The Eye: is a delicate organ. 1. the skull bones that
form the eye orbit (cavity) serve to protect more than half
of the eyeball at the back ( dorsally ) . 2. The lids and the eyelashes
aid in protecting the eye at the front part ( anteriorly ). 3. The
tears wash away small foreign objects that may enter the lid area. 4. A
sac lined with an epithelial membrane separates the front of the eye from
the eyeball proper and aids in the destruction of some of the pathogenic
bacteria that may enter from the outside.
The eyeball has three separate coats or tunics. The outermost layer
is called the sclera (skle'rah) and is made of firm, though,
connective tissue. It is commonly referred to as the white of the
eye. The second tunic of the eyeball is known as the choroid
(ko'roid) coat. It is heavily pigmented, preventing light rays from
scattering and reflection (bouncing) off the inner surface of the eye.
The choroid coat might be compared to the dull black lining of a camera.
It is made of a delicate network of connective tissue containing much dark
brown pigment and interlaced with many blood vessels. The innermost
coat, called the retina (ret'i-nah), includes some ten different
layers of nerve cells, including the end organs commonly called the rods
and cones. Theses are the receptors for the sense
of vision. The rods are sensitive to white and black. The cones
are sensitive to color. As far as is known, there are three types
of cones, each of which is sensitive to one of the primary colors (red,
yellow and blue). Persons who completely lack cones are totally color
blind; those who lack one type of cone are partially color blind.
Color blindness is and inherited condition and occurs almost exclusively
in males.
Light rays pass through a series of transparent, colorless eye parts.
On the way they undergo a process of bending known as refraction.
This refracting of the light rays makes it possible for light from a very
large area to be focused upon a very small surface, the retina, where the
receptors are located. The following are, in order from outside in,
the transparent refracting parts, or media, of the eye:
1.) The cornea (kor'ne-ah) is a forward continuation of the
outer coat, but it is transparent and colorless, whereas the sclera is
opaque and white. 2.) The aqueous humor, a watery fluid
which fills much of the eyeball in front of the lens, helps to maintain
the slight forward curve in the cornea. 3.) The crystalline
lens is a circular structure made of a jellylike material.
4.) The vitreous body fills the entire space behind the lens
and is necessary to keep the eyeball in its spherical shape.
The cornea is referred to frequently as the "window" of the eye.
It bulges forward slightly and is the most important refracting structure.
injuries caused by foreign objects or by infection may result in scar formation
in the cornea and a resulting area of opacity through which light rays
cannot pass. If such of the pupil (the hole in the center of the
colored part of the eye), blindness may be the result. The
cornea may be transplanted; eye banks store corneas obtained from donors
immediately after death, or in some cases, before death.
The next light-bending medium is the aqueous humor, followed by the
crystalline lens. The lens has two bulging surfaces, so it may be
best described as biconvex. During youth, the lens is elastic and
therefore is an important part of the system of accommodation to near vision.
In the process of accommodation the lens becomes thicker and thus bends
the light rays a greater amount, as is required for near objects.
With aging the lens loses its elasticity, and therefore its ability to
adjust by thickening, resulting in what is known as the
old eye, or presbyopia (prs-be-o'pe-ah).
The last of these transparent refracting parts of the eye is the vitreous
body. As in the case of the aqueous humor it is important in maintaining
the ball-like shape of the eyeball as well as aiding in refraction.
The vitreous body is not replaceable; an injury that causes a loss of an
appreciable amount of the jelly like vitreous material will cause collapse
of the eyeball, an operation called enucleation (e-nu-kle-a'shun).
Muscles of the eye: Certain muscles are inside the eyeball itself,
and therefore are described as intrinsic (in-trin'sik), while
others are attached to bones of the eye orbit as well as to the sclera
and are called extrinsic (eks-trin'sik) muscles.
