NiaChus: Anatomy of the eye

Anatomy of the eye

The cornea is the clear surface of the outer eye. It is about ½ mm thick and consists of five layers: epithelium, Bowman’s membrane, stroma, Descemet’s membrane and the endothelium layer. It has two main functions. First, it acts as a barrier preventing germs, dirt and other harmful material from entering the inner eye. Secondly, the cornea acts as the eye's outermost lens. It functions like a window that controls and focuses the entry of light into the eye. The cornea contributes between 65-75 percent of the eye's total focusing power and when light strikes the cornea, it bends, or refracts, the incoming light onto the lens.

The iris gives the eye its colour. This colour is genetically determined. It is made up of three layers of connective tissue and muscle fibers: endothelium, stroma and the epithelium. Its main function is to control the amount of light that is let into the eye. In bright light the muscles contract causing the opening at the centre of the iris (the pupil) to constrict. In dim light the muscles dilate allowing more light into the eye.

The pupil is the opening at the centre of the iris that lets light into the eye. It changes size in response to light levels.

The lens is a transparent structure of about 5 mm thick with a diameter of about 9 mm and is positioned directly behind the iris. It is made of proteins called crystallins. Its function is to focus light onto the retina. The lens is flexible and its curvature is controlled by the nervous system through a system of muscles around the lens. Changing the curvature of the lens allows focus on objects at different distances. The lens is encased in a capsule and suspended within the eye by zonule fibres.

The vitreous gel (also known as the vitreous humour) is a clear, thick, substance that fills the centre of the eye. It is comprised mainly of water and makes up approximately 2/3 of the eye's volume, giving it form and shape. It is contact with the retina and help keeps the retina in place.

The retina is a multi-layered sensory tissue of neural cells that lines the back of the inside of the eye. It contains 3 layers of nerve cells including the outermost layer of sensory photoreceptor cells that capture light rays and convert them into electrical impulses which are transmitted by the optic nerve to the brain. Photoreceptors comprise two types: rods and cones. Each retina comprises approximately 125 million rods. These are responsible for peripheral vision and function best in dimlight. There are approximately 6.5 million cones in a human eye and these are more concentrated in the macula, most densely in the fovea. Cones are essential for vision in bright light and for seeing colours. The outer layer of the retina is known as the retinal pigment epithelium (RPE) layer. This layer helps nourish the photoreceptor cells and is attached to the choroid which provides the RPE with this nourishment which includes oxygen. The innermost layer of the choroid is known as Bruch’s membrane.

The macula is situated roughly at the centre of the retina. It is the focus for incoming light and as such is responsible for central vision and the ability to see detail. It has a diameter of approximately 1.5mm.

The fovea is a small pit of around 0.3mm near the centre of the macula which has the highest concentration of cone cells and is free of rod cells.

The optic nerve is the nerve that transmits visual information in the form of electrical impulses from the retina to the brain. It connects to the back of the eye near to the macula. The photoreceptor cells of the retina are not present in the optic nerve. As a result this creates a blind spot in our field of vision at the point on the retina where the optic nerve leads back into the brain. This is not normally noticeable because the vision of one eye overlaps with that of the other.

The Anatomy of the Eye

The human eye is a complex anatomical device that remarkably demonstrates the architectural wonders of the human body. Like a camera, the eye is able to refract light and produce a focused image that can stimulate neural responses and enable the ability to see. In Lesson 6, we will focus on the physics of sight. We will use our understanding of refraction and image formation to understand the means by which the human eye produces images of distant and nearby objects. Additionally, we will investigate some of the common vision problems that plague humans and the customary solutions to those problems. As we proceed through Lesson 6, we will apply our understanding of refraction and lenses to the physics of sight.

The eye is essentially an opaque eyeball filled with a water-like fluid. In the front of the eyeball is a transparent opening known as the cornea. The cornea is a thin membrane that has an index of refraction of approximately 1.38. The cornea has the dual purpose of protecting the eye and refracting light as it enters the eye. After light passes through the cornea, a portion of

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it passes through an opening known as the pupil. Rather than being an actual part of the eye's anatomy, the pupil is merely an opening. The pupil is the black portion in the middle of the eyeball. Its black appearance is attributed to the fact that the light that the pupil allows to enter the eye is absorbed on the retina (and elsewhere) and does not exit the eye. Thus, as you sight at another person's pupil opening, no light is exiting their pupil and coming to your eye; subsequently, the pupil appears black.

