How does the human eye work. What are the main parts of the eye. How do different eye structures contribute to vision. What role does the brain play in visual processing. How do common eye conditions affect eye anatomy.

The Fascinating Structure of the Human Eye

The human eye is a marvel of biological engineering, comprising several intricate structures that work in harmony to provide us with the sense of sight. Let’s explore the key components of this remarkable organ and their functions.

The Cornea: The Eye’s Clear Window

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. It plays a crucial role in focusing light as it enters the eye. How does the cornea contribute to vision. The cornea acts as the eye’s outermost lens, responsible for about two-thirds of the eye’s total focusing power. It bends light to help the eye focus on objects at different distances.

The Iris and Pupil: Regulating Light Entry

The iris is the colored part of the eye that surrounds the pupil. It contains muscles that allow it to adjust the size of the pupil, controlling the amount of light that enters the eye. How does the iris adapt to different lighting conditions. In bright light, the iris constricts the pupil to reduce the amount of light entering the eye, while in dim light, it dilates the pupil to allow more light in, much like the aperture of a camera.

The Eye’s Focusing Mechanism: Lens and Ciliary Body

The lens and ciliary body work together to fine-tune the eye’s focus, allowing us to see objects clearly at various distances.

The Lens: Adjusting Focus

The lens is a transparent, biconvex structure located behind the iris. Its primary function is to focus light onto the retina. How does the lens change shape to focus on objects at different distances. The lens can change its shape through a process called accommodation. When focusing on nearby objects, the lens becomes more convex, increasing its optical power. For distant objects, it flattens, reducing its optical power.

The Ciliary Body: Controlling Lens Shape

The ciliary body is a ring of muscle tissue that surrounds the lens. It plays a vital role in the accommodation process. How does the ciliary body affect lens shape. When the ciliary muscles contract, they release tension on the suspensory ligaments that hold the lens in place, allowing the lens to become more rounded for near vision. When the muscles relax, the ligaments tighten, flattening the lens for distance vision.

The Retina: Converting Light into Neural Signals

The retina is a light-sensitive layer of tissue lining the inner surface of the eye. It’s responsible for converting light into electrical signals that can be interpreted by the brain.

Photoreceptor Cells: Rods and Cones

The retina contains two types of photoreceptor cells: rods and cones. How do rods and cones differ in function. Rods are more numerous and are sensitive to light levels, allowing us to see in dim light conditions but not in color. Cones, on the other hand, are responsible for color vision and operate best in bright light.

The Macula and Fovea: Centers of Sharp Vision

The macula is a small, highly sensitive part of the retina responsible for detailed central vision. At its center lies the fovea, which provides the sharpest visual acuity. Why is the fovea crucial for detailed vision. The fovea contains the highest concentration of cone cells, allowing for high-resolution color vision and the ability to discern fine details.

The Optic Nerve: Bridging Eye and Brain

The optic nerve is a bundle of more than a million nerve fibers that carries visual information from the retina to the brain.

Signal Transmission

How does visual information travel from the eye to the brain. Electrical impulses generated by the retina’s photoreceptor cells are transmitted through the optic nerve to the visual cortex of the brain. This process occurs at incredible speed, allowing for near-instantaneous visual perception.

The Blind Spot

The point where the optic nerve exits the eye creates a small area devoid of photoreceptors, known as the blind spot. Why don’t we normally notice this blind spot in our vision. The brain fills in this gap using information from the surrounding areas and the other eye, creating a seamless visual experience.

Supporting Structures: Sclera, Choroid, and Vitreous Humor

Several other structures play important roles in maintaining the eye’s shape and function.

The Sclera: The Eye’s Protective Outer Layer

The sclera is the white, outer layer of the eyeball. What is the main function of the sclera. It provides structure and protection for the inner components of the eye, maintaining its shape and offering a point of attachment for the extraocular muscles that control eye movement.

The Choroid: Nourishing the Eye

The choroid is a layer of blood vessels between the retina and the sclera. How does the choroid support eye function. It supplies oxygen and nutrients to the outer layers of the retina and helps maintain the proper temperature of the eye.

Vitreous Humor: Maintaining Eye Shape

The vitreous humor is a clear, gel-like substance that fills the space between the lens and the retina. What role does the vitreous humor play in eye health. It helps maintain the eye’s shape and provides a clear medium for light to pass through on its way to the retina. It also acts as a shock absorber, protecting the delicate structures of the eye from minor impacts.

The Visual Pathway: From Eye to Brain

Vision is not complete until the brain processes the information received from the eyes. Let’s explore how visual information is transmitted and interpreted.

