Villi

VILLI, villi (Latin: villi), a special anatomical feature of certain organs; thus, the mucous membrane of the small intestine and the synovial membranes of joints are densely covered with villi, giving their surface a velvety appearance. Abundant formation of villi occurs on the outer membrane of the fetus, which is therefore also called the villous membrane; these villi then constitute the most important part of the placenta. Villous structure is characteristic of the so-called Pacchionian granulations of the brain's soft membranes. Villi represent one of the principles of construction of tissues that have a pronounced function of absorption, and to some extent secretion (for the functional and histological features of various villi, see the respective organs). Under pathological conditions, villous formations are sometimes also observed, but here they lack any definite significance or structure, and the term is used only because of external and even remote similarity with villi; thus, in cases of fibrinous pericarditis, when the epicardium is strewn with fibrinous films and threads, one speaks of cor villosum, which, however, is more commonly designated as 'hairy heart.'-Arachnoid villi. See Pacchionian granulations.