The retina is the light-sensitive tissue that lines the inside of the eye. The retina functions in a manner similar to film in a camera. The optical elements within the eye focus an image onto the retina of the eye, initiating a series of chemical and electrical events within the retina. Nerve fibers within the retina send electrical signals to the brain, which then interprets these signals as visual images.
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| Retina |
The human retina is located on the inner surface of the posterior two-thirds to three-quarters of the eye. The eye itself is a mostly hollow organ, roughly spherical in shape. In adults, the eye measures approximately 22 mm in diameter. The walls of the eye consist of the firm outermost coat, comprised of the white sclera in the posterior three-quarters of the eye and the clear cornea in the anterior one-quarter of the eye. The middle layer consists of the uveal tract made up of the choroid posteriorly and the ciliary body and iris anteriorly. The retina is the innermost layer. It lines the entire posterior portion of the eye with the exception of the area of the optic nerve and extends anteriorly to end 360 degrees circumferentially at the ora serrata.
The total area of the retina is approximately 1,100 mm2 . The circumferential diameter at the equator of the adult eye averages 69 mm. The retinal distance circumferentially passing posteriorly from a point on the ora serrata to a point on the ora serrata 180 degrees away is approximately 50 mm. The average healthy retina is 250-µm thick immediately adjacent to the temporal margin of the optic nerve.
The retina thickens to approximately 400 µm in the macular area around the fovea and thins to 150 µm in the fovea. This difference in thickness in the central and noncentral portions of the macula can be appreciated when the eye is examined with an ophthalmoscope. The retina thins as it approaches the equatorial region of the eye and further thins to 80 µm at the ora serrata.
More details: 📖 The Retina: A Guide to Self-Assessment
Retinal nerve fibers exit the eye through the optic nerve, located nasally and on the same plane as the anatomical center of the retina. There is no retinal tissue overlying the optic nerve head. The optic nerve head or optic disc is oval in shape and measures approximately 1.75 mm. vertically and 1.5 mm horizontally. The center of the optic disc is located 4.5 mm to 5 mm nasal to the anatomical center of the retina.
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| When an ophthalmologist uses an ophthalmoscope to look into your eye he sees the following view of the retina |
The center of the retina provides the greatest resolving power of the eye. This area, responsible for central vision, is known as the macula. The center of the macula is called the fovea.
The inner surface of the retina is adjacent to the vitreous of the eye. The outermost layer of the retina, the retinal pigment epithelium, is tightly attached to the choroid.
Assuming that the ocular media (cornea, anterior chamber, lens, and vitreous) are not cloudy, the living retina can be examined using a direct or with a high diopter biconvex retinal lens at the slit lamp. In addition, the retina may be photographed using a retinal camera. The arterioles and venules of the retina are the only blood vessels whose wall can be directly examined in the living human without an incision.
The retina, with the exception of the blood vessels coursing through it, is transparent to the examiner up to its outer layer, the retinal pigment epithelium. The transparent portion of the retina is known as the neurosensory retina. The examiner sees the neurosensory retina against the background orange color of the melanin containing retinal pigment epithelium and blood-filled choroidal layer of the eye.
Blood supply of the retina
There are two circulations to the retina, both supplied by the ophthalmic artery, the first branch of the internal carotid artery on each side. The outer and middle retinal layers, including the outer plexiform and outer nuclear layers, the photoreceptors, and the retinal pigment epithelium, are nourished by branches of the posterior ciliary arteries, which enter the back of the eye outside the optic nerve.
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| Blood supply of the retina |
These vessels also supply the choroidal layer, external to the retina. The inner retina is supplied by the central retinal artery, the branch of the ophthalmic artery that enters the optic nerve 4 mm posterior to the eye. The central retinal artery has 4 main branches within the retina.
These vessels emerge from the optic nerve head and run radially away from the optic nerve.
The temporal vessels curve towards and around the fovea. The macular vessels arise from branches of the superior temporal and inferior temporal arteries. There is a blood vessel-free and capillary-free zone of approximately 500 µm in diameter in the area of the fovea.
In about 1 out of 5 people, the inner layer of the macula is dually supplied by cilioretinal arteries branching from the posterior ciliary arteries.
Retinal blood vessels maintain the blood-retinal barrier due to nonfenestration of the vascular endothelium. Choroidal endothelial vascular cells are fenestrated.
The arterial intraretinal branches then supply capillary networks, which subsequently drain into venules and then into the central retinal vein. The arterial circulation emanating from the posterior ciliary arteries drain out of the eye through one or two vortex veins in each of the 4 quadrants of the eyeball. The vortex veins penetrate the sclera and merge into the ophthalmic vein.
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