Section 2: Facial and Skull Anatomy

Chapter 8: The Skeletal System - Section Two: Axial Skeleton

Transition from Cranial to Facial Bones

  • Focus: From cranial bones to facial bones in the axial skeleton.

Ethmoid Bone

  • Location: Central part of the midface.
  • Function: Forms the superior aspect of the nasal cavity.
  • Structures:
    • Crista Galli:
    • Description: A bony projection on the ethmoid bone.
    • Function: Attachment site for cranial dura mater; stabilizes brain within the skull, preventing excessive movement.
    • Cribriform Plate:
    • Description: A thin, bony plate with tiny holes.
    • Function: Allows the olfactory nerve to pass from the nasal cavity to the brain for the sense of smell.
    • Perpendicular Plate:
    • Function: Forms part of the nasal septum, dividing the nasal cavity into left and right sides.
    • Ethmoidal Labyrinth and Ethmoidal Sinus Cells:
    • Description: Numerous small sinus cavities located in the ethmoid bone.
    • Function: Part of the ethmoidal sinuses, most numerous within the nasal cavity.

Zygomatic Bone

  • Description: Known as the cheekbone.
  • Function: Forms the lateral and inferior wall of the orbit (eye socket).
  • Connections:
    • Connects with:
    • Frontal Bone: Superiorly.
    • Temporal Bone: Via the temporal process.
    • Maxilla: Laterally.
  • Structural Importance: Important in connecting facial bones to the skull.

Lacrimal Bone

  • Description: The smallest bone in the skull (excluding middle ear bones).
  • Function: Forms the medial wall of the orbit (eye socket).
  • Component: Contains a lacrimal groove for the tear duct which carries tears from the lacrimal gland to the nasal cavity.

Palatine Bone

  • Location: Situated at the posterior aspect of the hard palate and part of the nasal cavity.
  • Structure: Contributes to the inferior part of the orbital cavity.
  • Parts:
    • Forms a portion of the hard palate and the lateral wall of the nasal cavity.

Maxillary Bone (Maxilla)

  • Description: The most prominent bone of the midface.
  • ** Functions**:
    • Holds the teeth and supports other facial structures.
    • Forms the lateral aspect of the nasal cavity and part of the orbit.
    • Major contributor to the hard palate and the anterior part of the roof of the mouth.
  • Key Structures:
    • Infraorbital Foramen: Exits for the infraorbital nerve supplying sensation to the face.
    • Incisive Foramen: Entry point for nerves supplying sensation to the front teeth.
  • Clinical Relevance: Issues may lead to cleft palate or other dental complications.
    • Cleft Palate: Incomplete fusion of the maxilla leading to a direct connection between the mouth and nasal cavity.
    • Exophthalmos: Protrusion of the eyes due to maxillary deformation (hypoplasia).

Mandible (Jawbone)

  • Description: Lower jaw with a specific shape.
  • Anatomy:
    • TMJ: The temporomandibular joint where the mandible articulates with the skull.
    • Structures:
    • Coronoid Process: Anterior part of the mandible; muscle attachment.
    • Ramus: Vertical part connecting the body to the skull.
    • Mental Foramen: Foramen for the mental nerve providing sensation to the chin.
    • Clinical Attention: Susceptibility to fractures leads to immobilization techniques (e.g., wiring jaws).

Cranial Fossa

  • Types of cranial fossa in the skull from superior to inferior:
    • Anterior Cranial Fossa: Predominantly formed by the frontal bone; houses the frontal lobe.
    • Contributions: Lesser wings of sphenoid bilaterally.
    • Middle Cranial Fossa: Mostly temporal bone; houses temporal lobes and pituitary gland.
    • Contributions: Greater wings of sphenoid bone.
    • Posterior Cranial Fossa: Encloses the cerebellum and brainstem; contains foramen magnum through which the spinal cord passes.

Orbit (Eye Socket)

  • Complex structure composed of 7 bones:
    • Ethmoid, Zygomatic, Lacrimal, Maxilla, Frontal, Sphenoid (greater wing), Palatine.
  • Gaps in the orbit:
    • Superior Orbital Fissure: Allows entry of cranial nerves II, III, IV, and VI, as well as the ophthalmic veins.
    • Inferior Orbital Fissure: Passage for infraorbital nerve and vessels.

