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The large spheroid space in back of the lens the center of the eyeball is filled with vitreous humor, a jellylike substance. Accessory structures of the eye are the lacrimal gland and its ducts in the upper lid, which bathe the eye with tears tears, watery secretion of the lacrimal gland, which is located at the outer corner of the eye socket immediately above the eyeball.

Tearing, or lacrimation, is a continuous and largely involuntary process stimulated by the autonomic nervous system. Click the link for more information. The eye is protected from dust and dirt by the eyelashes, eyelid, and eyebrows. Six muscles extend from the eyesocket to the eyeball, enabling it to move in various directions. In addition to errors of refraction astigmatism astigmatism , type of faulty vision caused by a nonuniform curvature in the refractive surfaces—usually the cornea, less frequently the lens—of the eye.

As a result, light rays do not all come to a single focal point on the retina.

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It is caused by a defect of refraction in which the image is focused behind the retina of the eye rather than upon it, either because the eyeball is Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it. Strabismus is a condition in which the eye turns in or out because of an imbalance in the eye musculature. A cornea damaged by accident or illness can sometimes be corrected by excimer laser or surgically replaced with a healthy one from a deceased person.

Experimental retinal implants, consisting of electrode arrays that receive visual data from an external camera, have been used to partially restore sight to persons with damaged retinas, enabling some recognition of shapes, light and dark areas, and motion. Eyes that are used in various ways for surgical repairs are supplied by eye banks.

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People can arrange to have their eyes donated to such organizations after their death. The camera type of eye, which forms excellent images, is found in all vertebrates, in cephalopods such as the squid and octopus , and in some spiders. In each of those groups the camera type of eye evolved independently.

In some species, e. Simple eyes, or ocelli, are found in a great variety of invertebrate animals, including flatworms, annelid worms such as the earthworm , mollusks, crustaceans, and insects. An ocellus has a layer of photosensitive cells that can set up impulses in nerve fibers; the more advanced types also have a rigid lens for concentrating light on this layer. Simple eyes can perceive light and dark, enabling the animal to perceive the location and movement of objects.

They form no image, or a very poor one. The compound eye is found in a large number of arthropods, including various species of insects, crustaceans, centipedes, and millipedes. A compound eye consists of from 12 to over 1, tubular units, called ommatidia, each with a rigid lens and photosensitive cells; each omnatidium is surrounded by pigment cells and receives only the light from its own lens.

The lenses fit together on the surface of the eye, forming the large, many-faceted structure that can be seen, for example, in the fly. Each ommatidium supplies a small piece of the image perceived by the animal. The compound eye creates a poor image and cannot perceive small or distant objects; however, it is superior to the camera eye in its ability to discriminate brief flashes of light and movement, and in some insects e.

Because arthropods are so numerous, the compound eye is the commonest type of animal eye. An aggregation of photoreceptor cells together with any associated optical structures. Eyes occur almost universally among animals, and are possessed by some species of virtually every major animal phylum. However, the complexity of eyes varies greatly, and this sense organ undoubtedly evolved independently a number of times within the animal kingdom.

The simplest invertebrate organs that might be considered to be eyes are clusters of photoreceptor cells located on the surface of the body. Pigment cells are usually interspersed among the photoreceptors, giving the eye a red or black color. Accessory structures, such as the lens and cornea, are usually absent. Simple eyes of this type, called pigment spot ocelli, are found in such invertebrates as jellyfish, flatworms, and sea stars. The most basic image-forming type of invertebrate eye probably arose from such patches of photoreceptor cells by an in-sinking of the sensory epithelium to form a cup, which may have become closed in conjunction with the evolution of a cornea and lens.

Such an evolutionary history is clearly suggested by the embryology and comparative anatomy of many invertebrates. In bilateral cephalic invertebrates, the eyes are typically paired and located at the anterior end of the body. Although one pair is usual, as in mollusks and many arthropods, multiple pairs are not uncommon. Some polychaete annelids have 4 eyes, and scorpions may have as many as The greatest number of eyes is found in marine flatworms, where there may be over ocelli scattered over the dorsal anterior surface and along the sides of the body.

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The occurrence of eyes on parts of the body other than the head is usually correlated with radial symmetry or unusual modes of existence. The primitive function of animal eyes was merely to provide information regarding the intensity, direction, and duration of environmental light. The perception of objects is dependent upon several factors, namely, the number of photoreceptors in the retina, the quality of the optics, and central processing of visual information.

