If the teeth are not worn down, the tips soon grow past each other and continue to grow in spiral form. The result is that the upper teeth may grow backward and upward, sometimes piercing the roof of the mouth, and the lower teeth may grow upward in front of the nose. This problem may be caused by injury, soft diet, or disease. The space between the incisors and the cheek teeth permits maximum utilization of the gnawing front teeth and the manipulation of food when passed back to the grinding teeth. A fold of velvety or furred skin is present between the incisors and the rest of the mouth.
Some rodents have either internal or external cheek pouches that open near the angle of the mouth.
The external pouches are fur-lined, and the animal can turn them wrong side out to clean them. The tongue, which is short and compressed, cannot be protruded beyond the incisors in many genera. In addition to the dental characters, rodents have other anatomical features in common. The bones of the lower arm, the radius and ulna, are distinct, and the elbow joint permits free motion of the forearm. The hand usually has five fingers, though the thumb may be vestigial or absent.