The eye is a sense organ. The light rays enter the dark center of the eye, the pupil. Before entering the eye through the pupil, light passes through the cornea. The cornea is a fibrous, transparent tissue that extends over the pupil and colored portion of the eye. The function of the cornea is to bend, or refract, the rays of light, so they are focused properly on the sensitive receptor cells in the posterior (located in the back, or toward the rear of ) region of the eye. The normal, healthy cornea has no blood vessels but receives nourishment from blood vessels near its junction with the opaque white of the eye, the sclera. The sclera is tough, fibrous, supportive, connective tissue that extends from the cornea on the anterior (located in the front) surface of the eyeball to the optic nerve in the back of the eye. The choroid is a dark brown membrane inside the sclera. It contains many blood vessels that supply nutrients to the eye. The choroid is continuous with the pigment-containing iris and the ciliary body on the anterior surface of the eye.
The ciliary body, on each side of the lens, contains muscles that adjust the shape and thickness of the lens. These changes in the shape of the lens cause refraction of light rays. Refraction is the bending of rays as they pass through the cornea, lens, and other tissues. Muscles of the ciliary body produce flattening of the lens (for distant vision) and thickening and rounding (for close vision). This refractory adjustment is accommodation.
Besides regulating the shape of the lens, the ciliary body also secrets a fluid called aqueous humor, which is found in the anterior chamber of the eye.
Aqueous humor maintains the shape of the anterior portion of the eye and nourishes the structures in that region. The fluid is consistently produced and leaves the eye through a canal that carries it into the bloodstream. Another cavity of the eye is the vitreous chamber, which is a large region behind the lens filled with a soft, jelly-like material, the vitreous humor. Vitreous humor maintains the shape of the eyeball and is nor constantly reformed. Both the aqueous humor and the vitreous humor further refract light rays.
The retina is the thin, delicate, and sensitive nerve layer of the eye. As light energy, in the form of waves, travels through the eye, it is refracted (by the cornea, lens, and fluids), so it focuses on sensitive receptor cells of the retina called the rods and cones. There are approximately 6.5 million cones and 120 million rods in the retina. The cones function in bright levels of light and are responsible for color and central vision. Rods function at reduced levels of light and are responsible for peripheral vision.
Light energy, when focused on the retina, causes a chemical change in the rods and cones, initiating nerve impulses that then travel from the eye to the brain via the optic nerve. The region in the eye where the optic nerve meets the retina is called the optic disc. Because there no light receptor cells in the optic disc, it is known as the blind spot of the eye. The macula is a small, oval, yellowish area on the side of the optic disc. It contains a central depression called the fovea centralis, which is composed largely of cones and is the location of the sharpest vision of the eye. If a portion of the fovea or macula is damaged, vision is reduced and central-vision blindness occurs.