The human nervous system contains two major categories, the central and peripheral nervous systems, with the brain and spinal cord belonging to the central nervous system and the nerves of the trunk and extremities belonging to the peripheral nerves. Of the 12 pairs of cranial nerves connected to the brain, 11 are peripheral nerves and the optic nerve is the only cranial nerve that belongs to the central nerve. Both the optic nerve and the brain originate from the embryonic ectoderm, and the sheath wrapped around the surface of the optic nerve is the three layers of meninges that continue with the brain. More importantly, the myelin sheath wrapped around each nerve fiber in the optic nerve is the oligodendrocytes of the central nervous system, not the Schwann cells of the peripheral nervous system. There are still many medical books that refer to the optic nerve as a peripheral nerve, which is wrong.