Axial Skeleton: Skull & Hyoid Bone Study Notes

Axial Skeleton

Overview of the Axial Skeleton

The axial skeleton forms the central axis of the human body and provides structural support and protection for vital organs. It is distinct from the appendicular skeleton, which includes the limbs and girdles.

• Definition: The axial skeleton consists of bones along the body's midline, including the skull, vertebral column, and thoracic cage.

• Main Components: Skull (cranium and facial bones), vertebral column, ribs, and sternum.

• Number of Bones: There are 80 bones in the axial skeleton.

• Function: Protects the brain, spinal cord, and thoracic organs; provides attachment for muscles.

• Comparison: The appendicular skeleton contains 126 bones and includes the limbs and girdles.

• Bilateral Symmetry: Most bones are paired and replicated on each side of the body.

Example: The skull is part of the axial skeleton, while the humerus (arm bone) is part of the appendicular skeleton.

Skull & Hyoid Bone

Structure and Components of the Skull

The skull is the most complex part of the human skeleton, composed of 22 bones and several associated elements. It protects the brain and forms the structure of the face.

• Cranium: The portion of the skull that encloses the brain.

• Facial Bones: Bones that form the structure of the face.

• Associated Elements: Includes the hyoid bone and auditory ossicles.

• Functions: Protection, muscle attachment, and support for sensory organs.

Example: The mandible is the only movable bone of the skull.

Classification of Skull Bones

The bones of the skull are classified into cranial and facial bones, each with specific functions and locations.

• Cranial Bones (8): Frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid.

• Facial Bones (14): Mandible, maxillae (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal conchae (2), vomer.

• Auditory Ossicles (6): Malleus (2), incus (2), stapes (2).

• Hyoid Bone (1): Located in the neck, supports the tongue.

Additional info: The skull also contains several foramina (openings) for nerves and blood vessels.

Major Cavities and Spaces of the Skull

Skull Cavities and Foramina

The skull contains several cavities and spaces that house organs and allow passage of nerves and vessels.

• Cranial Cavity: Encloses the brain.

• Orbits: House the eyes.

• Nasal Cavity: Passage for air and olfactory structures.

• Oral Cavity: Contains the mouth and associated structures.

• Foramina: Openings for nerves and blood vessels (e.g., foramen magnum for spinal cord).

Example: The foramen magnum is a large opening in the occipital bone for the passage of the medulla oblongata.

Major Bony Markings of the Skull

Bony Markings and Muscle Attachments

Bony markings on the skull serve as attachment points for muscles and ligaments, and as passageways for nerves and vessels.

• Processes: Projections such as the mastoid process (temporal bone) and coronoid process (mandible).

• Foramina: Openings such as the mental foramen (mandible) and supraorbital foramen (frontal bone).

• Ridges and Lines: Temporal lines for muscle attachment.

• Fossae: Depressions such as the hypophyseal fossa (sphenoid bone).

Example: The zygomatic arch is formed by the zygomatic bone and temporal bone, serving as an attachment for the masseter muscle.

Skull Joints and Sutures

Types of Skull Joints

The skull bones are joined by immovable joints called sutures, except for the mandible, which forms a movable joint at the temporomandibular joint (TMJ).

• Sutures: Fibrous joints that fuse skull bones (e.g., coronal, sagittal, lambdoid sutures).

• Fontanelles: Soft spots in the fetal and infant skull that allow for growth.

• TMJ (Temporomandibular Joint): The only movable joint of the skull, allowing jaw movement.

Example: The sagittal suture joins the two parietal bones along the midline of the skull.

Comparison Table: Axial vs. Appendicular Skeleton

The following table summarizes the main differences between the axial and appendicular skeletons.

Feature	Axial Skeleton	Appendicular Skeleton
Main Components	Skull, vertebral column, thoracic cage	Limb bones, pectoral and pelvic girdles
Number of Bones	80	126
Function	Protection, support, muscle attachment	Movement, manipulation of environment
Location	Midline of body	Lateral to axial skeleton

Key Terms and Definitions

• Axial Skeleton: Central part of the skeleton, including the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Bones of the limbs and girdles.

• Cranium: Skull bones that encase the brain.

• Mandible: Lower jawbone; only movable skull bone.

• Foramen: Opening in bone for passage of nerves and vessels.

• Suture: Immovable joint between skull bones.

• Fontanelle: Soft spot in infant skull.

• TMJ: Temporomandibular joint; movable joint of the jaw.