Nasal Complex and Sinuses

  • Major bones:
    • Ethmoid, Sphenoid, Lacrimal, Maxilla.
  • Turbinates or Conchae: Folds of tissue in the nasal cavity assist with airflow regulation and warming.
  • Types of Sinuses:
    • Ethmoidal Sinuses: Series of air cells within the ethmoid bone.
    • Maxillary Sinuses: Paired sinuses located within the maxilla.
    • Frontal Sinuses: Develop during childhood, often empirical at around 8-9 years.
    • Sphenoidal Sinuses: Located towards the back of the sphenoid bone.

Hyoid Bone

  • Description: Unique bone not articulating with other bones.
  • Function: Provides attachment for muscles involved in swallowing and speech.
  • Relevance: Commonly broken in cases of strangulation, significant in forensic medicine.

Bones of the Middle Ear

  • Key Structures:
    • Malleus (Hammer)
    • Incus (Anvil)
    • Stapes (Stirrup)
  • Function: Transmit vibrations from the eardrum to the cochlea of the inner ear.
    • Articulation: Each connects to the next, transmitting sound effectively.

Sexual Dimorphism in Skull Shape

  • Distinctions between male and female skulls:
    • Male skull: More prominent brow, squarer mental protuberance, thicker bone structure.
    • Female skull: More smooth, triangular mental protuberance, and a more vertical frontal bone.

Infant and Fetal Skull

  • Developmental importance: Skull flexibility allows for passage through the birth canal.
  • Fontanelles:
    • Anterior Fontanelle: Soft spot at the front of the skull, closes around 15 months.
    • Posterior Fontanelle: Closes a few days after birth.

Vertebral Column and Trunk Bones

  • Composed of:
    • Cervical: 7 vertebrae, allow neck movement.
    • Thoracic: 12 vertebrae, articulate with ribs.
    • Lumbar: 5 larger vertebrae, support lower back.
    • Sacrum: 5 fused vertebrae forming the posterior pelvis.
    • Coccyx: 4 fused vertebrae (tailbone).
  • Intervertebral Discs: Shock absorbers between vertebrae, made of fibrocartilage.
  • Curvatures:
    • Lordosis: Increased lumbar curvature.
    • Kyphosis: Increased thoracic curvature.
    • Scoliosis: Lateral curvature of the spine.

Structure of an Individual Vertebra

  • Key features:
    • Vertebral Body: Anterior part supporting weight.
    • Pedicles and Laminae: Form the vertebral arch, protect spinal cord.
    • Processes: Adaptation sites for ligaments and muscles.
    • Facets: Articular surfaces allowing vertebral movement.

Sacrum and Coccyx

  • Sacrum: Fusion of five vertebrae; connects iliac bones of pelvis.
  • Coccyx: Fusion of four small vertebrae at the base of the vertebral column, no spinal nerves.

Thoracic Cage

  • Components:
    • Sternum:
    • Manubrium: Top section where ribs 1 and 2 attach.
    • Body: Middle section attaching others.
    • Xiphoid Process: Small, variable prominent structure at the bottom.
    • Ribs:
    • True ribs: Directly attach to sternum (ribs 1-7).
    • False ribs: Indirectly connect to the sternum (ribs 8-10).
    • Floating ribs: Not attached (ribs 11-12).
  • Articulations:
    • Costal facets on vertebrae for rib attachment.

Pelvic Girdle

  • Structure: Composed of the ilium, ischium, and pubis.
  • Sacrum: Joins posteriorly with sacroiliac joints.
  • Function: Supports pelvic organs, facilitates childbirth pathway in females.
  • Sexual Dimorphism: Female pelvis is broader with a larger inlet for childbirth, while the male pelvis is narrower and more triangular.

Conclusion

  • The axial skeleton is crucial for structural support, protection of vital organs, and assisting with movement in conjunction with the appendicular skeleton.