Human Skeletal System: Axial Skeleton – 2

In the part 1 of Human Axial Skeleton, we covered all the bones till hyoid bone. In this post we shall proceed with the vertebral column.

Vertebral column

Vertebral column also known as the vertebrae or backbone houses the spinal cord. It is made up of individual units called vertebrae which are packed together with a cushioning of intervertebral discs present between adjacent vertebrae from the second vertebrae to sacrum. These vertebrae articulate in such a way that provides flexibility to the spine cord to endure curvatures. The total number of vertebrae in a child are 33, but in an adult, sacral and coccygeal vertebrae fuse and the total number is reduced to 26. All the curves in vertebral column are fully developed by the age of 10.

Structure of a typical human vertebrae

1-Vertebral body: Disc-shaped structure present on the anterior side of vertebrae. It is rugged on superior and inferior portions to facilitate the attachment of intervertebral discs.

2-Vertebral arch: The body of vertebrae extends posteriorly into two, thick bony projections called the pedicles. Pedicles unite with a flat lamina and to form vertebral arch. The vertebral arch and the body form a structure with a foramen at the center called the vertebral foramen and the combination of all vertebral foramen forms the vertebral canal through which the spinal cord passes.

3-Processes: The vertebral arch gives rise to seven processes.

Transverse process: Two transverse processes extend laterally on each side from the vertebral arch.

Spinous process: It extends posteriorly from the vertebral arch.

Articular processes: These processes facilitate intervertebral articulation. Each vertebra has four articular processes; two on the superior side and two present inferiorly. The superior articular process of one vertebra articulates with the inferior articular processes of the vertebrae lying above. The articulating surfaces of the articular processes are known as facets.

Picture 8: Structure of a typical vertebra
Picture Reference : Source

Types of vertebrae

1-Cervical vertebrae: There are 7 cervical vertebrae. They have the largest vertebral foramen among the other types as they receive the broadest portion of the spinal cord. The transverse processes of cervical vertebrae have transverse foramen for passage of blood vessels. The spinous process of 2-6 cervical vertebrae is often branched into two. The cervical vertebrae 3-6 represent typical cervical vertebrae, first two are structurally different:

Atlas: It is the first cervical vertebra differs in structure as it lacks the body and spinous process. Its shape is just like a ring of bone with two large lateral masses having facets that provide site for articulation of occipital condyles of the skull. Its transverse process and foramina are considerably larger.

Axis: It is the second cervical vertebrae having a body. On its anterior side, is a large rounded bony protuberance called the odontoid process or dens which articulates on the inner circumference of the atlas between the lateral masses. This articulation allows side-by-side movement of t6he skull.

2-Thoracic vertebrae: These are relatively larger than the cervical vertebrae. The spinous process of 1-10 cervical vertebrae are long and project inferiorly, while that of the rest two are relatively short and project posteriorly. A specific feature of thoracic vertebrae are the costal facets/demifacets (if the tubercle of rib articulates with the notch of a single vertebra, the notch is called a facet and if it articulates with the notch of two vertebrae, the notch is called a demifacet) for the articulation of ribs (except for 11 and 12 thoracic vertebrae). 1^st thoracic vertebra has a superior facet and an inferior demifacets, 2^nd-8^th thoracic vertebrae have superior and inferior demifacets, 9^th thoracic vertebra has a demifacets and thoracic vertebrae 10-12 have facets on either side for articulation with the head of ribs.

3-Lumbar Vertebrae: There are 5 lumbar vertebrae. They have short, thick and broad spinous processes. The superior articular processes of lumbar vertebra are directed medially while the inferior articular processes are directed laterally and not inferiorly. They have thickest intervertebral discs.

4-Sacrum: Sacrum, in an adult, is triangular in shape and is fused. Initially, there are 5 sacral vertebrae which begin to fuse during the age of 16-18 years and are completely fused by the age of 30. It forms a portion of pelvis, which otherwise is known as the pelvic girdle. The fusion of sacral vertebrae is clearly marked by four transverse lines present anteriorly. The antero-lateral end of sacrum has four pairs of anterior sacral foramina. At the superior end, laterally, sacrum has two raised protuberances or wings called the sacral ala.

