Cornea – The anterior clear part of the eye. It is the first clear “window” through which the light enters the eye. The transparency of the cornea is important in order for vision to be clear. Though it has many functions, one of the most important is to bend (refract) light entering the eye toward the lens, which in turn additionally focuses the light to the retina. The cornea also has a protective function.
Anterior Chamber – A term used to describe the area outlined in the front by the corneal endothelium (back surface of the cornea) and in the back by the crystalline lens. Aqueous fluid circulates throughout the anterior chamber.
Iris – A thin circular disc that gives our eyes their “color." There is a circular aperture (hole) in the center of the iris called the pupil. The pupil varies greatly in size under different levels of light. Its purpose is to regulate the amount of light that reaches the retina by closing up in bright light and getting larger in dim light. It also gets smaller when the eye is focused for near tasks. This occurs to sharpen the focus by diminishing spherical aberration. Eye color is controlled by the amount of pigment deposited in the layers of the iris. Blue eyes have less pigment deposited and brown eyes have more pigment in the iris. It is true that most caucasian babies are born with blue eyes. As time goes on, an infant’s eye color can change.
Lens – This structure located between the pupil and vitreous. It acts as the second “clear” window of the eye. The lens differs from the cornea in that it has the ability to change shape in order to focus images at varying distances (accommodate) becoming shorter and fatter for near vision (more convex) and thin and long for far vision. A cloudiness or yellowing of the lens is called a cataract.
Sclera – The “white” part of the eye is actually a fibrous tunic which makes up about 5/6 of the eye’s outer coat. It functions to protect the structures within the eye and maintain the shape of the eye. Due to its fragile nature in younger children, it may have a “bluish” color. The sclera has many blood vessels and nerves running through it, but the most important vessel is the optic nerve
Aqueous – Fluid that bathes structures in the front 1/3 of the eye. This fluid comes from the blood vessels in the ciliary body processes and through a recycling circulation process flows from the posterior chamber into the anterior chamber. Eventually it returns to the blood system draining out through structures including the Canal of Schlemm. Some glaucoma medicine is designed to reduce the formation of new aqueous fluid to help decrease the pressure inside the eye. Most glaucoma eye drops increase the ability of the eye to filter more aqueous fluid out to the blood stream through Sclemm's Canal.
Ciliary Body – Muscle which functions to control accommodation of the lens. It is connected to the lens by zonular fibers. When focusing fine detail up close, the ciliary body contracts, the zonular fibers relax causing the lens to become more convex and move slightly forward. This creates a built-in zoom lens allowing the eyes to change focal power from far to near. As we get older, the crystalline lens of the eye hardens and has difficulty changing its shape. By about the age of 45, most people have lost some ability to change focal power from far vision to near vision because of this hardening of the lens. At this point, a bifocal is needed to see up close. This condition is called presbyopia and worsens even further until all ability to focus at near is lost unless a person is myopic and can remove their optical prescription and see at near. The epithelium of the ciliary body secretes aqueous fluid into the anterior chamber of the eye.
Canal of Schlemm – A vascular structure which spans the circumference of the anterior chamber and functions to return aqueous humor back to the general blood circulation of the body.
Choroid – A thin membrane that consists largely of blood vessels that nourish the outer part of the retina. It is the most posterior part of the vascular coat (sheet) of the eye and is located between the sclera and the retina. It is difficult to estimate the thickness of the choroid because of the numerous vessels. It is thickest in the macular area, posteriorly (about 0.22 mm) and thinnest anteriorly (about 0.1 mm) near the ora serrata where it ends along with the retina.
Ora Serrata – The outer edge of retinal attachment to the ciliary body. This area marks the outer limits of seeable retina. The ora is less developed and thinner than the central portion of the retina. It is also the location of the strongest attachment of vitreous base to retina and therefore is a common site for retinal detachments.
Sclera – The “white” part of the eye is actually a fibrous tunic which comprises the 5/6 of the eye’s outer coat. It functions to protect the intra-ocular structures and sustain the shape of the eye. Due to its fragile nature in younger children, it may have a “bluish” color. The sclera is penetrated by blood vessels and nerves, but the most important penetration is the optic nerve (see below).
The retina is absolutely important to the function of the eye and is the site for many eye diseases and disorders such as macular degeneration, diabetic retinopathy, and retinal detachments.
Macular Degeneration is the most common complication that occurs in the macula and is one of the leading causes of blindness. There are new treatments for wet macular degeneration, the most devastating form of macular degeneration.
Fovea - This area is located in the center of the macula. The fovea is responsible for sharp central vision which is necessary for reading, watching television, driving, and any activity where visual detail is of primary importance.
Optic Nerve – Is the collective input of information from ganglion cells which are carrying information from the retinal photoreceptors. The optic nerve carries this information to the brain where the signals are interpreted allowing visual perception. Since it does not have any photoreceptors (i.e. rods and cones), the optic nerve corresponds with our natural blind spot. The optic nerve is the structure damaged by glaucoma which ultimately leads to blindness if not treated properly.
Central Retinal Vein – The main blood vessel that carries blood away from the eye. Veins appear darker and wider than arteries when viewed by special ophthalmic instruments (e.g. ophthalmoscopes) looking into the eyes.