Like the aperture of a camera, the size of the pupil opening can be adjusted by the dilation of the iris. The iris is the colored part of the eye - being blue for some people and brown for others (and so forth); it is a diaphragm that is capable of stretching and reducing the size of the opening. In bright-light situations, the iris adjusts its size to reduce the pupil opening and limit the amount of light that enters the eye. And in dim-light situations, the iris adjusts so as to maximize the size of the pupil opening and increase the amount of light that enters the eye.

Light that passes through the pupil opening, will enter the crystalline lens. The crystalline lens is made of layers of a fibrous material that has an index of refraction of roughly 1.40. Unlike the lens on a camera, the lens of the eye is able to change its shape and thus serves to fine-tune the vision process. The lens is attached to the ciliary muscles. These muscles relax and contract in order to change the shape of the lens. By carefully adjusting the lenses shape, the ciliary muscles assist the eye in the critical task of producing an image on the back of the eyeball.

The inner surface of the eye is known as the retina. The retina contains the rods and cones that serve the task of detecting the intensity and the frequency of the incoming light. An adult eye is typically equipped with up to 120 million rods that detect the intensity of light and about 6 million cones that detect the frequency of light. These rods and cones send nerve impulses to the brain. The nerve impulses travel through a network of nerve cells. There are as many as one million neural pathways from the rods and cones to the brain. This network of nerve cells is bundled together to form the optic nerve on the very back of the eyeball.

Each part of the eye plays a distinct part in enabling humans to see. The ultimate goal of such an anatomy is to allow humans to focus images on the back of the retina. This task is discussed in the next part of Lesson 6.

Eye Health Center

Image Collection:

Human Anatomy

Picture of the Eyes

The eye is a slightly asymmetrical globe, about an inch in diameter. The front part of the eye (the part you see in the mirror) includes:

• The iris (the pigmented part)

• The cornea (a clear dome over the iris)

• The pupil (the black circular opening in the iris that lets light in)

• The sclera (the white part)

• The conjunctiva (an invisible, clear layer of tissue covering the front of the eye, except the cornea)

Just behind the iris and pupil lies the lens, which helps to focus light on the back of the eye. Most of the eye is filled with a clear gel called the vitreous. Light projects through the pupil and the lens to the back of the eye. The inside lining of the eye is covered by special light-sensing cells that are collectively called the retina. The retina converts light into electrical impulses. Behind the eye, the optic nerve carries these impulses to the brain. The macula is a small sensitive area within the retina that gives central vision. It is located in the center of the retina and contains the fovea, a small depression or pit at the center of the macula that gives the clearest vision.

Eye color is created by the amount and type of pigment in the iris. Multiple genes inherited from each parent determine a person’s eye color.

Eye Conditions

Age-related macular degeneration: A loss of central vision in both eyes.

Amblyopia (lazy eye): One eye sees better than the other, a problem of childhood development. The weaker eye may or may not “wander.” The weaker eye is called the "lazy eye."

Astigmatism: A defect that causes an inability to properly focus light onto the retina. Astigmatism causes blurry vision that can be corrected with glasses or contact lenses.

Black eye: Swelling and discoloration around the eye as a result of injury to the face.

Blepharitis: Inflammation of the eyelids near the eyelashes. Blepharitis is a common cause of itching or a feeling of grit in the eyes.

Cataract: A clouding of the lens, which hinders the passage of light through the lens.

Chalazion: An oil-making gland gets blocked and swells into a bump. Often confused with styes, chalazions are not caused by infections.

Conjunctivitis: Also known as "pinkeye,” conjunctivitis is an infection or inflammation of the conjunctiva, the clear layer that covers the front of the eye. It is usually caused by allergies, a virus, or a bacterial infection.

Corneal abrasion: A scratch on the clear part of the front of the eye. Pain, light sensitivity, or a feeling of grit in the eye are the usual symptoms.

