Rugae is a term used in anatomy that refers to a series of ridges produced by folding of the wall of an organ. Most commonly the term is applied to the internal surface of the stomach (gastric rugae). The function of the rugae is to allow the stomach, or other tissue, to expand when needed. The stomach must be able to expand and contract, sometimes relatively rapidly, to accommodate people at mealtimes and as they digest. When the stomach is not full, the rugae are folds in the tissue. However, as the stomach fills it expands by unfolding the rugae. When the stomach empties again, it refolds and the rugae increase to their former size.