The eye is a sensory organ that detects light and is one of the most complex of structures within the human body. The eye functions similar to that of a camera. Just as light passes through a lens, which is then focused at the back of the camera, the eye focuses light through a lens onto the back of the eye, where it is converted to nerve impulses that the brain perceives as sight.
Visible Portion
The visible portion of the eye includes the sclera, also known as the white of the eye, the conjunctiva, the iris, the cornea and the pupil. The sclera is the tissue that makes up the entire globe of the eye from the edge of the iris back to the optic nerve. The iris is the colored portion of the eye and is controlled by muscles that open or close the aperture in the center of the iris, known as the pupil. This aperture allows light to enter the eye. The entire iris and pupil are covered with a tough, clear covering called the cornea, which is surrounded by another clear protective layer called the conjunctiva. Beneath the cornea is a liquid substance, called aqueous humor, which supports the cornea and the front part of the eye
Interior Portion
Behind the iris and pupil lies the lens, the retina and the optic nerve. Tiny muscles, called ciliary bodies, are connected to the edge of the lens by fibers known as zonules. The ciliary bodies support and help to adjust the lens' focus. The retina lines at the back of the eye. Within the retina are photoreceptors. The optic nerve is at the very back of the eye. A jelly-like liquid, called vitreous humor, supports the shape of the eyeball. Blood vessels within the eye, which connect to the central retinal artery and vein, provide oxygen and nutrients to eye tissues.
Function
As light passes through the pupil, the iris adjusts the size of the pupil to allow just enough light in to prevent damage to the eye from overexposure. The light then passes through the lens, which focuses the light on the retina. The photoreceptors within the retina sense the light image on the retina and transmit impulses through the optic nerve to the brain, which then interprets the stimuli.
Color and Brightness
Special photoreceptors within the retina, called cones, detect different wavelengths of light. The visual center within the brain perceives the information transmitted by cones as color. Another type of photoreceptor, called rods, detect variances in brightness. The brain interprets this information as light and dark. The combined stimuli are perceived as vision.
Movement
Six muscles attached to the outside of each eye control its movements, both up and down and left and right. These extraocular muscles are positioned at equal distances around the eye. They are very strong and move rapidly to provide fast acceleration and accurate stopping to point the eyes in a given direction.
Binocular Vision
The human body is equipped with two eyes to provide binocular vision. The extraocular muscles of both eyes work in tandem to concentrate their focus on a given object. This type of vision allows for increased depth and distance perception.
