The eyes are organs that refract (bend) and focus incoming light rays onto the sensitive photoreceptors at the back of each eye. The eye contains the receptors for vision and a refracting system that focuses light rays on the receptors in the retina, with the accessory structures of the eye, then later return to the eye itself and the physiology of vision.
Nerve impulses from the stimulated photoreceptors are conveyed through visual pathways within the brain to the occipital lobes of the cerebrum, where the sense of vision is perceived. The specialized photoreceptor cells can respond to an incredible 1 billion different stimuli each second. Further, these cells are sensitive to about 10 million gradations of light intensity and 7 million different shades of color. The eyes are anteriorly positioned on the skull and set just far enough apart to achieve binocular vision when focusing on an object. This three-dimensional perspective allows a person to assess depth. Often likened to a camera, the eyes are responsible for approximately 80% of all knowledge that is assimilated.
Visual stimuli help us form specific detailed visual images of objects in our environment. The sense of vision uses visual receptors (photoreceptors) in the eyes to detect light, color, and movement. Before describing the structures of the eye, we briefly examine the accessory structures that support and protect the eye’s exposed surface.
Accessory Structures of the Eye.
The accessory structures of the eye provide a superficial covering over its anterior exposed surface (conjunctiva), prevent foreign objects from coming in contact with the eye. Protective structures include the bony orbit, eyebrow, facial muscles, eyelids, eyelashes, conjunctiva, and the lacrimal apparatus that produces tears. Eyeball movements depend on the actions of the extrinsic ocular muscles that arise from the orbit and insert on the outer layer of the eyeball.
Orbit is a bony depression in the skull. Each eyeball is positioned in a Orbit.
Figure 1.External Anatomy of the Orbital Region.
Eyebrows consist of short, thick hairs positioned transversely above both eyes along the superior orbital ridges (fig.1). Eyebrows shade the eyes from the sun and prevent perspiration or falling particles from getting into the eyes. Underneath the skin of each eyebrow is the orbital portion of the orbicularis oculi muscle and a portion of the corrugator supercilli muscle . Contraction of either of these muscles causes the eyebrow to move, often reflexively, to protect the eye.
Eyelids and Eyelashes
The eyelids, or palpebrae ,close periodically to moisten the eye with tears, sweep debris from the surface, block foreign objects from the eye, and prevent visual stimuli from disturbing our sleep. The two eyelids are separated by the palpebral fissure, and the corners where they meet are called the medial and lateral commissures (canthi). The eyelid consists largely of the orbicularis oculi muscle covered with skin. It also has a supportive connective tissue layer called the tarsal plate. Within this plate are 20 to 25 tarsal glands that open along the edge of the eyelid. They secrete an oil that coats the eye and reduces tear evaporation. The eyelashes are guard hairs that help to keep debris from the eye. Touching the eyelashes stimulates hair receptors and triggers the blink reflex.
Each eyelid supports a row of eyelashes that protects the eye from airborne particles. The shaft of each eyelash is surrounded by a root hair plexus that makes the hair sensitive.
Eyelashes of the upper lid are long and turn upward, whereas those of the lower lid are short and turn downward. In addition to the layers of the skin and the underlying connective tissue and orbicularis oculi muscle fibers, each eyelid contains a tarsal plate, tarsal glands, and conjunctiva. The tarsal plates, composed of dense regular connective tissue, are important in maintaining the shape of the eyelids . Specialized sebaceous glands called tarsal glands are embedded in the tarsal plates along the exposed inner surfaces of the eyelids. The ducts of the tarsal glands open onto the edges of the eyelids, and their oily secretions help keep the eyelids from sticking to each other. Modified sweat glands called ciliary glands are also located within the eyelids, along with additional sebaceous glands at the bases of the hair follicles of the eyelashes. An infection of these sebaceous glands is referred to as a sty (
A specialized stratified squamous epithelium termed the conjunctiva forms a continuous lining of the external, anterior surface of the eye (the ocular conjunctiva ) and the internal surface of the eyelid.The space formed by the junction of the ocular conjunctiva and the palpebral conjunctiva is called the conjunctiva fornix. Eye movement is controlled by extrinsic muscles of the eye. The conjunctiva contains numerous goblet cells, which lubricate and moisten the eye. In addition, the conjunctiva houses numerous blood vessels that supply the avascular sclera (“white”) of the eye, as well as abundant free nerve endings that detect foreign objects as they contact the eye. Also Conjunctiva it’s a transparent membrane; it covers and protects the cornea.
