zone that contains smooth muscle and the ciliary processes and the pars plana, which lies more posteriorly. The ciliary processes in the pars plicata are connected to the lens zonules (annular ligament of Zinn). When the ciliary muscle contracts the entire structure moves inward and anteriorly. This relaxes the zonules, which in turn releases the tension on the lens, allowing for accommodation. Aqueous is also secreted by the ciliary body from the nonpigmented cells along the apex. Pigmented epithelial cells continue posterior in the ciliary body, which will eventually merge with the retinal pigment epithelium. The pars plana is approximately 4 mm wide and extends from the pars plicata to the termination of the retina, which is called the ora serrata. The ora serrata is located approximately 6 mm from the limbus. The pars plana is the safety location for the injection of intravitreal medications.

Posterior segment

The retina is formed from neuroectoderm and forms the innermost layer of the posterior eye. The sensory retina is a highly specialized, multilayered arrangement.17 There are nine layers within the retina: three layers of nuclei, three layers of fibers, two limiting membranes, and the outer segments of the rods and cones18 (8). The most anterior layer is the nerve fiber layer made up of the axons of the ganglion cells. These axons will join to form the optic nerve and exit through an opening in the sclera. Ganglion cells respond to impulses from bipolar cells, which receive their signals from the rods and cones. Amacrine cells located within the inner plexiform layer of the retina further modulate the signal sent to the ganglion cells. Other cells which modulate the signal and support the structures within the retina include

horizontal cells in the outer plexiform layer as well as Müller cells. Underlying the sensory retina is a layer of pigmented cells called the retinal pigment epithelium. The basal lamina of this layer forms Bruch’s membrane. The apical portion extends in villous processes to surround the outer segments of the rods and cones. The function of the retinal pigment epithelium is to absorb excess light that has entered the eye, metabolize vitamin A and rhodopsin, support the metabolic functions of the rods and cones, and form the blood—retinal barrier. The richly vascular choriocapillaris underlies the retinal pigment epithelium. It supplies the outer half of the retina as well as the retinal pigment

epithelium. The inner retina is supplied by the central retinal artery. Separation of the retina from the retinal pigment epithelium is called a retinal detachment.

The retina is further divided into sections by geography. The macular region is approximately 4.5 mm in diameter. It is further located within the temporal vascular arcades. Cones are the predominate structure. Within the macula is a 1.5 mm depression called the fovea. There is absence of the other layers of the retina, so that light falls on the cone outer segments. This area provides the best visual resolution in the eye (9).