The intrinsic muscles are found in two circular structures: 1.)
the iris , the colored or pigmented part of the eye, which
has a central opening called the pupil. (The size of the
pupil is governed by the action of two sets of muscles, one of which is
arranged in a circular fashion, while the other extends in a radial manner
resembling the spokes of a wheel. 2.) The ciliary body,
which is shaped some-what like a flattened ring with a hole that is the
size of the outer edge of the iris. This muscle alters the shape
of the lens.
The purpose of the iris is to regulate the amount of light entering
the eye. If a strong light is flashed in the eye, the circular muscle
fibers of the iris, which form a sphincter, contract and thus reduce the
size of the pupil. On the other hand, if the light is very dim, the
radial involuntary iris muscles, which are attached at the outer edge,
contract; the opening is pulled outward and thus enlarged. This pupillary
enlargement is known as dilation (di-la'shun).
The pupil changes size, too, according to whether one is looking at
a near object or a distant one. Viewing a near object causes the
pupil to become smaller; a far view will cause it to enlarge.
The muscle of the ciliary body is similar in direction and method of
action to the radial muscle of the iris. When the ciliary muscle
contracts, it removes the tension on the suspensory ligament of the lens.
The elastic lens then recoils and becomes thicker in much the same way
that a rubber band would thicken if a pull on it were released. This
action changes the focus of the lens, and thus adjusts the eye for either
long views or close-ups.
The six extrinsic muscles connected with each eye are ribbon like and
extend forward from the apex of the orbit behind the eyeball. One
end of each muscle is attached to a bone of the skull, while the other
end is attached to the white (sclera) of the eye. These muscles pull
on the eyeball in a coordinated fashion that causes the two eyes to move
together in order to center on one visual field. There is another
muscle located within the orbit which is attached to the upper eyelid.
When this muscle contracts, it keeps the eye open.
Nerve supple to the eye: The two sensory nerves of the eye are:
1.) The optic nerve, which carries visual impulses received
by the rods and cones in the retina to the brain. This is the second
cranial nerve. 2.) The ophthalmic nerve which carries impulses
of pain, touch and temperature from the eye and temperature from the eye
and surrounding parts. It is a branch of the fifth (trigeminal) cranial
nerve.
The optic nerve is connected with the eyeball a little toward the medial
or nasal side of the eye at the back. At this region there are no
rods and cones; and so this part, which is a circular white area, is called
the blind spot, known also as the optic disk.
There is a tiny depressed area in the retina called the fovea centralis
(fo've-ah sen--tra'lis), which is the clearest point of vision.
There are three nerves that carry motor fibers to the muscles of the
eyeball. The largest is the oculomotor nerve, which
supplies motor fibers, voluntary and involuntary, to all the muscles but
two. The other two nerves, the trochlear and the
abducens, supply one voluntary muscle each.
The lacrimal apparatus and the conjunctiva sac: The lacrimal
gland produces tears, and is located above the eye toward one side; that
is, it is superior and lateral to the eyeball. Tiny tubes carry the
tears to the front surface of the eyeball, where they serve to constantly
wash the sac that separates the front part of the eyeball from the larger
back portion. This sac is lined with a membrane called the conjunctiva
(kon-junk-ti'-vah). The conjunctiva lines the eyelids and is reflected
onto the front of the eyeball. It is kept moist by the tears flowing
across the front of the eye. Tears then are carried into tiny openings
near the nasal corner of the eye. From these openings tears are carried
by tubes that drain into the nose via, the nasolacrimal duct. A slight
excess of tears, or lacrimation (lak-re-ma'shun), causes
nose blowing; and a greater overproduction of tears results in an overflow
onto the face.
Eye Infections
Conjunctivitis: inflammation of the membrane that lines
the eyelids and covers the front of the eyeball. it may be acute or chronic,
and may be caused by a variety of irritants and pathogens. "Pinkeye"
is an acute conjunctivitis that is highly contagious and is caused
by cocci or bacilli in most cases. Sometimes irritants such as wind
and excessive glare, for example from snow, may cause an inflammation that
then may cause a susceptibility to bacterial infection. In the case
of the contagious epidemic form, children should be kept at home until
the infection has been cured.
