Download Drawing and Description of Skull: Frontal, Parietal, Occipital and

Drawing and Description of Skull:
Frontal, Parietal, Occipital and Temporal
Introduction
Osteology, the study of bone, is indispensable with physical anthropology which
is concerned with human origin, evolution and variation. During the course of evolution,
certain changes have taken place in the skeleton of man to attain the present form as
evident in the fossil records. To understand the phenomena one should be familiar with
the characteristics of different bones for which a sound knowledge of osteology is
required.
Frontal Bone
The frontal bone is a flat bone that resembles a cockleshell in form. It consists of two
portions – a vertical portion, the squama and an orbital or horizontal portion. The main
part of the frontal bone is formed by the squama. It corresponds to the region of the
forehead and also forms the anterior wall of the calvaria. It has external surface that is
convex and smooth while the internal surface is concave and deeply hollowed out. This
part is bounded below by the nasal notch in the middle and supra orbital margins on
each side; on the above by the parietal border and on either side by a prominent ridge
that is continuous with the temporal lines. The part of the external surface behind this
ridge and below the temporal line forms part of the temporal fossa.
Fig.1 Frontal bone
The external surface of the squama usually exhibits, in the lower part of the median
plane, the remains of the frontal or metopic suture; in infancy this suture divides the
bone into two, a condition which may persist throughout life (in 9% of cases). On either
side of this , about 3 cm above the supra orbital margin, is a rounded elevation, the
frontal tuber (or eminence ). Immediately below the frontal tuber but separated from the
supra orbital margin by a shallow groove, are two arched elevations- the supercilliary
arches which are joined together by a smooth elevation called the glabella. Beneath
each supercilliary arch is a curved and prominent margin, the supra orbital margin. It
forms the upper border of the orbit and also separates the squama from the orbital
portion of the bone. Its lateral part (2/3 of the margin) is sharp while the medial part (1/3
of the margin) is rounded. At the junction of these two is the supra orbital notch which is
sometimes converted into a foramen. Medial to it is another smaller frontal foramen or
notch. The supra orbital margin ends laterally in the strong and prominent zygomatic
process. Running upward and backward from this process is a well marked line, the
temporal line. The part of the frontal bone that lies below and behind the temporal line
forms the anterior part of the temporal fossa. Towards the middle, between the
supraorbital margins the squama projects downward to a level below the level of the
zygomatic process and it is known as the nasal part. It bears a rough, uneven interval,
the nasal notch. From the center of the notch the nasal part projects downward and
forward and ends below in a sharp spine.The internal surface of the squama is concave
and is marked in its upper part by a median groove, the sagittal sulcus, the edges of
which unite below to form a ridge, the frontal crest. The crest ends below in a small
notch and articulates with the ethmoid to form a foramen called the foramen cecum. The
size of the foramen cecum varies in different subjects. On either side of the middle line
the bone bears depressions for the convolutions of the brain.The orbital or horizontal
part consists of two thin triangular plates, the orbital plates, which are separated from
one another by a wide gap, the ethmoidal notch. The superior surface of the orbital
plates forms the greater part of the floor of the anterior cranial fossa, while the inferior
surface forms the greater portion of the vault / roof of the orbital cavity. The inferior
surface is smooth and concave and its lateral part shows a shallow depression the
lacrimal fossa for the lacrimal gland. Near the nasal part, the antero-medial part of the
inferior surface bears a small depression the trochlear fossa. The two orbital plates are
separated by the ethmoidal notch whose margins bear several depressions which form
the upper parts of ethmoidal air cells. The ethmoidal air cells become complete when
united with corresponding half-cells on the upper surface of the ethmoid. In front of the
ethmoidal notch, on either side of the frontal spine, are the openings of the frontal air
sinuses.
Articulation:
The frontal bone articulates with twelve bones- on the posterior side with the two
parietals and the greater wing of the sphenoid and through its zygomatic process with
the zygomaytic bone. The nasal part articulates with the two nasal bones and with the
frontal process of the maxillae. The spine and the orbital part join the ethmoid. The
orbital part also articulates with the greater and lesser wings of the sphenoid and with
the two lacrimals.
The Parietal Bone
A human cranium has two parietal bones– left and right. These two parietal bones unite
to form the greater part of the roof and side walls of the cranial cavity. Each bone is
irregularly quadrilateral in form; and has two surfaces, four borders, and four angles.
