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.
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.
1. Inflammation and infection of structures in the eyelids, as for
example, sty formation, in which oil glands on the lid edges become
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.
-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.
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