http://www.healthline.com/vpvideo/glaucoma

Glaucoma is a group of eye conditions in which high pressure inside your eye damages the optic nerve, which transmits visual information to the brain. The eye's inner pressure depends on the amount of fluid in the eye. Increased intraocular pressure in glaucoma usually results from a build up of fluid called aqueous humor. Normally, production and removal of this fluid is in balance. It is produced by ciliary bodies in the area behind the iris called the posterior chamber. It flows up the back side of the iris, out of the pupil, into the area in front of the iris, called the anterior chamber. It leaves the eye at the angle where the iris and cornea meet, flowing through spongy tissue called the trabecular network into a channel that is part of the venous bloodstream. In closed-angle glaucoma, the iris bulges forward and blocks the fluid from reaching the network. The fluid builds up and intraocular pressure increases, usually suddenly. In open-angle glaucoma, the trabecular network is partially blocked so the fluid drains out too slowly. As fluid backs up, pressure in the eye slowly increases. There may be substantial vision loss before the problem is identified.

https://www.nei.nih.gov/health/glaucoma/glaucoma_facts

http://www.brightfocus.org/glaucoma/brightfocus-insights/optic-nerve-degeneration.html