Glaucoma is a chronic, degenerative optic neuropathy that can be distinguished from most other forms of acquired optic neuropathy by the characteristic appearance of the optic nerve. In glaucoma, the neuroretinal rim of the optic nerve becomes progressively thinner, thereby enlarging the optic-nerve cup. This phenomenon is referred to as optic-nerve cupping. Its cause is the loss of retinal ganglion cell axons, glaucoma frequently occurs without an elevation of intraocular pressure, the disease is nonetheless classified according to anterior-segment variations that can elevate intraocular pressure. The anterior segment of the eye has its own circulatory system, which nourishes the crystalline lens and cornea, both of which lack a blood supply. Aqueous humor, produced by the ciliary body, circulates throughout the anterior chamber and drains through the trabecular meshwork in the iridocorneal angle, which is the angle formed by the iris and cornea. Elevated intraocular pressure does not result from increased aqueous humor production but rather from reduced aqueous outflow.