vomiting, ejection of food and other matter from the stomach through the mouth, often preceded by nausea. The process is initiated by stimulation of the vomiting center of the brain by nerve impulses from the gastrointestinal tract or other part of the body. The vomiting center then sends out nerve impulses that precipitate spasmodic muscular contractions of the stomach wall and downward spasms of the diaphragm. The pressure generated then forces up the contents of the stomach. The vomiting mechanism may be in response to local irritation (diseases or disorders of the gastrointestinal tract, overburdening of the capacity and digestive capabilities of the stomach, ingestion of harmful foods or substances) or result from a metabolic disturbance (as in pregnancy) or from disorders or stimulation of the nervous system (e.g., migraine, motion sickness, infectious disease, brain tumor or injury, disagreeable odors). Vomiting may also be a reflex action to other spasmodic conditions (whooping cough, gagging).

Forcible ejection of the stomach contents from the mouth, usually following nausea. Causes include illness, motion sickness, certain drugs, inner ear disorders, and head injury. Vomiting may occur without nausea (e.g., after extreme exertion). Two centres in the brain's medulla oblongata are believed to control it; the vomiting centre initiates and controls a series of muscle contractions beginning at the small intestine and moving through the stomach and esophagus. This reaction may be set off by the chemoreceptor trigger zone, stimulated by many toxins and drugs, to rid the body of them, or by stimuli from various parts of the body that may be stressed or diseased. Severe vomiting may cause dehydration, malnutrition, or esophageal wall rupture. Vomiting of blood may be a sign of bleeding ulcer or other upper digestive tract disorders. Seealso bulimia.

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