The globe is a fluid filled structure with aqueous humor being produced by the ciliary body and exiting via the trabecular meshwork into Schlemm’s canal. The balance between aqueous formation and drainage gives rise to the intra-ocular pressure (IOP). Pooled data from large epidemiological studies indicate that the mean IOP in the population is approximately 16 mm of Hg ie the pressure inside the average eye is 16 mm of Hg above atmospheric pressure (760 mm of Hg). Taking two standard deviations from the mean allows us to conclude that 95% of the population have an IOP between 10 – 21 mm of Hg.