Anatomy Chapter 5: Axial Skeleton

Axial Skeleton

• The vertebral column, or spine, forms the central axis of the skeleton and extends from the base of the skull to the tip of the coccyx.
• The vertebral column has three main functions: protection, support, and movement.
• It provides attachment for the deep muscles of the back and ribs laterally.

Vertebral Column

• The vertebral column consists of approximately 33 bones called vertebrae, separated by intervertebral discs.
• The vertebrae are divided into five groups: cervical, thoracic, lumbar, sacral, and coccygeal.
• There are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae, and 3-5 coccygeal vertebrae.

General Structure of a Vertebra

• A typical vertebra is composed of two main parts: an anterior mass of bone called the body and a posterior ring-like portion called the vertebral arch.
• The vertebral body forms the anterior part of each vertebra and is the weight-bearing component.
• The vertebral arch forms the lateral and posterior aspect of each vertebra and, in combination with the vertebral body, forms an enclosed hole called the vertebral foramen.

Vertebral Body

• The size of the vertebral body increases as one descends down the vertebral column.
• Adjacent vertebral bodies are separated by intervertebral discs, which are filled with a gelatinous substance called the nucleus pulposus.

Vertebral Arch

• The vertebral arch has several bony prominences that act as attachment sites for muscles and ligaments.
• The bony prominences include transverse processes, pedicles, lamina, articular processes, and spinous processes.

Cervical Vertebrae

• Cervical vertebrae have three main distinguishing features: bifid spinous process, transverse foramina, and triangular vertebral foramen.
• The first two cervical vertebrae (C1 and C2) are unique to allow for the movement of the head.
• The atlas (C1) is a ring-like structure with no body and a very short spinous process.
• The axis (C2) has a strong conical process called the dens or odontoid process.

Thoracic Vertebrae

• The body of the thoracic vertebrae increases in size from the first to the twelfth vertebrae.
• The thoracic vertebrae have costal facets for articulation with the shaft of the ribs.
• They have two demi facets, superiorly and inferiorly placed on either side of their vertebral body, which articulate with the heads of two different ribs.

Lumbar Vertebrae

• Lumbar vertebrae have very large vertebral bodies that are kidney-shaped.
• They are the largest and sturdiest examples of all vertebrae due to their role in supporting the weight of the upper body.
• The body of the fifth lumbar segment is considerably deeper in front than behind, which gives it a wedge shape.

Sacrum

• The sacrum is formed by the fusion of the five sacral vertebral segments into a curved triangular bone.
• The sacrum is wedged between the iliac bones of the pelvis.
• At each side of the sacral base is a large wing-like lateral mass called the ala.
• The anterior and posterior walls of the sacral canal are perforated by four pairs of pelvic sacral foramina.

Coccyx

• The coccyx is composed of three to five (usually four) rudimentary vertebrae that have a tendency to fuse into one bone in adults.

Vertebral Curvature

• The curvature of the vertebral column increases its strength, flexibility, and ability to absorb shock.
• Adult curvature is classified as primary and secondary curvature.
• Primary curvature includes the thoracic curve and the sacrococcygeal curve.
• Secondary curvature includes the cervical curve and the lumbar curve.

Spinal Deformity

• An abnormal alignment or curve in the bony vertebral column that forms the spine.