The gastrointestinal tract starts in the oral cavity (mouth) where your teeth grind and chew food, breaking it into small manageable pieces. This chewing process, known as mastication, is dependent upon powerful muscles (masseter and temporalis), as well as smaller muscles that permit fine control; they move the mandible (lower jawbone) against the upper jaw and enable crushing of relatively hard food. Mastication causes exocrine glands under the tongue and in the back of the mouth to secrete a watery liquid called saliva which performs two essential functions. It moistens and compacts the chewed food so your tongue can roll it into a ball (bolus) and push it to the back of your mouth for swallowing and easy passage through the pharynx and esophagus. In addition, saliva contains digestive enzymes (eg. salivary amylase) which begin the breakdown of carbohydrates. Mastication and saliva secretion work in harmony: chewing increases the surface area of foods which helps to accelerate the breakdown of starch molecules into simple sugars by the digestive enzymes. Almost no protein or fat digestion occurs in the mouth, except for the release of lingual lipase an enzyme secreted by Ebner’s glands on the dorsal surface of the tongue.
The actions of the teeth and tongue prepare food for swallowing. When you are ready to swallow, the tongue pushes a piece of chewed food (a bolus) toward the back of your throat and into the opening of the esophagus – the tube which connects to the stomach. To prevent food in the throat from rising into the nasal cavity or moving down the windpipe (trachea), the act of swallowing triggers two involuntary events. The soft palate (the back of the roof of the mouth) closes off the nasal cavity while the epiglottis, a flap of cartilage attached to the root of the tongue tilts downward to seal the trachea. The swallowing procedure is regulated by nerves in the medulla oblongata and pons. The reflex is instigated by receptors in the throat as a bolus of food is pushed to the back of the mouth by the tongue.