The SKULL
By Firas Al-Hameed
Thi-Qar Medical School
Skull
• Supports the face and forms a protective cavity for the brain.
• Skull bones are formed by intramembranous ossification, and
joined by sutures (fibrous joints).
• Two groups: those of the cranium (which consist of the cranial
roof and cranial base) and those of the face.
• Cranial bones: 8 bones
• Facial bones: 14
CRANIUM
(NEUROCRANIUM)
• It encloses and protects the brain,
meninges and cerebral
vasculature.
• Subdivided into a roof (known as
the calvarium), and a base
• Calvarium
: Comprised of the
frontal, occipital and two parietal
bones.
• Cranial base
: Comprised of six
bones – the frontal, sphenoid,
ethmoid, occipital, parietal and
temporal bones
• Most bones of the calvaria consist of internal and
external tables or layers of compact bone, separated by
diploë.
• The
diploë
is cancellous bone containing red bone
marrow during life, through which run canals formed by
diploic veins.
• The diploë in a dried calvaria is not red because the
protein was removed during preparation of the cranium.
• The internal table of bone is thinner than the external
table
• In some areas there is only a thin plate of compact bone
with no diploë
• The inner surface of the skull-cap is concave
• Presents depressions for the convolutions of
the cerebrum, together with numerous
furrows for branches of the meningeal vessels.
• Along the middle line is a longitudinal groove
which lodges the superior sagittal sinus, and
its margins afford attachment to the falx
cerebri.
• On either side of it are several depressions for
the arachnoid granulations
The frontal bone
• Involved in three regions of the head, such are the forehead, the orbit and the nose.
• Squamous part (forehead).
• The largest area of the frontal bone.
• It comprises the frontal sinuses which are situated superior to the orbit on the
medial side and are separated by a septum.
• The supraorbital margin contains the supraorbital notch where the supraorbital
vessels and nerve pass through.
• Orbital part: forms the roof of the orbit and the ethmoidal sinuses.
• Nasal part It is connected to the frontal processes of the maxilla and the nasal bones,
forming the stem of the nose.
• The
frontal suture
is a fibrous joint that divides the two halves of the frontal bone of
the skull in infants and children. Typically, it completely fuses between three and nine
months of age, with the two halves of the frontal bone being fused together. It is also
called the metopic suture
The parietal bones
• Sagittal border
• located superiorly.
• thickest and longest out of all of them.
• The two parietal bones articulate at the sagittal borders with each other to form the
sagittal suture.
• Squamosal border
• Situated inferiorly.
• The squamosal border comes in contact with three bony structures. From anterior to
posterior these are the greater wing of the sphenoid bone and the squamous and
petrous parts of temporal bone. These articulations form the sphenoparietal and
parietomastoid sutures.
• Frontal border
• located anteriorly.
• It is the most serrated margin of the parietal bone.
• It comes in contact with the frontal bone to form the superolateral half of the coronal
suture.
• Occipital border
• located posteriorly.
• forms the inferolateral half of the lambdoid suture by articulating with the occipital
bone.
Surfaces
• The external surface is smooth and convex. It
has several important features:
• Superior temporal line
• an arch that travels between the frontal and
occipital borders of the parietal bone.
• It represents the attachment point of the temporal
fascia.
• Inferior temporal line
• an identical arch to the previous one but located
more inferiorly.
• It represents the origin of the temporal muscle.
The internal surface
• It contains several grooves that house various blood vessels;
• A thin and highly branched groove for middle meningeal artery.
• A broader groove for superior sagittal sinus.
• A small portion of the groove for sigmoid sinus.
• The parietal foramen is located on the posterosuperior aspect of the
parietal bone. Transmits branches of the superior sagittal sinus and
occipital artery.
The Sphenoid Bone
• Means wedge-shaped (butterfly-shaped)
• It consists of a body, paired greater wings and lesser
wings, and two pterygoid processes.
Body
• Contains the sphenoidal sinuses
• The body articulates with the ethmoid bone
anteriorly
• The superior surface of the sphenoid body contains
some important bony landmarks:
• Sella turcica – a saddle-shaped depression. It has
three parts:
• Tuberculum sellae – forms the anterior wall of the sella
turcica, and the posterior aspect of the chiasmatic
groove.
