Functional morphology of the nictitating membrane in the domestic cat

Morphological differences between the corneal and palpebral surfaces of the nictitating membrane of the domestic cat were examined using histology, histochemistry and the scanning electron microscope. Both surfaces are covered by one or two layers of epithelial cells. The epithelium contains numerous goblet cells, particularly on the palpebral side. These cells revealed binding sites for six lectins as well as positive PAS reactivity, and alcianophilia at both pH 2.5 and pH 3.5. Numerous lymph follicles located on the corneal side make the epithelium look like lymphoepithelial tissue. Accumulations of lymphatic tissue may also be found on the palpebral side. Scanning electron micrographs showed microvilli on the epithelial surfaces. The function of the cat nictitating membrane is discussed on the basis of our findings and those of other authors. Movements of the nictitating membrane alternate between active and passive phases of gliding. The membrane as a whole effectively protects the nasal portion of the bulb from dust, dehydration and pathogens.