When you’re eating a slice of pizza, the first place food enters is the oral cavity and it is actually where digestion starts. Here, we get to taste the pizza and form a bolus of food to swallow for further digestion in the gut. Many structures work together to chew and swallow like the lips, teeth, and palate, so let’s explore them!
Now, the oral cavity itself is divided into the oral vestibule and the oral cavity proper. The vestibule is the compartment between the anterior and lateral aspects of the teeth and gingiva and the posterior and medial aspects of the lips and cheeks.
Moving further inside is the oral cavity proper, bounded by the dental arches anteriorly and laterally, and the palate superiorly. The oral cavity proper is mainly occupied by the tongue.
Now let’s get back to that slice of pizza. To eat it, first you open your mouth, and the opening through which the food enters is called the oral fissure.
Once the pizza’s inside the oral cavity, you use the lips, which act like valves, to keep the oral fissure closed and the food inside while you’re chewing. The lips, and the muscles within it, also help us exhale through the mouth, whistle, speak, or kiss.
Now, the lips extend from the nasolabial sulci and the nares to the mentolabial sulcus, which is just inferior to the lips, but not all of its surface looks the same.
The most remarkable feature is the part we’re actually used to calling “lips”: a thin, hairless, reddish skin, called the transitional zone, because it represents the transition between the skin of the face and the inner labial mucosa.