The retina is a thin layer of neural cells that lines the back of the eyeball of vertebrates and some cephalopods. In vertebrate embryonic development, the retina and the optic nerve originate as outgrowths of the developing brain. Hence, the retina is part of the central nervous system (CNS). It is the only part of the CNS that can be imaged directly. The vertebrate retina contains photoreceptor cells (rods and cones) that respond to light; the resulting neural signals then undergo complex processing by other neurons of the retina. The retinal output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve. Several important features of visual perception can be traced to the retinal encoding and processing of light. The unique structure of the blood vessels in the retina has been used for biometric identification.