The palpebral fissure is the space between the upper and lower eyelids. The shape of the palpebral fissure is elliptical when the eyes are open. The commissures (canthi) of the eye are the medial and lateral angles where the eyelids come together.
The commissures of the eye are the medial and lateral angles where the eyelids come together. The medial commissure, which is broader than the lateral commissure, is characterized by a small, reddish, fleshy elevation called the lacrimal caruncle.The lacrimal caruncle contains sebaceous and sudoriferous glands; it produces the whitish secretion, commonly called “sleep dust” that sometimes collects during sleep.
The iris is the most anterior portion of the vascular coat of the eye and is a circular vertically standing plate with a round aperture called the pupil.It control the amount of light entering the eye. The basis of the iris consists of connective tissue with the architecture of a lattice in which vessels have been fitted radially from the periphery to the pupil. These vessels are the sole carriers of elastic elements and together with the connective tissue form an elastic skeleton of the iris, permitting it to change easily in size. The actual movements of the iris itself are accomplished by the muscle system lodged within the stroma. This system consists of smooth muscle fibres which are partly arranged in a ring around the pupil to form the sphincter of the pupil and partly fan out radially from the pupillary aperture to form the dilator of the pupil . Both muscles are interrelated and affect each other: the sphincter stretches the dilator, while the dilator straightens out the sphincter. Because of this, each muscle returns to its initial position, which explains the rapidity of the movements of the iris. This integral muscle system has a punctum fixum on the ciliary body.
CONES are sensitive to light of high intensity (bright light) and colour. Cone cells, which are about 7 million per eye, provide daylight color vision and greater visual acuity. The photoreceptors synapse with bipolar neurons, which in turn synapse with the ganglion neurons. The axons of ganglion neurons leave the eye as the optic nerve. Cone cells are concentrated in a depression near the center of the retina called the fovea centralis, which is the area of keenest vision.
RODS are sensitive and functions in dim light. Rod cells are over 100 million per eye and are more slender and elongated than cone cells . Rod cells are positioned on the peripheral parts of the retina, where they respond to dim light for black-and-white vision. They also respond to form and movement but provide poor visual acuity.
FOVEA is a region where the cones are packed together. The fovea is directly opposite of the lens, is a most sensitive part of the retina.
Contain ciliary muscles that contract to contol the shape of the lens.
Is a hole an opening in the iris, which allow light to enter the eye. 8. SCLERA
This is the outermost layer of the eye. This layer protects, support and maintains the shape of the eyeball. The sclera continues and become a transparent layer at the front of the eye from cornea.
- CORNEA, It is a transparent front of the eyeball covered by a thin membrane known as conjunctiva, it is convex to reflect light also allow light to pass through.
This is a layer next to the sclerotic layer. Choroid layer extends to the front of the eye to form the Ciliary body and Iris. The pigment of the choroid absorbs stray rays of light to prevent reflection of light within the eye.
This is the area in retina through which optic nerve leaves the eyeball. The blind spot has neither rod nor cones. So images from object falling
THE LACRIMAL APPARATUS.