Trachoma: also known as, granular conjunctivitis,
it is caused by Chlamydia trachomatis, which is a type of bacteria.
This disease was formerly quite common in the mountains of the southern
United States, and among the American Indians. It is still prevalent
in the Far East, in Egypt and in Southern Europe. This disease is
characterized by the formation of granules on the lids, which may cause
such serious irritation of the cornea that blindness can result.
Better hygiene and the use of antibiotic drugs have reduced the prevalence
and seriousness of this infection.
Ophthalmia neonatorum: an eye infection of the newborn
infant. It is caused by the entrance of gonococci into the conjunctival
sac. Neglect of this infection may cause blindness. Prevention
by the instillation of an appropriate antiseptic such as a silver nitrate
solution or penicillin is routine in hospitals at the time of delivery
of the infant.
The iris, the choroid coat, the ciliary body and other parts
of the eyeball may become infected by a number of different organisms.
Such disorders are likely to be very serious, fortunately they are not
very common. Syphilis spirochetes, tubercle bacilli and variety of
cocci may cause these painful infections. They may follow sinus infections,
tonsillitis, conjunctivitis and numerous other disorders. The care
of these conditions usually should be in the hands of an ophthalmologist,
a physician who specializes in disorders of the eye.
Eye Defects and Eyestrain
Hyperopia: a defect that is often responsible for eyestrain
in children is farsightedness or hyperopia. In this condition the
light rays are not bent sharply enough to focus on the retina, with
the result that the eye cannot focus properly on nearby objects.
The eyeball may be too short, so that the actual focal point is behind
the retina. This is normal in the infant, but usually corrects itself
by the time the child uses his eyes more for near vision. To a certain
extent it is possible to use the ciliary muscle in the process of thickening
the lens to focus objects on the too-near retina. However, this causes
constant strain. Visual tests may not show that the condition exists
unless drops which paralyze the ciliary muscles are used. Hence any
suggestion of eyestrain should lead to consulting a specialist who has
a license to practice medicine and who will use the drops as necessary.
Myopia: or nearsightedness is another defect of
development. In this case the eyeball is too long, or the bending
of the light rays is too sharp, so that the focal point is in front of
the retina. Objects that are a distance away appear blurred, and
may appear clear only if brought very near the eye. Only by the use
of lenses that will throw the point of focus back can this disorder be
corrected. In some young people this nearsightedness becomes worse
each year. It was thought by some that much use of the eyes for school
work may have played a part, but any reading or other close work suffer
from this problem to as great a degree. Careful attention to
good hygiene and changing of glasses as indicated by the ophthalmologist
are desirable.
Astigmatism: This condition is due to irregularity
in the curvature of the cornea or the lens. The surfaces do not bend
the light rays the same amount, resulting in blurred vision with severe
eyestrain. Astigmatism often is found in combination with hyperopia
or myopia, so a careful eye examination and properly fitted glasses will
reduce or prevent eyestrain.
Strabismus: Strabismus means that the muscles of the eyeballs
do not coordinate, so that the two eyes do not work together. There
are several different kinds of strabismus (in another sense it means squint),
but the cross-eyed type in which the eyeball is pulled inward (medially)
is fairly common and is found early in life. Care by a skilled ophthalmologist
as soon as possible may result in restoration of muscle balance.
Each patient is treated as his needs indicate. In some, glasses and
exercises may correct the defect, while in others surgery may be required.
If correction is not accomplished early, the affected eye may become blinded,
since the brain has a way of cutting out the confusing double image, and
the eye suffers from disuse.
Cataract: one of the most common cause of blindness. A
cataract involves the lens or its capsule so that it loses its transparency.
