ANATOMY OF THE SKULL

DMR YEAR 1 SEM 2

Cranium

• The cranium (skull) is the skeleton of the head.

• A series of bones form its two parts;
1. The neurocranium
2. Viscerocranium.
Neurocranium
• The neurocranium (cranial vault) is the bony covering (case) of the
brain and its membranous coverings, the cranial meninges. It also
contains proximal parts of the cranial nerves and the vasculature of
the brain.
• The neurocranium has a dome-like roof, the calvaria (skullcap), and a
floor or cranial base (basicranium).
• The neurocranium in adults is formed by a series of eight bones:
i. four singular bones centered on the midline (frontal, ethmoidal,
sphenoidal, and occipital)
ii. two sets of bones occurring as bilateral pairs (temporal and parietal).
The skull
Anatomy of the skull
Anatomy of the skull
Shape of cranial bones
• The bones forming the calvaria are primarily flat bones (frontal,
temporal, and parietal) formed by intramembranous ossification of
head mesenchyme from the neural crest.
• The bones contributing to the cranial base are primarily irregular
bones with substantial flat portions (sphenoidal and temporal)
formed by endochondral ossification of cartilage (chondrocranium) or
from more than one type of ossification.
• The ethmoid bone is an irregular bone that makes a relatively minor
midline contribution to the neurocranium but is primarily part of the
viscerocranium.
 Shape of cranial bones
• The flat bones and flat portions of the bones forming the
neurocranium are curved; with convex external and concave internal
surfaces.

Joints of the skull

• Most calvarial bones are united by fibrous interlocking sutures

• During childhood, some bones (sphenoid and occipital) are united by

hyaline cartilage (synchondroses).
 Features on the cranium
• The foramen magnum; a large opening in the cranial base where spinal cord is
continuous with the brain.
• The orbitomeatal or Frankfort horizontal plane; a standard craniometric
reference where the inferior margin of the orbit and the superior margin of the
external acoustic meatus lie in the same horizontal .
• The temporal fossa ; is the region of the lateral aspect of the cranium superior
to the zygomatic arch and inferior to the temporal lines.
• The supraorbital notch, the infraorbital foramen, and the mental foramen are
approximately in a vertical line
 Pneumatized cranial bones
• Several bones of the cranium are pneumatized bones. They include;
1. Frontal
2. Maxilla
3. Sphenoid
4. Ethmoid
• They contain air spaces (air cells or larger sinuses), presumably to
decrease bone weight.
• The total volume of the air spaces in these bones increases with age.
• They are lined by a mucosal layer which when inflamed leads to a
condition called sinusitis
The viscerocranium
• The viscerocranium (facial skeleton) comprises the
facial bones that forms the anterior part of the
cranium and consists of the;
1. bones surrounding the mouth (upper and lower
jaws)
2. nose/nasal cavity
3. most of the orbits (eye sockets or orbital cavities).
Viscerocranium
• It consists of 15 irregular bones:
a) 3 singular bones centered on or lying in the midline
(mandible, ethmoid, and vomer)
b) 6 bones occurring as bilateral pairs (maxillae; inferior
nasal conchae; and zygomatic, palatine, nasal, and
lacrimal bones).
Viscerocranium
• The maxillae and mandible house the teeth that is, they provide the
sockets and supporting bone for the maxillary and mandibular teeth.
• The maxillae contribute the greatest part of the upper facial skeleton,
forming the skeleton of the upper jaw, which is fixed to the cranial
base.
• The mandible forms the skeleton of the lower jaw, which is movable
because it articulates with the cranial base at the temporomandibular
joints (TMJs).
Facial Aspect of the Cranium

• Features of the anterior or facial (frontal) aspect of the cranium are

the;
1. frontal bone
2. zygomatic bones,
3. orbits,
4. nasal region,
5. maxillae
6. mandible.
• The frontal bone, specifically its squamous (flat) part, forms the
skeleton of the forehead, articulating inferiorly with the nasal and
zygomatic bones.
Facial Aspect of the Cranium
• The glabella; a smooth slightly depressed area between the
superciliary arches.
• The metopic suture; remnant of the frontal suture, is visible in the
midline of the glabella in some adults

• The frontal suture divides the frontal bones of the fetal cranium.
• The intersection of the frontal and nasal bones is the nasion, which in
most people is related to a distinctly depressed area (bridge of the
nose).
Palpable bony structures
The skull
 The frontal bone
• The frontal bone also articulates with the lacrimal, ethmoid, and
sphenoids, and a horizontal portion of bone (the orbital part) forms both
the roof of the orbit and part of the floor of the anterior part of the
cranial cavity.
• The supraorbital margin of the frontal bone, the angular boundary
between the squamous and the orbital parts, has a supraorbital foramen
or notch in some crania for passage of the supraorbital nerve and vessels.
• Just superior to the supraorbital margin is a ridge, the superciliary arch,
that extends laterally on each side from the glabella. The prominence of
this ridge, deep to the eyebrows, is generally greater in males. Within the
orbits are the superior and inferior orbital fissures and optic canals.
 The zygomatic bones
• The zygomatic bones (cheek bones, malar bones), forming the prominences

of the cheeks, lie on the inferolateral sides of the orbits and rest on the

maxillae.

