(Redirected from Eyes)
'Eyes' are
organs of
vision that detect
light. Different kinds of light-sensitive organs are found in a variety of
organisms. The simplest eyes do nothing but detect whether the surroundings are light or
dark, while more complex eyes can distinguish shapes and
colors. The visual fields of some such complex eyes largely overlap, to allow better
depth perception (
binocular vision), as in
humans; and others are placed so as to minimize the overlap, such as in
rabbits and
chameleons.
Varieties
In most
vertebrates and some
mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of
cells known as the
retina at the rear of the eye, where the light is detected and converted into electrical signals. These are then transmitted to the
brain via the
optic nerve. Such eyes are typically roughly spherical, filled with a
transparent gel-like substance called the
vitreous humour, with a focusing
lens and often an
iris which regulates the intensity of the light that enters the eye. The eyes of
cephalopods,
fish,
amphibians and
snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a
camera focuses.
Compound eyes are found among the
arthropods and are composed of many simple facets which give a pixelated image (not multiple images, as is often believed). Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360-degree field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many
Strepsiptera, have compound eyes composed of a few facets each, with a retina capable of creating an image, which does provide multiple-image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain, providing very wide-angle, high-resolution images.
Possessing detailed
hyperspectral color vision, the
Mantis shrimp has been reported to have the world's most complex color vision system.
[1] Trilobites, which are now extinct, had unique compound eyes. They used clear
calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.
Some of the simplest eyes, called
ocelli, can be found in animals like
snails, who cannot actually "see" in the normal sense. They do have
photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct
sunlight. Jumping spiders have simple eyes that are so large, supported by an array of other, smaller eyes, that they can get enough visual input to hunt and pounce on their prey. Some insect
larvae, like
caterpillars, have a different type of simple eye (
stemmata) which gives a rough image.
Evolution of eyes
Main articles: Evolution of the eye
Diagram of major stages in the eye's
evolution
The common origin (
monophyly) of all animal eyes is established by shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye evolved some 540 million years ago.
[2][3][4] The majority of the advancements in early eyes are believed to have taken only a few million years to develop, as the first predator to gain true imaging would have touched off an "arms race",
[5] or rather, a phylogenetic radiation from the species with that first proto-eye, among the descendents of which, there may well have been an "arms race". Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed in parallel.
Eyes in various animals show adaptation to their requirements. For example,
birds of prey have much greater visual acuity than humans, and some can see
ultraviolet light. The different forms of eyes in, for example,
vertebrates and
mollusks are often cited as examples of
parallel evolution, despite their distant common ancestry.
The earliest eyes, called "eyespots", were simple patches of
photoreceptor cells, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the lightsource.
[6] This gradually changed as the eyespot depressed into a shallow "cup" shape, granting the ability to slightly discriminate directional brightness by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective
pinhole camera that was capable of slightly distinguishing dim shapes.
[7]
The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialize into a transparent humour that optimized color filtering, blocked harmful radiation, improved the eye's
refractive index, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent
crystallin protein.
[8]
The gap between tissue layers naturally formed a biconvex shape, an ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the
cornea and
iris. Separation of the forward layer again forms a humour, the
aqueous humour. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes.
Anatomy of the mammalian eye
Three layers
The structure of the
mammalian eye can be divided into three main layers or ''tunics'' whose names reflect their basic functions: the
fibrous tunic, the
vascular tunic, and the
nervous tunic.
[9][10][11]
★ The fibrous tunic, also known as the ''tunica fibrosa oculi'', is the outer layer of the eyeball consisting of the
cornea and
sclera.
[12] The sclera gives the eye most of its white color. It consists of dense
connective tissue filled with the protein
collagen to both protect the inner components of the eye and maintain its shape.
[13]
★ The vascular tunic, also known as the ''tunica vasculosa oculi'', is the middle vascularized layer which includes the
iris,
ciliary body, and
choroid.
The choroid contains
blood vessels that supply the retinal cells with necessary
oxygen and remove the waste products of
respiration. The choroid gives the inner eye a dark color, which prevents disruptive reflections within the eye.
★ The nervous tunic, also known as the ''tunica nervosa oculi'', is the inner sensory which includes the
retina.
