The white, fibrous, protective outer layer of the eye, composed primarily of collagen and elastic fibers. The sclera varies in thinkness from about 0.3 mm to 1 mm, and is covered by a thin layer of tinnue called the conjunctiva.
IrisIt is the iris that gives the visible eye its color and individuality. The iris consists of pigmented bands of muscle that surround the pupil. It also contracts or expands the pupil in response to light, extreme emotion, and, occasionally, pharmacologic substances
LensThe lens is a dense, transparent structure made of proteins, located just behind the iris and pupil. Its curvature and density cause light waves to bend as they travel through the lens. After passing through the lens, the light waves converge and project an inverted image on the retina. The retina changes the light waves to electric impulses that then travel along the optic nerve to the brain. This entire series of events results in a visual and understandable representation of light and color.
The lens also changes shape in order to focus on objects at various distances. The older one becomes, the stiffer and more inflexible the lens becomes. Oftentimes, the lens becomes so stiff that it cannot change its shape sufficiently to accommodate both images of objects up close and far away. This is why the need for bifocals or reading glasses are common as people age.
New, thin layers continually grow over old layers of the lens. These layers of cells grow inward from the edges but never form in the center of the lens. If the cells at the center of the lens grow cloudy, cataracts develop. Natural lenses can be replaced with artificial lenses in a relatively fast and easy surgical procedure, making cataracts an easily treatable disease.
VitreousThe vitreous is a clear, gel-like substance that fills the area between the lens and the retina. The pressure of the vitreous allows the eye to hold its shape. It also keeps structures like the retina in place.
With age, the composition of the vitreous changes. It may liquefy, allowing small bits of debris to float freely. Sometimes, miniscule collagen fibers clump together within the vitreous. Both create black, spidery flecks called floaters seen in the field of vision. Floaters are generally harmless but will remain within the vitreous unless surgically removed. The changing composition of the vitreous may also cause it to pull on the retina, sometimes resulting in a macular hole or a detached retina.
Rods and ConesThe two types of light-sensitive cells of the retina are rods and cones. Named because of their cylindrical shape, rods are interspersed throughout the retina. They are highly sensitive to low light and shadow and allow for at least some degree of nighttime vision.
Cones are present throughout the retina also, but are very highly concentrated in the fovea of the macula. Different cones are sensitive to different wavelengths and allow one to see vivid colors and sharp detail in conditions of bright light. There are 20 times more rods than cones.
PupilThe opening at the center of the iris through which light enters the eye. The iris causes the pupil to dilate and contract in response to stimuli such as light, extreme emotion, and certain drugs.
The pupil is in direct line with the macula, allowing light to be concentrated where vision is clearest. The iris will contract the pupil in conditions of bright light and will widen the pupil in low light in order to optimize vision.
ChoroidThe choroid is the nourishing layer of the eye located between the retina and the sclera, and is filled with tiny blood vessels. The choroid supplies the outer layer of the retina with blood. In the womb, the choroid gives rise to all other structures of the eye.
MaculaAt the back of the eye, directly in line with the pupil, is an area of the retina called the macula. In the center of the macula is an indentation called the fovea that is packed with cones, cells that require high levels of light in order to function. As light enters the eye through the pupil, it is concentrated into the area of the macula and the fovea. Because of the cones that comprise the macula, central vision is crisp, clear, detailed and in color.
The macula, an area just 5mm in diameter (roughly the thickness of a fingernail), is responsible for the vision that is necessary for many daily activities. Any condition that impairs this area will adversely affect central vision and the ability to carry out many activities that are important to daily life.
FundusIn ophthalmology, the fundus describes the back interior surface of the eye, opposite the lens, and includes the anatomical structures of the retina, optic disc, macula and fovea.
AnatomyDiagrams and descriptions of the parts of the eye.
Our eyes allow us to perceive light, dark, and color in astonishing detail, revealing the beauty and complexity of the world around us. Perhaps even more extraordinary is the fact that this organ of vision measures just 24mm (0.95 inches) and weighs 7.5 grams (0.25 ounces)!
The eye's many complex structures, some more obvious than others, are mostly unknown outside of the medical field. In an attempt to eliminate any mysteries, our encyclopedia offers easy to understand definitions of most of the terms you're likely to hear in your ophthalmologist's office, and cross-references them with other relevant terms when appropriate.
Indocyanine Green (ICG) AngiographyIndocyanine Green (ICG) Imaging is similar to fluorescein angiography (FA). Like FA, it is a useful tool for the diagnosis and treatment of several forms of macular degeneration and retinopathy. It is more effective than FA in producing images of blood vessel growth and damage in the deeper choroid layer behind the retina. It is also able to produce a better image than FA if blood is present in the macula.
The process is generally safe, with a minimum of discomfort, and is usually done in the doctor’s office. It involves dilating the eyes being photographed, and an injection of the green dye, usually in the arm, which circulates throughout the body, including the choroidal layer of the eye. A flash illuminates the eye and the resulting images are stored on a digital camera system. The fluorescent light that the indocyanine green dye emits can help to identify abnormalities in the circulation of the choroid. A series of images, or a movie of the dye moving through the vessels of the choroid, will help the ophthalmologist to decide upon the best treatment to prevent further vision loss.
Most ophthalmologists consider ICG angiography to be an adjunctive or secondary test which adds further information to the clinical picture, and is often done in addition to a fluorescein angiogram.
Below is an example of a typical Indocyanine Green angiogram, revealing an area of leakage (the bright area) just outside the macula in the center of the image. The optic nerve can be seen to the left.
Amsler GridThe Amsler grid is a simple square chart with a dot in the middle of what resembles graph paper, and can be used to test ones central visual field while at home. The lines are usually black, and the background white, though sometimes the colors are reversed. The test is a diagnostic tool that aids in the detection of visual disturbances caused by changes in the retina, particularly the macula (e.g. macular degeneration, Epiretinal membrane), as well as the optic nerve and the visual pathway to the brain.
When taking the test, the patient will be asked to look at the grid and concentrate on the dot in the middle, using one eye at a time. If the lines near the dot appear wavy, blurred, or are missing, macular degeneration, and the loss of central vision, may be occurring. The distorted image of the grid is not an optical illusion but is evidence that some photosensitive cells in the macula may have been destroyed, obscured (usually by blood) or displaced (as in a retinal detachment).