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: Consists of 80 bones, including the skull, vertebral column, and thoracic cage. Forms the longitudinal axis of the body.

• Appendicular Skeleton: Composed of the bones of the limbs and girdles 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 and a base. The cranial vault forms the superior, lateral, and posterior portions, while the base forms the inferior aspect. Internally, the base is divided into three fossae: anterior, middle, and posterior, which house the brain. The skull also contains other cavities such as the orbits, nasal cavity, and paranasal sinuses.

• Cranial Vault (Calvaria): Superior and lateral portion.

• Cranial Base: Inferior portion, divided into fossae.

• Orbits: House the eyeballs.

• Nasal Cavity: Passage for air and olfaction.

• Paranasal Sinuses: Lighten the skull and amplify voice.

Skull Bones and Markings

The skull has 85 named openings (foramina, canals, fissures) for passage of nerves and blood vessels. Key markings include:

• Foramina: Openings for blood vessels and nerves.

• Processes: Projections for muscle attachment.

• Sutures: Immovable joints connecting skull bones.

Cranial Bones

The cranium comprises eight bones: frontal, parietal (left and right), occipital, temporal (left and right), sphenoid, and ethmoid. Each bone has distinct features and articulations.

• Frontal Bone: Forms the forehead and superior wall of the orbits.

• Parietal Bones: Form most of the superior and lateral aspects of the cranial vault.

• Occipital Bone: Forms the posterior wall and base of the skull.

• Temporal Bones: Make up the inferolateral aspects and parts of the cranial base.

• Sphenoid Bone: Keystone bone, articulates with all other cranial bones.

• Ethmoid Bone: Deepest skull bone, forms part of the nasal cavity and orbits.

Facial Bones

The facial skeleton is made up of 14 bones, including the mandible, maxillae, zygomatic, nasal, lacrimal, palatine, vomer, and inferior nasal conchae. These bones form the framework of the face and provide cavities for sensory organs.

• Mandible: Largest, strongest bone of the face; forms the lower jaw.

• Maxillae: Form the upper jaw and central facial skeleton.

• Zygomatic Bones: Form the cheekbones.

• Nasal Bones: Form the bridge of the nose.

• Lacrimal Bones: Form the medial walls of the orbits.

• Palatine Bones: Form part of the hard palate and nasal cavity.

• Vomer: Forms part of the nasal septum.

• Inferior Nasal Conchae: Form part of the lateral walls of the nasal cavity.

Bony Boundaries of the Orbits and Nasal Cavity

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

• 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

Paranasal sinuses are hollow portions of bones surrounding the nasal cavity.

• Functions: Lighten the skull, give resonance and amplification to the voice.

• Locations: Frontal, ethmoidal, sphenoidal, and maxillary bones.

The Vertebral Column

The vertebral column extends from the skull to the pelvis and functions to transmit weight, protect the spinal cord, and provide attachment points for ribs and muscles.

• Regions: Cervical (7), thoracic (12), lumbar (5), sacrum (5 fused), coccyx (4 fused).

• Curvatures: Cervical and lumbar (convex anteriorly), thoracic and sacral (concave anteriorly).

• Intervertebral Discs: Cushion vertebrae and allow flexibility.

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 of Vertebrae

• 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

The thoracic cage forms a protective cage for major organs and is composed 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

• True Ribs (1-7): Attach directly to the sternum via costal cartilage.

• False Ribs (8-12): Attach indirectly or not at all; floating ribs (11-12) do not attach to the sternum.

The Appendicular Skeleton

Pectoral (Shoulder) Girdle

The pectoral girdle is composed 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

The pelvic girdle is composed of fused bones: ilium, ischium, and pubis. It supports the weight of the upper body and protects organs.

• Ilium: Superior region, attachment for gluteal muscles.

• Ischium: Posteroinferior region, ischial tuberosity bears weight when sitting.

• Pubis: Anterior region, pubic crest, obturator foramen.

Gender Differences of the Pelvis

• Female Pelvis: Wider, shallower, lighter, rounder for childbearing.

• Male Pelvis: Narrower, deeper, heavier.

Bones of the Lower Limb

• Femur: Thigh bone, largest and strongest.

• Patella: Knee cap, protects knee joint.

• Tibia: Medial, weight-bearing.

• Fibula: Lateral, does not bear weight.

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

Arches of the Foot

• Medial Longitudinal Arch: Talus is the keystone.

• Lateral Longitudinal Arch: Cuboid is the key.

• Transverse Arch: Formed by metatarsals and tarsals.

Joints

Classification of Joints

Joints, or articulations, hold bones together and allow for mobility. They are classified functionally and structurally.

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

• Structural Classification: Fibrous, cartilaginous, synovial.

Fibrous Joints

• Suture: Only found in the skull, immovable.

• Syndesmosis: Bones connected by ligaments, variable movement.

• Gomphosis: Peg-in-socket, e.g., tooth in alveolar socket.

Cartilaginous Joints

• Synchondrosis: Hyaline cartilage unites bones, immovable.

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

Synovial Joints

• Articular Cartilage: Covers ends of bones.

• Joint Capsule: Encloses joint cavity filled with synovial fluid.

• Ligaments: Reinforce the joint.

• Bursae: Sacs filled with synovial fluid, reduce friction.

• Tendon Sheath: Elongated bursa around a tendon.

Factors Affecting Synovial Joint Stability

• Shape of articular surfaces

• Number and positioning of ligaments

• Muscle tone

Types of Synovial Joints (Based on Shape)

• Plane, hinge, pivot, condyloid, saddle, ball-and-socket

Inflammatory Conditions Associated with Joints

• Bursitis: Inflammation of a bursa.

• Tendonitis: Inflammation of tendon sheaths.

• Arthritis: Over 100 types, most widespread crippling disease.

• Osteoarthritis: Most common, related to aging.

• Rheumatoid Arthritis: Autoimmune, leads to deformities.

Clinical and Lifestyle Considerations

Bone Health and Exercise

• Mechanical Stress: Stimulates bone deposition and increases density.

• Weight-Bearing Exercise: Essential for maintaining bone health.

• Hormones: Estrogen, testosterone, and parathyroid hormone regulate bone density.

Bone Imaging Techniques

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

• MRI: Cross-sectional images for complex fractures.

• CT Scans: Detailed bone structure analysis.

Prevention and Treatment of Bone Disorders

• Medications: Bisphosphonates, SERMs, hormone therapy.

• Nutrition: Adequate calcium and vitamin D intake.

• Screening: DEXA scans for bone density.

Rehabilitation and Therapy

• Physical Therapy: Improves strength, flexibility, and balance.

• Occupational Therapy: Regains independence in daily activities.

• Post-Fracture Rehabilitation: Restores function and mobility.

Summary Table: Major Bones of the Axial and Appendicular Skeleton

Key Equations

• Bone Density Calculation:

Additional info:

• Some clinical and lifestyle content was inferred to provide a comprehensive, exam-ready study guide.