Close-up of the eye of a patient who has recently undergone a trabeculectomy operation to relieve glaucoma. This procedure involves the removal of a small segment of eye tissue, visible here as the dark notch in the outer part of the iris at the 11 o'clock position, to reduce the intraocular pressure, the pressure within the eye. Glaucoma describes the consequences of an increase in this pressure. Primary glaucoma may arise in the absence of any other eye disease; secondary glaucoma results from the interruption of normal circulation of the aqueous humour within the eye by a pre-existing eye disorder.