Lizards (Order Squamata)

In lizards, retraction of the globe, elevation of the lower eyelid and movement of the nictitating membrane can occur relatively independently of each other:

1) Nictitating membrane blink with minimal movement of the eyelids. 

Nictitating membrane blink with minimal movement of the eyelids in a Komodo dragon (Varanus komodoensis). What movement there is can be attributed to retraction of the eyeball causing the eyelids to sink a little into the orbit.

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Stills from video showing minimal eyelid movement.

Further examples of nictitating membrane movement with minimal eyelid closure are provided below.

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Komodo dragon. At 120ms, the eyeball has retracted causing the upper and lower eyelids to sink into the orbit and the nictitating membrane has covered the cornea.

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Lace monitor (Varanus varius). Nictitating membrane blink.

Eye movements and a nictitating membrane blink in a frilled lizard (Chlamydosaurus kingii)

Nictitating membrane blink with slight elevation of the upper lid, on head turn in a perentie (Varanus giganteus). Played back at 10% speed.

2) Elevation of the lower eyelid with no retraction of the eyeball. 

Lower eyelid elevation with no globe retraction in a Komodo dragon.

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Stills from video.

3) Elevation of the lower eyelid with retraction of the eyeball. 

Figure 17.png

Lower lid elevation and globe retraction in a Komodo dragon. 

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Globe retraction and lower lid elevation during a blink in a lace monitor.

Lower lid elevation following a meal in a drowsy perentie (Varanus giganteus). 

4. Retraction of the eyeball with narrowing of the palpebral fissure. 

Globe retraction in a Komodo dragon

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The palpebral fissure narrows as the eyeball retracts and the eyelids sink into the orbit.

The anatomical basis of blinking with the nictitating membrane in lizards is shown below (Walls 1963).

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Medial view of the lizard eyeball. The tendon of the nictitating membrane (n) is pulled by the bursalis (B) (equivalent to quadratus in turtles) and bursalis retractor (B.r) muscles. The retractor bulbi (R.b) pulls on the globe which in turn pulls on the tendon of the nictitating membrane (n). None of these muscles is connected to the lower lid.

In summary, in lizards the globe can retract without involving the lower eyelid. The nictitating membrane is also able to move independently from the lower lid, unlike turtles. But movement of the nictitating membrane is always accompanied by globe retraction. In lizards, some blinks primarily involve globe retraction with movement of the nictitating membrane. In other blinks, the lower eyelid elevates with or without retraction of the globe. Walls (1963) refers to a muscle akin to but not homologous with orbicularis oculi in mammals, which elevates the lower eyelid.

Innervation of the muscles involved in blinking:

Retractor bulbi: Abducens nerve (VIth cranial nerve)

Bursalis: Abducens nerve

Bursalis retractor: --

Levator of the lower eyelid: --

Comments on the evolution of blinking in lizards

Lizards, which are predominantly terrestrial, have two types of blink. Nictitating membrane blinks clean and lubricate the cornea with secretions from the Harderian and lacrimal glands. Lower lid blinks with globe retraction protect the eye from injury.

Reference

Walls, GL. 1963. The vertebrate eye and its adaptive radiation. New York and London: Hafner.

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