Bones of the Skull

The human cranium

• 22 bones interconnected through immobile sutures

• Divided into:

  • Neurocranium

    • The calvaria (The dome), which is the superior portion of the cranium

    • The base (The floor of the skull)

  • Viscerocranium

• Cranial cavity

  • Continuous with the vertebral canal through a large opening in the base, called the foramen magnum

  • The upper (domed) part

  • The base of the skull

Cranial sutures

• Synarthroses (fibrous, strong, immobile bands between the bones of a fully grown adult)

• Developed after birth over the course of 2 years

• Flexible in infancy and early childhood

• Most important:

  • Coronal suture (between the two parietal bones and the posterior margin of the frontal bone)

  • Sagittal suture (median sagittal plane between the two parietal bones)

  • Lambdoid suture (between the superior margin of the occipital bone and the two parietal bones)

  • Squamous suture (lies between each temporal bone and the superior parietal bone)

Fontanelles

• Soft membranous gaps between cranial bones of the calvaria —→ present until age 2

• Function: To allow the rapidly growing brain to expand

• 6 fontanelles at the cranial joints

  • Posterior fontanel

    • At the lambdoid suture

  • Anterior fontanel

    • At the coronal suture

  • Posterolateral fontanel (mastoid)

  • Anterolateral fontanel (sphenoid)

• Craniosynostosis is a pathological process leading to complete ossification prematurely

Fontanelle closure

• Fontanelles also allow the bone plates to flex and thus the child’s head is able to pass through the birth canal

• The posterior fontanelle is the first to close

  • Happens around 2 to 3 months postpartum

• The sphenoid fontanelle is the second to close

  • Happens around 6 months after birth

• The mastoid fontanelle is the third to close

  • 6 to 18 months after birth

• The anterior fontanelle is the last to close

  • 12 to 18 months after birth

Neurocranium

• Composed of:

  • Calvaria, which is the membranous part of the roof of the skull formed by flat bones and slightly curved bones

  • Cranial base, the cartilaginous part

• Deliminated the cranial cavity

  • The cranial cavity houses the cerebral hemispheres, cerebellum, and brain stem

• The cranial cavity communicates below with the vertebral canal through foramen magnum

Calvaria

• Upper part of the neurocranium

• Composed of flat bones

  • Frontal bone (unpaired)

  • Occipital bone (unpaired)

  • Paired parietal bones

  • Paired temporal bones

• Smooth outer convex surface

• Inner concave surface

Calvaria bone structure

• Composed of flat bones

• Two layers of compact bone tissue

  • The inner layer is thinner than the outer.

  • The diploe separates the two compact layers (some areas lack diploe)

    • Diploe is a cancellous (spongy) bine tissue containing bone marrow and diploic veins

Frontal bone

• The forehead and superior rim of the orbit

• Pneumatized bone

  • Inside the bone there are two cavities filled with air, called the frontal sinuses

• Superior to each orbital rim

  • Supraciliary arch (more evident in men)

  • Supraorbital notch

    • The eponymous nerve and vessels pass through

• Glabella: A small depression between the two arches

• Partially forms the medial rim of the orbit

• Laterally, the frontal bone extends with the frontal process

  • Forms a joint with the zygomatic bone

Frontal bone articulation

• With the parietal bones, through the coronal suture

• With the greater wing of the sphenoid

• With the frontal process of the zygomatic bone, through the zygomatic process

• With the nasal and lacrimal bones

Parietal bone

• Paired quadrilateral bone

• Forms the roof and sides of the cranium

• Composed of:

  • 2 surfaces (Inner and outer)

  • 4 angles

  • 4 margin

• Margins

  • Frontal, with the coral suture

  • Occipital, with the lambdoid suture

  • Sagittal, with the eponymous suture

  • Squamous, with the identical suture

• Angles

  • Frontal

  • Sphenoid

  • Mastoid

  • Occipital

Occipital bone

• Trapezoidal curved bone

• Main bone of the occiput

• Overlies the occipital lobes

• 4 parts

  • Squamous

    • Smooth portion located centrally

  • Basilar

    • Participates in the formation of the cranial floor

  • Two lateral parts

• Contains a large opening through which the spinal cord passes: Foramen magnum

Inner occipital surface

• Base of the posterior cranial fossa

• Exposes the basilar portion of the bone

• Foramen magnum is bound

  • Anteriorly by the clivus

  • Posteriorly by the internal occipital crest

  • Laterally by the jugular tubercles and hypoglossal canals

• A transverse ridge and the occipital crest divide the surface into 4 depressions.

