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CRANIUM
03. 02.2014
Kaan Yücel
M.D., Ph.D.
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Dr.Kaan Yücel
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Cranium
The skeleton of the head is the skull. We rather use the ancient Greek term “cranium”, e.g. the cranial nerves.
The skull has 22 bones, excluding the ossicles of the ear. Except for the mandible, which forms the lower jaw, the
bones of the skull are attached to each other by sutures, are immobile, and form the cranium.
The part that is covering the cranial cavity and the brain in it is called neurocranium. The skeleton of the face
is called viscerocranium or facial skeleton. It is the lower part of the cranium.
Out of the 22 bones in the skull, 8 of them are in the neurocranium. They are:
•
1 Frontal bone; the bone in the front of the head
•
1 Occipital bone; the bone at the back of the head
•
2 Parietal bones; “paries” means wall, and these though bones are on the lateral sides of the skull.
•
2 Temporal bones; “temple” has two meanings “time” and “temple”. Time can make more sense for
the temporal bones, as where they are the hair becomes grey first.
•
1 Sphenoid bone in the middle (Greek sphēnoeidēs wedge-shaped)
•
1 Ethmoid bone again in the middle (In Moore’s textbook it is part of the facial skeleton,though)
The skeleton of your face is made up by the remaining 14 bones of the cranium.
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. The butterfly-shaped middle cranial fossa has a central part
composed of the sella turcicae on the body of the sphenoid and large, depressed lateral parts on each side. 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 posterior cranial fossa, the largest and deepest
of the three cranial fossae 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.”
Sutura is that form of articulation where the contiguous margins of the bones are united by a thin layer of
fibrous tissue; it is met with only in the skull. The major suturae in the skull are; coronal, lambdoid, and sagittal
suturues.
The skeleton of your face is made up by the remaining 14 bones of the cranium
They are:
•
Two Nasal bones
•
Two Maxillæ
•
Mandible
•
Two Lacrimal bones
•
Two Zygomatic bones
•
Two Palatines
•
Two Inferior Nasal Conchæ
•
Vomer
The viscerocranium forms the anterior part of the cranium and consists of the bones surrounding the mouth
(upper and lower jaws), nose/nasal cavity, and most of the orbits (eye sockets or orbital cavities). The
viscerocranium consists of 14 irregular bones: 2 singular bones centered on or lying in the midline (mandible and
vomer) and 6 bones occurring as bilateral pairs (maxillae; inferior nasal conchae; and zygomatic, palatine, nasal,
and lacrimal bones).
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Cranium
1. SKULL
The skeleton of the head is the skull. We rather use the ancient Greek term “cranium”, e.g. the cranial
nerves. The skull has 22 bones, excluding the ossicles of the ear. Except for the mandible, which forms the lower
jaw, the bones of the skull are attached to each other by sutures, are immobile, and form the cranium. Suture is
also a term used in surgical practices as “surgical stitching of a wound”. Actually suture in anatomy is a type of
articulation where two articulation surfaces come together along a line, just like you sew them with a needle.
We can divide the cranium into two or three parts. Generally into two!
Let’s see, there is one part enclosing the brain; protecting the brain, and there is another part which
makes the skeleton of your face. The part that is covering the cranial cavity and the brain in it is called
neurocranium. The skeleton of the face is called viscerocranium or facial skeleton. It is the lower part of the
cranium. Here the “viscera” means organ, and on your face there is a list of organs; your mouth, your nose, your
eyes. The prefix neuro in the term neurocranium just refers to the “nerve” telling you that this part of the
skeleton of the head covers the brain and meninges (the membrane covering the brain) within the cranial cavity.
A third part of the skeleton of the head? The part that covers the upper part of the head; calvaria
(skullcap) “kafatası” in Turkish might be considered as a third part. If you add it into the neurocranium, then the
cranium has two parts. Ok? So the neurocranium has one roof (calvaria) and one floor; the base (base of the
skull) basicranium.
Question: which bones make up the neurocranium? Out of the 22 bones of the cranium, 8 of them belong
to the neurocranium. Some of them are paired which means you can find one on the right side, and one on the
left side, and some of them are single.
 1 Frontal bone; the bone in the front of the head
 1 Occipital bone; the bone at the back of the head
