of the auditory system (Fig. 24.1). The external and middle ear collect and conduct sound energy to the internal ear, where auditory sensory receptors convert that energy into electrical impulses. The sensory receptors of the vestibular system are also located in the internal ear. These receptors respond to gravity and movement of the head.
The ear develops from surface ectoderm and components of the first and second pharyngeal arch
Embryologically, the functions of the ear, hearing and balance, are elaborated from an invagination of surface ectoderm that appears on each side of the myelencephalon. This invagination forms the octic vesicle (otocyst), which sinks deep to the surface ectoderm into the underlying mesenchyme (Fig. 24.2). The otic vesicle serves as a pri-mordium for development of the epithelia that line the membranous labyrinth of the internal ear. Later, development of the first and part of the second pharyngeal arch provides structures that augment hearing. The endodermal component of the first pouch gives rise to the tubotym-panic recess, which ultimately develops into the auditoiy tube (Eustachian tube) and the middle ear and its epithelial lining. The corresponding ectodermal outgrowth of the first pharyngeal groove gives rise to the external acoustic meatus and its epithelial lining (see Fig. 24.2). The connective tissue part of the pharyngeal arches produces the ossicles. The malleus and incus develop from the first pharyngeal arch, and the stapes from the second pharyngeal arch. The sensory epithelia of the membranous labyrinth that originates from the otic vesicle link with cranial nerve VIII, which is an outgrowth of the central nervous system. The cartilaginous, bony, and muscular structures of the ear develop from the mesenchyme surrounding these early epithelia.
auricle semicircular canals vestibule semicircular canals vestibule
vestibulocochlear nerve internal acoustic meatus auricle
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