Accommodation is the process by which the

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

changes

optical power In optics, optical power (also referred to as dioptric power, refractive power, focal power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the ...

to maintain a clear image or

focus Focus (: foci or focuses) may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film *Focus (2001 film), ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel *Focus (2015 ...

on an object as its distance varies. In this, distances vary for individuals from the

far point In visual perception, the far point is the farthest point at which an object can be placed (along the optical axis of the eye) for its image to be focused on the retina within the eye's accommodation. It is sometimes described as the farthest ...

—the maximum distance from the eye for which a clear image of an object can be seen, to the

near point In visual perception, the near point is the closest point at which an object can be placed and still form a focused image on the retina, within the eye's accommodation range. The other limit to the eye's accommodation range is the far point. A n ...

—the minimum distance for a clear image. Accommodation usually acts like a

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

, including part of the

accommodation-convergence reflex The accommodation reflex (or ''accommodation-convergence reflex'') is a reflex action of the eye, in response to focus (optics), focusing on a near object, then looking at a distant object (and vice versa), comprising coordinated changes in verg ...

, but it can also be consciously controlled. The main ways animals may change focus are: * Changing the shape of the lens. * Changing the position of the lens relative to the retina. * Changing the axial length of the eyeball. * Changing the shape of the cornea.

Focusing mechanisms

Focusing the light scattered by objects in a three dimensional environment into a two dimensional collection of individual bright points of light requires the light to be bent. To get a good image of these points of light on a defined area requires a precise systematic bending of light called

refraction In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...

. The

real image {{citations needed, date=June 2019 In optics, an ''image'' is defined as the collection of focus points of light rays coming from an object. A real image is the collection of focus points actually made by converging/diverging rays, while a ...

formed from millions of these points of light is what animals see using their

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

s. Very even systematic curvature of parts of the cornea and

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

produces this systematic bending of light onto the retina. Virtual eye showing the contribution to focus of different components

Due to the nature of optics the focused image on the retina is always inverted relative to the object. Different animals live in different environments having different refractive indices involving water, air and often both. The eyes are therefore required to bend light by different amounts leading to different mechanisms of focus being used in different environments. The air/cornea interface involves a larger difference in refractive index than hydrated structures within the eye. As a result, animals living in air have most of the bending of light achieved at the air/cornea interface with the lens being involved in finer focus of the image. Generally mammals, birds and reptiles living in air vary their eyes' optical power by subtly and precisely changing the shape of the elastic lens using the

ciliary body The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliary ...

. The small difference in refractive index between water and the hydrated cornea means fish and amphibians need to bend the light more using the internal structures of the eye. Therefore, eyes evolved in water have a mechanism involving changing the distance between a rigid rounder more refractive lens and the retina using less uniform

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

s rather than subtly changing the shape of the lens itself using circularly arranged muscles.

Land based animals and the shape changing lens

Varying forms of direct experimental proof outlined in this article show that most non-aquatic vertebrates achieve focus, at least in part, by changing the shapes of their lenses. What is less well understood is how the subtle, precise and very quick changes in lens shape are made. Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animal. When considering vertebrates, aspects of all models may play varying roles in lens focus. The models can be broadly divided into two camps: those models that stress the importance of external forces acting on a more passively elastic lens and other models that include forces that may be generated by the lens internally.

External forces

The model of a shape changing lens of humans was proposed by Thomas Young in a lecture on the 27th Nov 1800. Others such as

Hermann von Helmholtz Hermann Ludwig Ferdinand von Helmholtz (; ; 31 August 1821 – 8 September 1894; "von" since 1883) was a German physicist and physician who made significant contributions in several scientific fields, particularly hydrodynamic stability. The ...

and

Thomas Henry Huxley Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist who specialized in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution. The stor ...

refined the model in the mid-1800s explaining how the

ciliary muscle The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscleSchachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) . in the eye's middle layer, the uvea ( vascular layer). It controls accommodation for vie ...

contracts, rounding the lens to focus near. The model may be summarized like this: normally the lens is held under tension by its suspending ligaments and capsule being pulled tight by the pressure of the eyeball. At short focal distance the ciliary muscle contracts, stretching the ciliary body and relieving some of the tension on the suspensory ligaments, allowing the elastic lens to become more spherical, increasing refractive power. Changing focus to an object at a greater distance requires a thinner less curved lens. This is achieved by relaxing some of the sphincter-like ciliary muscles allowing the ciliarly body to spring back, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. There is a problem with the Helmholtz model in that despite mathematical models being tried, none has come close enough to working using only the Helmholtz mechanisms. Schachar model of focus

In 1992, Ronald Schachar proposed a model for land based vertebrates that was not well received. The theory allows mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. In this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles, while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. These multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. Not only changing focus, but also correcting for lens aberrations that might otherwise result from the changing shape while better fitting mathematical modeling. The "

catenary In physics and geometry, a catenary ( , ) is the curve that an idealized hanging chain or wire rope, cable assumes under its own weight when supported only at its ends in a uniform gravitational field. The catenary curve has a U-like shape, ...