 2 Parietal bones; “paries” means wall, and these though bones are on the lateral sides of the skull.
 2 Temporal bones; “temple” has two meanings “time” and “temple”. Time can make more sense for the
temporal bones, as where they are the hair becomes grey first.
 1 Sphenoid bone in the middle (Greek sphēnoeidēs wedge-shaped)
 1 Ethmoid bone again in the middle
As you see only the temporal bones and the parietal bones above them are paried (bilateral), and the
other four bones are single (unilateral) which make the eight bones of the neurocranium.
The neurocranium has a dome-like roof, the calvaria (skullcap), and a floor or cranial base (basicranium).
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The bones forming the calvaria are mainly the paired temporal and parietal bones, and parts of the
unpaired frontal, sphenoid, and occipital bones. The frontal bone, parietal bones, and occipital bone make up the
superior part of the calvaria or the calva (skullcap).
The bones forming the base of the cranium are mainly parts of the sphenoid, temporal, and occipital
bones.
The ethmoid bone is an irregular bone that makes a relatively minor midline contribution to the
neurocranium but is also part of the viscerocranium. Watch out! Although there is only a minor contribution of
the ethmoid bone to the neurocranium, it is counted under the bones of the neurocranium. Clue: Smelling.
The skeleton of your face (viscerocranium; as we have three organs: from superior to inferior- eyes, nose,
tongue) is made up by the remaining 14 bones of the cranium.
Figure 1. Skull bones (lateral view)
http://images.tutorvista.com/content/locomotion-animals/human-skull-structure.jpeg
2. BONES OF THE NEUROCRANIUM
2.1. FRONTAL BONE (OS FRONTALE)
Figure 2. Frontal bone
http://www.bleaching-dental.com/img/news/126.jpg
The biggest part of the brain (one third of a brain
hemisphere); the frontal lobe mostly resides on the frontal lobe.
The frontal bone forms the forehead. It also contributes to the
formation of two cavities; the orbital cavity where the eyes are
located and the nasal cavity (the cavity inside your nose).
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The frontal bone consists of two portions:
squama (etymology: Latin, squama: scale; squama frontalis)- vertical portion
corresponding with the region of the forehead
orbital portion (frontal orbit; orbita frontalis)– horizontal partion
enters into the formation of the roofs of the orbital and nasal cavities
Figure 3. Frontal bone, squamous part
http://virtual.yosemite.cc.ca.us/rdroual/Lecture%20Notes/Unit%202/chapter_6_axial_skeleton_copy%20with%20figures.htm
So you should take the frontal bone as two pieces; one flat surface (squama) and one horizontal surface
forming the roof of the orbit (the nest for the eye). From now on, if a cranial bone has a flat, smooth surface, it
will be named as squamous part (just like the one in the occipital bone, back of the head; squama means scale;
just like a scale of a fish).
2.2. PARIETAL BONES
The two parietal bones unite and form the sides and roof of the cranium. Each bone is irregularly
quadrilateral in form. The external surface is convex, smooth, and marked near the center by an eminence, the
parietal eminence (tuber parietale). Crossing the middle of the bone in an arched direction are two curved lines,
the superior and inferior temporal lines.
Figure 4. Parietal bones (anterior view)
http://aftabphysio.blogspot.com/2010/09/bones-of-skull.html
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2.3. TEMPORAL BONES
The temporal bones are situated at the sides and base of the skull. The temporal bone has the temporal
lobe on which is important for a long list of functions including memory, emotional memory, hearing. It has the
canal that goes to the ear. The temporal bone contributes most of the lower portion of the lateral wall of the
cranium. Each temporal bone has three parts which are separated from each other by a cartilaginous tissue in
the newborn. Later on, the three parts are united and become as one single bone.
1) Squamous part
2) Tympanic part
3) Petromastoid part
The squamous part has the appearance of a large flat plate, forms the anterior and superior parts of the
temporal bone, contributes to the lateral wall of the cranium. The zygomatic process is an anterior bony
projection from the lower surface of the squamous part of the temporal bone that initially projects laterally and
then curves anteriorly to articulate with the temporal process of the zygomatic bone to form the zygomatic arch.
The squamous part lies just lateral to the greater wing of the sphenoid. It participates in the
temporomandibular joint. It contains the mandibular fossa, which is a concavity where the head of the mandible
