by Dr. Gary Farr on July 22, 2003
Introduction - The Parietal Bone
The Cranium or Skull: A General Overview
The Cranium, also called the "Skull," describes the skeleton of the head, face and mandible.
It is a portion of the axial skeleton, or that portion associated with the central nervous system. Those portions of the skeleton not associated with the central nervous system, are associated with the appendicular skeleton or the extremities (i.e., the arms and legs).
The axial skeleton consists of the cranium, all the osseous elements of the vertebral column, the ribs, and the sternum.
In an adult, various of the bones of the cranium are paired left and a right, while others, which cross the mid-sagittal plane, are unpaired. Furthermore, the bones of the skull are classified as those which are called cranial Bones, or contribute to that portion surrounding the brain, or facial bones (i.e., those which do not assist in forming the braincase).
Detailed Description
The skull is one of the principle groups of bones in the human anatomy. The skull consists of twenty-six bones: eight bones form the cranium, which houses the brain and ear ossicles, plus fourteen facial bones, which form the front of the face, jaw, nose, orbits, and the roof of the mouth, three more bones make up the inner ear ossicles, and one more, the hyoid bone, is in the neck and is attached to the temporal bone by ligaments and anchors the tongue. The skull also contains a dental arcade of teeth, which are technically not bones, though they do share some of the compositional characteristics of bone tissue. Children may grow twenty deciduous (non-permanent) teeth, which will eventually fall out and be replaced by the permanent teeth (32 of them in the average adult). The bones of the skull include the frontal bone (which makes up the forehead and roof of the orbits), the occipital bone (which forms the back and base of the skull), two parietal bones (which form the roof and upper sides of the skull), and two temporal bones (which form the lower sides of the skull and house the inner ear ossicles). The lower rearmost part of each temporal bone is called the mastoid process, but because it is separated from the temporal bone, proper, by a suture, it is often considered a separate bone. The sphenoid bone forms the central base of the skull and spans the skull from side to side, the greater wings forming side plates of the skull. The sections of the ethmoid bone are positioned between the orbits, forming the walls and roof of the nasal cavity, while the three middle ear ossicles (stapes, malleus, and incus) are located within the temporal bones on each side of the skull. The U-shaped hyoid bone is found in the neck, and is attached by ligaments to the temporal bones. In the face, the two maxillary bones form much of the orbits, nose, upper jaw and roof of the mouth, while the malar (zygomatic) bones form the cheeks. The lachrymal bones are located on the inner sides of the orbits and are attached to the ethmoid and maxillary bones. Within the nasal cavity, the vomer is located in the low center and forms the thin flat bone of the nasal septum, while two inferior urbinates form the lower sides of the cavity and two palate bones form the floor of the nasal cavity as well as the roof of the mouth. The mandible is the only movable part of the skull, forming the lower jaw and mounting the teeth. The bones of the skull, with the exception of the mandible, are held together by very thin sutures, or seams, in which the periosteum of the individual bones interweave with each other, and are cemented by a fibrous, connective tissue. In the newborn, these sutures are not yet developed, with the bones being attached by cartilage which ossifies over time as the bones of the skull fuse together. The most evident external sutures of the cranium include the coronal suture, joining the frontal and parietal bones, the sagittal suture, joining the parietal bones to each other, the lambdoid suture, joining the occipital and parietal bones, the squamous suture, joining the temporal and sphenoid bones to the parietal bone on each side of the skull. The pterion is the short segment of the suture joining the squamous and parietal bones. The bones of the skull also feature a number of sinuses (cavities) and foramina (the plural of foramen, meaning hole or opening). Four pairs of sinuses flank the nasal cavity (and are therefore called paranasal sinuses). Two are found in the maxillary bone, and are called maxillary sinuses. The sphenoid bone forms two paranasal sinuses called the sphenoids, and the ethmoid bone forms the two paranasal sinuses called ethmoids. Additionally, the frontal sinuses are located in the frontal bone just behind the roof of each orbit. The foramen magnum is a large, round opening in the base of the skull which admits the spinal cord, while at the base of each temporal bone is the external auditory meatus, which serve as the auditory canals. Just above each orbit in the frontal bone is a small notch or hole, called a supraorbital foramen, and just below each orbit, in the maxillary bone, is an infraorbital foramen. Two more openings, one on each side of the skull, can be found in the frontal processes of the malar (zygomatic) bones, and are called, therefore, zygomatofacial foramina. On each side of the mandible, just below the lower canines, are the mental foramina. These facial foramina serve to admit blood vessels and nerves through and into the bone. The teeth are mounted in the maxillary bone and the mandible, and are brought together for chewing by the hinge-like motion of the mandible (the lower jawbone). An average adult will have thirty-two teeth, evenly arrayed on the maxilla and mandible.
