The Skeletal System: Structure, Function, and Anatomy

The Skeletal System

Parts of the Skeletal System

The skeletal system is composed of bones, joints, ligaments, and cartilages, which together provide structure, protection, and movement for the human body.

• Bones (skeleton): The rigid organs forming the framework of the body.

• Joints: Points where two or more bones meet, allowing for movement and flexibility.

• Ligaments: Strong connective tissues that connect bones to other bones at joints.

• Cartilages: Flexible connective tissue found in joints, rib cage, ear, nose, bronchial tubes, and intervertebral discs.

Subdivisions of the Skeleton

• Axial skeleton: Consists of the skull, vertebral column, and bony thorax; forms the longitudinal axis of the body.

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

Functions of Bones

Bones serve several essential functions in the body:

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

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

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

• Mineral and fat storage: Store minerals such as calcium and phosphorus, and fats in bone marrow.

• Blood cell formation: Hematopoiesis occurs in the red marrow of certain bones.

Bones of the Human Body

Overview

• The adult human skeleton consists of 206 bones.

• Two basic types of bone tissue:

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

  • Spongy bone: Composed of small needle-like pieces of bone and many open spaces.

Classification of Bones by Shape

Bones are classified based on their shapes, which relate to their functions.

• Long bones:

  • Typically longer than they are wide.

  • Have a shaft with heads at both ends.

  • Contain mostly compact bone.

  • Examples: Femur, humerus.

• Short bones:

  • Generally cube-shaped and contain mostly spongy bone.

  • Examples: Carpals (wrist), tarsals (ankle).

• Flat bones:

  • Thin, flattened, and usually curved.

  • Two thin layers of compact bone surround a layer of spongy bone.

  • Examples: Skull, ribs, sternum.

• Irregular bones:

  • Irregular shape, do not fit into other bone classification categories.

  • Examples: Vertebrae, hip bones.

Anatomy of a Long Bone

Long bones have a distinct structure that supports their function in movement and support.

• Diaphysis: Shaft composed of compact bone.

• Epiphysis: Ends of the bone, mostly spongy bone.

• Periosteum: Outer covering of the diaphysis, a fibrous connective tissue membrane.

• Sharpey's fibers: Secure periosteum to underlying bone.

• Arteries: Supply bone cells with nutrients.

• Articular cartilage: Covers the external surface of the epiphyses, made of hyaline cartilage, decreases friction at joint surfaces.

• Epiphyseal line: Remnant of the epiphyseal plate, seen in adult bones.

• Medullary cavity: Cavity inside the shaft; contains yellow marrow (fat) in adults and red marrow (for blood cell formation) in infants.

Bone Markings

Bone markings are surface features that serve as sites for muscle, tendon, and ligament attachment, or as passages for nerves and blood vessels.

• Projections or processes: Grow out from the bone surface.

• Depressions or cavities: Indentations in the bone.

Microscopic Anatomy of Bone

Bones are composed of repeating structural units called osteons or Haversian systems.

• Osteon (Haversian system): Unit containing a central canal and matrix rings.

• Central (Haversian) canal: Opening in the center of an osteon, carries blood vessels and nerves.

• Perforating (Volkmann's) canal: Canal perpendicular to the central canal, carries blood vessels and nerves.

• Lacunae: Cavities containing bone cells (osteocytes), arranged in concentric rings.

• Lamellae: Rings around the central canal, sites of lacunae.

• Canaliculi: Tiny canals radiating from the central canal to lacunae, forming a transport system connecting all bone cells to a nutrient supply.

Formation and Growth of the Human Skeleton

Cartilage and Bone Formation

During development, most cartilage is replaced by bone, but some cartilage remains in isolated areas (e.g., bridge of the nose, parts of ribs, joints).

Bone Growth (Ossification)

• Epiphyseal plates: Allow for lengthwise growth of long bones during childhood.

• New cartilage is continuously formed, older cartilage becomes ossified.

• Cartilage is broken down, and enclosed cartilage is digested away, opening up a medullary cavity.

