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Home >>Zoology Dictionary >> Exocrine Gland - Eye muscles
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Exocrine Gland - (See Gland).
Exopterygota - (=Hemimetabola) Insects whose wings develop outside the body and become progressively larger at each moult. Metamorphosis is incomplete and the young stages are nymhps resembling the adults in all but size and the small wings.
Exoskeleton - Skeleton that is outside the body (e.g) of crabs, insects, and snails, Muscles are attached to the inside.
Extensor Muscle - One that stretches alimb (e.g) the triceps.
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Extra Outside - E.g. extra cellular digestion-digestion outside the cell.
Eyes Organ of sight. Eyes vary form simple cells capable only of detecting light and dark, to the highly elaborate sense organs of verebrates. Most insects and other arthropods have compound eyes made up of many separate elements called ommatidia.
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Cephalopods have elacoars eyes which are remarkably similar in almost every way to vertebrate eyes. The human eye is described here' as a typical vertebrate eye.
Each eye is a hollow sphere embedded in its orbit in the skill. The eye wall has three main layers, a tough. fibrous outer coat the sclera; a layer inside this containing pigment and blood vessels the choroid; and an inner lining the retina which contains the light sensitive cells, the nerve fibres leading from the retina to form the optic nerve, and nerve cells that connect the receptors and the nerve fibres.
At the front of the eye the three layers are modified. The sclera, which is visible as the white of the eye, is transparent and forms the cornea, whose outer surface is covered by a thin, transparent protective layer the conjunctiva. The latter is a continuation of the eyelid lining. The choroid is modiffied to form the iris the visible blue or brown pigmented part of the eye which in its centre has an aperture, the pupil.
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The swollen outer part of the iris is the ciliary body which contains muscle fibres whose action changes the shape of the lens. This is a transparent crystalline structure which is suspended from the ciliary body by the suspensory ligament. The retina ends just behind the attachment of the suspensory ligament.
The iris divides the part of the eye in front of the lens into anterior and posterior chambers. These are filled with a watery fiuid the aqueous humour, and the hind part of the eye.
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The tear glands keep the cornea moist, nourish it and also wash away grit and dust. The eye is moved in its orbit by the action of six eye muscles. These of each eye are coordinated, so normally both eyes are moved inthe same direction.Light enters the hind chamber of the eye after passing through the cornea, the pupil and the lens. The cornea forms an important part of the focusing mechanism, bending the light far more than the lens does. The latter produces a sharp image of the object on the retina.
The iris is equivalent to the diaphragm of a camera. It has two sets of music firbres, one arranged radially (in a similar manner to the spokes of a bicycle wheel) and the other in a circle in the inner part of the iris. The radial fibres contract in dim light enlarging the pupil and thus allowing as much of the available light as possible to enter the eye.
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Contraction of the circular fibres reduces the size of the pupil and cuts down the amount of light that enters the eye as in bright conditions (in daylight the pupil is much smaller than it is at dusk).
The shape of the lens can be changed so that the images of either near or distant objects can be focused sharply on the retina. The lens consists of a 'plastic' arrangement of fibres surrounded by a thin capsule. The fibres of the suspensory ligament are inserted in this . If the tension on the ligament is altered the shape of the lens changes. When the eye is relaxed or looking at a distant object, the elastic force of the sclera. pulls on the suspensory ligament and stretches the lens capsule, causing the lens to flatten and become thinner. When the eye is doing close work the fleiliary muscles contract; the pull of the sclera is resisted and the tension on the suspensory ligament is relaxed, thus allowing the tens to become fatter.
The structure of the retina is extremely complicated. The light sensitive cells have their tips touching the pigment layer while their bases connect with nerve fibres. The nerve fibres run over the inner surface of the retina and join up to form the optic nerve. Light entering the eye therefore has to pass through the retinal nerve cellsbefore it can stimulate the receptors. The retina is said to be inverted.
This is the main difference between the vertebrate eye and that of cephalopods; the cephalopod retina is not inverted.
Where the optic nerve leaves the eye there are no light sensitive cells. Light falling on this region is not perceived. Consequently it is known as the blind spot.
The light sensitive cells are of two kinds, called rods and cones. Most of the cones are concentrated in a small round depression, a short distance to the side of the blind spot. This is called the yellow spot or fovea centrails. The fovea is yellow because the cells of the retina at this point contain a yellow pigment. It is the region where the light is principally focused and only the part of an image which falls upon the fovea it seen sharply. This is because the cones are very small and set close together. They are sensitive to colour. The whole os the rest of the retina contains rods, sensitive to weak light but not showing colour. For this reason we see no colours by moonlight. It is of interest that many nocturnal animals have few cones in their retina and some (e.g. bats) have an all rod retina.
The sensitivity to light depends upon the pigment visual purple in the ends of the receptor cells. When light falls on the retina visual purple undergoes chemical change. It is thought that its breakdown into other compounds stimulates the rods and so signals pass from them by way of the ganglion cells to fibres of the optic nerve and hence to the brain. The latter interprets these signals as light. In the dark the chemical change is reversed and visual purple is reconstituted. Presumably a similar chemical change stimulates the cones, though no substance has positively been shown to undergo such a change.
Eye Compound - Compound eyes are the main organs of sight in insects and cruslacens. The outer part of each eye consists of numerous tiny lonses or facets varying in number from ten to thirty thousand in different insects. These are usually hexagonal, forming a honeycomb like mosaic. Each facet is at the top of a cone shaped tube (ommatidium) at the bottom of which are the light receptive cells.
All the ommatidia together produce a mosaic os sports oflight, each spot representing the part of the field of view in the with a particular ommatidium. In effect each picture consists of a series of dots rather like a printed picture in a newspaper. The picture an insect sees, however, is indistinct. Insects can certainly distinguish shapes and can recognise certain patterns, but their eyes are best suited to pick out moving objects or those moving across their flight path. A dragonfly, for example, is an expert at catching its prey on the wing. In most insects the fields of view of both compound eyes overlap so that they have stereoscopic vision somewhat like our own.
Eye Muscles - The eye ball is moved in its sockct by six small muscles; the superior and inferior oblique muscles, and the internal, external anterior, and posterior rectus muscles. These muscles are supplied by the cranial nerves.
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