Eye. Artwork of a section through a healthy human eyeball. Light passes through the cornea (white arch, far left), the eye's main light-focusing structure. The iris (red fingers, centre left) contracts or relaxes to allow the ideal amount of light through the pupil, the iris' central hole. The light is finely focused by the lens (white/ blue oval, centre left), before hitting the retina that lines the inside of the eyeball. Signals from the retina's light-sensitive nerve cells travel along the optic nerve (yellow, lower right) to the brain for processing. The hyaloid canal runs through the vitreous humour, the eyeball's jelly filling, from the lens to the optic nerve.