Parietal Bones of Human Skull | Skeletal System | Biology

In this article we will discuss about:- 1. Anatomical Structure and Surfaces of Parietal Bones 2. Angles of the Parietal Bones 3. Parts.

Anatomical Structure and Surfaces of Parietal Bones:

There are two parietal bones present on the top of skull, develops from membrane. Each bone comes in contact – to all the lobes of cerebral hemisphere.

It has two surfaces – External and internal surfaces.

Four borders – Anterior, posterior, superior and inferior borders.

Four angles – Antero superior, antero inferior, posterosuperior and postero inferior angles.

External Surface:

It is convex.

It shows:

i. Parietal Tuberosity:

Lies near the centre of the bone – it is the site of maximum convexity.

ii. Vertex:

It forms the maximum prominent point on the top of skull.

iii. Temporal Lines:

They are two superior and inferior temporal lines – gives attachment to temporal fascia and temporalis muscle.

iv. Parietal Emissary Foramen:

It lies anterior to posterior border but lateral to the superior border. An emissary vein passes through it, connecting the superior sagittal sinus with the veins of the scalp.

v. Internal Surface:

Is concave and is grooved by the anterior and posterior branches of middle meningeal vessels.

Along the superior border – groove for superior sagittal sinus is present. Along this groove depressions or pits lie, lodging arachnoid granulations.

Inner aspect of postero inferior angle has a groove for the transverse sinus.

vi. Anterior Border:

Is thick and serrated, articulates with the frontal bone and forms coronal suture.

vii. Posterior Border:

Also thick and serrated, articulates with squamous part of occipital bone and form lambdoid suture.

Lambda is the meeting point of two parietal and occipital bone, i.e., junction of sagittal and lambdoid suture. In the foetal skull it is repre­sented as the posterior fontenelle.

viii.Superior Border:

Is thick and serrated articulates with the parietal bone of opposite side at the sagittal suture.

ix. Inferior Border:

Is concave and divided into three parts. The anterior part articulates with the greater wing of sphenoid at pterion.

i. Intermediate part articulates with the squamous part of the temporal bone.

ii. Posterior part is the thickest part articulates with the mastoid part of temporal bone.

Angles of the Parietal Bones:

a. Antero-Superior Angle:

Lies where sagittal and coronal sutures meet, and forms bregma. In foetus it forms anterior fontenelle.

b. Antero Inferior Angle:

Joins at pterion – This represents antero lateral fontenelle of the foetus.

At this point, following features are seen:

i. Anterior branch of middle meningeal artery.

ii. Lateral sulcus of cerebrum divides into three parts.

iii. Motor speech area lies.

c. Postero Superior Angle:

Represents lambda and posterior fontenelle of the foetus.

d. Postero Inferior Angle:

Takes part in formation of asterion. It is the meeting point of parietal, occipital and temporal bones. In foetus, it forms postero lateral fontenelle.

e. Ossification:

Two primary centers appear at 7th week of intra uterine life. One is situated above the parietal tuberosity and another lies on the tuberosity. Both centers fuse and ossification spreads.

Ossification is of two types- Membranous and Cartilaginous type.

(a) Primary centre of ossification appears above the external occipital protuberance at 8th week of intra uterine life in membrane.

(b) Below the external occipital protuberance cartilagenous type of ossification – Two centres for squamous part appear at 7th week, Two centres for occipital condyle appear at 8th week, One centre for basilar part appears at 6th week.

(c) Occipital condyles unite with the lower squa­mous part by 2nd year.

(d) The basilar part unites with the occipital condyles by 6th year.

f. Occipital Bone:

It is unpaired skull bone, lies posteriorly. It bounds the posterior cranial fossa and has a large foramen called foramen magnum.

Parts of the Parietal Bones:

1. Squamous part

2. Condylar part

3. Basilar part.

1. Squamous Part:

Is situated above and posterior to foramen magnum. It is the largest part, has a convex posterior surface and concave anterior surface (inner).

a. External Occipital Protuberance (Inion):

An elevation lies in the middle of external surface. Deep to it lies confluence of sinuses. The trapezius muscle and ligamentum nuchae is attached to it.

b. Highest Nuchal Line:

Runs laterally on either sides of the inion, commences from external occipital protuberance.

c. Superior Nuchal Line:

On either side of external occipital protuberance, a curved line situated called superior nuchal line lies just below the highest nuchal line. It separates the scalp from the back of neck and gives attachment to trapezius and occipital belly of the occipito frontalis muscle.

d. External Occipital Crest:

It extends from the external occipital protuberance to the posterior border of the foramen magnum, lies in the midline. It gives attachment to the ligamentum nuchae.

e. Inferior Nuchal Line:

Extends from the middle of the occipital crest, runs below the superior nuchal line, and gives insertion to rectus capitis posterior minor.