• Bone replaces cartilage through the action of osteoblasts (bone-forming cells).

• Bones are remodeled and lengthened until growth stops, in response to:

  • Blood calcium levels

  • Pull of gravity and muscles on the skeleton

• Bones grow in width (appositional growth).

Types of Bone Cells

• Osteocytes: Mature bone cells.

• Osteoblasts: Bone-forming cells.

• Osteoclasts: Bone-destroying cells; break down bone matrix for remodeling and release of calcium in response to parathyroid hormone.

Bone remodeling is performed by both osteoblasts and osteoclasts.

Bone Fractures

• Fracture: Break in a bone.

• Types of bone fractures:

  • Closed (simple) fracture: Break that does not penetrate the skin.

  • Open (compound) fracture: Broken bone penetrates through the skin.

• Repair of bone fractures:

  1. Hematoma (blood-filled swelling) is formed.

  2. Break is splinted by fibrocartilage to form a callus.

  3. Fibrocartilage callus is replaced by a bony callus.

  4. Bony callus is remodeled to form a permanent patch.

The Axial Skeleton

Overview

• Forms the longitudinal axis of the body.

• Divided into three parts:

  • Skull

  • Vertebral column

  • Bony thorax

The Skull

• Consists of two sets of bones:

  • Cranium

  • Facial bones

• Bones are joined by sutures (immovable joints).

• Only the mandible is attached by a freely movable joint.

• Paranasal sinuses: Hollow portions of bones surrounding the nasal cavity; functions include lightening the skull and giving resonance/amplification to the voice.

• Hyoid bone: The only bone that does not articulate with another bone; serves as a moveable base for the tongue and aids in swallowing and speech.

• Fetal skull: Large compared to the infant's total body length; fontanels allow the brain to grow and convert to bone within 24 months after birth.

The Vertebral Column

• Each vertebra is named according to its location.

• There are 24 single vertebral bones separated by intervertebral discs:

  • 7 cervical vertebrae (neck)

  • 12 thoracic vertebrae (chest)

  • 5 lumbar vertebrae (lower back)

• 9 vertebrae fuse to form two composite bones:

  • Sacrum: Formed by fusion of five vertebrae.

  • Coccyx: Formed by fusion of three to five vertebrae; "tailbone" or remnant of a tail.

• The spine has a normal curvature:

  • Primary curvatures: Thoracic and sacral regions; present from birth.

  • Secondary curvatures: Cervical and lumbar regions; develop after birth.

The Bony Thorax

• Forms a cage to protect major organs.

• Consists of three parts:

  • Sternum

  • Ribs (true ribs, false ribs, floating ribs)

  • Thoracic vertebrae

The Appendicular Skeleton

Overview

• Composed of 126 bones.

• Includes limbs (appendages), pectoral girdle, and pelvic girdle.

The Pectoral (Shoulder) Girdle

• Composed of two bones:

  • Clavicle (collarbone)

  • Scapula (shoulder blade)

• These bones allow the upper limb to have exceptionally free movement.

Bones of the Upper Limbs

• Humerus: Forms the arm; single bone.

• The forearm has two bones:

  • Ulna: Medial bone in anatomical position.

  • Radius: Lateral bone in anatomical position.

• The hand:

  • Carpals: Wrist bones.

  • Metacarpals: Palm bones.

  • Phalanges: Finger bones.

Bones of the Pelvic Girdle

• Formed by two coxal (ossa coxae) bones.

• Each coxal bone is composed of three fused bones:

  • Ilium

  • Ischium

  • Pubis

• The total weight of the upper body rests on the pelvis, which protects several organs (urinary bladder, part of the large intestine, reproductive organs).

Gender Differences of the Pelvis

• The female pelvis is larger and more circular.

• The female pelvis as a whole is shallower, and the bones are lighter and thinner.

• The female ilia flare more laterally.

• The female sacrum is shorter and less curved.

• The female ischial spines are shorter and farther apart; thus the outlet is larger.

