Chapter 21:Visual and Auditory Systems
STRUCTURES AND FUNCTIONS
The visual system includes external tissues and structures surrounding the eye.
o External structures include the eyebrows, eyelids, eyelashes, lacrimal system,
conjunctiva, cornea, sclera, and extraocular muscles.
o Internal structures include the iris, lens, ciliary body, choroid, and retina.
The cornea, aqueous humor, lens, and vitreous must all remain clear for light to reach the retina
and stimulate the photoreceptor cells.
Refraction is the ability of the eye to bend light rays so that they fall on the retina. When light
does not focus properly, it is called refractive error.
Types of refractive errors are myopia (nearsightedness) and hyperopia (farsightedness).
Astigmatism is caused by corneal unevenness resulting in visual distortion. Presbyopia is a type
of hyperopia due to aging.
The auditory system consists of peripheral and central systems.
o Peripheral system includes the external, middle, and inner ear and is involved with
sound reception and perception.
o The central system (brain and its pathways) integrates and assigns meaning to what is
Presbycusis can result from aging or insults from a variety of sources. Tinnitus, or ringing in the
ears, may accompany the hearing loss that results from the aging process.
External and middle ear portions conduct and amplify sound waves from the environment.