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The atlas is the topmost vertebra and the axis (the vertebra below it) forms the joint connecting the skull and spine. The atlas and axis are specialized to allow a greater range of motion than normal vertebrae. They are responsible for the nodding and rotation movements of the head.
In anatomy, the axis (from Latin axis, "axle") is the second cervical vertebra (C2) of the spine, immediately inferior to the atlas, upon which the head rests. The spinal cord passes through the axis. The defining feature of the axis is its strong bony protrusion known as the dens, which rises from the superior aspect of the bone.
The atlas (C1) and axis (C2) are the two topmost vertebrae. The atlas (C1) is the topmost vertebra, and along with the axis forms the joint connecting the skull and spine. It lacks a vertebral body, spinous process, and discs either superior or inferior to it. It is ring-like and consists of an anterior arch, posterior arch, and two lateral masses.
The atlantoaxial articular capsules are thick and loose, and connect the margins of the lateral masses of the atlas with those of the posterior articular surfaces of the axis. Each is strengthened at its posterior and medial part by an accessory ligament, which is attached below to the body of the axis near the base of the odontoid process ...
The anterior atlantoaxial ligament is a strong membrane, fixed above the lower border of the anterior arch of the atlas; below, to the front of the body of the axis.. It is strengthened in the middle line by a rounded cord, which connects the tubercle on the anterior arch of the atlas to the body of the axis.
The distance between the atlas and the occipital condyles, the atlanto-occipital interval (AOI), should measure less than 4 mm, and is better assessed on coronal images. [ 4 ] The Powers ratio was formerly used, which was the tip of the basion to the spinolaminar line, divided by the distance from the tip of the opisthion to the midpoint of the ...
The cruciate ligament of the atlas (cruciform ligament) is a cross-shaped (thus the name) ligament in the neck forming part of the atlanto-axial joint. It consists of the transverse ligament of atlas, a superior longitudinal band, and an inferior longitudinal band. The cruciate ligament of the atlas prevents abnormal movement of the atlanto ...
(b) slight lateral motion, lateroflexion, to one or other side around the anteroposterior axis. Flexion is produced mainly by the action of the longi capitis and recti capitis anteriores ; extension by the recti capitis posteriores major and minor , the obliquus capitis superior , the semispinalis capitis , splenius capitis ...