| Anatomy of the Eye |
The
eye is a complex sensory organ specialized for the gathering of visual information.
Each eye includes a lens system to focus the image, a layer of photosensitive
cells, and a network of cells and nerves that collect, process and transmit
visual information to the brain, all surrounded by a fibrous protective globe.
The eyes are housed in protective bony structures of the skull called the orbits.
Each eye is composed of a tough outer layer, the sclera and cornea; a middle
layer, the choroid, ciliary body and iris; and the inner layer of nerve tissue
called the retina. The photosensitive retina connects to the brain via the optic
nerve.
Special Terms for eye anatomy
Orbit: This cone shaped cavity in the skull is formed by the frontal, maxilla, zygomatic, sphenoid, ethmoid, lacrimal and palatine bones. These bones are thin and are often subject to fractures. The eye occupies the front portion on the cavity with the rest being filled with fat, nerves, blood vessels, muscle and the lacrimal (tear) gland.
Anterior chamber: This front
chamber of the three found in the eyeball itself is found between the cornea
and the iris.
Osterior chamber: This chamber occupies space between the iris and the
lens.
Aqueous humor: This fluid
fills the anterior and posterior chambers. It is made by cells of the ciliary
body and circulates from the posterior chamber, over the lens, to the anterior
chamber and exits via the Canal of Schlemm. It is similar to plasma but contains
very little protein.
Vitreous space: This chamber occupies the space behind the lens to the
retina. It is filled with a gelatinous substance called the vitreous body.
Sclera: This a fibrous protective covering of the eyeball that is seen as the white of the eye. It is made mostly of the structural protein collagen.
Cornea: Transparent, colorless anterior one sixth of the covering of the eyeball. The cornea consists of 5 layers with the outer one being a layer of epithelium composed of 5-6 layers of cells that turnover every 7 days or so. This layer and the internal endothelium are responsible for keeping the cornea transparent. They do this by keeping the middle layers relatively dehydrated which keeps the parallel collagen fibers present there from being opaque. The cornea contains no blood vessels and gets its nutrients from those in the anterior chamber fluid and surrounding vessels.
Schlemm's canal: This canal drains fluid from the anterior chamber. Obstruction of this canal leads to an increased intraocular pressure which can damage the retina. This condition is also called glaucoma.
Choroid: This is the vascular layer of the eye.
Retinal pigment epithelium: This layer lying betwen the retina and choroid, contains melanocytes that make for its characteristic black color. This pigment allows the inside of the eye to absorb stray rays of light like the black paint inside a camera or within a dark room.
Ciliary body: This is an expansion of the choroid at the front of the eye at the level of the lens. The lens is attached to the ciliary body which has smooth muscle within it. Contraction of this smooth muscle changes the shape of the lens and allows the eye to focus on objects. Part of the ciliary body is specialized to make aqueous humor.
Iris: Another expansion of
the choroid that partially covers the lens formed by pigment cells, fibroblasts,
blood vessels, contractile pigment cells. The pigment found in the iris prevents
light from entering the eye except through the pupil. The melanocytes (pigmented
cells) are responsible for the color of the eyes. If there is little pigment
in the cells the light reflected from the choroid at the back of the eye will
make the iris appear blue. As the amount of pigment increases the iris appears
greenish-blue, gray or brown.
Pupil: Round opening in the iris that allows light to pass through. Pupil
size changes based on the amount of light present. More light causes the pupil
to contract while the pupil widens in the dark to collect as much light as possible.
Lens: This biconvex structure is very elastic, at least in the young.
With age the lens looses its elasticity and therefore the ability to focus on
near objects decreases. The center of the lens is formed by elongated cells
(fibers) that have lost all organelles and are filled with special proteins
called crystallins. These fibers are replaced throughout life but the regeneration
slows down with age. Mature cataracts occur when these fibers accumulate pigment
granules so that they are less transparent.
Retina: The photosensitive part of the eye lies between the vitreous
body and the choroid layer at the back of the eyeball and is a complex network
of photosensitive cells and various types of neurons. Light that reaches the
retina must travel through several layers of transparent neurons at the front
of the retina before reaching the photosensitive rods and cones. The photosensitive
part of rods and cones is housed in extensions of the cells that look just like
their names. The rods are responsible for black and white vision while the cones
detect color. The neurons within the retina act to integrate the visual signals
received by the rods and cones and transfer the information to the optic nerve.
Fovea: This specialized area of the retina is for the most acute vision.
When a person is actively focusing or attending to an object the eyes are moved
so that the image is focused on the fovea. It is thinner, containing only the
cones necessary for detection of sharp images. The cones in this area are long
and thin resembling rods so that they can be closely packed. Blood vessels are
absent as well. Each foveal cone is directly connected to a neuron in the optic
nerve.
Blind spot: There are no photosensitive cells in this area of the retina.
This is where the optic nerve and blood vessels enter the eye through the optic
disc.
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