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: Consists of 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 interaction with the environment.

The Axial Skeleton

Skull: Structure and Function

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

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

• Facial Bones: Form the framework of the face, house sensory organ cavities, and anchor muscles for facial expression.

• 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 for sensory organs and passageways 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.

• Openings: Foramina, canals, and fissures allow 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 support 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: Markings and Identification

Skull bones have distinct markings for muscle attachment, passage of nerves and vessels, and articulation.

• Foramina: Openings for blood vessels and nerves.

• Processes: Projections for muscle attachment.

• Sutures: Immovable joints connecting skull bones.

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 26 irregular bones in five major 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 natural curvatures that enhance shock absorption and flexibility.

• Cervical and Lumbar: Convex anteriorly

• Thoracic and Sacral: Concave anteriorly

• Intervertebral Discs: Cushion vertebrae and allow movement

Abnormal Spinal Curvatures

• Scoliosis: Abnormal lateral rotation, may cause breathing difficulties

• Kyphosis: Excessive thoracic curvature (hunchback)

• Lordosis: Excessive lumbar curvature (swayback)

Structure of a Typical Vertebra

• Body: Main weight-bearing structure

• Vertebral Arch: Formed by pedicles and laminae

• Processes: Spinous, transverse, superior and inferior articular processes

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 spinous processes

The Thoracic Cage

Bones and Functions

The thoracic cage protects vital organs and supports the shoulder girdle.

• Sternum: Manubrium, body, xiphoid process

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

• Thoracic Vertebrae: Articulate with ribs

True vs. False Ribs

• True Ribs: Attach directly to sternum via costal cartilage

• False Ribs: Attach indirectly or not at all

• Floating Ribs: No attachment to sternum

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

• Humerus: Arm bone, articulates with scapula, radius, and ulna

• Ulna: Forms elbow joint, olecranon and coronoid processes

• Radius: Major contribution to wrist joint

• Hand: 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.

• Femur: Thigh bone, largest and strongest

• Patella: Knee cap, protects knee joint

• Tibia: Medial, weight-bearing

• Fibula: Lateral, non-weight-bearing

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

Arches of the Foot

• Medial and Lateral Longitudinal Arches: Support weight, maintained by bone shape, ligaments, and tendons

• Transverse Arch: Distributes weight

Joints: Classification and Structure

Functional and Structural Classification

Joints are classified by their movement and structure.

• 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

• 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

• Articular Cartilage: Covers bone ends

• Joint Capsule: Encloses joint cavity

• Synovial Fluid: Lubricates joint

• Ligaments: Reinforce joint

• Bursae and Tendon Sheaths: Reduce friction

Factors Affecting Synovial Joint Stability

• Articular Surface Shape

• Ligament Number and Position

• Muscle Tone

Inflammatory Conditions

• Bursitis: Inflammation of bursae

• Tendonitis: Inflammation of tendon sheaths

• Arthritis: Over 100 types, including osteoarthritis and rheumatoid arthritis

Bone Health and Clinical Considerations

Importance of Load-Bearing Activities

• Mechanical Stress: Stimulates bone deposition and increases density

• Physical Activity: Reduces risk of osteoporosis and fractures

Bone Imaging Techniques

• X-rays: Detailed images of bone and soft tissues

• MRI: Cross-sectional images for complex fractures

• CT Scans: Detailed bone structure

Hormones and Bone Health

• Estrogen: Promotes bone deposition, inhibits resorption

• Testosterone: Increases bone density

• Parathyroid Hormone (PTH): Regulates calcium, stimulates bone resorption

Exercise Guidelines and Screening

• Weight-Bearing Exercises: Walking, running, weightlifting

• Bone Density Screening: DEXA scan for osteoporosis risk

Rehabilitation and Therapy

• Occupational Therapy: Regains independence

• Physical Therapy: Improves strength and balance

• Post-Fracture Rehabilitation: Restores function and mobility

Summary Table: Axial vs. Appendicular Skeleton

Summary Table: Types of Joints

Summary Table: Major Bones of the Skull