On the posterior side, sacrum has a median sacral crest formed by the fusion of spinous processes of the upper sacral vertebrae and lateral sacral crest formed by the fusion of transverse process of sacral vertebrae. Four pairs of sacral foramina are also present on the posterior side. A hollow canal formed by the fusion of sacral vertebrae, the sacral canal, is a continuation of the vertebral canal. The lamina of fifth or sometimes of the fourth sacral vertebra seldom fails to meet leaving an opening at the inferior end called the sacral hiatus. The sacral hiatus is guarded by two sacral cornu, the inferior articular process of the fifth sacral vertebra.

On both the lateral surfaces sacrum has large auricular surface for articulation with illium. On the posterior side of auricular surface, inferior to sacral ala, lies the sacral tuberosity. On the posterior side, sacrum has two supra-articular surfaces which articulate with the inferior articular surface of the fifth lumbar vertebra. The base of sacrum articulates with the body of fifth lumbar vertebra. The narrow inferior region of sacrum is known as the apex.

5-Coccyx: Coccyx marks the tail of vertebral column. It is formed by the fusion of four coccygeal vertebrae. The fusion of coccygeal vertebrae is relatively delayed than that of sacral vertebrae and occurs between the age of 20-30 years. Coccyx has a pair of coccygeal cornu formed by the pedicles and superior articular processes of the first coccygeal vertebrae that articulate with the base of sacrum. The lateral surface of coccyx has transverse processes which is largest for the first coccygeal vertebrae.

Sternum

Sternum is a flat bone present at the center of the chest. It provides articulation facets for ribs (only first ten). The approximate length of sternum is about 15 cm and may vary. It consists of three parts- the manubrium, the body and the xyphoid process.

1-Manubrium: Manubrium is present at the superior end of sternum and articulates with sternum at a point called sternal angle. Important regions of manubrium include:

Suprasternal notch: On the superior surface of the manubrium is a depression called suprasternal notch.

Clavicular notch: Lateral to suprasternal notch, are the clavicular notches present of on either side of manubrium. They articulate with the medial clavicle.

Articulation with ribs: Manubrium articulates with the costal cartilages of first and second ribs at its lateral surface on either side.

2-The Body: The body of sternum articulates with the articular surfaces of 2^nd-10^th pairs of ribs either directly or via their costal cartilages.

3-Xyphoid process: Xyphoid process is the tail of sternum pointed at its inferior end. It is formerly made up of hyaline cartilage and is completely ossified by or during the age of 40s. It does not articulate with the ribs.

Ribs

There are 12 pair of ribs that articulate vertebral column on posterior end and with sternum on anterior end (except last two pairs).

1-True ribs: The first seven pairs of ribs that articulate directly with the sternum via their costal cartilages are called true ribs.

2-False ribs: The next five pairs of ribs that either indirectly articulate with or do not articulate with sternum are called false ribs.

3-Vertebrochondral ribs: The cartilages of ribs of 8^th-10^th pairs of ribs are fused with each other and form a common branch that articulates with the cartilage of the 7^th pair of ribs. These ribs are called vertebra-chondral ribs.

4-Floating ribs: The last two pairs of ribs that do not articulate with the sternum at their anterior ends are called floating ribs.

Structure of a typical vertebrae

1-Head: The head of a rib is its posterior end having a superior and inferior articular facet which articulate with either facets or demifacets present in the vertebrae.

2-Neck: It is a narrow constriction present lateral to the head of ribs.

3-Tubercle: A small protuberance present superior to neck that articulates with the facets present on the transverse process of vertebrae.

4-Shaft/Body: The shaft of ribs takes a sharp curve beyond the neck at a point called costal angle. The inner surface of the body of ribs has a depression called costal groove. At the anterior end, body has a facet for costal cartilage through which it articulates with sternum. The arch formed curved body of the rib is called costal arch.

References

1-Gerard Tortora and Bryan Derrickson, Principles of Anatomy and Physiology, Volume 1, International Student Version

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