Chapter 6: The Skeletal System: Axial Division

The Skeletal System: Axial Division

Introduction to the Axial Skeleton

• Functions: The axial skeleton serves as a fundamental framework performing several critical roles:

  • Support and Protection: It supports and safeguards organs located within the dorsal (cranial and vertebral) and ventral (thoracic and abdominopelvic) body cavities.

  • Special Sense Organs: It houses and protects the specialized sense organs responsible for taste, smell, hearing, balance, and vision.

  • Muscle Attachment: It provides crucial attachment sites for muscles involved in:

    • Adjusting the posture of the head, neck, and trunk.

    • Facilitating the movement of the thoracic cage for respiration.

    • Stabilizing the appendicular skeleton.

• Components and Bone Count: The axial skeleton is a major division of the human skeletal system, which totals $206$ bones. The axial component consists of $80$ bones, specifically:

  • Skull and Associated Bones: total $29$ bones

    • Skull: total $22$ bones

      • Cranium: $8$ bones

      • Face: $14$ bones

    • Associated Bones: total $7$ bones

      • Auditory Ossicles: $6$ (3 per ear)

      • Hyoid: $1$

  • Vertebral Column: total $24-26$ bones (depending on sacrum/coccyx fusion)

    • Vertebrae: $24$

    • Sacrum: $1$

    • Coccyx: $1$

  • Thoracic Cage: total $25$ bones

    • Sternum: $1$

    • Ribs: $24$ (12 pairs)

The Skull and Associated Bones

Bones of the Cranium

• The cranial cavity is a fluid-filled chamber designed to support and protect the brain.

• It is composed of the following $8$ bones:

  • Occipital bone (posterior base of the skull).

  • Parietal bones ($2$) (form the superior and lateral aspects of the skull).

  • Frontal bone (forms the forehead).

  • Temporal bones ($2$) (form the inferior and lateral aspects of the skull).

  • Sphenoid bone (a butterfly-shaped bone at the base of the skull).

  • Ethmoid bone (situated between the orbits).

Bones of the Face

• The skull contains a total of $14$ facial bones.

• Paired Facial Bones ($6$ pairs, $12$ total):

  • Maxillae (upper jaw bones).

  • Palatine bones (form the posterior part of the hard palate).

  • Nasal bones (form the bridge of the nose).

  • Zygomatic bones (cheekbones).

  • Lacrimal bones (form part of the medial walls of the orbits).

  • Inferior nasal conchae (thin, curved bones within the nasal cavity).

• Single Facial Bones ($2$ total):

  • Vomer (forms part of the nasal septum).

  • Mandible (the lower jaw bone, the only movable bone of the skull).

The Nasal Complex and Paranasal Sinuses

• Paranasal sinuses are interconnected hollow spaces found within several skull bones:

  • Frontal bone

  • Ethmoid bone

  • Sphenoid bone

  • Maxillary bones

• Functions of Paranasal Sinuses: These air-filled spaces contribute to:

  • Reducing the overall weight of the skull.

  • Producing mucus.

  • Allowing air to resonate, which is essential for voice production.

• Specific Sinuses: The paranasal sinuses are individually named:

  • Frontal sinus

  • Maxillary sinus

  • Sphenoidal sinus

  • Ethmoidal air cells

The Hyoid Bone

• The hyoid bone is a single bone, uniquely situated in the anterior neck.

• It does not articulate directly with any other bone but is suspended by ligaments and muscles.

• It functions as an attachment point for muscles of the tongue, pharynx, and larynx, playing a crucial role in swallowing and speech.

Review of the Skull Bones

• Total skull bones: $22$

  • Cranium: $8$

    • Occipital ($1$)

    • Parietal ($2$)

    • Frontal ($1$)

    • Temporal ($2$)

    • Sphenoid ($1$)

    • Ethmoid ($1$)

  • Facial Bones: $14$

    • Paired: Maxillae ($2$), Palatine ($2$), Nasal ($2$), Zygomatic ($2$), Lacrimal ($2$), Inferior Nasal Conchae ($2$)

    • Single: Vomer ($1$), Mandible ($1$)

The Skulls of Infants, Children, and Adults

• Infant skulls feature fontanelles, which are fibrous membranes connecting cranial bones. These allow for skull flexibility during birth and brain growth, eventually ossifying to form sutures.

