File Name: parts of human eye and their function .zip
Basically, the role of the eye is to convert light into electrical signals called nerve impulses that the brain converts into images of our surroundings. Light rays pass through the pupil in the cornea. Aqueous humour — maintains the pressure in your eye and nourishes the cornea and the lens by supplying amino acids and glucose, as well as vitamin C.
The lens is a transparent biconvex structure in the eye that, along with the cornea , helps to refract light to be focused on the retina. By changing shape, it functions to change the focal length of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina.
This adjustment of the lens is known as accommodation see also below. Accommodation is similar to the focusing of a photographic camera via movement of its lenses. The lens is more flat on its anterior side than on its posterior side. The lens is also known as the aquula Latin, a little stream , dim. In humans, the refractive power of the lens in its natural environment is approximately 18 dioptres , roughly one-third of the eye's total power.
The lens is part of the anterior segment of the human eye. In front of the lens is the iris , which regulates the amount of light entering into the eye. The lens is suspended in place by the suspensory ligament of the lens , a ring of fibrous tissue that attaches to the lens at its equator   and connects it to the ciliary body.
Posterior to the lens is the vitreous body , which, along with the aqueous humor on the anterior surface, bathes the lens. The lens has an ellipsoid , biconvex shape. The anterior surface is less curved than the posterior. The lens has three main parts: the lens capsule , the lens epithelium, and the lens fibers.
The lens capsule forms the outermost layer of the lens and the lens fibers form the bulk of the interior of the lens. The cells of the lens epithelium, located between the lens capsule and the outermost layer of lens fibers, are found only on the anterior side of the lens. The lens itself lacks nerves, blood vessels, or connective tissue. The lens capsule is a smooth, transparent basement membrane that completely surrounds the lens. The capsule is elastic and is composed of collagen.
It is synthesized by the lens epithelium and its main components are type IV collagen and sulfated glycosaminoglycans GAGs. The capsule varies from 2 to 28 micrometres in thickness, being thickest near the equator and thinnest near the posterior pole. The lens epithelium, located in the anterior portion of the lens between the lens capsule and the lens fibers, is a simple cuboidal epithelium. The cells of the lens epithelium also serve as the progenitors for new lens fibers.
It constantly lays down fibers in the embryo, fetus, infant, and adult, and continues to lay down fibers for lifelong growth. The lens fibers form the bulk of the lens. If cut along the equator, it appears as a honeycomb. The middle of each fiber lies on the equator. The lens fibers are linked together via gap junctions and interdigitations of the cells that resemble "ball and socket" forms. The lens is split into regions depending on the age of the lens fibers of a particular layer.
Moving outwards from the central, oldest layer, the lens is split into an embryonic nucleus, the fetal nucleus, the adult nucleus, and the outer cortex. New lens fibers, generated from the lens epithelium, are added to the outer cortex.
Mature lens fibers have no organelles or nuclei. Unlike the rest of the eye, which is derived mostly from the neural ectoderm , the lens is derived from the surface ectoderm. The first stage of lens differentiation takes place when the optic vesicle , which is formed from outpocketings in the neural ectoderm, comes in proximity to the surface ectoderm.
The optic vesicle induces nearby surface ectoderm to form the lens placode. As development progresses, the lens placode begins to deepen and invaginate. As the placode continues to deepen, the opening to the surface ectoderm constricts and the lens cells forms a structure known as the lens vesicle. The cells of the anterior portion of the lens vesicle give rise to the lens epithelium. Additional secondary fibers are derived from lens epithelial cells located toward the equatorial region of the lens.
These cells lengthen anteriorly and posteriorly to encircle the primary fibers. The new fibers grow longer than those of the primary layer, but as the lens gets larger, the ends of the newer fibers cannot reach the posterior or anterior poles of the lens. The lens fibers that do not reach the poles form tight, interdigitating seams with neighboring fibers. These seams are readily visible and are termed sutures. The suture patterns become more complex as more layers of lens fibers are added to the outer portion of the lens.
The lens continues to grow after birth, with the new secondary fibers being added as outer layers. New lens fibers are generated from the equatorial cells of the lens epithelium, in a region referred to as the germinative zone.
The lens epithelial cells elongate, lose contact with the capsule and epithelium, synthesize crystallin , and then finally lose their nuclei enucleate as they become mature lens fibers. From development through early adulthood, the addition of secondary lens fibers results in the lens growing more ellipsoid in shape; after about age 20, however, the lens grows rounder with time and the iris is very important for this development.
Several proteins control the embryonic development of the lens: among these, primarily, PAX6 , considered the master regulator gene of this organ. In many aquatic vertebrates, the lens is considerably thicker, almost spherical, to increase the refraction. This difference compensates for the smaller angle of refraction between the eye's cornea and the watery medium, as they have similar refractive indices.
