Inverted versus non-inverted<unk>  The vertebrate<unk>  is inverted in the sense that the light sensing cells are in the back of the<unk> , so that light has to pass through layers of neurons and capillaries before it reaches the rods and cones. The ganglion cells, whose axons form the optic nerve, are at the front of the<unk> ; therefore the optic nerve must cross through the<unk>  en route to the brain. In this region there are no photoreceptors, giving rise to the blind spot. In contrast, in the cephalopod<unk>  the photoreceptors are in front, with processing neurons and capillaries behind them. Because of this, cephalopods do not have a blind spot. Although the overlying neural tissue is partly transparent, and the accompanying glial cells have been shown to act as fibre-optic channels to transport photons directly to the photoreceptors, light scattering does occur. Some vertebrates, including humans, have an area of the central<unk>  adapted for high-acuity vision. This area, termed the fovea centralis, is avascular (does not have blood vessels), and has minimal neural tissue in front of the photoreceptors, thereby minimizing light scattering. The cephalopods have a non-inverted<unk>  which is comparable in resolving power to the eyes of many vertebrates. Squid eyes do not have an analog of the vertebrate<unk> l pigment epithelium (RPE). Although their photoreceptors contain a protein,<unk> chrome, that recycles<unk> l and replicates one of the functions of the vertebrate RPE, one could argue that cephalopod photoreceptors are not maintained as well as in vertebrates and that, as a result, the useful lifetime of photoreceptors in invertebrates is much shorter than in vertebrates. Having easily replaced stalk-eyes (some lobsters) or<unk> e (some spiders, such as Deinopis) rarely occurs. The cephalopod<unk>  does not originate as an outgrowth of the brain, as the vertebrate one does. It is arguable that this difference shows that vertebrate and cephalopod eyes are not homologous but have evolved separately. From an evolutionary perspective, a more complex structure such as the inverted<unk>  can generally come about as a consequence of two alternate processes: (a) an advantageous "good" compromise between competing functional limitations, or (b) as a historical maladaptive relic of the convoluted path of organ evolution and transformation. Vision is an important adaptation in higher vertebrates. A third view of the "inverted" vertebrate eye is that it combines two benefits: the maintenance of the photoreceptors mentioned above, and the reduction in light intensity necessary to avoid blinding the photoreceptors, which are based on the extremely sensitive eyes of the ancestors of modern hagfishes (a fish that lives in very deep, dark water).<unk> l layers Section of<unk>  bipolar and horizontal cells (yellow layer), then to the amacrine cells and ganglion cells (purple layer), then to the optic nerve fibres. The signals are processed in these layers. First, the signals start as raw outputs of points in the rod and cone cells. Then the nerve layers identify simple shapes, such as bright points surrounded by dark points, edges, and movement. (Based on a drawing by Ramón y Cajal, 1911.) Illustration of the distribution of cone cells in the fovea of an individual with normal colour vision (left), and a colourblind (protanopic)<unk> . Note that the center of the fovea holds very few blue-sensitive cones. Distribution of rods and cones along a line passing through the fovea and the blind spot of a human eye The vertebrate<unk>  has ten distinct layers. From closest to farthest from the vitreous body: # Inner limiting membrane – basement membrane elaborated by Müller cells. # Nerve fibre layer – axons of the ganglion cell bodies (note that a thin layer of Müller cell footplates exists between this layer and the inner limiting membrane). # Ganglion cell layer – contains nuclei of ganglion cells, the axons of which become the optic nerve fibres, and some displaced amacrine cells. # Inner plexiform layer – contains the synapse between the bipolar cell axons and the dendrites of the ganglion and amacrine cells. # Inner nuclear layer – contains the nuclei and surrounding cell bodies (perikarya) of the amacrine cells, bipolar cells, and horizontal cells. # Outer plexiform layer – projections of rods and cones ending in the rod spherule and cone pedicle, respectively. These make synapses with dendrites of bipolar cells and horizontal cells. In the macular region, this is known as the Fiber layer of Henle. # Outer nuclear layer – cell bodies of rods and cones. # External limiting