Axial Skeleton: Vertebral Column, Sternum, and Ribs

The Vertebral Column: Anatomy and Divisions

Overview of the Vertebral Column (Spine/Backbone/Spinal Column)

• Begins with $33$ individual vertebrae that fuse during development to become $24$ distinct vertebrae in the adult.

• Each vertebra articulates with one superior and one inferior vertebra, forming a flexible column.

• Key Functions:

  • Provides vertical support for the body.

  • Supports the weight of the head.

  • Helps maintain an upright body position.

  • Houses and protects the delicate spinal cord.

Vertebral Anatomy: Common Structural Features

All vertebrae share several fundamental structural components:

• Body: A thick, anterior, weight-bearing structure. This is the largest part of the vertebra, especially prominent in the lower regions.

• Vertebral Arch: Located posterior to the body, it encloses the spinal cord.

  • Pedicle: Originates from the posterolateral margins of the vertebral body, connecting the arch to the body.

  • Lamina: Extends posteromedially from the posterior edge of the pedicle, forming the posterior portion of the vertebral arch. (Refer to Page $4$ for a superior view illustrating lamina and pedicle).

• Vertebral Foramen: The opening enclosed by the vertebral body and the vertebral arch. This space directly contains the spinal cord at each segmental level.

• Vertebral Canal: Formed by the stacking of all individual vertebral foramina. This continuous canal runs the length of the vertebral column and houses the entire spinal cord.

• Transverse Process: A lateral projection extending from the vertebral arch, providing attachment points for muscles and ligaments.

• Spinous Process: A prominent posterior projection extending from the vertebral arch, also serving as an attachment point for muscles and ligaments. (Refer to Page $4$ for a superior view illustrating transverse and spinous processes).

Divisions of the Vertebral Column and Naming Convention

The vertebral column is divided into five distinct regions, totaling $24$ mobile vertebrae plus the fused sacrum and coccyx.

• Mnemonic for remembering the number of vertebrae in mobile regions: Think "when you eat meals":

  • $7$ - Cervical (Breakfast)

  • $12$ - Thoracic (Lunch)

  • $5$ - Lumbar (Dinner)

• The five divisions from superior to inferior are:

  • Cervical Vertebrae: $7$ (denoted $C<em>1$ to $C</em>7$)

  • Thoracic Vertebrae: $12$ (denoted $T<em>1$ to $T</em>{12}$)

  • Lumbar Vertebrae: $5$ (denoted $L<em>1$ to $L</em>5$)

  • Sacral/Sacrum: $5$ fused vertebrae forming a single bone (denoted $S<em>1$ to $S</em>5$ when referring to segments, or simply "Sacrum" as a bone).

  • Coccyx: Variable number, typically $1-5$ fused segments (often $4$), commonly forming a single bone or several small fused anes.

• Identification System: Vertebrae are identified by a capital letter representing their region, followed by a numerical subscript or number indicating their sequence from superior to inferior locations.

  • Examples: $T<em>{12}$ (thoracic vertebra $12$), $L</em>3$ (lumbar vertebra $3$), $S<em>2$ (sacral vertebra $2$ segment), $C</em>3$ (cervical vertebra $3$).

Specific Vertebral Regions: Distinctive Features

Cervical Vertebrae ($C<em>1-C</em>7$)

• There are $7$ cervical vertebrae, forming the bones of the neck.

• Their primary role is to support only the weight of the head.

• Distinguishing Features:

  • Transverse Foramen: A unique opening within each transverse process that houses the vertebral artery, which supplies blood to the brain.

  • Bifid Spinous Process: Most cervical vertebrae ($C<em>2$ to $C</em>6$) have a spinous process that splits into two parts at its tip.

• Specialized Cervical Vertebrae:

  • Atlas ($C_1$): Named for its role in supporting the head (like the mythological figure). It is a ring-shaped vertebra without a body or spinous process. It articulates with the occipital condyles of the skull and permits the nodding motion of the head (the "yes" movement).

  • Axis ($C_2$): Features a superior projection called the dens (or odontoid process), which extends upward into the vertebral foramen of the atlas. This articulation allows the atlas (and thus the head) to rotate around the dens, permitting the side-to-side head shaking motion (the "no" movement).

  • Vertebra Prominens ($C_7$): This is typically the most inferior cervical vertebra and has a long, non-bifid spinous process that is easily palpated (felt) through the skin at the base of the neck, making it a useful landmark.

Thoracic Vertebrae ($T<em>1-T</em>{12}$)

• There are $12$ thoracic vertebrae, forming the superior region of the back.

• Distinguishing Features:

  • Possess a long, downward-pointing spinous process.

  • Have facets on their bodies and transverse processes for articulation with the ribs.

  • Often described as looking like a "giraffe" in a lateral or superior view due to their prominent spinous and transverse processes.