The Individual Bones
Each of the bones of the cranium posses a number of distinctive features which not only allow the bone to be identified, but also permit its exact location and orientation in the body to be determined (i.e., as a left or right, medial- lateral, posterior-anterior, inferior-superior, etc.). Additionally, this article contains various QuickTime VR movies in order to assist you in viewing a real three dimensional view of the bones.
The following list shows how the bones in the cranium are arranged.
Paired Cranial Bones:
- Parietals
- Temporals
Unpaired Cranial Bones:
- Frontal
- Occipital
- Sphenoid
- Ethmoid
Paired Facial Bones:
- Lacrimals
- Nasals
- Zygomatics
- Maxillae
- Palatines
- Inferior Nasal Conchae
Unpaired Facial Bones:
- Vomer
- Mandible
- Hyoid
The following diagrams show the bones that this article will cover. Click on the name of the bone or the area of the skull on the bone you want information about. There are 15 pages to this article.
The parietal bones form, by their union, the sides and roof of the cranium. Each bone is irregularly quadrilateral in form, and has two surfaces, four borders, and four angles.
To view a QuickTime VR movie of the parietal bone click 
Surfaces.—The external surface is convex, smooth, and marked near the center by an eminence, the parietal eminence (tuber parietale), which indicates the point where ossification commenced. Crossing the middle of the bone in an arched direction are two curved lines, the superior and inferior temporal lines; the former gives attachment to the temporal fascia, and the latter indicates the upper limit of the muscular origin of the Temporalis. Above these lines the bone is covered by the galea aponeurotica; below them it forms part of the temporal fossa, and affords attachment to the Temporalis muscle. At the back part and close to the upper or sagittal border is the parietal foramen, which transmits a vein to the superior sagittal sinus, and sometimes a small branch of the occipital artery; it is not constantly present, and its size varies considerably.
The internal surface is concave; it presents depressions corresponding to the cerebral convolutions, and numerous furrows for the ramifications of the middle meningeal vessel; 27 the latter run upward and backward from the sphenoidal angle, and from the central and posterior part of the squamous border. Along the upper margin is a shallow groove, which, together with that on the opposite parietal, forms a channel, the sagittal sulcus, for the superior sagittal sinus; the edges of the sulcus afford attachment to the falx cerebri. Near the groove are several depressions, best marked in the skulls of old persons, for the arachnoid granulations (Pacchionian bodies). In the groove is the internal opening of the parietal foramen when that aperture exists.
Borders.—The sagittal border, the longest and thickest, is dentated and articulates with its fellow of the opposite side, forming the sagittal suture. The squamous border is divided into three parts: of these, the anterior is thin and pointed, bevelled at the expense of the outer surface, and overlapped by the tip of the great wing of the sphenoid; the middle portion is arched, bevelled at the expense of the outer surface, and overlapped by the squama of the temporal; the posterior part is thick and serrated for articulation with the mastoid portion of the temporal. The frontal border is deeply serrated, and bevelled at the expense of the outer surface above and of the inner below; it articulates with the frontal bone, forming onehalf of the coronal suture. The occipital border, deeply denticulated, articulates with the occipital, forming one-half of the lambdoidal suture.
