The Eyes:
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The Eyes are undoubtedly the most sensitive and delicate organs that we possess and perhaps the most amazing; they present us with the window through which we view the world and are responsible for 4/5 of all the information our brain receives, which is probably why we rely on our Eyesight more than any other sense.

We use our Eyes in almost every activity we perform, whether reading, working, watching television, writing a letter, driving a car and in countless other ways; most people probably would agree that sight is the sense they value more than all the rest.

The Eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals and transmits these signals to the brain through complex neural pathways that connect the Eye via the optic nerve to the visual cortex and other areas of the brain.

The Eye allows us to see and interpret the shapes, colours and dimensions of objects in the world by processing the light they reflect or emit; the Eye is able to detect bright light or dim light, but it cannot sense objects when light is absent; fluctuations in the intensity of incoming light change the size of the Eye’s pupil; as the light entering the Eye becomes brighter, the pupil will constrict, get smaller, due to the pupillary light response and as the entering light becomes dimmer, the pupil will dilate, get larger.

The average newborn’s Eyeball is about 18 millimeters in diameter, from front to back, the axial length; in an infant, the Eye grows slightly to a length of approximately 19½ millimeters; the Eye continues to grow, gradually, to a length of about 24 to 25 millimeters, or about 1 inch, in adulthood; a ping-pong ball is about 1½ inch in diameter, which makes the average adult Eyeball about 2/3 the size of a ping-pong ball.

The human eye consists of several structures and amongst the most important are the cornea, conjunctiva, iris, crystalline lens, vitreous humor, retina, macula, optic nerve and extraocular muscles.

The Eyeball is set in a protective cone-shaped cavity in the skull called the 'Orbit' or 'Socket'; this bony orbit also enlarges as the Eye grows; the orbit is surrounded by layers of soft, fatty tissue; these layers protect the Eye and enable it to turn easily; traversing the fatty tissue are three pairs of extraocular muscles, which regulate the motion of each Eye; the medial & lateral rectus muscles, the superior & inferior rectus muscles and the superior & inferior oblique muscles.

Light waves from an object, such as a tree, enter the Eye first through the cornea, which is the clear dome at the front of the Eye; it is like a window that allows light to enter the Eye; the light then progresses through the pupil, the circular opening in the center of the coloured iris; initially, the light waves are bent or converged first by the cornea and then further by the crystalline lens, located immediately behind the iris and the pupil, to a nodal point located immediately behind the back surface of the lens; at that point, the image becomes reversed, turned backwards and inverted,turned upside-down.


The light continues through the vitreous humor, the clear gel that makes up about 80% of the Eye’s volume and then, ideally, back to a clear focus on the retina, behind the vitreous; the small central area of the retina is the macula, which provides the best vision of any location in the retina; if the Eye is considered to be a type of camera, albeit, an extremely complex one, the retina is equivalent to the film inside of the camera, registering the tiny photons of light interacting with it.

Within the layers of the retina, light impulses are changed into electrical signals; then they are sent through the optic nerve, along the visual pathway, to the occipital cortex at the posterior of the brain; here, the electrical signals are interpreted, or seen, by the brain as a visual image, which means that we do not actually see with our Eyes but, rather with our brains; our Eyes are merely the beginning of the visual process.

Visual Acuity:

This is often referred to as the Snellen Acuity because the chart and letters are named after a 19th-century Dutch ophthalmologist Hermann Snellen (1834 to 1908), who created them as a test of visual acuity; a person's visual acuity is an indication of the clarity or clearness of their vision; it is a measurement of how well a person sees; the word 'Acuity' comes from the Latin 'Acuitas', which means sharpness.

20/20 or 6/6 Visual Acuity:

The reason why the number '20' is used in visual acuity measurements is because in the US, the standard length of an eye exam room, that is the distance from the patient to a Far Acuity Chart, is about 20 feet; and the reason why the number '6' is used is because in the UK meters are used instead of feet and a typical eye exam room is about 6 meters long; 6 meters is 19.685 feet, which is close to 20 feet and is usually considered to be close enough to optical infinity; therefore, instead of using 20/20 for normal vision, a notation of 6/6 is used in the UK.

When the room is not large enough, a machine is used to project a virtual chart onto a highly reflective screen; it is projected in such a way as to make the patient believe that the chart is the correct distance away; when this type of examination room is set up, care should be taken in calibrating the size of the letters on the virtual visual acuity chart, so that the letters look the same size as they would if an actual chart was set up 20 feet, or 6 metres, away.

The Size of the Letters:.

Someone with 20/20 or 6/6 vision should be just able to decipher a letter that subtends a visual angle of 5 minutes of arc, written 5', at the eye; 5' of arc is 5/60 of a degree, since there are 60' of arc in 1 degree; what this means is that if you draw a line from the top of a 20/20 letter to the eye and another line from the bottom of the letter to the eye, the size of the angle at the intersection of these two lines at the eye is 5' of arc.

