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Анатомия на окото За да попадне светлината в окото, първо трябва да премине през роговицата - най-предната прозрачна част на зрителния орган. Тя осигурява шестдесет процента от диоптричната (пречупвателната) сила на окото. След пречупването на светлинните лъчи от роговицата те преминават през зеницата. Тя представлява отвор на ириса, променящ се според интензивността на светлината. Ирисът от своя страна придава цвета на окото. След зеницата светлината преминава през лещата, която има способността да променя пречупвателната си сила в зависимост от отдалечеността на наблюдавания предмет, като по този начин спомага, получаваният образ винаги да бъде на фокус. При нормалните очи след лещата светлината преминава през стъкловидното тяло - гелообразна структура, която изпълва окото отвътре и му придава сферична форма. Най-накрая образът от наблюдавания предмет попада върху ретината във фокусиран вид. Централната част на ретината, отговаряща за ясното виждане, се нарича макула. Ретината е изградена от милиони светлочувствителни клетки, най-вътрешната част от които, с дългите си нервни окончания, образуват снопа на зрителния нерв. Зрителният нерв предава зрителните дразнения към мозъка, където те се възприемат като образ.
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Eye anatomy The eye is a complex organ made up of several physical and chemical elements. It’s also tightly involved with the nervous system. Any damage to one of these components will alter the way you see the world and might even cause partial or total loss of sight. Understanding the way these elements interact is the first step in helping make you choice to keep them in perfect working order. The Eyelid The eyelid protects the front part of the eye. The lid helps keep the eye clean and moist by blinking several times a minute. Eyelids can also shut automatically to protect the eye from possible danger such as bright light or a foreign object. The Sclera The white part of the eyeball is called the sclera. It acts as a protective outer layer of the eye. It is tough, opaque and fibrous and contains both collagen and elastic fibre. Six muscles, which control the movement of the eye, connect to the sclera. The optic nerve is attached to the sclera at the very back of the eye. The Cornea The cornea is the transparent front part of the eye that covers the pupil and the iris. It is a transparent dome that sits in front of the coloured part of the eye. When light strikes the eye, the first part it reaches is the cornea. The cornea controls the amount of light entering the eye. It provides 2/3 of the eye's focusing power. It does not have blood vessel, but contains many nerves. In fact, there are more nerve endings in the cornea than anywhere else in the body. The cornea has three main layers: - The most superficial layer is the epithelium. - The stroma is the middle and thickest layer. - The endothelium, located between the stroma and the aqueous humour, works as a pump, expelling excess water back into the eye. The Iris The iris is a circular band of muscles that controls the size of the pupil. Light reaches the iris and the pupil after having been reflected by the cornea. The iris shrinks the pupil if there is too much light and widens it if there is not enough. This is an involuntary function controlled by the brain. The pigmentation of the iris gives "colour" to the eye. Eye colour is created by the amount and type of pigment in the iris. Blue eyes have the least amount of pigment, brown eyes have the most. The most common eye colours are brown, hazel, blue and green. Multiple genes inherited from each parent determine a person’s eye color. Eye colours around the world It is estimated that: - 55 % of the world’s population has brown eyes. - 8 % of the world’s population (mainly in Europe) has blue eyes. - 5-8% of the world’s population has hazel eyes. - 2 % of the world's population has green eyes. - Silver eye color, most common in eastern European countries, is one of the rarer eye colors worldwide. - Amber eyes are very rare worldwide, and are most common in Asia and South American countries. Light reaches the iris and the pupil after having been reflected by the cornea. The iris shrinks the pupil if there is too much light and widens it if there is not enough. This is an involuntary function controlled by the brain. The Pupil The pupil is the black circle in the center of the iris. It is really an opening in the iris. The pupil appears black because most of the light entering the pupil is absorbed by the tissues within the eye. The pupil allows light to enter the retina after having passed through the lens. The Lens Set behind the pupil and the iris, the lens is a transparent tissue that further refracts light passing through the eye. The lens adapts itself to create a finer image. It flattens to properly focus light received from distant objects and gets rounder for nearer objects. The ciliary muscles are responsible for altering the shape of the lens to allow focusing images of objects at different distances. As we age the lens loses some of its elasticity so we are no longer able to focus on objects up close. This condition is called presbyopia and it’s the reason the majority of people need reading glasses when they reach their 40s. A cataract is a clouding of the lens of the eye that affects vision. Most cataracts are related to aging. By age 80, more than half of all North Americans either have a cataract or have had cataract surgery. The Retina Once past the lens, light strikes the retina, located at the back of the eye. The retina uses special cells that are highly responsive to light, rods and cones. Rods are responsible for vision in dim light and are out of commission in bright light. They process images in black and white and are highly sensitive. They are responsible for night vision. Cones are active in bright light. They discriminate colours and are responsible for fine details. The retina has three types of cones. Each cone type is sensitive to one of three different colors — red, green, or blue. There are about 120 million rods and 7 million cones in each eye. The retina evolved as an outgrowth of the brain itself. As such, it kept its connections with the brain proper through a bundle of fibres—the optic nerve. The macula is a small central area of the retina that provides vision for fine work and reading. Within the macula are the fovea and foveola which contain a high density of cones. Damage to the macula will result in loss of central vision. The Optic Nerve The Optic Nerve acts like a cable connecting the eye with the brain. Each optic nerve contains between 770,000 and 1.7 million nerve fibres that are outgrowths of retinal cells. COMPLEXITY REQUIRES KNOWLEDGE AND EXPERIENCE Your eyes are one of the most complex organs in your body. As such, they require the best care available. Vision MD’s team of ophthalmologists, optometrists and eye care professionals provide a comprehensive array of health services, from routine eye exams to cutting-edge diagnosis and treatment of eye disease. Don’t take your vision for granted. Most eye diseases, if detected early, can be treated in order to reduce and sometimes prevent vision loss. Why delay? Schedule your annual check up today.