 The Sense of Sight Each eye socket is shaped by seven bones that are lined with periosteum, adipose tissue for additional protection and insulation, blood vessels, nerves, and connective tissues. The eyelids also protect the eyeballs. Lacrimal, glands, moisten and lubricate the eyes, and extrinsic muscles move the eyes. The eye is a hollow, spherical structure with three distinct layers. The outer layer, or tunic, is largely composed of connective tissue with a thin layer of epithelium. The anterior one-sixth of the outer tunic is the cornea. It is transparent and bulges forward to help focus entering light waves. The rest of the outer layer, the sclera, is the white portion of the eye. It is tough, full of collagen and elastic fibers, and is continuous with the dura mater. The optic nerves and blood vessels pierce the sclera in the back of the outer tunic. The middle layer is known as the vascular tunic. It includes the iris, the coroid coat, and the ciliary body. Blood vessels pervade the coroid coat, which loosely joins the sclera. The coroid coat forms the posterior five-sixth of the eye globe. Many melanocytes in this coat produce pigment to absorb excess light. The ciliary body is the thickest part of the middle tunic and helps form the anterior of the eye. Many strong fibers hold the lens in place. Ciliary muscles help the lens to focus. The lens is directly behind the iris and pupil. The iris extends forward to the ciliary body and lies between the cornea and the lens. The iris is composed of smooth muscle fibers and connective tissue. The pupil is a circular opening in the center of the iris. The inner surface of the ciliary body secretes a watery fluid called aqueous humor, which circulates into the posterior chamber of the anterior cavity behind the iris. The fluid flows from this space through a circular opening in the center of the iris called the pupil and into the anterior chamber of the anterior cavity between the cornea and the lens. The iris divides the space called the anterior chamber among the cornea, ciliary body, and lens. The inner tunic, the retina, is thin and delicate and contains the photoreceptors. It is continuous with the optic nerve at the back of the eye. Complex groups of retinal neurons transmit the sense of sight. The foveus centralis produces the sharpest vision. The posterior cavity, enclosed by the lens, ciliary body, and retina contains a jelly-like vitreous humor to help support the internal structure of the eye. The photoreceptors of the eyes are modified neurons. The two types of modified neurons are the more numerous rods and the less numerous cones of the retina. Rods and cones are associated with a layer of pigmented epithelium. The epithelial pigment of the retina absorbs those light waves that are not absorbed by the rods and cones and together with the absorption of light by the coroids coat of the middle tunic keep light from reflecting off of the surface of the eye. Visual pigments are synthesized from vitamin A. Rods produce colorless vision and are far more sensitive to light than our cones. For this reason, rods provide vision in dim light. The cones are less sensitive to light but can detect colors and provide sharp images. The difference in sharpness is due to the convergence of rod fibers. Cone fibers do not converge onto sensory cells. The area of sharpest vision, the foveus centralis, contains no rods, only cones with few or no converging fibers. The complex structures of the eye allow for a wealth of color and light. The optic nerves lead from the eyes to the brain. The optic chiasma helps to provide stereo vision. Congratulations! You have completed this learning activity, The Sense of Sight.