Axial Skeleton:#8 Vertebrae, Ribs, Sternum, and Hyoid Bone

Lab Exercise #8: Axial Skeleton

Overview of Vertebrae

• The vertebral column consists of individual vertebrae.

• Lower vertebrae are generally larger than upper vertebrae, supporting more weight.

• Intervertebral discs: Fibrocartilaginous pads located between individual vertebrae.

• Vertebral foramen: A large opening through which the spinal cord passes.

• Body: The anterior, weight-bearing part of the vertebra, in contact with the intervertebral disc.

• Vertebral arch: The posterior section of the vertebra, located behind the body.

  • Consists of two pedicles and two lamina.

  • Pedicles: Extend from the body of the vertebra to the two transverse processes.

  • Lamina: Broad, flat structures situated between the transverse process and the dorsal spinous process.

• Superior articular process and superior articular facet: Structures that articulate (form a joint) with the vertebra located above it.

• Inferior articular process and inferior articular facet: Structures that articulate with the vertebra located below it.

Vertebrae Classification (Top to Bottom)

• Cervical vertebrae ($C<em>1$ - $C</em>7$):

  • There are $7$ cervical vertebrae.

  • Mnemonic: "I eat breakfast at $07:00$ AM" (referring to $7$ vertebrae).

• Thoracic vertebrae ($T<em>1$ - $T</em>{12}$):

  • There are $12$ thoracic vertebrae.

  • Mnemonic: "I eat lunch at $12:00$ (Noon)" (referring to $12$ vertebrae).

• Lumbar vertebrae ($L<em>1$ - $L</em>5$):

  • There are $5$ lumbar vertebrae.

  • Mnemonic: "I eat dinner at $05:00$ PM" (referring to $5$ vertebrae).

Spinal Curvatures

There are $4$ main spinal curvatures, which provide balance in an upright posture and offer space for internal organs like the lungs:

• Cervical: Convex curvature.

• Thoracic: Concave curvature.

• Lumbar: Convex curvature.

• Sacral (pelvic): Concave curvature.

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

• Unique feature: They are the only vertebrae that possess $3$ foramina:

  • One vertebral foramen.

  • Two transverse foramina (one on each side) through which arteries and veins pass.

• $C<em>2$ - $C</em>6$: Typically have bifid spinous processes, meaning they are split in two.

• $C_7$ (Vertebra prominens): Characterized by a spinous process that projects sharply in a posterior direction and is often palpable.

• The body of cervical vertebrae is less massive compared to inferior vertebrae, as they bear less weight.

Atlas & Axis ($C<em>1$ & $C</em>2$)

• Atlas ($C_1$):

  • The only vertebra without a body.

  • Its name comes from Atlas, a titan in Greek mythology who held up the world, reflecting its function of supporting the head.

  • Joins with the head and allows for the "Yes" nodding movement.

• Axis ($C_2$):

  • Features a prominent superior projection called the dens (odontoid process).

  • The dens runs superiorly through the atlas.

  • This articulation allows the axis to move on the atlas, facilitating the rotational "No" movement of the head.

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

• Articulation points for ribs: Markings on their lateral posterior bodies.

  • Rib (Costal) facet: A marking present when only one rib attaches to the vertebra.

  • Demifacet: A marking present when two ribs attach to the vertebra. The head of a rib typically attaches to a demifacet.

• Have longer spinous processes that angle more inferiorly.

• Their overall appearance is often compared to that of a Giraffe.

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

• Distinguishing features: They do NOT have transverse foramina or rib facets.

• Spinous processes tend to be more horizontal.

• The lumbar body is considerably larger due to its role in carrying more weight.

• Their overall appearance is often compared to that of a Moose.

Sacrum

• A large, wedge-shaped bone formed by the fusion of $5$ sacral vertebrae.

• Transverse lines: Visible fusion lines on both the posterior and anterior surfaces.

• Anterior surface:

  • Features a shallow depression.

  • Anterior sacral foramina: Two rows of holes on the anterior surface, for nerves and blood vessels.

• Posterior surface:

  • Posterior sacral foramina: Two rows of holes on the posterior surface.

  • Median sacral crest: Formed by the fused spinous processes.

  • Lateral sacral crests: Formed by the fused transverse processes.

• Sacral promontory: A prominent rim on the anterior superior part of the sacrum.

• Ala: Wing-like lateral projections located lateral to the promontory.

• Articular or Auricular (ear-shaped) surfaces: Located lateral to the ala.

  • Each articulates with the ilium to form the sacroiliac joint, which helps distribute forces applied to the sacrum.

• Sacral canal: A continuation of the vertebral canal, running through the sacrum, with superior and inferior openings.

• Superior articular processes and facets: Join with the $L_5$ vertebra.

• Sacral hiatus: An opening at the inferior part of the sacrum, a common site for epidural injections.

Coccyx

• Commonly known as the "tail bone".

• Represents the terminal portion of the vertebral column.

• Typically composed of $4$ fused coccygeal vertebrae.

Hyoid Bone

• Location: Found at the junction of the floor of the mouth and the neck.

• Often referred to as the "floating bone" because it is not directly articulated with any other bone.

• Anchored by muscles from the anterior, posterior, and inferior directions.

• Aids tongue movement and swallowing.

• Body: Located in the center of the bone.

• Greater and Lesser Horn (Cornu):

  • The greater horn is more posterior.

  • The lesser horn is more anterior.

Ribs

• There are a total of $12$ paired ribs, which, along with the sternum, form the thoracic cage.

• True ribs: The first $7$ pairs ($1$ through $7$).

  • Each attaches directly to the sternum by its own costal cartilage.

• False ribs: Ribs $8$ through $12$.

  • Do NOT attach directly to the sternum by their own cartilage; instead, their cartilages typically fuse with the cartilage of the rib above.

• Floating ribs: Ribs $11$ and $12$ (a subset of false ribs).

  • Do NOT attach to the sternum at all; they terminate in the muscles of the abdominal wall.

Rib Structures

• Head: The posterior end of the rib that articulates with the body of one or more thoracic vertebrae.

• Neck: The constricted region located near the head of the rib.

• Tubercle: A process near the neck and head of ribs $1$ through $10$ that articulates with the transverse process of a thoracic vertebra.

• Shaft: The main, elongated part of the rib.

• Angle or Costal angle: A prominent posterior bend in ribs $2$ through $10$, located near the head of the rib.

• Sternal end: The anterior end of the rib that attaches to a costal cartilage before joining the sternum.

• Costal groove: A depression that runs along the inferior edge of each rib, providing passage for nerves and vessels.

Sternum (Breastbone)

• Composed of $3$ fused bones:

  1. Manubrium: The superior segment of the sternum.

     • Jugular (Suprasternal) notch: A depression at the superior border of the manubrium.

     • Clavicular notches: Lateral articulation sites on the manubrium for the clavicles (collarbones).

  2. Body or Gladiolus: The main, middle portion of the sternum.

     • Sternal angle (Angle of Louis): The joint between the manubrium and the body, an important anatomical landmark, often palpable.

     • Costal notches: Articulation sites for the costal cartilages of the ribs.

  3. Xiphoid process: The narrow, blade-like, and most inferior segment of the sternum. It is primarily cartilaginous in youth and ossifies later in life.