It maintains intraocular pressure and helps maintain the global shape of the eye. The ciliary muscles are very important for visual accomodation. The choroid is a thick, vascular, darkly pigmented layer that coats the retina and helps nourish both the sclera and retina. The third coat or innermost layer of the eye is the retina. It is very sensitive to light and contains photo receptive rods and cones.
Cones are important for visual acuity and color vision and are most numerous in nocturnal birds.
The retina of birds is different from mammals in that it is thick and has no blood vessels. The pecten is a vascular, folded structure projecting from the avian retina. It is thought to aid the choroid layer in supplying the retina with nutrients and in transporting oxygen and carbon dioxide. The optic nerve centers the retina in the lower quadrant of the back of the eye. The area of sharpest visual accuity of the retina is in the macular disk.