PLATE 107 • EYE IV: SCLERA, CORNEA, AND LENSLFLFCEnCEnDCSCSBCjEpCjEpCEpCEpBCEpCEpCEnCEnDCSNBCEpCEpBVBVCSCSSCjEpCjEpCjCjCjCjEpSBVCSACCEpBSCCEnDACNCjEpCEpBCEpBDCEnEnCSLFLCSSSS
OVERVIEW OF THE EAR / 928 EXTERNAL EAR / 928 MIDDLE EAR / 929 INTERNAL EAR / 932 Structures of the Bony Labyrinth / 932 Structures of the Membranous Labyrinth / 933 Sound Perception / 941 Innervation of the Internal Ear / 942 Blood Vessels of the Membranous Labyrinth / 945 Folder 25.1 Clinical Correlation: Otosclerosis / 933 Folder 25.2 Clinical Correlation: Hearing Loss— Vestibular Dysfunction / 934 Folder 25.3 Clinical Correlation: Vertigo / 937
The ear is a three-chambered sensory organ that functions as an auditory 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 and vestibular systems (Fig. 25.1). 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 respond to gravity and movement of the head. They are responsible for the sense of balance and equilibrium and help to coordinate movements of the head and eyes.
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 otic vesicle (otocyst), which sinks deep to the surface ectoderm into the underlying mesenchyme (Fig. 25.2). The otic vesicle serves as a primordium 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 tubotympanic recess, which ultimately develops into the auditory 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. 25.2). 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.
The auricle is the external component of the ear that collects and amplifies sound.
The auricle (pinna) is the oval appendage that projects from the lateral surface of the head. The characteristic shape of the auricle is determined by an internal supporting structure of elastic cartilage. Thin skin with hair follicles, sweat glands, and sebaceous glands cover the auricle. The auricle is considered a nearly vestigial structure in humans, compared with its development and role in other animals. However, it is an essential component in sound localization and amplification.
The external acoustic meatus conducts sounds to the tympanic membrane.