The Skeletal System: Structure, Classification, and Bone Markings

The Skeletal System

Overview of the Skeletal System

The skeletal system is a complex framework of bones, joints, cartilages, and ligaments that provides structural support, protection, and movement for the human body. It is divided into two main subdivisions: the axial skeleton and the appendicular skeleton.

• Axial skeleton: Includes the skull, vertebral column, and thoracic cage.

• Appendicular skeleton: Comprises the limbs and girdles (pectoral and pelvic) that attach them to the axial skeleton.

Functions of Bones

Major Functions

Bones serve several essential functions in the body:

• Support: Provide a rigid framework that supports the body and cradles soft organs.

• Protection: Protect vital organs (e.g., skull protects the brain, rib cage protects thoracic organs).

• Movement: Serve as levers for muscles to produce movement.

• Mineral and Fat Storage: Store minerals such as calcium and phosphorus, and fats in the marrow cavities.

• Blood Cell Formation (Hematopoiesis): Occurs in red bone marrow.

Classification of Bones

Types of Bone Tissue

Bones are composed of two main types of osseous tissue:

• Compact bone: Dense, smooth, and homogeneous tissue forming the outer layer of bones.

• Spongy bone: Consists of small needlelike pieces called trabeculae and many open spaces.

Bone Shapes

Bones are classified by shape into four groups:

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

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

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

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

Structure of Bone

Gross Anatomy of a Long Bone

Long bones have a characteristic structure that supports their function:

• Diaphysis: Shaft, composed of compact bone.

• Epiphyses: Ends of the bone, mostly spongy bone covered by a thin layer of compact bone.

• Periosteum: Fibrous connective tissue membrane covering the diaphysis, secured by Sharpey's fibers.

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

• Epiphyseal plate/line: Growth plate of hyaline cartilage in children; becomes the epiphyseal line in adults.

• Medullary cavity: Central cavity in the diaphysis; contains yellow marrow (fat) in adults and red marrow (blood cell formation) in children.

• Endosteum: Membrane lining the inner surface of the shaft.

Microscopic Anatomy of Bone

Bone tissue is organized into structural units for strength and nutrient delivery:

• Osteon (Haversian system): The structural and functional unit of compact bone, consisting of concentric lamellae around a central canal.

• Lamellae: Rings of bone matrix within an osteon.

• Lacunae: Small cavities housing osteocytes (mature bone cells).

• Canaliculi: Tiny canals connecting lacunae, allowing for nutrient and waste exchange.

• Perforating (Volkmann's) canals: Canals perpendicular to the central canal, carrying blood vessels and nerves.

Comparison of Compact and Spongy Bone

특징	치밀골 (Compact Bone)	해면골 (Spongy Bone)
기본 구조 단위	오스테온 (Osteon)	골소주 (Trabeculae)
혈관 중심관	있음 (하버스관)	없음 (주변 골수에서 직접 흡수)
현미경적 형태	각 한 동나무 나이테 구조	여러방향 같은 격자형 구조
주요 역할 기능	직선적인 압력과 비틀림 저항	사방에서 오는 충격 흡수 및 분산

Bone Markings

Types and Functions of Bone Markings

Bone markings are specific features on bones that serve as sites for muscle, tendon, and ligament attachment, or as passages for nerves and blood vessels. They are categorized as projections (processes) or depressions (cavities).

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

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

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

Additional info:

• Bones are organs composed of not only osseous tissue but also fibrous tissue, cartilage, adipose tissue, and blood.

• Bone markings are essential for understanding muscle attachment, joint formation, and passage of nerves and vessels.