In reptiles and birds , the ciliary body touches the lens with a number of pads on its inner surface, in addition to the zonular fibres.
These pads compress and release the lens to modify its shape while focusing on objects at different distances; the zonular fibres perform this function in mammals. In fish and amphibians , the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye. In cartilaginous fish , the zonular fibres are replaced by a membrane, including a small muscle at the underside of the lens.
This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts , by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process , and serves to pull the lens backwards from the relaxed position to focus on distant objects. While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not homologous with those of either type of fish.
In frogs , there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle. In the most primitive vertebrates, the lampreys and hagfish , the lens is not attached to the outer surface of the eyeball at all. There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea.
To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards. The lens is flexible and its curvature is controlled by ciliary muscles through the zonules. By changing the curvature of the lens, one can focus the eye on objects at different distances from it. This process is called accommodation. At short focal distance the ciliary muscle contracts, zonule fibers loosen, and the lens thickens, resulting in a rounder shape and thus high refractive power.
Changing focus to an object at a greater distance requires the relaxation of the lens and thus increasing the focal distance. The refractive index of human lens varies from approximately 1.
Aquatic animals must rely entirely on their lens for both focusing and to provide almost the entire refractive power of the eye as the water-cornea interface does not have a large enough difference in indices of refraction to provide significant refractive power. As such, lenses in aquatic eyes tend to be much rounder and harder. Crystallins tend to form soluble, high-molecular weight aggregates that pack tightly in lens fibers, thus increasing the index of refraction of the lens while maintaining its transparency.
Another important factor in maintaining the transparency of the lens is the absence of light-scattering organelles such as the nucleus , endoplasmic reticulum , and mitochondria within the mature lens fibers. The pigment responsible for blocking the light is 3-hydroxykynurenine glucoside, a product of tryptophan catabolism in the lens epithelium. The lens is metabolically active and requires nourishment in order to maintain its growth and transparency. Compared to other tissues in the eye, however, the lens has considerably lower energy demands.
By nine weeks into human development, the lens is surrounded and nourished by a net of vessels, the tunica vasculosa lentis , which is derived from the hyaloid artery. After regression of the hyaloid artery, the lens receives all its nourishment from the aqueous humor. Glucose is the primary energy source for the lens.
Section through the margin of the lens, showing the transition of the epithelium into the lens fibers. This svg file was configured so that the rays, diaphragm and crystalline lens are easily modified . From Wikipedia, the free encyclopedia. Lens Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens.
Medical portal. Retrieved The Eye: Basic Sciences in Practice. London: W. Saunders Company Ltd. Edinburgh: Mosby. Experimental Eye Research. Vertebrates: Comparative anatomy, function, evolution 5th ed. The Vertebrate Body. Optics , 2nd ed. Amino Acids. Progress in Retinal and Eye Research. Progress in Biophysics and Molecular Biology.
Wood and Roger J. Truscott March British Journal of Ophthalmology. Perceptual and Motor Skills.
The structures and functions of the eyes are complex. Each eye constantly adjusts the amount of light it lets in, focuses on objects near and far, and produces continuous images that are instantly transmitted to the brain. The orbit is the bony cavity that contains the eyeball, muscles, nerves, and blood vessels, as well as the structures that produce and drain tears. Each orbit is a pear-shaped structure that is formed by several bones. The outer covering of the eyeball consists of a relatively tough, white layer called the sclera or white of the eye. Near the front of the eye, in the area protected by the eyelids, the sclera is covered by a thin, transparent membrane conjunctiva , which runs to the edge of the cornea.
Their purpose is to protect the eye from foreign bod- ies and limit the The cornea and lens are the main eye components that refract. (bend) light rays on the.
Human Eye: working of human eye, Persistence of vision, Power of accommodation of human eye, Defects of vision. The Human Eye: It is a natural optical instrument which is used to see the objects by human beings. It is like a camera which has a lens and screen system.
Human eye , in humans, specialized sense organ capable of receiving visual images, which are then carried to the brain. The eye is protected from mechanical injury by being enclosed in a socket, or orbit, which is made up of portions of several of the bones of the skull to form a four-sided pyramid, the apex of which points back into the head. Thus, the floor of the orbit is made up of parts of the maxilla, zygomatic, and palatine bones, while the roof is made up of the orbital plate of the frontal bone and, behind this, by the lesser wing of the sphenoid. The optic foramen , the opening through which the optic nerve runs back into the brain and the large ophthalmic artery enters the orbit, is at the nasal side of the apex; the superior orbital fissure is a larger hole through which pass large veins and nerves. These nerves may carry nonvisual sensory messages—e.
Both modes require fibroblast growth factor 2 FGF2. So I'm just drawing that in. Move your left hand to unblock your left eye and the gap re-appears.
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