The lacrimal system
It is not possible to discuss blinking without making some reference to the lacrimal system and tear fluid, ‘the land animal’s substitute for an ocean’ (Walls 1962). Tear fluid is produced by the lacrimal and/or Harderian glands, depending on the species, and some smaller glands; the Harderian gland is also known as the gland of the nictitating membrane. Most of the tear fluid is lost by evaporation, excess tears draining through puncti which lead via the nasolacrimal duct into the nasal cavity. Tear fluid contains dissolved gases, glucose and protein necessary for metabolism of the corneal cells which have no blood supply. Tear fluid also has bactericidal properties.
The tear film has three layers: a mucous layer which sticks the film to the cornea and is secreted by conjunctival goblet cells, an aqueous layer which transports oxygen and carbon dioxide between the air and the cornea and is secreted by the lacrimal gland, and an oily lipid layer which reduces evaporation of the aqueous layer and is secreted by the Harderian gland or, in the case of higher primates, the Meibomian glands, modified sebaceous glands which line the edges of the lids.
The Harderian gland occurs in all groups of terrestrial vertebrates (Payne 1994): frogs, reptiles, birds and mammals. It is absent in bats, cows, horses, terrestrial carnivores and higher primates including man. The gland is attached to the medial part of the orbit. Its duct opens on the surface of the nictitating membrane. Depending on the species, its secretions may be mucous, serous or lipid or a mixture of each. It has a rich nerve supply and may have a role in the immune system. In some species it is larger than the eye. The Harderian gland is prominent in amphibians, reptiles and birds, groups in which the lacrimal gland is poorly developed.
In mammals other than rodents, the lacrimal gland is dominant. This gland lies next to the superior temporal quadrant of the eyeball in the anterior part of the orbit. Its dozen ducts open under the upper lid.
Reference
Payne, A.P. 1994. "The harderian gland: a tercentennial review." J. Anat. 185:1-49.
Walls, GL. 1963. The vertebrate eye and its adaptive radiation. New York and London: Hafner.