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Our eyes constantly scan the world around us. An image appearing in the peripheral field of vision is rapidly centered onto the fovea by a jerky movement of the eyes. These rapid eye movments are called saccades. During a saccade the eyes move at angular velocities of between 200 and 600° S-I, In contrast, when watching a race or when playing a ball game, the eye follows the object of interest keeping its position on the retina fairly constant. This smooth tracking of an object is called a pursuit movement, which can have a maximum angular velocity of about 50° s-1. These two types of eye movement can be combined, for example when looking out of a moving vehicle. One object is first fixated and followed until the eyes reach the limit of their travel. The eyes then flick to fixate another object and follow that, and so on. The continuous switching of the point of fixation is seen as a pursuit movement followed by a saccade and then another pursuit movement. This pattern is known as optokinetic nystagmus.
If an object, such as a pencil, is fixated and then moved around the visual field both eyes track the object. These are known as conjugate eye movements. If the pencil is moved first away from the face and then towards it, the two eyes move in mirror image fashion to keep the image in focus on each retina. When the object approaches the eyes; the visual axes converge and as the object moves away they progressively diverge until they are parallel with each other. These eye movements are called vergence movements. If the movements of the two eyes are not properly coordinated, double vision (diplopia) results. The various kinds of eye movements are controlled by the oculomotor system which controls the extrinsic muscles that suspend the eye in the orbit. The position of each eye is controlled by six extraocular muscles. These are innervated by the third, fourth, and sixth cranial nerves (the oculomotor, trochlear, and abducent nerves). The lateral and medial rectus muscles control the sideways movements, the superior and inferior oblique muscles control diagonal movements, and the superior and inferior rectus muscles control the up and down movements. The eye movements require full coordination of all of the extraocular muscles, with activation of synergists and an appropriate degree of inhibition of antagonists. If we look to one side, the right for example, the right lateral rectus and the left medial rectus are both activated while the right medial rectus and le lateral rectus are inhibited. Diagonal movements involve more muscles and will require an even greater complexity of control, It is not surprising, therefore, that about 10 per cent of aft motoneurons are employed in serving the extraocular eye muscles. These muscles have the smallest motor units in t body, only 5-10 muscle fibers per motoneuron. This permits high degree of precision in the control of eye position. Our visual systems perform all kinds of amazing jobs, from finding constellations in the night sky, to picking out just the right strawberry in the supermarket, to tracking a fly ball into a waiting glove. Most animals and many plants are photosensitive; that is, they can detect different light intensities. Some accomplish this with single cells or with simple eyes that do not form images but do allow the organism to react to light by moving toward or away from it. In order for an eye to transmit more information about the world, however, it must have a way of forming an image, a representation of the scene being viewed. Higher invertebrates and virtually all vertebrates have complex, image-forming eyes, and we will "focus" on the refracting eye found in the octopus and in all vertebrates. Arthropods have compound eyes, which have greater depth of focus than refracting eyes, but which sacrifice resolving power or acuity. To see an approximation of what the world looks like to a honeybee (an arthropod). Our eyes, like those of many animals, detect a just narrow range of all the wavelengths of electromagnetic radiation, that between 360 and 740 nanometers; this is called light, or the visible spectrum. Figure below for a comparison of the entire electromagnetic and the visible spectra.
The eye even has its own special cleaning system. Above the outer corner of each eye are the lacrimal glands, and they make a unique cleaning fluid - tears! Every time you blink your eye, a tiny bit of tear fluid comes out of your upper eyelid. It washes away any germs, dust, or other particles that don't belong in your eye. It also keeps your eye from drying out. Then the fluid drains out of your eye by going into the lacrimal duct (this is also called the tear duct). Sometimes the eye needs to make even more tear fluid than it normally does. If you've ever been poked in the eye by mistake or been in a dusty or smoky area, your eyes may have worked double time to protect themselves by making lots of tears. These tears helped to keep your eyes from becoming injured or dried out. While crying, eyes get a message from the brain to make you cry, and the lacrimal glands made many, many tears. The lachrymal glands and tear fluid: Each orbit is endowed with lachrymal glands which provide a constant secretion of tear fluid that serves to lubricate the movement of the eyelids and keep the outer surface of the cornea moist, so providing a good optical surface. The lachrymal glands are innervated by the parasympathetic outflow of the facial nerve (cranial nerve VII). Tear fluid has a pH similar to that of plasma and is isotonic with blood. It possesses a mucolytic enzyme called lysozyme that has a bactericidal action. Under normal circumstances about 1 ml of tear fluid is produced each day, most of which is lost by evaporation, the remainder is drained into the nasal cavity via the tear duct. Irritation of the corneal surface (the conjunctiva) increases the production of tear fluid and this helps to flush away noxious agents.
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Source(s): All above information
& images are based on information collected from chapter on
eyes from the book Human Physiology by Gillian Pocock and
Christophor D. Richards, Vision Spectrum topic (information & images) is from an article written Dr. Chudler, faculty.washington.edu/chudler and other topics are from various sources. All rights reserved by respective owners. For our full credit and copyright information please view our Credit List.
Disclaimer: Any information displayed here is just for educational
purposes, and may not be taken as an expert advice and should not
be applied in life without consulting your eye doctor/specialist. We here
by take no responsiblity of the accuracy of the above content as they have
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Did you know ?
A cataract needs to be removed only when vision loss interferes with your everyday activities, such as driving, reading, or watching TV. [submit fact]
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