Anatomy of the Skull and Axial/Appendicular Skeleton

Axial vs. Appendicular Skeleton

  • Axial Skeleton: The central part of the skeleton, encompassing bones that protect the brain and lungs. It includes:
    • Skull
    • Hyoid bone
    • Ossicles
    • Vertebrae
    • Ribs
    • Sternum
  • Appendicular Skeleton: Comprises the extremities and girdles. Common test mistake: The clavicle is part of the appendicular, not axial, skeleton because it is part of the pectoral girdle.
    • Pectoral Girdle: Includes the scapula and clavicle.
    • Pelvic Girdle: Refers to the pelvic bone.

The Skull

  • Composition: The skull (cranium) consists of 22 bones.
    • 8 Cranial bones
    • 14 Facial bones

Cranial Bones

  • Function: Primarily protect the brain and form the cranial cavity.
  • Specific Bones & Features:
    • Frontal Bone: Forms the forehead.
      • Initially two bones that fuse, leaving a metopic suture in some.
      • Has a squamous part (flat part).
      • Includes the supraorbital foramen and supraorbital ridge.
      • Shapes the groove of the eye (part of the eye socket).
    • Parietal Bones: Form the skull caps.
      • Touch all four major sutures: lambdoid, squamous, coronal, and sagittal sutures.
    • Temporal Bone: Located where the ear is.
      • Landmarks:
        • Squamous part: The flat area.
        • Zygomatic arch/portion: Forms an arch behind the cheekbone.
        • Acoustic/Auditory meatus: Ear canal opening.
        • Styloid process: A pointy, pen-like protuberance below the meatus.
        • Mastoid process: A large protuberance often asked about in relation to the temporal bone.
    • Occipital Bone: Located at the back of the head.
      • Landmarks:
        • External Occipital Protuberance (EOP): A protuberance at the back of the head, above where it dips in.
        • Foramen Magnum: The largest foramen in the body; allows the spinal cord to exit the brain and connect to the vertebrae.
        • Occipital Condyles: Two structures articulating with the first cervical vertebra.
        • Hypoglossal Canal: Passageway for the hypoglossal nerve (cranial nerve 12), which controls tongue movement.
    • Sphenoid Bone: A deep skull bone, resembling a butterfly or moth in shape. It forms part of the floor of the skull and has numerous passageways.
      • Main Parts:
        • Lesser wings (green in diagram).
        • Body (blue in diagram).
        • Pterygoid processes (yellow in diagram, starts with 'P', pronounced 'terygoid'): Extend into the mouth/pharynx for attachment of mastication muscles (medial and lateral pterygoids).
        • Greater wings (red in diagram).
      • Foramina & Structures:
        • Superior Orbital Fissure: Allows passage for nerves like the trochlear nerve.
        • Optic Canal: Where the optic nerve exits.
        • Foramen Rotundum, Foramen Ovale, Foramen Spinosum.
        • Hypophyseal Fossa (Sella Turcica): A dip in the body of the sphenoid that houses the pituitary gland (also known as hypophysis).
    • Ethmoid Bone: Deep bone, forms the top part of the nasal cavity and part of the floor of the skull. It holds the olfactory bulb (olfactory nerve).
      • Landmarks:
        • Crista Galli: A pointy, rooster-crest-like projection.
        • Cribriform Plate: A flat, perforated area on either side of the crista galli, allowing olfactory nerves to pass into the nasal cavity to pick up scent particles.
        • Ethmoidal Air Cells: These are the ethmoidal sinuses.
        • Superior and Middle Nasal Conchae: Located on the lateral sides, they help moisten and warm inhaled air.
        • Perpendicular Plate of the Ethmoid: Forms the superior portion of the nasal septum.

Facial Bones

  • Function: Provide attachment for facial expression muscles, aid in breathing and digestion (forming parts of the mouth and respiratory system).
  • Specific Bones & Features:
    • Nasal Bones (2): Form the bridge of the nose, supporting the nasal cartilage and nasal opening.
    • Maxilla (2): The upper jaw, holding the upper teeth. Articulates with the frontal bone and forms the anterior portion of the hard palate.
      • Infraorbital Foramen: Located within the maxilla.
      • Alveolar Processes: Articulations for the teeth in the bone.
    • Zygomatic Bones (2): The cheekbones, forming the lateral side of the orbit (eye socket).
    • Mandible (1): The lower jaw; the only movable bone in the skull.
      • Parts: Ramus, angle, and body.
      • Ramus Features:
        • Mandibular Condyles: Where the temporomandibular joint (TMJ) attaches.
        • Mandibular Notch: Between the condyles and coronoid processes.
        • Coronoid Processes: Two pointy anterior processes of the notch.
      • Body Features:
        • Mental Foramen: Two foramens located on the body.
    • Palatine Bones (2): Form the posterior aspect of the hard palate (behind the maxilla).
    • Lacrimal Bones (2): Located on the medial aspect of the orbit.
      • Lacrimal Fossa: An opening for the lacrimal sac, which collects tears and drains them into the nasal cavity (explaining a runny nose during crying).
    • Inferior Nasal Conchae (2): The bottom turbinates in the nose, also helping to moisten and warm inhaled air.
    • Vomer (1): Forms the inferior portion of the nasal septum, articulating with the perpendicular plate of the ethmoid to form the complete septum.

Skull Cavities

  • Nasal Cavity
  • Orbits: Eye sockets.
  • Paranasal Sinuses: Four types.
    • Frontal
    • Maxillary
    • Ethmoid (Ethmoidal air cells)
    • Sphenoid
  • Small Cavities in the Ear: House organs for hearing and equilibrium (e.g., organ of Corti).

Sutures of the Skull

  • Definition: Immovable joints that fuse skull bones together.
  • Four Major Sutures:
    • Coronal Suture: Separates the frontal bone from the parietal bones. Runs across the crown of the head (like a crown) where the soft spot (bregma) is in infants.
    • Sagittal Suture: Separates the two parietal bones from each other, splitting the skull into right and left halves.
    • Squamous Suture: Separates the parietal bones from the temporal bone on the side of the head. Squamous means flat, referring to the flat area where it's located.
    • Lambdoid Suture: Separates the parietal bones from the occipital bone at the back of the head. Mnemonic: Limbo (going backward) relates to Lambdoid (at the back).

Muscles of the Skull

  • Muscles attached to the facial bones produce facial expressions and facilitate mastication (chewing).

Practice

  • Note: While an osteology class would delve into more minute landmarks (e.g., occipital angle, frontal angle), the focus here is on major, identifiable features for typical anatomy courses. For lab tests, identification of parts on bones is expected; for lecture tests, multiple-choice questions on landmarks are common.