The orbital fat and bony walls support and provide protection for the eye and orbital tissues [1].

The orbital fat acts as a semi-fluid padding that cushions the eye.

The inferior and medial orbital walls are thin. They are readily fractured on blunt trauma, providing some shock absorption and orbital decompression to protect the eye from injury [2, 3].

The eyelids provide a mechanical barrier between the eye and external environment, rapidly closing on reflexive or voluntary blinking [4].

Cilia (modified fine hairs) on the eyelid skin are highly sensitive to airborne particles; when stimulated, they elicit a blink reflex [5].

The corneoscleral shell provides tensile strength to the globe [6].

Dense corneal innervation allows for rapid blink and withdrawal reflexes.

Corneal innervation also provides trophic factors that promote epithelial healing [7, 8].

A normal Bell’s phenomenon provides involuntary upward rotation of the globe on lid closure, removing the cornea from noxious stimuli [9].

Tear flow increases dramatically in response to mechanical or noxious stimuli [10].

This causes dilution and washout of the irritant.

The corneal epithelium is 5–7 layers thick with cells adjoined by desmosomes [11, 12].

Tight junctions (zonulae occludens) surround the most superficial corneal epithelial cells providing a low conductance barrier to fluid and solutes [13].

The tear film has several bacteriostatic properties [14]:

These act as physical barriers against ocular penetration by microbial pathogens.

Descemet’s membrane is resistant to proteolysis in severe corneal infections, maintaining the integrity of the globe [17].

The dazzle reflex: bright light induces reflexive blinking.

Rapid pupil constriction in response to bright light limits excessive radiation exposure to the ocular media internal to the iris [18].

Absorption of nonvisible optic radiation prevents harmful levels of EMR from damaging the eye.

The cornea and sclera absorb ultraviolet (UV)-B, UV-C, infrared (IR)-B, and IR-C [19–21].

The crystalline lens absorbs UV-A.

Antioxidants in the lens and macula prevent excessive UV-induced oxidative damage.

The yellow macular carotenoid xanthophyll pigments in Henle’s fibre layer absorb short wavelength radiation [22]. They minimize blue light incident to the fovea and reduce chromatic aberration and glare.

Hemoglobin and melanin, principally found in the choroid, absorb excessive light and IR radiation. This results in excessive heat generation; the choroidal circulation acts as a heat sink to dissipate thermal energy [23].

Eyelid closure provides a barrier function elicited by voluntary or reflexive blinking [4, 16].

The eyelids apply gentle posterior pressure on the globe to counteract forward pressure from orbital tissues behind the globe.

Blinking distributes tears across the ocular surface and promotes drainage of tears via the lacrimal pump mechanism [25, 26].

The eyelid contains glands with secretions that add to the tear film.

In adults, the normal interpalpebral fissure height is 8–11 mm; the horizontal palpebral fissure length is 27–30 mm.

The upper lid margin rests 1.5–2 mm below the limbus; the lower rests on the limbus [27, 28].