Also the individual parts of the letter subtend a visual angle of 1' of arc at the eye; it does not matter how far away something is from the eye, if it subtends an angle of 5' of arc at the eye, then a person with 20/20 visual acuity will just be able to distinguish what it is.

A person with 20/20 vision could stand 30 feet away from a test chart and just decipher a 20/30 letter on the chart, since at that distance a 20/30 letter would subtend an angle of 5' of arc at the person’s eye; that same person could stand 80 feet away from the chart and be able to decipher a 20/80 letter, or 200 feet away to be able to decipher a 20/200 letter.

Someone with 20/20 visual acuity does not have perfect vision, since it is quite possible to see better than 20/20; the less the bottom number in the visual acuity ratio, the better the acuity and the greater the bottom number, the worse the acuity; therefore, 20/15 acuity is better than 20/20 acuity, and 20/30 acuity is worse than 20/20 acuity; 20/15 acuity is equivalent to 6/4.5 acuity, whilst 20/30 acuity is the same as 6/9 acuity.

Although 20/20 is classed as normal visual acuity for most people, it is possible and, in fact, very common to be able to see better than that; for instance, many people have 20/15 visual acuity; a person with 20/15 acuity can stand 20 feet away from an object and see it as well as another person with 20/20 acuity moving up to 15 feet away from the same object to view it.

You can use the same rationale when considering someone with less than 20/20 acuity; consider a person with 20/40 visual acuity, which is what someone needs in most states to acquire a driver’s license, if a person with 20/20 acuity can just read a sign which is 60 feet down the road, the person with 20/40 acuity would have to be 30 feet away to read the same sign and a person with 20/15 acuity would have to be 80 feet away and a person with 20/10 acuity would have to be 120 feet away, to read the same sign.

Near Visual Acuity:

A person’s visual acuity is not only tested at a far distance, but also at a near distance; this is done by holding a Nearpoint Snellen Acuity Card (NSAC) at 40 centimeters, about 16 inches away; though, some NSACs are calibrated for 35 centimeters, about 14 inches away; just as on a Far Acuity Chart, a 20/20 letter on a NSAC subtends a visual angle at the eye of 5' of arc; a NSAC has small paragraphs for the patient to read and each paragraph is progressively smaller in order to test how good your near distance vision is.

Without a lens correction, a Myopic 'Nearsighted' person will generally have better visual acuity at near than at far, whilst a Hyperopic 'Farsighted' person will generally have better acuity at far than at near; for many, until they reach the early to mid-40s, a person with 20/20 far acuity usually has 20/20 near acuity; however, once presbyopia sets in, the uncorrected near visual acuity decreases, creating the need for reading glasses or bifocals.



Optical Infinity:

When an eye is looking at a far away distance, such as at the horizon, or at the moon, or at a star, the rays of light entering the eye are virtually parallel and the crystalline lens of the eye is thin and relaxed because, essentially, there is zero accommodation; accommodation is the way that the eye increases its optical power, the degree to which the lens converges or diverges light; this is necessary to produce a clear image, a focus, on an object when it draws near the eye.

A lens that is more convex, fatter in the middle, would refract more light rays than a less convex lens, thinner lens; the lens can change shape because the cells of the lens contain an elastic crystalline protein; the young human eye can change focus from distance to seven centimeters from the eye in 350 milliseconds; the eye focuses on a given object by changing the shape of the eye lens through accommodation, which is controlled by ciliary muscle,which is attached to the lens; light from a single point of a distant object and light from a single point of a near object can be seen clearly when the curvature of the lens changes.


When an optometrist, or an ophthalmologist, examines and performs a refraction on someone’s eyes, it is optimal for the object being viewed, presumably using an acuity chart, to be as far away as possible from the patient; this is so that the incoming rays of light are as close to parallel as possible and the amount of accommodation of the crystalline lens of the eye will be negligible.


Understanding the GOS2 Patient's Optical Prescription Or Statement Form:

In many places in the UK, a patient is given a filled in GOS2 form on completion of their eye test, which should contain sufficient details in order to create the correct lensed spectacles for that patient; the only thing is most patients do not understand what all the details mean; if you would like a detailed explanation of the form then click on the link: The GOS2 Form


THE HUMAN EYE


Eye Terminology - In Clockwise Order Around the Image:

Visual Axis - The line of vision; a straight line joining the fovea of the Eye with the Eye’s fixation point.

Bulbar Conjunctiva - The clear mucous membrane that covers the sclera on the front of the Eyeball.

Cornea - The transparent, anterior, dome shaped portion of the Eyeball that covers the iris and pupil, acting like a window which admits light into the Eye.

Pupil - The contractile, usually round aperture in the iris of the Eye, which allows light to pass into the crystalline lens.

