precise and informative anatomy of cranial bones

1. ANATOMY OF THE CRANIAL BONES.
The skull consists of 8 cranial bones and 14 facial bones.
The eight bones of the cranium articulate firmly with one
another to enclose and protect the brain and sensory organs.
The 14 facial bones form the framework for the facial region
and support the teeth.The facial bones,with the exception of
the mandible (“jawbone”),are also firmly interlocked with one
another and the cranial bones.The skull has several cavities.
The cranial cavity is the largest. The nasal cavity is formed by
both cranial and facial bones and is partitioned into two
chambers, or nasal fossae, by a nasal septum of bone and
cartilage. Four sets of paranasal sinuses, located within the
bones surrounding the nasal area, communicate via ducts into
the nasal cavity. Middle- and inner-ear cavities are positioned
inferior to the cranial cavity and house the organs of
hearing and balance. The two orbits for the eyeballs are
formed by facial and cranial bones. The oral, or buccal
cavity (mouth), which is only partially formed by bone, is
completely within the facial region.
During fetal development and infancy, the bones of
the cranium are separated by fibrous unions. There are also
six large areas of connective tissue membrane that cover the
gaps between the developing bones. These membranous
sheets are called fontanelles, meaning “little fountains.” The
name derives from the fact that a baby’s pulse can be felt
surging in these “soft spots” on the skull. The fontanelles
permit the skull to undergo changes in shape, called molding,
during parturition (childbirth), and they accommodate the
rapid growth of the brain during infancy. Ossification of the
fontanelles is normally complete by 20 to 24 months of age.
The following fontanelles are present:

2. 1. Anterior (frontal) fontanelle,fonticulus anterior. The
anterior fontanel is diamond-shaped and is the most
prominent. It is located
on the anteromedian portion of the skull.
2. Posterior (occipital) fontanelle,fonticulus posterior. The
posterior fontanel is positioned at the back of the skull on the
median line. It is also diamond-shaped, but smaller than the
anterior
fontanelle.
3. Anterolateral (sphenoid) fontanelles,fonticulus
sphenoidalis. The paired anterolateral fontanels are found on
both sides of the skull, directly lateral to the anterior
fontanelle. They are relatively small and irregularly shaped.
4. Posterolateral (mastoid) fontanelles,fonticulus
mastoideus. The paired posterolateral fontanelles, also
irregularly shaped, are located on the posterolateral sides of
the skull.
A prominent sagittal suture extends the
anteroposterior median length of the skull between the
anterior and posterior fontanelles. A coronal
suture extends from the anterior fontanel to the

3. anterolateral fontanel. A lambdoid suture extends
from the posterior fontanel to the posterolateral fontanelle.
A squamous suture connects the posterolateral
fontanelle to the anterolateral fontanelle.
Clinical application:During normal childbirth, the fetal skull
comes under tremendous pressure. Bones may even
shift, altering the shape of the skull. A
common occurrence during molding of the
fetal skull is for the occipital bone to be
repositioned under the two parietal bones. In
addition, one parietal bone may shift so as to
overlap the other. This makes delivery easier
for the mother. If a baby is born breech
(buttocks first), these shifts do not occur.
Delivery becomes much more difficult, often
requiring the use of forceps.
CRANIAL BONES
The cranial bones enclose and protect the brain and
associated sensory organs. They consist of one frontal, two
parietals, two temporals, one occipital, one sphenoid, and one
ethmoid.
THE FRONTAL BONE
The frontal bone forms the anterior roof of the cranium,the
forehead,the roof of the nasal cavity and the superior arches of
the orbits,which contain the eyeballs. The frontal bone
develops in two halves that grow together.

4. Generally,they are completely fused by age 5 or 6. A suture
sometimes persists between these two portions and is
referred to as a metopic suture.This makes them unpaired.
The frontal bone has three parts:
1.Squamous part,squama frontalis:a vertical convex part of
the bone,which consists of
the external,internal and temporal surfaces. The squamous
part bears the following structures:
 Frontal tuber,tuber frontale: a paired,most prominent
elevation of the squamous part.
 Superciliary arch,arcus: a paired, prominent, rounded
arch situated below and medial to the frontal tuber.
 Glabella,glabella: an unpaired area between the
superciliary arches and above the base of the nose.
 Supra-orbital margin,margo supraorbitalis: a
paired,prominent area on the border with the orbital part.
 Supra-orbital notch,incisura supraorbitalis:resides
medially on the supra-orbital margin .
 The frontal notch,incisura frontalis:a shallow notch that
resides medially from the supra-orbital notch.
 Zygomatic process,processus zygomaticus: a directed
downward,lateral portion of the supra-orbital margin.
 Temporal line,linea temporalis: arches upward and
backward from the zygomatic process.
 Frontal crest,crista frontalis: unpaired, located in the
middle of the of the inferior portion of the bone.
2.Orbital part,pars orbitalis:o horizontal part,which contains
the orbital and cerebral surfaces.It bears the following parts:

