What is the structure of the human eye? How does the visual apparatus function? Discover the intricate anatomy and remarkable capabilities of this remarkable sense organ.

Anatomy of the Visual Apparatus

The human eye is a highly specialized sense organ capable of receiving visual images, which are then transmitted to the brain. The eye is protected from mechanical injury by being enclosed in a socket, or orbit, formed by various bones of the skull. The optic foramen, through which the optic nerve runs back into the brain, and the superior orbital fissure, through which large veins and nerves pass, are important openings in this bony structure.

The eyeball and its functional muscles are surrounded by a layer of orbital fat that acts as a cushion, allowing for smooth rotation of the eyeball. The front surface of the eyeball, the cornea, must remain moist, which is achieved by the eyelids sweeping secretions from the lacrimal apparatus and other glands over the surface at regular intervals.

The Eyelids and Conjunctiva

The eyelids are folds of tissue covering the front of the orbit, with an almond-shaped aperture when the eye is open. The lid can be divided into four layers: the skin, a muscular layer, a fibrous layer, and the innermost layer, a portion of the conjunctiva.

The conjunctiva is a mucous membrane that serves to attach the eyeball to the orbit and lids, while still allowing for a considerable degree of eyeball rotation. The conjunctiva lines the lids and then bends back over the surface of the eyeball, constituting an outer covering to the forward part of the eye and terminating at the cornea.

The Fibrous Layer and Muscles of the Lids

The fibrous layer of the eyelid, which gives it mechanical stability, is made up of the thick, rigid tarsal plates and the thinner palpebral fascia. These two components are collectively known as the septum orbitale, which covers the entire opening of the orbit when the lids are closed.

Closure of the lids is achieved by the contraction of the orbicularis muscle, a single oval sheet of muscle surrounding the orbit and extending into the lids. This muscle is divided into orbital and palpebral portions, with the palpebral portion primarily responsible for lid closure.

The Lacrimal System

The lacrimal system is responsible for producing, distributing, and draining the tears that keep the eye moist and protected. This system includes the lacrimal gland, which produces tears, and the lacrimal ducts, which drain the excess tears from the eye.

The lacrimal gland is located in the upper, outer part of the orbit, and the tears it produces are distributed across the surface of the eye by the blinking action of the eyelids. The excess tears are then drained through the lacrimal ducts, which lead to the nasal cavity.

The Vascular Supply of the Eye

The eye is supplied with blood by several arteries, including the ophthalmic artery, which enters the orbit through the optic foramen. This artery and its branches provide blood to the various structures of the eye, including the conjunctiva, the muscles, and the inner structures of the eyeball.

The veins of the eye, on the other hand, drain the blood from the eye and its surrounding structures, ultimately leading to the cavernous sinus, a large venous structure at the base of the brain.

The Innervation of the Eye

The eye and its associated structures are innervated by several cranial nerves, including the optic nerve, which carries visual information from the retina to the brain, and the oculomotor, trochlear, and abducens nerves, which control the movement of the eye muscles.

In addition, the eye is also innervated by the trigeminal nerve, which provides sensory information, and the sympathetic and parasympathetic nervous systems, which control various autonomic functions of the eye, such as pupil size and tear production.

The Remarkable Capabilities of the Human Eye

The human eye is a remarkable sense organ, capable of detecting light, color, and motion, and converting these physical stimuli into electrical signals that the brain can interpret and understand. The eye’s ability to focus, adapt to different lighting conditions, and perceive depth and distance are all essential for our visual experience.

Moreover, the eye’s remarkable resilience and self-healing abilities, as well as its intricate anatomical structure, have long fascinated scientists and researchers, who continue to uncover new insights about this incredible organ.

Human eye | Definition, Anatomy, Diagram, Function, & Facts

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human eye, in humans, specialized sense organ capable of receiving visual images, which are then carried to the brain.

