Skeletal System Overview

Introduction to the Skeletal System

The skeletal system is a complex framework of bones, cartilage, and joints that provides support, protection, and movement for the human body. It is divided into two main divisions: the axial skeleton and the appendicular skeleton.

• Axial skeleton: Consists of 80 bones, including the skull, vertebral column, and thoracic cage. It forms the central axis of the body.

• Appendicular skeleton: Composed of 126 bones, including the limbs and girdles that attach them to the axial skeleton.

• Functions: Support, protection of vital organs, movement (in conjunction with muscles), mineral storage, and blood cell formation.

The Skull

Structure and Function

The skull is composed of cranial and facial bones. It protects the brain, provides attachment sites for muscles, and forms the structure of the face.

• Cranial bones: Enclose and protect the brain; provide attachment for head and neck muscles.

• Facial bones: Form the framework of the face, house sense organs, provide openings for food and air, and anchor facial muscles.

Cranial Sutures

Sutures are immovable joints that connect the bones of the skull. They allow for growth during childhood and fuse in adulthood.

• Coronal suture: Between frontal and parietal bones.

• Sagittal suture: Between right and left parietal bones.

• Lambdoid suture: Between parietal bones and occipital bone.

• Squamous suture: Between parietal and temporal bones.

Major Cranial Bones and Features

• Frontal bone: Forms the forehead and superior part of the orbit.

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

• Occipital bone: Forms the posterior base of the skull; contains the foramen magnum for passage of the spinal cord.

• Temporal bones (2): Form the sides and base of the cranium; house structures of the ears.

• Sphenoid bone: Spans the width of the cranial floor; contains the sella turcica for the pituitary gland.

• Ethmoid bone: Forms part of the nasal cavity and the orbits; contains the cribriform plate for olfactory nerves.

Facial Bones

• Maxillae (2): Form the upper jaw and part of the hard palate.

• Mandible: Lower jawbone; only movable bone of the skull.

• Zygomatic bones (2): Cheekbones; form part of the orbit.

• Nasal bones (2): Form the bridge of the nose.

• Lacrimal bones (2): Form part of the medial wall of the orbit.

• Palatine bones (2): Form part of the hard palate and nasal cavity.

• Vomer: Forms part of the nasal septum.

• Inferior nasal conchae (2): Form part of the lateral walls of the nasal cavity.

Skull Cavities and Openings

• Cranial cavity: Houses the brain.

• Orbits: Eye sockets formed by seven bones (frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, ethmoid).

• Nasal cavity: Formed by several bones; contains the nasal septum and conchae.

• Paranasal sinuses: Air-filled spaces in the frontal, sphenoid, ethmoid, and maxillary bones; lighten the skull and enhance voice resonance.

• Foramina, canals, and fissures: Openings for nerves and blood vessels (e.g., foramen magnum, optic canal, jugular foramen).

Paranasal Sinuses

Paranasal sinuses are mucosa-lined, air-filled spaces that communicate with the nasal cavity. They help humidify and warm air, lighten the skull, and enhance resonance of the voice.

• Frontal sinus

• Ethmoid sinus

• Sphenoid sinus

• Maxillary sinus

Cleft Palate

A cleft palate occurs when the right and left palatine processes of the maxilla fail to fuse medially, leaving an opening between the oral and nasal cavities. This can lead to feeding and respiratory difficulties and increases the risk of aspiration pneumonia.

• Prevention: Prenatal vitamins containing folic acid can reduce risk.

• Treatment: Surgical repair is typically performed in infancy.

Vertebral Column

Structure and Regions

The vertebral column, or spine, supports the trunk, protects the spinal cord, and provides attachment points for ribs and muscles. It consists of 26 bones in adults.