The lacrimal apparatus consists of the lacrimal gland, which secretes the lacrimal fluid (tears), and a series of ducts that drain the lacrimal fluid into the nasal cavity (fig. 2). The lacrimal gland, which is about the size and shape of an almond, is located in the superolateral portion of the orbit. It is a compound tubuloacinar gland that secretes lacrimal fluid through several excretory lacrimal ductules into the conjunctival sac of the upper eyelid. With each blink of the eyelids, lacrimal fluid is spread over the surface of the eye much like windshield wipers spread windshield washing fluid. Lacrimal fluid drains into two small openings, called lacrimal puncta, on both sides of the lacrimal caruncle. From here, the lacrimal fluid drains through the superior and inferior lacrimal canaliculi into the lacrimal sac and continues through the nasolacrimal duct to the inferior meatus of the nasal cavity (fig. 2). Lacrimal fluid is a lubricating mucus secretion that contains a bactericidal substance called lysozyme. Lysozyme reduces the likelihood of infections. Normally, about 1 milliliter of lacrimal fluid is produced each day by the lacrimal gland of each eye. If irritating substances, such as particles of sand or chemicals from onions, make contact with the conjunctiva, the lacrimal glands secrete greater volumes. The extra lacrimal fluid protects the eye by diluting and washing away the irritating substance.
Tears are produced by the lacrimal glands, located at the upper, outer corner of the eyeball, within the orbit (Fig.2). Secretion of tears occurs constantly, but is increased by the presence of irritating chemicals (onion vapors, for example) or dust, and in certain emotional situations (sad or happy). Small ducts take tears to the anterior of the eyeball, and blinking spreads the tears and washes the surface of the eye. Tears are mostly water, with about 1% sodium chloride, similar to other body fluids. Tears also contain lysozyme, an enzyme that inhibits the growth of most bacteria on the wet, warm surface of the eye. At the medial corner of the eyelids are two small openings into the superior and inferior lacrimal canals. These ducts take tears to the lacrimal sac (in the lacrimal bone), which leads to the nasolacrimal duct, which empties tears into the nasal cavity. This is why crying often makes the nose run.
Fig.2. Lacrimal apparatus.
The eye (oculus) consists of the eyeball and the auxiliary apparatus surrounding it. The eyeball is spherical in shape and is situated in the eye socket. The anterior pole, corresponding to the most convex point on the cornea, and the posterior pole, located lateral to the exit of the optic nerve are distinguished in the eyeball. The straight line connecting both poles is called the optic axis, or the external axis of the. The part lying between the posterior surface of the cornea and the retina is called the internal axis of the eye. This axis intersects at a sharp angle with the so-called visual line (linea visus) which passes from the object of vision, through the nodal point, to the place of the best vision in the central pit of the retina. The lines connecting both poles along the circumference of the eyeball form meridians, whereas the plane perpendicular to the optic axis is the equator of the eyeball dividing it into the anterior and posterior halves. The horizontal diameter of the equator is slightly shorter than the external optic axis (the latter is 24 mm and the former 23.6 mm); its vertical diameter is even shorter (23.3 mm). The internal optic axis of a normal eye is 21.3 mm; in myopic eyes it is longer, and in long-sighted people (hypermetropic eyes) it is shorter. Consequently, the focus of converging rays in myopic people is in front of the retina, and in hypermetropic people it is behind the retina. To achieve clear vision, the hypermetropic people must always resort to accommodation. To relieve such anomalies of sight, adequate correction by means of eyeglasses is essential.
The coats(Tunic) of the eyeball.
The eyeball has three coats(Tunic) surrounding its inner nucleus; a fibrous outer coat, a vascular middle coat, and an inner reticular coat (the retina).
The fibrous tunic is the outer layer of the eyeball. It is divided into two regions: the posterior five-sixths is the opaque sclera and the anterior one-sixth is the transparent cornea .The toughened sclera is the white of the eye. It is composed of tightly bound elastic and collagenous fibers that give shape to the eyeball and protect its inner structures. It also provides a site for attachment of the extrinsic ocular muscles. The sclera is avascular but does contain sensory receptors for pain. The large optic nerve exits through the sclera at the back of the eyeball. The transparent cornea is convex, so that it refracts (bends in a converging pattern) incoming light rays. The transparency of the cornea is due to tightly packed, avascular dense connective tissue. Also, the relatively few cells that are present in the cornea are arranged in unusually regular patterns. The circumferential edge of the cornea is continuous structurally with the sclera. The outer surface of the cornea is covered with a thin, nonkeratinized stratified squamous epithelial layer called the anterior corneal epithelium, which is actually a continuation of the bulbar conjunctiva of the sclera (see fig. 3).
