In this article we will discuss about the various parts of sphenoid bone which belongs to human skull.
It resembles a bat or butterfly with outstretched wings. It takes part in the formation of orbit, middle cranial fossa, infra temporal, temporal and pterygopalatine fossae. It is a pneumatic bone – containing sphenoidal air sinus in its body.
Parts of the Sphenoid Bone:
It has- A centrally placed body, A pair of greater wings, A pair of lesser wings, A pair of pterygoid processes.
Body:
Contains a pair of sphenoidal air sinuses separated by a septum.
It shows following surfaces:
1. Superior Surface:
Shows a small bony projection from the anterior part which articulates with the cribriform plate of ethmoid bone – it is called ethmoidal spine.
Jugum Sphenoidale:
Is smooth lies behind the ethmoidal spine, laterally limited by the olfactory grooves lodging olfactory tract.
Sulcus Chiasmaticus:
Is a transverse groove lies behind the jugum sphenoidale, laterally it ends to the optic canal. Optic chiasma lies postero superior to the sulcus.
Tuberculum Sellae:
It is an oval elevation situated in between chiasmatic sulcus and sella turcica.
Sella Turcica:
It is a deep depression behind the tuberculum sellae. It lodges hypophysis cerebri. The upper surface of sella is covered by the diaphragma sellae.
Dorsum sellae limited above and laterally by posterior clinoid process – forms posterior boundary of fossa.
Posterior clinoid process gives attachments to the attached border of tentorium cerebelli.
Petrosal Processes:
Are the lateral parts of the dorsum sellae – articulates with the apex of petrous temporal bone.
Clivus:
It is the downward sloping posterior surface of dorsum sellae. It articulates with the basilar part of occipital bone. Pons is related to clivus.
Inferior Surface of Body:
Is also called nasal surface – it forms the part of the roof of the nasal cavity. In the midline a bony process called rostrum – articulates with the vomer, in the nasal septum.
Anterior Surface of Body:
Sphenoidal crest is present in the midline – articulates with the perpendicular plate of the ethmoid bone and vomer.
Lateral Surface of the Body:
Fuses with the pterygoid process and greater wing of sphenoid bone. Carotid sulcus is present lodging internal carotid artery and cavernous sinus.
2. Greater Wing of Sphenoid:
Arise from body, directed upwards and laterally. Posteriorly it articulates with the petrous and squamous part of the temporal bone. It has three surfaces – superior, inferior and lateral surface.
Superior surface forms the floor of the middle cranial fossa and has foramina – rotundum, foramen ovale and foramen spinosum.
Foramen Rotundum:
Is directed forwards and opens into pterygo palatine fossa – it transmits maxillary nerve.
Foramen Ovale:
Is oval, large foramen communicates middle cranial fossa to infra temporal fossa.
It transmits:
1. Sensory and motor roots of mandibular nerve
2. Accessory middle meningeal artery
3. Lesser superficial petrosal nerve
4. Emissary vein – communicating cavernous sinus and pterygoid venous plexus.
Foramen Spinosum:
It communicates middle cranial fossa and infra temporal fossa and transmits middle meningeal vessels and nervi spinosus.
Foramen of Vesalius:
Not always present. It transmits an emissary vein connecting cavernous sinus and pterygoid venous plexus, when present.
Lateral Surface of Greater Wing:
Is divided into temporal and infra temporal surfaces by the infra temporal crest. Temporal surface gives origin to temporalis muscle and takes part in the formation of the temporal fossa.
Infra Temporal Surface:
Forms the roof of the infra temporal fossa and inferior openings of the foramina are present. Spine of the sphenoid is present gives attachment to the spheno mandibular ligament and is related to chorda tympani nerve medially and auriculotemporal nerve laterally.
Infra temporal crest gives origin to lateral pterygoid muscle.
