Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptor cells in conscious vision connect light to movement. In higher organisms the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.
The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment.
Famous quotes containing the word eye:
“The eye is the casement at which the heart generally looks out. Many a woman who will not show herself at the door, has tipt the sly, the intelligible wink from the window.”
—Samuel Richardson (16891761)
“Let every eye negotiate for itself,
And trust no agent.”
—William Shakespeare (15641616)
“Will this thought rise on those who will meet my face no more,
He was one who had an eye for such mysteries?”
—Thomas Hardy (18401928)