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| − | ==Introduction==
| + | Common mydriatics have no effect on the reptilian eye because both the iris sphincter and ciliary muscle contain striated (or skeletal) rather than smooth muscle. This means that they are under conscious control. Unlike mammals, the consensual pupillary response is absent, the cornea does not contain a Descemets membrane and scleral ossicles are often present. This renders the ocular examination more challenging but more importantly, the parasympatholytic topical drugs (that reduce the activity of the parasympathetic nervous system) used to dilate the pupil (mydriasis) in mammals are totally ineffective. |
| − | [[Image:Panther_Chameleon.jpg|300px|thumb|right|'''Panther chameleon spectacle''' (© Tom Junek, Wikimedia Commons)]]
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| − | [[Image:Lizard_skull_mandible.jpg|150px|thumb|right|'''Parietal eye on the dorsum of the head''' (Copyright © RVC)]]
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| − | Common [[Mydriatic|mydriatics]] have no effect on the reptilian eye because both the iris sphincter and ciliary muscle contain striated (or skeletal) rather than smooth muscle. This means that they are under conscious control. Therefore, unlike mammals, the consensual pupillary response is absent, the cornea does not contain a Descemets membrane and scleral ossicles are often present. This renders the ocular examination more challenging but more importantly, the parasympatholytic topical drugs (that reduce the activity of the parasympathetic nervous system) used to dilate the pupil [[Mydriatic|mydriasis]]) in mammals are totally ineffective. | |
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| − | Another characteristic of the lizard eye is its ability to occupy both scotopic (light limited) and photopic (light rich) environments. Indeed, the morphology of the eye adapts and varies according to activity in these different light environments.
| + | A well developed parietal eye (with a lens and retina) is found in some lizards including the Green iguana. It functions in hormone regulation. |
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| − | * '''For information on examination of the eye, see''' [[Lizard Physical Examination|Lizard Physical Examination]].
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| − | ==Parietal Eye==
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| − | [[File:Anolis carolinensis parietal eye.JPG|180px|thumb|right|'''The Parietal Eye of a Lizard (Wikimedia Commons)]]
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| − | Also Known As: '''''Pineal Eye''''' — '''''Third Eye'''''
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| − | A well developed parietal "eye" (with a rudimentary lens and retina) is found on the '''dorsal midline''' in some lizards, including the [[Green Iguana|green iguana]], [[Stump-tailed Skink|blue-tongued skink]] and [[Water Dragon|water dragon]]. | |
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| − | This sensory organ is connected to the central [[Lizard Nervous System|nervous system]] and the pineal gland by the small parietal nerve. It functions in [[Lizard Endocrine System|hormone production]] (including [[Lizard Reproduction|reproduction]]) and thermoregulation (by acting as a light dosimeter). It detects both UV light and heat. Although sensitive to changes in light, it cannot form images. By detecting '''light and dark''' it allows lizards to detect the '''movement of predators'''. Sometimes referred to as "pineal eye" or "third eye", it it visible as an opalescent gray spot on the top of some lizard's heads.
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| | ==Eyelids== | | ==Eyelids== |
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| − | They are usually present. The lower lid is movable, moving upwards to close the eye; in some lizards, this lid might be transparent, allowing vision even when the lids are closed. Lizard also have a well developed, semi-transparent, highly mobile third eyelid which is called the 'nictitating membrane'; this membrane can close across the eye even while the eyelids remain open.
| + | In some chameleon species, the eyelids have fused to produce a circular, immobile and transparent dermal aperture called a spectacle. It create an impervious barrier to topically applied medications. Lizard species lacking a spectacle have a well developed, semi-transparent, highly mobile third eyelid which is also called the 'nictitating membrane'; this membrane can close across the eye even while the eyelids remain open. |
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| − | In some [[Chameleon|chameleon]] species as well as some geckos and the oscellated skink (''Ablepharus sp.'') the eyelids have fused to produce a circular, immobile and transparent dermal aperture called a spectacle. It create an impervious barrier to topically applied medications.