The retina contains the photosensitive
rod and
cone cells and associated neurons. To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It does have two points at which it is different; the
fovea and
optic disc. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for
color vision in humans, and enables high acuity, such as is necessary in
reading. The optic disc, sometimes referred to as the anatomical
blind spot, is a point on the
retina where the optic nerve pierces the retina to connect to the nerve cells on its inside. No photosensitive cells whatsoever exist at this point, it is thus "blind". Squids and Octupi don't have this blind spot, however.
Anterior and posterior segments
The mammalian eye can also be divided into two main segments: the
anterior segment and the
posterior segment.
[16]
Anterior segment
The anterior segment is the front third of the eye that includes the structures in front of the
vitreous humour: the
cornea,
iris,
ciliary body, and
lens.
Within the anterior segment are two fluid-filled spaces: the
anterior chamber and the
posterior chamber. The anterior chamber is the space between the posterior surface of the cornea (i.e. the
corneal endothelium) and the iris, whereas the posterior chamber is between the iris and the front face of the vitreous.
The cornea and lens help to
converge light rays to focus onto the retina. The lens, behind the iris, is a , springy disk which focuses light, through the second humour, onto the
retina. It is attached to the
ciliary body via a ring of
suspensory ligaments known as the
Zonule of Zinn. To clearly see an object far away, the
ciliary muscle is relaxed, which stretches the fibers connecting it with the lens, flattening the lens. When the ciliary muscle contracts, the tension of the fibers decrease (imagine that the distance between the tip of a triangle to its base, is less than the tip of the triangle to the other two tips.) which lets the lens bounce back a more convex and round shape. Humans gradually lose this flexibility with age, resulting in the inability to focus on nearby objects, which is known as
presbyopia. There are other
refraction errors arising from the shape of the cornea and lens, and from the length of the eyeball. These include
myopia,
hyperopia, and
astigmatism. The iris, between the lens and the first humour, is a pigmented ring of fibrovascular tissue and muscle fibres. Light must first pass though the centre of the iris, the
pupil. The size of the pupil is actively adjusted by the circular and radial muscles to maintain a relatively constant level of light entering the eye. Too much light being let in could damage the retina; too little light makes sight difficult.
All of the individual components through which light travels within the eye before reaching the retina are transparent, minimising dimming of the light. Light enters the eye from an external medium such as air or water, passes through the cornea, and into the first of two humours, the
aqueous humour. Most of the light refraction occurs at the cornea which has a fixed curvature. The first humour is a clear mass which connects the cornea with the lens of the eye, helps maintain the convex shape of the cornea (necessary to the
convergence of light at the lens) and provides the
corneal endothelium with nutrients.
Posterior segment
Diagram of a human eye; note that not all eyes have the same anatomy as a human eye.
The posterior segment is the back two-thirds of the eye that includes the
anterior hyaloid membrane and all structures behind it: the
vitreous humor,
retina,
choroid, and
optic nerve.
[18]
On the other side of the lens is the second humour, the
vitreous humour, which is bounded on all sides: by the
lens,
ciliary body, suspensory ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. In some animals, the retina contains a reflective layer (the
tapetum lucidum) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions.
Light from a single point of a distant object and light from a single point of a near object being brought to a focus
Extraocular anatomy
In many species, the eyes are inset in the portion of the skull known as the
orbits or eyesockets. This placement of the eyes helps to protect them from injury.
In humans, the
eyebrows redirect flowing substances (such as rainwater or sweat) away from the eye. Water in the eye can alter the refractive properties of the eye and blur vision. It can also wash away the tear fluid—along with it the protective lipid layer—and can alter corneal physiology, due to
osmotic differences between tear fluid and freshwater. This is made apparent when swimming in freshwater pools, as the osmotic gradient draws "pool water" into the corneal tissue (the pool water is
hypotonic), causing
edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. It can be reversed by irrigating the eye with
hypertonic saline which osmotically draws the excess water out of the eye.
In many animals, including humans,
eyelids wipe the eye and prevent dehydration. They spread
tears on the eyes, which contains substances which help fight
bacterial infection as part of the
immune system.
Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above and below water. Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye, or a bright light) by covering the eyes, and/or by turning the eyes away from the threat.
Blinking the eyes is, of course, also a
reflex.
In many animals, including humans,
eyelashes prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to tears and subsequent blurred vision.
Other articles regarding eye anatomy
Annulus of Zinn,
Conjunctiva,
Macula,
Nictitating membrane,
Schlemm's canal,
Trabecular meshwork.
Cytology
This image clearly shows the
pupil and
iris of the human eye.
The structure of the mammalian eye owes itself completely to the task of focusing
light onto the
retina. This light causes
chemical changes in the
photosensitive cells of the retina, the products of which trigger
nerve impulses which travel to the brain.
The retina contains two forms of photosensitive cells important to vision—
rods and
cones. Though structurally and metabolically similar, their function is quite different. Rod cells are highly sensitive to light allowing them to respond in dim light and dark conditions, however, they cannot detect color. These are the cells which allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). This is why the darker conditions become, the less color objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different
wavelengths of light, which allows an organism to see color.
The differences are useful; apart from enabling sight in both dim and light conditions, humans have given them further application. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. This gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires looking at things. Its requirement for high intensity light does cause problems for
astronomers, as they cannot see dim stars, or other objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" (
averted vision) where rods also exist, and where the light ''is'' sufficient to stimulate cells, allowing the individual to observe distant stars.
Rods and cones are both photosensitive, but respond differently to different frequencies of light. They both contain different pigmented
photoreceptor proteins. Rod cells contain the protein
rhodopsin and cone cells contain different proteins for each color-range. The process through which these proteins go is quite similar—upon being subjected to
electromagnetic radiation of a particular wavelength and intensity, the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into
opsin and
retinal; iodopsin of cones breaks down into
photopsin and retinal. The opsin in both opens
ion channels on the
cell membrane which leads to
hyperpolarization, this hyperpolarization of the cell leads to a release of
transmitter molecules at the
synapse.
This is the reason why cones and rods enable organisms to see in dark and light conditions—each of the photoreceptor proteins requires a different light intensity to break down into the constituent products. Further,
synaptic convergence means that several rod cells are connected to a single
bipolar cell, which then connects to a single
ganglion cell by which information is relayed to the
visual cortex. This is in direct contrast to the situation with cones, where each cone cell is connected to a single bipolar cell. This results in the high visual acuity, or the high ability to distinguish between detail, of cone cells and not rods. If a ray of light were to reach just one rod cell this may not be enough to hyperpolarize the connected bipolar cell. But because several "converge" onto a bipolar cell, enough
transmitter molecules reach the
synapse of the bipolar cell to hyperpolarize it.
Furthermore, color is distinguishable when breaking down the iodopsin of cone cells because there are three forms of this protein. One form is broken down by the particular EM wavelength that is red light, another green light, and lastly blue light. In simple terms, this allows human beings to see
red, green and blue light. If all three forms of cones are stimulated equally, then white is seen. If none are stimulated, black is seen. Most of the time however, the three forms are stimulated to different extents—resulting in different colors being seen. If, for example, the red and green cones are stimulated to the same extent, and no blue cones are stimulated,
yellow is seen. For this reason red, green and blue are called
primary colors and the colors obtained by mixing two of them,
secondary colors. The secondary colors can be further complemented with primary colors to see
tertiary colors.
Acuity
Main articles: Visual acuity
Visual acuity can be measured with several different metrics.
Cycles per
degree (CPD) measures how much an eye can differentiate one object from another in terms of degree angles. It is essentially no different from
angular resolution. To measure CPD, first draw a series of black and white lines of equal width on a grid (similar to a bar code). Next, place the observer at a distance such that the sides of the grid appear one degree apart. If the grid is 1 meter away, then the grid should be about 8.7 millimeters wide. Finally, increase the number of lines and decrease the width of each line until the grid appears as a solid grey block. In one degree, a human would not be able to distinguish more than about 12 lines without the lines blurring together. So a human can resolve distances of about 0.93 millimeters at a distance of one meter. A horse can resolve about 17 CPD (0.66 mm at 1 m) and a rat can resolve about 1 CPD (8.7 mm at 1 m).
