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K. van Doorn, J. G. Sivak; Threat-Induced Variations in Blood Flow in the Reptilian Spectacle. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1315. doi: https://doi.org/.
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The eyes of snakes and certain lizards are covered by a "spectacle" consisting of fused, transparent eyelids and a hard ocular scale. This presents a problem for the oxygenation of the cornea, as the scale is impermeable to atmospheric oxygen. The spectacles of reptiles are therefore vascularized, the vessels having transparent walls, though of course the erythrocytes flowing through them are not transparent. We here present research that suggests the dynamics of flow can be adjusted to maximize clarity in times of visual need.
The presence or absence of blood flow through the spectacle vessels of 3 coachwhip snakes (Masticophis flagellum, Colubridae) was recorded during 12 sessions of 1-2 hour durations with an infrared slit-lamp video system while at rest and as threatening visual stimuli (an experimenter active within the room for up to 8 minutes, or an object moving within 1 meter of them) were presented.
At rest, flow through the spectacle vessels follows a cyclical on-off pattern, the durations of which are relatively consistent within sessions but vary between (p<0.000; temperature, time of day, and time since or until next moulting appear to be factors). With the visual presentation of threatening stimuli, flow is halted for significantly greater durations (p<0.000) than the no-flow times in the baseline cycle, with a return to the cyclical on-off pattern following the removal of the threat (eg. in Fig. 1).
The reason for the regular cyclical pattern at rest is unknown though it may conceivably be related to several factors, particularly supply to the tissues and thermoregulation. The presentation of a threat induces vasoconstriction of the spectacle vessels. Whether this is a response specific to aiding vision or a systemic sympathetic response (ie. "fight or flight") is not known, although the functional effect on vision remains the same; the vision of spectacled reptiles may be improved during threatening situations.
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