Handout on occipital bone : helpful for viva examination

Occipital

1.       Anatomical  points:

·         Occipital condyles are directed  downwards

·         Basilar part is directed forwards and upwards

·         External surface of squamous part is convex and directed backward

2.       Morphological type : Flat bone

3.       Structure of bone : inner and outer compact and middle diploe

4.       Ossification :  area of occipital bone above the highest nuchal line  is arisen from membranous ossification and rest of the parts of occipital bone is arisen from cartilaginous ossification

5.       Development : paraxial mesoderm

6.       Parts : Squamous part , basilar part and condylar part

Squamous part :

1.       Two surfaces : external and internal

External surface of squamous part

·         External occipital protuberance : gives attachment to the highest point of the ligamentum nuchae

·         Highest nuchal line : gives attachment to epicranial aponeurosis :

·         Superior nuchal line:  its

·          medial part attached with origin of trapezius muscle and

·          lateral part from before backwards attached with origin of occipital belly of occipito frontalis muscle and insertion of sternocleidomastoid and splenius capitis

·         Inferior nuchal line: medially:  rectus capitis posterior minor and

                                     Laterally rectus capitis posterior major  

·         External occipital crest : gives attachment of ligamentum nuchae

·         Area between superior and inferior nuchal line :

·          medially insertion of semispinalis capitis and

·          laterally obliquus capitis superior  

·         Area between inferior nuchal line and foramen magnum

·         Medially attachment of rectus capitis posterior minor and

·         laterally attachment of rectus capitis posterior major

Internal surface of squamous part of occipital bone

Internal occipital protuberance : an irregular elevation opposite the external occipital protuberance

Sagittal sulcus : lodges superior sagittal sinus and its margin of sulcus attached with falx cerebri

Transverse sulcus  (right and left)  : lodges  transverse sinuses  and its margin of sulcus attached with tentorium cerebella

Right transverse sinus is the continuous of superior sagittal sinus and

left transverse sinus is the continuous of straight sinus

Internal occipital crest lodges occipital sinus and its margin attached with falx cerebelli.

Fossae :

Two upper triangular fossae : lodge occipital lobes of the cerebrum with meninges

Two lower quadrilateral fossae : lodge cerebellar hemispheres

Vermian fossa: a small depression which lodge vermis of the cerebellum

Angles:

Superior angle :  is meeting points of occipital bone and two parietal bone

Lateral angle : is the meeting points of occipital bone, parietal bone and mastoid part of temporal bone

Two lambdoid borders: they are articulates with posterior border of occipital bone

Two mastoid borders: they are articulates with mastoid part of temporal bone

Basilar part of occipital bones

Parts

2.        Three surfaces and three borders

3.       Anterior surface: articulate with body of sphenoid , forming primary cartilaginous joint , ossifies by the end of the 25^th years

4.       Smooth Superior surface (also known as clivus) it is related with meninges , ligament , lower part of pons and medulla oblongata , near the foramen magnum it is related with following structures above downward

·         Membrana tectoria

1.       Upper band of crucial ligament

2.       Apical ligament

3.       Anterior atlanto –occipital membrane

Inferior surface of basilar part of occipital bone :

Pharyngeal tubercle: a small middle bony elevation attached with fibrous raphe of pharynx and superior constrictor muscle

Lateral to pharyngeal tubercle : insertion of longus capitis muscles

Behind the attachement of longus capitis or in front of occipital condyles : insertion of rectus capitis anterior

Borders of basilar part of occipital bones

Posterior border is formed the anterior margin of foramen magnum

Two Lateral borders : articulate with petrous part of temporal bone

Condylar part of occipital bone

Parts

Superior surface : contain jugular tubercle an oval eminence overlying the hypoglossal canal

Inferior surface:

Two Occipital condyles : articulate with superior articular processes of atlas forming atlanto –occipital joint

Hypoglossal canals : they are  situated anterior part of each condyle

 transmits :

1.       Hypoglossal nerve

2.       Meningeal branch of ascending pharyngeal artery 

3.       Emissary vein connecting basilar venous plexus with pterygoid venous plexus

Condylar fossa : a depression behind the condyle

Condylar canal occationally a foramen present within the condylar fossa which transmit emissary vein connect sigmoid sinus with veins of suboccipital triangle

Jugular process : it is situated just lateral to occipital condyles

Anterior surface of jugular process bears jugular notch which form jugular foramen by articulating with petrous part of temporal bone

Jugular foramen : transmitted

1.       Anterior compartment  :  inferior petrosal sinus

2.       Middle compartment : glossopharyngeal

Vagus nerve , accessory nerve 

Meningeal branch of ascending pharyngeal artery

Emissary vein

3.       Posterior compartment :

Sigmoid sinus continued as internal jugular vein

Foramen magnum:

Smaller anterior compartment transmits

1.       Apical ligament

2.       Upper band of cruciate ligament

3.       Membrane tectoria

Larger posterior compartment transmits

1.       Lower end of medulla oblongata

2.       Right and left vertebral artery

3.       Anterior and posterior spinal artery

4.       Spinal root of accessory nerve

5.       Sympathelic plexus of nerves

6.       Veins joining the venous plexus of medulla oblongata

7.       Lower part of tonsil of cerebellum

Jugular foramen:

1. glossopharyngeal nerve : mixed nerve : sensory supply motor supply
2. vagus nerve : mixed nerve : sensory supply  motor supply
3. Cranial root of accessory nerves: motor nerve. supply
4. Internal jugular vein