A
diopter is the unit of measure of
optical power.
Spectral response
Human eyes respond to light with wavelength in the range of approximately 400 to 700 nm. Other animals have other ranges, with many such as birds including a significant
ultraviolet (shorter than 400 nm) response.
Dynamic range
The retina has a static
contrast ratio of around 100:1 (about 6 1/2
stops). As soon as the eye moves (
saccades) it re-adjusts its exposure both chemically and by adjusting the iris. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the
Purkinje effect) are mostly complete in thirty minutes. Hence, a dynamic
contrast ratio of about 1,000,000:1 (about 20
stops) is possible. The process is nonlinear and multifaceted, so an interruption by light nearly starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.
Equivalent resolution
The maximum resolution of the human eye in good light is approximately 1.6 minute of arc per line pair, or 1.25 line per minute of arc.
[19] Assuming two pixels per line pair (one pixel per line) and a square field of 120 degrees, this would be equivalent to approximately 120×60×1.25 = 9000 pixels in each of the X and Y dimensions, or about 81
megapixels.
However, the human eye itself has only a small spot of sharp vision in the middle of the retina, the
fovea centralis, the rest of the field of view being progressively lower resolution as it gets further from the fovea. The angle of the sharp vision being just a few degrees in the middle of the view, the sharp area thus barely achieves even a single megapixel resolution. The experience of wide sharp human vision is in fact based on turning the eyes towards the current point of interest in the field of view, the brain thus perceiving an observation of a wide sharp field of view.
The narrow beam of sharp vision is easy to test by putting a fingertip on a newspaper and trying to read the text while staring at the fingertip — it is very difficult to read text that's just a few centimeters away from the fingertip.
Eye movement
MRI scan of human eye
The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second (Westheimer and McKee, 1954). Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities.
Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.
Extraocular muscles
Main articles: Extraocular muscles
Each eye has six
muscles that control its movements: the
lateral rectus, the
medial rectus, the
inferior rectus, the
superior rectus, the
inferior oblique, and the
superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn. This is an almost pure rotation, with only about one millimeter of translation.
[20] Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye. Once the human eye sustains damage to the optic nerve, the impulses will not be taken to the brain. Eye transplants can happen but the person receiving the transplant will not be able to see. As for the optic nerve, once it is damaged it cannot be fixed.
Rapid eye movement
Main articles: Rapid eye movement
Rapid eye movement, or REM for short, typically refers to the stage during
sleep during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.
Saccades
Main articles: Saccade
Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain.
Microsaccades
Main articles: Microsaccade
Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.
Vestibulo-ocular reflex
Main articles: Vestibulo-ocular reflex
The
vestibulo-ocular reflex is a
reflex eye movement that stabilizes images on the
retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa.
Smooth pursuit movement
Main articles: Pursuit movement
The eyes can also follow a moving object around. This is less accurate than the vestibulo-ocular reflex as it requires the brain to process incoming visual information and supply
feedback. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.
It is more difficult to visually estimate speed in low light conditions or while moving, unless there is another point of reference for determining speed.
Optokinetic reflex
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a short moment (through smooth pursuit), until the tree moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the tree (through a saccade).
Vergence movement
Main articles: Vergence
The two eyes converge to point to the same object.
When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate 'towards each other' (
convergence), while for an object farther away they rotate 'away from each other' (
divergence). Exaggerated convergence is called ''cross eyed viewing'' (focusing on the nose for example) . When looking into the distance, or when 'staring into nothingness', the eyes neither converge nor diverge.
Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation.
Accommodation
Main articles: Accommodation (eye)
To see clearly, the lens will be pulled flatter or allowed to regain its thicker form.
Diseases, disorders, and age-related changes
Main articles: List of eye diseases and disorders,
List of systemic diseases with ocular manifestations
The
stye is a common irritating inflammation of the eyelid.
There are many diseases, disorders, and age-related changes that may affect the eyes and surrounding structures.
As the eye ages certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of aging eye diseases. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability (
presbyopia). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates also decreases with age. Because of the smaller pupil size, older eyes receive much less light at the retina. In comparison to younger people, it is as though older persons wear medium-density sunglasses in bright light and extremely dark glasses in dim light. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting. Certain ocular diseases can come from sexually transmitted diseases such as herpes and genital warts. If contact between eye and area of infection occurs, the STD will be transmitted to the eye.
