Land-vertebrate eyes {eye} are spherical and focus images on retina.
eye muscles
Eye muscles exert constant tension against movement, so effort required to move eyes or hold them in position is directly proportional to eye position. Midbrain oculomotor nucleus sends, in oculomotor nerve, to inferior oblique muscle below eyeball, superior rectus muscle above eyeball, inferior rectus muscle below eyeball, and medial rectus muscle on inside. Pons abducens nucleus sends, in abducens nerve, to lateral rectus muscle on outside. Caudal midbrain trochlear nucleus sends, in trochlear nerve, to superior oblique muscle around light path from above eyeball.
eye muscles: convergence
Eyes converge toward each other as object gets nearer than 10 meters.
eye muscles: zero-gravity
In zero-gravity environment, eye resting position shifts upward, but people are not aware of shift.
fiber projection
Removing embryonic eye and re-implanting it in rotated positions does not change nerve fiber projections from retina onto visual cortex.
Horseshoe crab (Limulus) eye {simple eye} can only detect light intensity, not direction. Input/output equation uses relation between Green function and covariance, because synaptic transmission is probabilistic.
Most mammals and birds have tissue fold {inner eyelid} {palpebra tertia} that, when eye retracts, comes down from above eye to cover cornea. Inner eyelid has outside mucous membrane {conjunctiva}, inner-side lymphoid follicles, and lacrimal gland.
Reptiles and other vertebrates have transparent membrane {nictitating membrane}| that can cover and uncover eye.
Eye has transparent cells {cornea}| protruding in front. Cornea provides two-thirds of light refraction. Cornea has no blood vessels and absorbs nutrients from aqueous humor. Cornea has many nerves. Non-spherical-cornea astigmatism distorts vision. Corneas can transplant without rejection.
Elastic and transparent cell layers {lens, eye} {crystalline lens} attach to ciliary muscles that change lens shape. To become transparent, lens cells destroy all cell organelles, leaving only protein {crystallin} and outer membrane. Lens cells are all the same. They align and interlock [Weale, 1978]. Lens shape accommodates when objects are less than four feet away. Lens maximum magnification is 15.
Sphincter muscles in a colored ring {iris, eye}| close pupils. When iris is translucent, light scattering causes blue color. In mammals, autonomic nervous system controls pupil smooth muscles. In birds, striate muscles control pupil opening.
Eye has opening {pupil}| into eye. In bright light, pupil is 2 mm diameter. At twilight, pupil is 10 mm diameter. Iris sphincter muscles open and close pupils. Pupil reflex goes from one eye to the other.
Eyeball has insides {fundus, eye}.
Liquid {aqueous humor}| can be in anterior chamber behind cornea and nourish cornea and lens.
Liquid {vitreous humor}| fills main eyeball chamber between lens and retina.
Eyeball has outer white opaque connective-tissue layer {sclera}|.
Eye regions {trochlea}| can have eye muscles.
Eyeball has inner blood-vessel layer {choroid}.
Between retina and choroid is a cell layer {retinal pigment epithelium} (RPE) and Bruch's membrane. RPE cells maintain rods and cones by absorbing used molecules.
Retinal-pigment epithelium and membrane {Bruch's membrane} {Bruch membrane} are between retina and choroid.
At back inner eyeball, visual receptor-cell layers {retina}| have 90 million rod cells, one million cones, and one million optic nerve axons.
cell types
Retina has 50 cell types.
cell types: clustering
Retina has clusters of same cone type. Retina areas can lack cone types. Fovea has few short-wavelength cones.
development
Retina grows by adding cell rings to periphery. Oldest eye part is at center, near where optic nerve fibers leave retina. In early development, contralateral optic nerve fibers cross over to connect to optic tectum. In early development, optic nerve fibers and brain regions have topographic maps. After maturation, axons can no longer alter connections.
processing
Retina cells separate information about shape, reflectance, illumination, and viewpoint.
Ganglion-cell axons leave retina at region {blindspot}| medial to fovea [DeWeerd et al., 1995] [Finger, 1994] [Fiorani, 1992] [Komatsu and Murakami, 1994] [Komatsu et al., 2000] [Murakami et al., 1997].
Cone cells are Long-wavelength, Middle-wavelength, or Short-wavelength. Outside fovea, cones can form two-dimensional arrays {color-receptor array} with L M S cones in equilateral triangles. Receptor rows have ...S-M-L-S-M-L-S... Receptor rows above, and receptor rows below, are offset a half step: ...-L-S-M-L-S-M-.../...S-M-L-S-M-L-S.../...-L-S-M-L-S-M-...
hexagons
Cones have six different cones around them in hexagons: three of one cone and three of other cone. No matter what order the three cones have, ...S-M-L-S-M..., ...S-L-M-S-L..., or ...M-L-S-M-L..., M and L are beside each other and S always faces L-M pair, allowing red+green brightness, red-green opponency, and yellow-blue opponency. L receptors work with three surrounding M receptors and three surrounding S receptors. M receptors work with three surrounding L receptors and three surrounding S receptors. S receptors work with six surrounding L+M receptor pairs, which are from three equilateral triangles, so each S has three surrounding L and three surrounding M receptors.
In all directions, fovea has alternating long-wavelength and middle-wavelength cones: ...-L-M-L-M-.
Primates have central retinal region {fovea}| that tracks motions and detects self-motion. Retinal periphery detects spatial orientation. Fovea contains 10,000 neurons in a two-degree circle. Fovea has no rods. Fovea center has no short-wavelength cones. Fovea has patches of only medium-wavelength cones or only long-wavelength cones. Fovea has no blood vessels, which pass around fovea.
Retinal layers {inner plexiform layer} can have bipolar-cell and amacrine-cell axons and ganglion-cell dendrites. There are ten inner plexiform layers.
Near retina center is a yellow-pigmented region {macula lutea}| {yellow spot}. Yellow pigment increases with age. If incident light changes spectra, people can briefly see macula image {Maxwell spot}.
1-Consciousness-Sense-Vision-Anatomy
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Date Modified: 2022.0225