Basic Eye Anatomy

The ability to see is one of the most important senses a human possesses. The organ responsible for sight - the eye - is one of the most complex in the human body. It allows conscious light perception and vision, including the ability to distinguish 10 million colors and the perception of depth. Take a look at the parts that make up the eye, and how they work together to provide sight.
A transparent, gelatinous fluid located in the anterior chamber of the eye, the space between the lens and the cornea. It provides nutrition for eye tissue, as well as inflation for expansion of the cornea which helps protect against dust, wind, pollen grains and some pathogens.
Lying between the retina and the sclera, this layer contains blood vessels and connective tissues. It provides oxygen and nourishement to the outer layers of the retina. The choroid is thickest in the rear of the eye.
The transparent front part of the eye that covers the iris, pupil, and anterior chamber. The cornea, with the anterior chamber and lens, refracts light.
A thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil (the opening in the iris) and thus the amount of light reaching the retina. The color of the iris gives the eye its color.
A crystalline, transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. By changing shape, the lens functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina.
The extraocular muscles are the six muscles that control movement of the eye. The actions of the six muscles responsible for eye movement depend on the position of the eye at the time of muscle contraction.
A light-sensitive layer of tissue, lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina (through the cornea and lens), similar to film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These are sent to various visual centres of the brain through the fibres of the optic nerve.
Also known as the white of the eye, the sclera is the opaque, fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In humans the whole sclera is white, contrasting with the coloured iris, but in other mammals the visible part of the sclera matches the colour of the iris, so the white part does not normally show.
The transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye. It is present at birth and does not change much over the course of aging. Unlike the fluid in the frontal parts of the eye (aqueous humour) which is continuously replenished, the gel in the vitreous chamber is stagnant.
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Light
Iris
Dilate
Constrict
Distance
Near
Far

Rods & Cones

Rods and cones are specialized neurons, contained in the retina, that are capable of phototransduction - the process by which light is converted into electrical signals.

Rods are very sensitive to light and help provide vision in low-light conditions, due to the higher levels of pigmentation. Cones require brighter, more direct light, but work together to provide color vision.

There are approximately 120 million rods and 6 million cones in each eye, with the cones being more concentrated toward the rear of the retina.
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Putting It All Together

In the retina, the photo-receptor cells synapse (or connect) directly onto receptors, which in turn synapse onto ganglion cells of the outermost layer, which then conduct impulses to the brain.

Lying at the rear of the brain, the visual cortex is the largest system in the human brain and is responsible for processing the visual image. The human mind recognizes objects with highest contrast to their surroundings first.
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