Biology of the Eyes | Merck Manual Consumer Version

The eyes are able to adjust the amount
of light that enters, focus on objects, and instantly transmit images to the
brain. Light enters the eye through the cornea which helps focus light on the
retina at the back of the eye. Light then travels through the pupil. The iris is
the colored area that surrounds the pupil which controls the amount of
light that enters by constricting and dilating. Behind the iris sits the lens.
The lens focuses light onto the retina by changing its shape, muscles attached
to the lens cause it to become thicker to focus on nearby objects and thinner
to focus on distant objects. The retina contains cells that sense light called
photoreceptors and blood vessels to supply them. The nerve fibers from the
photoreceptors are bundled together to form the optic nerve which carries
signals to the brain. The fluid in the eye the aqueous humour nourishes the eye and maintains pressure. During glaucoma and imbalance between fluid production
and drainage increases pressure and causes optical nerve damage that leads
to loss of vision. The conjunctiva is the membrane that lines the white of the eye
and the eyelid. Conjunctivitis occurs when this membrane becomes inflamed.