The mouth

The process of digestion starts in the mouth, where the food and drink enter the body (ingestion). The mouth is sealed by the lips, that keep anything from leaving (or escaping) the cavity. Even before taking the first bite, the smell of the food causes the salivary glands in the mouth to start secreting saliva. There are 3 major pairs of salivary glands. The parotid gland, located anteriorly beneath the ears, between the skin and the masseter (the muscle that raises the lower jaw), drains its contents into the mouth via the parotid duct. The submandibular gland, located midway along the inner side of the the lower jaw, empties via the submandibular duct onto the floor of the mouth. Likewise, the sublingual gland, located on the side of the tongue, also empties onto the floor of the mouth via several sublingual ducts. Saliva contains enzymes (protein based molecules that speed up chemical reactions) that start breaking down carbohydrates (one of the principal types of nutrients consisting mainly of sugars and starches, used as energy sources to the body) into glucose or monosaccharide (simple sugars). This process is called hydrolysis. The teeth bite off and chew the food (mastication) into a soft pulp that mixes with the saliva so that it is easy to swallow. The sharp canine teeth tear up the food and the wide, flat molars grind it. A tooth is a hard structure set in the upper (maxilla) and lower jaw (mandible), held in place by periodontal membranes. The visible part of a tooth is the crown, and the anchor that extends into the jawbone is the root. The crown is covered with enamel, which is the hardest substance in the human body, and the root is covered with a bone-like tissue called cementum. Under the enamel and the cementum is the dentin, the largest part of the tooth. In the middle of the tooth are found the nerves and bloodvessels. During the mastication process the tongue, that consists of groups of muscles, pushes the food to the teeth so the teeth can grind it up. In the pharynx (the upper section of the alimentary canal), behind the tongue is the epiglottis, a flap of cartilage that allows air to pass into the larynx (voice box) and lungs. During swallowing the epiglottis folds backward to cover the entrance to the larynx, in order to prevent food and drink from entering the windpipe. After swallowing the epiglottis returns to its original upright position.