Optic nerve
The optic nerve connects the eye to the brain. The optic nerve carries the impulses formed by the retina, the nerve layer that lines the back of the eye and senses light and creates impulses. These impulses are dispatched through the optic nerve to the brain, which interprets them as images. Using an ophthalmoscope, the head of the optic nerve can be easily seen. It can be viewed as the only visible part of the brain (or extension of it). The optic nerve is the second cranial nerve. The cranial nerve emerge from or enter the skull (the cranium), as opposed to the spinal nerves which emerge from the vertebral column. There are twelve cranial nerves. In terms of its embryonic development, the optic nerve is a part of the central nervous system (CNS) rather than a peripheral nerve. The word "optic" comes from the Greek "optikos", pertaining to sight. Aside from the optic nerve, the eye has a number of other components. These include the cornea, iris, pupil, lens, retina, macula, and vitreous. The cornea is the clear front window of the eye that transmits and focuses light into the eye. The iris is the colored part of the eye that helps regulate the amount of light that enters the eye. The pupil is the dark aperture in the iris that determines how much light is let into the eye. The lens is the transparent structure inside the eye that focuses light rays onto the retina. The retina is, as mentioned, the nerve layer that lines the back of the eye, senses light and creates impulses that go through the optic nerve to the brain. The macula is a small area in the retina that contains special light-sensitive cells and allows us to see fine details clearly. The vitreous humor is a clear, jelly-like substance that fills the middle of the eye. In sum, the optic nerve is uniquely a part of both the eye and the brain. It is embryologically the brain's envoy to the eye and functionally the eye's envoy to the brain.

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optic nerve n either of the pair of sensory nerves that comprise the second pair of cranial nerves, arise from the ventral part of the diencephalon, form an optic chiasma before passing to the eye and spreading over the anterior surface of the retina, and conduct visual stimuli to the brain called also second cranial nerve

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the second cranial nerves (II), which is responsible for vision. Each nerve contains about one million fibres that receive information from the rod and cone cells of the retina. It passes into the skull behind the eyeball to reach the optic chiasma, after which the visual pathway continues to the cortex of the occipital lobe of the brain on each side.

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nervus opticus.

Medical dictionary. 2011.

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