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The Eye

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Eyes are the organs of vision. They detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptor cells in conscious vision connect light to movement.

In higher organisms the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.[1] Image-resolving eyes are present in molluscs, chordates and arthropods.[2]

The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms.[citation needed] From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment.

Eye. Overview[edit] Complex eyes can distinguish shapes and colours.


The visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception. In other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses, which have monocular vision. Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye.

Each sensor has its own lens and photosensitive cell(s). Possessing detailed hyperspectral colour vision, the Mantis shrimp has been reported to have the world's most complex colour vision system.[7] Trilobites, which are now extinct, had unique compound eyes. In contrast to compound eyes, simple eyes are those that have a single lens. Evolution[edit] Overview.

Evolution of the Eye

Types of Eyes. Nutrients. Relationship to life requirements. Visual acuity. Perception of color. Pigmentation. Eyeball. Mammalian Eye. Cephalopod Eye. Mollusc Eye. Arthropod Eye. Tapetum lucidum. Reflection of camera flash from tapetum lucidum In darkness, eyeshine reveals this raccoon Similar adaptations occur in some species of spiders, although these are not the result of a tapetum lucidum.

Tapetum lucidum

Most primates, including humans, lack a tapetum lucidum, and compensate for this by perceptive recognition methods. Eyeshine[edit] White eyeshine occurs in many fish, especially walleye; blue eyeshine occurs in many mammals such as horses; green eyeshine occurs in mammals such as cats, dogs, and raccoons; and red eyeshine occurs in coyote, rodents, opossums and birds.

A three-month-old black Labrador puppy with apparent eye shine Despite it being present in some primates, the human eye has no tapetum lucidum, hence no eyeshine. Blue-eyed cats and dogs[edit] Odd-eyed cat with eyeshine, plus red-eye effect in one eye Classification[edit] A classification of anatomical variants of tapeta lucida[3] defines 4 types: Nictitating membrane. The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility.

Nictitating membrane

Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye. Some mammals, such as camels, polar bears, seals and aardvarks, have full nictitating membranes. Often called a third eyelid or haw, it may be referred to in scientific terminology as the plica semilunaris, membrana nictitans or palpebra tertia. The nictitating membrane (mid-blink) of a bald eagle Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball. Eye movement. Eye movement in two seconds.

Eye movement

Arthropod eye. "Fly eye" redirects here.

Arthropod eye

For Calvin Harris's record label, see Fly Eye Records. Adaptation (eye) In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.

Adaptation (eye)

The eye takes approximately 20–30 minutes to fully adapt from bright sunlight to complete darkness and become ten thousand to one million times more sensitive than at full daylight. In this process, the eye's perception of color changes as well (this is called the Purkinje effect). However, it takes approximately five minutes for the eye to adapt to bright sunlight from darkness. This is due to cones obtaining more sensitivity when first entering the dark for the first five minutes but the rods take over after five or more minutes.[1] Dark adaptation is far quicker and deeper in young people than the elderly. [2] Visual Response to Darkness A minor mechanism of adaptation is the pupillary light reflex, adjusting the amount of light that reaches the retina.

Above a certain luminance level (about 0.03 cd/m2), the cone mechanism is involved in mediating vision; photopic vision. 1. 2.

Outdated theories on vision