The vascular coat of the eyeball is rich in vessels, soft, dark-coloured by the pigment contained in it. It lies immediately under the sclera and consists of three parts: the choroid, the ciliary body, and the iris. The choroid (chorioidea) is the posterior largest segment of the vascular coat. Due to the constant movement of the choroid in accommodation a slit-like lymphatic perichoroidal space (spatium perichorioideale) is formed here between the layers. The ciliary body (corpus ciliare), the anterior thickened part of the vascular tunic, is arranged in the shape of a circular swelling in the region where the sclera is continuous with the cornea. Its posterior edge, which forms the ciliary ring (orbiculus ciliaris), is continuous with the choroid. This place corresponds to the retinal ora serrata (see below). In front the ciliary body is connected with the external edge of the iris. Anteriorly of the ciliary ring the ciliary body carries about 70 fine radially arranged whitish ciliary processes (processus ciliares).
Figure 3.The internal structure of the eye.
The inner layer (tunica interna) consists of the retina, which internally lines the posterior two-thirds of the eyeball.
3.Internal Tunic (Retina)
The retina covers the choroid as the innermost layer of the eye. The internal layer of the eye wall, called the retina or either the internal tunic or neural tunic, is composed of two layers: an outer pigmented layer and an inner neural layer. The pigmented layer is immediately internal to the choroid and attached to it. This layer provides vitamin A for photoreceptor cells. Light rays that pass through the inner layer are absorbed in this outer layer. The neural layer houses all of the photoreceptors and their associated neurons. This layer of the retina is responsible for receiving light rays and converting them into nerve impulses that are transmitted to the brain. The retina extends posteriorly from the ora serrata to line the internal posterior surface of the eye wall. The ora serrata is the jagged margin between the photosensitive posterior region of the retina and the nonphotosensitive anterior region of the retina that continues anteriorly to cover ciliary body and the posterior side of the iris.
The retina lines the posterior two-thirds of the eyeball and contains the visual receptors, the rods and cones . Rods detect only the presence of light, whereas cones detect colors, which are the different wavelengths of visible light. Cones are most abundant in the center of the retina, especially an area called the macula lutea directly behind the center of the lens on what is called the visual axis. Rods are proportionally more abundant toward the periphery, or edge, of the retina. Our best vision in dim light or at night, for which we depend on the rods, is at the sides of our visual fields. Cones are most abundant in the center of the retina, especially an area called the macula lutea directly behind the center of the lens on what is called the visual axis. The fovea, which contains only cones, is a small depression in the macula and is the area for best color vision.
When light strikes the retina, the rods and cones generate impulses. These impulses are carried by ganglion neurons, which all converge at the optic disc and pass through the wall of the eyeball as the optic nerve. There are no rods or cones in the optic disc, so this part of the retina is sometimes called the “blind spot.” We are not aware of a blind spot in our field of vision, however, in part because the eyes are constantly moving, and in part because the brain “fills in” the blank spot to create a “complete” picture.
Rods and Cones(Photo receptors).
LOSS OF VISION FOR AGED PEOPLE..
An important cause of vision loss for people over 65 years of age is age-related macular degeneration (AMD), that is, loss of central vision, and some cases seem to have a genetic component. In the dry form of AMD, small fatty deposits impair circulation to the macula, and cells die from lack of oxygen. In the wet form of AMD, abnormal blood vessels begin leaking into the retina, and cells in the macula die from the damaging effects of blood outside its vessels. The macula, the center of the visual field, is the part of the retina we use most: for reading, for driving, for recognizing people, and for any kind of close work. People of all ages should be aware of this condition and that smoking and exposure to ultraviolet rays are risk factors.