5.  Fossa for lacrimal gland,fossa glandulae
lacrimalis:resides near the zygomatic process in the
upper,lateral angle of the orbit.
 Trochlear fovea,fovea trochlearis:a small depression
located medially.
 Trochlear spine,spina trochlearis:lies near the trochlear
fovea.
 Cerebral yokes,juga cerebralia:also reside on the
cerebral surface between the impressions for cerebral
gyri.
3.Nasal part,pars nasalis:an unpaired part located in the
middle between two orbital parts surrounding the ethmoidal
notch.It also has the following parts:
 Nasal spine,spina nasalis which lies in the middle.
 opening of the frontal sinus,apertura sinus frontalis: a
paired orifice leading into the frontal sinus.
 Ethmoidal notch,incisura ethmoidalis resides in the
middle and separates orbital parts.
 Frontal sinus,sinus frontalis: a sizeable air cavity within
the lower portion of the squamous part.The frontal sinus
is separated by a bony septum into two cavities, which
communicate with the nasal cavity.
THE PARIETAL BONE
The two parietal bones form the upper sides and roof of the
cranium
.The coronal suture separates the
frontal bone from the parietal bones, and the sagittal
suture along the superior midline separates the right and left
parietals from each other.
Borders of parietal bone

6.  sagittal border,margo sagittalis: the upper margin of the
bone,which articulates with the opposite parietal bone to
form the sagittal suture.
 Frontal border,margo frontalis: a directed forward
border,which articulates with frontal bone to form the
coronal suture.
 Occipital border,margo occipitalis: a directed backward
border, which articulates with the occipital bone forming
the lambdoid suture.
 Squamous border,margo squamosus: the
fourth,oblique,directed downward border of the parietal
bone. It overlaps with the squamous part of the temporal
bone.
Angles of the parietal bone
 Frontal angle,angulus frontalis the anterior,upper,straight
angle,which articulates with the frontal bone.
 Sphenoidal angle,angulus sphenoidalis the
anterior,lower,sharper angle,which articulates with the
greater wing of the sphenoid bone.
 Occipital angle,angulus occipitalis the
posterior,lower,rounded angle,which articulates with the
occipital bone.
 Mastoid angle,angulus mastoideus: the
posterior,upper,blunt angle,which articulates with the
mastoid process of the temporal bone.

7. TEMPORAL BONE
The two temporal bones form the lower sides of the cranium.
Each temporal bone is joined to its adjacent
parietal bone by the squamous suture.
Structurally, each temporal bone has four
parts.
1. Squamous part,pars squamosa. The squamous part is the
flattened plate of bone at the sides of the skull.
Projecting forward is a zygomatic process that
forms the posterior portion of the zygomatic
arch. On the inferior surface of the squamous
part is the cuplike mandibular fossa,
which forms a joint with the condyle of the
mandible. This articulation is the
temporomandibular joint.

8. 2.Tympanic part,pars tympanica. The tympanic part of the
temporal bone contains the external acoustic
meatus, or ear canal, which is posterior to the
mandibular fossa. A thin, pointed styloid
process projects inferiorly from the tympanic
part.External acoustic opening which resides
in the centre where the parts of the temporal
bone articulate.A tympanosquamous fissure
separates the tympanic part from the
squamous part.It is split into two parts by the
plate of the petrous part:petrosquamous fissure
and petrotympanic fissure.The
tympanomastoid fissure separates the
tympanic part from the mastoid process.
The Tympanic Cavity,cavitas
tympani
The tympanic cavity is the air filled cavity of
irregular shape situated within the petrous part
of the temporal bone.It is invested with the
mucous membrane.
walls of the tympanic cavity:The tympanic
cavity has six walls,namely;
 The tegmental wall,paries tegmentalis is the wall
related tot he tegmen tympani.