The Vertebral Column

• Structure: The adult vertebral column is comprised of $26$ individual bones:

  • $24$ individual vertebrae (cervical, thoracic, lumbar).

  • $1$ sacrum (formed by the fusion of $5$ vertebrae).

  • $1$ coccyx (formed by the fusion of $3-5$ rudimentary vertebrae).

• Functions: The vertebral column performs several vital functions for the body:

  • Spinal Cord Protection: It encloses and meticulously protects the delicate spinal cord.

  • Skull Support: It provides essential support for the skull.

  • Weight Bearing: It supports the considerable weight of the head, neck, and trunk.

  • Weight Transfer: It efficiently transfers this body weight to the lower limbs.

  • Postural Maintenance: It plays a key role in maintaining the upright position of the body.

• Vertebral Regions: The vertebral column is anatomically divided into distinct regions from superior to inferior:

  • Cervical Vertebrae: $7$ (C1-C7)

  • Thoracic Vertebrae: $12$ (T1-T12)

  • Lumbar Vertebrae: $5$ (L1-L5)

  • Sacral Vertebrae: $1$ (a single bone formed by the fusion of $5$ vertebrae)

  • Coccygeal Vertebrae: $1$ (a single bone formed by the fusion of $3-5$ vertebrae)

Spinal Curves

• Definition: Spinal curves are natural anterior and posterior curvatures of the vertebral column that are crucial for weight transfer and flexibility. They are named according to the region in which they occur:

  • Cervical curve

  • Thoracic curve

  • Lumbar curve

  • Sacral curve

• Primary Curves (Accommodation Curves):

  • These are posteriorly sweeping curves found in the thoracic and sacral regions.

  • They develop before birth.

  • Their primary purpose is to provide sufficient room for the abdominopelvic viscera within the body cavities.

• Secondary Curves (Compensation Curves):

  • These are anteriorly sweeping curves that develop in the cervical and lumbar regions.

  • They develop after birth during infancy and toddlerhood.

  • Cervical Curve: Develops as an infant learns to hold his or her head up.

  • Lumbar Curve: Develops as a toddler begins to walk.

General Vertebral Anatomy

• Vertebral Body: The main, anterior weight-bearing portion.

• Vertebral Arch: Posterior to the body, composed of pedicles and laminae.

• Vertebral Foramen: The opening enclosed by the vertebral body and arch, through which the spinal cord passes.

• Spinous Process: A projection extending posteriorly from the vertebral arch.

• Transverse Processes: Projections extending laterally from the vertebral arch.

• Superior and Inferior Articular Processes: Projections bearing articular facets that allow articulation with adjacent vertebrae.

• Intervertebral Foramen: Openings between adjacent vertebrae allowing for the passage of spinal nerves.

• Intervertebral Discs: Fibrocartilaginous pads located between vertebral bodies, acting as shock absorbers and allowing flexibility.

Typical Cervical Vertebrae (C3-C6)

• Number: There are $7$ total cervical vertebrae (C1-C7).

• Size: They are the smallest and most superior vertebrae.

• Spinous Process: Relatively stumpy and often feature a bifid process (split tip).

• Costal Processes: Extra extensions of bone from the ventrolateral body that attach to the transverse processes.

• Transverse Foramina: Holes located within the transverse processes, formed between the costal process and the transverse process, allowing passage of vertebral arteries and veins.

The Atlas (C1)

• Unique Feature: The atlas is unique because it has no vertebral body.

• Articulation: It articulates cranially with the occipital condyles of the skull.

• Movement: This specific articulation is responsible for the