Function of the Retina
The retina has three layers of neurons . The layer closest to the choroid contains the rod cells and cone cells; the middle layer contains bipolar cells; and the innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and the cone cells are sensitive to light, and therefore light must penetrate to the back of the retina before they are stimulated. The rod cells and the cone cells synapse with the bipolar cells, which in turn synapse with ganglion cells that initiate nerve impulses. In fact, the retina has as many as 150 million rod cells and 6 million cone cells but only one million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell.
As signals pass to bipolar cells and ganglion cells, integration occurs. Each ganglion cell receives signals from rod cells covering about one square millimeter of retina. This region is the ganglion cell’s receptive field. Some time ago, scientists discovered that a ganglion cell is stimulated only by nerve impulses received from the center of its receptive field; otherwise, it is inhibited. If all the rod cells in the receptive field receive light, the ganglion cell responds in a neutral way that is, it reacts only weakly or perhaps not at all. This supports the hypothesis that considerable processing occurs in the retina before nerve impulses are sent to the brain. Additional integration occurs in the visual areas of the cerebral cortex.
Blind Spot provides an opportunity to point out that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area.
From the Retina to the Visual Cortex , sensory fibers from the ganglion cells in the retina assemble to form the optic nerves. The optic nerves carry nerve impulses from the eyes to the optic chiasma. The optic chiasma has an X-shape formed by a crossing over of some of the optic nerve fibers. At the chiasma, fibers from the right half of each retina converge and continue on together in the right optic tract, and fibers from the left half of each retina converge and continue on together in the left optic tract. The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) in the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the primary visual areas of the occipital lobes.
The occipital lobes are a part of the cerebral cortex. The visual cortex consists of the primary visual area and the visual association areas of the occipital lobes. Notice that the image arriving at the thalamus, and therefore the primary visual areas, has been split because the left optic tract carries information about the right portion of the visual field and the right optic tract carries information about the left portion of the visual field. Therefore, the right and left visual cortex must communicate with each other for us to see the entire visual field. Also, because the image is inverted and reversed (see Fig. 4).The visual association areas are believed to rebuild the field and give us an understanding of it.
Figure 4. Optic chiasma.
The sclera is the thickest layer and is made of fibrous connective tissue that is visible as the white of the eye. The most anterior portion is the cornea, which differs from the rest of the sclera in that it is transparent. The cornea has no capillaries, covers the iris and pupil inside the eye, and is the first part of the eye that refracts, or bends, light rays.
The choroid layer contains blood vessels and a dark blue pigment (derived from melanin) that absorbs light within the eyeball and thereby prevents glare (just as does the black interior of a camera). The anterior portion of the choroid is modified into more specialized structures: the ciliary body and the iris.
The ciliary body (muscle) is a circular muscle that surrounds the edge of the lens and is connected to the lens by suspensory ligaments. The lens is made of a transparent, elastic protein, and, like the cornea, has no capillaries . The shape of the lens is changed by the ciliary muscle, which enables the eye to focus light from objects at varying distances from the eye.
Blood Supply to the Eyeball.
Both the choroid and the retina are richly supplied with blood. Two ciliary arteries pierce the sclera at the posterior aspect of the eyeball and traverse the choroid to the ciliary body and base of the iris. Although the ciliary arteries enter the eyeball independently, they anastomose (connect) extensively throughout the choroid. The central artery (central retinal artery) branches from the ophthalmic artery and enters the eyeball in contact with the optic nerve. As the central artery passes through the optic disc, it divides into superior and inferior branches, each of which then divides into temporal and nasal branches to serve the inner layers of the retina (see fig. 3). The central vein drains blood from the eyeball through the optic disc. The branches of the central artery can be observed within the eyeball through an ophthalmoscope (An instrument).
Cavities and Chambers of the Eyeball.