The Visual Cortex

The visual cortex is the part of the brain responsible for processing visual information. How does the brain interpret visual signals. The visual cortex receives electrical impulses from the optic nerve and processes them to create the images we perceive. Different areas of the visual cortex are specialized for processing various aspects of vision, such as color, motion, and form.

Binocular Vision

Humans have two eyes, which provides us with binocular vision. What advantages does binocular vision offer. Binocular vision allows for depth perception and stereopsis, giving us the ability to perceive three-dimensional space and judge distances accurately. The brain combines slightly different images from each eye to create a single, unified 3D perception of the world.

Common Eye Conditions and Their Impact on Eye Anatomy

Understanding eye anatomy helps in comprehending various eye conditions and their effects on vision.

Myopia and Hyperopia

Myopia (nearsightedness) and hyperopia (farsightedness) are common refractive errors. How do these conditions affect the eye’s focusing ability. In myopia, the eye is too long or the cornea too curved, causing light to focus in front of the retina. In hyperopia, the eye is too short or the cornea too flat, causing light to focus behind the retina. Both conditions can be corrected with glasses, contact lenses, or refractive surgery.

Glaucoma

Glaucoma is a group of eye conditions that damage the optic nerve. What causes glaucoma, and how does it affect vision. Glaucoma is often associated with increased intraocular pressure, which can damage the optic nerve fibers. This damage leads to progressive vision loss, typically starting with peripheral vision.

Cataracts

Cataracts involve the clouding of the eye’s natural lens. How do cataracts impact vision. As the lens becomes cloudy, less light can pass through, resulting in blurred vision, glare sensitivity, and difficulty seeing in low light conditions. Cataracts can be treated by surgically replacing the clouded lens with an artificial intraocular lens.

Age-Related Macular Degeneration (AMD)

AMD affects the macula, the central part of the retina responsible for sharp, detailed vision. How does AMD progress, and what are its effects on vision. AMD can be dry (atrophic) or wet (neovascular). In dry AMD, the macula slowly deteriorates, while wet AMD involves the growth of abnormal blood vessels under the retina. Both types can lead to central vision loss, affecting activities like reading and recognizing faces.

The Eye’s Remarkable Adaptations

The human eye has evolved several remarkable adaptations that enhance our visual capabilities.

Dark Adaptation

Our eyes can adjust to see in a wide range of light conditions. How long does it take for eyes to fully adapt to darkness. Full dark adaptation can take up to 30 minutes. During this process, the pupils dilate, the retina becomes more sensitive to light, and the eye switches from cone-dominated to rod-dominated vision.

Rapid Eye Movements (Saccades)

Our eyes are constantly making rapid, jerky movements called saccades. What is the purpose of these movements. Saccades allow us to quickly scan our environment and focus on different points of interest. These movements happen so quickly that our brain creates a stable, continuous image of our surroundings.

Tear Production and the Corneal Reflex

The eye has built-in mechanisms to protect itself. How do tears and the corneal reflex contribute to eye health. Tears not only lubricate the eye but also contain antibacterial compounds that help protect against infections. The corneal reflex, which causes us to blink when something approaches the eye, helps prevent damage to this delicate organ.

In conclusion, the human eye is an incredibly complex and efficient organ, perfectly adapted to provide us with the sense of sight. From the cornea’s clear window to the retina’s light-sensitive cells, each component plays a crucial role in the visual process. Understanding the anatomy and function of the eye not only satisfies our curiosity about how we see the world but also helps us appreciate the importance of maintaining good eye health and seeking appropriate care when problems arise.

