any structural anomaly of the retroocular sebaceous gland found within the orbit of mammals that possess a nictitating membrane (third eyelid); the chief products of the gland vary between different groups of vertebrates, and epithelial cells possess granules or vacuoles whose contents may be mucous, serous or lipid; in rodents, the Harderian gland is especially large and secretes lipids (by a merocrine mechanism), melatonin and porphyrins; ascribed functions include a role in lubrication and protection for the eyeball and nictitating membrane (by lipids), photoprotection (by porphyrin), photoreception (by regulation of the incidence of light on the retina), and thermoregulation (in some rodents) of the sebaceous gland located behind the eyeball that excretes fluid that facilitates movement of the nictitating membrane
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