Iris - The opaque muscular contractile diaphragm that is suspended in the aqueous humor in front of the lens of the Eye; perforated by the pupil and continuous peripherally with the ciliary body; possesses a deeply pigmented posterior surface, which excludes the passage of light except through the pupil and a coloured anterior surface which determines the colour of the Eye.

Crystalline Lens - The biconvex transparent structure located immediately posterior to the iris of the Eye which changes shape, flattens and thickens, to focus the incoming light from objects far away and near; can develop a cloudy, even opaque cataract with age and a cumulative absorption of ultraviolet radiation.

Suspensory ligament of Lens - Also known as the zonules of Zinn, a ring of fibrous strands, composed mainly of elastin microfibrils, connecting the ciliary body with the crystalline lens of the Eye; it holds the lens in place, stretching and loosening, due to relaxation and contraction of the ciliary muscle, to change the shape of the lens to focus far and near 'Accommodation'.

Sclera - The dense fibrous opaque white outer coat enclosing the Eyeball, except the part covered by the cornea.

Medial Rectus Muscle An extraocular muscle in the orbit, originating in the annulus of Zinn; innervated by the oculomotor nerve 'The Cranial Nerve III'; it adducts the Eye.

Retinal Arteries - The arteries in the retina, which are salmon to red in colour and are about 1 1/3 to 2 times the diameter of the retinal veins.

Retinal Veins - The veins in the retina, which are salmon to orange in colour and are about 1/2 to 3/4 the diameter of the retinal arteries.

Dura Mater - The outer meningeal layer of the optic nerve; it fuses with the sclera where the optic nerve enters the Eye.

Optic Nerve - 'The Cranial Nerve II'; the sensory nerve which carries electrical impulses from visual stimuli in the retina out of the Eye, across the optic chiasm and to the ventral part of the diencephalon, on their way to the visual cortex in the occipital cortex of the brain for interpretation.

Optic Disc - The optic nerve head in the Eye, in which no photoreceptors are present, thus resulting in a blind spot in the visual field.

Macula Lutea - The small yellowish area, lying slightly lateral to the center of the retina, that constitutes the region of maximum visual acuity and is made up almost wholly of retinal cones.

Fovea Centralis - A small shallow depression or pit at the center of the macula, caused by an almost complete absence of inner retinal layers, containing only cones, no rods, and which affords the most acute vision.

Foveola - The center of the fovea.

Sclera - The dense fibrous opaque white outer coat enclosing the Eyeball, except the part covered by the cornea.

Choroid Coat - A vascular membrane containing large branched pigment cells that lies between the retina and the sclera of the Eye.

Retina - A layer of nervous tissue, covering the back 2/3 of the Eyeball, in which stimulation by light initiates an electrochemical reaction in which electrical impulses are transmitted to the brain, producing the sensation of vision; actually an extension of the brain, formed embryonically from brain tissue and connected to the brain proper by the optic nerve.

Vitreous Chamber - Also known as the Vitreous Humor is the transparent gelatinous mass occupying the posterior compartment,the space between the crystalline lens and the retina of the Eye, which is enclosed by a delicate hyaloid membrane; composed of water '99%', collagen fibrils, highly hydrated hyaluronic acid, halocytes, inorganic salts, sugar and ascorbic acid; produced by halocytes located peripherally in the vitreous body.

Lateral Rectus Muscle - An extraocular muscle in the orbit, originating in the annulus of Zinn; innervated by the abducens nerve 'The Cranial Nerve VI'; abducts the Eye.

Ora Seratta - The serrated junction between the retina and the ciliary body; marks the transition from the simple non-photosensitive area of the retina to the complex, multi-layered photosensitive region.

Ciliary Body - An annular,ring-like, structure on the inner surface of the anterior wall of the Eyeball, contained within the uveal tract and composed largely of the ciliary muscle and bearing the ciliary processes.

Canal of Schlemm - A circular canal lying in the substance of the sclero-corneal junction of the Eye and draining the aqueous humor from the anterior chamber into the veins draining the Eyeball.

Anterior Cavity - The space in the Eye bounded in front by the cornea and in back by the iris and middle part of the lens; contains the aqueous humor.

Anterior Chamber - This is the fluid-filled space inside the Eye between the iris and the cornea's innermost surface, a narrow space inside the Eye, which contains aqueous humor.

Posterior Chamber - This is the fluid-filled space immediately behind the peripheral part of iris but in front of the suspensory ligament of the lens, a narrow space inside the Eye, which contains aqueous humor.

Aqueous Humor - The transparent fluid occupying the anterior compartment, the space between the cornea and the crystalline lens, of the Eye; produced by ciliary epithelium and circulates into the posterior chamber, between the iris and the crystalline lens, through the pupil, into the anterior chamber, between the cornea and the iris, and out of the Eye through the trabecular meshwork and canal of Schlemm; it nourishes the lens and epithelial cells.


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