The orbicularis oris muscle is a complex of skeletal muscle fibres around the mouth that closes and shapes the lips. Its contraction can press the lips together or project them forward. Compare sealing the mouth while holding water with rounding the lips to whistle: both use the same muscular region to change the outlet's shape. The visible lip includes other tissues besides the muscle that moves it.[ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section 11.3, Muscles That Create Facial Expression, figures 11.7–11.8
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