Dentin
The layer beneath enamel that forms the bulk of tooth structure and contains nerve endings.
Dentin makes up the majority of tooth structure and lies beneath the enamel. Unlike enamel, dentin is a living tissue with microscopic tubules that connect to the dental pulp. This is why exposed dentin causes sensitivity. Dentin is softer than enamel and more susceptible to decay when exposed. It has a yellow color, which is why teeth with thin enamel appear more yellow.