The medial part of the interval between superior and inferior nuchal lines – semispinalis capitis is inserted. Lateral part of this area gives insertion for obliquus capitis superior.

Inner Surface of Squamous Part:

Main features are:

i. Internal occipital protuberance – centrally placed.

ii. From here cruciform shaped lines (sulcus) radiate superiorly, inferiorly and transversely, lodging superior sagittal sinus above right and left transverse sinus and occipital sinus from below. Margins give attachment to falx cerebri superiorly, tentorium cerebelli transversely and falx cerebelli inferiorly.

Internal Occipital Crest:

Is the lower vertical part from internal occipital protuberance to the posterior margin of the foramen magnum it forms a triangular depression called vermian fossa. It gives attachment to falx cerebelli and lodges occipital sinus.

Angles of the Squamous Part:

Superior angle joins lambda, the lateral angles meet the mastoid part of the temporal bone and postero inferior angle of parietal bone at asterion.

Borders of the Squamous Part:

Lambdoid border joins with the posterior border of parietal bone and forms lambdoid suture, mastoid border articulates with the mastoid part of the temporal bone.

2. Condylar Part:

Lies lateral to occipital condyles. Each part shows superior and inferior surfaces.

Features:

On inferior surface of condylar parts are:

1. Occipital condyle

2. Condylar fossa

3. Condylar canal

4. Hypoglossal canal

5. Jugular process.

1. Occipital Condyles:

These are oval or kidney shaped structure situated below the foramen magnum.

2. Condylar Fossa:

It is a depression situated behind the occipital condyles, occasionally condylar canal opens into this fossa.

3. Condylar Canal:

Not always present. When present it transmits an emissary vein connecting sigmoid sinus and sub occipital plexus of veins.

4. Hypoglossal Canal:

Lies at the junction of basilar part and occipital condyle.

It transmits:

a. Hypoglossal nerve.

b. Meningeal branch of ascending pharyngeal artery.

c. Emissary vein – connecting basilar plexus of veins with the pharyngeal venous plexus.

5. Jugular Process:

It lies lateral to occipital condyle, quadrangular shaped bar of bone.

The upper and anterior part of the jugular process is grooved by the sigmoid sinus called jugular notch. Notch is converted into a foramen by petrous part of temporal bone – called jugular foramen. It transmits – sigmoid sinus, inferior petrosal sinus, glossopharyngeal (IX), vagus (X) and (XIth) accessory cranial nerves, an emissary vein and meningeal branch of occipital artery.

Superior surface of condylar part – shows a smooth elevation called jugular tubercle which is grooved by IXth, Xth and XIth cranial nerves.

Foramen Magnum:

Oval shaped largest foramen, bounded anteriorly by basilar part of occipital bone, posteriorly by squamous part and laterally by condylar part of occipital bone.

Communication:

It communicates posterior cra­nial fossa with the vertebral canal.

Structures Passing:

Foramen is divided into anterior and posterior compartments by the attachment of alar ligaments.

Anterior Compartment Transmits:

i. Apical ligament

ii. Upper part of cruciate ligament

iii. Membrana tectoria.

Posterior Compartment Transmits:

i. Lower part of medulla oblongata to become spinal cord.

ii. Meninges and cerebro spinal fluid.

iii. Tonsil of cerebellum.

iv. Right and left vertebral arteries.

v. Anterior (one) and two posterior spinal arteries.

vi. Spinal root of accessory nerve.

vii. Sympathetic plexus around the blood vessels.

viii. Veins accompanying the arteries.

ix. First teeth of ligamentum denticulatum.

Along the anterior border of foramen mag­num the anterior atlanto occipital membrane is attached and along the posterior border – posterior atlanto occipital membrane is at­tached.

3. Basilar Part:

Extends forwards and upwards from the foramen magnum. It is thick and narrow anteriorly articulates with the body of sphenoid to form spheno occipital synchondroses.

Superior Surface (Clivus):

It is concave, related to pons and medulla. Vertebral arteries, basilar artery and basilar plexus of veins are present on clivus.

The upper surface gives attachments to the ligaments entering the cranium through foramen magnum are:

a. Membrana tectoria

b. Superior band of cruciate ligament

c. Apical ligament.

The lower surface of basilar part shows pharyngeal tubercle, gives attachment to median pharyngeal raphe and superior constrictor of pharynx.