In the vertebrate eye the retina is the innermost of three coats. It is composed of photoreceptors cells, known as rods and cones, which are arranged radially to the surface of the retina. The middle coat is the choroid, a black-pigmented light screening layer that also carries blood tissues to and from the eye. (In many vertebrates a thin film of crystalline light-screening material coats the choroid, which makes the eyes of these animals shine in near-darkness.) The outermost layer (not shown) is the sclera, which is fibrous in man but cartilagenous in many other mammals. In the front of the eye these coats are modified structurally. The sclera merges with the transparent cornea. The choroid coat continues as the sometimes pigmented iris, which encloses the pupil. The spaces between lens and cornea are filled with a lymphlike aqueous humor, and the space between lens and retina contains a jellylike vitreous humor.