The Skeletal System: Structure, Classification, and Bone Markings

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The Skeletal System

Functions of Bones

The skeletal system is essential for providing structural support, protecting internal organs, enabling movement, storing minerals and fats, and facilitating blood cell formation.

Support: Bones form the framework that supports the body.
Protection: Bones such as the skull and rib cage protect vital organs (e.g., brain, heart, lungs).
Movement: Skeletal muscles attach to bones, allowing movement at joints.
Storage: Bones store minerals (calcium, phosphorus) and fats (in marrow cavities).
Hematopoiesis: Blood cell formation occurs in the bone marrow.

Classification of Bones

Bones are classified by their shape and the type of tissue they contain. The adult skeleton consists of 206 bones, which are categorized as follows:

Compact bone: Dense, smooth, and homogeneous tissue.
Spongy bone: Composed of small needlelike pieces and many open spaces.

Bones are also classified by shape:

Long bones: Longer than wide, mostly compact bone (e.g., femur, humerus).
Flat bones: Thin, flattened, usually curved, with spongy bone sandwiched between compact bone.
Short bones: Cube-shaped, mostly spongy bone (e.g., wrist, ankle).
Irregular bones: Complex shapes (e.g., vertebrae).

Flat bone structure: spongy bone sandwiched between compact bone

Structure of Long Bones

Long bones have a shaft (diaphysis) and two ends (epiphyses). The diaphysis is composed of compact bone, while the epiphyses contain spongy bone. The medullary cavity within the diaphysis contains yellow bone marrow in adults.

Periosteum: A fibrous membrane covering the bone.
Endosteum: Lines the medullary cavity.
Articular cartilage: Covers the epiphyses, reducing friction at joints.

Long bone structure: yellow marrow, compact bone, periosteum, nutrient arteries Long bone structure: articular cartilage, spongy bone, compact bone

Microscopic Anatomy of Bone

Bone tissue is organized into structural units called osteons (Haversian systems). Compact bone contains concentric rings (lamellae) of matrix surrounding a central canal. Spongy bone is made of trabeculae and open spaces filled with marrow, blood vessels, and nerves.

Osteocytes: Mature bone cells located in lacunae.
Lacunae: Small cavities housing osteocytes.
Lamellae: Concentric circles of matrix.
Canaliculi: Tiny canals connecting lacunae.

Microscopic structure of bone: osteon, lamellae, central canal, periosteum

Bone Markings

Projections and Processes

Bone markings are features that serve as sites for muscle and ligament attachment or as passageways for blood vessels and nerves. Projections and processes include:

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

Table of bone markings: projections for muscle and ligament attachment

Projections That Help Form Joints

Name	Description	Illustration
Head	Bony expansion carried on a narrow neck	Femur, humerus
Facet	Smooth, nearly flat articular surface	Vertebra
Condyle	Rounded articular projection	Mandible
Ramus	Arm-like bar of bone	Mandible

Table of bone markings: projections that help form joints

Depressions and Openings

Name	Description	Illustration
Groove	Furrow	Skull
Fissure	Narrow, slitlike opening	Skull
Foramen	Round or oval opening through a bone	Skull
Notch	Indentation at the edge of a structure	Skull
Meatus	Canal or tunnel-like passageway	Skull
Sinus	Cavity within a bone, filled with air and lined with mucous membrane	Skull
Fossa	Shallow, basinlike depression in a bone, often serving as an articular surface	Skull

Table of bone markings: depressions and openings

Microscopic Structure of Compact Bone

Osteon (Haversian System)

The osteon is the fundamental unit of compact bone, consisting of concentric lamellae surrounding a central canal. Blood vessels and nerves run through the central canal, supplying the bone tissue.

Perforating (Volkmann's) canals: Run perpendicular to the central canal, connecting blood and nerve supply.
Periosteum: The outer fibrous layer covering the bone.

Microscopic structure of bone: osteon, lamellae, central canal, periosteum

Microscopic Structure of Spongy Bone

Trabeculae and Open Spaces

Spongy bone is composed of trabeculae, which are small, needlelike pieces of bone. The open spaces between trabeculae are filled with bone marrow, blood vessels, and nerves, contributing to the lightweight nature of spongy bone.

Trabeculae: Provide structural support and house marrow.
Open spaces: Allow for the passage of blood vessels and nerves.

Spongy bone structure: trabeculae and open spaces

Summary Table: Bone Markings

Purpose of Table

This table summarizes the main types of bone markings, their descriptions, and their anatomical illustrations. Bone markings are essential for understanding muscle attachment, joint formation, and passageways for nerves and blood vessels.

Type	Description	Example
Projection	Site of muscle/ligament attachment	Tuberosity, crest, trochanter
Joint formation	Articular surface	Head, facet, condyle
Depression/opening	Passage for vessels/nerves	Foramen, meatus, sinus