articulates with the base of the skull.
The tympanic part of the temporal bone is immediately below the origin of the zygomatic process from
the squamous part of the temporal bone. The external acoustic opening (pore) is the entrance to the external
acoustic meatus (canal), which leads to the tympanic membrane (eardrum). On the lateral edge of the temporal
bone lies the cone-shaped mastoid process projecting from its inferior surface. The mastoid process is a
prominent structure and is the point of attachment for several muscles.
Immediately lateral to the basilar part of the occipital bone is the petrous part of the temporal bone.
The apex forms one of the boundaries of the foramen lacerum, an irregular opening filled in life with cartilage.
The large opening between the occipital bone and the petrous part of the temporal bone is the jugular
foramen.This foramen is very important as major structures pass through this foramen. The vein draining the
brain exits the skull through the jugular foramen. Three of the 12 pairs of cranial nerves pass through the jugular
foramen and go to their destinations exiting the cranium. Anterosuperior to the jugular foramen is the internal
acoustic meatus for the passage of two other cranial nerves. One of them is the nerve for the muscles of the
face, and the other is good for the hearing and balance.
The styloid process is needle-shaped bone marking. It projects from the lower border of the temporal
bone anteromedial to the mastoid process. The styloid process is a point of attachment for numerous muscles
and ligaments. The stylomastoid foramen, transmitting the nerve for the muscles of the face lies (CN VII; Facial
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nerve – CN= Cranial nerve) posterior to the base of the styloid process; between the styloid process and the
mastoid process.
Figure 5. Temporal bone
http://medicinembbs.blogspot.com/2011/02/skull-anatomy.html
2.4. SPHENOID BONE
The sphenoid bone is situated at the base of the skull in front of the temporal bones and basilar part of
the occipital bone. It somewhat resembles a bat with its wings extended, and is divided into a median portion;
body, two great and two small wings extending outward from the sides of the body, and two pterygoid processes
which project from it below.
Just anterior to each anterior clinoid process is a circular opening in the lesser wing of the sphenoid (the
optic canal), through which the ophthalmic artery and optic nerve [II] pass as they exit the cranial cavity to enter
the orbit. The optic canals are usually included in the middle cranial fossa.
The sella turcica[e] (L. Turkish saddle) 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,”
and 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.
The sella turcica is composed of three parts:
1) The tuberculum sellae (horn of saddle): a variable slight to prominent median elevation forming the
posterior boundary of the chiasmatic sulcus (optic groove) and the anterior boundary of the hypophysial fossa.
It lies behind the chiasmatic groove. On both ends of the tuberculum sellae are middle clinoid processes.
2) The hypophysial fossa (pituitary fossa): a median depression (seat of saddle) in the body of the sphenoid
that accommodates the pituitary gland (L. hypophysis).
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3) The dorsum sellae (back of 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.
On each side of the body of the sphenoid, four foramina perforate the roots of the cerebral surfaces of the
greater wings of the sphenoids:
Superior orbital fissure: Located between the greater and the lesser wings, it opens anteriorly into the orbit.
Foramen rotundum (round foramen): Located posterior to the medial end of the superior orbital fissure.
Foramen ovale (oval foramen): A large foramen posterolateral to the foramen rotundum.
Foramen spinosum (spinous foramen): Located posterolateral to the foramen ovale.
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 partly closed
by a cartilage plate.
Figure 6. Sphenoid bone (anterior view)
http://virtual.yosemite.cc.ca.us/rdroual/Lecture%20Notes/Unit%202/chapter_6_axial_skeleton_copy%20with%20figures.htm
Figure 7. Foramina in the sphenoid bone (superior view) and other openings
http://medchrome.com/wp-content/uploads/2011/05/skull-superior.jpg
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2.5. OCCIPITAL BONE
The occipital bone is situated at the back and lower part of the cranium. It is trapezoid in shape and
curved on itself. It is pierced by a large oval aperture, the foramen magnum, through which the cranial cavity
communicates with the vertebral canal.
The foramen magnum is the most prominent feature of the cranial base. The major structures passing
through this large foramen:

