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 axial skeleton forms the longitudinal axis of the body, supporting the head, neck, and trunk, and protecting the brain, spinal cord, and thoracic organs. The appendicular skeleton facilitates movement and supports the limbs.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Includes the pectoral girdle, pelvic girdle, and limbs.

• Functions: Protection, support, movement, and attachment for muscles.

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, contain cavities for sensory organs, and anchor muscles for facial expression.

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

• Facial Bones: Form the face, house sensory cavities, and secure teeth.

• 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 middle and internal ear, nasal cavity, orbits, and 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.

Skull Bones and Markings

• Foramina: Openings for blood vessels and nerves.

• Processes: Projections for muscle attachment.

• Sutures: Immovable joints connecting skull bones.

Cranial Bones: Identification and Features

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

• Frontal Bone: Forms the forehead, superior wall of orbits, and contains frontal sinuses.

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

• Occipital Bone: Forms the posterior wall, contains the foramen magnum, occipital condyles, and nuchal lines.

• Temporal Bones: Make up inferolateral aspects, contain zygomatic, mastoid, and styloid processes, and house ear cavities.

• Sphenoid Bone: Keystone bone, articulates with all cranial bones, contains sella turcica and sinuses.

• Ethmoid Bone: Deepest skull bone, forms part of the nasal cavity and orbits, contains cribriform plates and ethmoidal air cells.

Facial Bones: Identification and Features

The facial skeleton is made up of 14 bones, including the mandible, maxillae, zygomatic, nasal, lacrimal, palatine, vomer, and inferior nasal conchae.

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

• Maxillae: Form the upper jaw and central facial skeleton; contain maxillary sinuses.

• Zygomatic Bones: Form cheekbones and margins of orbits.

• Nasal Bones: Form the bridge of the nose.

• Lacrimal Bones: Form medial walls of orbits; house lacrimal sac.

• Palatine Bones: Complete the hard palate and form part of the nasal cavity and orbits.

• 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

• 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.

The Vertebral Column

Structure and Regions

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. It consists of 26 irregular bones in five major regions: cervical (7), thoracic (12), lumbar (5), sacrum (1, fused), and coccyx (1, fused).

• Cervical: 7 vertebrae in the neck.

• Thoracic: 12 vertebrae in the upper back.

• Lumbar: 5 vertebrae in the lower back.

• Sacrum: 5 fused vertebrae in the pelvis.

• Coccyx: 3-5 fused vertebrae at the 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, increase resilience, flexibility, and shock absorption.

• Spinal Curvatures: Maintain balance and absorb shock.

• Intervertebral Discs: Cushion vertebrae and allow movement.

Abnormal Spinal Curvatures

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

• Kyphosis: Abnormal dorsal thoracic curvature (hunchback).

• Lordosis: Accentuated 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, with transverse foramina and 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 forms a protective cage for major organs and is composed of the sternum, ribs, and thoracic vertebrae.

• Sternum: Manubrium, body, and xiphoid process.

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

• Functions: Protection, support, and respiration.

True vs. False Ribs

• True Ribs: Attach directly to the sternum via their own costal cartilage.

• False Ribs: Attach indirectly or not at all; floating ribs do not attach to the sternum.

The Appendicular Skeleton

Pectoral Girdle and Upper Limb

The pectoral girdle consists of the clavicle and scapula, allowing for free movement of the upper limb. The upper limb includes the humerus, ulna, radius, 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 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, and pubis. The lower limb includes the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges.

• Ilium: Superior region of hip bone.

• Ischium: Posteroinferior part; ischial tuberosity bears weight when sitting.

• Pubis: Anterior portion; pubic crest and obturator foramen.

• Femur: Largest, strongest bone; articulates with hip bone.

• Patella: Knee cap; protects knee joint.

• Tibia: Medial, weight-bearing bone.

• Fibula: Lateral, finer, non-weight-bearing bone.

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

Arches of the Foot

• Medial Longitudinal Arch: Talus is the keystone.

• Lateral Longitudinal Arch: Cuboid bone is the key.

• Transverse Arch: Formed by metatarsals and tarsals.

Joints: Classification and Structure

Functional and Structural Classification

Joints, or articulations, hold bones together and allow for mobility. They are classified functionally as synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable). Structurally, joints are fibrous, cartilaginous, or synovial.

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

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

• Synovial Joints: Freely movable; have a joint cavity filled with synovial fluid.

Features of Synovial Joints

• Articular Cartilage: Covers ends of bones.

• Joint Capsule: Encloses joint surfaces.

• Synovial Fluid: Lubricates the joint.

• Ligaments: Reinforce the joint.

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

• Tendon Sheath: Elongated bursa around a tendon.

Factors Affecting 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 and rheumatoid arthritis.

Bone Health and Clinical Considerations

Importance of Load-Bearing Activities

• Mechanical Stress: Stimulates bone deposition and increases density.

• Bone Density: Maintained by regular physical activity.

Bone Imaging Techniques

• X-rays: Detailed images for diagnosing fractures and abnormalities.

• MRI: Cross-sectional images for complex fractures.

• CT Scans: Detailed bone structure analysis.

Hormones and Bone Health

• Estrogen: Promotes bone deposition; decreased levels increase osteoporosis risk.

• Testosterone: Important for bone health in men.

• Parathyroid Hormone (PTH): Regulates calcium levels and 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 in daily activities.

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

• Post-Fracture Rehabilitation: Restores function and mobility.

Summary Table: Major Bones of the Axial and Appendicular Skeleton

Key Equations and Concepts

• Bone Density Equation: $\text{Bone Density} = \frac{\text{Bone Mass}}{\text{Bone Volume}}$

• Mechanical Stress: $\text{Stress} = \frac{\text{Force}}{\text{Area}}$

Conclusion

The skeletal system is essential for protection, support, movement, and mineral storage. Understanding the structure and function of the axial and appendicular skeleton, as well as the classification and features of joints, is fundamental for anatomy and physiology students. Regular exercise, proper nutrition, and screening are crucial for maintaining bone health and preventing disorders such as osteoporosis and arthritis.