The mucosa, forming a continuous layer that protects the internal membranes from the outside, where the external environment goes into or through the body, as in both the cavities (respiratory system and genital-urinary system) and the intestinal tract. The external skin and the internal mucosa meet at the orifices, forming mucoepithelial membranes.
A membrane lining the cavities and canals of the body kept moist by the secretion of mucus.