• A small zygomaticofacial foramen pierces the lateral aspect of each bone.

• The zygomatic bones articulate with the frontal, sphenoid, and temporal

bones and the maxillae.

 Nasal bone
• Inferior to the nasal bones is the pear-shaped piriform aperture, the anterior

nasal opening in the cranium.

• The bony nasal septum can be observed through this aperture, dividing the

nasal cavity into right and left parts.

• On the lateral wall of each nasal cavity are curved bony plates, the nasal

conchae.
The maxillae
• Form the upper jaw; their alveolar processes include the tooth
sockets (alveoli) and constitute the supporting bone for the maxillary
teeth.
• The maxillae surround most of the piriform aperture and form the
infraorbital margins medially.
• They have a broad connection with the zygomatic bones laterally and
an infraorbital foramen inferior to each orbit for passage of the
infraorbital nerve and vessels.
• The two maxillae are united at the intermaxillary suture in the median
plane.
The Maxillae
 The mandible
• The mandible is a U-shaped bone with an alveolar process that supports the

mandibular teeth. It consists of a horizontal part, the body, and a vertical part,

the ramus.

• Inferior to the second premolar teeth are the mental foramina for the mental

nerves and vessels. The mental protuberance, forming the prominence of the

chin, is a triangular bony elevation inferior to the mandibular symphysis , the

osseous union where the halves of the infantile mandible fuse.

Mandible
Lateral Aspect of the Cranium

• The lateral aspect of the cranium is formed by both the neurocranium

and the viscerocranium.
• The main features of the neurocranial part include the temporal
fossa, the external acoustic opening, and the mastoid process of the
temporal bone.
• The main features of the viscerocranial part include the infratemporal
fossa, zygomatic arch, and lateral aspects of the maxilla and
mandible.
The temporal fossa
• The temporal fossa is bounded superiorly and posteriorly by the superior and

inferior temporal lines, anteriorly by the frontal and zygomatic bones, and

inferiorly by the zygomatic arch.

• The superior border of this arch corresponds to the inferior limit of the cerebral

hemisphere of the brain.

• The zygomatic arch is formed by the union of the temporal process of the

zygomatic bone and the zygomatic process of the temporal bone.

 Landmark, shape and location
Pterion Junction of the greater wing of the sphenoid, squamous
temporal, frontal, and parietal bones; overlies course of
anterior division of middle meningeal artery. It is a weak point
of the skull, injury at the pterion can damage the middle
meningeal artery and lead to intracranial hemorrhage

Lambda Point on calvaria at junction of lambdoid and sagittal sutures

Bregma Point on calvaria at junction of coronal and sagittal sutures

Vertex Superior point of neurocranium, in the middle with the cranium

oriented in anatomical (orbitomeatal or Frankfort) plane

Asterion Star shaped; located at junction of three sutures:

parietomastoid, occipitomastoid, and lambdoid
Normal skull of the newborn
• In the anterior part of the temporal fossa, superior to the midpoint of
the zygomatic arch, is a clinically important area of bone junctions:
the pterion.
• It is usually indicated by an H-shaped formation of sutures that unite
the frontal, parietal, sphenoid (greater wing), and temporal bones.
Less commonly, the frontal and temporal bones articulate; sometimes
all four bones meet at a point.
• The external acoustic opening; is the entrance to the external
acoustic meatus (canal), which leads to the tympanic membrane
(eardrum).
 The temporal bone
• The mastoid process of the temporal bone is posteroinferior to the external

acoustic pore. Anteromedial to the mastoid process is the styloid process of

the temporal bone, a slender needle-like, pointed projection.

• The infratemporal fossa is an irregular space inferior and deep to the

zygomatic arch and the mandible and posterior to the maxilla.

Parts of the temporal bone
The temporal bone

• The groove for the cartilaginous part of the pharyngotympanic (auditory)

tube lies medial to the spine of the sphenoid, inferior to the junction of the
greater wing of the sphenoid and the petrous part of the temporal bone.