The interior of the eyeball is separated by the lens and its associated lens capsule into an anterior cavity and a posterior cavity . The anterior cavity is subdivided by the iris into an anterior chamber and a posterior chamber . The anterior chamber is located between the cornea and the iris. The posterior chamber is located between the iris and the suspensory ligament and lens. The anterior and posterior chambers connect through the pupil and are filled with a watery fluid called aqueous humor. The constant production of aqueous humor maintains an intraocular pressure of about 12 mmHg within the anterior and posterior chambers. Aqueous humor also provides nutrients and oxygen to the avascular lens and cornea. An estimated 5.5 ml of aqueous humor is secreted each day from the vascular epithelium of the ciliary body. From its site of secretion within the posterior chamber, the aqueous humor passes through the pupil into the anterior chamber. From here, it drains from the eyeball through the scleral venous sinus (canal of Schlemm) into the bloodstream. The scleral venous sinus is located at the junction of the cornea and iris. The large posterior cavity is filled with a transparent jellylike vitreous humor. Vitreous humor contributes to the intraocular pressure that maintains the shape of the eyeball and holds the retina against the choroid. Unlike aqueous humor, vitreous humor is not continuously produced; rather, it is formed prenatally. Additional vitreous humor forms as a person’s eyes become larger through normal body growth.
The optical components of the eye are transparent elements that admit light rays, bend (refract) them, and focus images on the retina. They include the cornea, aqueous humor, lens, and vitreous body.
The lens is a strong, yet deformable, transparent structure bounded by a dense, fibrous, elastic capsule.The lens is composed of flattened, tightly compressed cells called lens fibers. It is suspended behind the pupil by a fibrous ring called the suspensory ligament which attaches it to the ciliary body. Internally, it is composed of precisely arranged layers of cells that have lost their organelles and are filled completely by a protein called crystallin .The lens focuses incoming light onto the retina, and its shape determines the degree of light refraction. The suspensory ligaments attach to the lens capsule at its periphery, where they transmit tension that enables the lens to change shape. The tension in the suspensory ligaments comes from contraction of the ciliary muscles in the ciliary body. When the ciliary muscles relax, the ciliary body moves posteriorly, away from the lens, and so the tension on the suspensory ligaments increases. This constant tension causes the lens to flatten . A flattened lens is necessary for far vision . This shape of the lens is the “default” position of the lens.
The lens of the eye is normally transparent but may become opaque; this cloudiness or opacity is called a cataract. Cataract formation is most common among elderly people. With age, the proteins of the lens break down and lose their transparency. Longterm exposure to ultraviolet light (sunlight) seems to be a contributing factor, as is smoking. The cloudy lens does not refract light properly, and blurry vision throughout the visual field is the result. Small cataracts may be destroyed by laser surgery. Artificial lenses are available, and may be surgically implanted to replace an extensively cloudy lens. The artificial lens is not adjustable, however, and the person may require glasses or contact lenses for vision at certain distances
2.The aqueous humor
The aqueous humor is a serous fluid secreted by the ciliary body into a space between the iris and lens called the posterior chamber (fig. 3). It flows through the pupil forward into the anterior chamber, a space between the cornea and iris. From here, it is reabsorbed by a ringlike vessel called the scleral venous sinus (canal of Schlemm45). Normally the rate of reabsorption balances the rate of secretion.
3.The vitreous body.
The vitreous body (vitreous humor) is a transparent jelly that fills the large space behind the lens. An oblique channel through this body, called the hyaloid canal, is the remnant of a hyaloid artery present in the embryo.
Mechanism of Vision.
The visual process begins when light rays enter the eye, become focused on the retina, and produce a tiny inverted image. The cornea refracts incoming light rays toward the optical axis of the eye, and the lens makes relatively slight adjustments to fine focus the image.