Lumbar Vertebrae ($L<em>1-L</em>5$)

• There are $5$ lumbar vertebrae, forming the inferior, concave ("small") region of the back.

• Distinguishing Features:

  • They are the largest of the vertebrae, reflecting their role in weight-bearing.

  • Have a very thick, robust vertebral body.

  • Possess a short, blunt spinous process that projects posteriorly.

  • Bear most of the weight of the body.

  • Often described as looking like a "moose" in a lateral or superior view due to their large bodies and distinctive processes.

Sacral Vertebrae (Sacrum)

• Initially $5$ individual sacral vertebrae that fuse into a single, large, triangular bone called the sacrum in adulthood.

• It is anteriorly curved.

• Key Features:

  • Sacral Canal: A continuation of the vertebral canal, containing the sacral nerves.

  • Sacral Hiatus: The inferior opening of the sacral canal.

  • Auricular Surface: An ear-shaped surface for articulation with the ilium of the pelvis.

  • Median Sacral Crest: A ridge formed by the fused spinous processes.

  • Apex: The inferior, narrow tip of the sacrum.

  • Base: The superior, broad end of the sacrum.

  • Transverse Ridges: Lines indicating the fusion points of the original sacral vertebrae.

Coccyx

• Commonly called the "tailbone."

• Ranges in number from $1-5$ small, individual vertebrae, most commonly $4$, which begin to unite during puberty.

• May ultimately fuse with the sacrum.

• Sexual Dimorphism:

  • In males, the coccyx typically projects anteriorly.

  • In females, it typically projects inferiorly, which is significant for childbirth.

Spinal Curvatures

The vertebral column exhibits four distinct curvatures that provide flexibility and better support the body's weight compared to a straight spine.

• The Four Curves: Cervical, Thoracic, Lumbar, and Sacral curvatures.

• Developmental Sequence: These curves appear sequentially during fetal, newborn, and child development.

  • Primary Curvatures (present during fetal development): These curves retain the $C$-shape of the fetal spine.

    • Thoracic Curve: A posterior curve.

    • Sacral Curve: A posterior curve.

  • Secondary Curvatures (forms after birth): These are anterior curves that develop as a result of muscle activity and upright posture.

    • Cervical Curve: Forms when an infant is able to hold up its head (around $3 months$).

    • Lumbar Curve: Forms when a child is able to stand and walk (around $12-18 months$), shifting the center of gravity over the legs.

Sternum (Breastbone)

• A flat bone located on the anterior midline of the thoracic wall.

• Components of the Sternum:

  • Manubrium: The widest and most superior portion of the sternum. It articulates with the clavicles (collarbones) and the first pair of ribs.

  • Body: The longest part of the sternum. It articulates with the manubrium at the sternal angle (a palpable landmark). Costal cartilages from ribs $2-7$ attach directly to the body.

  • Xiphoid Process: The small, triangular cartilaginous projection at the inferior tip of the sternum. It doesn't fully ossify (turn to bone) until approximately age $40$. Due to its soft tissue attachment, it is a critical landmark for CPR hand placement to avoid injury to underlying organs.

Ribs

• There are $12$ pairs of ribs, originating posteriorly on the thoracic vertebrae.

• They form the protective cage of the thorax.

• Classification of Ribs based on Sternum Attachment:

  • True Ribs (Ribs $1-7$): These ribs connect individually and directly to the sternum via their own cartilaginous extensions, known as costal cartilages.

  • False Ribs (Ribs $8-12$): The costal cartilages of these ribs do not attach directly to the sternum.

    • Ribs $8-10$: Their costal cartilages indirectly attach to the sternum by fusing with the costal cartilage of the rib superior to them (they join the cartilage of rib $7$).

    • Ribs $11-12$ (Floating Ribs): These ribs have no direct or indirect connection to the sternum. Their costal cartilages terminate within the musculature of the lateral body wall.

• Rib Anatomy:

  • Head: The posterior end of the rib that articulates with the vertebral bodies.

    • Superior Facet: Articulates with the body of the vertebra superior to its own number.

    • Inferior Facet: Articulates with the body of its numerically corresponding vertebra.

  • Neck: The constricted portion just lateral to the head.

  • Tubercle: A small bump on the posterior side of the rib, where the neck joins the body. It has an articular facet for articulation with the transverse process of the numerically corresponding vertebra.

  • Body (Shaft): The main curved portion of the rib.

  • Costal Angle: The point where the rib sharply bends anteriorly and inferiorly.

  • Costal Groove: A groove on the inferior and internal surface of the rib body that protects the intercostal nerve and vessels.

  • Facet for Costal Cartilage: The anterior end of the rib, which connects to the costal cartilage for sternal or indirect attachment.

  • Intercostal Space: The space between adjacent ribs, occupied by intercostal muscles, nerves, and vessels.