Image formation has evolved as an additional capacity of the eyes of some invertebrates. The number of photoreceptor cells composing the retinal surface is of primary importance, since each photoreceptor cell or group of cells acts as the detector for one point of light. An image is formed by the retina through the association of points of light of varying intensity, much as an image is produced by an array of pixels on a computer monitor. The ability of an eye to form an image and the coarseness or fineness of the image are, therefore, dependent upon the number of points of light that are distinguished which, in turn, is dependent upon the number of photoreceptor cells composing the retina.

A large number of photoreceptor cells must be present to produce even a coarse image. The great majority of invertebrate eyes cannot form a detailed image because they do not possess a sufficient number of photoreceptor cells.

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The number of photoreceptor cells might be sufficient to detect movement of an object, but is inadequate to provide much information about the object's form. See Photoreception. The focusing mechanisms of invertebrate eyes vary considerably. The focus of arthropod eyes tends to be fixed, that is, the distance between the optical apparatus and the retina cannot be changed. Thus objects are in focus only at a certain distance from the eye, determined by the distance between the lens and the retina.

The oceanic family of swimming polychaete worms, Alciopidae, have eyes of that are focused hydrostatically. A bulb to one side of the eye injects fluid into the space between the retina and the lens, forcing the lens outward. Another mechanism is employed in octopods whereby lens movement is brought about by a ciliary muscle attached to the lens as in aquatic vertebrates, like fish. The compound eye of crustaceans, insects, centipedes, and horseshoe crabs has a sufficiently different construction from that of other invertebrates to warrant separate discussion.

The structural unit of the compound eye is called an ommatidium see illustration. The outer end of the ommatidium is composed of a cornea, which appears on the surface of the eye as a facet. Beneath the cornea is an elongated, tapered crystalline cone; in many compound eyes the cornea and cone together function as a lens. The receptor element at the inner end of the ommatidium is composed of one or more central translucent cylinders rhabdome , around which are located several photoreceptor cells typically 7 or 8.

The rhabdome is the initial photoreceptive element, and it in turn stimulates the adjacent photoreceptor cells to depolarize. The photoreceptor element of each ommatidium functions as a unit and can respond only to one point of light. Thus image formation is dependent upon the number of photoreceptor units present.

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The number of ommatidia composing a compound eye varies greatly. Pigment granules surround the ommatidium proximally and distally, forming a light screen that separates one ommatidium from another. The pigment granules migrate, depending upon the amount of light. In bright light the ommatidium is adapted by funneling light directly down to the rhabdome, by extending the pigment screen, so that light received by one ommatidium is prevented from stimulating the rhabdome of another.

Under these conditions the image produced is said to be appositional, or mosaic. The term mosaic has been misinterpreted to mean that a given ommatidium forms a separate image, even if only a part of the image. In general, however, the compound eyes function like any other eye—each photoreceptor unit represents one point in visual space.

It is not obvious whether or not compound eyes have any special advantages over other eye designs, despite their universal occurrence in crustaceans and insects. However, in many arthropods the total corneal surface is greatly convex, resulting in a wide visual field.

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Many invertebrate eyes are capable of seeing and analyzing patterns of polarized light in nature. This capacity reaches its apex in compound eyes, as well as in the simple eyes of cephalopods. Cuttlefish are known to communicate with each other with displays produced on their body surfaces that are visible only to animals that have polarization vision.

Most invertebrates with polarization vision, however, use this ability to navigate with the assistance of patterns of polarization in the sky that occur naturally due to scattering of sunlight by the atmosphere. Bees and ants can find their way back to their nests or hives using only these celestial polarization cues. See Eye vertebrate. A sense organ that acts as a photoreceptor capable of image formation. The eye of vertebrates is constructed along a basic anatomical pattern which, in the diversification of animals, has undergone a variety of structural and functional modifications associated with different ecologies and modes of living.

Often compared with a camera, the vertebrate eye is conveniently described in terms of its wall, cavities, and lens see illustration. The wall of the eye consists of three distinct layers or tunics which, from outward to inward, are termed the fibrous, vascular, and sensory tunics. This continuous, outermost fibrous tunic comprises a transparent anterior portion, the cornea, and a tough posterior portion, the sclera. In the human, the cornea represents about one-sixth of the fibrous tunic, the sclera five-sixths.

The vertebrate cornea exhibits very few modifications in structure regardless of environmental influences. Its major constituent is connective tissue both cells and fibers , regularly arranged and bordered on both anterior and posterior surfaces by an epithelium.

