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Human Sight

We live primarily in visual world, and sight is our dominant sense. Although we can rely on our eyes to bring us many of the sights of the external world, they are not able to reveal everything. We see only objects that emit or are illuminated by light waves in our receptive range, which is only 1/70 of the entire electromagnetic spectrum.
It’s divided into three cavities: anterior chamber (the region between the cornea and iris), posterior chamber (between the iris and the lens) and vitreous chamber (occupies 80% of the eyeball – the entire space behind the lens).
The first two chambers, anterior and posterior, are both filled with aqueous humor, a thin, watery fluid that is essentially an ultra filtrate of blood similar to cerebrospinal fluid. It’s largely responsible for maintaining a constant pressure within the eyeball. It also provides oxygen, glucose and amino acids for the lens and cornea.
The vitreous chamber is filled with vitreous humor; a gelatinous substance, which is, actually modified connective tissue. It protects eyeball from collapsing as a result of external pressure. The wall of the eyeball consists of three layers: supporting, vascular and retinal.
Supporting layer consists mainly of though fibrous connective tissue. The posterior segment, which makes up 5/6 of the tough outer layer, is the opaque white sclera (Gr. skleros, hard), which gives the eyeball its shape and protects the delicate inner layers.
The anterior segment of the supporting layer is the transparent cornea. It bulges slightly. The light enters the eye through the cornea. The sclera and cornea are continuous. The cornea of this supporting layer contains no blood vessels.
Because the middle layer of the eyeball contains many blood vessels, it is called the vascular layer. The dark color of the middle layer is produced by pigments that help to lightproof the wall of the eye by absorbing stray light and reducing reflections. The posterior 2/3 of the vascular layer consists of a thin membrane called the choroids, which is essentially a layer of blood vessels and connective tissue between the sclera and the retina.
The vascular layer becomes thickened toward the anterior portion to form the ciliary body.

The smooth muscles in it (ciliary muscles) contract to ease the tension on the suspensory ligament of the lens, which consists of fibrils that extend from ciliary processes to the lens.
The lens of the eye is a flexible, transparent, colorless, avascular body of epithelial cells behind the iris. The lens is held in place by the suspensory ligament of the lens and by the ciliary processes. The shape of the lens can be adjusted so that objects at different distances can be brought into focus on the retina. This mechanism is called accommodation. The lens loses much of its elasticity with aging, making it difficult to focus efficiently without corrective eyeglasses.
The anterior extension of the choroids is a thin muscular layer called the iris (gr. Rainbow) because it is the colored part of eyeball that can be seen through the cornea. In the center of the iris is an adjustable circular aperture, the pupil. It appears black because most of the light that enters the eye is not reflected outward. The iris, acting as a diaphragm, is able to regulate the amount of light entering the eye because it contains smooth muscles that contract or dilate in an involuntary reflex in response of the light available, causing the pupil to become larger or smaller. This mechanism is called adaptation.
The innermost layer of the eyeball is the retinal layer.
The retina
It is an egg-shaped, multi-layered, light sensitive membrane, containing network of specialized nerve cells. It is connected to the brain by a circuit of over million neurons in the optic nerves. The retina has a thick layer and a thin layer. The thick layer is nervous tissue, called the neuroretina that connects with the optic nerve. Behind it is a thin layer of pigmented epithelium that prevents reflection from the back of the retina. The function of the neuroretina is to receive focused light waves and convert them into nerve impulses that can be conveyed to the brain and converted into visual perceptions. The neuroretina doesn’t extend into anterior portion of the eyeball, where the light could not be focused on it.
The neuroretina consists of highly specialized photoreceptor nerve cells, the rods and cones (cones are color-sensitive, the rods only “see” black and white). The outer segment of a rod or cone consists most of the elements necessary (including light-sensitive photo pigments) to absorb light and produce a receptor (generator) potential. The inner segments contain mitochondria, Golgi apparatuses, endoplasmatic reticulum, the nucleus, and other structures necessary for generating energy and renewing molecules in outer segment.

