The eye is a specialized sense organ a part of the nervous system. In senses photons (light) and transmits information to the brain. The white of the eye is known as the sclera it is the outermost part of the eye, underneath it lies the choroid which supplies the retina with blood (oxygen, nutrients). [neurons have the most mitochondrial activity of the cells and require lots of ATP to synthesize and integrate neurotransmitters/ other signals].
Photoreceptors
The Retina contains the photo-receptors (rods, and cones). Cones are used to sense bright lights and colours, while rods sense dim light and are mainly responsible for peripheral vision. Rods contain a single rod visual pigment (rhodopsin), while cones contain several types of visual pigments able to sense red, green and blue pigments. The photoreceptors transduce the photon signal into an action potential that synapses onto a bipolar cell, which then synapses onto a ganglion cell. The ganglion cells bundle together and exit through the the optic nerve to the brain. This region of bundled ganglion axons lacks photoreceptors and is also known as a Blind spot.
Fovea
There is a region in the retina with a high concentration of cones. It is called the fovea, due to the high concentration of cones, the fovea is responsible for high acuity vision.
Focus
So far we have discussed the sensory portion of the eye, but how do we get light into the eye? The cornea, the clear dome shaped covering of the front most part of the eye, refracts, or focuses light. Too much light can destroy neuronal cells (think of your vision, when coming back inside from a couple hours in the snow). Your iris (muscle) contracts to reduce the amount of light entering the eye, or dilates to increase the amount of light entering the eye, and controls the size of the pupil (the black center of the iris). Light passes through the pupil and onto the lens, which can be controlled by ciliary muscles to change shape or focal length. This is important in focusing a clear image onto the retina, to get an accurate representation of the object to your brain.