The mouth is the opening to the oral cavity, the first chamber of the digestive tract. The teeth of the oral cavity aid in the mechanical breakup of food. There are several types of teeth: incisors (four in each jaw) for biting food, canines (one on either side of the incisors, for a total of two in each jaw) for tearing food, and premolars and molars (two and three, respectively, behind each of the two canines in each jaw) for grinding and crushing food. Incisors and canines are adapted for chewing meat. Premolars and molars are adapted for chewing plant materials. These teeth break down the non-digestible cellulose casing around plant cells so that digestive enzymes can act on the cells' contents.
In the oral cavity, saliva dissolves food and acts as a lubricant. It also begins the breakdown of starch via the enzyme amylase and contains agents which destroy the harmful microbes regularly introduced into the mouth. The tongue forms food into a ball-like mass of food, called a bolus, and, then, pushes it through the pharynx into the esophagus. This process, called swallowing, requires a complex set of reflexes to close off the trachea and nasal passages, which also empty into the pharynx, in order to prevent choking.
The esophagus, a long tube connecting the pharynx to the stomach, pushes food along quickly via waves of muscular contraction called peristalsis. As a bolus contacts the circular muscles of the esophagus, it causes the muscles to contract, thus, pushing the bolus toward the stomach. When waves of peristaltic contraction reach the
