retinas. This process, called convergence, producesclear, three-dimensional vision.HEARINGThe ear is the primary organ of hearing. Its majorparts are illustrated in figure 1-52. The ear is dividedinto three parts: the external, middle, and inner ear.External EarThe external (outer) ear is composed of two parts,the auricle and the external auditory canal(fig. 1-52). The auricle, or pinna, is a cartilaginousstructure located on each side of the head. The auriclecollects sound waves from the environment, which arethen conducted by the external auditory canal to theeardrum. The lining of the external auditory canalcontains glands that secrete a wax-like substancecalled cerumen. Cerumen aids in protecting theeardrum against foreign bodies and microorganisms.The tympanic membrane, or eardrum, is an ovalsheet of fibrous epithelial tissue that stretches acrossthe inner end of the external auditory canal. Theeardrum separates the outer and middle ear. The soundwaves cause the eardrum to vibrate, and this vibrationtransfers the sounds from the external environment tothe auditory ossicles.Middle EarThe middle ear is a cavity in the temporal bone,lined with epithelium. It contains three auditoryossicles—the malleus (hammer), the incus (anvil), andthe stapes (stirrup)—which transmit vibrations fromthe tympanic membrane to the fluid in the inner ear1-47HM3f0151OPTICNERVEIMAGE ONRETINAOBJECTIRISLENSPUPILCORNEAFigure 1-51.—The vision process.SCLERACHOROIDRETINAOPTIC DISK(BLIND SPOT)NASALSIDETEMPORALSIDEFOVEA CENTRALIS(AREA OF SHARPEST VISION)HM3f0150Figure 1-50.—Ophthalmoscope view of the eye.
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