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The atlanto-axial joint is supposedly a pivot joint. I assumed it would be considered a ball-and-socket joint, based on a) the physical shape of the dens, and b) the degrees of motion of the cranium relative to the spinal column.

A pivot joint is distinct from a typical ball-and-socket joint because it only allows for a single degree of freedom; yaw - to rotate about it's own axis or diameter, like a spindle. A typical ball-and-socket joint allows at least one additional plane of limited movement.

I can think of (or observe) at least three degrees of freedom of the cranium, relative to the spinal column; pitch & roll. Pitch allows you to direct your gaze from the ceiling, to the floor. Roll allows lateral movement in a similar manner, so you can point your left ear to the ground, and then your right.

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The additional degrees of freedom are provided by the entire cervical spine, rather than the atlantoaxial joint alone, which is the joint between C1 and C2. What you're describing as roll (when you point one ear to the ground), is provided by contraction of the sternocleidomastoid. The sternocleidomastoid muscle pulls the mastoid process (behind the ear) down and in, toward the fixed clavicle and sternum. The joint that provides lateral flexion in response is the entire cervical spine.

Here you can see the sternocleidomastoid muscle (the SCM):

enter image description here

And here, a drawing with a reasonable approximation of the way the cervical spine responds to lateral flexion (due to contraction of the SCM). I can't find a good c-spine film at the moment to demonstrate it in a human person, rather than an artists imagination. But this drawing is accurate to my recollection. You can see the space between C1 and C2 on the right is quite limited (that atlas and the axis are in close apposition). C2 and C3, as well as the other cervical vertebrae, have more space between them on the right: enter image description here

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