Anatomy & Physiology I and II

Anatomy & Physiology Study Guide The posterior cavity of the eye contains the vitreous body, a gelatinous mass. The vitreous body helps stabilize the shape of the eye. Unlike the aqueous humor, the vitreous body is formed during development of the eye and is not replaced. The Lens The lens lies posterior to the cornea. The main function of the lens is to focus the visual image on the photoreceptors by changing its shape. The lens consists of concentric layers of cells that are precisely organized. The cells in the interior of the lens are called lens fibers . These cells are highly specialized cells and have lost all their organelles. They are slender and elongate and are filled with transparent proteins called crystallins , which are responsible for both the clarity and the focusing power of the lens. When the lens loses its transparency, this abnormality is known as a cataract . Refraction The retina has about 130 million photoreceptors, each monitoring light striking a specific site on the retina. Visual images result from the processing of information from all the receptors. The lens of the eye must focus the arriving image. If the rays are not perfectly focused, the image is blurry. Focusing normally occurs in two steps: as light passes first through the cornea and then the lens. Light is refracted, or bent, when it travels from one medium to another medium with a different density. In the human eye, the greatest amount of refraction occurs when light passes from the air into the corneal tissues, which have a density close to that of water. When you open your eyes under water, you cannot see clearly because refraction at the corneal surface has been largely eliminated; light passes unbent from one watery medium to another. The space from the center of the lens to its focal point is the focal distance of the lens. Whether in the eye or in a camera, the focal distance is determined by two factors: the distance from the object to the lens, and the shape of the lens. Accommodation Images are focused on the retina by changing the shape of the lens to keep the focal length constant; a process called accommodation. During accommodation, the lens becomes rounder to focus the image of a nearby object on the retina; the lens flattens with focusing on a distant object. The inner limit of clear vision, called the near point of vision, is calculated by the degree of elasticity in the lens. Children can usually focus on something 7–9 cm from the eye, but the lens tends gradually becomes stiffer and less responsive. A young adult can focus on objects 15–20 cm away. As aging occurs, this distance increases; the near point at age 60 is typically about 83 cm. If light traveling through the cornea and lens is not refracted properly, the visual image will be distorted. In the condition called astigmatism , the degree of curvature in the cornea or lens varies from one axis to another. Image Reversal An object in view is a complex light source that must be regarded as a large number of solitary points. Light from each point is centered on the retina. The result is the creation of a miniature image of the original, but the image arrives upside down and reversed. The brain compensates for this image reversal, and we are not aware of any difference between the orientation of the image on the retina and that of the object. ©2018 Achieve Test Prep Page 193 of 367