Chapter 7: The Skeleton – Axial Skeleton and Cranial Bones

The Skeletal System

Composition of the Skeleton

The skeletal system forms the structural framework of the human body, providing support, protection, and facilitating movement. It is composed of:

• Bones: Rigid organs that form the majority of the skeleton.

• Cartilage: Flexible connective tissue found in joints, rib cage, ear, nose, bronchial tubes, and intervertebral discs.

• Ligaments: Bands of dense connective tissue that connect bones to other bones.

• Tendons: Connect muscles to bones (though primarily considered part of the muscular system, they interact closely with the skeleton).

Functions of the skeletal system include:

• Support for the body and soft organs

• Protection of vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs)

• Movement, as bones serve as levers for muscles

• Mineral storage (especially calcium and phosphate)

• Blood cell formation (hematopoiesis) in bone marrow

• Triglyceride (fat) storage in yellow bone marrow

The Axial Skeleton

Overview and Major Components

The axial skeleton forms the long axis of the body and provides support and protection for the brain, spinal cord, and organs in the thorax. It consists of:

• Skull: Protects the brain and forms the structure of the face.

• Vertebral column: Supports the body and protects the spinal cord.

• Thoracic cage (rib cage): Protects the heart and lungs.

The axial skeleton does not include the limbs or the girdles that attach them to the trunk (these are part of the appendicular skeleton).

Anterior and Posterior Views:

• Anterior view: Shows the front of the skeleton, highlighting the facial bones, sternum, and ribs.

• Posterior view: Shows the back, including the occipital bone of the skull, vertebral column, and scapulae (shoulder blades, part of appendicular skeleton).

The Skull

Cranial and Facial Bones

The skull is a complex bony structure that protects the brain and forms the face. It is divided into two main parts:

• Cranial bones: Enclose and protect the brain, forming the cranial cavity.

• Facial bones: Form the framework of the face, contain cavities for the sense organs, provide openings for air and food, and secure the teeth.

Appearance:

• Cranial bones are generally curved and thick, forming a protective case.

• Facial bones are more varied in shape, forming the contours and features of the face.

Divisions and Sutures of the Skull

• Cranial division: Includes 8 bones (frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid).

• Facial division: Includes 14 bones (maxilla (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal concha (2), mandible, vomer).

• Sutures: Immovable joints that connect the bones of the skull. Major sutures include:

  • Coronal suture: Between frontal and parietal bones

  • Sagittal suture: Between the two parietal bones

  • Lambdoid suture: Between parietal and occipital bones

  • Squamous suture: Between parietal and temporal bones

Overview of Skull Geography

Key Features and Landmarks

The geography of the skull refers to the arrangement and location of its bones and features. Important landmarks include:

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

• Processes: Projections for muscle attachment (e.g., mastoid process, styloid process).

• Fossae: Depressions that accommodate brain regions (e.g., anterior, middle, and posterior cranial fossae).

• Sinuses: Air-filled cavities within certain bones (e.g., frontal sinus, maxillary sinus).

These features can be identified from various views (anterior, lateral, superior, inferior, and posterior) as shown in textbooks and lab manuals.

Cranium

Detailed Description of Each Cranial Bone

The cranium consists of 8 bones that encase and protect the brain. Each bone has unique features and locations:

• Frontal bone: Forms the forehead and the roofs of the orbits (eye sockets).

• Parietal bones (2): Form the superior and lateral aspects of the skull.

• Temporal bones (2): Form the sides and base of the skull; house structures of the ears.

• Occipital bone: Forms the posterior and base of the skull; contains the foramen magnum for the spinal cord.

• Sphenoid bone: Butterfly-shaped bone at the base of the skull; forms part of the cranial floor and orbits.

• Ethmoid bone: Located between the eyes; forms part of the nasal cavity and the nasal septum.

There are 8 cranial bones in total. Each can be located from various anatomical views as illustrated in textbooks and lab manuals.

Cranial Bone	Location	Key Features
Frontal	Forehead, superior orbits	Supraorbital foramen, frontal sinus
Parietal (2)	Superior/lateral skull	Joined by sagittal suture
Temporal (2)	Inferior/lateral skull	Mastoid process, external acoustic meatus
Occipital	Posterior skull, base	Foramen magnum, occipital condyles
Sphenoid	Base of skull, behind eyes	Sella turcica, greater/lesser wings
Ethmoid	Between orbits, nasal cavity	Cribriform plate, perpendicular plate

Example: The foramen magnum in the occipital bone allows passage of the spinal cord from the brain to the vertebral column.

Additional info: The cranium not only protects the brain but also provides attachment points for head and neck muscles, and houses structures for hearing and balance.