Final answer:
The crown of a tooth includes an outer layer of enamel and an inner layer of dentin surrounding a pulp cavity. The divisions into thirds of the tooth's crown were not specified, but it can be structurally divided into incisal, middle, and cervical thirds.
Step-by-step explanation:
The crown of a tooth is the visible part above the gum line. The structure of the crown includes an outer layer of enamel, which is the hardest substance in the human body. Inside the enamel is a layer of dentin, which is a bone-like tissue. This encases the vital inner pulp cavity that houses nerves and blood vessels.
While you did not provide a specific answer to the thirds into which a tooth can be divided, based on anatomical division, we may infer that the tooth can be divided into thirds based on different functional and structural aspects such as the incisal (or occlusal) third, the middle third, and the cervical third when considering the crown from the perspective of its height. However, for a precise and universally accepted division into thirds, additional context or conventions from dental anatomy textbooks or lectures would be needed.