Orbital Surface of the Greater Wing:
Forms a part of lateral wall of the orbit and is quadrangular shaped. Medial border bounds the superior orbital fissure inferiorly. In the middle of this border a tubercle is situated – gives attachment to the common tendinous ring. Below the fissure this orbital surface is pierced by the foramen rotundum.
Pterion:
It is the meeting point of greater wing of sphenoid, parietal bone, frontal and temporal bones. It is deeply related to middle meningeal vessels.
Foramen Lacerum:
Is situated behind the greater wing of sphenoid but lies anterior to the apex of petrous temporal bone.
3. Lesser Wing of Sphenoid:
Arises from the body and is triangular in shape, forms a part of floor of anterior cranial fossa and roof of the orbit. Superior orbital fissure lies below it. Superior surface is related to frontal lobe and inferior surface gives origin to levator palpebrae superioris and superior oblique muscles.
Spheno parietal sinus (venous) is related to its posterior border. Medially this border projects to form the anterior clinoid process.
Medial end of the lesser wing divides and encloses the optic canal – which transmits – optic nerve with meninges and ophthalmic artery with its sympathetic plexus.
Superior Orbital Fissure:
Communicates orbit and middle cranial fossa. It is triangular in shape situated at the apex of the orbit.
Boundaries:
Superior – Lesser wing of sphenoid.
Inferior – Greater wing of sphenoid.
Medial – Body of sphenoid.
Lateral – Frontal bone.
Structures Passing through Superior Orbital Fissure:
Common tendinous ring divides this fissure into three parts:
A. Lateral Part:
It transmits from lateral to medial:
(a) Recurrent branch of lacrimal artery
(b) Lacrimal nerve
(c) Frontal nerve
(d) Trochlear nerve
(e) Superior ophthalmic vein.
B. Intermediate Part:
Within the tendinous ring – transmits:
(a) Upper division of oculomtor nerve.
(b) Nasociliary nerve.
(c) Lower division of oculomotor nerve.
(d) Abducent nerve.
C. Medial Part:
Transmits inferior ophthalmic vein.
4. Pterygoid Process:
It arises where the greater wing meets the body of sphenoid. It has medial and lateral pterygoid plates – separated by pterygoid fossa postero inferiorly. Anteriorly both plates fuse in its upper part and forms posterior boundary of pterygo palatine fossa.
Scaphoid Fossa:
Is shallow depression lies at the upper end of medial pterygoid plate – gives origin to levator veli palatini muscle.
Lateral Pterygoid Plate:
Forms medial boundary of infra temporal fossa. Its lateral surface gives origin to lateral pterygoid muscle, and medial surface gives origin to medial pterygoid muscle. The upper part of the anterior border forms posterior boundary of pterygo maxillary fissure.
Medial Pterygoid Plate:
Is larger, its lower end forms pterygoid hamulus, gives origin to superior constrictor muscle of the pharynx and pterygo mandibular raphe is attached to it. Medial pterygoid forms lateral wall of posterior nasal opening. Pharyngo basilar fascia is attached to its posterior border.
The upper part of medial pterygoid plate forms the vaginal process – articulates with the sphenoidal process of palatine bone and with vomer. Between vaginal process and sphenoidal process of palatine bone a palato vaginal canal lies which transmits pharyngeal branches of pterygo palatine ganglion and maxillary artery.
Sphenoidal Concha:
A pair of curved thin plates is situated along the anterior aspect of the floor of sphenoidal sinus. It has a vertical portion situated anteriorly and a horizontal portion situated posteriorly.
Anterior vertical portion of both sides meet in the midline and forms sphenoidal crest.
Posterior horizontal portion is triangular forms roof of the nose and bounds spheno palatine foramen.
Ossification:
The tuberculum sellae divides the developing sphenoid into pre-sphenoidal and post- sphenoidal parts. Two parts remain separate upto 7-8 months of intra uterine life.
Pre-sphenoidal part forms tuberculum sellae and lesser wings sphenoidal concha.
Post-sphenoidal part forms pterygoid process, greater wing of sphenoid, sellaturcica and dorsum sellae.