Anatomy of the visual apparatus

Structures auxiliary to the eye

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.g., pain—or they may be motor nerves controlling the muscles of the eye. There are other fissures and canals transmitting nerves and blood vessels. The eyeball and its functional muscles are surrounded by a layer of orbital fat that acts much like a cushion, permitting a smooth rotation of the eyeball about a virtually fixed point, the centre of rotation. The protrusion of the eyeballs—proptosis—in exophthalmic goitre is caused by the collection of fluid in the orbital fatty tissue.

It is vitally important that the front surface of the eyeball, the cornea, remain moist. This is achieved by the eyelids, which during waking hours sweep the secretions of the lacrimal apparatus and other glands over the surface at regular intervals and which during sleep cover the eyes and prevent evaporation. The lids have the additional function of preventing injuries from foreign bodies, through the operation of the blink reflex. The lids are essentially folds of tissue covering the front of the orbit and, when the eye is open, leaving an almond-shaped aperture. The points of the almond are called canthi; that nearest the nose is the inner canthus, and the other is the outer canthus. The lid may be divided into four layers: (1) the skin, containing glands that open onto the surface of the lid margin, and the eyelashes; (2) a muscular layer containing principally the orbicularis oculi muscle, responsible for lid closure; (3) a fibrous layer that gives the lid its mechanical stability, its principal portions being the tarsal plates, which border directly upon the opening between the lids, called the palpebral aperture; and (4) the innermost layer of the lid, a portion of the conjunctiva. The conjunctiva is a mucous membrane that serves to attach the eyeball to the orbit and lids but permits a considerable degree of rotation of the eyeball in the orbit.

The conjunctiva

The conjunctiva lines the lids and then bends back over the surface of the eyeball, constituting an outer covering to the forward part of this and terminating at the transparent region of the eye, the cornea. The portion that lines the lids is called the palpebral portion of the conjunctiva; the portion covering the white of the eyeball is called the bulbar conjunctiva. Between the bulbar and the palpebral conjunctiva there are two loose, redundant portions forming recesses that project back toward the equator of the globe. These recesses are called the upper and lower fornices, or conjunctival sacs; it is the looseness of the conjunctiva at these points that makes movements of lids and eyeball possible.

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The fibrous layer

The fibrous layer, which gives the lid its mechanical stability, is made up of the thick, and relatively rigid, tarsal plates, bordering directly on the palpebral aperture, and the much thinner palpebral fascia, or sheet of connective tissue; the two together are called the septum orbitale. When the lids are closed, the whole opening of the orbit is covered by this septum. Two ligaments, the medial and lateral palpebral ligaments, attached to the orbit and to the septum orbitale, stabilize the position of the lids in relation to the globe. The medial ligament is by far the stronger.

The muscles of the lids

Closure of the lids is achieved by contraction of the orbicularis muscle, a single oval sheet of muscle extending from the regions of the forehead and face and surrounding the orbit into the lids. It is divided into orbital and palpebral portions, and it is essentially the palpebral portion, within the lid, that causes lid closure. The palpebral portion passes across the lids from a ligament called the medial palpebral ligament and from the neighbouring bone of the orbit in a series of half ellipses that meet outside the outer corner of the eye, the lateral canthus, to form a band of fibres called the lateral palpebral raphe. Additional parts of the orbicularis have been given separate names—namely, Horner’s muscle and the muscle of Riolan; they come into close relation with the lacrimal apparatus and assist in drainage of the tears. The muscle of Riolan, lying close to the lid margins, contributes to keeping the lids in close apposition. The orbital portion of the orbicularis is not normally concerned with blinking, which may be carried out entirely by the palpebral portion; however, it is concerned with closing the eyes tightly. The skin of the forehead, temple, and cheek is then drawn toward the medial (nose) side of the orbit, and the radiating furrows, formed by this action of the orbital portion, eventually lead to the so-called crow’s feet of elderly persons. It must be appreciated that the two portions can be activated independently; thus, the orbital portion may contract, causing a furrowing of the brows that reduces the amount of light entering from above, while the palpebral portion remains relaxed and allows the eyes to remain open.