• The female pubic arch is more rounded because the angle of the pubic arch is greater.

Bones of the Lower Limbs

• Femur: Thigh bone; the heaviest, strongest bone in the body.

• The lower leg has two bones:

  • Tibia: Shinbone; larger and medially oriented.

  • Fibula: Thin and sticklike.

• The foot:

  • Tarsals: Ankle bones; two largest are the calcaneus (heelbone) and talus.

  • Metatarsals: Sole bones.

  • Phalanges: Toe bones.

  • Arches of the foot: Bones are arranged to form three strong arches (two longitudinal, one transverse).

Joints

Functions of Joints

• Hold bones together.

• Allow for mobility.

Classification of Joints

Joints are classified functionally and structurally.

• Functional classification:

  • Synarthroses: Immovable joints.

  • Amphiarthroses: Slightly movable joints.

  • Diarthroses: Freely movable joints.

• Structural classification:

  • Fibrous joints: Bones united by fibrous tissue; generally immovable.

    • Sutures: Immovable joints (e.g., skull).

    • Syndesmoses: Allow more movement than sutures (e.g., distal end of tibia and fibula).

  • Cartilaginous joints: Bones connected by cartilage; immovable or slightly movable.

    • Examples: Pubic symphysis, intervertebral joints.

  • Synovial joints: Articulating bones are separated by a joint cavity filled with synovial fluid; freely movable.

Structure Associated with Synovial Joints

• Bursae: Flattened fibrous sacs lined with synovial membranes, filled with synovial fluid; not part of the joint but reduce friction.

• Tendon sheath: Elongated bursa that wraps around a tendon.

Inflammatory Conditions Associated with Joints

• Bursitis: Inflammation of a bursa, usually caused by a blow or friction.

• Tendonitis: Inflammation of tendon sheaths.

• Arthritis: Inflammatory or degenerative diseases of joints; over 100 different types.

Clinical Forms of Arthritis

• Osteoarthritis: Most common chronic arthritis; probably related to normal aging processes.

• Rheumatoid arthritis: Autoimmune disease; immune system attacks joints, leading to deformities.

• Gouty arthritis: Inflammation caused by deposition of uric acid crystals from the blood; can usually be controlled with diet.

Skeletal Changes Throughout Life

• Fontanels are completely replaced with bone within two years after birth.

• Ossification centers appear in the 2-week-old fetus.

• Long bones are formed of hyaline cartilage; flat bones begin as fibrous membranes.

• Fontanels remain until around age 2.

• Adolescence: Epiphyseal plates become ossified and long bone growth ends.

• Size of cranium in relationship to body:

  • At 2 years old, skull is large in proportion to body.

  • At 8-9 years old, skull is near adult size and proportion.

  • Between ages 6 and 11, face grows out from the skull.

• Curvatures of the spine:

  • Primary curvatures: Present at birth, convex posteriorly.

  • Secondary curvatures: Associated with child's later development, convex anteriorly.

  • Abnormal spinal curvatures (scoliosis, lordosis) are often congenital.

• Osteoporosis: Bone-thinning disease affecting 50% of women over age 65 and 20% of men over age 70; makes bones fragile and susceptible to fracture.

Table: Classification of Joints

Type	Structure	Mobility	Example
Fibrous	Fibrous tissue	Immovable or slightly movable	Sutures (skull), Syndesmoses (distal tibia/fibula)
Cartilaginous	Cartilage	Immovable or slightly movable	Pubic symphysis, Intervertebral joints
Synovial	Joint cavity with synovial fluid	Freely movable	Knee, elbow, shoulder

Key Equations

• Bone Remodeling Equation: $\text{Bone Remodeling} = \text{Osteoblast Activity} - \text{Osteoclast Activity}$

• Calcium Homeostasis: $\text{Blood Calcium Level} \propto \text{Bone Deposition} - \text{Bone Resorption}$

Additional info: Some details, such as the specific names of bones and clinical conditions, were expanded for clarity and completeness.