A. The Eye A1. Eye in detail EYE ANATOMY A guide to the many parts of the human eye and how they function. The ability to see is dependent on the actions of several structures in and around the eyeball. The graphic below lists many of the essential components of the eye's optical system. When you look at an object, light rays are reflected from the object to the cornea , which is where the miracle begins. The light rays are bent, refracted and focused by the cornea, lens , and vitreous . The lens' job is to make sure the rays come to a sharp focus on the retina . The resulting image on the retina is upside-down. Here at the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve , to the brain, where the image is translated and perceived in an upright position! Think of the eye as a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract, or focus the light and a film (retina) on which to focus the rays. If any one or more of these components is not functioning correctly, the result is a poor picture. The retina represents the film in our camera. It captures the image and sends it to the brain to be developed. The macula is the highly sensitive area of the retina. The macula is responsible for our critical focusing vision. It is the part of the retina most used. We use our macula to read or to stare intently at an object. http://www.stlukeseye.com/Anatomy.asp Angle Structures The area in the anterior chamber where the cornea and iris join is known as the angle. This is comprised of several structures that make up the eye's drainage system. The angle structures include: the outermost part of the iris, the front of the ciliary body, the trabecular meshwork, and the Canal of Schlemm. Aqueous is formed in the ciliary body behind the iris. It flows through the pupillary space into the anterior chamber. From there, the fluid travels into the angle structures and drains from the eye. As the aqueous fluid leaves the angle, it passes through a filter called the trabecular meshwork. After leaving the trabecular meshwork, the aqueous travels through a tiny channel in the sclera called the Canal of Schlemm. The aqueous flows into other tiny channels and eventually into the eye's blood vessels. The production and drainage of aqueous fluid determines the eye's intraocular pressure (IOP). Aqueous Humor The aqueous is the thin, watery fluid that fills the space between the cornea and the iris (anterior chamber). It is continually produced by the ciliary body, the part of the eye that lies just behind the iris. This fluid nourishes the cornea and the lens and gives the front of the eye its form and shape. Choroid The choroid lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye. The choroid connects with the ciliary body toward the front of the eye and is attached to edges of the optic nerve at the back of the eye. Ciliary Body The ciliary body lies just behind the iris. Attached to the ciliary body are tiny fiber "guy wires" called zonules. The crystalline lens is suspended inside the eye by the zonular fibers. Nourishment for the ciliary body comes from blood vessels which also supply the iris. One function of the ciliary body is the production of aqueous humor, the clear fluid that fills the front of the eye. It also controls accommodation by changing the shape of the crystalline lens. When the ciliary body contracts, the zonules relax. This allows the lens to thicken, increasing the eye's ability to focus up close. When looking at a distant object, the ciliary body relaxes, causing the zonules to contract. The lens becomes thinner, adjusting the eye's focus for distance vision. With age, everyone develops a condition known as presbyopia. This occurs as the ciliary body muscle and lens gradually lose elasticity, causing difficulty reading. Conjunctiva The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye. It begins at the outer edge of the cornea, covering the visible part of the sclera, and lining the inside of the eyelids. It is nourished by tiny blood vessels that are nearly invisible to the naked eye. The conjunctiva also secretes oils and mucous that moisten and lubricate the eye. Cornea The cornea is the transparent, dome-shaped window covering the front of the eye. It is a powerful refracting surface, providing 2/3 of the eye's focusing power. Like the crystal on a watch, it gives us a clear window to look through. Because there are no blood vessels in the cornea, it is normally clear and has a shiny surface. The cornea is extremely sensitive - there are more nerve endings in the cornea than anywhere else in the body. The adult cornea is only about 1/2 millimeter thick and is comprised of 5 layers: epithelium, Bowman's membrane, stroma, Descemet's membrane and the endothelium. The layers of the cornea The epithelium is layer of cells that cover the surface of the cornea. It is only about 5-6 cell layers thick and quickly regenerates when the cornea is injured. If the injury penetrates more deeply into the cornea, it may leave a scar. Scars leave opaque areas, causing the corneal to lose its clarity and luster. Boman's membrane lies just beneath the epithelium. Because this layer is very tough and difficult to penetrate, it protects the cornea from injury. The stroma is the thickest layer and lies just beneath Bowman's. It is composed of tiny collagen fibrils that run parallel to each other. This special formation of the collagen fibrils gives the cornea its clarity. Descemet's membrane lies between the stroma and the endothelium. The endothelium is just underneath Descemet's and is only one cell layer thick. This layer pumps water from the cornea, keeping it clear. If damaged or disease, these cells will not regenerate. Tiny vessels at the outermost edge of the cornea provide nourishment, along with the aqueous and tear film. Extraocular Muscles The six tiny muscles that surround the eye and control its movements are known as the extraocular muscles (EOMs). The primary function of the four rectus muscles is to control the eye's movements from left to right and up and down. The two oblique muscles move the eye rotate the eyes inward and outward. All six muscles work in unison to move the eye. As one contracts, the opposing muscle relaxes, creating smooth movements. In addition to the muscles of one eye working together in a coordinated effort, the muscles of both eyes work in unison so that the eyes are always aligned. Eyelids The eyelids protect the eyes from the environment, injury and light. They also maintain a smooth corneal surface by spreading tears evenly over the eye. The lids are composed of an outer layer of skin, a middle layer of muscle and tissue that gives them form, and an inner layer of moist conjunctival tissue. Several muscles work together to control the actions of the lids. Located in the middle layer of the eyelid is the orbicularis oculi, a circular muscle that closes the lids. The levator muscle is attached inside the upper lid and elevates it. A smooth muscle called Mueller's gives the lids tone and helps maintain elasticity. Tiny oil-producing meibomian glands line the inner edge of the lids. These glands produce oil that lubricates the eye. Rows of lashes protect the eyes from the elements and debris. Not only do the eyelids provide protection and moisture, they display expression and emotions that are an important part of our individuality. Iris The colored part of the eye is called the iris. It controls light levels inside the eye similar to the aperture on a camera. The round opening in the center of the iris is called the pupil. The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size. The sphincter muscle lies around the very edge of the pupil. In bright light, the sphincter contracts, causing the pupil to constrict. The dilator muscle runs radially through the iris, like spokes on a wheel. This muscle dilates the eye in dim lighting. The iris is flat and divides the front of the eye (anterior chamber) from the back of the eye (posterior chamber). Its color comes from microscopic pigment cells called melanin. The color, texture, and patterns of each person's iris are as unique as a fingerprint. Lens The crystalline lens is located just behind the iris. Its purpose is to focus light onto the retina. The nucleus, the innermost part of the lens, is surrounded by softer material called the cortex. The lens is encased in a capsular-like bag and suspended within the eye by tiny "guy wires" called zonules. In young people, the lens changes shape to adjust for close or distance vision. This is called accommodation. With age, the lens gradually hardens, diminishing the ability to accommodate. Macula he macula is located roughly in the center of the retina, temporal to the optic nerve. It is a small and highly sensitive part of the retina responsible for detailed central vision. The fovea is the very center of the macula.