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 supports the body, protects vital organs, and enables movement.

• Axial Skeleton: Consists of 80 bones, including the skull, vertebral column, and thoracic cage. It forms the longitudinal axis of the body, supports the head, neck, and trunk, and protects the brain, spinal cord, and thoracic organs.

• Appendicular Skeleton: Includes the bones of the limbs and girdles, facilitating 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 framework of the face. Most skull bones are flat and joined by immovable joints called sutures.

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

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

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

Cranial Vault and Base

The cranium is divided into a vault (calvaria) 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 different regions of the brain.

• Cranial Vault: Superior, lateral, and posterior skull, including the forehead.

• Cranial Base: Inferior skull, divided into fossae for brain regions.

Skull Cavities and Openings

The skull contains several cavities, including the cranial cavity, orbits, nasal cavity, and paranasal sinuses. It also has 85 named openings (foramina, canals, fissures) for passage of nerves and blood vessels.

• Orbits: House the eyeballs.

• Nasal Cavity: Passage for air and olfactory structures.

• Paranasal Sinuses: Air-filled spaces that lighten the skull and enhance voice resonance.

• Foramina: Openings for nerves and vessels.

Major Cranial Bones and Markings

The cranium consists of eight bones: frontal, parietal (2), occipital, temporal (2), sphenoid, and ethmoid. Each bone has distinct markings and functions.

• Frontal Bone: Forms the forehead, superior wall of orbits, and anterior cranial fossa. Contains the glabella and frontal sinuses.

• Parietal Bones: Form the superior and lateral aspects of the cranial vault. Articulated by coronal, sagittal, lambdoid, and squamous sutures.

• Occipital Bone: Forms the posterior skull and cranial fossa. Features include the foramen magnum, occipital condyles, and nuchal lines.

• Temporal Bones: Make up the inferolateral skull and cranial base. Include squamous, tympanic, and petrous parts, with processes and foramina for muscle attachment and nerve passage.

• Sphenoid Bone: Keystone bone articulating with all cranial bones. Contains sphenoidal sinuses, sella turcica, and multiple foramina.

• Ethmoid Bone: Deepest skull bone, forms the roof of the nasal cavity, nasal septum, and medial wall of orbits.

Facial Bones

The facial skeleton consists of 14 bones, including the mandible, maxillae, zygomatic, nasal, lacrimal, palatine, vomer, and inferior nasal conchae. These bones form the face, house sensory organs, and provide attachment points for muscles.

• 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 Orbits and Nasal Cavity

The orbits and nasal cavity are formed by multiple bones, providing protection and structural 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.

The Vertebral Column

Structure and Regions

The vertebral column, or spine, extends from the skull to the pelvis and consists of 26 irregular bones in five major 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: 1 bone (fusion of 5 vertebrae, pelvis)

• Coccyx: 1 bone (fusion of 3-5 vertebrae, terminus)

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

Abnormal curvatures can result from congenital defects, disease, or poor posture.

• Scoliosis: Abnormal lateral rotation, often in thoracic region.

• Kyphosis: Excessive dorsal thoracic curvature (hunchback).

• Lordosis: Excessive lumbar curvature (swayback).

Structure of a Typical Vertebra

Each vertebra consists of a body, vertebral arch (pedicles and laminae), and several processes (spinous, transverse, superior and inferior articular).

• Cervical Vertebrae: Smallest, with transverse foramina and bifid spinous processes.

• Thoracic Vertebrae: Heart-shaped body, articular facets for ribs, long spinous processes.

• Lumbar Vertebrae: 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: 12 pairs; true ribs (1-7) attach directly to sternum, false ribs (8-12) attach indirectly or not at all, floating ribs (11-12) do not attach to sternum.

• Functions: Protection, support, respiration.

The Appendicular Skeleton

Pectoral Girdle and Upper Limb

The pectoral girdle consists of the clavicle and scapula, allowing free movement of the upper limb. The upper limb includes the humerus, radius, ulna, carpals, metacarpals, and phalanges.

• 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 contributor 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) forming the hip bone. The lower limb includes the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges.

• Ilium: Superior region of hip bone.

• Ischium: Posteroinferior part, bears weight when sitting.

• Pubis: Anterior portion, forms pubic crest.

• Femur: Thigh bone, largest and strongest.

• Patella: Knee cap, protects knee joint.

• Tibia: Medial, weight-bearing bone.

• Fibula: Lateral, does not bear weight.

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

Arches of the Foot

The foot has three arches (medial, lateral, transverse) maintained by bone shapes, ligaments, and tendons. They support weight and provide flexibility.

• Medial Longitudinal Arch: Talus is the keystone.

• Lateral Longitudinal Arch: Cuboid is the key.

• Transverse Arch: Formed by metatarsals and tarsals.

Joints: Classification and Structure

Functional and Structural Classification

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

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

• Structural: Fibrous (immovable), cartilaginous (immovable or slightly movable), synovial (freely movable).

Fibrous Joints

Bones united by fibrous tissue; mostly immovable. Types include sutures (skull), syndesmosis (ligaments), and gomphosis (tooth socket).

Cartilaginous Joints

Bones connected by cartilage. Types include synchondrosis (hyaline cartilage, immovable) and symphysis (fibrocartilage, slightly movable).

• Examples: Pubic symphysis, intervertebral joints.

Synovial Joints

Freely movable joints with a cavity filled with synovial fluid. Features include articular cartilage, fibrous capsule, synovial membrane, ligaments, bursae, and tendon sheaths.

• Articular Cartilage: Covers bone ends.

• Joint Capsule: Encloses joint surfaces.

• Synovial Fluid: Lubricates joint.

• Bursae: Sacs reducing friction.

• Tendon Sheath: Elongated bursa around tendons.

Factors Affecting Synovial Joint 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; includes osteoarthritis (aging), rheumatoid arthritis (autoimmune).

Clinical Relevance: Bone Health

Osteoporosis and Bone Density

Osteoporosis is a condition characterized by decreased bone density, increasing fracture risk. It is common in postmenopausal women due to decreased estrogen levels.

• Risk Factors: Age, genetics, lifestyle, decreased estrogen.

• Prevention: Calcium, vitamin D, weight-bearing exercise, avoiding smoking and excessive alcohol.

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

Bone Imaging Techniques

• X-rays: Detailed images for fractures and tumors.

• MRI: Cross-sectional images for complex fractures.

• CT Scans: Detailed bone structure analysis.

• DEXA Scan: Measures bone density.

Exercise and Rehabilitation

• Weight-Bearing Exercise: Stimulates bone deposition.

• Resistance Training: Strengthens muscles and bones.

• Flexibility Exercise: Improves range of motion.

• Rehabilitation: Physical and occupational therapy restore function after fractures.

Hormonal Regulation of Bone Health

• Estrogen: Promotes bone deposition, inhibits resorption.

• Testosterone: Increases bone density.

• Parathyroid Hormone (PTH): Regulates calcium, stimulates bone resorption when blood calcium is low.

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