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| − | ==Pupil==
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| − | The pupil is usually round and relatively immobile in diurnal species and is usually slit-like in nocturnal species. Geckos, such as the [[Tokay Gecko|Tokay gecko]], have a serrated pupillary opening resembling a series of small holes when the pupil is completely closed. This specificity allows for acute vision even in dim light.
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| − | ==Behaviour== | + | ==The pupil== |
| | + | The pupil is usually round and relatively immobile in diurnal species and is usually slit-like in nocturnal species. |
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| − | When threatened or molested by other animals, such as dogs, coyotes or foxes, certain horned lizards (''Phrynosoma cornutum, P. coronatum and P. solare'') will respond by squirting a spectacular amount of blood from their eyes. They accomplish this by constricting venous outflow from the head which increases blood pressure and causes leakage from the ocular venous sinuses.
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| − | * '''For more information on behaviour, see''' [[Lizard Behaviour|Lizard Behaviour]].
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| − | {{Learning
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| − | |flashcards = [[Reptiles and Amphibians Q&A 16]]
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| − | }}
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| | ==References== | | ==References== |
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| − | * Engbretson, G.A. & Lent, C.M. (1976). Parietal eye of the lizard: neuronal photoresponses and feedback from the pineal gland. Proceedings of the National Academy of Sciences 73, 654-657 | + | * Mader, D.R. (2005). ''Reptile Medicine and Surgery''. Saunders. pp. 1264. ISBN 072169327X |
| − | * Fowkes, R (2008) '''Endocrinology of Special Species''' BVetMed Integrated Course ''RVC''
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| − | * Frye, FL & Williams, DL (1995) '''Self-Assessment Colour Review - Reptiles & Amphibians''' ''Manson''
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| | * Girling, S.J. (2004). ''BSAVA Manual of Reptiles''. British Small Animal Veterinary Association. pp. 350. ISBN 0905214757 | | * Girling, S.J. (2004). ''BSAVA Manual of Reptiles''. British Small Animal Veterinary Association. pp. 350. ISBN 0905214757 |
| − | * Hall, M.I. (2008). Comparative analysis of the size and shape of the lizard eye. Zoology 111, 62-75
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| − | * Mader, D.R. (2005). ''Reptile Medicine and Surgery''. Saunders. pp. 1264. ISBN 072169327X
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| − | {{review}}
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| − | ==Webinars==
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| − | <rss max="10" highlight="none">https://www.thewebinarvet.com/ophthalmology/webinars/feed</rss>
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| − | [[Category:Lizard Anatomy|E]]
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| − | [[Category:Expert Review - Exotics]]
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Common mydriatics have no effect on the reptilian eye because both the iris sphincter and ciliary muscle contain striated (or skeletal) rather than smooth muscle. This means that they are under conscious control. Unlike mammals, the consensual pupillary response is absent, the cornea does not contain a Descemets membrane and scleral ossicles are often present. This renders the ocular examination more challenging but more importantly, the parasympatholytic topical drugs (that reduce the activity of the parasympathetic nervous system) used to dilate the pupil (mydriasis) in mammals are totally ineffective.
A well developed parietal eye (with a lens and retina) is found in some lizards including the Green iguana. It functions in hormone regulation.
Eyelids
In some chameleon species, the eyelids have fused to produce a circular, immobile and transparent dermal aperture called a spectacle. It create an impervious barrier to topically applied medications. Lizard species lacking a spectacle have a well developed, semi-transparent, highly mobile third eyelid which is also called the 'nictitating membrane'; this membrane can close across the eye even while the eyelids remain open.
The pupil
The pupil is usually round and relatively immobile in diurnal species and is usually slit-like in nocturnal species.
References
- Mader, D.R. (2005). Reptile Medicine and Surgery. Saunders. pp. 1264. ISBN 072169327X
- Girling, S.J. (2004). BSAVA Manual of Reptiles. British Small Animal Veterinary Association. pp. 350. ISBN 0905214757
