Chapter 7 The Sensory Systems 267 for the avascular (without blood vessels) lens and cornea, and also helps maintain normal intraocular pressure (pressure inside the eye). The posterior chamber of the eye is filled with the gel-like vitreous (VIT-ree-us) humor, which also contributes to intraocular pressure. When at rest, the eye is focused for distance vision. For the eye to clearly view objects closer than about 20 feet, the muscles of the ciliary body contract to change the shape of the lens. This process of contraction, known as accommodation, makes the lens thicker, enabling it to focus incoming light rays on the surface of the retina. After about 40 years of age, the ability of the ciliary body muscles to appropriately contract diminishes. In the absence of other visual corrections, this causes people in the post-40 age group to need reading glasses for up-close vision. Check Your Understanding 1. What do tarsal glands produce? 2. Describe two different ways by which tears clean the eyes. 3. Name the three layers of the eye. 4. Explain the purpose of aqueous and vitreous humors. Vision You see an object when the light that is reflected from that object passes through your cornea, pupil, and lens to your retina. The rods and cones in the retina are stimulated and pass impulses to the optic nerve, which transmits sensory signals to the brain. The optic nerves from the eyes cross at the optic chiasma (kigh-AZ-ma). The nerve fibers exiting the optic chiasma are called optic tracts. The optic tracts carry visual stimuli to the occipital lobe of the brain. Check Your Understanding 1. Which nerve is responsible for transmitting sensory signals to the brain? 2. Which part of the brain processes sensory signals from the eyes? You may have heard of the “blind spot” on the retina. The physiological blind spot on each retina is called the optic disc. The optic disc is the junction between the optic nerve and the eye. Because there are no rods and cones in the optic disc, this tiny area is unable to transmit visual information—hence the term blind spot. Under normal circumstances, you do not perceive the blind spot because the brain fills in the visual information from the other eye. Because of the separation between the two eyes, the blind spots are missing different pieces of the combined visual field. The lens of the eye is located behind the iris. It is a transparent, flexible, crystal-like structure curved outward on both sides. The lens is held in place by tiny suspensory ligaments that surround it. These ligaments attach to the ciliary body, which merges with the choroid layer. The lens separates the anterior and posterior chambers of the eye. The anterior chamber is filled with the clear, watery aqueous (AY- kwee-us) humor. Continually secreted by the choroid, the aqueous humor provides nutrients © Body Scientific International Figure 7.5 Rods and cones in the retina. Which types of cells—cones or rods—are activated in dim light? Cone Rod Pigmented part of retina Bipolar neurons Nerve ganglions Copyright Goodheart-Willcox Co., Inc.