[21]
With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as
ectropion,
entropion,
dermatochalasis, and
ptosis. The vitreous gel undergoes liquefaction (
posterior vitreous detachment or PVD) and its opacities—visible as
floaters—gradually increase in number.
Various
eye care professionals, including
ophthalmologists,
optometrists, and
opticians, are involved in the treatment and management of ocular and vision disorders. A
Snellen chart is one type of
eye chart used to measure
visual acuity. At the conclusion of an
eye examination, an eye doctor may provide the patient with an
eyeglass prescription for
corrective lenses
Cuisine
In many countries, stuffed
cow's eyes are considered a
delicacy. They are made by first removing the vitreous humor, lens, cornea, and iris, then are usually boiled. Cow eyes are often stuffed with varieties of
coleslaw,
beef, and even
cream cheese.
Seal eyes are a delicacy for the
Inuit.
See also
★
Eye exam
★
Infant vision
References
1. http://www.nwf.org/nationalwildlife/article.cfm?issueID=77&articleID=1114
2. Halder, G., Callaerts, P. and Gehring, W.J. (1995). "New perspectives on eye evolution." ''Curr. Opin. Genet. Dev.'' 5 (pp. 602–609).
3. Halder, G., Callaerts, P. and Gehring, W.J. (1995). "Induction of ectopic eyes by targeted expression of the ''eyeless'' gene in ''Drosophila''". ''Science'' 267 (pp. 1788–1792).
4. Tomarev, S.I., Callaerts, P., Kos, L., Zinovieva, R., Halder, G., Gehring, W., and Piatigorsky, J. (1997). "Squid ''Pax-6'' and eye development." Proc. Natl. Acad. Sci. USA, 94 (pp. 2421–2426).
5. Conway-Morris, S. (1998). ''The Crucible of Creation''. Oxford: Oxford University Press.
6. Land, M.F. and Fernald, Russell D. (1992). "The evolution of eyes." ''Annu Rev Neurosci'' 15 (pp. 1–29).
7. Eye-Evolution?
8. Fernald, Russell D. (2001). The Evolution of Eyes: Where Do Lenses Come From? ''Karger Gazette'' 64: "The Eye in Focus".
9. "The Eye." Accessed October 23, 2006.
10. "General Anatomy of the Eye." Accessed October 23, 2006.
11. "Eye Anatomy and Function." Accessed October 23, 2006.
12. Cline D; Hofstetter HW; Griffin JR. ''Dictionary of Visual Science''. 4th ed. Butterworth-Heinemann, Boston 1997. ISBN 0-7506-9895-0
13. http://www.bartleby.com/107/225.html
14. Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainsville, Florida: Triad Publishing Company, 1990.
15. "Medline Encyclopedia: Eye." Accessed October 25, 2006.
16. http://www.e-sunbear.com/anatomy_02.html
17. "Departments. Anterior segment."
Cantabrian Institute of Ophthalmology.
18. Posterior segment anatomy
19. Optical System Design, , , , McGraw-Hill Professional, 2000,
20. Roger H.S. Carpenter (1988); ''Movements of the Eyes (2nd ed.)''. Pion Ltd, London. ISBN 0-85086-109-8.
21. AgingEye Times
★
Anatomy
★
Kandel ER, Schwartz JH, Jessell TM. ''
Principles of Neural Science'', 4th ed. McGraw-Hill, New York (2000). ISBN 0-8385-7701-6
External links
★
DJO | Digital Journal of Ophthalmology
★
Glossary of Eye Conditions
★
Evolution of the Eye
★
Diagram of the eye
★
Webvision. The organisation of the retina and visual system.
★
VisionSimulations.com | Images and vision simulators of various diseases and conditions of the eye
★
Eyes and computers.
★
Eyeatlas online (ophthalmological images) by Umberto Benelli, MD, PhD
★
ClarkVision's estimation of the resolution of the eye
★
Video: Vision and How Our Eyes Work
★
Summary of eye diseases and disorders
★
Your Baby's Eyes.
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