covering, lining, or separating layer of tissue. Cell membrane separates cell protoplasm from surrounding medium or from other cells. In animal body, fibrous membrane, with parallel fibers, provides attachments or support. Serous membrane, with smooth surface, lines or covers organs to prevent binding and friction. Mucous membrane lines organs directly or indirectly open to the air; its glands and cells secrete a protective and lubrication fluid, the mucus brain.