The structure indicated is the optic nerve (cranial nerve II).
The optic nerve is the second cranial nerve and is responsible for vision. Similar to the first cranial nerve (olfactory nerve), the optic nerve is not a true nerve but is rather an extension of the brain (diencephalon), which means it is technically a part of the central, rather than peripheral nervous system.
The primary sensory neurons are the bipolar cells of the retina; these synapse on ganglion cells, the axons of which form the optic nerve which then passes via the optic canal to join with the contralateral optic nerve at the optic chiasm. At the optic chiasm, the fibres from the nasal retina crossover to the contralateral side to pass within the optic tract to the lateral geniculate body. The fibres from the temporal retina remain within the ipsilateral optic tract. The optic tract then extends to the lateral geniculate bodies of the thalamus and from here the pathway continues to the visual cortex of the occipital lobe as the optic radiation.
Learn more about the cranial nerves in this anatomy tutorial.