Lateral masses of C1

These are the lateral boney masses on the superior side of the Atlas (C1, First cervical vertebral body) that articulate with the occipital condyles of the skull at the Occipital-C1 articulation. 50% of the flexion/extension in the cervical spine takes place at the joint between the occipital condyles and the lateral masses of C1. This is part of the craniocervical junction.