Osteology Notes

The vertebral column consists of a total of $26$ bones: $24$ individual vertebrae plus two fused bones (the sacrum and the coccyx).
Vertebrae are organized into regions: seven cervical (C1–C7), twelve thoracic (T1–T12), and five lumbar (L1–L5). The sacrum comprises five fused vertebrae, and the coccyx comprises four fused vertebrae.
Functions: supports the skull and ribs, facilitates trunk movements, and provides attachment points for upper and lower limbs.

Key features present in most vertebrae: body, vertebral arch, spinous process, transverse processes, superior articular process, inferior articular process, vertebral foramen, intervertebral disc between adjacent vertebral bodies, intervertebral foramina for spinal nerves, nucleus pulposus (inner region) and annulus fibrosus (outer ring).
Zygapophysial joints (facet joints) formed by articulation of superior and inferior articular processes.
Intervertebral discs: pads of fibrocartilage that separate stacked vertebral bodies.

Composition: outer annulus fibrosus (fibrocartilaginous ring) and inner nucleus pulposus (gelatinous region).
Function: shock absorption and load distribution across the vertebral column.
End of spinal cord and cauda equina: within the vertebral canal; below the end of the cord, the cauda equina continues.
Disc bulging/herniation: bulging disc can impinge on neural elements; discs are commonly referred to as herniated or slipped discs in pathology.

General features: relatively small bodies; transverse processes with transverse foramina that transmit the vertebral artery.
Atlas (C1): no body; only bony contact between skull and vertebral column; articulates with the occipital bone.
- Superior articular facets are cup-shaped to receive the occipital condyles.
Axis (C2): characterized by the dens (odontoid process) projecting upward through the anterior arch of C1.
- Dens acts as a pivot for C1, enabling head rotation on the neck.
Typical cervical vertebra anatomy applies to C3–C7.

Primary curvature: anteriorly concave (thoracic kyphosis).
Number: $T1$ – $T12$ (12 vertebrae).
Features: facets on the body and the transverse processes for articulation with ribs; spinous processes are long and slender and oriented inferiorly.
Each thoracic vertebra articulates with its corresponding rib via costal facets on the body and the transverse process.

Number: $L1$ – $L5$ (5 vertebrae).
Key features: absence of facets for ribs; no transverse foramina.
Body: large and robust; spinous processes are stout and square (often described as moose-like).
Curvature: lumbar spine has a secondary curve (develops to support weight bearing).

Sacrum: fused $S1$ – $S5$ ; forms a strong posterior wall of the pelvis and links the vertebral column to the pelvis.
- Sacral canal runs within the sacrum; sacral foramina transmit sacral nerves.
- Lateral sacral crests and other surface features provide attachment sites for ligaments and muscles.
Coccyx: fused remnants of $3–4$ vertebrae; tailbone.

Anterior longitudinal ligament: runs along the anterior surfaces of the vertebral bodies from the skull to the sacrum.
Posterior longitudinal ligament: runs within the vertebral canal along the posterior aspect of the vertebral bodies, from skull to sacrum.
Supraspinous ligament: runs along the tips of the spinous processes.
Interspinous ligaments: run between adjacent spinous processes.
Ligamentum flavum: connects laminae between adjacent vertebrae.
Function: these ligaments stabilize the spine and limit movements that could damage the spinal cord; whiplash is often discussed in relation to hyperextension injury of the neck.

Bony thorax protects thoracic organs and provides an attachment framework for muscles.
Ribs: 1–7 true ribs; 8–10 false ribs; 11–12 floating ribs.
Sternum (breastbone): composed of three parts—manubrium, body, and xiphoid process.
Anterior wall: ribs and costal cartilage.
Posterior wall: vertebrae and posterior aspects of the ribs.
Lateral walls: ribs.
Function: protection of thoracic organs (heart and lungs) and a rigid framework for respiration; sternum serves as a central attachment point for muscles.

Pectoral girdle consists of clavicle and scapula, which anchor the upper limb to the axial skeleton.
Sternoclavicular joint provides attachment for muscles moving the arm via the clavicle.
Clavicle: a doubly curved long bone (collarbone); curves laterally and anteriorly for roughly half its length, then forms a smooth posterior curve to articulate with the scapula.
The flat acromial end of the clavicle is broader than the sternal end.
Scapula: large, triangular bone (shoulder blade) surrounded by muscles that move the arm.

Upper limb divisions: brachium (upper arm) contains the humerus; Antebrachium (forearm) contains the radius and ulna; Hand contains carpals, metacarpals, and phalanges.
Carpals: eight bones in the wrist arranged in two rows.
Metacarpals: five bones in the palm.
Phalanges: fourteen bones in the fingers; each finger has proximal, middle, and distal phalanges, except the thumb (pollex) which has only proximal and distal phalanges.
Thumb is also known as the pollex.

Radius and ulna form the forearm with an interosseous membrane that binds them together and allows forearm rotation.
Radial head dislocation is a common pediatric injury known as nursemaid's elbow.

Distal phalanges include the tips of the fingers.
Intermediate (middle) and proximal phalanges are present in all fingers except the thumb (pollex).
Metacarpals lie in the palm; carpals form the wrist.

Disc degeneration and herniation: changes in the nucleus pulposus and annulus fibrosus can decrease shock absorption and compress neural structures.
Spinal curvatures: the four curves (cervical lordosis, thoracic kyphosis, lumbar lordosis, sacral kyphosis) provide flexibility and distribute mechanical loads during movement and standing.
The rib cage provides protection for thoracic organs, supports respiration, and serves as an anchor for muscles of the chest and back.

Vertebral counts: $7$ cervical, $12$ thoracic, $5$ lumbar; sacrum $5$ fused; coccyx $4$ fused -> total $26$ bones.
Atlas: $C1$ ; Axis: $C2$ with the dens forming a pivot for C1.
Intervertebral disc components: $ext{annulus fibrosus}$ and $ext{nucleus pulposus}$ .
True ribs: $1–7$ ; False ribs: $8–10$ ; Floating ribs: $11–12$ .
Pollex: the thumb.
Distal/proximal/middle phalanges and metacarpals: bones of the hand; carpals: $8$ bones in the wrist.

The vertebral column’s structure underpins posture, mobility, and protection of the spinal cord.
Curvatures and intervertebral discs together distribute axial loads during standing, walking, and lifting.
The thoracic cage protects vital organs and facilitates breathing mechanics.
The shoulder girdle and upper limb bones enable a wide range of arm movements while maintaining axial attachments via the sternoclavicular joint.
Common injuries (e.g., disk herniation, nursemaid’s elbow) illustrate functional anatomy in clinical scenarios.