Anatomy of the Eye | Kellogg Eye Center

• Choroid
  Layer containing blood vessels that lines the back of the eye and is located between the retina (the inner light-sensitive layer) and the sclera (the outer white eye wall).  
• Ciliary Body
  Structure containing muscle and is located behind the iris, which focuses the lens.
• Cornea
  The clear front window of the eye which transmits and focuses (i.e., sharpness or clarity) light into the eye. Corrective laser surgery reshapes the cornea, changing the focus.
• Fovea
  The center of the macula which provides the sharp vision.
• Iris
  The colored part of the eye which helps regulate the amount of light entering the eye. When there is bright light, the iris closes the pupil to let in less light. And when there is low light, the iris opens up the pupil to let in more light.
• Lens
  Focuses light rays onto the retina. The lens is transparent, and can be replaced if necessary. Our lens deteriorates as we age, resulting in the need for reading glasses. Intraocular lenses are used to replace lenses clouded by cataracts.
• Macula
  The area in the retina that contains special light-sensitive cells. In the macula these light-sensitive cells allow us to see fine details clearly in the center of our visual field. The deterioration of the macula is a common condition as we get older (age related macular degeneration or ARMD).
• Optic Nerve
  A bundle of more than a million nerve fibers carrying visual messages from the retina to the brain. (In order to see, we must have light and our eyes must be connected to the brain.) Your brain actually controls what you see, since it combines images. The retina sees images upside down but the brain turns images right side up. This reversal of the images that we see is much like a mirror in a camera. Glaucoma is one of the most common eye conditions related to optic nerve damage.
• Pupil
  The dark center opening in the middle of the iris. The pupil changes size to adjust for the amount of light available (smaller for bright light and larger for low light). This opening and closing of light into the eye is much like the aperture in most 35 mm cameras which lets in more or less light depending upon the conditions.
• Retina
  The nerve layer lining the back of the eye. The retina senses light and creates electrical impulses that are sent through the optic nerve to the brain.
• Sclera
  The white outer coat of the eye, surrounding the iris.
• Vitreous Humor
  The, clear, gelatinous substance filling the central cavity of the eye.

The five senses include sight, sound, taste, hearing and touch. Sight, like the other senses is closely related to other parts of our anatomy. The eye is connected to the brain and dependent upon the brain to interpret what we see.

How we see depends upon the transfer of light. Light passes through the front of the eye (cornea) to the lens. The cornea and the lens help to focus the light rays onto the back of the eye (retina). The cells in the retina absorb and convert the light to electrochemical impulses which are transferred along the optic nerve and then to the brain.

The eye works much the same as a camera. The shutter of a camera can close or open depending upon the amount of light needed to expose the film in the back of the camera. The eye, like the camera shutter, operates in the same way. The iris and the pupil control how much light to let into the back of the eye. When it is very dark, our pupils are very large, letting in more light. The lens of a camera is able to focus on objects far away and up close with the help of mirrors and other mechanical devices. The lens of the eye helps us to focus but sometimes needs some additional help in order to focus clearly. Glasses, contact lenses, and artificial lenses all help us to see more clearly.  

Anatomy of the eye – Moorfields Eye Hospital

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HomeYour eye healthAnatomy of the eye

Our eyes might be small, but they provide us with what many people consider to be the most important of our senses – vision.

How vision works

Vision occurs when light enters the eye through the pupil. With help from other important structures in the eye, like the iris and cornea, the appropriate amount of light is directed towards the lens.

Just like a lens in a camera sends a message to produce a film, the lens in the eye ‘refracts’ (bends) incoming light onto the retina. The retina is made up by millions of specialised cells known as rods and cones, which work together to transform the image into electrical energy, which is sent to the optic disk on the retina and transferred via electrical impulses along the optic nerve to be processed by the brain.

Anatomy of the eye

What makes up an eye

• Iris: regulates the amount of light that enters your eye. It forms the coloured, visible part of your eye in front of the lens. Light enters through a central opening called the pupil.
• Pupil: the circular opening in the centre of the iris through which light passes into the lens of the eye. The iris controls widening and narrowing (dilation and constriction) of the pupil.
• Cornea: the transparent circular part of the front of the eyeball. It refracts the light entering the eye onto the lens, which then focuses it onto the retina. The cornea contains no blood vessels and is extremely sensitive to pain.
• Lens: a transparent structure situated behind your pupil.  It is enclosed in a thin transparent capsule and helps to refract incoming light and focus it onto the retina. A cataract is when the lens becomes cloudy, and a cataract operation involves the replacement of the cloudy lens with an artificial plastic lens.
• Choroid: the middle layer of the eye between the retina and the sclera. It also contains a pigment that absorbs excess light so preventing blurring of vision.
• Ciliary body: the part of the eye that connects the choroid to the iris.
• Retina: a light sensitive layer that lines the interior of the eye. It is composed of light sensitive cells known as rods and cones. The human eye contains about 125 million rods, which are necessary for seeing in dim light. Cones, on the other hand, function best in bright light.  There are between 6 and 7 million cones in the eye and they are essential for receiving a sharp accurate image and for distinguishing colours. The retina works much in the same way as film in a camera.
• Macula: a yellow spot on the retina at the back of the eye which surrounds the fovea.
• Fovea: forms a small indentation at the centre of the macula and is the area with the greatest concentration of cone cells. When the eye is directed at an object, the part of the image that is focused on the fovea is the image most accurately registered by the brain.
• Optic disc: the visible (when the eye is examined) portion of the optic nerve, also found on the retina. The optic disc identifies the start of the optic nerve where messages from cone and rod cells leave the eye via nerve fibres to the optic centre of the brain. This area is also known as the ‘blind spot’.
• Optic nerve: leaves the eye at the optic disc and transfers all the visual information to the brain.
• Sclera: the white part of the eye, a tough covering with which the cornea forms the external protective coat of the eye.
• Rod cells are one of the two types of light-sensitive cells in the retina of the eye. There are about 125 million rods, which are necessary for seeing in dim light.
• Cone cells are the second type of light sensitive cells in the retina of the eye. The human retina contains between six and seven million cones; they function best in bright light and are essential for acute vision (receiving a sharp accurate image). It is thought that there are three types of cones, each sensitive to the wavelength of a different primary colour – red, green or blue. Other colours are seen as combinations of these primary colours. 

