The Human Skeleton

Overview of the Skeleton

The human skeleton is a structural framework composed of bones, cartilages, joints, and ligaments. It is divided into two major divisions: the axial skeleton and the appendicular skeleton. The skeleton accounts for approximately 20% of body mass and serves to support, protect, and facilitate movement.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage. Forms the longitudinal axis of the body.

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

The Axial Skeleton

Skull: Structure and Function

The skull is formed by two sets of bones: cranial bones and facial bones. Cranial bones enclose the brain and provide attachment sites for head and neck muscles, while facial bones form the framework of the face and house cavities for sensory organs.

• Cranial Bones: Protect the brain and provide muscle attachment.

• Facial Bones: Form the face, secure teeth, 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 (calvaria) and a base. The cranial base is internally divided into three fossae: anterior, middle, and posterior, which house different regions of the brain. The skull also contains other cavities such as the orbits, nasal cavity, and paranasal sinuses.

• Cranial Vault: Forms the superior, lateral, and posterior portions of the skull.

• Cranial Base: Forms the inferior aspect and is divided into fossae.

• Openings: Foramina, canals, and fissures provide passageways for nerves and blood vessels.

Bony Boundaries of the Orbits and Nasal Cavity

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

• 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 8 cranial and 14 facial bones. Important markings include foramina (openings), processes (projections), and sutures (joints).

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

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

The Vertebral Column

Structure and Regions

The vertebral column extends from the skull to the pelvis and consists of 33 vertebrae divided into five regions. It transmits weight, protects the spinal cord, and provides attachment points for ribs and muscles.

• 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: cervical and lumbar (convex anteriorly), thoracic and sacral (concave anteriorly). These curvatures, along with intervertebral discs, provide shock absorption and flexibility.

• Spinal Curvatures: Increase resilience and flexibility, help maintain balance.

• Intervertebral Discs: Cushion vertebrae, allow bending and twisting.

Abnormal Spinal Curvatures

• Scoliosis: Abnormal lateral rotation, may cause breathing difficulties.

• Kyphosis: Dorsal thoracic curvature, common in osteoporosis.

• Lordosis: Accentuation of lumbar curvature, seen in pregnancy or obesity.

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 of the Thoracic Cage

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: Heart and lungs.

• Support: Shoulder girdle.

• Respiration: Expands and contracts thoracic cavity.

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

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

• Clavicle: Collarbone, fractures anteriorly to protect arteries.

• Scapula: Shoulder blade, triangular with three borders.

Bones of the Upper Limb

• 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

• Hip Bones: Ilium, ischium, pubis fused at acetabulum.

• Functions: Supports body weight, protects organs.

• Female Pelvis: Wider, shallower, adapted for childbirth.

Bones of the Lower Limb

• Femur: Thigh bone, largest and strongest.

• Patella: Knee cap, protects 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 shapes, ligaments, and tendons.

• Transverse Arch: Redistributes weight.

Joints

Classification and Structure

• Fibrous Joints: Bones united by fibrous tissue, mostly immovable.

• Cartilaginous Joints: Bones connected by cartilage, immovable or slightly movable.

• Synovial Joints: Freely movable, separated by a joint cavity filled with synovial fluid.

Functional Classification

• Synarthroses: Immovable joints.

• Amphiarthroses: Slightly movable joints.

• Diarthroses: Freely movable joints.

Features of Synovial Joints

• Articular Cartilage: Covers bone ends.

• Joint Capsule: Encloses joint surfaces.

• Ligaments: Reinforce the joint.

• Bursae and Tendon Sheaths: Reduce friction.

Factors Affecting Stability

• Articular Surface Shape

• Ligament Number and Position

• Muscle Tone

Inflammatory Conditions

• Bursitis: Inflammation of a bursa.

• Tendonitis: Inflammation of tendon sheaths.

• Arthritis: Over 100 types, most common is osteoarthritis.

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: Diagnose fractures and abnormalities.

• MRI: Cross-sectional images for complex fractures.

• CT Scans: Detailed bone structure analysis.

Hormones and Bone Health

• Estrogen: Promotes bone deposition, inhibits resorption.

• Testosterone: Increases bone density.

• Parathyroid Hormone (PTH): Regulates calcium levels.

Exercise Guidelines

• Weight-Bearing Exercises: Walking, running, weightlifting.

• Resistance Training: Strengthens muscles and bones.

• Flexibility Exercises: Improve range of motion.

Screening and Rehabilitation

• Bone Density Screening: DEXA scan for osteoporosis risk.

• Physical and Occupational Therapy: Restore function after fracture.

Case Study: Osteoporosis in Women

• Risk Factors: Decreased estrogen, age, genetics, lifestyle.

• Prevention: Calcium, vitamin D, exercise, avoid smoking/alcohol.

• Treatments: Medications, hormone therapy.

Homeostatic Imbalances

• Mastoiditis: Infection of mastoid process, risk of spreading to brain.

• Flatfoot: Loss of medial arch, risk factors include age, obesity, high-impact activities.

Additional info: The notes above expand on brief points with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.