Angles.—The frontal angle is practically a right angle, and corresponds with the point of meeting of the sagittal and coronal sutures; this point is named the bregma; in the fetal skull and for about a year and a half after birth this region is membranous, and is called the anterior fontanelle. The sphenoidal angle, thin and acute, is received into the interval between the frontal bone and the great wing of the sphenoid. Its inner surface is marked by a deep groove, sometimes a canal, for the anterior divisions of the middle meningeal artery. The occipital angle is rounded and corresponds with the point of meeting of the sagittal and lambdoidal sutures—a point which is termed the lambda; in the fetus this part of the skull is membranous, and is called the posterior fontanelle. The mastoid angle is truncated; it articulates with the occipital bone and with the mastoid portion of the temporal, and presents on its inner surface a broad, shallow groove which lodges part of the transverse sinus. The point of meeting of this angle with the occipital and the mastoid part of the temporal is named the asterion.
Ossification.—The parietal bone is ossified in membrane from a single center, which appears at the parietal eminence about the eighth week of fetal life. Ossification gradually extends in a radial manner from the center toward the margins of the bone; the angles are consequently the parts last formed, and it is here that the fontanelles exist. Occasionally the parietal bone is divided into two parts, upper and lower, by an antero-posterior suture.
Articulations.—The parietal articulates with five bones: the opposite parietal, the occipital, frontal, temporal, and sphenoid.
The Occiput
The occipital bone consists of a large squamous, or
flattened portion separated from a small thick basal portion by the foramen
magnum on either side of which is a left or right occipital condyle. The
occipital condyles articulate with the first cervical vertebrae (the atlas).
Externally, the squamous portion of the bone possesses superior, middle, and
inferior nuchal lines to which the muscles at the back of the neck are attached.
The external occipital protuberance lies on the superior nuchal line in the mid-sagittal
plain. Lateral to each occipital condyle are the condylar fossae and foramen
while the hypoglossal canal is medial to them.
Internally, are the sagittal and transverse sulci, or grooves which converge at
the confluence of sinuses. A single internal occipital protuberance or cruciform
eminence is also found in this area. Running inferior from the eminence to the
foramen magnum is the internal occipital crest which separates the cerebellar
fossae. The transverse sulci assist in directing the developing jugular vein to
the jugular notch on either side of the basilar portion of the occipital.
To view a QuickTime VR movie of the occipital bone click
The occipital touches, or articulates with, the following bones:
- Parietals
- Temporals
- Sphenoid
- Atlas - The atlas is not part of the skull. It is the first of the seven cervical vertebrae and the one upon which the base of the skull sits. It is the bone around which the skull rotates, hence the name "atlas."
The Frontal Bone
To view a QuickTime VR movie of the frontal bone click
The frontal bone may be divided into two main portions, a vertical squamous
portion which articulates with the paired parietals along the coronal suture and
forms the forehead, and two orbital plates, which contribute to the ceiling and
lateral walls of the left and right eye orbits. On the external surface the
squamous portion frequently possesses a left and right frontal eminence.
Additionally, the bone possesses two supra-orbital ridges (i.e., superciliary or
brow ridges) which are bumps above each of the eye orbits. In early hominids
these ridges formed a torus or large shelf-like process protruding from above
the eyes. Associated with each superior orbital margin of the eye orbit the
frontal bone may posses a supra-orbital notch or if completely surrounded by
bone, a supra-orbital foramen. Above the fronto-nasal suture which allows
articulation between the frontal and nasal bones there is generally a trace of
the vertical metopic suture. In early life the metopic suture divided the
frontal bone into left and right halfs. With in the bone, and above and the
metopic suture, is the frontal sinus. The left and right frontal crest, begins
at each zygomatic process of the frontal bone, and provides the anterior origin
of the temporal line to which the left and right temporal muscle is attached.
Internally, the frontal bone possesses the median sagittal (i.e., sagittal-frontal)
crest which separates the two frontal hemispheres of the brain.
The Temporal Bones
The Temporal Bones
To view a QuickTime VR movie of the temporal bones click
The temporal bone is another paired cranial bone which
is difficult to describe due to its various features, and projections. It
consists of two major portions, the squamous portion, which is flat or fan-like
and projects superiorly from the other, very thick and rugged portion, the
petrosal portion.