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Opening of the eye is not just the result of passive relaxation of the orbicularis muscle but also is the effect of the contraction of the levator palpebrae superioris muscle of the upper lid. This muscle takes origin with the extraocular muscles at the apex of the orbit as a narrow tendon and runs forward into the upper lid as a broad tendon, the levator aponeurosis, which is attached to the forward surface of the tarsus and the skin covering the upper lid. Contraction of the muscle causes elevation of the upper eyelid. The nervous connections of this muscle are closely related to those of the extraocular muscle required to elevate the eye, so that when the eye looks upward the upper eyelid tends to move up in unison.

The orbicularis and levator are striated muscles under voluntary control. The lids also contain smooth (involuntary) muscle fibres that are activated by the sympathetic division of the autonomic system and tend to widen the palpebral fissure (the eye opening) by elevation of the upper, and depression of the lower, lid.

In addition to the muscles already described, other facial muscles often cooperate in the act of lid closure or opening. Thus, the corrugator supercilii muscles pull the eyebrows toward the bridge of the nose, making a projecting “roof” over the medial angle of the eye and producing characteristic furrows in the forehead; the roof is used primarily to protect the eye from the glare of the sun. The pyramidalis, or procerus, muscles occupy the bridge of the nose; they arise from the lower portion of the nasal bones and are attached to the skin of the lower part of the forehead on either side of the midline; they pull the skin into transverse furrows. In lid opening, the frontalis muscle, arising high on the forehead, midway between the coronal suture, a seam across the top of the skull, and the orbital margin, is attached to the skin of the eyebrows. Contraction therefore causes the eyebrows to rise and opposes the action of the orbital portion of the orbicularis; the muscle is especially used when one gazes upward. It is also brought into action when vision is rendered difficult either by distance or the absence of sufficient light.

The outermost layer of the lid is the skin, with features not greatly different from skin on the rest of the body, with the possible exception of large pigment cells, which, although found elsewhere, are much more numerous in the skin of the lids. The cells may wander, and it is these movements of the pigment cells that determine the changes in coloration seen in some people with alterations in health. The skin has sweat glands and hairs. As the junction between skin and conjunctiva is approached, the hairs change their character to become eyelashes.

The glandular apparatus

The eye is kept moist by secretions of the lacrimal glands (tear glands). These almond-shaped glands under the upper lids extend inward from the outer corner of each eye. Each gland has two portions. One portion is in a shallow depression in the part of the eye socket formed by the frontal bone. The other portion projects into the back part of the upper lid. The ducts from each gland, three to 12 in number, open into the superior conjunctival fornix, or sac. From the fornix, the tears flow down across the eye and into the puncta lacrimalia, small openings at the margin of each eyelid near its inner corner. The puncta are openings into the lacrimal ducts; these carry the tears into the lacrimal sacs, the dilated upper ends of the nasolacrimal ducts, which carry the tears into the nose.

The evaporation of the tears as they flow across the eye is largely prevented by the secretion of oily and mucous material by other glands. Thus, the meibomian, or tarsal glands, consist of a row of elongated glands extending through the tarsal plates; they secrete an oil that emerges onto the surface of the lid margin and acts as a barrier for the tear fluid, which accumulates in the grooves between the eyeball and the lid barriers.

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11 differences between a lens and a human eye

Your eyes can be compared to camera lenses.

But the perception of an image by a person and a camera has a number of differences, which you need to know not only the photographer, but also you as a model.

First difference.

You have two eyes, each of which perceives the image from different “points of view”. Then the brain adds up one common three-dimensional picture from them. Therefore, we perceive the world as voluminous, in three dimensions. Why does a drunk see double? Pictures don’t match…

The camera has only one eye – the lens, so in photographs it forms a picture in two dimensions, that is, a flat one. The picture does not have a third dimension – depth, or perspective. The volume of the image, the perspective are obtained with the help of halftones, the properties of various textures and some technical means.