9.  The jugular wall,paries jugularies related to the jugular
fossa.
 The labyrinthine wall,paries labyrinthicum is the
medial wall related to the bony labyrinth.
 The membranous wall,paries membranaceus is a small
lateral wall scaled with the tympanic membrane.
 The carotid wall,paries caroticus is the wall related to
the carotid canal.The tympanic opening of the auditory
tube and the opening of the caroticotympanic caniculi
can be distinguished on this wall.
 The mastoid wall,paries mastoideus is the posterior wall
related to the mastoid process of the temporal bone.
3. Mastoid part,pars mastoidea. The mastoid process, a
rounded projection posterior to the external acoustic meatus,
accounts for the mass of the mastoid part. The mastoid
foramen is directly posterior to the mastoid process. The
stylomastoid foramen, located between the mastoid and
styloid processes , provides the passage for part of the facial
nerve.

10. 4. Petrous part(pyramid),pars tetrosa. The petrous part can
be seen in the floor of the cranium. The structures of the
middle ear and inner ear are housed in this dense part of the
temporal bone. The carotid canal and the jugular foramen
border on the medial side of the petrous part at the junction of
the temporal and occipital bones. The carotid canal allows
blood into the brain via the internal carotid artery, and the
jugular foramen lets blood drain from the brain via the
internal jugular vein. Three cranial nerves also pass through
the jugular foramen.The following surfaces are distinguished
in the pyramid:
 Anterior surface,facies anterior: faces forward,upward
and laterally.
 Posterior surface,facies posterior:faces backward and
medially.
 Inferior surface,facies inferior:faces downward towards
the area of the external surface of the cranial base.
The three surfaces of the petrous part are bounded by three
borders:
 Anterior border of the petrous part,margo anterior partis
pertrosae:the shortest border,which connects the squama
with the apex of the pyramid.
 Superior border of the petrous part,margo superior patris
pertrosae:saparates the anterior and posterior surfaces of
the pyramid.
 Posterior border of the petrous part,margo posterior
patris petrosae:separates the posterior and inferior
surfaces of the pyramid.
Canals of the temporal bone

11.  Facial canal,canalis nervi facialis:It transmits the facial
nerve.The facial canal begins at the bottom of the
internal acoustic meatus and ends in the stylomastoiss
foramen.
 Canal for chorda tympani,canaliculis chordae
tympani: It gives passage to a thin nerve(chorda
tympani).
 Carotid canal,canalis caroticus: It transmits the internal
carotid artery.The carotid canal begins on the inferior
surface of the pyramid.
 Caroticotympanic canal,canaliculus
caroticotympanici:begin on the posterior of the carotid
canal and enter the tympanic cavity penetrating its
anterior wall.Its gives passage to the corresponding
nerves and vessels.
 Musculotubal canal,canalis musculotubarius:It exits the
tympanic cavity.The musculotubal canal resides in the
angle between the anterior margin of the pyramid and the
squamous part of the temporal bone. A horizontal septum
divides the musculotubal canal into two:
 Canal for tensor tympani,semicanalis m. tensor
tympani,situated above.
 Canal for auditory tube,semicanalis tubae
auditivae,resides below the previous canal and transmits
the auditory(Eustachian) tube,which establishes the
communication between the tympanic cavity and the
nasal part of the pharynx.
 Tympanic canal,canaliculis tympanicus:begins on the
inferior surface of the pyramid in the petrous fossula and
runs vertically upward penetrating the inferior wall of the
tympanic cavity.It houses the tympanic nerve.
 Mastoid canal,canaliculis mastoideus:begins at the
bottom of the jugular fossa,crosses the facial canal in its

12. lower portion and opens into the tympanomastoid
fissure.It gives passage to the auricular branch of the
vagus nerve.
Clinical application:The mastoid process of the temporal
bone can be easily palpated as a bony knob immediately
behind the earlobe. This process contains a number of small
air-filled spaces called mastoid cells that can become infected
in mastoiditis, as a result, for example,of a prolonged middle-
ear infection.
OCCIPITAL BONE
The occipital bone forms the posterior and most of the base of
the skull. It articulates with the parietal bones at the lambdoid
suture. The foramen magnum is the large hole in the occipital
bone through which the spinal cord passes to attach to the
brain stem. On each side of the foramen magnum are the
occipital condyles, which articulate with the first vertebra (the
atlas) of the vertebral column. At the anterolateral edge of the
occipital condyle is the hypoglossal canal, through which the
hypoglossal nerve passes. A condyloid canal lies posterior to
the occipital condyle . The external occipital protuberance is a
prominent posterior projection on the occipital bone that can
be felt as a definite bump just under the skin. The superior
nuchal line is a ridge of bone extending laterally from the
occipital protuberance to the mastoid part of the temporal
bone. Sutural bones are small clusters of irregularly shaped
bones that frequently occur along the lambdoid suture.
The occipital bone has the following parts:
 Basilar part,pars basilaris, an unpaired portion located in
front of the foramen magnum.
 Lateral part,pars lateralis, a paired part situated on each
side of the foramen magnum.

