THE SPINE AND MOVEMENT
Altogether there are 149 joints in the spine. As well as those which link the vertebrae to each other, there are those which link the spine to other structures. For instance, the first cervical vertebra is joined to the part of the skull called the occiput, and the sacrum is joined to the parts of the hip bones known as the ilia, forming the two sacroiliac joints. The ribs are joined to the transverse processes of the thoracic vertebrae.
The bony basin of the pelvis holds and protects many vital soft structures in the abdomen, such as the intestines and the female reproductive organs. Being articulated with the hip joints, it transfers weight down the legs to the feet. The tilt of the pelvis is an important factor for the balance of the spine.
The individual shapes of the vertebrae govern the direction of their movements, permitting certain movements and not others. For example, the first cervical vertebra (called the atlas), allows the head to nod backwards and forwards, and to tilt sideways. The second (called the axis) has a knob which fits into a socket through the atlas into the base of the skull: it allows the head to rotate, that is to turn to the left and right.
The thoracic vertebrae also allow backward, forward and sideways movement, and most of them also permit rotation – but only to a limited extent, because they are anchored to the rib cage. The lumbar vertebrae permit backward, forward and sideways movement only.
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