membrane – layer that separates the inner segment portions of the photoreceptors from their cell nuclei. # Inner segment / outer segment layer – inner segments and outer segments of rods and cones. The outer segments contain a highly specialized light-sensing apparatus. #<unk> l pigment epithelium – single layer of cuboidal epithelial cells (with extrusions not shown in diagram). This layer is closest to the choroid, and provides nourishment and supportive functions to the neural<unk> , The black pigment melanin in the pigment layer prevents light reflection throughout the globe of the eyeball; this is extremely important for clear vision. These layers can be grouped into 4 main processing stages: photoreception; transmission to bipolar cells; transmission to ganglion cells, which also contain photoreceptors, the photosensitive ganglion cells; and transmission along the optic nerve. At each synaptic stage there are also laterally connecting horizontal and amacrine cells. The optic nerve is a central tract of many axons of ganglion cells connecting primarily to the lateral geniculate body, a visual relay station in the diencephalon (the rear of the forebrain). It also projects to the superior colliculus, the suprachiasmatic nucleus, and the nucleus of the optic tract. It passes through the other layers, creating the optic disc in primates. Additional structures, not directly associated with vision, are found as outgrowths of the<unk>  in some vertebrate groups. In birds, the pecten is a vascular structure of complex shape that projects from the<unk>  into the vitreous humour; it supplies oxygen and nutrients to the eye, and may also aid in vision. Reptiles have a similar, but much simpler, structure. In adult humans, the entire<unk>  is approximately 72% of a sphere about 22 mm in diameter. The entire<unk>  contains about 7 million cones and 75 to 150 million rods. The optic disc, a part of the<unk>  sometimes called "the blind spot" because it lacks photoreceptors, is located at the optic papilla, where the optic-nerve fibres leave the eye. It appears as an oval white area of 3 mm². Temporal (in the direction of the temples) to this disc is the macula, at whose centre is the fovea, a pit that is responsible for our sharp central vision but is actually less sensitive to light because of its lack of rods. Human and non-human primates possess one fovea, as opposed to certain bird species, such as hawks, who are bifoviate, and dogs and cats, who possess no fovea but a central band known as the visual streak. Around the fovea extends the central<unk>  for about 6 mm and then the peripheral<unk> . The farthest edge of the<unk>  is defined by the ora serrata. The distance from one ora to the other (or macula), the most sensitive area along the horizontal meridian is about 32 mm. In section, the<unk>  is no more than 0.5 mm thick. It has three layers of nerve cells and two of synapses, including the unique ribbon synapse. The optic nerve carries the ganglion cell axons to the brain, and the blood vessels that supply the<unk> . The ganglion cells lie innermost in the eye while the photoreceptive cells lie beyond. Because of this counter-intuitive arrangement, light must first pass through and around the ganglion cells and through the thickness of the<unk> , (including its capillary vessels, not shown) before reaching the rods and cones. Light is absorbed by the<unk> l pigment epithelium or the choroid (both of which are opaque). The white blood cells in the capillaries in front of the photoreceptors can be perceived as tiny bright moving dots when looking into blue light. This is known as the blue field entoptic phenomenon (or Scheerer's phenomenon). Between the ganglion cell layer and the rods and cones there are two layers of neuropils where synaptic contacts are made. The neuropil layers are the outer plexiform layer and the inner plexiform layer. In the outer neuropil layer, the rods and cones connect to the vertically running bipolar cells, and the horizontally oriented horizontal cells connect to ganglion cells. The central<unk>  predominantly contains cones, while the peripheral<unk>  predominantly contains rods. In total, there are about seven million cones and a hundred million rods. At the centre of the macula is the foveal pit where the cones are narrow and long, and, arranged in a hexagonal mosaic, the most dense, in contradistinction to the much fatter cones located more peripherally in the<unk> . At the foveal pit the other<unk> l layers are displaced, before building up along the
Retina
Forebrain
Hindbrain
Barrel cortex
Brainstem
Auditory cortex
Electuary
Diencephalon