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

口腔表由角化皮覆盖的结构包括硬腭、舌的背侧（）牙龈。

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.

如果牙龈退缩牙龈牙根间形成牙斑，酸就可以直接侵犯牙骨质，牙釉质相比牙骨质更软，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.

牙冠是牙齿突出牙龈，可以被直视的部分，它被牙釉质覆盖；牙釉质的矿物质含量极高，是人体最坚硬的组织。