The digestive system functions to receive, store, and break down food to absorb nutrients. Both physical and chemical digestion processes begin in the mouth. The presence of food in the mouth causes the salivary glands to deliver saliva into the mouth. Often even the smell or anticipation of food will cause activation of the salivary glands. Digestive enzymes in saliva begin to moisten, soften, and dissolve the food before passing it to the next stage of the digestive system. Saliva also contains mucin, which protects the mouth from abrasion, and buffers, which prevent dental cavities by neutralizing acid in the mouth. The tongue is used to taste and manipulate food during chewing.

Fish, amphibians, and reptiles principally use teeth to grip prey and prevent it from escaping until it can be swallowed whole. Birds have no teeth, but many have bills with...