Introduction:
The pivot joint between the atlas and axis is a crucial joint in the vertebral column that allows for rotation of the head. This joint is classified as a synovial joint.

Synovial Joints:
Synovial joints are the most common type of joint in the body and are characterized by the presence of a synovial cavity. These joints are highly mobile and allow for various types of movements, such as flexion, extension, rotation, abduction, and adduction. Examples of synovial joints include the knee, shoulder, hip, and elbow joints.

Pivot Joint:
A pivot joint is a type of synovial joint that allows for rotation around a central axis. It consists of a rounded or pointed surface of one bone that fits into a ring or notch of another bone. The joint between the atlas (C1) and axis (C2) is a specific example of a pivot joint.

Atlas and Axis:
The atlas (C1) and axis (C2) are the first two vertebrae in the vertebral column. The atlas is responsible for supporting the weight of the head and allowing for nodding movements. The axis, on the other hand, has a unique feature called the dens or odontoid process, which acts as a pivot for rotation of the atlas and head.

Anatomical Structure:
In the pivot joint between the atlas and axis, the rounded surface of the dens fits into the ring formed by the anterior arch of the atlas. This arrangement allows for the rotation of the head, as the atlas rotates around the dens. The joint is supported by ligaments and surrounded by a synovial capsule that contains synovial fluid, which lubricates and nourishes the joint.

Function:
The pivot joint between the atlas and axis is responsible for the rotatory movements of the head. This joint allows for the shaking of the head to indicate "no" and the rotation of the head to look over the shoulder.

Conclusion:
In conclusion, the pivot joint between the atlas and axis is a synovial joint that allows for rotation of the head. This joint is crucial for various movements of the head and neck and is supported by ligaments and synovial fluid.