Anatomy & Physiology

Anatomy & Physiology Study Guide disc. The optic disc has no photoreceptors or other structures typical of the rest of the retina. Because light striking this area goes unnoticed, the optic disc is commonly called the blind spot. The Chambers of th Eye The eye contains a large posterior cavity, or vitreous chamber, and a smaller anterior cavity. The anterior cavity is subdivided into the anterior chamber, which extends from the cornea to the iris, and a posterior chamber, between the iris and the ciliary body and lens. The fluid aqueous humor fills the anterior and posterior chambers of the eye. Aqueous humor is a fluid that circulates within the anterior cavity, passing from the posterior to the anterior chamber through the pupil. It also freely diffuses through the vitreous body and across the surface of the retina. Aqueous humor forms through active secretion by epithelial cells of the ciliary body’s ciliary processes. It supplies an important route for nutrient and waste transport and forms a fluid cushion. The eye is filled with fluid, and the fluid pressure in the aqueous humor helps to maintain the shape of the eye. 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. Acco modation 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 Achieve Page 193 of 368 ©2018