The Eye

Information About Vision Loss And Blindness The Eye and Its Parts > 12 Signs of Vision Loss Myths About Vision Eye Conditions Education and Training Adaptive Technology Reading Adaptive Products Home Adaptations Organizations Recreation Meeting Someone All about Braille Eye Safety Tips E - Mail Me Updated 9/02/07 Drawings of the Eye Drawing of the eye Cross section drawing of the eye - (side view) with major parts labeled. Drawing of the eye Cross section drawing of the eye - (rear view). Drawing of the eye in its socket Cut-away view of the eye in its socket showing the: bony socket, orbital muscles, eyelids and eyelashes. Drawing of the eye The lacrimal system - (tear ducts) produce tears to clean, moisten and lubricate the eyes and then drains the excess fluid into the nose. Drawing of the optic pathways Our Eyes and brain divide what we see into a right and left half. In the drawing above, light gray represents the left half; dark gray represents the right half. The eyes invert the image and the left side of what we see ends up in the right side of our brain and visa versa. This all works out because the right side of our brain controls the left side of our bodies and visa versa. Anterior Chamber The space between the cornea and iris filled with Aqueous Humor. Aqueous Humor A water like fluid, produced by the ciliary body, it fills the front of the eye between the lens and cornea and provides the cornea and lens with oxygen and nutrients. It drains back into the blood stream through the canals of schlemm. Brain The brain is where the electrical signals sent from our eyes are processed into vision. Damage to the brain can lead to vision loss if the visual cortex or optic pathways are damaged. The majority of nerve fibers in the optic tract connect to the LGN. Several nerve fibers leave the optic tract before the LGN to connect to sub cortical structures through out the brain. These parts of the brain regulate things like: eye and head movements, pupillary light reflex - (pupil size), and circadian rhythms - (light/dark cycle). Damage to these parts of the brain often leads to vision disorders too. Canals of Schlemm These canals are located around the perimeter of the iris. They allow aqueous fluid to drain back into the blood stream. The Trabecular Meshwork along with the Canals of Schlemm regulate the eyes internal pressure. In the eye disease called glaucoma, these canals become blocked leading to increased pressure. The increased pressure, from this condition, destroys the optic nerve. Choroid The choroid is a layer of blood vessels between the retina and sclera; it supplies blood to the retina. In the disease called Macular Degeneration, abnormal blood vessels grow into the space between the retina and choroid damaging the macula. Ciliary Body This is where the Aqueous Humor is produced. Ciliary Muscle The eye can bring the fine print in a phone book into focus, or focus in on the moon over ¼ million miles away. The ciliary muscle changes the shape of the lens - (this is called accommodation). It relaxes to flatten the lens for distance vision; for close work it contracts rounding out the lens. Everyone will develop an eye condition called presbyopia. As we age, the ciliary muscle and crystalline lens lose their elasticity. This is why most people need reading glasses by their 40's. Conjunctiva The conjunctiva is a thin, clear membrane covering the front of the eye and inner eyelids. Cells in this lining produce mucous that helps to lubricate the eye. This is the eyes first layer of protection against infection. Inflammation of this membrane is called conjunctivitis, or pink eye. Cornea The cornea is a clear, dome-shaped surface that covers the front of the eye. It is the first and most powerful lens in the eye's optical system. To keep it transparent the cornea contains no blood vessels. Tears that flow over it and aqueous humor in the chamber behind it keep it nourished. When you hear of eye banks and eye transplants, it is the cornea that is being replaced. The cornea can be damaged from: accidents, infections, and genetic defects. Crystalline Lens The eye's crystalline lens works like the adjustable lens in a camera. Positioned just behind the cornea; it is responsible for keeping images in focus on the retina. It is adjustable for distance and close work. A cataract is the lens clouding up. This happens to most people as they age. A few people are even born with cataracts. Modern surgery has all but eliminated cataracts as a cause of blindness in the developed world. Eyeball The eye is like a little video camera measuring about 1 inch or 2.5 cm. in diameter. If someone's eyeball is larger then this, they will be nearsighted (myopic); if it is smaller then this, they will be farsighted (hyperopic). Having two eyes gives us binocular vision - (depth perception). This is due to the fusing of both images in the visual cortex. Eyelashes and Eyebrows These specialized hairs protect the eyes from particles that may injure them. They form a screen to keep dust and insects out. Anything touching them triggers the eyelids to blink. Eyelids Our eyelids protect and lubricate our eyes. Small oil-producing glands line the inner edge of our eyelids. These oils mix with tears when we blink, keeping the eye moist and clean. Eye Socket The orbit or eye socket is a cone-shaped bony cavity that protects the eye. The socket is padded with fatty tissue that allows the eye to move easily. Fovea - (small pit) The fovea is an indentation in the center of the macula. Its diameter is only 1.5 mm or about 1/16 inch. This small part of our retina is responsible for our highest visual acuity. It is the center of our central vision. Lacrimal Gland - (Tear Duct) This gland continually releases tears and other protective fluids onto the surface of the eye. It lubricates and keeps the cornea from becoming dehydrated. Lacrimal Sac The lacrimal sac is a tiny pump that drains tears and other debris from the eye. The fluids flow down the nasolacrimal duct into the nose where they help keep the nasal linings moist. This is why your nose runs when you cry. Lateral Geniculate Nucleus - (LGN) This part of the brain acts as a relay station; it decodes visual information from the optic tract before sending it to the visual cortex for final processing. Lens Sack or Capsule During modern cataract surgery the outer membrane of the lens is left in