The canine tooth and the lateral incisor at the center of this image have been severely damaged due to attrition. This type of damage is due to grinding of teeth over time. The teeth of the upper and lower jaw are ground down. Of this constant grinding goes on for many years, the tooth can be severely damaged. At first, the enamel is worn down. This usually takes longer time because of is is a very hard tissue. Once the occlusal/incisal enamel is gone, the dentine is worn down. The dentine is much softer the enamel. Eventually, the tooth wears down to the pulp cavity. Usually, the pulp produces tertiary dentine to protect itself and keep the tooth sealed. tertiary dentine can bee seen at the center of the canine tooth as a brown spot. In patients with massive grinding, or of the pulp tissue is necrotic, the pulp may also be exposed leaving the pulp cavity open to the oral cavity.
Take a look at tertiary dentine in a histologic section.