Epithelium - n. The superficial layer of cells lining the alimentary
canal and all its appendages, all glands and their ducts, blood vessels
and lymphatics, serous cavities, etc. It often includes the epidermis
(i. e., keratin-producing epithelial cells), and it is sometimes
restricted to the alimentary canal, the glands and their appendages, --
the term endothelium being applied to the lining membrane of the blood
vessels, lymphatics, and serous cavities.