oculi and the eyelids (blinking) moves this fluid moved across the surface of the
conjunctiva to the medialinferior aspect. Here, the lacrimal fluid is collected and
delivered into the nasal chamber by the nasal lacrimal duct.
d. Eyebrow. The eyebrow is a special group of hairs above the orbit. The
eyebrow serves to keep rain and perspiration away from the bulbus oculi.
e. Optic Nerve. Neurons carry information from the photoreceptors of the
nervous retina. They leave the bulbus oculi at the blind spot. At the optic nerve, or
second cranial nerve, the neurons pass to the rear of the orbit. There, the optic nerve
exits through the optic canal into the cranial cavity. Beneath the brain, the optic nerves
from both sides join to form the optic chiasma, in which half of the neurons from each
optic nerve cross to the opposite side. Rom the optic chiasma, the right and left optic
tracts proceed to the brain proper.
5-4. DISEASES/CONDITIONS AFFECTING THE EYE
a. Myopia ("Near-Sightedness"). In myopia the image from distant objects are
focused in front of the retina. Myopia is caused by a lens that is too strong. Although
the ciliary muscle is completely relaxed, the light rays entering the eye are not properly
bent to be focused on the retina. This type of lens condition can be corrected by the
use of a concave lens. Figure 5-3a illustrates this condition and correction with a
concave lens.
b. Hypermetropia (Hyperopia)("Far-Sightedness"). In hypermetropia, the
parallel light rays entering the eye are not bent sufficiently by the lens and the image is
focused behind the retina. In hypermetropia, the bulbus oculi is too short or the lens
system is too weak when the ciliary muscle is relaxed. A convex lens is used to correct
this condition. Figure 5-3b illustrates this condition and its correction with a convex
lens.
c. Astigmatism. Astigmatism occurs when the light rays passing through an
astigmatic lens are not all focused at the same point. A malformed lens or cornea
causes astigmatism. A specially designed lens can be used to help correct this
condition.
d. Glaucoma. Glaucoma is a common cause of blindness. In glaucoma, the
intraocular pressure becomes too great and causes damage to the retina and optic
nerve. The intraocular pressure of a normal person is approximately 15 to 20 mm Hg
(millimeters of mercury), while the intraocular pressure of a person with glaucoma can
reach from 80 to 90 mm Hg. As the intraocular pressure increases, damage is done to
the delicate tissues of the eye. The retinal artery, which enters the bulbus oculi at the
optic disk, becomes increasingly compressed. Hence, nutrition to the retina is reduced-
-damage to the retina and optic nerve follow. Glaucoma can be either of a sudden
onset or of a slow onset. Glaucoma results from the high pressure caused by reduced
drainage of a fluid (aqueous humor). Because of the decreased drainage and
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