Eye Anatomy Introduction Even though the eye is small, only about 1 inch in diameter, it serves a very important function - sense of sight. Vision is the most used of the 5 senses and is one of the primary means that we use to gather information from our surroundings. The eye is often compared to a camera. Each gathers light and then transforms that light into a �picture.� Both also have lenses to focus the incoming light. A camera uses the film to create a picture, whereas the eye uses a specialized layer of cells, called the retina, to produce an image. Orbit The orbit is the eye socket, which is formed by the cheekbone, the forehead, the temple, and the side of the nose. The eye is cushioned within the orbit by pads of fat. The orbit also contains the lacrimal gland that is located underneath the outer portion of the upper eyelid. The lacrimal gland produces tears that help lubricate and moisten the eye, as well as flush away any foreign matter that may enter the eye. The tears drain away from the eye through the nasolacrimal duct, which is located at the inner corner of the eye into the nasal cavity. Eyelids and Eyelashes The eyelids serve to protect the eye from foreign matter, such as dust, dirt, as well as bright light that might damage the eye. When you blink, the eyelids also help spread tears over the surface of your eye, keeping the eye moist and comfortable. The eyelashes help filter out foreign matter, including dust and debris, and prevent it from getting into the eye. Conjunctiva The conjunctiva is a thin, clear layer of skin covering the front of the�eye, including the sclera and the inside of the eyelids.�The conjunctiva keeps bacteria and foreign material from getting behind the eye. Sclera The white part of your eye that you see when you look at yourself in the mirror is the front part of the sclera. However, the sclera, a tough, leather-like tissue, also extends around the eye, the sclera surrounds the eye and gives the eye its shape. The sclera is also attached to the extraocular muscles, which, in turn, move the eye left and right, up and down, and diagonally. Cornea The cornea is a clear layer at the front and center of the eye. In fact, the cornea is so clear that you may not even realize it is there.�The cornea�is located just in front of the iris, which is the colored part of your eye. The main purpose of the cornea is to help focus light as it enters the eye. If you wear contact lenses, the contact lens rests on your cornea. Anterior Chamber The anterior chamber is the fluid-filled space immediately behind the cornea and in front of the iris. The fluid that fills this chamber is called the aqueous humor. The aqueous humor helps to nourish the cornea and the lens. Iris and Pupil The iris, which is the colored part of your eye, controls the amount of light that enters the eye.�The iris�is a ring shaped tissue with a central opening, called the pupil. The iris has a ring of muscle fibers around the pupil, which, when contracted, causes the pupil to constrict (become smaller) in bright light. Another set of muscle fibers radiate outward from the pupil, which causes the pupil to dilate (become larger) in dim light or darkness. Anterior Chamber Angle/Trabecular Meshwork The anterior chamber angle and the trabecular meshwork are located where the cornea meets the iris. The trabecular meshwork is important because it is the site where the aqueous humor drains out of the eye. If the aqueous humor cannot properly drain out of the eye, the pressure can build up inside the eye, causing optic nerve damage and eventually vision loss, a condition known as glaucoma. Posterior Chamber The posterior chamber is the fluid-filled space immediately behind the iris but in front of the lens. The fluid that fills this chamber is called the aqueous humor. The aqueous humor helps to nourish the cornea and the lens. Lens The lens is a clear, flexible structure that is located just behind the iris and the pupil. A ring of muscular tissue, called the ciliary body, surrounds the lens. Together, the lens and the ciliary body help control fine focusing of light as it passes through the eye. Vitreous Cavity The vitreous cavity is located behind the lens and in front of the retina. It is filled with a gel-like fluid, called the vitreous humor. The vitreous humor helps maintain the shape of the eye. Retina/Macula/Choroid The retina acts like the film in a camera to create an image. To do this, the retina, a specialized layer of cells, converts light signals into nerve signals. After light signals are converted into nerve signals, the retina sends these signals to the optic nerve, which carries the signals to the brain. There, the brain helps process the image. The retina is primarily made up of 2 distinct types of cells: rods and cones. Rods are more sensitive to light; therefore, they allow you to see in low light situations but do not allow you to see color. Cones, on the other hand, allow you to see color but require more light. The macula is located in the central part of the retina.�It is the area of the retina that is responsible for giving you sharp central vision. The choroid is a layer of tissue that separates the retina and the sclera. It is mostly made up of blood vessels. The choroid helps nourish the retina. Optic Nerve The optic nerve, a bundle of over 1 million nerve fibers, is responsible for transmitting nerve signals from the eye to the brain. These nerve signals contain information on an image for processing by the brain. The front surface of the optic nerve, which is visible on the retina, is called the optic disc. Extraocular Muscles Six extraocular muscles are attached to each eye to help move the eye left and right, up and down, and diagonally.