The Skeletal System

Overview

The skeletal system forms the structural framework of the human body, providing support, protection, and facilitating movement. It is composed of bones, cartilages, and joints, and accounts for approximately 20% of total body mass.

• Support: Provides the body’s framework and supports soft tissues.

• Protection: Shields vital organs such as the brain, heart, and lungs.

• Movement: Serves as levers for muscles to act upon.

• Mineral Storage: Reservoir for minerals, especially calcium and phosphate.

• Blood Cell Formation: Hematopoiesis occurs in red marrow cavities of certain bones.

Major Subdivisions of the Skeleton

Axial and Appendicular Skeleton

The human skeleton is divided into two major subdivisions: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Consists of 80 bones forming the longitudinal axis of the body. It includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Composed of 126 bones of the limbs and girdles (pectoral and pelvic) that attach them to the axial skeleton.

Functions of the Axial Skeleton:

• Supports and protects the brain, spinal cord, and thoracic organs.

• Forms the longitudinal axis of the body.

• Provides attachment points for head, neck, and trunk muscles.

Skull: Cranium and Facial Bones

Cranium

The cranium encloses the brain and provides attachment for head and neck muscles. It is composed of eight bones:

• Frontal (1): Forms the forehead and superior part of the orbits.

• Parietal (2): Form the superior and lateral aspects of the skull.

• Occipital (1): Forms the posterior part and base of the skull; contains the foramen magnum.

• Temporal (2): Form the inferolateral aspects of the skull and parts of the cranial floor.

• Sphenoid (1): Spans the width of the middle cranial fossa.

• Ethmoid (1): Forms part of the nasal cavity and the orbits.

Sutures: Immovable joints that connect the bones of the skull, such as the coronal, sagittal, lambdoid, and squamous sutures.

Facial Bones

The facial skeleton consists of 14 bones, which form the framework of the face, contain cavities for special sense organs, and provide openings for air and food passage.

• Nasal (2)

• Lacrimal (2)

• Inferior nasal conchae (2)

• Maxilla (2): Upper jaw, holds upper teeth.

• Zygomatic (2): Cheekbones.

• Palatine (2): Posterior part of the hard palate.

• Vomer (1): Forms part of the nasal septum.

• Mandible (1): Lower jaw, only moveable bone of the skull.

Other Skull Features

• Orbits: Bony cavities that encase the eyes and lacrimal glands, formed by seven bones.

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

• Hyoid Bone: Not part of the skull, but located in the anterior neck; does not articulate with any other bone and serves as a movable base for the tongue.

Vertebral Column

Structure and Regions

The vertebral column, or spine, extends from the skull to the pelvis and is composed of 26 irregular bones (vertebrae) in five major regions:

• Cervical (7): Neck region.

• Thoracic (12): Upper back, articulates with ribs.

• Lumbar (5): Lower back, bears most of the body's weight.

• Sacrum (1): Formed from fusion of five vertebrae, articulates with hip bones.

• Coccyx (1): Formed from fusion of three to five vertebrae, forms the terminus of the column.

Curvatures

• Cervical and lumbar curvatures: Concave posteriorly.

• Thoracic and sacral curvatures: Convex posteriorly.

• Curvatures increase the spine’s resilience and flexibility.

Intervertebral Discs

• Structure: Cushion-like pads composed of a nucleus pulposus (inner gelatinous core) and anulus fibrosus (outer collar of collagen and fibrocartilage).

• Function: Absorb shock and allow flexibility.

• Clinical Note: Herniated discs occur when the nucleus pulposus protrudes through the anulus fibrosus, potentially compressing spinal nerves.

Vertebral Structure

• Body: Anterior weight-bearing region.

• Vertebral arch: Encloses the vertebral foramen.

• Processes: Spinous (posterior), transverse (lateral), and articular (superior/inferior) processes for muscle and ligament attachment.

Special Vertebrae

• Atlas (C1): Supports the skull, allows nodding "yes" motion.

• Axis (C2): Has the dens (odontoid process) for rotation, allows shaking head "no" motion.

• Vertebra prominens (C7): Notable for its prominent spinous process.

Thoracic Cage

Components and Functions

The thoracic cage protects vital organs and supports the shoulder girdles and upper limbs. It consists of:

• Sternum: Breastbone, composed of the manubrium, body, and xiphoid process.

• Ribs: 12 pairs, attached posteriorly to thoracic vertebrae.

• Costal cartilages: Connect ribs to sternum.

Classification of Ribs

Appendicular Skeleton

Pectoral Girdle and Upper Extremities

• Pectoral Girdle: Consists of clavicles (collarbones) and scapulae (shoulder blades); attaches upper limbs to trunk.

• Upper Limb Bones:

  • Humerus: Only bone of the arm; articulates with scapula, radius, and ulna.

  • Radius and Ulna: Forearm bones; articulate with each other at radioulnar joints.

  • Carpals: Eight bones in the wrist.

  • Metacarpals: Five bones in the palm.

  • Phalanges: Fourteen bones in the fingers; thumb (pollex) has two, other fingers have three each.

Pelvic Girdle and Lower Extremities

• Pelvic Girdle: Formed by two hip bones (coxal bones), sacrum, and coccyx; attaches lower limbs to trunk and supports pelvic organs.

• Lower Limb Bones:

  • Femur: Thigh bone, longest and strongest bone in the body.

  • Patella: Kneecap, protects knee joint.

  • Tibia and Fibula: Parallel bones of the leg; tibia is weight-bearing, fibula is not.

  • Tarsals: Seven bones in the ankle; talus and calcaneus bear most weight.

  • Metatarsals: Five bones in the foot.

  • Phalanges: Fourteen bones in the toes; hallux (big toe) has two, others have three each.

Summary Table: Major Divisions and Bones of the Skeleton

Key Terms

• Foramen magnum: Large opening in the occipital bone for the spinal cord.

• Sutures: Immovable joints between skull bones.

• Acetabulum: Hip socket receiving the head of the femur.

• Manubrium, body, xiphoid process: Parts of the sternum.

• Pollex: Thumb.

• Hallux: Big toe.

Additional info: Some clinical notes (e.g., herniated discs, IM injection landmarks, carpal tunnel syndrome) were inferred from standard anatomy teaching and are relevant for practical understanding.