The Retina

    The retina, a 5 square centimeter area in the back of the eye, is where all light detection takes place. The retina is a network of nerves connected to over 100 million photo-sensitive rods and cones. The signals created by these rods and cones are then sent via the optic nerve to the brain.

    The top layer of the retina surprisingly does not interpret the light that strikes it. This layer, called the Plexiform layer, is a web of optical nerves. These nerves carry the signals of rods and cones to the optic nerve. This web is located between the photo-sensitive cells and the vitreous humor so that the web's cells can be nourished. Fortunately, these cells are nearly transparent so only minimally interfere with light striking the photo-sensitive cells.

    The bottom layer of the retina is called the choroid. The choroid serves a double purpose: nourishment and absorption. The choroid carries blood to the retina and the humors, providing nourishment to the eye. In addition, the choroid absorbs any light that strikes it. This is extremely important, because light that passes through the rods and cones does not reflect back. If the light did reflect, the photo-sensitive cells would receive the light message twice, and would think that there was twice as much light as there really was.

	Here's some evidence: The eye pupil appears black. This is because all light that enters the pupil is absorbed by either the photo-sensitive layer or the choroid. Since no light is reflected, the pupil appears extremely dark or black. A sudden burst of intense light cannot be absorbed completely by the photo-sensitive layer and the choroid. When a flash photograph is taken, much of the light is reflected back. The light reflected is red because of the blood in the choroid, causing red eye! The choroids of cats are slightly different from those of humans: they reflect light instead of absorbing it, allowing the cat to see better in dimmer light. This accounts for cats' eyes often being described as luminous.

Rods and Cones

    The final layer of the retina is the photo-sensitive layer, made up of rods and cones. When they are exposed to light, rods and cones send signals to the plexiform layer. This signal is then transferred via the optic nerve to the brain to be interpreted.

Rods and Cones react to different levels of light. Cones are photopic, which means that they respond to high light levels. Rods are scotopic, which means they respond to low levels of light. Depending on the amount of light striking the retina, one kind of cell will become more responsive, while the other will "turn off." The human eye has about 7 million cones and 120 million rods. Nocturnal animals, such as cats, have more rods and fewer cones. Many diurnal animals, such as pigeons, only have cones.     Each rod and cone is made up of four segments. The cell nucleus keeps the cell alive and causes cell reproduction. The inner segment manufactures a light sensitive chemical made up of Vitamin A and protein.  The outer segment uses this chemical to absorb light. When light strikes the chemical, it undergoes a reaction in the fourth segment, the synaptic ending. This reaction produces a neuro-transmitter that is then sent to the plexiform layer and the brain.

   Cones are the cells that we use to distinguish color. This has several very interesting effects. Almost all cones are located at the middle of the retina, so it is very difficult to distinguish color in our peripheral vision. In addition, it makes it impossible to see color in low levels of light when the cones are "turned off."

The Blind Spot

    The Optic Nerve receives all of the signals created in the eye and transfers them to the brain to be processed. The nerve enters in the back of the eye, and where it enters there are neither rods nor cones. This results in a "blind spot," an area where our eyes cannot see. Though this area is rarely noticed because the brain "fills in" the blind spot, an interesting trick can be done to demonstrate its existence.

	Now you see it, now you don't!
	Close one eye. If your right eye is open, stare at the left dot above. If your left eye is open, stare at the right dot above. Put your eye as close to the monitor as possible, then slowly move your head back. Continue to stare at the one dot. When you are approximately a foot from your monitor the other dot will suddenly disappear! It has entered your blind spot!

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