266 Introduction to Anatomy & Physiology Two sets of muscles within the iris work to control the amount of light admitted to the eye. The sphincter pupillae contracts in the presence of bright light or when the eye focuses on an object within close range, causing the pupil to become smaller. In the presence of dim light or when the eye focuses on a distant object, the dilator pupillae muscle contracts, causing dilation (enlargement) of the pupil. The innermost layer of the eye, the retina, is located only around the posterior portion of the eye, anterior to the choroid. The retina is composed of two layers. The outer, or pigmented, layer includes pigmented cells that absorb light, store vitamin A, and serve as phagocytes to remove any damaged receptor cells on the inner layer. The inner, or neural, layer of the retina is dense in specialized, light-sensitive nerve endings. These nerve endings send impulses through the optic nerve to the occipital lobe of the brain, where visual images are interpreted. The sensory cells in the retina are called rods and cones (Figure 7.5). The rods are activated in dim light and are most densely distributed around the periphery of the retina. They provide peripheral vision and enable perception of shades of gray in dim light. The cones are sensitive to bright light and also provide color vision. They are most densely distributed in the center of the retina, with decreasing distribution moving toward the periphery. Nerve ganglions (GAYNG- glee-ahnz) and bipolar neurons (discussed in Chapter 6) provide connections between the retina and the rods and cones. © Body Scientific International Figure 7.4 Internal structures of the eye. How would your vision be affected if the iris lacked sphincter pupillae and dilator pupillae muscles to control the amount of light admitted to the eye? Conjunctiva Aqueous humor (liquid filling) Cornea Pupil Lens Iris Suspensory ligaments Ciliary body Sphincter pupillae Vitreous humor (liquid filling) Choroid layer Retina Sclera Optic nerve Copyright Goodheart-Willcox Co., Inc.