The Skeletal System: Structure, Features, and Organization

The Skeletal System

Overview of the Skeleton

The human skeleton provides the structural framework for the body, supporting movement, protecting organs, and serving as a site for hematopoiesis and mineral storage. It is divided into two main regions: the axial and appendicular skeletons.

• Axial skeleton: Forms the central supporting axis of the body and includes the skull, vertebrae, sternum, ribs, sacrum, and hyoid bone.

• Appendicular skeleton: Comprises the pectoral girdle, upper limbs, pelvic girdle, and lower limbs.

• Bone count: Adults typically have 206 bones, while infants have about 270 bones, which decrease in number as bones fuse during development.

Anatomical Features (Markings) of Bones

Bone Markings: Types and Functions

Bones display various anatomical features, known as markings, which serve as sites for articulation, muscle attachment, or passage of nerves and blood vessels. These features are classified as projections, depressions, and passages/cavities.

Type	Name	Description and Example
Joint Surfaces	Condyle	Rounded knob that articulates with another bone (e.g., occipital condyles of the skull)
Joint Surfaces	Facet	Smooth, flat, slightly concave or convex articular surface (e.g., articular facets of vertebrae)
Joint Surfaces	Head	Prominent expanded end of a bone (e.g., head of the femur)
Projections	Crest	Narrow ridge (e.g., iliac crest of the pelvis)
Projections	Epicondyle	Expanded region superior to a condyle (e.g., medial epicondyle of the femur)
Projections	Line	Slightly raised, elongated ridge (e.g., nuchal lines of the skull)
Projections	Process	Bony prominence (e.g., mastoid process of the skull)
Projections	Protuberance	Outgrowth or protruding part (e.g., mental protuberance of the chin)
Projections	Spine	Sharp, slender, or narrow process (e.g., spine of the scapula)
Projections	Tubercle/Tuberosity	Small/large rounded process (e.g., greater tubercle of the humerus, tibial tuberosity)
Depressions	Alveolus	Pit or socket (e.g., tooth socket)
Depressions	Fossa	Shallow, broad, or elongated basin (e.g., mandibular fossa)
Depressions	Fovea	Small pit (e.g., fovea capitis of the femur)
Depressions	Sulcus	Groove for a tendon, nerve, or blood vessel (e.g., intertubercular sulcus of the humerus)
Passages/Cavities	Canal	Tubular passage or tunnel (e.g., carotid canal of the skull)
Passages/Cavities	Fissure	Slit through a bone (e.g., orbital fissure)
Passages/Cavities	Foramen	Hole through a bone (e.g., foramen magnum of the skull)
Passages/Cavities	Meatus	Cavity or canal (e.g., external acoustic meatus)
Passages/Cavities	Sinus	Air-filled space in a bone (e.g., frontal sinus)

Articulated vs. Disarticulated Skeletons

• Articulated skeleton: Bones are connected to show spatial relationships.

• Disarticulated bones: Bones are separated to study surface features and markings in detail.

• Bone markings: Include ridges, spines, bumps, depressions, canals, pores, slits, cavities, and articular surfaces.

The Skull

Complexity and Structure

The skull is the most complex part of the skeleton, composed of multiple bones joined by immovable joints called sutures. It houses and protects the brain and forms the structure of the face.

• Sutures: Immovable joints connecting skull bones.

• Cavities of the skull:

  • Cranial cavity (brain case)

  • Orbits (eye sockets)

  • Nasal cavity

  • Oral (buccal) cavity

  • Middle- and inner-ear cavities

  • Paranasal sinuses

Example: The paranasal sinuses (frontal, sphenoid, ethmoid, and maxillary) are air-filled spaces lined by mucous membrane, which lighten the skull and add resonance to the voice.