Light stimuli are detected by photoreceptors in the retina.The stimulation of photoreceptors by incoming light causes a change in the rods and cones. These photoreceptor cells, in turn, signal the change to the bipolar cells, resulting in the stimulation of the ganglion cells and the generation of a nerve impulse. The visual image that is formed is a result of the processing and summation of information as it is collected and integrated in the retina. Due to this continual processing and integration, there are fewer cells in each layer, from photoreceptor to ganglionic neuron. Ganglionic axons converge to form the optic nerve. Optic nerves project from each eye through paired optic foramina and converge at the optic chiasm immediately anterior to the pituitary gland . Only the ganglionic axons originating from the medial region of each retina cross to the opposite side of the brain at the optic chiasm. The optic chiasm is a flattened structure anterior to the infundibulum that is the location of decussation (or crossing) of optic nerve axons. Ganglionic axons originating from the lateral region of each retina remain on the same side of the brain and do not cross. Optic tracts form laterally from the optic chiasm as a composite of ganglionic axons originating from the retinas of each eye. Upon entry into the brain, some axons within each optic tract project to the superior colliculi. Collectively, these projecting axons and the indirect motor pathway they stimulate are called the tectal system . This system coordinates the movements of the eyes, head, and neck in responding to visual stimuli. The remainder of the optic tract axons extend to the thalamus, specifically to the lateral geniculate nucleus , where visual information is processed within each thalamic body. Neurons from the thalamus project to the primary visual cortex of the occipital lobe for interpretation of incoming visual stimuli.
A condition in which the eyeball is too long. Light rays come into focus before they reach the retina and begin to diverge again by the time they fall on it. A short sighted person focus distance objects properly. A person who have this problem can only focus near objects clearly. This is because the light rays of distance object converge at a point in front of the retina.Myopia it Corrected with concave lenses, which cause light rays to diverge slightly before entering the eye.
Hyperopia/ Hypermetropia / Farsightedness.
This is a condition in which the eyeball is too short. The retina lies in front of the focal point of the lens, and the light rays have not yet come into focus when they reach the retina. Causes the greatest difficulty when viewing nearby objects. Corrected with convex lenses, which cause light rays to converge slightly before entering the eye.
Reduced ability to accommodate for near vision with age because of declining flexibility of the lens. Results in difficulty in reading and doing close handwork. Corrected with reading glasses or bifocal lenses. Astigmatism Inability to simultaneously focus light rays that enter the eye on different planes. Focusing on vertical lines, such as the edge of a door, may cause horizontal lines, such as a tabletop, to go out of focus. Caused by a deviation in the shape of the cornea so that it is shaped like the back of a spoon rather than like part of a sphere. Corrected with “cylindrical” lenses, which refract light more in one plane than another.Under this condition, the lens cannot change its shape. It is brought about by loss in elasticity of lens and ciliary muscle due to old age can be corrected by the use of convex lenses.
The lens gradually becomes cloudy so that light cannot pass through easily and the person cannot see properly. It may become gradually worse. The lens may have to be removed by operation and can be replaced by a plastic lens inside the eye.
This defect is common in old people, glaucoma is caused by pressure in the eye.
This is the genetic disorder in which a certain color cannot be distinguished by man. A common type is red green blindness, individual is not in position to determine/distinguish between red and green color.
This is the inability to see well in dim light or at night, is usually caused by a deficiency of vitamin A, although some night blindness may occur with aging. Vitamin A is necessary for the synthesis of rhodopsin in the rods. Without sufficient vitamin A, there is not enough rhodopsin present to respond to low levels of light. Color blindness is a genetic disorder in which one of the three sets of cones is lacking or nonfunctional. Total color blindness, the inability to see any colors at all, is very rare. The most common form is red-green color blindness, which is the inability to distinguish between these colors. If either the red cones or green cones are nonfunctional, the person will still see most colors, but will not have the contrast that the non-working set of cones would provide. So red and green shades will look somewhat similar, without the definite difference most of us see. This is a sex-linked trait; the recessive gene is on the X chromosome. A woman with one gene for color blindness and a gene for normal color vision on her other X chromosome will not be color blind but may pass the gene for color blindness to her children. A man with a gene for color blindness on his X chromosome has no gene at all for color vision on his Y chromosome and will be color blind.