place. The artificial intraocular lens is placed in this sack. Iris This is the colored part of the eye: brown, green, blue, etc. It is a ring of muscle fibers located behind the cornea and in front of the lens. It contracts and expands, opening and closing the pupil, in response to the brightness of surrounding light. Just as the aperture in a camera protects the film from over exposure, the iris of the eye helps protect the sensitive retina. Macula - (yellow spot) This part of the retina is the most sensitive. Its diameter is only 7 mm or about 1/4 inch. It is responsible for our central, or reading vision. This part of the retina gives us 20/20 vision. Without the macula, you would be blind - Legally Blind that is. People with eye diseases like Macular Degeneration have vision from 20/200 to 20/800. Optic Chiasm This is the first part of the brain to receive visual input. Each eye takes a slightly different picture of the world. At the optic chiasm each picture is divided in half. The outer left and right halves continue back toward the visual cortex. The inner left and right halves cross over to the other side of the brain then continue back toward the visual cortex. See Drawing of optic pathways. Optic Disk The optic disk is the spot on the retina where the optic nerve leaves the eye. There are no sensory cells here, creating a blind spot. Each eye covers for the blind spot of the other eye and the brain fills in the missing information. Optic Nerve Each optic nerve has about 1.2 million nerve fibers. This is the cable connecting the eye to the brain. Optic Tract The nerves that connect the optic chiasm to the LGN are called the optic tract. If one of these tracts is damaged, vision will be lost in one side of each eye. Orbital Muscles Six muscles are in charge of eye movement. Four of these move the eye up, down, left and right. The other two control the twisting motion of the eye when we tilt our head. Defects in these muscles and the nerves that control them lead to conditions like Nystagmus and Amblyopia (Lazy Eye). Photoreceptor Cells The retina is composed of two types of photoreceptor cells. When light falls on one of these cells, it causes a chemical reaction that sends an electrical signal to the brain. Cone cells give us our detailed color daytime vision. There are 6 million of them in each human eye. Most of them are located in the central retina - macula fovea area. There are three types of cone cells: one sensitive to red light, another to green light, and the third sensitive to blue light. Rod cells are about 500 times more sensitive to light then cone cells; they give us our dim light or night vision. They are also more sensitive to motion then cone cells. There are 120 million rod cells in the human eye. Most rod cells are located in our peripheral or side vision. Posterior Chamber The space between the iris and the front of the lens filled with Aqueous Humor. Pupil The pupil is the hole in the center of the iris that light passes through. The iris muscles control its size. Retina The retina is the film of the eye. It converts light rays into electrical signals and sends them to the brain through the optic nerve. The sides of the retina are responsible for our peripheral vision. The center area, called the macula, is used for our fine central vision and color vision. The retina is where most the problems leading to vision loss Occur. Three of the leading causes of blindness, from retina damage, are Retinitis Pigmentosa, Macular Degeneration and Diabetic Retinopathy. Retinal Blood Vessels A doctor can see the blood vessels that supply the retina when he looks into your eyes. These vessels are in the choroid just beneath the retina. Abnormal blood vessel growth and leaking blood vessels are the cause of vision loss in eye conditions like, Diabetic Retinopathy, ROP, and Macular Degeneration. Retinal Pigment Epithelium - (RPE) The RPE is a layer of cells between the retina and choroid. The inside of a camera is panted black to absorb scattered and reflected light. The black pigment known as melanin in the RPE dose the same job for the eye. The RPE gets rid of waste products produced by the photoreceptor cells. As we age, the RPE can sometimes lose its ability to process this waste. Deposits of this waste, called drusen, can distort and damage the retina leading to an eye condition called dry macular degeneration. Sclera The sclera is the white, tough wall of the eye. It along with internal fluid pressure keeps the eyes shape and protects its delicate internal parts. Uvea The uvea is the middle Vascular layer of the eye. It is made up of three parts: the iris, ciliary body and chorid. Uveitis is the inflammation (or swelling) of these parts of the eye. Visual Axis The Visual Axis is an imaginary line drawn through the center of the pupil to the center of the Fovea. The orbital muscles keep the visual axis of both eyes aligned on the center of what you are looking at (fixation point). An eye condition called Strabismus - (misaligned eyes) results when the orbital muscles fail to keep the eyes in alignment. Any damage to eye structures along this axis leads to severe vision loss. Visual Cortex The part of the brain that processes and combines visual information from both eyes and converts it into sight. Damage to the visual cortex results in a condition called cortical blindness. Visual Fields The retina of each eye has two sections the nasal retina - (nose side) and temporal retina - (ear side). For example: with your right eye, you see the right half of the world with your nasal retina; you see the left half of the world with your temporal retina. The picture your eye takes is flipped left for right and upside down; its up to the brain to sort things out. Vitreous Cavity The space between the lens and retina filled with the gel like Vitreous Humor. Vitreous Humor The vitreous humor is a jelly like liquid that fills most of the eye (from the lens back). As we age it changes from a gel to a liquid and gradually shrinks separating from the retina. This is when people start seeing floaters, dark specs in their vision. This is a normal sign of aging, but in a few cases the retina can become detached as the vitreous separates. Zonules Zonules are hundreds of string like fibers that hold the lens suspended in position and enable it to change shape for near or distant vision. http://www.99main.com/~charlief/Blindness.htm