DENTITION
Most rodents have between 16 and 22 teeth but have only one pair of upper incisors, whereas rabbits have 28 teeth and two pairs. There are no canine teeth and the space between the incisors and molars is called the diastema (Fig.
7.8). The incisor teeth are open rooted (aradicular) and constantly grow from the base, producing a tooth like the segment of a circle. The outer surface is harder than the inner dentin, so this creates the chisel-shaped selfsharpening edge. There is no nerve in the tooth except at the base. When the rodent is gnawing, cheek folds can be pulled into the diastema and this enables the animal to gnaw at hard objects without swallowing sharp material (Hurst 1999; Nowak 1999b, 1999c; Vaughan 1986b, 1986c; Wiggs & Lobprise 1995).The cheek teeth are composed of hard enamel covering the outer layer and some internal loops surrounding a center of softer dentine. The enamel, being harder, wears more slowly and this results in the creation of ridges on the crown of the teeth, the patterns of which aid in the classification of rodent species. Herbivorous species that feed on coarse vegetation, such as the rabbit, guinea pig, and chinchilla, have open rooted (aradicular) molars that are constantly ground down by attrition. In contrast, omnivores like the rat have molars that cease growing at 4 months (Hurst 1999; Nowak 1999b; Wiggs & Lobprise 1995).