The Mouth and the Digestive System
The mouth begins the digestive process. Food entering the mouth is chewed using the teeth. The chewing process is also known as mastication. Chewing uses muscles in the mandible (lower jawbone) to close the lower jaw against the upper jaw. These muscles are very strong, capable of crushing relatively hard food. The teeth tear, cut, chop, and grind food into smaller pieces that can be swallowed.
The process of chewing causes glands under the tongue, and in the back of the mouth, to secrete saliva. The chewing then mixes the food with the saliva. Saliva provides three benefits. It moistens and compacts the chewed food so the tongue can roll it into a ball, called a bolus. It also lubricates the food, making it easier to swallow. Lastly, saliva contains digestive enzymes. Salivary amylase begins the breakdown of carbohydrates (e.g., starch). Salivary lipase begins the breakdown of fat. The salivary glands also secrete lysozyme that kills bacteria.
Once the food is fully chewed and formed into a bolis, muscles in the mouth and tongue push the bolis to the back of the throat. When the food reaches the throat, a reflex action causes swallowing. Swallowing causes the soft palate to close off the nasal cavity, and the epiglottis to close of the windpipe, to ensure that the food enters the esophagus on its way to the stomach.