An eye is a part of the body specialized for catching light and translating it into nerve impulses. Defined in such basic terms, eyes have been around for a very long time. The fossils of primitive arthropods (ancestors of insects, crustaceans and spiders), 530 million years old, show clear signs of eyes, and the first eye-like organs probably date back to the very beginning of multicellular life, 600 million years ago.
In the depths of the ocean, in underground rivers, inside the bodies of other animals, there are creatures without eyes. And organisms such as corals and sea anemones, which simply stay still and grow, have no need of eyes. But wherever there is light and a reason to move around, animals have eyes.
Scrutiny of the fossil record, and of the diversity of extant animals, suggests that eyes have been ‘invented’ by natural selection at least 40 times during the evolution of life on earth. ‘Re-invented’ might be more accurate, because there is genetic evidence that there may have been a single origin for all eyes. A gene called Pax-6, which has a very similar, ‘conserved’ DNA sequence in a vast range of animals, from fruit flies to human beings, appears to determine when and where eyes form during development.
Light, the diet of eyes, constitutes a tiny part of the entire spectrum of electromagnetic radiation. The various forms of such radiation (ranging from that associated with mains electricity through to X-rays and gamma rays) differ in the frequency at which their electrical and magnetic fields oscillate, and therefore the distance travelled during one complete cycle of oscillation — the wavelength.
The sun is a powerful source of electromagnetic energy. The sun's rays that reach the surface of the earth belong mainly to the so-called optical band. This extends from about 1 mm to 100 nanometres in wavelength (a nanometre is one billionth of a metre). The longer wavelengths constitute infrared radiation, which we feel as warmth, and the shortest are ultraviolet, which can be damaging to the skin and to the eyes. Visible radiation — true light — lies in the middle of the range, extending from about 400 to 750 nanometres. All our visual experience rests on the detection of this narrow band of wavelengths.
Sources of light (the sun and other visible stars, lamps, camera flashbulbs, etc.) appear bright because the radiation that they emit enters the eye directly. But most of the light that reaches our eyes has been reflected from the surfaces of objects around us: that is how we see those objects. The brightness and colours of surfaces are determined by the amount of light they reflect and the wavelength composition of that reflected light.
These facts now seem self-evident, but the whole process was deeply mysterious to the ancient Greeks. Plato imagined that some sort of ‘spirit’ streamed out from the eyes to palpate the world. Aristotle, ever contrary, suggested that light flowed into the eyes. Isaac Newton's famous experiments with prisms demonstrated that white sunlight is composed of a rainbow of different sorts of light, appearing to be of different colours, from violet, through blue, green and yellow to red. Newton concluded that these forms of light vibrate at different frequencies. Light of short wavelength appears blue, the longest visible wavelengths appear red.
The most essential feature of a true eye is a mechanism for catching the energy of light and using it to trigger a chemical reaction. This is achieved in all eyes by substances called photopigments. Every photopigment consists of a derivative of vitamin A (retinal) linked to a protein molecule called an opsin. Here again is evidence of the antiquity of seeing: opsins are similar in structure and are encoded by very similar, conserved genes throughout the animal world. Even certain bacteria have photopigments in their membranes. The capacity to catch light was one of the first tricks discovered in the story of life.
In a famous passage in The Origin of Species, Charles Darwin played Devil's (or perhaps God's?) Advocate:
To suppose that the eye, with all its inimitable contrivances … could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.But he went on to argue:
Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor … then the difficulty of believing that a perfect and complex eye could be formed by natural selection … should not be considered as subversive of the theory.
There is indeed evidence for numerous, useful gradations of eye. The earliest eye-like organs, still found in present-day limpets, probably consisted of depressions lined with cells containing photopigment, which were connected to the nervous system. Such photosensitive pit organs can detect the presence of light and even give crude information about its direction (enabling the organism to turn towards or away from the light — phototropism). But proper eyes have something more — a system to focus the light to form an image on the array of photoreceptive cells. Like telescopes, microscopes and cameras, almost all eyes form images with lenses or mirrors. Humans, indeed all seeing vertebrates, have so-called simple eyes: they have a single lens system, focusing light to form one, continuous image of the outside world. But all sighted insects have compound eyes, consisting of a mosaic of tube-shaped optical lens systems, with photoreceptors at the base of each tube. Scallops have eyes that form images with reflective mirrors. And the primitive mollusc Nautilus has an eye with only a pinhole at the front, forming a crude image on the array of photoreceptors inside.