" model of lens focus proposed by

Coleman Coleman may refer to: Places Antarctica * Coleman Glacier (Antarctica) * Coleman Peak, Ross Island Canada * Coleman, Alberta * Coleman, Ontario * Coleman, Prince Edward Island United Kingdom * Coleman, Leicester, England United States * C ...

demands less tension on the ligaments suspending the lens. Rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. The lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change its curve when the poles are moved closer together. This model requires precise fluid movement of the lens front only rather than trying to change the shape of the lens as a whole. While this concept may be involved in the focusing it has been shown by Scheimpflug photography that the rear of the lens also changes shape in the living eye.

Internal forces

When Thomas Young proposed the changing of the human lens' shape as the mechanism for focal accommodation in 1801, he thought the lens may be a muscle capable of contraction. This type of model is termed intracapsular accommodation as it relies on activity within the lens. In a 1911 Nobel lecture Allvar Gullstrand spoke on "How I found the intracapsular mechanism of accommodation" and this aspect of lens focusing continues to be investigated. Young spent time searching for the nerves that could stimulate the lens to contract without success. Since that time it has become clear the lens is not a simple muscle stimulated by a nerve so the 1909 Helmholtz model took precedence. Pre-twentieth century investigators did not have the benefit of many later discoveries and techniques. Membrane proteins such as

aquaporin Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of ...

s which allow water to flow into and out of cells are the most abundant membrane protein in the lens.

Connexin Connexins (Cx)TC# 1.A.24, or gap junction proteins, are structurally related transmembrane proteins that assemble to form vertebrate gap junctions. An entirely different family of proteins, the innexins, forms gap junctions in invertebrates. Eac ...

s which allow electrical coupling of cells are also prevalent. Electron microscopy and immunofluorescent microscopy show fiber cells to be highly variable in structure and composition. Magnetic resonance imaging confirms a layering in the lens that may allow for different refractive plans within it. The

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This index gradient enhances the

of the lens. As more is learned about mammalian lens structure from ''in situ'' Scheimpflug photography, MRI and physiological investigations, it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle, but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. The accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. The age related changes in the human lens may also be related to changes in the water dynamics in the lens.

Human eyes

The young human eye can change focus from distance (infinity) to as near as 6.5 cm from the eye. This dramatic change in focal power of the eye of approximately 15 dioptres (the reciprocal of focal length in metres) occurs as a consequence of a reduction in zonular tension induced by

contraction. This process can occur in as little as 224 ± 30 milliseconds in bright light. The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude can decline so that the near point of the eye is more remote than the reading distance. When this occurs the patient is presbyopic. Once presbyopia occurs, those who are

emmetropic Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the ciliary muscle in a relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and eye lens and the axi ...

(i.e., do not require optical correction for distance vision) will need an optical aid for near vision; those who are

myopic Myopia, also known as near-sightedness and short-sightedness, is an eye condition where light from distant objects focuses in front of, instead of on, the retina. As a result, distant objects appear blurry, while close objects appear normal. ...

(nearsighted and require an optical correction for distance or far vision), will find that they see better at near without their distance correction; and those who are

hyperopic Far-sightedness, also known as long-sightedness, hypermetropia, and hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blur is due to incoming light being focused behind, instead of ...

(farsighted) will find that they may need a correction for both distance and near vision. Note that these effects are most noticeable when the

pupil The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black becau ...

is large; i.e. in dim light. The age-related decline in accommodation occurs almost universally to less than 2 dioptres by the time a person reaches 45 to 50 years, by which time most of the population will have noticed a decrease in their ability to focus on close objects and hence require glasses for reading or bifocal lenses. Accommodation decreases to about 1 dioptre at the age of 70 years. The dependency of accommodation amplitude on age is graphically summarized by Duane's classical curves.

=Amplitude of accommodation

= The amplitude of accommodation is a clinical measurement that describes the maximum potential increase in

that an

can achieve in adjusting its focus. It refers to a certain range of object distances for which the retinal image is as sharply focused as possible. Amplitude of accommodation is measured during routine eye-examination. The closest that a normal eye can focus is typically about 10 cm for a child or young adult. Accommodation then decreases gradually with age, effectively finishing just after age fifty. The average amplitude of accommodation, in

diopters A dioptre ( British spelling) or (American spelling), symbol dpt or D, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, . It is normally used to express the optical power of a lens or curved mi ...

