Illustration of the avian eye anatomy. The structure of a birds eye is similar to that of humans and other vertebrates, with a clear cornea forming a bulge (left) in front of the lens (oval, white). The layers of the eyeball are the retina (inner, yellow), choroid (middle, pink) and sclera (outer, beige). A thin sheet of muscle, the iris (brown stripes), with a central opening, the pupil, can be altered to control the amount of light entering the eye. The structure unique to birds (and some reptiles) is the pecten (rhomboid, red-brown, centre-right). This is a nonsensory, highly vascular, pigmented structure that is believed to both nourish the retina and control the pH of the vitreous body (gel-like liquid between the lens and the retina).