The Nasalis muscle has two main parts, the transverse or compressor part (also known as compressor naris), which constricts the nostril, and the alar or dilator part (also known as dilator naris), which flares the nostril. The compressor part of nasalis originates in the upper jaw near the canine tooth and inserts into nasal cartilage on the bridge of the  nose, each side mixing with the other (thus transverse). When it contracts, it tends to draw the nostril wings towards the septum. The dilator part originates in the upper jaw and cartilage of the nose and inserts in skin of the nostril. When it contracts, it pulls the nostril wings away from the septum. (Depressor septii is considered by some to be a part of nasalis.)

Nasalis is innervated by the lower zygomatic and buccal branches of the facial (VII) nerve and receives blood from the facial artery.