Without taking this into account, taking pictures in a birch grove in cloudy weather, you run the risk of being photographed against the backdrop of a birch fence. For the same reason, in the photographs we look seven to ten kilograms heavier than we actually are…

The second difference.

The human eye tends to automatically focus on the subject on which we stopped our eyes. This is called accommodation. The same role is played by the wipers on the windshield of a car during rain for the driver – they sharpen the image of the road with a traffic inspector standing on it.

With the camera, this is done either manually or automatically, but in any case, such a concept, often used in photographic art and photographic technology, arises as the depth of field of the image. This characteristic shows between which boundaries, determined by the far and near distance from the camera, the image will be sharp. And outside of these boundaries – vague.

When shooting against the background of an architectural monument, you may find yourself in the picture against the background of blurry colored spots, or you yourself risk becoming such a spot against the background of a historical monument.

The third difference.

The human eye perceives objects, cars, animals, people in motion. The camera stops moving. Goethe also said something about stopping a moment. To make the picture look dynamic, photographers use various means, both technical and compositional. The expression of your face is constantly changing due to facial expressions and movement, that is, it turns out that you have many faces at different points in time. The lens is able to snatch out of this variety only specific individual moments.

If you are photographed running during a sporting event, looking at the frame, you might think that you are working as a mannequin in the window of a sporting goods store. If you feel like blinking, and at that moment you are photographed, then half-closed eyelids will make you the last drunkard in the picture, although maybe in life you don’t drink anything stronger than kefir.

— Do you know why there is a double-headed eagle on the coat of arms?

– Because when he was photographed, he turned his head.

In a sense, the image in a photograph does not obey many laws of physics, such as gravity. Jumping up, in the photo you will hang in the air for a very long time until you fade. The wind will blow your hat off endlessly. And even such natural phenomena as lightning or a short circuit will take forever in the picture!

The fourth difference.

You are accustomed to seeing yourself in a mirror, that is, your mirror, “reverse” image.

The photograph gives you a real image, and in some cases, with obvious asymmetry of the face, facial expressions, the presence of birthmarks on the face, etc., you may have a suspicion that you are not depicted in the photo.

Look at yourself in the mirror through another mirror, unless, of course, it is in the room of laughter – and you will no longer have problems.

– How did you guess that dad got paid?

– And he shaves the bathroom mirror!

The fifth difference.

The eye and the camera perceive the human figure in different ways.

Although your gaze moves around the image chaotically, the maximum attention falls on the semantic centers. Such centers are images of a person or an animal, even if the picture is a landscape or any objects are depicted on it, including technology. However, there are some priorities here too. The faces of people or the muzzles of animals, when perceived, mean more than secondary sexual characteristics, then come, in terms of significance, the figure, the interior.

Also, the semantic center of the image is any pronounced gesture that reflects the emotional state of a particular model.

When shooting portraits, any emotion that is clearly expressed on the face – laughter, screaming, crying, grimace – also instantly becomes the center of the composition, regardless of body position.

Attractive women attract attention.

Looking at the portrait, the viewer focuses his attention mainly on the compositional centers: the eyes, mouth, nose of a person or animal.

Men usually perceive women’s legs, face, secondary sexual characteristics as separate from everything else. At the same time, their significance, depending on the character, may exceed the face of the model. These parts of the body, as it were, fill the entire plane of the frame, despite the fact that they actually do not occupy such a large area, and are the semantic centers of the image.

Women pay the main attention to the face of a man, his eyes, only then – to the figure.

The movement of a person in a photograph, his facial expressions, charm or lack of it, personal likes or dislikes of the viewer sometimes create such different images of the same person in the photograph and in the viewer’s head after comprehending the perception of the image, that one can say that these are completely different people.

Therefore, keep in mind that how the photographer perceives you can be very different from what happens in the picture. If the photographer is inexperienced, then shooting correctly is your task as a model, and not an easy one.

Sixth difference.

The human eye automatically adjusts to the illumination of the subject. This is called adaptation. Remember, when you move from a bright room to a dark one, at first you don’t see anything, but then your eyes get used to it, and you start looking in the dark for a black cat, which may not be there.

