Anatomy Notes: Axial Skeleton - Sutures, Sinuses, Fontanels, Hyoid, and Vertebral Column

Cranial Sutures, Sinuses, and Fontanels

• Squamous Suture: Separates the parietal bone from the temporal bone.
• Lambdoid Suture: Separates the parietal bone from the occipital bone.
• Paranasal Sinuses:
  • Frontal Sinus: Located within the frontal bone.
  • Sphenoid Sinus: Located within the sphenoid bone.
  • Ethmoid Air Cells (Ethmoid Sinus): Located within the ethmoid bone.
  • Maxillary Sinus: Located within the maxillary bone.
• Fontanels (Soft Spots in Infants):
  • Anterior Fontanel: The most commonly observed soft spot, located at the junction of the frontal and parietal bones.
  • Posterior Fontanel: Located at the junction of the sagittal suture and the lambdoid suture.
  • Sphenoidal Fontanel: Located on the sides of the head, near the sphenoid area, posterior to the eyes.

Hyoid Bone

• Unique Articulation: The hyoid bone is the only bone in the body that does not articulate directly with any other bone.
• Support & Structure: It is supported and held in place by muscles.
• Functions:
  • Supports the tongue and its associated structures.
  • Provides attachment for muscles of the anterior neck.
  • Provides attachment for muscles of the pharynx.
  • Helps keep the oropharynx open.
  • Assists with walking.
• Landmarks: Has two greater horns located posteriorly.
• Muscle Attachments: Many muscles of the anterior neck and pharynx (e.g., thyrohyoids, stylohyoids, mylohyoids) attach to the hyoid bone, facilitating swallowing and talking.

Vertebral Column (Spine/Backbone/Spinal Column)

• Functions:
  • Protects the spinal cord.
  • Serves as numerous points of attachment for muscles.
  • Provides support for the pelvic girdle (via the sacrum).
  • Supports the head and trunk.
• Curvatures:
  • The vertebral column is naturally curved, not completely straight. These anterior and posterior curves are normal.
  • Benefits of Curves: Increase strength, help maintain balance, and aid in shock absorption (e.g., during walking, running).
• Divisions of the Vertebral Column:
  • Cervical Vertebrae: $7$ bones, located in the neck region. (Mnemonic: Breakfast at $7$)
  • Thoracic Vertebrae: $12$ bones, located in the mid-back region. (Mnemonic: Lunch at $12$)
  • Lumbar Vertebrae: $5$ bones, located in the low-back region. (Mnemonic: Dinner at $5$)
  • Sacrum: $5$ vertebrae fused together, considered immovable. Articulates with the pelvis.
  • Coccyx: $4$ vertebrae fused together, considered immovable, forming the tailbone.
• Types of Curves:
  • Primary Curves (Kyphotic Curves): Present from birth.
    • Kyphosis (Kyphotic) refers to a forward bending or hunchback-like curve.
    • Locations: Thoracic and Sacral regions (Curves 2 and 4 in typical diagrams).
  • Secondary Curves (Lordotic Curves): Develop after birth as a child begins to lift their head and walk.
    • Lordosis (Lordotic) refers to an anterior arch or swayback-like curve.
    • Locations: Cervical and Lumbar regions (Curves 1 and 3 in typical diagrams).
  • Adult Curves: An adult spine typically has four curves: two primary (thoracic and sacral kyphotic) and two secondary (cervical and lumbar lordotic).
• Spinal Conditions (Exaggerated Curves):
  • Hyperkyphosis: An exaggerated posterior curve (hunchback).
    • Example: Ankylosing spondylitis (an autoimmune disease, often in young men, where vertebral ligaments calcify, leading to hunchback).
  • Hyperlordosis: An exaggerated anterior curve (swayback).
    • Causes: Mainly pregnancy, or excessive anterior abdominal load.
  • Scoliosis: A lateral curvature of the spine.
    • Types: Thoracic only, lumbar only, thoracolumbar, or combined (forming an 'S' curve).
    • Etiology: Often idiopathic (unknown cause), sometimes juvenile.
    • Progression: Tends to stop progressing around puberty.
• Intervertebral Discs:
  • Location: Found between adjacent vertebrae.
  • Composition:
    • Annulus Fibrosus: The strong, outer fibrous ring, made of fibrocartilage.
    • Nucleus Pulposus: The soft, gelatinous (juicy) inner core, allowing for shock absorption.
  • Functions: Form strong joints, act as shock absorbers, and allow for motion of the vertebral column ($5-7$ degrees per segment in the neck).
• Herniated Disc Stages:
  • Disc Degeneration: Annulus fibrosus weakens, but no significant herniation or tears. May involve a bulge.
  • Prolapse/Bulge: Nucleus pulposus herniates, but the annulus fibrosus is still intact, though stretched.
  • Extrusion: The annulus fibrosus tears, and the nucleus pulposus permeates out of its normal area into the spinal canal.
  • Sequestration: A piece of the nucleus pulposus breaks off (separates) from the main disc and floats freely within the spinal canal.
• Typical Vertebrae Structure:
  • Body: The main, weight-bearing anterior portion. Intervertebral discs sit on top of the body.
  • Vertebral Arch: Posterior to the body, surrounds the spinal cord.
  • Vertebral Foramen: The opening created by the vertebral arch, through which the spinal cord passes.
  • Processes (Attachment Points for Muscles/Ligaments):
    • Spinous Process: Posterior projection.
    • Transverse Processes: Two lateral projections.
    • Pedicle: Connects the body to the transverse process.
    • Lamina: Connects the spinous process to the transverse process. (Laminectomy: surgical removal of lamina to create space for spinal cord).
    • Facet Joints (Articular Facets): Flat surfaces for articulation between superior and inferior articular processes of adjacent vertebrae. Prone to arthritis.
• Differentiating Vertebrae Types:
  • Cervical Vertebrae (C1-C7):
    • Key Features: Three foramina (holes): one large vertebral foramen and two transverse foramina (in the transverse processes). No other vertebrae have transverse foramina.
    • Spinous Process: Often bifid (forked or C-shaped,