Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Sense Organs. Analyzers
The Organ of Vision. The Visual Analyzer
The Optical System of the Eye

Visual Perception begins with the projection of an image onto the retina and the excitation of its receptor Cells: rod and cone neurocytes—rods and cones. The Optical System of the eye, which consists of light-refracting and accommodation apparatuses, ensures the PROJECTION OF THE image onto the retina.

The light-refracting apparatus of the eye integrates the cornea, aqueous humor, crystalline lens, and vitreous body. These are transparent structures that refract light as it passes from one medium to another (air — cornea — surface of the lens). The strongest refraction of light occurs in the cornea.

The accommodation apparatus is formed by the ciliary body, iris, and crystalline lens. These structures direct light rays emanating from observed objects onto the retina, specifically into the area of its macula (fovea centralis). In humans, the main structural mechanism of accommodation is provided by the lens and the ciliary body. Changes in the curvature of the lens are regulated by the complexly arranged Muscle of the ciliary body. When the muscle bundles contract, the tension of the zonule fibers attached to the lens capsule slackens. Free from the limiting pressure of its capsule, the lens becomes more convex, which increases its refractive power. When the ciliary muscle relaxes, the zonular fibers become taut, the lens flattens, and its refractive power decreases. With the help of the ciliary muscle, the lens constantly changes its curvature, adapting the eye for clear Vision of objects at various distances. This property of the lens is known as accommodation.

On its way to the light-sensitive retina, light passes through a series of transparent light-refracting media of the eye. Acting as a Diaphragm, the pupil constricts or dilates under the action of its Muscles, allowing a smaller or larger beam of light to pass into the eye onto the most sensitive area of the retina—the macula.

The Eyeball refracts parallel light rays and focuses them precisely on the retina. If the refractive power of the cornea or lens is weakened, light rays converge to a focus behind the retina, a condition known as hyperopia (farsightedness). In hyperopia, a person sees distant objects well, but those close up poorly. Conversely, when the refractive power of the transparent media of the eye is excessively high, light rays converge at a point in front of the retina rather than on it. This leads to myopia (nearsightedness), where a person sees close objects clearly, while distant ones appear blurred. Both hyperopia and myopia are corrected using eyeglasses with biconvex or biconcave lenses.