The longissimus capitis is a muscle located in the back of the neck. It is part of the erector spinae group, which helps support and move the spine. This muscle originates from the upper thoracic vertebrae and inserts into the base of the skull, specifically the mastoid process. Its primary function is to extend and rotate the head. This muscle plays a crucial role in maintaining posture and facilitating head movements. When the longissimus capitis contracts, it allows for actions such as tilting the head backward or turning it to the side. Proper functioning of this muscle is important for activities like looking around and maintaining balance.