• Depressions in the squamous part of the temporal bone, called the

mandibular fossae, accommodate the mandibular condyles when the
mouth is closed. The cranial base is formed posteriorly by the occipital
bone, which articulates with the sphenoid anteriorly.
 Occipital Aspect of the Cranium
• The posterior or occipital aspect of the cranium is composed of the
occiput (L. back of head, the convex posterior protuberance of the
occipital bone), parts of the parietal bones, and mastoid parts of the
temporal bones.
• The external occipital protuberance, is usually easily palpable in the
median plane; however, occasionally (especially in females) it may be
inconspicuous.
• A craniometric point (a point used in making cranial measurements
and comparisons) defined by the tip of the external protuberance is the
inion
Occipital Aspect of the Cranium
Occipital Aspect of the Cranium

• The external occipital crest descends from the protuberance toward

the foramen magnum, the large opening in the basal part of the
occipital bone.
• The superior nuchal line, marking the superior limit of the neck,
extends laterally from each side of the protuberance; the inferior
nuchal line is less distinct. In the center of the occiput, the lambda
indicates the junction of the sagittal and lambdoid sutures.
• The lambda can sometimes be felt as a depression. One or more
sutural bones (accessory bones) may be located at the lambda or
near the mastoid process.
 Superior Aspect of the Cranium
• The superior (vertical) aspect of the cranium (L. norma superioris or
verticalis), usually somewhat oval in form, broadens posterolaterally at
the parietal eminences.
• In some people, frontal eminences are also visible, giving the calvaria
an almost square appearance.
• The coronal suture separates the frontal and parietal bones, the
sagittal suture separates the parietal bones, and the lambdoid suture
separates the parietal and temporal bones from the occipital bone.
• The bregma is the landmark formed by the intersection of the sagittal
and coronal sutures. The vertex, the most superior point of the
calvaria, is near the midpoint of the sagittal suture.
 Superior Aspect of the Cranium
• The parietal foramen is a small, inconstant aperture located posteriorly in

the parietal bone near the sagittal suture; paired parietal foramina may be

present.

• Most irregular, highly variable foramina that occur in the neurocranium are

emissary foramina that transmit emissary veins, veins connecting scalp veins

to the venous sinuses of the dura mater.

 External Surface of the Cranial Base
• The cranial base (basicranium) is the inferior portion of the
neurocranium (floor of the cranial cavity) and viscerocranium minus the
mandible.
• The external surface of the cranial base features the alveolar arch of the
maxillae (the free border of the alveolar processes surrounding and
supporting the maxillary teeth); the palatine processes of the maxillae;
and the palatine, sphenoid, vomer, temporal, and occipital bones.
• The hard palate (bony palate) is formed by the palatine processes of the
maxillae anteriorly and the horizontal plates of the palatine bones
posteriorly.
The sphenoid bone
• Wedged between the frontal, temporal, and occipital bones is the sphenoid,
an irregular unpaired bone that consists of a body and three pairs of
processes: greater wings, lesser wings, and pterygoid processes.

• The greater and lesser wings of the sphenoid spread laterally from the
lateral aspects of the body of the bone.

• The pterygoid processes, consisting of lateral and medial pterygoid plates,

extend inferiorly on each side of the sphenoid from the junction of the body
and greater wings.
 The occipital bone
• The four parts of the occipital bone are arranged around the foramen
magnum, the most conspicuous feature of the cranial base.
• The major structures passing through this large foramen are the spinal
cord (where it becomes continuous with the medulla oblongata of the
brain), the meninges (coverings) of the brain and spinal cord, the
vertebral arteries, the anterior and posterior spinal arteries, and the
accessory nerve (CN XI).
• On the lateral parts of the occipital bone are two large protuberances,
the occipital condyles, by which the cranium articulates with the
vertebral column.
• The large opening between the occipital bone and the petrous part of
the temporal bone is the jugular foramen, from which the internal
jugular vein (IJV) and several cranial nerves emerge from the cranium.
• Superolateral to the jugular foramen is the internal acoustic meatus, a
passage for CN VII and CN VIII. The entrance to the carotid canal for
the internal carotid artery is just anterior to the jugular foramen.
• The mastoid processes provide for muscle attachments. The
stylomastoid foramen, transmitting the facial nerve (CN VII) and
stylomastoid artery, lies posterior to the base of the styloid process.
 Internal Surface of the Cranial Base
• The internal surface of the cranial base has three large depressions that
lie at different levels: the anterior, middle, and posterior cranial fossae ,
which form the bowl-shaped floor of the cranial cavity.
• Anterior cranial fossa is at the highest level, and the posterior cranial
fossa is at the lowest level.
Anterior Cranial Fossa
• The inferior and anterior parts of the frontal lobes of the brain occupy
the anterior cranial fossa, the shallowest of the three cranial fossae.
• The fossa is formed by the frontal bone anteriorly, the ethmoid bone in
the middle, and the body and lesser wings of the sphenoid posteriorly.
Anterior Cranial Fossa