, for a patient of a given age was estimated by Hofstetter in 1950 to be 18.5 − (0.30 × patient age in years) with the minimum amplitude of accommodation as 15 − (0.25 × age in years), and the maximum as 25 − (0.40 × age in years). However, Hofstetter's work was based on data from two early surveys which, although widely cited, used methodology with considerable inherent error. (Donders, Sheard, Duane, Turner for reference)

=Theories on how humans focus

= * Helmholtz—The most widely held theory of accommodation is that proposed by

in 1855. When viewing a far object, the circularly arranged

relaxes allowing the

zonules The zonule of Zinn () (Zinn's membrane, ciliary zonule) (after Johann Gottfried Zinn) is a ring of fibrous strands forming a wikt:zonule#Noun, zonule (little band) that connects the ciliary body with the crystalline lens (anatomy), lens of the eye ...

and suspensory ligaments to pull on the lens, flattening it. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the

sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...

. When viewing a near object, the ciliary muscles contract (resisting the outward pressure on the sclera) causing the lens zonules to slacken which allows the lens to spring back into a thicker, more convex, form. * Schachar—Ronald A. Schachar proposed in 1992 what has been called a "rather bizarre geometric theory" which claims that focus by the human lens is associated with increased tension on the lens via the equatorial zonules; that when the ciliary muscle contracts, equatorial zonular tension is increased, causing the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing. The increased equatorial zonular tension keeps the lens stable and flattens the peripheral lens surface during accommodation. As a consequence, gravity does not affect the amplitude of accommodation and primary

spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. This phenomenon commonly affects lenses and curved mirrors, as these components are often shaped in a spherical ...

shifts in the negative direction during accommodation. The theory has not found much independent support. * Catenary—

D. Jackson Coleman D. Jackson Coleman is a professor of clinical ophthalmology at NewYork-Presbyterian Hospital at The Edward S. Harkness Eye Institute of Columbia University. He is the former John Milton McLean Professor of Ophthalmology and chairman emeritus at W ...

proposes that the lens, zonule and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye. Ciliary muscle contraction initiates a pressure gradient between the vitreous and aqueous compartments that support the anterior lens shape. It is in this lens shape that the mechanically reproducible state of a steep radius of curvature in the center of the lens with slight flattening of the peripheral anterior lens, i.e. the shape, in cross section, of a

occurs. The anterior capsule and the zonule form a trampoline shape or hammock shaped surface that is totally reproducible depending on the circular dimensions, i.e. the diameter of the ciliary body (Müeller's muscle). The ciliary body thus directs the shape like the pylons of a suspension bridge, but does not need to support an equatorial traction force to flatten the lens.

=Induced effects of accommodation

= When humans accommodate to a near object, they also

converge Converge may refer to: * Converge (band), American hardcore punk band * Converge (Baptist denomination), American national evangelical Baptist body * Limit (mathematics) In mathematics, a limit is the value that a function (or sequence) app ...

their eyes and constrict their

s. The combination of these three movements (accommodation, convergence and

miosis Miosis, or myosis (), is excessive constriction of the pupil.Farlex medical dictionary
citing: ...

) is under the control of the Edinger-Westphal nucleus and is referred to as the ''near triad'', or

accommodation reflex The accommodation reflex (or ''accommodation-convergence reflex'') is a reflex action of the eye, in response to focusing on a near object, then looking at a distant object (and vice versa), comprising coordinated changes in vergence, lens shap ...

. While it is well understood that proper convergence is necessary to prevent

diplopia Diplopia is the simultaneous perception of two images of a single object that may be displaced in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often voluntary. However, when occ ...

, the functional role of the pupillary constriction remains less clear. Arguably, it may increase the

depth of field The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus. Factors affecting depth ...

by reducing the

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

of the eye, and thus reduce the amount of accommodation needed to bring the image in focus on the retina. There is a measurable ratio ( Matthiessen's ratio) between how much convergence takes place because of accommodation (AC/A ratio, CA/C ratio). Abnormalities with this can lead to

binocular vision Binocular vision is seeing with two eyes. The Field_of_view, field of view that can be surveyed with two eyes is greater than with one eye. To the extent that the visual fields of the two eyes overlap, #Depth, binocular depth can be perceived. Th ...

problems.

=Anomalies of accommodation described in humans

= There are many types of accommodation anomalies. It can be broadly classified into two, decreased accommodation and increased accommodation. Decreased accommodation may occur due to physiological (presbyopia), pharmacological (cycloplegia) or pathological. Excessive accommodation and spasm of accommodation are types of increased accommodation.

=Presbyopia

Presbyopia Presbyopia is a physiological insufficiency of optical Accommodation (vertebrate eye), accommodation associated with the aging of the human eye, eye; it results in progressively worsening ability to focus clearly on close objects. Also known as ...

, physiological insufficiency of accommodation due to age related changes in lens (decreased elasticity and increased hardness) and ciliary muscle power is the commonest form of accommodative dysfunction. It will cause gradual decrease in near vision.

