Lecture 25 - Skeletal System: Axial Skeleton - Skull and Hyoid

Axial Skeleton: Skull and Hyoid Bone

Introduction

  • The adult human skeleton consists of 206 bones.
  • The axial skeleton includes the skull, which comprises facial bones and the cranial case.

Axial vs. Appendicular Skeleton

  • There are about 206 bones in the adult body; children have more bones that fuse over time.
  • Bones are divided into two divisions:
    • Axial skeleton: 80 bones along the longitudinal axis (head, neck, and trunk).
      • Includes the skull, hyoid bone, vertebrae, ribs, sternum, and auditory ossicles.
    • Appendicular skeleton: 126 bones.
      • Includes upper and lower limbs, and pelvic and pectoral girdles (attachment sites between axial and appendicular skeleton).

Bone Surface Markings

  • Bones have surface markings like ridges, projections, openings, and depressions (see table 7.2).

  • Ridges:

    • e.g., spine of the scapula.
    • Attachment points for muscles.

  • Projections:

    • Attachment points for muscles and ligaments.
    • Examples: process, tuberosity, tubercles.

  • Openings:

    • Foramen (holes) e.g., foramina of the cribriform plate.
    • Canals (tunnels through bone).

  • Depressions:

    • Fossas (shallow depressions).
    • Notches and grooves.

  • Openings and depressions are areas for blood/nerve supply.

  • General Terms:

    • Body: largest part of the bone.
    • Neck: connects head to the body.
    • Head: end of the bone.
    • Margin/border: outside portion of the bone.
    • Condyle: smooth, rounded articular surface (forms a joint).
    • Facet: small, flattened articular surface.

  • Passageways for blood vessels and nerves are openings or depressions.

  • Muscle/tendon and ligament attachments occur on projections and ridges.

Skull

  • Protects the brain and supports organs (vision, hearing, taste, smell).

  • Made up of 22 separate bones connected by immovable joints.

    • Includes six auditory ossicles (malleus, incus, stapes) in the middle ear.
    • Hyoid bone is associated but not attached to the skull.

  • Two Portions:

    • Cranial bones (8 bones): form the brain case.
      • Paired: Parietal and temporal bones.
      • Single: Frontal, occipital, sphenoid, and ethmoid bones.
    • Facial bones (14 bones):
      • Paired: Maxillary, zygomatic, palatine, lacrimal, nasal, inferior nasal conchae.
      • Single: Vomer and mandible.

  • Mandible: only movable bone of the skull, articulating with the temporal bone.

Lateral View of Skull

  • Frontal Bone:
    • Forms the forehead, roof of the orbit, and anterior cranial floor.
    • Originally two bones that fuse by age six.
  • Parietal Bone:
    • Two parietal bones: left and right.
    • Make up most of the sides and roof of the cranial cavity.
  • Occipital Bone:
    • Posterior and inferior portion/base of the cranium.
  • Temporal Bone:
    • Two temporal bones: one on each side of the skull.
    • Inferior lateral aspects and part of the floor of the cranial cavity.
    • Features:
      • Mastoid process: bony feature behind the ear, attachment point for neck muscles.
      • Styloid process: attachment point for muscles of the tongue and neck.
      • External auditory canal/meatus (acoustic meatus): hole leading to the middle and inner ear.
      • Zygomatic process: part of the cheekbone.
  • Sphenoid Bone:
    • Bat or butterfly-shaped, mostly internal.
    • Portion of the lateral skull, cranial floor and orbit
    • Articulates with all other cranial bones.
  • Ethmoid Bone:
    • Small part visible on the orbit.
    • Contains the cribriform plate (separates nasal cavity from cranial cavity).
  • Facial Bones:
    • Nasal Bones:
      • Form the upper bridge of the nose and anterior roof of the nasal cavity.
    • Lacrimal Bones:
      • Smallest facial bones, contain the lacrimal sac canal.
    • Zygomatic Bones:
      • Form the cheekbones and portions of the orbit.
      • Temporal process: feature of the zygomatic bone.
    • Temporal process of zygomatic bone + zygomatic process of temporal bone = zygomatic arch (cheekbone).
    • Maxilla (Maxillary Bones):
      • Form the upper jaw, floors of the orbit, some of the nasal cavity, and most of the hard palate.
      • Hard palate: roof of the mouth, separating nasal and oral cavities.
    • Mandible:
      • Largest and strongest bone of the skull, articulates with the temporal bone at the mandibular fossa (only movable bone of the skull).

Anterior View of Skull

  • Orbit of the eye:
    • Features the sphenoid and ethmoid bones.
  • Ethmoid Bone:
    • Perpendicular plate: superior and posterior portion of the nasal septum.
    • Middle nasal conchae: ridge on the ethmoid bone.
    • Superior nasal conchae: ridge on the ethmoid bone, hidden under the nasal bone.
    • Cribriform plate: contains foramina for olfactory neurons.
  • Palatine Bone:
    • Small piece visible in the anterior view.
  • Lacrimal Bone:
    • Makes up a small portion of the orbit.
  • Supraorbital margins:
    • Ridges on the top of the orbit in the frontal bone, protect the eye.
  • Nasal Cavity:
    • Superior and middle nasal conchae (ethmoid bone).
    • Inferior nasal conchae (separate bone).
    • Create turbulent airflow, trap air particles, and move odorants to olfactory epithelium.
  • Vomer:
    • Forms the inferior and posterior region of the nasal septum.
  • Nasal septum:
    • Posterior side: vomer and perpendicular plate.
    • Anterior side: hyaline cartilage.
  • Alveolar processes:
    • Spaces in the maxillary bones and mandible where teeth fit.

Inferior View of Skull

  • Mandible removed for better visibility.
  • Sphenoid Bone:
    • Butterfly or bat-shaped, large portion of the cranial floor.
  • Vomer:
    • Portion of the nasal septum.
  • Zygomatic Bone:
    • Temporal process forms the zygomatic arch with the temporal bone's zygomatic process.
  • Maxillary Bones (Maxilla):
    • Palatine processes form a large portion of the roof of the mouth and fuse during development.
    • Improper fusion results in cleft lip/palate.
    • Cleft palate: space between oral and nasal cavities, which can be life-threatening.
  • Palatine Bone:
    • L-shaped (horizontal plate and vertical portion).
    • Horizontal plate and palatine process of maxillary bones form the hard palate.
  • Temporal Bone:
    • Features: mastoid process, external auditory meatus, styloid process, mandibular fossa.
    • Jugular foramen: veins drain blood from the brain.
    • Carotid foramen: arteries enter the cranial cavity.
  • Occipital Bone:
    • External occipital protuberance: bump on the back of the skull, joins to ligaments that hold the head upright.
    • Foramen magnum: spinal cord connects to the brainstem.
    • Occipital condyles: articulate with the first cervical vertebrae, allow nodding and tilting head.

Bones of the Orbit

  • Seven bones form the orbit:
    • Roof: frontal and sphenoid bones.
    • Lateral wall: zygomatic and sphenoid bones.
    • Floor: maxilla, zygomatic, and palatine bones.
    • Medial wall: maxilla, lacrimal, ethmoid, and sphenoid bones.
  • The orbit is cone-shaped and bony features provide attachment points for extrinsic eye muscles.

Hyoid Bone

  • Located below the chin, associated with the skull but not part of it.
  • Unpaired floating bone: does not articulate with any other bone.
  • U-shaped bone with a body and projections.
  • Attachment point for tongue muscles and neck muscles that elevate the larynx during speech and swallowing.