The eye is a specialized organ to provide sight. The various parts of the eye work to assist in this process. There are two eyes per human (many more if you are a spider or a scallop). The paired human eyes allow for improved depth perception. It takes two eyes for a human to best determine how far an object lies in front of him or her. Each eye has a slightly different view and the difference between the two images is used by the brain to make a three-dimensional (3-D) image. Test: you can see the difference in depth perception by trying to thread a needle or perform a similar fine task with one eye compared with two eyes. If the two eyes are not lined up straight, a person sees two images (double vision).
Shape of the eye
The shape of the eye is fairly round like a ball. This design helps the eye determine what direction light is coming from (see video: evolution of the eye). In near-sighted eyes, the eye becomes somewhat elongated (like an egg). This appears to be due to an adaptive response of the eye to aid in focusing at near. However, the elongation of the eye causes the tissues to stretch and this predisposes the eye to retinal detachment.
The Tear Film
This is an often-overlooked part of the eye. The tears form a smooth surface over the front of the cornea and help avoid scattering of light as it enters the eye. The tear film also provides nutrition, protection (antibacterial), and cleansing of the surface of the eye. The tear film is composed of three parts/layers: the aqueous (water), mucin, and lipid layers. The aqueous component is produced by lacrimal glands that rest in the eye socket behind a bone under the eye brow. The mucin layer is produced by specialized cells (goblet cells) in the conjunctiva. The mucin mixes with the aqueous layer and adds structure to the tears helping them to remain on the eye surface. The lipid layer (outer surface) produced by meibomian glands along the edge of the eye lids. The lipid helps protect against evaporation of the tears from the surface of the eye. Various conditions and diseases interfere with one or more layers of the tear film leading to dry eyes and reduced vision.
The Conjunctiva
The conjunctiva (not labelled on image) is a smooth moveable outer “skin” overlying the white sclera (see below). The conjunctiva acts as a physical barrier against entry of infectious organisms, and contains many blood vessels which dilate and become inflamed if infection or foreign objects threaten to damage the eye. The smooth surface of the conjunctiva and loose attachment to the underlying sclera helps the eye turn smoothly.
The Sclera
The sclera is the tough outer coat of the eye. It provides structural support and protection of the sensitive tissues (like the retina) inside the eye. The sclera does allow for the outflow of water that is produced by the ciliary body. It also allows medications to pass into the eye. Nerves and blood vessels penetrate the sclera to reach the inside of the eye.
The Cornea
The cornea is the clear window at the front of the eyeball that allows light to enter the eye. It is continuous with the sclera. Most of the focusing of light happens at the corneal surface/tear film. As this outer layer of the eye is so critical for sight, there are many nerve endings in the cornea making it the most sensitive part of the eye to touch. The outer-most layer is the epithelium: a smooth surface to transmit light with tightly layered cells difficult for infections to penetrate. The epithelium acts as a barrier to keep the water-filled tears from entering the cornea as this would make the cornea lose its clarity. The middle layer of the cornea is called the stroma. It occupies most of the cornea with orderly layers of protein fibers to transmit light and minimize scattering. There is very little water in the stroma because the inner-most layer of the cornea (the endothelium) pumps out water from the cornea into the eye. When the cornea becomes hazy or opaque, a corneal transplant may be performed.
The Iris and Pupil
The iris is responsible for the color of the eye as seen from the outside. A circular opening in the iris is called the pupil, which is seen as a black spot in the center of the iris. Iris muscles expand and contract to change the size of the pupil and alters the amount of light entering the eye. In bright light, the pupil constricts so as to prevent too much light from entering the eye. There is no specific function of iris color and iridology is not a true science. However, lighter-colored irises (irides) may not block as much light as dark irides. From a medical perspective, eyes with lighter color eyes may be at higher risk of developing macular degeneration. Darker eyes are seen among people who evolved nearer the equator. In birds, iris color appears to play a role in the mating ritual.
The lens
The lens inside the eye is made of specialized crystalline protein fibers that help to focus light and allow for flexibility of the lens in order to focus light from different distances from the eye. With age the eye loses its ability to focus light; therefore, reading glasses are needed by most people around the age of 40 years. When the clear lens turns foggy, the lens is called a cataract. Surgery may be performed to remove a cataract and replace it with a clear plastic lens implant.
The ciliary body (SILL-ee-air-ee)
The ciliary body has two functions inside the eye. It contains muscles to focus the lens (the ciliary muscles are attached to the lens by fine fibers called zonules). The ciliary body also has a pump that produces aqueous (water). The aqueous pump works to keep the eye inflated. The aqueous also provides nutrition to the structures inside the eye. There is a constant flow of aqueous into the eye via the ciliary body and out of the eye through the sclera (i.e. the trabeculum). If the trabecular drain becomes blocked, the pressure in the eye goes up and damage may occur to the optic nerve. This condition is called glaucoma.
The Vitreous
The vitreous is a clear gel that fills most of the eye. There are no blood vessels and very few cells in the vitreous. The clarity of the vitreous is important in order to allow light to pass from the lens to reach the retina. The vitreous is mainly composed of water, but there are fine protein fibers and a gel (hyaluronin) providing a unique structure. If the eye becomes cut from an accidental trauma, the vitreous gel may plug the hole in the sclera keeping the eye from deflating. The vitreous also appears to protect the lens; there are anti-oxidants in the vitreous gel that help keep the lens clear.
With age, the vitreous loses its gel-like quality and the protein fibers begin to clump together. When this happens, fiber-like floaters may appear in the vision. These floaters occur earlier in life in near-sighted eyes and in eyes following inflammation or trauma. Occasionally, the vitreous fibers will pull on the retina causing brief, streak-like flashes of light in the vision. The pulling may cause a retinal break (retinal tear), which may lead to retinal detachment. In other situations, the vitreous fibers may pull on the macula resulting in distortion of vision from vitreo-macular traction syndrome, epiretinal membrane, or macular hole.
The Retina
The retina is a light-sensitive layer of nerve tissue that lines the inside of the eye wall. It acts like the film in a camera. The central portion of the retina is called the macula. The macula is designed for central vision. When you read or see fine details, you move your eye so that light focuses on the macula. The rest of the retina is used for peripheral vision (side-vision). The peripheral vision is essential for walking around a room without bumping into things. The peripheral retina is also sensitive to detect movement in the environment. The blood supply to the inner retinal layers (ten layers in all) comes from retinal blood vessels that enter and exit the eye through the optic nerve. The outer retinal layers are supplies by blood vessels in the choroid (see below). Blockage of the blood supply (retinal artery occlusion or retinal vein occlusion) causes a blind spot in the vision or blurred vision. If the retina becomes detached from the eye wall, it does not function well, and surgery is required to recover vision. Aging may result in macular degeneration.
The Choroid
The choroid is a layer of tissue under the retina filled with blood vessels. This important blood vessel layer provides oxygen and nutrition to the retina. It also evacuates waste materials from the outer retina and acts as a heat sump, keeping the retina from overheating. The choroid may be affected by a number of problems including inflammation, blood vessel blockage, and central serous retinopathy.
The Optic Nerve
The optic nerve connects the nerve tissue of the retina to the nerve tissue in the brain, like wire in an electric circuit. The optic nerve is sensitive to the pressure inside the eye. High intra-ocular pressure may cause loss of vision or blindness from glaucoma. The optic nerve may also be affected by blocked blood flow, inflammation, and pressure from outside the eye (tumors and aneurysms).
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