microscopic processes of the inner membranes of a number of organs, chiefly in higher vertebrate animals and man.

(1) Processes of the intestinal mucous membrane in some amphibians, birds, and mammals. They are formed by intestinal epithelium and connective tissue rich in muscles, blood and lymph vessels, and nerves. There are about 4 million villi in the human intestine, and they have a total area of about 43 sq m. Their shape and size vary depending on the nature of the nutrients and the segment of the intestine. They are 0.3-0.5 mm high and about 0.1 mm thick. Villi are covered with a single layer of epithelium consisting of different kinds of cells. Food is actively absorbed through the villi. Muscle contractions of the villi help to mix food in the intestinal cavity and move nutrients arriving through the epithelium along the blood vessels (products of protein and carbohydrate decomposition) and lymph vessels (products of lipolysis). The epithelial cells of the villi are sloughed off and gradually replaced with new ones multiplying in crypts (invaginations of the mucous membrane). The surface of columnar epithelial cells of the villi, which faces the intestinal lumen, has cytoplasmatic processes, or microvilli, on whose external membranes so-called parietal (membrane) digestion takes place. Invertebrates have no villi. However, intestinal epithelium with a border having microvilli is common in many of them. Sometimes the microvilli responsible for parietal digestion cover the external body surface, especially in parasitic forms not having an intestine.

(2) Projections of the synovial membrane of joint capsules, covered with an epithelioid layer of synovial cells.

(3) Projections of one of the embryonic membranes, the chorion.