The eyes of cephalopods (squid and octopus) are uncannily like human eyes. They too are simple eyes, with muscles to make them move, and a pigmented iris. They have an internal lens and they form an image on a light-sensitive retina. But there the similarity ends. There are differences so fundamental in the anatomy of the retina that it is generally believed that cephalopod eyes arose independently of vertebrate eyes. The striking similarity of these unrelated organs is a dramatic example of convergent evolution — the separate emergence of similar structures to solve the same problem.
The adult human eye is roughly spherical, about 24 mm in diameter, with a transparent bulge — the cornea — at the front, around which the white of the eye — the sclera — is visible. The cornea has no blood supply; hence transplanted corneas (replacing damaged, opaque originals) are rarely rejected. The cornea provides two-thirds of the focusing power of the eye. Much of that power is lost, hence causing blurred vision, when the eye is immersed in water. Immediately behind the cornea is the anterior chamber, filled with fluid called aqueous humour. The continuous production of this fluid by epithelial cells generates a hydrostatic pressure inside the eye (normally about 15 mm of mercury). The fluid percolates out of the eye through an epithelial meshwork inside the rim of the cornea, enters the Canal of Schlemm, which runs beneath the surface of the eye, around the edge of the cornea, and drains into the veins of the eye. The potentially blinding condition of glaucoma, which is an elevation of intraocular pressure above about 25 mm of mercury, is usually caused by structural defects in the anterior chamber associated with extreme short-sight, or degenerative changes in old age.
Visible through the cornea of the eye is the pigmented epithelium of the iris (which gives the eye its characteristic, genetically determined, colour). Contraction of smooth muscle within the iris changes the diameter of the hole in the middle — the pupil. When the circular muscle around the edge of the aperture of the iris contracts, the pupil constricts. This muscle is controlled by parasympathetic nerves, coming from the midbrain. There are also radial muscle fibres in the iris, innervated by sympathetic nerves, which make the pupil enlarge or dilate.
The main function of the pupil is to optimize the quality of the retinal image and the retinal illumination. The eye is not a perfect optical system. It has many sources of error, which can reduce the sharpness of the image. The effects of spherical and chromatic aberration increase as the pupil expands. The blurring effects of diffraction (due to interaction of the wavefront of light with the pupil) become worse as the aperture decreases, as in all lens systems. In bright light, the pupil is usually just over 2 mm in diameter — providing an optimal compromise between diffraction and the aberrations. However, in very dim conditions, the pupil dilates, sacrificing image quality in order to catch enough light to sustain vision.
The pupil also serves as a signal to others. Generalized activation of the sympathetic system (the ‘fight or flight’ reaction) causes the pupil to dilate — a clear sign of anger or fear. Activation of the sympathetic nerves to the pupil also occurs in sexual arousal, and the sight of dilated pupils can itself be arousing. If people are asked to rank the attractiveness of photographs of the opposite sex with small pupils or large, they generally prefer the latter. Indeed, Belladonna (‘beautiful woman’), an extract of deadly nightshade containing the drug atropine, which blocks the action of parasympathetic nerves and hence dilates the pupil, used to be applied to the eyes as a cosmetic.
Immediately behind the pupil is the crystalline lens — a transparent protein gel in an elastic sac, which provides the additional optical power needed to bring light to a focus at the back of the eye. The part of the eyeball behind the lens is filled with a jelly called vitreous humour. The margin of the lens is attached to the ciliary body, which, like the iris, contains a circular muscle innervated by parasympathetic nerves. When this muscle contracts, the tension on the lens decreases and it becomes more spherical, and hence more optically powerful, focusing the eye on closer objects — a process called accommodation.
An ‘ideal’ (emmetropic) eye is exactly focused on the extreme distance when its ciliary muscle is completely relaxed, so that the entire range of additional power provided by accommodation is available to focus closer objects. During the first few years of life, the growth of the eyeball is normally regulated, in some uncertain way, according to the quality of the retinal image, so as to tend to make the eye emmetropic. Eyes that are not optimized in this way are said to have refractive errors (long sight or short sight).
