Cornea, Sclera, Iris and Pupil
Your eyeball is located inside a pear-shaped bony cavity known as the orbit. The orbit also holds muscles, nerves, blood vessels and structures that form and drain tears. The white part of the eyeball is called the sclera. The sclera is covered by a thin mucous membrane called the conjunctiva. The conjunctiva covers the back surface of the eyelids and runs to the edge the cornea, or the clear covering of the colored part of the eye. The cornea protects the front of the eyeball and helps light focus on the retina, which is located at the back of the eye. When light passes through the cornea, it then travels through the pupil, which is the black circle in the middle of the eye. When light enters the eye, the pupil dilates (enlarges) and constricts. The pupil size is controlled by the pupillary sphincter muscle as well as the dilator muscle. The circular, colored part of the eye is known as the iris. The iris determines how much light can enter the eye, depending on the environment. If the environment is dark, more light is allowed to enter the eye. If the environment is bright, less light is allowed into the eye. The anterior chamber is located between the iris and cornea. The anterior chamber is filled with transparent fluid that supplies the eye with oxygen, protein and glucose.
Lens
The lens is located behind the iris. The lens focuses the light that is allowed into the eye onto the retina. There is a group of muscles known as the ciliary muscles that allows the lens to thicken when it needs to focus on objects that are near and then thin when it needs to focus on objects that are further away. The posterior chamber is located between the iris and the lens. The posterior chamber produces the aqueous humor, which flows through the pupil and drains out of the eyeball.
Retina
The retina senses light and contains the macula, which has millions of photoreceptors (cells that sense light). The macula is the most sensitive part of the retina. The photoreceptors create visual images and more details. Nerve fibers from the photoreceptors are bundled onto the optic nerve, which is located at the back of the eye. Cones and rods are the two main types of photoreceptors. Cones are located mainly in the macula and create sharp, detailed and colored vision. Rods are located mainly in the peripheral areas of the retina and create night and side (peripheral) vision. The images that are created are then converted into electrical impulses, and the optic nerve carries those impulses to the brain.