The squamous portion assists in forming the squamous suture which separates the
temporal bone from the adjacent and partially underlaying parietal bone. The
petrosal portion contains the cavity of the middle ear and all the ear ossicles;
the malleus, incas and stapes. This portion projects anterior and medialy
beneath the skull. Projecting inferiorly from the petrosal portion is the
slender styloid process which is of variable length. The styloid process serves
as a muscle attachment for various thin muscles to the tongue and other
structures in the throat. Externaly the petrosal portion possesses the external
auditory meatus while internally there is an internal auditory meatus. Anterior
to the external meatus the zygomatic process has its origin. This process
projects forward toward the face and its articulation with the temporal process
of the zygomatic. Just anterior of the external meatus and inferior of the
origin of the zygomatic process is the glenoid or mandibular fossa which assists
in forming the shallow socket of the tempro-mandibular joint. Posterior to the
external auditory meatus is the inferiorly projecting mastoid process which
serves as an attachment for the sternocleidomasotid muscle. Above the mastoid
process is the supramastoid crest to which the posterior portion of the temporal
muscle is attached.
The temporals touch, or articulate with, the following bones:
- Occipital
- Sphenoid
- Parietals
- Zygomatics
- Mandible
The Sphenoid
The Sphenoid
To view a QuickTime VR movie of the sphenoid bone click
The sphenoid is one of the more difficult bones to describe and invision. It has
a number of features and projections, which allow it to be seen from various
views of the skull. It is a single bone that runs through the mid-sagittal plane
and aids to connect the cranial skeleton to the facial skeleton. It consists of
a hollow body, which contians the sphenoidal sinus, and three pairs of
projections: the more superior lesser wings, the intermediate greater wings, and
the most inferior projecting pterygoid processes. Internally upon the body is
the sella turcica where the pituitary gland rests in life. The smaller lesser
wings posssesses the optic foramen through which the optic or second cranial
nerve passes before giving rise to the eye. The supra-orbital fissure separates
the lesser wing superiorly from the greater wing below and can best be viewed on
the posterior wall of each eye orbit. The left and right greater wings assist in
forming the posterior wall of each of the eye orbits where it forms an orbital
plate. In addition the external surface of the greater wing can be viewed in the
the lateral view of the cranium in an area called the pterion region. Just
inferior to the supra-orbital fissure near the body of the sphenoid, each of the
greater wings also possess a foramen rotundum which in life transmits the
maxillary branch of the fifth, or trigeminal, cranial nerve. Each of these wings
also possesses a much larger foramen ovale more laterally, which transmits the
the mandibular branch of the same nerve. More posteriorly is the smallest of the
three pairs of foramena, the foramen spinosum which transmits the middle
meningial vessels and nerve to the tissues covering the brain.
The left and right pterygoid processes project inferiorly from near the junction
of each of the greater wings with the body of the sphenoid. These processes run
along the posterior portion of the nasal passage toward the palate. Each process
is formed from a medial and lateral pterygoid plate to which the respective
medial and lateral pterygoid muscle is attached during life. The muscles run
from these attachments to the internal, or medial surface, of the mandible in
the area of the gonial angle. In life the muscles assist in creating the
grinding motion associated with chewing.
The sphenoid touches, or articulates with, the following bones:
- Vomer
- Ethmoid
- Frontal
- Occipital
- Parietals
- Temporals
- Zygomatics
- Palatines
The Ethmoid Bone
The Ethmoid
If the sphenoid is the most difficult cranial bone to describe and invision, the
Ethmoid is the second most difficult. It has a number of features and
projections, but unlike the sphenoid it cannot be seen from various views of the
skull. Like the sphenoid, it is a single bone.
The "ethmoid bone" is located in front of the sphenoid bone. It consists of two
masses, one on each side of the nasal cavity, which are joined horizontally by
thin "cribriform plates." These plates form part of the roof of the nasal
cavity, and nerves (ethmoidal cells) associated with the sense of smell pass
through tiny openings in them. Portions of the ethmoid bone also form sections
of the cranial floor, eye sockets, and nasal cavity walls. A "perpendicular
plate" projects downward in the middle from the cribriform plates to form the
bulk of the nasal septum. Delicate scroll-shaped plates called "superior" and
"middle nasal conchae" project inward from the sides of the ethmoid bone toward
the perpendicular plate. These bones, which are called the "turbinate bones,"
support mucous membranes that line the nasal cavity.
