Study Guide: The Skeletal System (Chapter 5) – Structure, Classification, and Function

The Skeletal System

Overview and Functions

The skeletal system is a complex framework of bones, joints, cartilages, and ligaments that provides structural support, protection, movement, storage, and blood cell formation. It is divided into two main subdivisions: the axial skeleton (longitudinal axis) and the appendicular skeleton (limbs and girdles).

• Support: Bones form the body’s structural framework.

• Protection: Bones protect vital organs (e.g., skull protects the brain).

• Movement: Muscles attach to bones, enabling movement.

• Storage: Bones store minerals (calcium, phosphorus) and fats.

• Blood cell formation: Hematopoiesis occurs in bone marrow.

Classification of Bones

Types of Bones

Bones are classified by their shape and structure. The adult skeleton contains 206 bones, which are categorized as follows:

• Long bones: Longer than they are wide; mostly compact bone (e.g., humerus).

• Short bones: Cube-shaped; mostly spongy bone (e.g., talus).

• Flat bones: Thin, flattened, and usually curved; two thin layers of compact bone with spongy bone in between (e.g., sternum).

• Irregular bones: Complex shapes that do not fit other categories (e.g., vertebra).

Structure of Bone

Gross Anatomy of a Long Bone

Long bones have a distinct structure, including the diaphysis (shaft), epiphyses (ends), and various membranes and cavities.

• Diaphysis: Central shaft, composed of compact bone, covered by periosteum.

• Epiphysis: Ends of the bone, mostly spongy bone surrounded by compact bone.

• Articular cartilage: Hyaline cartilage covering epiphyses, reduces friction at joints.

• Epiphyseal plate/line: Plate of hyaline cartilage in growing bones; replaced by bone in adults (epiphyseal line).

• Medullary cavity: Hollow shaft containing yellow marrow (adults) or red marrow (infants).

• Periosteum: Fibrous membrane covering diaphysis, secured by Sharpey’s fibers.

• Arteries: Supply nutrients to bone cells.

Bone Remodeling

Bone is a dynamic tissue that undergoes continuous remodeling. Osteoclasts resorb bone, while osteoblasts deposit new bone, especially during growth and repair.

• Articular cartilage: Maintains smooth joint surfaces.

• Epiphyseal plate: Site of longitudinal growth in children.

• Osteoclasts: Cells that break down bone matrix.

• Osteoblasts: Cells that build new bone matrix.

Microscopic Anatomy of Bone

Bone tissue is organized into structural units called osteons (Haversian systems), which contain concentric lamellae, lacunae, and central canals.

• Osteon: Fundamental unit of compact bone.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Small spaces housing osteocytes.

• Central (Haversian) canal: Contains blood vessels and nerves.

• Canaliculi: Tiny channels connecting lacunae.

• Perforating (Volkmann’s) canal: Transverse canals connecting osteons.

Bone Markings

Surface Features of Bones

Bones have various markings that serve as sites for muscle, tendon, and ligament attachment, or as passages for nerves and blood vessels. These markings are classified as projections (processes) or depressions (cavities).

• Projections/processes: Grow out from bone surface (e.g., tuberosity, crest, trochanter).

• Depressions/cavities: Indentations in bone (e.g., fossa, foramen).

Name of bone marking	Description	Illustration
Tuberosity	Large, rounded projection; may be roughened	Femur, hip bone
Crest	Narrow ridge of bone; usually prominent	Hip bone
Trochanter	Very large, blunt, irregularly shaped process	Femur
Line	Narrow ridge of bone; less prominent than a crest	Femur
Tubercle	Small, rounded projection or process	Femur
Epicondyle	Raised area on or above a condyle	Femur
Spine	Sharp, slender, often pointed projection	Vertebra
Process	Any bony prominence	Vertebra

Name of bone marking	Description	Illustration
Head	Bony expansion carried on a narrow neck	Rib
Facet	Smooth, nearly flat articular surface	Rib
Condyle	Rounded articular projection	Mandible
Ramus	Armlike bar of bone	Mandible

Summary Table: Compact vs. Spongy Bone

Type	Structure	Location	Function
Compact bone	Dense, smooth, homogeneous	Diaphysis of long bones, outer layer of flat bones	Support, protection
Spongy bone	Small needle-like pieces, many open spaces	Epiphyses of long bones, interior of flat bones	Lightens bone, houses marrow

Additional info:

• Bone remodeling is regulated by hormones and mechanical stress.

• Bone markings are essential for understanding muscle attachment and joint function.