M.Sc. Human Anatomy Lecture 6: Cranial Vault

Interior View of the Skull

The internal surface of the cranial vault shows coronal, sagittal, and lambdoid sutures.
The 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 (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 the Base of the Skull

The floor of the cranial cavity is subdivided into three cranial fossae: anterior, middle, and posterior.
Each cranial fossa has anterior and posterior boundaries and is divided at the midline into right and left areas.
The shape of each cranial fossa matches the shape of the brain regions that it contains.

Anterior Cranial Fossa

The most anterior and shallowest of the three cranial fossae.
Overlies the orbits and contains the frontal lobes of the brain.
Anteriorly bounded 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.
The upward projecting crista galli of the ethmoid bone with cribriform plates at either side are located at the midline of the floor of the anterior cranial fossa.

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 (petrous portion of the temporal bones) 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 (a part of the sphenoid bone).
Openings in the middle cranial fossa include:
- Optic canal: provides a passage for the optic nerve into the orbit.
- Superior orbital fissure: a large, irregular opening into the posterior orbit.
- Foramen rotundum: a rounded opening for a major sensory nerve that supplies the cheek, nose, and upper teeth.
- Foramen ovale: a large, oval-shaped opening for a major sensory nerve to the lateral head, cheek, chin, and lower teeth.
- Foramen spinosum: a small opening for an important artery that supplies the covering layers surrounding the brain.

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, the opening that provides a passage for the spinal cord.
Openings in the posterior cranial fossa include:
- Internal acoustic meatus: provides a passage for the nerve from the hearing and equilibrium organs of the inner ear, and the nerve that supplies the muscles of the face.
- Hypoglossal canal: provides a passage for an important nerve to the tongue.
- Jugular foramen: a large, irregularly shaped opening for several cranial nerves from the brain (the 9th, 10th, 11th) and for all the venous return blood leaving the brain.

Neonatal Skull

The newborn skull, compared with the adult 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, being connected by fibrous tissue or cartilage.
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 posterior fontanelle is triangular and lies between the two parietal bones in front and the occipital bone behind.
The tympanic part of the temporal bone is a C-shaped ring at birth, and 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 when the child moves their head.
The mandible has right and left halves at birth, united in the midline with fibrous tissue, and 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 it is only after eruption of the permanent teeth that the angle of the mandible assumes the adult shape and the head and neck of the condyle grow so that the head comes to lie higher than the coronoid process.