The muscle indicated is the orbicularis oris muscle.
The orbicularis oris muscle is one of the muscles of facial expression belonging to the oral group. The muscles of facial expression can be organised into functional groups to provide a more structured approach to understanding the muscles. The functional groups include the following:
The oral group of muscles includes muscles that are responsible for moving the lips and the cheek. There are 11 muscles included in this functional group: orbicularis oris, depressor anguli oris, depressor labi inferioris, mentalis, risorius, buccinator, zygomaticus major, zygomaticus minor, levator labii superioris, levator labii superioris alaeque nasi, levator anguli oris.
The muscle fibres of the orbicularis oris form a circular ring around the mouth. Contraction of the orbicularis oris results in closing of the lips and narrowing of the mouth – “pursing of the lips”. It receives innervation from the buccal branch of the facial nerve (cranial nerve VII).
To learn more about the anatomy of the muscles of facial expression, watch this tutorial.