For medical purposes, it would be more accurate to say that the eyes are the windows to the brain. This is because observations of the eyes give so many clues about pathological changes taking place within the brain. The sensory nerve component is the visual pathway extending from the retina posteriorly into the brain. Light passes through the pupil at the front of the eye on to the retina at the back. A light shone into one eye should cause a fast constriction of the pupils in both eyes, a direct reaction in the pupil to which the light was applied and a consensual reaction in the opposite pupil. Glaucoma is becoming better understood, not just as a disturbance of vision with acute eye pain caused by raised intraocular pressure, but by the fact that beneath these symptoms hides a neurodegenerative disorder affecting retinal cells and their axons. Ptosis could be part of several neurological disorders.