- Retina
- The retina is the nerve layer that lines the back of the eye, senses light, and creates impulses that travel through the optic nerve to the brain. There is a small area, called the macula, in the retina that contains special light-sensitive cells. The macula allows us to see fine details clearly. The eye has a number of other components. These include the cornea, iris, pupil, lens, optic nerve and vitreous. The cornea is the clear front window of the eye that transmits and focuses light into the eye. The iris is the colored part of the eye that helps regulate the amount of light that enters the eye. The pupil is the dark aperture in the iris that determines how much light is let into the eye. The lens is the transparent structure inside the eye that focuses light rays onto the retina. The optic nerve is the nerve that connects the eye to the brain and carries the impulses formed by the retina to the visual cortex of the brain. The vitreous humor is a clear, jelly-like substance that fills the middle of the eye.
* * *Grossly, the r. consists of three parts: optic part of r., ciliary part of r., and iridial part of r.. The optic part, the physiologic portion that receives the visual light rays, is further divided into two parts, the pigmented part (pigment epithelium) and the nervous part, which are arranged in the following layers: 1) pigmented layer; 2) layer of inner and outer segments (of rods and cones); 3) outer limiting layer (actually a row of junctional complexes); 4) outer nuclear layer; 5) outer plexiform layer; 6) inner nuclear layer; 7) inner plexiform layer; 8) ganglionic (cell) layer; 9) layer of nerve fibers; and 10) inner limiting layer. Layers 2–10 compose the neural layer. At the posterior pole of the visual axis is the macula, in the center of which is the fovea, the area of acute vision. Here layers 6–9 and blood vessel s are absent, and only elongated cones are present. About 3 mm medial to the fovea is the optic disk, where axons of the ganglionic cells converge to form the optic nerve. The ciliary and iridial parts of the r. are forward prolongations of the pigmented layer and a layer of supporting columnar or epithelial cells over the ciliary body and the posterior surface of the iris, respectively. SYN: optomeninx. [Mediev. L. prob. fr. L. rete, a net]- detached r. SYN: retinal detachment.- fleck r. of Kandori [MIM*228990] an autosomal recessive disorder of the retinal pigment epithelium characterized by retinal flecks and night blindness, occurring among Japanese.- leopard r. SYN: tessellated fundus.- shot-silk r. the appearance of numerous wavelike, glistening reflexes, like the shimmer of silk, observed sometimes in the r. of a young person. SYN: shot-silk phenomenon, shot-silk reflex.- tigroid r. SYN: tessellated fundus.
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ret·i·na 'ret-ən-ə, 'ret-nə n, pl retinas or ret·i·nae -ən-.ē the sensory membrane that lines most of the large posterior chamber of the vertebrate eye, is composed of several layers including one containing the rods and cones, and functions as the immediate instrument of vision by receiving the image formed by the lens and converting it into chemical and nervous signals which reach the brain by way of the optic nerve* * *
n.the light-sensitive layer that lines the interior of the eye. The outer part of the retina (retinal pigment epithelium; RPE), next to the choroid, is pigmented to prevent the passage of light. The inner part, next to the cavity of the eyeball, contains rod and cone (light-sensitive cells) and their associated nerve fibres. A large number of cones is concentrated in a depression in the retina at the back of the eyeball called the fovea.• retinal adj.* * *
ret·i·na (retґĭ-nə) [L.] [TA] the innermost of the three tunics of the eyeball, surrounding the vitreous body and continuous posteriorly with the optic nerve. It is divided into the pars optica, which rests upon the choroid, the pars ciliaris, which rests upon the ciliary body, and the pars iridica, which rests upon the posterior surface of the iris. The pars optica is subdivided into an outer, pigmented layer (stratum pigmentosum) and an inner, transparent layer (stratum nervosum). The stratum nervosum is divided into nine layers, as follows (see illustration): (1) the internal limiting membrane; (2) the nerve fiber layer; (3) the layer of ganglion cells; (4) the inner plexiform layer; (5) the inner nuclear layer; (6) the outer plexiform layer; (7) the outer nuclear layer; (8) the external limiting membrane; (9) the layer of rods and cones. The layer of rods and cones is the percipient part of the retina, responding to visual stimuli by a photochemical reaction; it is connected with the nerve fiber layer by nerve fibers that join to form the optic nerve. In the center of the posterior part of the retina is the macula lutea, the most sensitive portion of the retina, with the fovea centralis at its center. About 0.25 cm inside the fovea is the point of entrance of the optic nerve and the central artery of the retina; at this point the retina is incomplete and forms the blind spot.
Retina.
Layers of the optic part of the retina.
Medical dictionary. 2011.