At the same time, due to the fact that in bright and gloomy lighting a person has two different visual systems, in semi-darkness he ceases to recognize colors. As they say, “in the dark, all cats are gray.”

The camera adjusts the brightness either manually or automatically using the aperture. This also needs to be taken into account, otherwise, when shooting, for example, on a sunny seashore, you run the risk of being a dark, illegible spot in the picture against the background of boundless sea distances, or vice versa. The same effect will be obtained if you are between the camera and the window or the sun.

The seventh difference.

A person perceives the surrounding world, including people, in a panoramic way, along with a landscape, interior or other background. In the picture, you can capture the model only against the background of a small part of the panorama, which is limited by the frame. This effect is often shown on TV when, starting a news program, they first show a panorama of the studio, and then the camera “runs in” on the announcer, “driving” him alone into the television “box”. Often the intention of the photographer and the model, if the frame is not taken into account, fails. For example, when shooting at a party among friends having fun, you run the risk of being surrounded by torn curtains and an empty table, and the magnificent carpets and antique furniture of your apartment will not get into the frame.

To achieve the desired effect during shooting, you need to imagine the size of the field of view of a person, and if the photo lens corresponds to it as much as possible, then it will be the most real and transmit the photographed objects without distortion. In the horizontal plane, the effective angle of vision in a person is 60 degrees, the total is 120 degrees; in the vertical plane relative to the horizon – 10 degrees up and 27 degrees down. This roughly corresponds to a lens focal length of 50mm when shooting on the popular 35mm film.

Eighth difference.

When you perceive a person or a landscape in a normal environment, you use, in addition to vision, also hearing, smell, touch, taste, and, perhaps, one of the many “sixth” senses.

The camera is not able to convey to the viewer, which is also the reason that the charm that was present during the shooting leaves the frame. Shooting in a flower greenhouse after a May thunderstorm, when the air is clean and transparent, the flowers are fragrant so that your head is spinning, and the photographer is your close friend, in whom you do not have a soul, with whom you feel good, easy and simple … However, you need to consider that all this will not be in the picture.

The ninth difference.

Film, especially color film, registers images much more clearly than the human eye. For example, we do not notice a slightly reddened nose in the wind, and in the photo it may look like the last alcoholic. We do not notice the blue veins that are slightly visible through the skin. In the photograph, they will be visible so clearly that they can be used to study the circulatory system, especially in older models. The same goes for body fat. Or, for example, we may not notice an inscription on paper made with a yellow felt-tip pen. It is very easy to read in the picture. Sit on a bench without noticing that it is freshly painted, and your back, like a photographic film, will register its image just as clearly.

The human eye has another peculiarity. Due to the fact that a natural analyzer is “built into it”, which constantly compares the image with a neutral gray color, the eye always makes adjustments for lighting and almost always adequately perceives colors, whether it is a sunny or cloudy day, electric lighting or candles . .. By the way, pictures are best shown on a neutral light gray background, then they will be most expressive due to the peculiarity of vision, which was described above. But if a person perceives colors adequately in any light, what cannot be said about their brightness. On a clear sunny day, he, as the brightest, perceives red, yellow, orange colors. At dusk, blue and green colors come out on top in terms of brightness.

Film is initially designed only for a certain type of lighting, expressed by the so-called color temperature, and perceives all colors in approximately the same way, regardless of the time of day and illumination.

Tenth difference.

The so-called “peeping effect”. It lies in the fact that the viewer can thoroughly examine the model depicted not in the photograph, which is in any, even the most “delicate” position. And at the same time, he does not risk anything. Whereas in life, moral principles and social rules come into play that dominate the viewer, who runs the risk of “running into” trouble.

For example, a girl whose skirt hem is pulled up by a gust of wind can be looked at as much as you like in the photo. If this happens in real life, then you can easily get a slap in the face.

Eleventh difference.

Pictures of memorable events and just working moments are designed to awaken our memory for many years.

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