The nerve that connects the eye to the brain and carries visual signals is called the optic nerve. It is the second of the 12 cranial nerves and is responsible for transmitting visual information from the retina to the brain. The optic nerve is mainly composed of retinal ganglion cell axons. In the human eye, the optic nerve receives light signals from about 125 million photoreceptor cells via two intermediate neuron types, bipolar and amacrine cells. The optic nerve carries these signals to the brain, where they are interpreted as visual images. The optic nerve starts in the optic disk, a group of cells in retinal tissue at the back of the eye, travels through the optic canal, and enters the skull. It then crosses paths with the opposite optic nerve to form an X-shaped structure called the optic chiasm and creates upper and lower pathways called the optic radiations. The optic radiations carry nerve messages to a part of the brain called the visual cortex, which processes sensory information for sight.