=Accommodative insufficiency

= Accommodative insufficiency is the condition where amplitude of accommodation of a person is lesser compared to physiological limits for their age. Premature sclerosis of lens or ciliary muscle weaknesses due to systemic or local cases may cause accommodative insufficiency. Accommodative insufficiency is further categorised into different categories.

=Ill-sustained accommodation

= Ill-sustained accommodation is a condition similar to accommodative insufficiency. In this, range of accommodation will be normal, but after excessive near work accommodative power will decrease.

=Paralysis of accommodation

= In paralysis of accommodation, amplitude of accommodation is either markedly reduced or completely absent (

cycloplegia Cycloplegia is paralysis of the ciliary muscle of the eye, resulting in a loss of accommodation. Because of the paralysis of the ciliary muscle, the curvature of the lens can no longer be adjusted to focus on nearby objects. This results in ...

). It may occur due to ciliary muscle paralysis or occulomotor nerve paralysis. Parasympatholytic drugs like atropine will also cause paralysis of accommodation.

=Unequal accommodation

= If there is amplitude of accommodation between the eyes differ 0.5 dioptre or more, it is considered as unequal. Organic diseases,

head trauma A head injury is any injury that results in trauma to the skull or brain. The terms ''traumatic brain injury'' and ''head injury'' are often used interchangeably in the medical literature. Because head injuries cover such a broad scope of inj ...

or functional

amblyopia Amblyopia, also called lazy eye, is a disorder of sight in which the brain fails to fully process input from one eye and over time favors the other eye. It results in decreased vision in an eye that typically appears normal in other aspects. Amb ...

may be responsible for unequal accommodation.

=Accommodative infacility

= Accommodative infacility is also known as accommodative inertia. In this condition there will be difficulty in changing accommodation from one point to other. There may be difficulty in adjusting focus from distance from near. It is a comparatively rare condition.

=Spasm of accommodation

Spasm of accommodation A spasm of accommodation (also known as a ciliary spasm, an accommodation, or accommodative spasm) is a condition in which the ciliary muscle of the eye remains in a constant state of contraction. Normal accommodation allows the eye to "accommodat ...

also known as ciliary spasm is a condition of abnormally excessive accommodation which is out of voluntary control of the person. Vision may be blurred due to induced

pseudomyopia Pseudomyopia (from ''ψεῦδο'', "pseudo": false; and ''μυωπία'' "myopia": near sight) occurs when a spasm of the ciliary muscle prevents the eye from focusing in the distance, sometimes intermittently; this is different from myopia which ...

=Accommodative excess

= Accommodative excess occurs when an individual uses more than normal accommodation for performing certain near work. Modern definitions simply regard it as an inability to relax accommodation readily.

Aquatic animals

Aquatic animals include some that also thrive in the air so focusing mechanisms vary more than in those that are only land based. Some whales and seals are able to focus above and below water having two areas of retina with high numbers of rods and cones rather than one as in humans. Having two high resolution area of retina presumably allows two axis of vision one for above and one for below water. In

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

s and

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, the ciliary body which supports the lens via suspensory ligaments also touches the lens with a number of pads on its inner surface. These pads compress and release the lens to modify its shape while focusing on objects at different distances; the suspensory ligaments usually perform this function in

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s. With

vision in fish Visual system, Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial animal, terrestrial vertebrates like bird vision, birds and mammals, but have a more spherical lens (anatomy), lens. ...

and

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye using a muscle called the retractor lentus. In

cartilaginous fish Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...

, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In

teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

s, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the ''falciform process'', and serves to pull the lens backwards from the relaxed position to focus on distant objects. While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not similar in either type of animal. In

frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

s, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle. In the simplest vertebrates, the

lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

s and

hagfish Hagfish, of the Class (biology), class Myxini (also known as Hyperotreti) and Order (biology), order Myxiniformes , are eel-shaped Agnatha, jawless fish (occasionally called slime eels). Hagfish are the only known living Animal, animals that h ...

, the lens is not attached to the outer surface of the eyeball at all. There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea. To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards. While not vertebrate, brief mention is made here of the convergent evolution of vertebrate and Molluscan eyes. The most complex Molluscan eye is the

Cephalopod eye Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions.Budelmann BU. "Cephalopod sense organs, nerves and the brain: Adaptations for high performance and life style." Marine and Freshwater Behav ...

which is superficially similar structure and function to a vertebrate eye, including accommodation, while differing in basic ways such as having a two part lens and no cornea. The fundamental requirements of optics must be filled by all eyes with lenses using the tissues at their disposal so superficially eyes all tend to look similar. It is the way optical requirements are met using different cell types and structural mechanisms that varies among animals.

References

External links

* —"Presbyopia: Cause and Treatment" * {{DEFAULTSORT:Accommodation (Eye) Eye Optometry Ophthalmology Vision