Swallowing mechanism, illustration. This sequence runs from top left to bottom right. The mouth without food in it is at top left. Food placed in the mouth is chewed to form a mass called a bolus (green) on the tongue (upper centre). As the swallowing action starts, the bolus is pushed towards the back of the mouth with the tongue (top right). As muscles in the throat contract, the tongue moves further back (bottom left) and the bolus moves down the throat (lower centre), with a flap called the epiglottis folding over to prevent it entering the trachea (windpipe). At bottom right, the bolus enters the oesophagus, where smooth muscle contractions as part of a process called peristalsis move it downwards to the stomach.