To view a QuickTime VR movie of the ethmoid bone click
The ethmoid touches, or articulates with, the following bones:
- Sphenoid
- Frontal
- Maxillae
- Palatines
- Vomer
- Lacrimals
The Lacrimal Bone
The Lacrimal Bone
The lacrimal bones are the smallest and most fragile of
the facial bones. They are paried left and right and assist in forming the
anterior portion of the medial wall of each eye orbit. They are basicaly
rectangular with two surfaces and four borders. Each of the borders articulate
with the bones that surround the lacrimal. The orbital or lateral surface
contributes to the eye orbit, while the medial surface assists in forming a
small portion of the nasal passage. The orbital surface possesses a sharp
superior-inferior running ridge called the posterior lacrimal crest which
divides this surface into an orbial plate and the lacrimal sulcus. The sulcus,
along with a contiguous sulcus on the maxillae, assists in forming the lacrimal
fossa which contains the lacrimal duct in life. The duct connects the medial
corner of the eye to the nasal passage and allows tears from the eye to be
shunted into the nasal passage.
The lacrimals touch, or articulate with, the following bones:
- Frontal
- Ethmoid
- Maxillae
The Nasal Bones
The Nasal Bones
Each of the nasal bones is a small rectangular bone
which together form the bridge of the nose above the nasal cavity also called
the piriform aperture. They articulate with each other by way of the internasal
suture and with the frontal bone superiorly by way of the fronto-nasal suture
just below the glabellar region of the frontal bone. The intersection of these
two sutures marks the anatomical landmark called nasion. Laterally, each of the
nasal bones articulates with the frontal process of the maxilla.
A nasal touches, or articulates, the following bones:
- Frontal
- Maxilla
- Nasal
The Zygomatic Bones
The Zygomatic Bones
Also called the malars or jugals, each cheek or
zygomatic bone possesses three major processes which articulate with the bones
which surround it. The frontal process of the zygomatic forms the lateral margin
and wall of the eye orbit and projects superiorly to articulate with the
zygomatic process of the frontal bone. This portion of the bone separates the
eye orbit from the temporal fossa and possesses a posterior projecting edge
called the marginal process. The temporal process of the zygomatic runs lateral
and posterior toward an articulation with the zygomatic process of the temporal
bone. Together these two processes assist in forming the zygomatic arch which
serves as the attachment for the masseter muscle in life, one of the primary
muscles used in mastication. The temporal muscle runs beneath the arch and is
also a primary mover of the mandible in chewing. The maxillary process of the
zygomatic articulates with the zygomatic portion of the maxilla by way of the
zygo-maxillary suture.
To view a QuickTime VR movie of the zygomatic bones click
The zygomatics touch, or articulate with, the following bones:
- Frontal
- Sphenoid
- Maxillae
- Temporals
The Maxillae
The Maxillae
The maxillae are the paired facial bones which contain
the upper dention and thus form the upper jaw. Each is basicly hollow with a
large maxillary sinus. A superior projection, the frontal process, assists in
forming the lateral margin of the nasal aperture and ends by articulating with
the frontal bone. An orbital plate forms the floor of the eye orbit, while the
zygomatic process articuates with the zygomatic bone. On the anteror surface of
the bone, near the maxillo-zygomatic suture, ther is an infra-orbital foramen.
The alveolar process of the maxilla contains the upper dentition and assists in
giving rise to the palatine portion which forms the anterior half of the hard
palate. The left and right maxillae articulate with one another by way of the
inter-maxillary suture. The superior end of this suture frequently terminates
with the nasal spine.
To view a QuickTime VR movie of the maxillae bones click
A maxilla touches, or articulates with, the following bones:
- Frontal
- Ethmoid
- Zygomatic
- Vomer
- Lacrimal
- Maxilla
- Nasal
- Palatine
- Mandible
The Palatine Bones
The Palatine Bones
The palatine bones are paired left and right and
articulate with one another in the mid-sagittal plane at the interpalatine
suture. Both bones assist in forming the posterior portion of the hard palate as
well as a portion of the nasal cavity. Each bone possesses a horizontal part,
with an inferior surface which forms the posterior portion of the hard palate
and a superior surface that assists in forming the posterior portion of the
floor of the nasal cavity. The vertical part of each contributes to the lateral
wall of the nasal cavity. Near the posterior junction of the vertical and
horizontal parts on the palatal surface is a palatine foramen. Each bone
possesses a number of processes and articular surfaces which touch the bones
that surround it.
