Final answer:
The lips are formed primarily by the orbicularis oris muscle and play a vital role in regulating the movements of the mouth, such as speaking and eating.
Step-by-step explanation:
The structure formed primarily by the orbicularis oris muscle is the lips. On the exterior, the lips transition from skin to a mucous membrane on the inside of the mouth, reflecting their dual composition. The orbicularis oris muscle itself is crucial in regulating what enters and leaves the mouth and is directly involved in actions like speaking and eating. Not only do the lips have an important functional aspect, but their substantial representation on the cerebral cortex suggests a possible reason for humans' attraction to activities such as kissing. This muscle, along with the buccinator muscles in the cheeks, aids in keeping food in the mouth while chewing and plays a role in speech articulation.