spinal cord (where it becomes continuous with the medulla oblongata of the brain)

meninges (coverings) of the brain and spinal cord

vertebral arteries

spinal accessory nerve (CN XI).
The four parts of the occipital bone are arranged around the foramen magnum:
1) The curved, expanded plate behind the foramen magnum is named the squama.
2) The thick, quadrilateral piece in front of the foramen is called the basilar part.
3) On either side of the foramen is the lateral (condylar) portion of the occipital bone.
The cranial base is formed posteriorly by the occipital bone, which articulates with the sphenoid bone anteriorly.
The external occipital protuberance, is usually easily palpable in the median plane; however, occasionally
(especially in females) it may not be prominent. The superior nuchal line marks the superior limit of the neck. It
extends laterally from each side of the protuberance. The inferior nuchal line is less distinct. On the lateral parts
of the occipital bone are two large protuberances, the occipital condyles. The cranium articulates with the
vertebral column by the occipital condyles.
2.6. ETHMOID BONE
Gk, ethmos, sieve sifter, eidos, form
The etmoid bone is exceedingly light and spongy, and cubical in shape; it is situated at the anterior part of
the base of the cranium, between the two orbits, at the roof of the nose, and contributes to each of these
cavities. It consists of four parts: a horizontal or cribriform plate, forming part of the base of the cranium; a
perpendicular plate, constituting part of the nasal septum; and two lateral ethmoidal labyrinths.
The crista galli (L. crest of the cock) is a thick, median ridge of bone posterior to the foramen cecum
(frontal bone), which projects superiorly from the ethmoid. On each side of the ridge called crista galli, located in
the frontal bone, 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. SMELLING!!!
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Figure 8. Ethmoid Bone
http://medical-dictionary.thefreedictionary.com/ethmoid+bone
Figure 9. Ethmoid bone’s location in the skull
http://www.daviddarling.info/images/ethmoid_bone.jpg
3. CRANIAL FOSSAE
The five bones that make up the skull base are the ethmoid, sphenoid, occipital, paired frontal, and paired
parietal bones. The skull base can be subdivided into 3 regions: the anterior, middle, and posterior cranial fossae.
3.1. ANTERIOR CRANIAL FOSSA
Parts of the frontal, ethmoid, and sphenoid bones form the anterior cranial fossa. The anterior cranial
fossa is above the nasal cavity and the orbits, and it is filled by the frontal lobes of the cerebral hemispheres
3.2. MIDDLE CRANIAL FOSSA
The middle cranial fossa consists of parts of the sphenoid and temporal bones. The butterfly-shaped
middle cranial fossa has a central part composed of the sella turcice on the body of the sphenoid and large,
depressed lateral parts on each side.
The middle cranial fossa is posteroinferior to the anterior 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
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the petrous parts of the temporal bones posteriorly. The lateral parts of the middle cranial fossa support the
temporal lobes of the brain.
3.3. POSTERIOR CRANIAL FOSSA
The posterior cranial fossa consists mostly of parts of the temporal and occipital bones with small
contributions from the sphenoid and parietal bones. It is the largest and deepest of the three cranial fossae and
contains the brainstem (midbrain, pons, and medulla) and the cerebellum.
Figure 10. Cranial fossae
http://tmjc.com.ne.kr/tmj/info/drinfo/images/tm6-6.jpg
4. SUTURA
Sutura is that form of articulation where the contiguous margins of the bones are united by a thin layer of fibrous
tissue; it is met with only in the skull.
4.1. CORONAL SUTURE
Parietal bones articulate with the frontal bone in their front, forming the coronal suture.
4.2. LAMBDOID SUTURE
Parietal bones articulate with the occipital in their behind, forming the lambdoid suture.
4.3 .SAGITTAL SUTURE
Parietal bone articulates with its the opposite side, forming the sagittal suture. Vertex, the most superior point of
the calvaria, is near the midpoint of the sagittal suture.
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.
5. FONTANELLES
The bones of the calvaria of a newborn infant are separated by membranous intervals. They include the
anterior and posterior fontanelles and the paired sphenoidal and mastoid fontanelles. Palpation of the
fontanelles during infancy, especially the anterior and posterior ones, enables physicians to determine the:
•
Progress of growth of the frontal and parietal bones.
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•
Degree of hydration of an infant (a depressed fontanelle indicates dehydration).
•
Level of intracranial pressure (a bulging fontanelle indicates increased pressure on the brain).
The anterior fontanelle, the largest one, is diamond or star shaped; it is bounded by the halves of the
frontal bone anteriorly and the parietal bones posteriorly. Thus it is located at the junction of the sagittal,
coronal, and frontal sutures, the future site of bregma. By 18 months of age, the surrounding bones have fused
and the anterior fontanelle is no longer clinically palpable.
The posterior fontanelle is triangular and bounded by the parietal bones anteriorly and the occipital bone
posteriorly. It is located at the junction of the lambdoid and sagittal sutures, the future site of lambda. The
posterior fontanelle begins to close during the first few months after birth; and by the end of the 1st year, it is
small and no longer clinically palpable.
The sphenoidal and mastoid fontanelles fuse during infancy and are less important clinically than the midline
fontanelles.
Figure 11. Fontanelles
http://medical-dictionary.thefreedictionary.com/fontanelle
6. SKELETON OF THE FACE
The lower and anterior part of the cranium is the facial skeleton (viscerocranium). The skeleton of the
face is made up by 14 bones.

