When you take a bite of food, you begin the mechanical phase of digestion. Incisors—sharp front teeth—cut the food. Then, the broad, flat surfaces of molars, or back teeth, grind it up. The tongue helps keep the food between the chewing surfaces of the upper and lower teeth by manipulating it against the hard palate, the bony, membrane-covered roof of the mouth. This structure is different from the soft palate, an area located just behind the hard palate. The soft palate is made of folded membranes and separates the mouth cavity from the nasal cavity.

Chemical digestion involves a change in the chemical nature of the nutrients. Salivary glands produce saliva (suh-LIE-vuh), a mixture of water, mucus, and a digestive enzyme called salivary amylase. Besides the many tiny salivary glands located in the lining of the mouth, there are three pairs of larger salivary glands, as shown in Figure 48-7. The salivary amylase begins the chemical digestion of carbohydrates by breaking down some starch into the disaccha-ride maltose.

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