The optic nerve is formed by axons of the nerve fiber layer. These nerve fibers develop from the ganglion cells to form the nerve fiber layer of the retina, extend posteriorly to form the optic nerve and extent toward the brain. Myelin develops from the brain outward to reach the eye a few weeks after birth. In the dog, myelin usually extends into the eye and a short distance along the nerve fiber layer. In the cat, myelin usually does not enter the eye. Dura mater and arachnoid of the brain extend along the optic nerve to blend into the posterior sclera.