Two Nasal bones

Two Maxillæ

Mandible

Two Lacrimal bones

Two Zygomatic bones

Two Palatines

Two Inferior Nasal Conchæ

Vomer
The viscerocranium forms the anterior part of the cranium and consists of the bones surrounding the mouth
(upper and lower jaws), nose/nasal cavity, and most of the orbits (eye sockets or orbital cavities). The
viscerocranium consists of 14 irregular bones: 2 singular bones centered on or lying in the midline (mandible and
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vomer) and 6 bones occurring as bilateral pairs (maxillae; inferior nasal conchae; and zygomatic, palatine, nasal,
and lacrimal bones).
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.
Several bones of the cranium (frontal, temporal, sphenoid, and ethmoid bones) are pneumatized bones,
which contain air spaces (air cells or large sinuses), presumably to decrease their weight. The total volume of the
air spaces in these bones increases with age.
HARD PALATE (BONY PALATE)
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 paired palatine processes of each maxilla meet in the midline at the intermaxillary suture, the paired
maxilla and the paired palatine bones meet at the palatomaxillary suture, and the paired horizontal plates of
each palatine bone meet in the midline at the interpalatine suture.
Figure 12. Hard palate
http://www.gla.ac.uk/ibls/US/cal/anatomy/cleftpalate/final/hardp975.htm
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