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The anterior epithelium is stratified, ectodermal in origin, and continuous with the conjunctival epithelium lining the eyelids. The transparency of the cornea is attributed to the geometric organization of its connective tissue elements, its constant state of deturgescence, and its chemical composition.

It is the first ocular component traversed by the incoming light. The sclera, a touch connective tissue tunic, provides support for the eye and serves for the attachment insertions of the muscles that move it. The limbus is located at the angle of the anterior chamber. Why not come and peruse our comprehensive range of natural history titles at our well stocked bookshop, where you can also receive our expert advice. Click here for details of our shop. We attend exhibitions at international conferences and congresses. We provide an exhibition service for scientific publishers. Full details can be provided on request.

View events that we are attending here. Request Image. Our customers have not yet submitted a review for this title - click here to be the first to write a review. The invertebrates possess remarkable adaptations in their visual systems, which have allowed them to conquer almost every imaginable habitat. The number, forms, and functions of the dorsal ocelli vary markedly throughout insect orders.

They tend to be larger and more strongly expressed in flying insects particularly bees, wasps, dragonflies and locusts , where they are typically found as a triplet. Two lateral ocelli are directed to the left and right of the head, respectively, while a central median ocellus is directed frontally. In some terrestrial insects e.

The unfortunately labelled "lateral ocelli" here refers to the sideways-facing position of the ocelli, which are of the dorsal type. They should not be confused with the lateral ocelli of some insect larvae see stemmata. A dorsal ocellus consists of a lens element cornea and a layer of photoreceptors rod cells. The ocellar lens may be strongly curved e.

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  4. The photoreceptor layer may e. The number of photoreceptors also varies widely, but may number in the hundreds or thousands for well-developed ocelli. Two somewhat unusual features of the ocelli are particularly notable and generally well conserved between insect orders. These two factors have led to the conclusion that the dorsal ocelli are incapable of perceiving form, and are thus solely suitable for light-metering functions. Given the large aperture and low f -number of the lens, as well as high convergence ratios and synaptic gains, the ocelli are generally considered to be far more sensitive to light than the compound eyes.

    Additionally, given the relatively simple neural arrangement of the eye small number of synapses between detector and effector , as well as the extremely large diameter of some ocellar interneurons often the largest diameter neurons in the animal's nervous system , the ocelli are typically considered to be "faster" than the compound eyes. One common theory of ocellar function in flying insects holds that they are used to assist in maintaining flight stability. Given their underfocused nature, wide fields of view, and high light-collecting ability, the ocelli are superbly adapted for measuring changes in the perceived brightness of the external world as an insect rolls or pitches around its body axis during flight.

    Corrective flight responses to light have been demonstrated in locusts [11] and dragonflies [12] in tethered flight. Other theories of ocellar function have ranged from roles as light adaptors or global excitatory organs to polarization sensors and circadian entrainers. Recent studies have shown the ocelli of some insects most notably the dragonfly, but also some wasps are capable of form vision, as the ocellar lens forms an image within, or close to, the photoreceptor layer. Further research has demonstrated these eyes not only resolve spatial details of the world, but also perceive motion.

    Dragonfly ocelli are especially highly developed and specialised visual organs, which may support the exceptional acrobatic abilities of these animals. Research on the ocelli is of high interest to designers of small unmanned aerial vehicles. Designers of these craft face many of the same challenges that insects face in maintaining stability in a three-dimensional world. Engineers are increasingly taking inspiration from insects to overcome these challenges.

    Stemmata singular stemma are a class of simple eyes. Many kinds of holometabolous larvae bear no other form of eyes until they enter their final stage of growth. Adults of several orders of hexapods also have stemmata, and never develop compound eyes at all. Examples include fleas , springtails , and Thysanura. Strepsiptera have clusters of simple eyes.

    Some other Arthropoda , such as some Myriapoda , rarely have any eyes other than stemmata at any stage of their lives exceptions include the large and well-developed compound eyes of Scutigera [19].

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    6. Behind each lens of a typical, functional stemma, lies a single cluster of photoreceptor cells, termed a retinula. The lens is biconvex , and the body of the stemma has a vitreous or crystalline core. Although stemmata are simple eyes, some kinds, such as those of the larvae of Lepidoptera and especially those of Tenthredinidae , a family of sawflies, are only simple in that they represent immature or embryonic forms of the compound eyes of the adult.

      They can possess a considerable degree of acuity and sensitivity, and can detect polarized light.