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Explanatory dictionary of ophthalmic terms | Eye examination and treatment in Novosibirsk at Glazka clinic

Accommodation (autofocus) – the ability of the eye to reflexively change its optical power for clear vision at different distances.

Myopia (nearsightedness) – strong refraction (optics of the eye), in which light rays are refracted in front of the retina. With a weak degree of myopia, vision is reduced in the distance, with medium and high – far and near.

A B C D E F G H I J K L M N O P R S T U F Y Z

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Addition (additive) – the difference between the power of the optical lens for distance and near.

Accommodation (autofocus) – the ability of the eye to reflexively change its optical power for clear vision at different distances.

Amblyopia (low vision) is a persistent decrease in visual acuity without visible anatomical and optical causes, which is difficult to optical correction. Read more…

Ametropia – disproportionate refraction (optics of the eye), in which the rays of light are refracted closer or further than the retina, which leads to reduced vision.

Anisometropia – inequality of the optical structure of the eyes.

Artifakia – the presence of an artificial lens in the eye.

Asthenopia – visual fatigue, which may precede already a pathological change in vision and even ophthalmic diseases.

Astigmatism – violation of the sphericity of the eye due to different refraction (optics) in two meridians (or several). Read more…

Optic atrophy – destruction of all or part of the nerve fibers, leading to a disruption in the transmission of information from the retina to the brain.

Aphakia – absence of the lens in the eye.

Basalioma is a tumor from the basal layer of the skin epithelium. Often develops in open areas of the body (face, eyelids, neck). It can become malignant, therefore it is subject to mandatory removal.

Binocular vision – perception of the outside world by the visual system of the two eyes.

Blepharitis – this name combines a large group of diseases, accompanied by inflammation of the edges of the eyelids.

AMD – age-related macular degeneration – a chronic progressive degenerative disease of the central zone of the retina. It leads to a gradual decrease and distortion of central vision. It develops more often in older women who smoke.

Hemianopsia – half or quarter loss in the field of view.

Heterophoria – imbalance of the eye muscles while maintaining binocular vision.

Hypermetropia (farsightedness) – weak refraction (optics of the eye), in which the rays of light are refracted behind the retina. Vision is reduced far and near. The prefix hyper- means hyperfunction, namely, a constant tension of accommodation to compensate for a defect. Read more…

Glaucoma is a group of heterogeneous diseases of the eye, accompanied by a constant or periodic increase in intraocular pressure, changes in the visual fields and irreversible death of the optic nerve fibers. It can be asymptomatic for a long time, it is detected during regular examinations by an ophthalmologist.

Neonatal dacryocystitis – inflammation of the lacrimal ducts of a newborn due to impaired patency of the nasolacrimal canal. It is characterized by lacrimation and purulent discharge from the eye.

Colorblindness – a hereditary or acquired feature of human vision, when one or more colors are not distinguished. It is named after the English scientist John Dalton, who first described this anomaly.

Diabetic retinopathy – one of the complications of diabetes, accompanied by damage to the vessels of the retina. It develops with a long course of diabetes mellitus and may be accompanied by a decrease in visual acuity, hemorrhages in the retina and even its detachment. With regular examinations and timely prescribed treatment, long-term preservation of high visual functions is possible.

Divergence – separation of the visual axes of the eyes when looking from a near object to a far one.

Retinal dystrophy is the general name for various degenerative changes in the retina of the eye. It is detected when examining the fundus after instillation of drops that dilate the pupils. Depending on the localization, it can be central and peripheral.

Cataract is a group of diseases characterized by a violation of the transparency of the lens and is accompanied by a decrease in visual acuity of varying severity. May be congenital or acquired.

