The muscles used in facial expression are unique in that they are attached to various parts of the facial skin. All the muscles of the face are supplied by branches of two main nerves, the right and left facial nerves, which arise from the brain stem. A group of four large muscles on each side moves the jaw for chewing and biting. The other muscles help provide the motions needed for grinding food between the molars. Many bundles of fibers running in various directions produce all these complex movements.
Humans can swallow water standing on their heads.
Special sets of muscle guard the larynx. Others alter the tension and position of the vocal cords, which are able to produce sound when the lungs force air between them. Its contraction greatly enlarges the chest and draws air into the lungs with each inspiration. This forceful compression is required to expel urine from the bladder, the contents of the colon and rectum during bowel movements, and the baby from a woman's uterus. In each of these cases, smooth muscles in the walls of the hollow organs also take part in the expulsion of the contents.