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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 'nictating 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. (20040. BSAVA Manual of Reptiles. British Small Animal Veterinary Association. pp. 350. ISBN 0905214757