<P> Teeth are complex structures made of materials specific to them . They are made of a bone - like material called dentin, which is covered by the hardest tissue in the body--enamel . Teeth have different shapes to deal with different aspects of mastication employed in tearing and chewing pieces of food into smaller and smaller pieces . This results in a much larger surface area for the action of digestive enzymes . The teeth are named after their particular roles in the process of mastication--incisors are used for cutting or biting off pieces of food; canines, are used for tearing, premolars and molars are used for chewing and grinding . Mastication of the food with the help of saliva and mucus results in the formation of a soft bolus which can then be swallowed to make its way down the upper gastrointestinal tract to the stomach . The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles . </P> <P> The epiglottis is a flap of elastic cartilage attached to the entrance of the larynx . It is covered with a mucous membrane and there are taste buds on its lingual surface which faces into the mouth . Its laryngeal surface faces into the larynx . The epiglottis functions to guard the entrance of the glottis, the opening between the vocal folds . It is normally pointed upward during breathing with its underside functioning as part of the pharynx, but during swallowing, the epiglottis folds down to a more horizontal position, with its upper side functioning as part of the pharynx . In this manner it prevents food from going into the trachea and instead directs it to the esophagus, which is behind . During swallowing, the backward motion of the tongue forces the epiglottis over the glottis' opening to prevent any food that is being swallowed from entering the larynx which leads to the lungs; the larynx is also pulled upwards to assist this process . Stimulation of the larynx by ingested matter produces a strong cough reflex in order to protect the lungs . </P> <P> The pharynx is a part of the conducting zone of the respiratory system and also a part of the digestive system . It is the part of the throat immediately behind the nasal cavity at the back of the mouth and above the esophagus and larynx . The pharynx is made up of three parts . The lower two parts--the oropharynx and the laryngopharynx are involved in the digestive system . The laryngopharynx connects to the esophagus and it serves as a passageway for both air and food . Air enters the larynx anteriorly but anything swallowed has priority and the passage of air is temporarily blocked . The pharynx is innervated by the pharyngeal plexus of the vagus nerve . Muscles in the pharynx push the food into the esophagus . The pharynx joins the esophagus at the oesophageal inlet which is located behind the cricoid cartilage . </P> <P> The esophagus, commonly known as the gullet, is an organ which consists of a muscular tube through which food passes from the pharynx to the stomach . The esophagus is continuous with the laryngeal part of the pharynx . It passes through the posterior mediastinum in the thorax and enters the stomach through a hole in the thoracic diaphragm--the esophageal hiatus, at the level of the tenth thoracic vertebra (T10). Its length averages 25 cm, varying with height . It is divided into cervical, thoracic and abdominal parts . The pharynx joins the esophagus at the esophageal inlet which is behind the cricoid cartilage . </P>

What are the parts of human digestive system