Sometimes the areas of opacity can be seen through a pupil which becomes
greatly enlarged because of reduction in the amount of light that can reach
the retina. In other cases there is gradual loss of vision, and frequent
changes in glasses may aid in maintaining useful vision for some time.
Removal of the lens may restore some vision, but the addition of a contact
lens usually is required to achieve satisfactory visual acuity, as well
as binocular vision which is desirable for driving a car, for example.
most persons will need reading glasses for close work.
Glaucoma: is another cause for blindness, particularly
in older persons, a condition characterized by excess pressure of the eye
fluid. Aqueous humor is being produced constantly from the blood;
and after circulation it is reabsorbed into the blood stream. Interference
with the normal reentry of this fluid to the blood stream leads to an increase
in pressure inside the eyeball. As in the case of cataract, glaucoma
usually progresses rather slowly, with vague visual disturbances and gradual
impairment of vision. Halos around lights, headaches and the need
for frequent changes of glasses (particularly by people over 40 ) are symptoms
that should be investigated by an ophthalmologist. There are different
forms of glaucoma, some occurring in the very young; and each type requires
a different management. Since continued high pressure of the aqueous
humor may cause destruction of the optic nerve fibers, it is important
to obtain continuous treatment beginning early in the disease to avoid
blindness.
Diabetes as a cause of blindness is increasing in the United States.
Disorders of the eye directly related to diabetes include 1) optic atrophy
in which the optic nerve fibers die, 2) cataracts, which occur earlier
and with greater frequency among diabetics, and 3) diabetic retinopathy,
in which the retina can be damaged by blood vessel hemorrhages and other
causes. ( Diabetic patients -note- it is important to have your eyes examine
frequently especially if you are experiencing above symptoms as well as
the ones below)
There are many other causes of blindness, and frequently these could
have been prevented. Injuries by pieces of glass and other sharp
objects are an important cause of eye damage. Industrial accidents
involving the eye have been greatly reduced by the use of protective goggles.
If an injury should occur, it is then very important to prevent infection.
Even a tiny scratch can become so seriously infected that blindness will
results.
The retina may become detached from the choroid and float into the
vitreous body. If neglected, blindness may result. Treatment
includes a sort of "spot welding" with an electric current or a weak laser
beam. a series of pinpoint scars reattach the retina. --See your
doctor or ophthalmologist.
Symptoms:
1. Inflammation and infection of structures in the eyelids, as for example, sty formation, in which oil glands on the lid edges become infected.
2. Excessive tear formation (lacrimation) and pain in the eyes.
3. Headaches and other nervous disturbances.
4. Digestive disturbances and loss of appetite with malnutrition.
5. Blurred vision
6. Flashing-light sensations, tiny objects floating in your eyes.
7. Sudden loss of central or peripheral vision may indicate
8. Difficulty distinguishing between red and green in dim light
9. Difficulty distinguishing objects in dim light
If you are experiencing any of these symptoms see your doctor and or
an ophthalmologist.
Treatment:
Eyestrain:
-Eye exercises or resting the eyes every 30 minutes helps relieve
eyestrain, especially if you work with computers.
-Smaller children should begin reading books in which the type is larger
and the letters are spaced relatively far apart to make them easier
to differentiate.
-Be certain that there is enough light without glare.
The table or desk on which the work is being done should be neither
too low nor too high
-Proper examination of the eyes and the use of adequate lenses are very
important. The notion that glasses will weaken the eyes has absolutely
no basis in fact.
For other vision problems see your ophthalmologist for proper treatment.
Call your doctor promptly if:
-Cut or blow to your eye affects your vision
-You have a foreign object in your eye that will not flush out with
water.
-Unusual sensitivity to bright light
-Flashing light sensations, tiny objects floating in your eyes, or a sudden loss of central or peripheral vision may indicate retinal detachment.
-Blurred vision, loss of consciousness, headache, and or dizziness.
