A layer of cementum covers the dentin at the level of the root. Finally, in the interior of the tooth is the pulp, which contains blood vessels and nerves. Believe it or not, the shape of the rat's front teeth is literally a work of art. The incisors of a rat are very different from those of other mammals, and this difference is what makes the rat such a good chewer. This dichotomy in structure, with the tooth being harder on one side than the other, gives rise to the specialized shape and function of the incisors.
Attrition is the normal wearing of the teeth due to functional activity.
The back and forth movement of the jaws during gnawing rapidly wears away the softer dentin on the interior surface of the incisor more so than the hard enamel on the labial side. As a result, a sharp, chisel-like edge is formed. The incisors grow at a rate of about 5 inches a year, and are continuously worn away, to be completely replaced every 40 to 50 days. There even appears to be a circadian rhythm associated with the eruption rate, with slower eruption taking place at night when the incisors are in more continual use, and thus experiencing more frequent functional occlusion. In the event of improper wear due to fracture of the opposing tooth or misalignment of the jaw, the formation of a screw-like helix may occur.