Rod cells, which are located in the retina, allow humans and nocturnal animals to see in low-light situations. They're named for their cylindrical shape, and are highly sensitive to light. They also provide peripheral vision, which is sight that occurs at the edge of the field of view.
Rods and cones are the two types of photosensitive nerve cells in the retina. Cones are used for day vision and, therefore, need a lot of light in order to function. They can distinguish between colors and provide detailed sight, or visual acuity. Different cones are responsible for recognizing different colors because different bands of light produce different color bands.
The second type of light-sensitive nerve cells in the retina are called rods. The highly photosensitive rod cells are responsible for night vision, and a lack of them causes a condition called night blindness. Unlike cones, rod cells can't distinguish between colors and don't provide much visual acuity.
Light passes into the eye through the cornea, which is the transparent film that covers the outside part of the eye. It covers the iris and pupils and refracts incoming light. The aqueous humor is a thick liquid that sits behind the cornea and in front of the lens.
Behind the lens is a large cavity that's filled with a gel called the vitreous humor. The retina is a band of light-sensitive tissue that wraps around the vitreous humor and lines the eye's inner surface. Inside this band of light-sensitive tissue is where rod cells and cone cells are found. The retina contains about seven million cones and over 100 million rods.
After light passes through the cornea, it goes through the aqueous humor to the lens, and then through the vitreous humor, finally reaching the light-sensing area of the retina. Once light reaches the retina, a chemical reaction occurs. The optic nerve receives information via electrical impulses sent by the chemicals produced.
The optic nerve, also called the cranial nerve two, transmits visual information to the optic, or vision, center in the brain. This vision center is located in the back of the brain, in the area called the occipital lobe. This part of the brain is in the back of the head, and is covered by the cranial bone known as the occipital bone. The process by which the visual organs gather information and transmit it to the brain, which forms psychological awareness, is called visual perception.
