The digestive tract consists of the mouth, throat, esophagus, stomach, small intestine, large intestine, rectum, and anus. The mouth is the entrance to both the digestive and the respiratory systems. The inside of the mouth is lined with mucous membrane. Ducts from the salivary glands in the cheeks, under the tongue, and under the jaw all empty into the mouth. On the floor of the mouth lies the tongue, which is used to taste and mix food.
Taste is sensed by taste buds on the surface of the tongue.
Smell is sensed by olfactory receptors high in the nose. The sense of taste is relatively simple, distinguishing only sweet, sour, salty, and bitter. The sense of smell is much more complex, distinguishing many subtle variations. Between meals, the flow of saliva washes away bacteria that can cause tooth decay and other disorders. Saliva also contains antibodies and enzymes, such as lysozyme, that break down proteins and attack bacteria directly.