13.  Squamous part,squama occipitalis, an unpaired larger
portion of the bone located the foramen.
SPHENOID BONE
The sphenoid bone forms part of the anterior base of the
cranium. This bone has a somewhat mothlike shape. It
consists of a body and laterally projecting greater and lesser
wings that form part of the orbit. The wedgelike body
contains the sphenoidal sinuses and a prominent saddlelike
depression, the sella turcica. Commonly called “Turk’s
saddle,” the sella turcica houses the pituitary gland. A pair of

14. pterygoid processes project inferiorly from the sphenoid bone
and help form the lateral walls of the nasal cavity.
Several foramina are associated with the sphenoid bone:
1. The optic canal is a large opening through the lesser wing
into the back of the orbit that provides passage for the optic
nerve and the ophthalmic artery.
2. The superior orbital fissure is a triangular opening between
the wings of the sphenoid bone that provides passage for the
ophthalmic nerve, a branch of the trigeminal nerve, and for
the oculomotor, trochlear, and abducens nerves.
3. The foramen ovale is an opening at the base of the lateral
pterygoid plate,through which the mandibular nerve passes.
4. The foramen spinosum is a small opening at the posterior
angle of the sphenoid bone that provides passage for the
middle meningeal vessels.
5. The foramen lacerum is an opening between the sphenoid
and the petrous part of the temporal bone, through which the
internal carotid artery and the meningeal branch of the
ascending pharyngeal artery pass.
6. The foramen rotundum is an opening just posterior to the
superior orbital fissure, at the junction of the anterior and
medial portions of the sphenoid bone. The maxillary nerve
passes through this foramen.

15. Clinical application:Located on the inferior side of the
cranium, the sphenoid bone would seem to be well protected
from trauma. Actually, just the opposite is true—and in fact
the sphenoid is the most frequently fractured bone of the
cranium. It has several broad, thin, platelike extensions that
are perforated by numerous foramina. A blow to almost
any portion of the skull causes the buoyed, fluid-filled brain to
rebound against the vulnerable sphenoid bone, often causing
it to fracture.
THE ETHMOID BONE
The ethmoid bone is located in the anterior portion of the
floor of the cranium between the orbits, where it forms the
roof of

16. the nasal cavity. An inferior projection of the ethmoid bone,
called the perpendicular plate, forms the superior part of the
nasal septum that separates the nasal cavity into two
chambers. Each chamber of the nasal cavity is referred to as a
nasal fossa. Flanking the perpendicular plate on each side is a
large but delicate mass of bone riddled with ethmoidal air
cells, collectively constituting the ethmoid sinus. A spine of
the perpendicular plate, the crista galli,projects superiorly into
the cranial cavity and serves as an attachment for the
meninges covering the brain. On both lateral walls of the
nasal cavity are two scroll-shaped plates of the ethmoid
bone, the superior and middle nasal conchae,also known as
turbinates. At right angles to the perpendicular plate, within
the floor of the cranium, is the cribriform plate, which has
numerous cribriform foramina for the passage of olfactory
nerves from the epithelial lining of the nasal cavity.

17. Clinical application:The moist,warm vascular lining within
the nasal cavity is susceptible to infections,particularly if a
person is not in good health. Infections of the nasal cavity can
spread to several surrounding areas. The paranasal sinuses
connect to the nasal cavity and are especially prone to
infection. The eyes may become reddened and swollen during
a nasal infection because of the connection of the
nasolacrimal duct,through which tears drain from the
anterior surface of the eye to the nasal cavity. Organisms may
spread via the auditory tube from the nasopharynx to the
middle ear. With prolonged nasal infections,organisms may
even ascend to the meninges covering the brain via the
sheaths of the olfactory nerves and pass through the
cribriform plate to cause meningitis.