The thin shell of the eyeball is made up of three layers or tunics: the outer sclera, consisting of strong protein fibres; the choroid, a spongy layer filled with blood vessels; and the retina or nervous layer. The retina forms as an outgrowth of the embryonic brain, and is strictly part of the central nervous system. It consists of alternating layers of nerve cell bodies and their processes and connections. The photoreceptors lie at the back of the retina, so light has to pass through all the other layers of transparent neurons to reach the outer segments of the receptors, containing membranous discs, packed with photopigment. The tips of the receptors are in contact with the pigment epithelium, which contains enzymes that regenerate photopigment that has been ‘bleached’ by light. Retinal detachment occurs when a gap opens between receptors and pigment epithelium.
There are two kinds of photoreceptor. The 120 million, thin rods, which operate in dim conditions, contain a photopigment called rhodopsin, which is most sensitive to short wavelength (blue) light. Each of the 6 million or so cones, which are more conical in shape, contains one of three pigments, absorbing maximally in the blue, yellow-green and red parts of the spectrum. The perceived colour of a light (whether a pure, monochromatic light or a mixture of wavelengths) depends on the relative stimulation of these three classes of cone receptor. There is a very dense concentration of slender cones in a dimple, called the fovea, in the centre of the retina. Shifting gaze towards something involves rotating the eyeball to bring the image of the object of interest on to this special region. It has very high visual acuity (the capacity to resolve fine patterns of light and dark, as in reading) and excellent colour vision.
The electrical potential inside an individual rod changes detectably when one of its millions of rhodopsin molecules absorbs a single photon or quantum of light (the smallest, indivisible unit of energy). This exquisite sensitivity is achieved by a ‘cascade’ of four chemical reactions, ‘amplifying’ the response. The final step is the constriction of ion channels that normally allow positive sodium ions to leak into the cell. Hence, light makes the voltage inside the photoreceptor more negative, which reduces the release of transmitter substance at synaptic connections at the foot of the receptor. Photoreceptors cannot produce nerve impulses: indeed, out of the five basic types of nerve cells in the retina, only those in the innermost layer, the ganglion cells, reliably produce full-blown impulses. The roughly 1.5 million ganglion cells have long axons, which run across the inner surface of the retina towards a round patch called the optic disc. Here the fibres form a bundle and plunge back through a hole in the sclera to form the optic nerve. This nerve connects the eye to the brain, which learns of the world through the chatter of impulses reaching it from the eyes.
The retina is not merely a passive device, like the film in a camera. Its complex network of nerve cells compresses the information flooding into the eye, and detects important features in the visual image. Most of our knowledge of retinal function has come from the study of anaesthetized animals, in which electrical responses can be recorded from individual nerve cells, while images are presented to the eye. Most ganglion cells (in cats and monkeys) do not respond well to overall illumination of the retina, but to a local difference in brightness (i.e. contrast) between their particular region of the retina and the surrounding area. Half respond to local brightening, half to local darkening. In monkeys, many of the ganglion cells are also ‘colour-selective’, responding best to one colour of light.
The eyes rest snugly in slippery cups of fat, inside the orbits — cavities in the skull on each side of the nose. Six thin, flat muscles, stretching from the back of the orbit to various parts of the eyeball, enable it to rotate extremely quickly (up to hundreds of degrees per second) around any axis perpendicular to the direction of gaze. Some torsional rotation around the axis of gaze is also possible. The precious front surface of the eye is well protected against blows and flying objects, by the bony brow ridges, the hairs of the eyebrows, the eyelids and their lashes. A delicate translucent epithelium — the conjunctiva — covers the visible part of the sclera, folds back under the lids and attaches all around the inner margins, thus sealing off the contents of the orbit.
Lacrimal glands, embedded in the upper eyelids and controlled by parasympathetic nerves, secrete watery tear fluid, which flows through tiny ducts on to the surface of the cornea. The fluid is distributed over the cornea by blinking — rapid closing of the eyelids, combined with upward rotation of the eyeball, which normally occurs spontaneously about 10 times a minute. This continuous bathing maintains the smoothness and transparency of the cornea, which are essential to its function. When the process is compromised, the painful condition of dry eye occurs. Tear fluid normally collects in the inner corner of the eye and drains down into the nose through a tube called the nasolacrimal duct, but can tumble over the lower lid and run down the cheeks when secretion increases, e.g. in windy conditions, or when the eye is irritated. Crying or weeping occurs as a result of excessive secretion of tears.