Diabetic retinopathy: High blood sugar damages blood vessels in the eye. Eventually, weakened blood vessels may overgrow the retina or bleed, threatening vision.

Diplopia (double vision): Seeing double can be caused by many serious conditions. Diplopia requires immediate medical attention.

Dry eye: Either the eyes don’t produce enough tears, or the tears are of poor quality. Dry eye can be caused by medical problems such as lupus, scleroderma, and Sjogren's syndrome.

Glaucoma: Increased pressure inside the eye slowly reduces vision. Peripheral vision is lost first, often going undetected for years.

Hyperopia (farsightedness): Inability to see near objects clearly. The eye is “too short” for the lens, or certain eye muscles have weakened with age.

Hyphema: Bleeding into the front of the eye, behind the cornea. Hyphema is usually caused by trauma.

Keratitis: Inflammation or infection of the cornea. Keratitis typically occurs after germs enter a corneal abrasion.

Myopia (nearsightedness): Inability to see clearly at a distance. The eye is “too long” for the lens, so light isn’t focused properly on the retina.

Optic neuritis: The optic nerve becomes inflamed, usually from an overactive immune system. Painful vision loss in one eye typically results.

Pterygium: A thickened conjunctival mass usually on the inner part of the eyeball. It may cover a part of the cornea, causing vision problems.

Retinal detachment: The retina comes loose from the back of the eye. Trauma and diabetes are common causes of this medical emergency.

Retinitis: Inflammation or infection of the retina. Retinitis may be a long-term genetic condition or result from a viral infection.

Scotoma: A blind or dark spot in the visual field.

Strabismus: The eyes do not point in the same direction. The brain may then favor one eye, causing decreased vision (amblyopia) in the other eye.

Stye: Bacteria infect the skin on the edge of the eyelid, creating a tender red bump.

Uveitis (iritis): The colored part of the eye becomes inflamed or infected. An overactive immune system, bacteria, or viruses can be responsible. Eye Health Center

Eye Tests

Tonometry: A test that measures pressure in the eye, called intraocular pressure. Tonometry is used to check for glaucoma.

Slit lamp examination: A physician or optometrist shines a vertical slit of light across your eye while examining through a magnifying glass. This general exam can detect many eye problems.

Fundoscopic exam: Dilating drops first widen the pupil. By shining bright light in the back of the eye, the examiner can view the retina.

Refraction: If vision is impaired, a series of lenses are placed before the eyes to determine the right corrective lens prescription.

Visual acuity test: Reading ever-smaller-sized letters across the room identifies distance vision problems. Reading up-close can identify problems with near vision.

Fluorescein angiography: A fluorescent dye is used to take a sequence of retinal images.

Regular adult eye exam: This collection of tests may include the ones mentioned above plus others, such as eye movement.

Eye Treatments

Contact lenses and glasses: Glasses or contact lenses correct refractive errors such as nearsightedness, farsightedness, and astigmatism.

LASIK (laser assisted in situ keratomileusis): A doctor cuts a flap in the cornea with a tiny saw. A laser reshapes the cornea’s surface, improving nearsightedness. Used for correcting myopia, hyperopia, and astigmatism.

Radial keratotomy (RK): A series of small incisions are made in the cornea to correct nearsightedness. Radial keratotomy is rarely used today.

Photorefractive keratectomy (PRK): A doctor rubs off the surface of the cornea, then uses a laser to improve nearsightedness. The cornea then heals and grows back.

LASEK (laser epithelial keratomileusis): Similar to LASIK, but no flap is cut in the cornea. Instead, the topmost layer of cornea cells is pulled off, allowing the laser to reshape the cornea.

Artificial tears: Eyedrops with similar composition to natural tears, used to treat dry or irritated eyes.

Cyclosporine eye drops (Restasis): When dry eye is from a condition called keratoconjunctivitis sicca, immune-suppressing eye drops could help.

Laser photocoagulation: A doctor uses a laser to burn blood vessels in the retina that are leaking or growing abnormally. Laser photocoagulation is most often done for diabetic retinopathy.

Cataract surgery: The cloudy cataract is removed from the lens and replaced by a manmade lens.