The eyes and their accessory structures are receptor organs that provide vision. As one of the most important sense organs of the body, the eyes provide most of the information about what we see, but also of what we learn from printed material. Similar to other sensory organs, the eyes are constructed to detect stimuli in the environment and to transmit those observations to the brain for visual interpretation.
The eye is a globe-shaped, hollow structure set within a bony cavity. The bony cavity, or orbit, houses the eyeball and associated structures, such as the eye muscles, nerves, and blood vessels. Most of the eyeball is protected from trauma by the orbit’s bony cavity.
The wall of the eyeball contains three layers: the (1) sclera, the white outer layer of the eyeball, is composed of fibrous connective tissue. On the most anterior portion of the eye, the sclera forms a transparent, domed structure called the (2) cornea.
The cornea also protects the front part of the eye from injury and is the first structure of the eye that refracts light rays. In addition, the cornea is avascular (without blood vessels or capillaries), but is well supplied with nerve endings, most of which are pain fibers. For this reason, some people can never adjust to wearing contact lenses.
Kerat/itis, a vision-threatening infection, can occur if contact lenses are not cleaned and disinfected properly. In some cases, laser kerat/o/tomy can be used to correct vision.
Doing so eliminates the need for contact lenses or glasses. Shallow, bloodless, hairline, radial incisions are made using a laser in the outer portion of the cornea, where they will not interfere with vision. This allows the cornea to flatten and helps to correct nearsightedness. About two-thirds of patients are able to eliminate the use of glasses or contact lenses by undergoing the surgical procedure called laser
The opening in the center of the iris is called the (8) pupil. The amount of light entering the eye is controlled by contractions and dilations of the pupil. Constriction of the pupil permits a sharper near vision. It is also a reflex that protects the retina from intense light. Label the pupil in Figure Eye structures.
Sensory receptors of vision, rods and cones, contain light - sensitive molecules (photopigments) that convert light energy into electrical impulses. Impulses generated by rods and cones are transmitted by retinal nerve fibers to the cortex of the brain.
Retinal nerve fibers unite at the (9) opticdisc and cut across through the wall of the eyeball as the (10) optic nerve. Because the optic disk has no rods or cones, it is known as the blind spot. Label the structures in Figure Eye structures. as you learn about the location and role these structures play in providing vision.
A stroke can prevent eye movement and cause paralysis of eye muscles. A person with paralysis of eye (muscles) has a condition calledophthalm/o/plegia
The (11) conjunctiva is a thin mucous-secreting membrane that lines the interior surface of the eyelids and the exposed anterior surface of the eyeballs. Conjuctiv/itis is often caused by allergies and is manifested by itchy, watery, red eyes.
The surgical procedure to remove wrinkles from the eyelids is known as blephar/o/plasty. This procedure is be performed for functional and cosmetic reasons. Xanth/opiais a condition in which objects that are not yellow appear to be yellow.
In normal vision, the lens focuses the visual image on the retina. Hyper/opia occurs when the lens focuses the visual image beyond the retina(see Figure Refraction of the eye.), causing difficulty in seeing objects that are close. This is a condition common in people over 40 years of age, but can be corrected with “reading” glasses.
People with hyper/opia (farsightedness) have difficulty seeing objects that are close. If the eyeball is too long, the visual image falls in front of the retina (see Figure Refraction of the eye), causing difficulty seeing objects that are far away.
Refraction of the eye.
Eyelids shade the eyes during sleep, protect them from excessive light and foreign objects, and spread lubricating secretions over the eyeballs. The (1) lacrimal gland is located above the outer corner of each eye. These glands produce tears, which keep the eyeballs moist. The (2) lacrimal sac collects and drains tears into the (3) nasolacrimal duct. Label the lacrimal structures in Figure Lacrimal apparatus.
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