Is it not remarkable that such mundane, though essential, physiological functions as lubrication of the cornea, enlargement of the pupil, fluttering of the eyelids, and movement of gaze have been ‘captured’ as potent, normally unconscious, social signals of extreme emotion. Metaphorically, eyes can dance, twinkle, drink (to me only!), even kill, as well as beginning the remarkable process of vision.
Further reading: Oyster, C. W. (1999). The Human Eye: Structure and Function.: Sinauer Associates, Sunderland, Mass.
Gregory, R. L. (1998). Eye and Brain: the Psychology of Seeing, 5th edition. Oxford University Press, Oxford.
See also accommodation; autonomic nervous system; blindness; blind spot; colour blindness; eye movements; gaze; nystagmus; radiation, non-ionizing; refractive errors; sensory receptors; squint; vision; weeping.
"eyes." The Oxford Companion to the Body. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/eyes
"eyes." The Oxford Companion to the Body. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/eyes
See also 14. ANATOMY ; 48. BLINDNESS ; 51. BODY, HUMAN ; 149. FACIAL FEATURES .
- achromatopsy, achromatopsia
- color blindness. Also called acritochromacy .
- a form of color blindness characterized by the inability to see blue.
- aniseikonia, anisoconia
- a defect of the eyesight in which the images on the retinas are different in size. —aniseikonic , adj.
- a defect of the eyesight in which each eye has a different power to refract light. Cf. isometropia . —anisometropic , adj.
- a defect in a lens, eye, or mirror that causes rays from one direction not to focus at one point. —astigmatic , adj.
- twitching of the eyelids.
- soreness or inflammation of the eyelids.
- Pathology. a drooping of the upper eyelid.
- an eyewash or other liquid preparation for the eyes. See also 350. REMEDIES .
- inflammation of the conjunctiva.
- red-green color blindness.
- a defect of the eyesight in which the retina does not respond to green. —deuteranope , n. —deuteranopic , adj.
- a form of color blindness in which the sufferer can perceive only two of the three primary colors.
- an instrument for measuring the refractive index of the lens of the eye.
- color blindness.
- emmetropia, emmetropy
- the normal refractive function of the eye in which light is focused exactly on the retina with the eye relaxed. —emmetropic , adj.
- a condition of the eyes in which while one eye focuses on the object viewed the other eye turns inward; cross-eye.
- a disease of the eyes, in which the pressure inside the eyeball increases, often resulting in blindness. —glaucomatous , adj.
- a condition of the eyes in which the sufferer can see clearly at night but has impaired vision during the day; day blindness.
- the condition of farsightedness. Also called hyperopia . —hypermetropic , adj.
- hypermetropia. —hyperopic , adj.
- Surgery. the making of an artificial pupil in the eye by transverse division of iris fibers.
- the state or quality of the eyes being equal in refraction. Cf. anisometropia.
- an inflamed condition of the cornea.
- the surgical process of corneal grafting.
- the process of surgical incision of the cornea.
- lacrymatory, lachrimatory
- a lacrymal vase or small vessel for storing shed tears.
- Iagophthalmia, lagophthalmus
- a persistent, abnormal retraction of the eyelid so that the eyeball is not covered during sleep. —lagophthalmic , adj.
- an instrument for testing the eyes to determine the ability to distinguish variations in color or intensity of light.
- the development of leucoma, a whitish clouding of the cornea caused by ulceration.
- soreness of the eyes; a bleary-eyed condition.
- study or examination of an object with the naked eye as contrasted with examination under the microscope.
- a defect of the eyesight in which what is viewed is greatly magnified.
- darkness or blackness of eyes, hair, or complexion.
- miosis, myosis
- abnormal constriction of the pupil of the eye, caused by drugs or illness. Cf. mydriasis . —miotic, myotic , adj.