• The greater part of the fossa is formed by the orbital parts of the frontal
bone, which support the frontal lobes of the brain and form the roofs of
the orbits. This surface shows sinuous impressions (brain markings) of the
orbital gyri (ridges) of the frontal lobes.
• The frontal crest is a median bony extension of the frontal bone. At its base
is the foramen cecum of the frontal bone, which gives passage to vessels
during fetal development but is insignificant postnatally.
• The crista galli is a thick, median ridge of bone posterior to the foramen
cecum, which projects superiorly from the ethmoid. On each side of this
ridge is the sieve-like cribriform plate of the ethmoid.
• Its numerous tiny foramina transmit the olfactory nerves (CN I) from the
olfactory areas of the nasal cavities to the olfactory bulbs of the brain,
which lie on this plate.
Internal Surface of the Cranial Base
Middle Cranial Fossa
• The bones forming the lateral parts of the fossa are the greater wings of
the sphenoid and squamous parts of the temporal bones laterally and the
petrous parts of the temporal bones posteriorly.
• The lateral parts of the middle cranial fossa support the temporal lobes of
the brain. The boundary between the middle and the posterior cranial
fossae is the superior border of the petrous part of the temporal bone
laterally and a flat plate of bone, the dorsum sellae of the sphenoid,
medially.
• The sella turcica is the saddle-like bony formation on the upper surface of
the body of the sphenoid, which is surrounded by the anterior and
posterior clinoid processes. Clinoid means bedpost, the four processes
(two anterior and two posterior) surround the hypophysial fossa, the bed
of the pituitary gland, like the posts of a four-poster bed.
Middle Cranial Fossa
 Foramina in the middle cranial fossae
• On each side of the body of the sphenoid a crescent of four foramina
perforate the roots of the greater wings of the sphenoids:
1. Superior orbital fissure: Located between the greater and lesser wings, it
communicates with the orbit and transmits the ophthalmic veins and
nerves (CN III, CN IV, CN V1, CN VI, and sympathetic fibers) entering the
orbit.
2. Foramen rotundum (round foramen): Located posterior to the medial
end of the superior orbital fissure, it transmits the maxillary nerve (CN V2)
that supplies the skin, teeth, and mucosa related to the maxilla (i.e., lining
the upper jaw and maxillary sinus).
3. Foramen ovale (oval foramen): A large foramen posterolateral to the
foramen rotundum, it opens inferiorly into the infratemporal fossa and
transmits the mandibular nerve (CN V3) and a small accessory meningeal
artery.
Foramina in the middle cranial fossae
• Foramen spinosum (spinous foramen): Located posterolateral to the
foramen ovale, it transmits the middle meningeal vessels and the
meningeal branch of the mandibular nerve.
• The foramen lacerum (lacerated or torn foramen) is not part of the
crescent of foramina. This ragged foramen lies posterolateral to the
hypophysial fossa and is an artifact of a dried cranium.
• In life, it is closed by a cartilage plate. Only some meningeal arterial
branches and small veins are transmitted vertically through the
cartilage, completely traversing this foramen.
The sella turcica is composed of three parts;

1. The tuberculum sellae (horn of the saddle): a variable slight to

prominent median elevation forming the posterior boundary of the
prechiasmatic sulcus and the anterior boundary of the hypophysial
fossa.
2. The hypophysial fossa (pituitary fossa): a median depression (seat
of the saddle) in the body of the sphenoid that accommodates the
pituitary gland.
3. The dorsum sellae (back of the saddle): a square plate of bone
projecting superiorly from the body of the sphenoid. It forms the
posterior boundary of the sella turcica, and its prominent
superolateral angles make up the posterior clinoid processes.
Sella turcica
Sella turcica
Posterior Cranial Fossa

• The posterior cranial fossa, the largest and deepest of the three cranial
fossae, lodges the cerebellum, pons, and medulla oblongata.
• The posterior cranial fossa is formed mostly by the occipital bone, but the
dorsum sellae of the sphenoid marks its anterior boundary centrally and
the petrous and mastoid parts of the temporal bones contribute its
anterolateral walls.
• From the dorsum sellae there is a marked incline, the clivus, in the center
of the anterior part of the fossa leading to the foramen magnum.
• Posterior to this large opening, the posterior cranial fossa is partly divided
by the internal occipital crest into bilateral large concave impressions, the
cerebellar fossae.
 Posterior Cranial Fossa
• At the base of the petrous ridge of the temporal bone is the jugular
foramen, which transmits several cranial nerves in addition to the sigmoid
sinus that exits the cranium as the IJV.

• Anterosuperior to the jugular foramen is the internal acoustic meatus for the
facial and vestibulocochlear nerves (CN VIII) and the labyrinthine artery. The
hypoglossal canal for the hypoglossal nerve (CN XII) is superior to the
anterolateral margin of the foramen magnum.
Posterior cranial fossa
THE END