Hearing

retinas. This process, called convergence, produces clear, three-dimensional vision. HEARING The ear is the primary organ of hearing. Its major parts are illustrated in figure 1-52. The ear is divided into three parts: the external, middle, and inner ear. External Ear The external (outer) ear is composed of two parts, the auricle and the external auditory canal (fig. 1-52). The auricle, or pinna, is a cartilaginous structure located on each side of the head. The auricle collects sound waves from the environment, which are then conducted by the external auditory canal to the eardrum. The lining of the external auditory canal contains glands that secrete a wax-like substance called cerumen. Cerumen aids in protecting the eardrum against foreign bodies and microorganisms. The tympanic membrane, or eardrum, is an oval sheet of fibrous epithelial tissue that stretches across the inner end of the external auditory canal. The eardrum separates the outer and middle ear. The sound waves cause the eardrum to vibrate, and this vibration transfers the sounds from the external environment to the auditory ossicles. Middle Ear The middle ear is a cavity in the temporal bone, lined with epithelium. It contains three auditory ossicles—the malleus (hammer), the incus (anvil), and the stapes (stirrup)—which transmit vibrations from the tympanic membrane to the fluid in the inner ear 1-47 HM3f0151 OPTIC NERVE IMAGE ON RETINA OBJECT IRIS LENS PUPIL CORNEA Figure 1-51.—The vision process. SCLERA CHOROID RETINA OPTIC DISK (BLIND SPOT) NASAL SIDE TEMPORAL SIDE FOVEA CENTRALIS (AREA OF SHARPEST VISION) HM3f0150 Figure 1-50.—Ophthalmoscope view of the eye.