A schematic of a human adult eye A schematic of a human adult eye. The adult eye can be subdivided into two major domains: the anterior and posterior eye. The anterior segment includes the tissues shown on the right, moving from distal to proximal, the cornea, aqueous humour, iris, and lens. The lens is attached to the globe of the eye via the ciliary muscles and trabecular meshwork, which are derived from the ciliary body. The ciliary muscles and trabecular meshwork comprise the ciliary apparatus. The trabecular meshwork regulates the intraocular pressure of the eye by regulating fluid movement between the anterior and posterior regions of the eye. The posterior segment is made up of the most proximal parts of the eye. These include the retina proper and the back of the eye where the optic nerve exits. On the outside of the globe is the sclera, which adds rigidity to the eye. Internal to this layer is the choroid, which is made up largely of blood vessels. The innermost tissue is the retina, which contains two major types of cells: an external layer one cell thick, the retinal pigment epithelium (RPE), and an internal, multilayered neuroretina. The globe is filled with the vitreous humor. Light enters the eye through the transparent cornea, through the pupil in the center of the iris, and is focused on the back of the neuroretina by the lens (see Fig. 240-2 for a photograph of the neuroretina). Light scatter is reduced by the retinal pigment epithelium. Photons are captured by the photoreceptors located at the outside of the neuroretina adjacent to the RPE, and the energy is transduced to electric signals. The signal is passed through interneurons of the retina to the ganglion cells at the inner part of the neuroretina. The axons of ganglion cells exit the eye at the optic disc and fasciculate to form the optic nerve which travels to the optic centres of the brain. The human eye contains a specialized region of the neuroretina called the fovea or macula, where cone photoreceptors are concentrated. Light is focused on the fovea, and the high cone number results in high acuity vision. (Figure modified from Timothy J. Peters and Co. Inc. Copyright 1994 by Tim Peters and Company, Inc., Peapack, NJ, USA. Used with permission.) Source: Transcription Factors in Eye Disease and Ocular Development, The Online Metabolic and Molecular Bases of Inherited Disease Citation: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. The Online Metabolic and Molecular Bases of Inherited Disease; 2014 Available at: https://ommbid.mhmedical.com/DownloadImage.aspx?image=/data/books/971/ch240fg1.png&sec=62662720&BookID=971&ChapterSecID=62662710&imagename= Accessed: October 25, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved