MEMBRANE, in anatomy and physiology, a thin sheet-like tissue, more or less elastic, varying in structure and vital properties. Mem branes absorb or secrete fluids, connect certain parts of the body, separate, envelop or form certain organs or act as partitions between two fluids or gases, permitting them to mingle. Mucous membranes line the canals, cavities and hollow organs which communicate externally by different apertures on the skin; for example, the digestive, respiratory and genitourinary tracts. These membranes are soft and velvety and have on their free surface cells for ab sorption or motion and in their substance follicles which secrete mucus for lubricating and other purposes. Such secretions are saliva, gastric juice and pancreatic juice. Serous membrances,. such as the peritoneum, pleura, the units' vaginales and pericardium, facilitate the motion of the organs they envelop (ab dominal digestive organs, the lungs and heart) by reason of the serum they secrete, and also maintain the shape of these organs. Allied to
the serous membranes are the synovial mem branes lining movable joints. By reason of their smoothness and by aid of their lubricating serum the ends of bones move readily upon one another. Fibrous membranes (for exam ple, periosteum, Jura mater and perimysium), not moistened by any particular fluid, augment the solidity of organs they envelop, retain them in position, favor their motion and form canals and rings for the passage of different organs. The membranes which envelop the brain and are extended to cover the spinal cord are called meninges. (See MENINGITIS). The membranes enclosing the fetus are called the placenta (q.v.). Other special membranes are Desce met's membrane, which is the fourth layer of the cornea of the eye, and Bowman's membrane which is the second.