Sclera (shown here as gray outer layer)
The sclera is a membrane of tendon in the eye, also known as the
white of the eye. Rugged and robust, the sclera works to protect the
inner, more sensitive parts of the eye like the retina and choroid. It
is about 0.03 of an inch thick except for where the four “straight” eye
muscles append, where the depth is no more than 0.01 of an inch. Four of
six eye muscles on each eye are known as “straight” eye muscles (Mm.
Recti) but all six control the movements of the eye. On top of the
sclera, the episclera is located. The episclera contains blood vessels
that nourish the sclera with oxygen and nutrients. These vessels are
thus visible on the surface of the white of the eye.
Choroid (shown here as orange middle layer)
The choroid is the layer located within the sclera. The layer
consists of a network of vessels that nourish the retina with oxygen and
nutrients. The macula and the anterior part of the optic nerve are
dependant on blood supply from the choroid. Macular degeneration
(specifically the “wet” type) is an eye condition that is directly
caused by abnormal blood vessel growth, which starts in the choroid and
go through the Bruchs Membrane to the retina (known as choroidal
neovascularization). Due to a weaker structure, these vessels have a
tendency to rupture or to bleed, thus causing edemasRetina (shown here as yellow inner layer)
The Retina refers to the light sensitive tissue in the back of the eyeball. The retina absorbs light that ultimately is sent as visual signals to the brain. The retina contains two kinds of light receptors, the cones and the rods. The cones, which are color sensitive, are located in the center of the retina and mainly absorb stronger light. The rods absorb softer light in black and white and are located peripheral to the fovea. A serious eye condition related to the retina is “amotio retinae” or retinal detachment. Common symptoms are sensations of light flashes in the field of vision or partial to full vision loss that, if left untreated, can lead to permanent vision loss. If any of these symptoms occur, immediate medical care is required.
Vitreous Body (shown here as red mass for clarity)
The vitreous body is a clear gel-like substance filling the
eyeball, behind the lens. Aqueous fluid fills the space in front of the
lens. When you get older, the vitreous body shrinks in volume. When the
vitreous body decreases in size it can detach from the retina. This is
referred to as vitreous body detachment and is in itself not a dangerous
phenomenon. It is only when the vitreous body drags along the retina
that complications can occur. This is called retinal detachment.
Macula (shown here as the yellow circle)
The macula is the area around the fovea. Closely-packed visual
cells in the macula result in high image resolution, or high visual
acuity. Around the macular area, visual cells are not as closely packed
as in the macula itself, but they are good enough for our peripheral
vision to work properly. A common eye disease amongst the elderly is
age-related macular degeneration (AMD). This condition can be divided
into two sub-categories: “dry” or “wet”. Wet AMD is treatable with
FDA-approved drugs such as Lucentis, but it affects the eye worse than
dry AMD, often leading to more serious cases of vision loss. Dry AMD is
an incurable eye disease and, unfortunately, the most common form of
AMD.
Fovea (shown here as green disc)
The fovea is the most central part of the macula. The visual cells
located in the fovea (cones) are packed tightest, resulting in optimal
sharpness of vision, perfect for reading or watching TV. Rods are packed
just outside the fovea, and are active in low light conditions. The
fovea covers about 5 degrees of the vision field.Optic Disc (marked here by black dot)
Also known as the optic nerve head or the blind spot, the optic disc is where the optic nerve attaches to the eye. All visual cell nerve threads, including some blood vessels, have their entrance to the eyeball here. The absence of visual cells in the optic disc causes the appearance of a blind spot in your field of vision.
Lens
The lens is where about 20 of the eye's 60 diopters obtain
diopteric power. The remainder of the eye's diopteric power is contained
in the cornea. Furthermore it is the lens that enables us to change the
focus according to different distances, e.g. when we change focus from a
TV screen across the living room to a newspaper in our hand. The lens
is attached to a mass of threads called zonula threads. The zonula
threads are then attached to the ciliary body. We can compare this optic
correlation with a bicycle wheel where the lens is the hub, the threads
the spokes and the ciliary body the rim. When we then want to focus on a
near object, a muscle in the ciliary body contracts. This way the
zonula threads can loosen up, allowing the lens to contract in diameter
and thicken, thus increasing its acuity. As a person becomes older, the
flexibility of the lens decreases. By the age of 45 the lens will often
have stiffened to the extent that normal reading vision, without
corrective glasses or lenses, is unattai nable. Cataracts are an
age-related deficiency in the lens that sooner or later affects
everybody in some form. The most common symptom associated with
cataracts is blurry vision. Cataracts can result in removal and
replacement of the lens. 
Optic nerveThe optic nerve sends signals from the eye to the different parts of the brain where the signals are interpreted into images. The optic nerve consists of about 1,000,000 nerve threads. The optic nerves from both eyes are reconnected behind the eyes so that everything that is seen in the right field of vision is sent to the left cerebral hemisphere and vice versa.
Pupil
The pupil is essentially just a hole in the iris. The reason why it's black is because the layer of pigment inside the eye absorbs major parts of the light, thus resulting in a darker shade. However, when pupils appear red in photos it is actually the color of the retina that is reflected.
Iris
The iris is the part of the eye that regulates the amount of light that enters the eye. With strong light, the iris sphincter muscles will contract the pupil. In darkness, the iris opens the pupil using the dilator muscles. When focusing on near objects, the pupil decreases in diameter, but it will expand when focusing on distant objects, a reflex known as the Accommodation Reflex. A smaller pupil enables better focal depth. A person’s eye color actually refers to the amount and type of pigments in his/her iris. The most common color is brown, while the least common is green.
Aqueous Fluid
Aqueous fluid (or aqueous humor) fills the front part of the eye,
between the lens and the cornea. This fluid is produced at the back of
the cilia body, then seeps through the pupil, into the anterior
chamber and ultimately is drained through the tubercular mesh work. The
aqueous fluid's main function is to supply the cornea and the lens with
nutrients and oxygen. The anterior chamber is the space located between
the iris and the cornea. If an imbalance occurs between fluid production
and outflow, thus increasing the amount of aqueous fluid, the pressure
in the eye inevitably increases. This is one of the contributing factors
that cause glaucoma.
