up-and-down and side-to-side movements of the lower jaw that assist in reducing particles of solid food, making them more easily swallowed; teeth usually act as the grinding and biting surface. In cats and dogs, food is reduced only to a size that permits easy swallowing. Cows and other cud-chewing animals diminish their food to a semifluid state. In humans, food is usually the size of a few cubic millimetres before it is swallowed, but the size is somewhat dependent on the nature of the food, personal habits, and early training. The amount of chewing has little effect on digestion, but inadequately broken meat or vegetable fibres may slow digestion.

The muscles controlling the jaw movements are voluntarily controlled. The act of chewing, however, may become a conditioned reflex stimulated by the presence of food in the mouth. The crushing force exerted by the adult molar teeth is between 75 and 200 pounds per square inch, while that of the incisors is 30 to 70 pounds per square inch. Reasons for chewing food are to soften the tough fibres and to expose them to enzymes necessary for digestion. When food is mixed with the saliva of the mouth, it becomes hydrated and permeated with salivary enzymes; food is also lubricated by the mucus in saliva, which makes it more easily swallowed.