Anatomy of the human retinaThe retina is roughly 0.5 mm thick and forms about three-quarters of a full sphere (being open at the front where light enters the eye). In the center of the retina is the optic disk, a circular to oval white area measuring about 2 × 1.5 mm across. This disk is where the axons of ganglion cells leave the eye to form the optic nerve. From the center of the optic disk radiate the major blood vessels of the retina. Displaced temporally (in the direction of the temples) from the optic disk is the macula, in the middle of which is the fovea, responsible for detailed vision.
The central part of the retina is dominated by cone cells, whereas the peripheral retina is dominated by rod cells. Altogether there are about 7 million cones and 100 million rods.
Like all vertebrate retinas, the human retina has three layers of nerve cell bodies and two layers of synapses. Through a quirk of vertebrate evolution, the ganglion cells lie innermost in the retina while the light-sensitive rods and cones are outermost, so that light first has to pass through the thickness of the retina before it reaches the photoreceptive cells.
Embryology of the retina
In cephalopods the retina is formed not as an outgrowth of the brain but from the external ectoderm and the light-sensitive cells are external to the nerve fibers. This reveals that, although vertebrate and cephalopod eyes have some striking similarities, they are biologically analogous rather than homologous.
Comparison of the vertebrate and cephalopod retinasAs mentioned, the vertebrate retina is back-to-front, or inverted, in that the light-sensing cells are on the back side of the retina, so that light has to travel through a layer of neurons before it gets to the photoreceptors. By contrast, the cephalopod retina is everted: the photoreceptors lie at the front of the retina, with processing neurons behind them. Because of this arrangement, cephalopods do not have a blind spot.
Related category ANATOMY AND PHYSIOLOGY
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