Bird eye. Cut-away computer illustration showing the anatomy of a typical bird's eye. The structure of a bird's eye is similar to that of humans and other vertebrates, with a clear cornea forming a bulge (right) in front of the lens (oval). Some birds have an additional set of muscles that can be used to change the shape of the cornea. The layers of the eyeball are the retina (inner), choroid (middle) and sclera (outer). The structure unique to birds (and some reptiles) is the pecten (protuberances at left). This is a projection of the choroid into the inside of the eyeball at the point where the optic nerve meets the eyeball. The pecten is thought to help nourish the retina.