The Human Skeleton: Overview

Divisions and Functions

The human skeleton is a structural framework composed of bones, cartilages, joints, and ligaments. It is divided into two major regions: the axial skeleton and the appendicular skeleton. The skeleton provides support, protection, and facilitates movement.

• Axial skeleton: Includes the skull, vertebral column, and thoracic cage. Forms the longitudinal axis of the body and protects vital organs.

• Appendicular skeleton: Includes the limbs and girdles, enabling movement and manipulation of the environment.

• Functions: Support, protection, movement, mineral storage, and blood cell formation.

The Axial Skeleton

Skull: Structure and Function

The skull is composed of cranial and facial bones, which protect the brain and form the framework of the face.

• Cranial bones: Enclose the brain and provide attachment sites for head and neck muscles.

• Facial bones: Form the face, house sensory organs, provide openings for air and food, secure teeth, and anchor facial muscles.

• Sutures: Immovable joints connecting skull bones, with a serrated appearance.

Skull Geography and Cavities

The cranium is divided into a vault and a base, with internal fossae supporting the brain. The skull contains multiple cavities and openings for nerves and blood vessels.

• Cranial vault (calvaria): Forms the superior, lateral, and posterior portions of the skull.

• Cranial base: Forms the inferior aspect, divided into anterior, middle, and posterior cranial fossae.

• Cavities: Middle/internal ear, nasal cavity, orbits, and sinuses.

• Openings: Foramina, canals, and fissures for passage of nerves and vessels.

Bony Boundaries of Orbits and Nasal Cavity

The orbits and nasal cavity are formed by multiple bones, providing protection and passageways for sensory organs.

• Orbits: Roof (frontal, sphenoid), lateral wall (zygomatic, sphenoid), floor (maxilla, zygomatic, palatine), medial wall (ethmoid, lacrimal, maxilla, sphenoid).

• Nasal cavity: Roof (nasal, frontal, ethmoid, sphenoid), floor (maxilla, palatine), lateral walls (maxilla, ethmoid, inferior nasal conchae, palatine), medial wall (ethmoid, vomer).

• Paranasal sinuses: Frontal, ethmoidal, sphenoidal, and maxillary sinuses lighten the skull and amplify voice.

Skull Bones and Markings

The skull consists of 22 bones, with distinct markings for muscle attachment and passage of nerves and vessels.

• Cranial bones: Frontal, parietal, occipital, temporal, sphenoid, ethmoid.

• Facial bones: Mandible, maxillae, zygomatic, nasal, lacrimal, palatine, vomer, inferior nasal conchae.

• Markings: Foramina (openings), processes (projections), sutures (joints).

The Vertebral Column

Structure and Regions

The vertebral column extends from the skull to the pelvis, transmitting weight and protecting the spinal cord. It consists of 33 vertebrae in five regions.

• Cervical: 7 vertebrae (neck)

• Thoracic: 12 vertebrae (upper back)

• Lumbar: 5 vertebrae (lower back)

• Sacrum: 5 fused vertebrae (pelvis)

• Coccyx: 4 fused vertebrae (tailbone)

Curvatures and Functions

The vertebral column exhibits four natural curvatures, contributing to shock absorption and flexibility.

• Cervical and lumbar: Convex anteriorly

• Thoracic and sacral: Concave anteriorly

• Functions: Shock absorption, balance, flexibility

Structure of a Typical Vertebra

• Body: Main weight-bearing structure

• Vertebral arch: Formed by pedicles and laminae

• Processes: Spinous, transverse, superior/inferior articular

Regional Features

• Cervical: Smallest, transverse foramina, bifid spinous processes

• Thoracic: Articular facets for ribs, heart-shaped body, long spinous processes

• Lumbar: Largest, kidney-shaped body, short blunt spinous processes

The Thoracic Cage

Structure and Function

The thoracic cage protects vital organs and supports the shoulder girdle. It consists of the sternum, ribs, and thoracic vertebrae.

• Sternum: Manubrium, body, xiphoid process

• Ribs: True ribs (1-7), false ribs (8-12), floating ribs (11-12)

• Functions: Protection, support, respiration

True vs. False Ribs

The Appendicular Skeleton

Pectoral Girdle and Upper Limb

The pectoral girdle consists of the clavicle and scapula, allowing free movement of the upper limb.

• Clavicle: Collarbone, fractures anteriorly to protect arteries

• Scapula: Shoulder blade, triangular with three borders

• Upper limb bones: Humerus (arm), radius and ulna (forearm), carpals (wrist), metacarpals (palm), phalanges (fingers)

Pelvic Girdle and Lower Limb

The pelvic girdle is composed of fused bones (ilium, ischium, pubis) and supports the weight of the upper body.

• Hip bones: Ilium, ischium, pubis

• Femur: Thigh bone, largest and strongest

• Patella: Knee cap, protects and improves leverage

• Tibia and fibula: Leg bones, tibia bears weight, fibula does not

• Foot: Tarsals (ankle), metatarsals (sole), phalanges (toes), arches support weight

Joints: Classification and Structure

Functional and Structural Classification

Joints are classified by their function and structure, determining their mobility.

• Functional: Synarthroses (immovable), amphiarthroses (slightly movable), diarthroses (freely movable)

• Structural: Fibrous, cartilaginous, synovial

Fibrous Joints

• Suture: Only in skull, immovable

• Syndesmosis: Bones connected by ligaments, variable movement

• Gomphosis: Peg-in-socket, e.g., teeth

Cartilaginous Joints

• Synchondrosis: Hyaline cartilage, immovable

• Symphysis: Fibrocartilage, slightly movable (e.g., pubic symphysis, intervertebral joints)

Synovial Joints

• Features: Articular cartilage, joint cavity, synovial fluid, fibrous capsule, synovial membrane, ligaments

• Bursae: Sacs filled with synovial fluid, reduce friction

• Tendon sheath: Elongated bursa around tendons

Factors Affecting Stability

• Shape of articular surfaces

• Number and positioning of ligaments

• Muscle tone

Types of Synovial Joints

• Classified by shape: plane, hinge, pivot, condyloid, saddle, ball-and-socket

Clinical Relevance: Bone Health and Disorders

Osteoporosis and Bone Density

• Risk factors: Age, genetics, lifestyle, decreased estrogen in postmenopausal women

• Prevention: Calcium, vitamin D, weight-bearing exercise, avoiding smoking/alcohol

• Screening: DEXA scan for bone density

• Treatments: Medications (bisphosphonates, SERMs, hormone therapy)

Rehabilitation and Therapy

• Physical and occupational therapy restore function after fractures

• Exercise guidelines: Weight-bearing, resistance, flexibility

Joint Disorders

• Bursitis: Inflammation of bursae

• Tendonitis: Inflammation of tendon sheaths

• Arthritis: Over 100 types, including osteoarthritis (aging), rheumatoid arthritis (autoimmune)

Summary Table: Axial vs. Appendicular Skeleton