All true teeth have certain characteristics in common and are composed of three layers. The innermost section of a tooth is the pulp, which consists of small blood vessels, cells, and a nerve. The pulp nourishes the dentine that is similar in composition to bone and comprises the bulk of each tooth. Enamel (illustrated above), the hardest substance of the body, covers the dentine of the tooth's crown. Due to the enamel, the teeth of mammals are the part most often fossilized and since the number, size, organization and shape of the teeth are different in every species, they are extremely useful for taxonomy.