• Hypophyseal fossa – the deepest part of the sella
turcica, where the
pituitary gland
is located.
• Dorsum sellae – forms the posterior wall of the sella
turcica.
• Chiasmatic groove – a sulcus formed by the optic
chiasm (where the optic nerves partially cross).
• Surrounded by the anterior and posterior clinoid
processes. They serve as attachment points for the
tentorium cerebelli, a membranous sheet that
divides the brain.
Greater Wing
• It contributes to three parts of the facial skeleton:
• Floor of the middle cranial fossa
• Lateral wall of the skull
• Posterolateral wall of the orbit
• There are three foramina present in the greater wing – the
foramen rotundum, foramen ovale and foramen spinosum.
Lesser Wing
• It separates the anterior cranial fossa from the middle cranial
fossa.
• There is a ‘slit-like’ gap between the lesser and greater wings of
the sphenoid – the superior orbital fissure.
Pterygoid Process
• descends inferiorly from the point of junction between the
sphenoid body and the greater wing. It consists of two parts:
• Medial pterygoid.
• Lateral pterygoid plate – site of origin of the medial and lateral
pterygoid muscles
Ethmoid bone
• Made up of three parts
• Cribriform plate, the perpendicular plate, and the ethmoidal
labyrinth.
• The cribriform plate
• Forms the roof of the nasal cavity.
• Pierced by numerous olfactory nerve fibres, which innervate
the nasal cavity with the sense of smell.
• Crista galli: Projects superiorly from the cribriform plate
which provides an attachment point for the falx cerebri
(sheet of dura mater that separates the two cerebral
hemispheres).
• Perpendicular plate: descends from the cribriform
plate. It forms the superior two-thirds of the nasal
septum.
• The ethmoid bone contains two ethmoidal labyrinths
which contain the ethmoidal air cells (sinuses).
Temporal bone
• Contributes to the lower lateral walls of the
skull.
• It contains the middle and inner portions of
the ear
• Crossed by the majority of the cranial
nerves.
• The lower portion of the bone articulates
with the mandible, forming the
temporomandibular joint of the jaw.
• Five constituent parts: The squamous,
tympanic and petromastoid parts, with the
zygomatic and styloid processes projecting
outwards
Petromastoid part
• Can be split into a mastoid and petrous
parts.
• Mastoid part:
• mastoid process, an inferior projection of
bone, palpable just behind the ear
• mastoid air cells. These are hollowed out
areas within the temporal bone. They act as
a reservoir of air, equalising the pressure
within the middle ear
• The petrous part is pyramidal shaped,
and lies at the base of temporal bone. It
contains the inner ear.
Attached muscles
Muscle
Site of Attachment
Description
Temporalis
Originates from the lower part of
squamous
Muscle of mastication
Masseter
Lateral zygomatic surface
Muscle of mastication
Sternocleidomastoid
Mastoid process
Superficial muscle of the neck. Involved in rotation of head
and flexion of neck. Important landmark for
the anterior and posterior cervical triangles.
Posterior belly of digastric
Mastoid process
A suprahyoid muscle. Involved in processes such as
swallowing.
Splenius capitis
Mastoid process
Strap-like muscle in the back of the neck. Involved in
movements such as shaking the head.
The pterion
• ‘H-shaped’ junction between
temporal, parietal, frontal and
sphenoid bones.
• The thinnest part of the skull.
• A fracture here can lacerate the
middle meningeal artery
(anterior branch), resulting in
an extradural haematoma.
Sutures of the Skull
• Unique to the skull. Immovable junctions between
bones, and fuse completely around the age of 20.
• Clinical importance: they can be points of potential
weakness in both childhood and adulthood.
• The main sutures in adulthood are:
• Coronal
suture which fuses the frontal bone with the
two parietal bones.
• Sagittal
suture which fuses both parietal bones to each
other.
• Lambdoid
suture which fuses the occipital bone to the
two parietal bones.
• In neonates, the incompletely fused suture joints
give rise to membranous gaps between the bones,
known as
fontanelles
.
• Frontal fontanelle (located at the junction of the coronal
and sagittal sutures)
• Occipital fontanelle (located at the junction of the
sagittal and lambdoid sutures).