The conjunctiva and nictitating membrane of the eyelids in some reptilian species: a SEM overview.

The conjunctiva and nictitating membrane of reptilian eyelids have been studied by means of SEM. The whole surface pattern of the eyelids can be subdivided into the centrally positioned A-zone near the rims and the surrounding B-zone, including the fornix conjunctivae. The A-zone is characterized by the presence of epithelial lamellae or similar arranged cellular protrusions. All conjunctival epithelial cells demonstrate protrusions of extent morphological variety. Numerous goblet-cells occur in the B-zone and progressively increase in density in the direction of the fornix conjunctivae and the epithelial cells exhibit decreasing protrusions. The lower eyelid incorporates the discus conjunctivae, a platelike formation. It consists of dense connective tissue in which the collagen fibres stretch out orthogonally in a plywood-like manner. The nictitating membrane can be considered as a conjunctival fold near the rostral angle of the eyelids. Its bulbar surface folds in a perpendicular direction.