  • At their intersection: internal occipital protuberance

External occipital view

• Each internal element has a correspondent on the the outer surface

  • External occipital crest

  • External occipital protuberance

  • Hypoglossal canal

• Three transverse nuchal lines

  • Highest

  • Superior

  • Inferior

• Two occipital condyles on each side of the foramen magnum —→ articulates with the atlas

Temporal bone

• Paired flat bones located on the sides of the cranium

• Houses structures of the ear

• Pneumatized bone

• Composed of 4 parts

  • Squamous

  • Zygomatic process

  • Tympanic and styloid

  • Petromastoid: with two further subdivisions, petrous and mastoid

Temporal bone components

• Squamous

  • Flat plate —→ anterior and superior parts

  • Articulates with the greater wing of the sphenoid and the parietal bone

• Zygomatic

  • Anterior projection, curves anteriorly

  • Forms the zygomatic arch with the temporal process of the zygomatic bone

• Tympanic

  • Below the zygomatic process and the squamous bone

  • External acoustic opening with the external acoustic meatus

• Petromastoid

  • Mastoid—→ posterior part

  • Mastoid and styloid process

  • petrous—→ the base

Cranial base

• The floor of the cranial cavity

• Two surfaces

  • Endocranium (Inner surface of the cranial cavity)

  • External cranial base

• Some bones of the calvaria

• Components:

  • Sphenoid bone

  • Ethmoid bone

  • Occipital bone

  • Frontal bone

  • Temporal bone

Ethmoid Bone

• Small, unpaired bone, irregular shape

• Separates the nasal from the cranial cavity

• Pneumatized bone

• Superior projection—→ crista galli

• Sieve-like structure on each side of the ridge allows passage of small olfactory fibers to the olfactory bulb → cribriform plate

• Forms a small portion of the orbit

• Articulates with the frontal and with sphenoid bones

Sphenoid Bone

• Butterfly-shaped, unpaired bone

• Location:

  • Anterior to the basilar portion of the occipital bone

  • Middle part of the neurocranium

• Pneumatized bone: sphenoidal sinuses

• Composed of:

  • Body

  • Greater wing

  • Lesser wing

  • Two pterygoid process

Sphenoid bone

• One of the most complex bones of the neurocranium —> numerous bone articulations

• Participates in formation of:

  • Nasal cavity

  • Orbits

  • Middle portion of the endocranium

• The body contains a depression

  • Sella turcica —> home of the pituitary gland

  • The saddle

• Optic nerves pass through the optic canal

Viscerocranium

• The facial skeleton, located anterior to the neurocranium

• Supports the facial features

  • Nose bridge

  • Cheeks

  • Jawline

• Comprised of 14 bones

• All bones are joined together through synarthroses, except the mandible.

Zygomatic bone

• Irregular paired bone

• Forms the face’s lateral prominences

• Part of the orbit inferior and lateral rims

• Composed of:

  • 4 processes

  • 3 surfaces

  • 4 borders

Nasal bone

• Small, obling, paired bones

• Forms the bridge of the nose

• Articulates medially through the internasal suture (synarthrosis)

• Each side has two surfaces: Inner and outer.

• Articulates

  • Superiorly with the frontal bone

  • Inferiorly with the maxillae

Maxillae

• Large paired bones: the upper jaw

• Pneumatized bones —→ maxillary sinuses

• Hold the upper teeth

• Involved in formation of:

  • Orbit

  • Nasal cavity

  • Palate

• Composed of:

  • Body

  • Four processes: zygomatic, frontal, alveolar, and palatine

Palatine Bone

• Irregular paired bones

• Alongside the maxillae, forms the hard palate

• Location: between maxilla and the pterygoid process of the sphenoid

• Fuse together on the midline

• Composed of two perpendicular plates

  • Horizontal plate: the hard palate

  • Vertical plate: nasal cavity lateral wall

Mandible

• Large bone forming the chin and jawline

• Unpaired

• Not an actual part of the skull

• Articulates with the temporal bone through the temporo-mandibular joint

• Hold the lower teeth

• Composed of:

  • Ramus (coronoid and condylar processes)

  • Quadrilateral body

  • Alveolar process