The inner segment also contains a synaptic terminal, which allows the photoreceptor cells to communicate chemically with other retinal cells.
Each eye has got about 125 million rods and 7 million cones. Most of the cones are concentrated in the center of the retina directly behind the lens in area called the macula lutea (lat. “yellow spot”), especially in a small, depressed, rod-free area called the fovea, or fovea centralis. The vision is sharpest and color perception is optimal on this places.
The portion of retina where the optic nerve exists from the eyeball contains neither rods nor cones; it is called the blind spot or optic disc.

Images from an object less than 6m away would normally be focused behind the retina instead of on it. To bring images into perfect focus on the retina is the role of the adjustable lens. This is accomplished by a reflex called accommodation. When you want to focus your eyes on an object close to you, you involuntarily contract the ciliary muscles in your eye, which pull the ciliary body slightly forward and inward, reducing the tension on the suspensory ligaments attached to the lens capsule. When the tension is reduced, the elastic lens becomes thicker. By the focusing the distant objects, the ciliary muscles are released.
As the lens becomes harder and less elastic with age, it becomes more and more difficult to focus on near objects, a condition known as presbyopia.

Human beings have binocular vision. Each eye receives an image from a slightly different angle and this creates the impression of distance, depth, and three-dimensionality. For this reason is this vision also called stereoscopic vision. In binocular vision, the two eyeballs turn slightly inward to focus on a close object so that both images fall on the corresponding points of both retinas at the same time. This action is called convergence. In order to produce a single image , the six pairs of extraocular muscles must move together with perfect coordination. As a result of convergence, the simultaneous stimulation of both retinas produces the perception of a single image in the occipital lobe of the cerebral cortex.

Accessory structures of the Eye
The eye is enclosed in an orbital cavity, or orbit, which protects it from external buffeting. The eyelids (palpebrae) are folds of skin that create an almond-shaped opening around the eyeball when the eye is open. The points of the almond, where the upper and lower eyelids meet, are called canthi.

The eyelid may be divided into four layers – the skin layer contains the eyelashes, the muscular layer contains the orbicularis oculi muscle, which lowers the eyelid to close the eye; the fibrous connective tissue layer contains many modified oil glands, whose secretions keep the eyelids from sticking together and the innermost layer is composed of a portion of the lining of the eyelid, the conjunctiva. Eyelids protect eyeball from dust and another harmful external objects.
The edges of the eyelids are lined with short, thick hairs, the eyelashes. They act as strainers to prevent foreign materials from entering the eye. The eyelids of each eye contain about 200 eyelashes. Eyebrows are thickened ridges of skin over the protruding frontal bone, covered with short , flattened hairs. They protect eye from sweat, excessive sunlight, and foreign materials.

The conjunctiva
It is a thin, transparent mucous membrane that lines the eyelids and bends back over the surface of the eyeball, terminating at the transparent cornea, which is uncovered. The portion that lines the eyelid is the palpebral conjunctiva, and the portion that covers the white of the eye is the bulbar conjunctiva. Between both portions of the conjunctiva are two recesses called the conjunctival sacs. The lacrimal apparatus
It is made up of the lacrimal gland, lacrimal sac, lacrimal ducts and nasolacrimal duct. The eyeball is kept moist by secretions of the lacrimal gland, located under the upper lateral eyelid and extending inward from the outer canthus of each eye. The eye blinks every 2 to 10 seconds, with each blink lasting only 0.3 to 0.4 second. Blinking stimulates the lacrimal gland to secrete a sterile fluid that serves at least four purposes. It washes foreign particles off the eye; it kills invading bacteria with a mild antibacterial enzyme, lysozyme; it distributes water and nutrients to the cornea and lens and it gives the eyeball a clear, moist and smooth surface.
The tears
The tears are composed of salts, water, the enzyme lysozyme and mucin (organic compounds produced by mucous membranes). In addition to the steady secretion of tears, reflex tears are produced in emergencies, as when the fumes from sliced onion irritate the eyes.
Approximately 3 to 12 lacrimal ducts lead from each gland onto the superior conjunctival sac in the upper eyelid. From there tears flow down across the eye into small openings near the inner canthus called lacrimal puncta. The puncta open into the lacrimal ducts, which drain excess tears from the area of the inner canthus to the lacrimal sac, the dilated upper end of the nasolacrimal duct. .

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