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.