The conjunctiva is a thin mucous membrane that lines the posterior surface of the lids and reflects forward at the fornix or recess onto the globe. Depending on the location, the epithelium of the conjunctiva is either stratified columnar or stratified squamous, and it is continuous with the corneal epithelium. The presence of conjunctival pigment is a protecting factor for development of squamous cell carcinomas, which are stimulated by ultraviolet radiation. Viruses are common etiologic agents of conjunctivitis in all domestic animals. The third eyelid is well developed in domestic animals, while in higher primates it is a vestigial structure, the plica semilunaris. The movement of the third eyelid across the globe consists of a passive component and an active component, the latter being found in birds and, perhaps, cats. In cattle, subconjunctival orbital fat is commonly observed dorsally when the dorsal lid is retracted, and the eye is rolled ventrally.