Skeletal Overview and the Axial Skeleton: Study Notes

Skeletal Overview

Classification and Structure of Bone & Cartilages

The human skeleton is constructed from two of the most supportive tissues: bone and cartilage. Bone forms the framework of the body, providing support and protection for organ systems, while cartilage is found in areas requiring flexibility and resilience.

• Bone Functions:

  • Provides a lever system for movement in conjunction with skeletal muscles

  • Acts as a storage depot for lipids and minerals (e.g., calcium, phosphate)

  • Site for hematopoiesis (blood cell formation) in the marrow

• Cartilage Functions:

  • Provides flexibility and resilience in specific locations (e.g., joints, rib cage)

• Joints (Articulations): Bones are connected at joints, allowing movement and stability.

Classification of Bone

Bones are classified by their shape and structure.

• Shape (Gross Anatomy):

  • Long bones (e.g., femur, humerus)

  • Short bones (e.g., carpals, tarsals)

  • Flat bones (e.g., skull, sternum)

  • Irregular bones (e.g., vertebrae, pelvis)

• Structure (Texture):

  • Compact bone: Dense, strong outer layer

  • Spongy (cancellous) bone: Porous, inner layer, often found at the ends of long bones and inside flat bones

The Skeleton

Divisions of the Skeleton

The skeleton is subdivided into two main divisions:

• Axial Skeleton: Composed of the skull, vertebral column, and thoracic cage

• Appendicular Skeleton: Includes the limbs and girdles (not covered in detail here)

Bone Markings

Types and Functions of Bone Markings

Bones are not smooth; they have various projections, depressions, ridges, and holes called bone markings. These serve as sites for muscle attachment, conduits for nerves and blood vessels, and help form joints.

• Projections: Sites for muscle and ligament attachment (e.g., tuberosity, crest, trochanter)

• Depressions and Openings: Allow passage of blood vessels and nerves (e.g., foramen, fossa)

• Joint Formation: Markings that help form joints (e.g., condyle, head)

Name of Bone Marking	Description	Illustrations
Tuberosity	Large rounded projection; may be roughened	Illustrated on ilium and femur
Crest	Narrow ridge of bone; usually prominent	Illustrated on ilium
Trochanter	Very large, blunt, irregularly shaped process	Illustrated on femur
Line	Narrow ridge of bone; less prominent than a crest	Illustrated on femur
Tubercle	Small rounded projection or process	Illustrated on femur
Epicondyle	Raised area on or above a condyle	Illustrated on femur
Spine	Sharp, slender, often pointed projection	Illustrated on vertebra
Process	Any bony prominence	Illustrated on vertebra

The Axial Skeleton

Skull

The skull is a complex structure composed of several bones joined by sutures. It protects the brain and forms the structure of the face.

• Major Bones: Frontal, parietal, temporal, occipital, sphenoid, ethmoid, nasal, zygomatic, maxilla, mandible

• Sutures: Immovable joints between skull bones (e.g., coronal, sagittal, lambdoid, squamous)

• Bone Markings: Processes, condyles, and foramina for muscle attachment and passage of nerves/vessels

Vertebral Column

The vertebral column (spine) is composed of individual vertebrae and provides support, protection for the spinal cord, and flexibility.

• Regions:

  • Cervical (7 vertebrae, C1–C7)

  • Thoracic (12 vertebrae, T1–T12)

  • Lumbar (5 vertebrae, L1–L5)

  • Sacral (5 fused vertebrae, sacrum)

  • Coccygeal (3–5 fused vertebrae, coccyx)

• Spinal Curves: Cervical and lumbar curves are concave; thoracic and sacral curves are convex

Special Cervical Vertebrae: Atlas and Axis

The first two cervical vertebrae are highly modified to allow special functions.

• Atlas (C1): Supports the skull, allows nodding motion

• Axis (C2): Has the odontoid process (dens), allows rotation of the head

• Unique Features: Lateral masses, anterior and posterior arches, transverse foramen

Thoracic Cage

The thoracic cage consists of the ribs, sternum, and thoracic vertebrae. It protects vital organs (heart, lungs) and supports the shoulder girdles and upper limbs.

• Rib Articulation: Ribs articulate with thoracic vertebrae and the sternum via costal cartilages

• Components: Sternum, ribs (true, false, floating), costal cartilages

Example: The rib cage expands during inhalation to allow lung expansion.