• Cervical vertebrae (7): Neck region (C1–C7)

• Thoracic vertebrae (12): Upper back, articulate with ribs (T1–T12)

• Lumbar vertebrae (5): Lower back (L1–L5)

• Sacrum (1): Five fused vertebrae

• Coccyx (1): Three to five fused vertebrae (tailbone)

Spinal Curvatures

• Primary curvatures: Thoracic and sacral; present at birth.

• Secondary curvatures: Cervical (develops as infant lifts head) and lumbar (develops as child begins to walk).

Abnormal Spinal Curvatures

• Scoliosis: Lateral curvature of the spine.

• Kyphosis: Exaggerated thoracic curvature (hunchback).

• Lordosis: Exaggerated lumbar curvature (swayback).

General Structure of Vertebrae

• Body (centrum): Weight-bearing region.

• Vertebral arch: Composed of pedicles and laminae; surrounds the vertebral foramen.

• Vertebral foramen: Canal for the spinal cord.

• Processes: Spinous, transverse, and articular processes for muscle and ligament attachment.

• Intervertebral foramina: Openings for spinal nerves to exit the vertebral column.

Regional Differences in Vertebrae

• Cervical vertebrae: Smallest and lightest; C1 (atlas) and C2 (axis) specialized for head movement. Transverse foramina for vertebral arteries.

• Thoracic vertebrae: Articulate with ribs; heart-shaped bodies; long, downward-pointing spinous processes.

• Lumbar vertebrae: Largest; kidney-shaped bodies; short, thick spinous processes.

• Sacrum: Triangular bone formed by fusion of five vertebrae; articulates with hip bones.

• Coccyx: Small, triangular bone; remnant of a tail.

Specialized Cervical Vertebrae

• Atlas (C1): Lacks a body and spinous process; articulates with occipital condyles to allow nodding motion (atlanto-occipital joint).

• Axis (C2): Has a dens (odontoid process) projecting superiorly; allows rotation of the head (atlanto-axial joint).

Intervertebral Discs

Intervertebral discs are pads of fibrocartilage between vertebrae that absorb shock and allow movement.

• Annulus fibrosus: Outer ring of fibrocartilage.

• Nucleus pulposus: Inner gelatinous core that provides elasticity and compressibility.

Thoracic Cage

Components and Functions

The thoracic cage protects vital organs, supports the shoulder girdles and upper limbs, and provides attachment points for muscles.

• Sternum: Flat bone at the anterior midline; consists of manubrium, body, and xiphoid process.

• Ribs (12 pairs): Attach posteriorly to thoracic vertebrae.

• True ribs (1–7): Attach directly to sternum via costal cartilage.

• False ribs (8–12): Attach indirectly or not at all to the sternum.

• Floating ribs (11–12): No anterior attachment.

Fontanelles

Structure and Function in Infants

Fontanelles are soft spots on a baby's skull where the bones have not yet fused. They allow for compression during birth and brain growth after birth.

• Anterior fontanelle: Largest; closes by 1.5–2 years of age.

• Posterior, mastoid, and sphenoidal fontanelles: Close within the first year.

• Clinical significance: Palpation can assess hydration and intracranial pressure.

Summary Table: Major Bones of the Axial Skeleton

Key Terms and Definitions

• Axial skeleton: The part of the skeleton forming the central axis, including the skull, vertebral column, and thoracic cage.

• Suture: An immovable joint between skull bones.

• Fontanelle: A soft spot on an infant's skull where bone formation is incomplete.

• Foramen: An opening in a bone for nerves and blood vessels.

• Intervertebral disc: Fibrocartilaginous pad between vertebrae.

• Dens (odontoid process): Upward projection from the axis (C2) allowing head rotation.

Example: Atlanto-Occipital and Atlanto-Axial Joints

• Atlanto-occipital joint: Between occipital condyles and atlas (C1); allows nodding "yes" motion.

• Atlanto-axial joint: Between atlas (C1) and axis (C2); allows rotation "no" motion.

Additional info: The above notes expand on the provided slides with standard anatomical definitions, clinical context, and logical grouping of fragmented points for clarity and completeness.