Keratitis – inflammation of the cornea (the outer transparent membrane of the eye). Accompanied by decreased vision, sharp pain, redness of the eye, photophobia.

Convergence – convergence of the visual axes of the eyes to the point of fixation.

Conjunctivitis – inflammation of the mucous membrane of the eye, which is accompanied by redness of the eye, the presence of discharge and a feeling of a foreign body. More often it is allergic, bacterial or viral. Usually does not lead to a decrease in visual acuity.

Interpupillary distance (for near) – the distance between the centers of the pupils when viewed at a distance of 40 cm. Measured with a special device with a pupillometer (not a ruler).

Interpupillary distance (for distance) – the distance between the centers of the pupils when viewed at a distance of 5m. Always more than the interpupillary distance for near by an average of 4 mm. It is measured by a special device with a pupillometer (not a ruler).

Myopia (nearsightedness) – strong refraction (optics of the eye), in which light rays are refracted in front of the retina. With a weak degree of myopia, vision is reduced in the distance, with medium and high – far and near. Read more…

Progressive myopia – myopia with gradual deterioration of distance vision due to the growth of the eye and/or optical enhancement of refraction (optics).

The optical power of the lens is a value that characterizes its refractive effect in diopters.

Optometry is a branch of ophthalmology dealing with the diagnosis and optical correction of visual defects.

Orthoptics – a complex hardware treatment to restore and strengthen binocular vision in strabismus.

Orthophoria – complete, perfect balance of the eye muscles.

Visual acuity (Visus) – the ability of the eye to separate perception of two points at the smallest angle of view in 1 min. The physiological norm is 1.0.

Retinal detachment – separation of the innermost layer of the retina from the choroid. It often develops in people with myopia, but may be the result of an eye injury, a complication of diabetes mellitus or hypertension. Without timely treatment, it leads to a permanent loss of visual functions.

Peripheral scotoma – loss on the periphery of the visual field.

PINA – habitually excessive accommodation tension – a long-term excessive accommodation tone at which myopia is detected. It differs from accommodation spasm in that corrected visual acuity remains high. It often develops in children and adolescents against the background of intense visual stress.

Pinguecula is a benign degenerative change in the conjunctiva of the eye. It is characterized by the appearance of a yellowish island more often on the nasal side of the conjunctiva.

Pleoptics – complex treatment of amblyopia (poor vision) in order to restore visual acuity.

Field of view – field of view with fixed eyes and head.

Presbyopia – age-related deterioration in near visual acuity due to loss of elasticity of the lens and weakening of the accommodative muscle after 40 years.

Progressive (multifocal) lens – a spectacle lens with a smoothly changing optics from top to bottom, allows you to wear one glasses and see at several distances.

Pterygium – a disease in which the conjunctiva grows on the cornea in the form of a triangle, often from the nasal side.

Refraction (optics of the eye) – refraction of light rays in the optically active media of the eye, which determines its optical structure.

Scotoma – loss in the field of view, not reaching its periphery.

Spasm of accommodation (false myopia, pseudomyopia) – increased optics of the eye with loss of accommodation ability to self-relaxation. With medical dilation of the pupils, the spasm of the accommodative muscle is removed, and refraction returns to normal.

Strabismus , heterotropia – strabismus, deviation of the visual axes of the eyes from a parallel, symmetrical position, leading to impaired binocular vision. Read more…

Strabology is a branch of ophthalmology dealing with the diagnosis and treatment of strabismus.

Narrowing of the field of vision – limitation of the field of vision in the periphery.

Phoria – muscle balance, balance of the eye muscles.

Fusion reserves are the reserve forces that the body has to maintain muscular balance and maintain binocular vision.

Fusion – the ability to merge images on the retinas of both eyes into a single visual image.

Chalazion is a chronic inflammation of the meibomian gland located in the thickness of the eyelid. It manifests itself in the form of a rounded, dense formation, a mobile formation.

Central scotoma – defect in the center of the visual field.

Esophoria – muscular imbalance of the eyes, causing latent convergent strabismus.

Exophoria – muscular imbalance of the eyes, causing latent divergent strabismus.

Emmetropia – proportionate refraction (optics of the eye) with a clear focus on the retina.

Epiphora – profuse lachrymation and lacrimation. Often accompanies neonatal dacryocystitis

Barley – acute purulent inflammation of the eyelash bulb and the gland adjacent to it, located in the thickness of the eyelid. May be accompanied by swelling, redness of the eyelid, deterioration of the general condition and fever. Often develops in children and people with impaired immunity.

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