Fig.2 Parietal Bone
The external surface is convex, smooth and marked near the center by an eminence,
the parietal eminence (or tuber), which corresponds to the point where ossification
commenced. Crossing the middle of the bone from the anterior border in an arched
direction are two curved lines, the superior and inferior temporal lines. The part of the
bone below the inferior temporal line forms part of the temporal fossa, and affords
attachment to the Temporalis muscle. Near the posterior part and close to the upper or
sagittal border is the parietal foramen, a small which transmits a vein to the superior
sagittal sinus, and sometimes a small branch of the occipital artery; which is not
present consistently and varies considerably in size.The internal surface which is
concave bears depressions corresponding to the cerebral convolutions and numerous
furrows for the ramifications of the middle meningeal vessel. Along the upper margin is
a shallow groove, which, together with the groove on the opposite parietal, forms a
channel, the sagittal sulcus, for the superior sagittal sinus. In the groove is found the
internal opening of the parietal foramen when that aperture exists. The postero-inferior
angle also bears a groove for part of sigmoid sinus.The bone has four borders the sagittal border, squamous border, frontal border and occipital border of which
the sagittal border is the longest and thickest. It is dentated and articulates with its
fellow of the opposite side to form the sagittal suture. The inferior or 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 or anterior border is
deeply serrated and it articulates with the frontal bone, forming one half of the coronal
suture. The occipital border, deeply denticulated, articulates with the occipital, forming
one-half of the lambdoidal suture.The four borders meet to form four angles– the frontal
angle, sphenoidal angle, mastoid angle and the occipiatal angle. The frontal angle is
practically a right angle and corresponds with the point of meeting of the sagittal and
coronal sutures. The sphenoidal angle which is thin and acute is received into the
interval between the frontal bone and the great wing of the sphenoid. 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. The mastoid angle is truncated
and articulates with the occipital bone and the mastoid portion of the temporal. It
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.
Articulations:
The parietal articulates with five bones: the opposite parietal, the occipital, frontal,
temporal, and sphenoid.
Side identification:
The side to which a given parietal bone belongs can be determined from the following
points.
1. The sagittal border is straight, while the inferior border is irregular (oblique) in
shape.
2. The external or lateral surface is convex , while the internal surface is concave
3. The antero-inferior (sphenoidal) angle is acute from this the front side of the bone
can be ascertained.
The occipital bone
The occipital bone is a flat bone that lies at the rear of the cranium. It is trapezoid in
shape and forms the back and lower part of the cranium. The bone is pierced by a large
oval aperture, the foramen magnum. The expanded part of the bone above and behind
the foramen magnum is the squamous part; the thick, somewhat quadrilateral piece in
front of the foramen is called the basilar part and the parts on either side of the foramen
are the lateral or condylar parts.The squamous part contributes to the posterior wall of
the cranial cavity. It has external surface and internal surface. The external surface is
convex and bears a prominence called the external occipital protuberance midway
between the summit of the bone and the foramen magnum. Running laterally from this
on either side are two curved lines, one a little above the other. The upper is named
the highest nuchal line and the lower is termed the superior nuchal line. The highest
nuchal line is often faintly marked and not always present. The part of the squama
which lies above the superior nuchal lines is rough and irregular and provides space for
the attachment of several muscles. Below the external occipital protuberance there is a
median ridge or crest, the external occipital crest. Running laterally from the crest
across either half of the nuchal plane is the inferior nuchal line. The internal surface of
the squama is deeply concave and marked by four fossae – two in the upper and two in
the lower. The upper two fossae are triangular and lodge the occipital lobes of the
cerebrum; the lower two are quadrilateral and accommodate the hemispheres of the
cerebellum. The four fossae meet to form the internal occipital protuberance. Above the
protuberance, between the upper two fossae, there is a median groove called the
sagittal sulcus. On either side of the internal occipital protuberance there is an equally
wide groove for the transverse sinus. Inferior to the protuberance, the part of the bone
that divides the two lower fossae is prominent and bears a crest called the internal
occipital crest.
Fig.3 occipital bone
The lateral part of the occipital bone has superior and inferior surfaces. Lateral to the
foramen magnum, the lower surface bears the occipital condyles for articulation with the
superior facets of the atlas. The condyles are oval or reniform in shape, and their
anterior extremities direct forward and medially. The articular surfaces of the condyles
are convex. There are two canals closely related to the occipital condyles. At the base
of either condyle the bone is tunnelled by a short canal, the hypoglossal canal . This
begins on the cranial surface of the bone immediately above the foramen magnum, and
is directed lateralward and forward above the condyle. Behind either condyle there is a
depression, the condyloid fossa, in which posterior condylar cannal is sometimes seen.
Lateral to the condyle is a quadrilateral plate of bone called the jugular process. This
process is excavated in front by the jugular notch and forms the posterior part of the
jugular foramen in the articulated skull. The under surface of the jugular process is
rough and gives attachment surface for muscles. The superior surface of the lateral part
forms part of the floor of the cranial fossa. It presents an oval eminence, the jugular
tubercle. The superior surface of the jugular process bears a deep groove which is
continuous with the jugular notch. The basilar part of the occipital bone is continuous
with the sphenoid but in case of a young cranium the two are separated by a cartilage.
The upper surface bears a broad, shallow groove which inclines upward forming the
median portion of the anterior wall of the posterior cranial fossa. The lower surface
bears the pharyngeal tubercle which gives attachment to the fibrous raphé of the
pharynx.
