Retina(redirected from Nervous tunic)
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retina(rĕt′ĭ-nă) plural.retinae [L.]
The retina is a light-sensitive membrane on which light rays are focused. It extends from the entrance point of the optic nerve anteriorly to the margin of the pupil, completely lining the interior of the eye. It consists of three parts. The pars optica, the nervous or sensory portion, extends from the optic disk forward to the ora serrata, a wavy line immediately behind the ciliary process; the pars ciliaris lines the inner surface of the ciliary process; and the pars iridica forms the posterior surface of the iris. Slightly lateral to the posterior pole of the eye is a small, oval, yellowish spot, the macula lutea, in the center of which is a depression, the fovea centralis. This region contains only cones and is the region of the most acute vision. About 3.5 mm nasally from the fovea is the optic papilla (optic disk), where nerve fibers from the retina make their exit and form the optic nerve. This region is devoid of rods and cones and is insensitive to light; hence it is named the blind spot.
The layers of the retina, in the order light strikes them, are the optic nerve fiber layer, ganglion cell layer, inner synaptic layer, bipolar cell layer, outer synaptic layer, layer of rods and cones, and pigment epithelium. See: illustration
The retina is normally red, reflecting blood flow, and is pale in anemia or ischemia.
The arteries are branches of a single central artery, which is a branch of the ophthalmic artery. The central artery enters at the center of the optic papilla and supplies the inner layers of the retina. The outer layers, including rods and cones, are nourished by capillaries of the choroid layer. The veins lack muscular coats. They parallel the arteries; blood leaves by a central vein that leads to the superior ophthalmic vein.
retinaThe complex membranous network of nerve cells, fibres and photoreceptors that lines the inside of the back of the eye and converts optical images formed by the lens system of the eye into nerve impulses. The retina contains colour-blind but very sensitive rods and colour-sensitive cones and a computing system that refines the signals produced by these. The impulses leave each retina by way of about one million nerve fibres that form the optic nerve.
retinathe lining of the interior of the vertebrate eye containing a concentration of photoreceptor cells known as rods and cones that are connected to the optic nerve via BIPOLAR CELLS. The retina lies immediately in front of the vascular choroid layer which nourishes it.
converse retina See inverted retina.
fleck retina Term referring to a retina with multiple, small or yellow spots, which are seen in various conditions: actinic keratopathy, drusen, fundus albipunctatus, fundus flavimaculatus.
inverted retina Term referring to the fact that the retina of vertebrates is orientated so that the light has to pass through all the neuronal layers before reaching the photo-receptors. However, the retina of invertebrates is normally orientated so that light passes first through the photoreceptors as it traverses the retina: such a retina is called a verted or converse retina.
lattice degeneration of the retina A vitreoretinal degeneration usually found between the equator and the ora serrata leading to a thinning of the retina and characterized by a lesion made up of fine white lines and some pigmentation. It may result in holes or tears and in rhegmatogenous retinal detachment by which time the patient usually complains of floaters. The condition is most common in myopes and often found in patients with Marfan's syndrome. See retinal break; retinal detachment; retinoschisis; Marfan's syndrome.
leopard retina See leopard fundus.
neurosensory retina This is composed of all the layers of the retina, except the outer pigmented layer (called retinal pigment epithelium). It comprises three main groups of neurons: (1) the photoreceptors, (2) the bipolar cells, and (3) the ganglion cells. In addition, there are other connecting neurons: the horizontal and amacrine cells. The neurosensory layer is derived embryologically from the inner layer of the optic cup whereas the pigmented layer is derived from the outer layer of the optic cup and they are separated by a potential space which facilitates their separation, as occurs in detached retina. Syn. sensory retina.
tessellated retina See tessellated fundus.
tigroid retina See tessellated fundus.
verted retina See inverted retina.
|Table R2 Some approximate retinal dimensions|
|structure||mm||degrees||distance from centre of foveola|
|nasal disc margin||5.5 mm or 18.5|
|temporal disc margin||3.5 mm or 12.5|
|centre of disc||4.6 mm or 15.5|
|*The figures given for the size of the disc are those corresponding to the blind spot. Anatomically the optic disc is slightly smaller.|