If the photoreceptors, rods or cones are destroyed, the individual will be blind, even if the rest of the visual pathway is undamaged. The most common cause of blindness in the Western world is age-related macular degeneration, which results in destruction of the macula lutea, a yellowish area in the central region of the retina. The macula lutea contains a concentration of cones, especially in the fovea centralis. Individuals with this condition have a distorted visual field: Blurriness or a blind spot is present, straight lines may look wavy, objects may appear larger or smaller than they are, and colors may look faded (Fig. 9B). There are two main forms of age-related macular degeneration. “Wet” macular degeneration means that abnormal growth of new blood vessels is evident in the region of the macula. The blood vessels leak serum and blood, and the retina becomes distorted, leading to severe scarring that completely destroys the macula. “Dry” macular degeneration is not accompanied by the growth of blood vessels, and visual loss is less dramatic. Heredity plays a role in the development of age-related macular degeneration: 15% of people with a family history of the condition develop the disease after age 60. Also, light-eyed people tend to be afflicted more frequently than dark-eyed people. Smoking, hypertension, and excessive sun exposure are possible contributing factors. A yearly eye examination assists in the early detection of many eye diseases, including macular degeneration, cataracts, and glaucoma. When an ophthalmologist presents an Amsler grid (a crosshatched pattern of straight lines) to someone with macular degeneration, the grid looks blurred, distorted, or discolored. Signs of the “wet” form can be detected by an examination of the retina and confirmed by a fluorescein angiogram. In this test, a number of pictures are taken of the macula lutea after an orange dye has been injected into a vein in the patient’s arm. Currently, the treatment for the “dry” form of macular degeneration is the use of vitamin and mineral supplements, which may help stem the disease. For example, research indicates that consumption of zinc may prevent further loss of vision. On the other hand, when the “wet” form of the disease is diagnosed early, laser treatment can sometimes stop the growth of blood vessels. Although people with age-related macular degeneration are classified as blind, they still have normal peripheral vision (outside the macula), which they can learn to use effectively. Because the periphery of the retina contains a high concentration of rods, vision there is less acute, and colors are not detected. But highpowered eyeglasses, magnifying devices, closed-circuit television, and special lamps can help patients see details more clearly. Accumulating evidence suggests that both macular degeneration and cataracts, which tend to occur in the elderly, are caused by long-term exposure to the ultraviolet rays of the sun. Therefore, everyone—especially people who live in sunny climates or work outdoors—should wear sunglasses that absorb ultraviolet light. Large lenses worn close to the eyes offer further protection. The Sunglass Association of America has devised a system for categorizing sunglasses, which is helpful.
IMPORTANT THING TO KNOW.
Normal visual acuity is referred to as 20/20; that is, the eye should and does clearly see an object 20 feet away. Nearsightedness (myopia) means that the eye sees near objects well but not distant ones. If an eye has 20/80 vision, this means that what the normal eye can see at 80 feet, the nearsighted eye can see only if the object is brought to 20 feet away. The nearsighted eye focuses images from distant objects in front of the retina, because the eyeball is too long or the lens too thick. These structural characteristics of the eye are hereditary. Correction requires a concave lens to spread out light rays before they strike the eye. Farsightedness (hyperopia) means that the eye sees distant objects well. Such an eye may have an acuity of 20/10, that is, it sees at 20 feet what the normal eye can see only at 10 feet. The farsighted eye focuses light from near objects “behind” the retina, because the eyeball is too short or the lens too thin. Correction requires a convex lens to converge light rays before they strike the eye. As we get older, most of us will become more farsighted (presbyopia). As the aging lens loses its elasticity, it is not as able to recoil and thicken for near vision, and glasses for reading are often necessary. Astigmatism is another error of refraction, caused by an irregular curvature of the cornea or lens that scatters light rays and blurs the image on the retina. Correction requires a lens ground specifically for the curvature of the individual eye.
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