Articulations:
The occipital articulates with six bones: the two parietals, the two temporals, the
sphenoid, and the atlas
.
Temporal bone
The temporal bone is an irregular bone. The temporal bones numbering two are
situated at the sides and base of the skull. Each bone consists of five parts, viz.,
the squama, the petrous, the mastoid, the tympanic and the styloid process.
The squama forms the lateral wall and base of the cranial cavity. It has internal and
external surfaces. Arising from its external aspect there is a zygomatic process whose
anterior end joins the temporal process of the zygomatic bone to form the zygomatic
arch. The zygomatic process is connected to the squama by two roots, the anterior and
posterior roots. The anterior root of the process, continuous with the lower border, is
short but broad and strong. It is directed towards the medial and ends in a rounded
eminence, the articular tubercle. Behind the anterior root is an oval depression forming
part of the mandibular fossa to receive the condyle of the mandible. The mandibular
fossa (glenoid fossa) is bounded in the front by the articular tubercle; behind, by the
tympanic part of the bone. There is another projection behind the mandibular fossa
called the postglenoid tubercle. The posterior root, an extension of the upper border, is
strongly marked and it runs backwards and upwards forming the supramastoidal crest
that is continuous with the temporal line. This crest separates the squama and mastoid
part of the temporal bone. Below the posterior root, there is an opening of a bony
tunnel called the external acoustic meatus which forms part of the external ear.
Between the posterior wall of the external acoustic meatus and the posterior root of the
zygomatic process there is an area called the suprameatal triangle or mastoid fossa.
The internal surface of the squama is concave and it forms the lateral wall and the
lateral portion of the middle cranial fossa. It bears depressions corresponding to the
convolutions of the temporal lobe of the brain, and grooves for the branches of vessels.
Fig.3 Temporal bone
The mastoid part of the temporal bone lies behind the external acoustic meatus and it
forms the posterior part of the bone. In the young it is separated from the squama but
fused in the later age and remnants of the suture may be seen in some adults. The
bone presents a large downward conical projection called the mastoid process. Medial
to the mastoid process there is a deep mastoid notch. Medial to this there is a shallow
furrow, the occipital groove. Near the anterior end of the notch is found the stylomastoid
notch. The bone is perforated by a large mastoid foramen near the posterior border.
The inner surface of the mastoid portion presents a deep, curved groove, the sigmoid
sulcus, which lodges part of the transverse sinus and the internal opening of the
mastoid foramen is also seen. The petrous part of the temporal bone is pyramidal and is
wedged in at the base of the craium between the sphenoid and occipital. It has a base,
an apex, three surfaces, and contains, in its interior, the essential parts of the organ of
hearing. The base is fused with the internal surfaces of the squama and mastoid portion
The apex, rough and uneven, is wedged into the angular interval between the posterior
border of the great wing of the sphenoid and the basilar part of the occipital; it presents
the anterior or internal orifice of the carotid canal. The anterior surface forms the sloping
posterior part of the floor of the middle cranial fossa. The posterior surface forms the
lateral part of the sloping anterior wall of the posterior cranial fossa. This surface
presents the opening of the internal acoustic meatus. The anterior and posterior
surfaces are separated by sharp superior border. The inferior surface is rough and
irregular and bears many prominent structures. Some of them are the carotid canal,
jugular fossa, jugular canal, stylomastoid foramen.The tympanic part of the temporal
bone is in the form of a plate called the tympanic plate. It lies between the mandibular
fossa and the external acoustic meatus. It forms the anterior wall, lower surface and the
lower part of the posterior wall of the external acoustic meatus. The margin of the plate
is rough that provides attachment to cartilages. On the back side, the tympanic part of
the temporal bone forms sheath for the base of the styloid process.The styloid part of
the temporal bone is found in the form of a pointed process. It is slender, pointed and
variable in length (usually about 2.5 cm). It is directed downwards and forwards. Its
base is enclosed by a sheath on the postero-inferior aspect of the tympanic plate.
Articulations:
The temporal articulates with five bones: the mandible, the zygomatic, the sphenoid, the
occipital and the parietal.
Side identification:
The side to which a given temporal bone belongs can be determined from the following
points.
1. The external acoustic meatus and the zygomatic process are found on the lateral
side of the bone.
2. The zygomatic process is directed towards the front.
3. The mastoid and styloid processes point downwards.
Conclusion
Osteology is one of the most useful techniques available for palaeoanthropology and
forensic anthropology. The reason for this importance is that both palaeoanthropology
and forensic anthropology deal with complete and fragmented human remains. It is
through osteology that researchers can determine the age, sex, stature, length of time
since death, probable racial affiliation, and activity patterns of the person to whom the
bone to be studied belonged. It also helps in the study of human evolution. The human
cranium and its parts are not only frequently encountered in archaeological remains but
also found to be the most telling of all of the bones.