Complex sentence: The normally sharp demarcation between the red of the lip and the adjacent normal skin, the vermillion border is important in dentistry and oral pathology because it is used as a marker to detect pathosis (disease), such as in actinic cheilitis.
Split into simpler sentences: The normally sharp demarcation between the red of the lip and the adjacent normal skin. The vermillion border is important in dentistry and oral pathology because it is used as a marker to detect pathosis (disease), such as in actinic cheilitis.