- monoblepsia, monoblepsis
- a defect of the eyesight in which vision is best when only one eye is open.
- a defect in which the retina cannot perceive color.
- abnormal dilatation of the pupil, the result of disease or the use of certain drugs. Cf. miosis . —mydriatic , adj.
- the condition of nearsightedness. —myopic , adj.
- miosis. —myotic , adj.
- the ability, sometimes pretended, to sight ships or land at great distances.
- nictitation, nictation
- the process of winking or blinking rapidly, as in certain birds or animals or as the result of a tic in humans.
- a condition of the eyes in which the sufferer can see clearly during the day or in bright light but has impaired vision at night or in poor light; night blindness.
- uncontrollable and rapid movement of the eyeball in any direction. —nystagmic , adj.
- a physician who specializes in ophthalmology.
- an abnormal fear of eyes.
- the branch of medical science that studies the eyes, their diseases and defects. —ophthalmologist , n. —ophthalmologic, ophthalmological , adj.
- a person who makes and sells glasses according to prescriptions prepared by an oculist or optometrist.
- an image on the retina caused by bleaching of the pupils.
- the act or practice of reproducing optograms.
- Archaic. the testing of the eyes for lenses.
- the practice or profession of testing eyes for defects in vision and the prescribing of corrective glasses. —optometrist , n. —optometrical , adj.
- type used in the testing of eyesight.
- the art of treating visual defects by exercise and retraining in Visual habits. —orthoptist , n. —orthoptic , adj.
- oxyopia, oxyopy
- an extremely heightened acuteness of the eyesight, resulting from increased sensibility of the retina.
- phantasmascope, phantascope
- an optical device that enables the viewer to converge the optical axes of the eyes and experience some of the phenomena of binocular vision.
- pain in the eyes caused by light.
- an abnormal fear of photalgia.
- vision, or the ability to see in bright light. Cf. scotopia . —photopic , adj.
- polyopia, polyopsia, polyopsy, polyopy
- multiple vision; the seeing of one object as more than one.
- a form of farsightedness that occurs in old age. Also called presbyopia, presbytia . Cf. hypermetropia . —presbytic , adj.
- a defect of the eyesight in which the retina does not respond to red. —protanope , n. —protanopic , adj.
- a method of determining the refractive error of an eye using an ophthalmoscope to illuminate the retina through the lens of the eye. Also called skiascopy . —retinoscopist , n.
- vision in dim light or darkness. Cf. photopia . —scotopic , adj.
- the inability of both eyes to focus on one object thereby producing the effect of squinting or cross-eyes. Also called strabismus . —strabismal, strabismic , adj.
- a diseased condition characterized by adhesion, especially the adhesion of the iris to the cornea.
- a contagious form of conjunctivitis, with the formation of inflammatory granules on the inner surface of the eyelid. —trachomatous , adj.
- a condition in which the hair, especially of the eyelashes, grows inward.
- a defect of the eyesight in which the retina does not respond to blue and yellow. —tritanope , n. —tritanopic , adj.
- an inflamed condition of the uvea. —uveitic , adj.
- xanthocyanopsy, xanthocyanopy
- a form of color blindness in which only yellows and blue can be perceived.
- xerophthalmia, xerophthalmy
- a form of conjunctivitis, the result of a deficiency of vitamin A, marked by a dry and dull condition of the eyeball. Also called xeroma .
- abnormal dryness, as of the eyes or skin. Also called xeransis . —xerotic , adj.
"Eyes." -Ologies and -Isms. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/eyes
"Eyes." -Ologies and -Isms. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/eyes
the eyes are the window of the soul it is in the eyes that a person's true nature can be discerned. The saying is recorded in English from the mid 16th century, but a similar idea is found in Latin in Cicero's Orator, ‘ut imago est animi voltus sic indices oculi [the face is a picture of the mind as the eyes are its interpreter].’
have eyes in the back of one's head know what is going on around one even when one cannot see it.
See also the buyer has need of a hundred eyes, eye, fields have eyes at field, four eyes see more than two, the scales fall from someone's eyes at scales2, pull the wool over someone's eyes.
"eyes." The Oxford Dictionary of Phrase and Fable. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/eyes
"eyes." The Oxford Dictionary of Phrase and Fable. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/eyes