The oral surfaces covered in keratinized epithelium include the hard palate, the dorsal surface, or top, of the tongue, and the gingiva.

口腔表面由角覆盖的结构包括硬腭、舌的背侧面（面）与龈。

If the gingiva recedes or plaque forms between the gingiva and the root, bacteria and acids have direct access to the cementum, which is much softer than enamel and dissolves more quickly.

如果龈退缩龈和根间形成斑，细菌和酸就可以直接侵犯骨质，与釉质骨质更软，溶解更快。

The crown is the visible part of the tooth that protrudes from the gingiva, and it's covered in enamel, which has such a high mineral content that it's the hardest substance in the human body.

冠是齿突出龈，可以被直视的部分，它被釉质覆盖；釉质的矿物质含量极高，是人体最坚硬的组织。