268 Introduction to Anatomy & Physiology Jane is concerned about her dad. When he comes home in the evening, he no longer watches the evening news on TV because he says that his eyes are aching. And the last time she rode in a car with him driving, she had to warn him a couple of times about something on the periphery of his vision that he apparently did not see. As you read this section, try to determine which of the following conditions Jane’s dad most likely has. A. Myopia B. Cataracts C. Glaucoma D. Macular degeneration CLINICAL CASE STUDY Injuries, Diseases, and Disorders of the Eye The eyes are so essential to daily life that any kind of injury or disease can have a major impact on a person’s life and well-being. This section describes some common injuries, disorders, and diseases that can affect the ways in which the eye functions. The etiology (cause), strategies for prevention, pathology (clinical characteristics), diagnosis (keys for identifying the condition), and common treatments for these disorders are summarized in Figure 7.6. The eyes begin to develop in the embryo during the 17th day after conception through a complex series of events. Optic vesicles form in the neural area and within five days create a cup-shaped beginning of the wall of the eyeball. This allows the retina and lens to begin formation. Over the next 2 to 3 weeks, the lens, iris, and cornea grow to completion. At four weeks, the orbits and extrinsic muscles begin to develop. During the sixth week, the lacrimal glands begin forming. The lacrimal glands, however, produce no tears until the third month after birth, which is why there are no tears when new infants cry. The sclera develops at seven weeks, followed by the formation of blood vessels. At eight weeks, the eyelids start to form. The maturing cornea becomes transparent when the eye becomes functional in the seventh month. Not until the eight month of pregnancy does the pupil develop the ability to constrict in response to light. During all this time the retina is evolving, with cells differentiating into rods and cones. Axons from the retinal ganglion cells group to form the optic nerve. The development of the retina occurs between 24 weeks gestation and 3 to 4 months after birth, when the optic nerve becomes fully myelinated. At birth, the rod cells allow newborns to see dark, light, and shades of gray. The ability to see colors does not occur until the cones mature at approximately 3 months of age. At birth, the infant’s eye is only about 65% of its full size. Changes during the first year include growth of the orbit, changes to the lens, and pigmentation of the iris. Eye movements are somewhat uncoordinated and corneal curvature is changing. Full color vision occurs between 5 and 7 months. Visual acuity and stereoscopic vision continue improving through about 15 to 18 months of age. Focus, tracking, and depth perception continue to develop through early and middle childhood. The ability of both eyes to focus on an object simultaneously becomes more fully developed by about age seven. Depending on eye growth after birth, the orbits may not fully mature until adolescence. Numerous characteristic changes occur to the lens of the eye as a person ages. The lens becomes stiffer, which causes most people in their 40s to acquire presbyopia, making it difficult to see objects closer than two feet without corrective lenses. Because the lens becomes less transparent, it is also more difficult to see well in dim light. The lens also tends to yellow, so colors appear differently. Other changes due to aging include slowed reaction of the pupil to changes in light level and a decrease in the number of cells transmitting visual information to the brain. These changes result in increased sensitivity to glare, difficulty in discerning fine details and shades of color, and difficulty with depth perception. A decrease in the number of cells producing lubricant to the eyes may cause dryness of the eyes. The eyes also become more susceptible to certain diseases and disorders, as described in the next section. Lifespan Review 1. Why do infants not produce tears when they cry? 2. What functions of the eyes are still developing at birth? 3. How do changes associated with aging affect vision in healthy elderly individuals? LIFE SPAN DEVELOPMENT: The Eyes and Vision Copyright Goodheart-Willcox Co., Inc.