Human Anatomy: Skeletal System - Cranial Vault

Internal Surface of Cranial Vault

• The internal surface of the cranial vault shows the following features:
• Coronal suture, sagittal suture, and lambdoid sutures
• Frontal crest lies anteriorly in the median plane
• Sagittal sulcus runs medially along the adjoining margins of parietal bones
• Deep irregular pits for arachnoid granulations called granular pits are seen on either side of the sagittal sulcus
• Vascular markings for branches of middle meningeal vessels are seen on either side

Internal Surface of Base of Skull

• The floor of the cranial cavity is subdivided into three cranial fossae:
• Anterior cranial fossa:
  • The most anterior and the shallowest of the three cranial fossae
  • Overlies the orbits and contains the frontal lobes of the brain
  • Bounded anteriorly by the frontal bone, which also forms the majority of its floor
  • The lesser wings of the sphenoid bone form the prominent ledge that marks the boundary between the anterior and middle cranial fossae
• Middle cranial fossa:
  • Deeper and situated posterior to the anterior fossa
  • Occupied by the temporal lobes of the brain
  • Extends from the lesser wings of the sphenoid bone anteriorly to the petrous ridges posteriorly
  • The petrous ridges give the middle cranial fossa a butterfly shape, making it narrow at the midline and broad laterally
  • Divided at the midline by the upward bony prominence of the Sella turcica
  • Openings for the passage of blood vessels and cranial nerves:
    • Optic canal
    • Superior orbital fissure
    • Foramen rotundum
    • Foramen ovale
    • Foramen spinosum
• Posterior cranial fossa:
  • The most posterior and deepest portion of the cranial cavity
  • Contains the cerebellum of the brain
  • Bounded anteriorly by the petrous ridges, while the occipital bone forms the floor and posterior wall
  • Divided at the midline by the large foramen magnum
  • Openings for the passage of blood vessels and cranial nerves:
    • Internal acoustic meatus
    • Hypoglossal canal
    • Jugular foramen

Neonatal Skull

• The newborn skull:
• Has a disproportionately large cranium relative to the face
• The bones of the skull are smooth and unilaminar
• Most of the skull bones are ossified at birth, but the process is incomplete, and the bones are mobile on each other
• The bones of the vault are not closely joined at sutures, as in the adult, but are separated by unossified membranous intervals called fontanelles
• Clinically, the anterior and posterior fontanelles are most important and are easily examined in the midline of the vault
• The anterior fontanelle is diamond-shaped and lies between the two halves of the frontal bone in front and the two parietal bones behind
• The fibrous membrane forming the floor of the anterior fontanelle is replaced by bone and is closed by 18 months of age
• The posterior fontanelle is triangular and lies between the two parietal bones in front and the occipital bone behind
• By the end of the 1st year, the fontanelle is usually closed and can no longer be palpated
• Other features of the newborn skull:
• The tympanic part of the temporal bone is a C-shaped ring at birth
• The external auditory meatus is almost entirely cartilaginous in the newborn
• The mastoid process is not present at birth and develops later in response to the pull of the sternocleidomastoid muscle
• The mandible has right and left halves at birth, united in the midline with fibrous tissue
• The two halves fuse at the symphysis menti by the end of the 1st year
• The angle of the mandible at birth is obtuse, and the head of the condyle being placed level with the upper margin of the body of the mandible