To view a QuickTime VR movie of the palantine bones click
A palatine touches, or articulates with, the following bones:
- Sphenoid
- Ethmoid
- Maxilla
- Vomer
- Palatine
The Inferior Nasal Conchae
The Inferior Nasal Concha
The inferior nasal concha is a very thin, porous, and
fragile, paired bone basically elongated and curled upon itself. It lays in the
horizontal plane and is attached to the lateral wall of the nasal cavity. By way
of the maxillary process on the bone's lateral surface, it is attached to the
maxilla, and by way of the lacrimal, ethmoid and palatine processes to each of
the bones which assist in forming the lateral wall of the nasal cavity. By
projecting into the nasal cavity, the medial surface of the inferior nasal
concha assists in increasing the surface area within the cavity and thus
increases the amount of mucus membrane and olfactory nerve endings exposed to
inhaled odors.
An inferior nasal concha touchs, or articulates with, the following bones:
- Ethmoid
- Lacrimal
- Maxilla
- Palatine
The Vomer Bone
The Vomer Bone
The vomer is a single relatively flat bone located in
the mid-sagittal plane. It articulates with the perpendicular plate of the
ethmoid superiorly and together aid in forming the nasal septum. While it is
frequently deflected slightly to the left or right, in general the septum is
aligned perpendicularly and divides the the nasal aperture into the the left and
right nasal passages. In addition to the perpendicular portion, superiorly the
vomer mushrooms out into a pair of alae which terminate and articulate with the
sphenoid in a heart shaped process. Inferiorly the vomer rests on both the
maxillae and the palatines.
The vomer touches, or articulates with, the following bones:
- Sphenoid
- Ethmoid
- Palatines
- Maxillae
The Mandible
The Mandible
The mandible or lower jaw consists to four major
portions, a left and right mandibular ramus and the left and right body. The
alveolar process of the body is that portion of the mandible which contains the
lower dentition. The junction of the ramus and the body occurs at the gonial
angle where externally one of the masseter muscles is attatched. The left and
right masseters make up a set of two sets of muscels used in chewing. At the
gonial angle on the internal surface the pterygoid attachements are found. These
attachements are for the medial and lateral pterygoid muscles which assist in
the grinding motion of chewing. The external surface of the mandibular body
possesses the mental foramen and at the midline, the mental protuberance or
chin. The internal surface of the body possesses the lingual foramen, the
mandibular canal, and the longitudinal running mylo-hyoid ridge. The genio
tubercle is located in the mid-sagittal plane on the internal surface of the
mandible. The superior margin of each ramus possesses both a mandibular condyle
or head, for articulaltion with the temporal bone at the tempro-mandibular
joint, and the coronoid process, for the attachement of the temporalis muscle
(one in the set of primary muscles used in mastication). The mandible
articulates with each of the maxillae by way of their contained respective lower
and upper dentition.
To view a QuickTime VR movie of the mandible bone click
The mandible touches, or articulates with, the following bones:
- Temporals
- Maxillae
The Hyoid Bone
The Hyoid Bone
The hyoid is a single small "u" shaped bone in the adult
which does not articulate with any other bone. It is suspended from the styloid
process of each temporal bone by means of the stylohyoid ligaments. It is
located in the mid-sagittal plane, at the front of the throat, and beneath the
mandible but above the larynx near the level of the third cervical vertebrae. It
is formed from three separate parts (i.e., the body,and the left and right
greater and lesser cornu) which fuse in early adulthood. The base of the "u"
shaped bone is located anteriorly while the cornu project posteriorly.
The hyoid is closely associated with the skull but is not strictly a part of it.
It anchors muscles, especially those of the tongue, and is the only bone in the
body which is not linked to another. The blood vessels into and out of the hyoid
bone are small branches of the external carotid artery, and are called "superior
thyroid arteries" and "superior thyroid veins."
The hyoid does not articulate with (i.e., touch) any other bones.
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