Here are your notes on the Sphenoid Bone of human skull!
The sphenoid bone is an unpaired pneumatic irregular bone, situated at the base of the skull, and wedged between the frontal and ethmoid bones in front, basi-occiput and petrous part of temporal bones behind, and squamous part of temporal bone on each side.
The sphenoid consists of a central body, and pairs of greater and lesser wings and pterygoid processes. It resembles a bat with wings outstretched and stands on pterygoid processes which represent its legs (Fig. 1.9, 1.10).
It is cuboidal in shape and contains a pair of sphenoidal air sinuses, separated by a median partition which is usually deflected to one side. The body presents six surfaces – superior, inferior, anterior, posterior and a pair of lateral surfaces.
Superior (cerebral) surface presents from before backwards the following features:
1. Ethmoidal spine—It forms a triangular projection between the two lesser wings and articulates with the posterior margin of the cribriform plate of ethmoid bone.
2. Jugum sphenoidale is a flattened plate of bone upon which rest on each side gyrus rectus of the frontal lobe and olfactory tract.
3. Sulcus chiasmaticus is a shallow transverse groove which leads on each side to the optic canal. Anterior part of the optic chiasma lies slightly above the sulcus.
4. Tuberculum sellae is the posterior limit of sulcus chiasmaticus, and gives anterior attachment to the diaphragma sellae which forms the dural roof of hypophyseal fossa. On each side the tuberculum presents a small projection, the middle clinoid process, which is connected to the anterior clinoid process by the carotido-clinoid ligament. The carotido-clinoid foramen thus formed between the ligament and the body of sphenoid transmits upturned course of internal carotid artery. Occasionally the carotido-clinoid ligament is converted into a bone.
5. Hypophyseal fossa is a deep depression which contains the hypophysis cerebri or pituitary gland. The floor of the fossa is related to the sphenoidal sinuses.
A small median cranio-pharyngeal canal is occasionally observed in the floor of the fossa and is believed to represent a remnant of Rathke’s pouch from which the anterior lobe of pituitary gland is developed. Arey (1950), however, opines that the canal appears secondarily for the passage of blood vessels.
6. Dorsum sellae is a square-like plate of bone which projects upward and presents a conical projection, the posterior clinoid process, on each side; the latter gives attachment to the anterior end of the attached margin of tentorium cerebelli and is connected with the anterior clinoid process of the lesser wing by the interclinoid ligament. The upper margin of dorsum sellae provides posterior attachment to the diaphrgma sellae.
Below the posterior clinoid process, the lateral margin of dorsum sellae presents a petrosal process which is connected to the tip of the petrous temporal by a petrosphenoid ligament; immediately below this ligament the abducent nerve pierces the dura mater and appears in the cavernous sinus.
Posterior surface of the dorsum sellae slopes downward and backward, and is continuous with the upper surface of the basi-occiput in adults to form the clivus, upon which rests the upper part of the pons.
The sella turcica resembles a Turkish saddle and is a collective name consisting of tuberculum sellae, hypophyseal fossa and dorsum sellae from before backwards.
Inferior surface of the body presents three features: Sphenoidal rostrum, pairs of sphenoidal conchae (lower part) and vaginal processes.
1. Sphenoidal rostrum projects downward as a median ridge and is continuous with the lower end of sphenoidal crest (vide infra.) It articulates with the grooved upper margin of the vomer between the diverging alae.
2. Lower horizontal part of each sphenoidal concha extends backward as a triangular plate by the side of the rostrum and articulates with the upper surface of the alae of vomer.
3. The vaginal process projects downward and medially as a narrow triangular shelf from the base of each medial pterygoid plate and lies below the horizontal part of the concha. Upper surface of the vaginal process articulates with the lateral margin of the alae of the vomer in an overlapping manner and forms a vomero-vaginal canal which does not transmit any significant structure.
Anterior margin of the vaginal process and the adjoining lower surface articulate with the upper surface of sphenoidal process of palatine bone and form the palatino-vaginal (pharyngeal) canal which transmits pharyngeal vessels and nerve. Therefore, vomerovaginal canal lies above, and palatino-vaginal canal below the vaginal process.
Anterior surface of the body presents three features: sphenoidal crest, a pair of sphenoidal conchae (upper vertical part) and articular rough area involving each concha.
1. Sphenoidal crest is placed vertically as a median ridge, articulates with the posterior border of perpendicular plate of ethmoid bone and forms a part of nasal septum.
2. Upper vertical parts of sphenoidal conchae lie on each side of the crest and complete the anterior surface of the body. Lower and medial part of each concha is smooth and presents a sphenoidal foramen through which each sphenoidal sinus communicates with the sphenoethmoidal recess of the superior meatus of nasal cavity.
3. Above and lateral to the sphenoidal foramen, a quadrilateral or triangular rough area affects each concha, and articulates with the posterior surface of the labyrinth of ethmoid and orbital process of the palatine bones.
Posterior surface of the body is rough and quadrilateral, and articulates by a plate of hyaline cartilage with the anterior surface of the basi-occiput forming a primary cartilaginous joint, which is replaced by bone usually between 20 and 25 years. If bony fusion takes place at an earlier age, say at 15 years, then the antero-posterior growth of the base of skull will be arrested so that the maxillary teeth of upper jaw fail to adjust with the teeth of lower jaw because the growth of the mandible takes place at the normal rate.
Each lateral surface of the body joins with the greater wing projecting laterally and with the pterygoid process extending downward. Above the root of greater wing, the lateral surface is marked by an italic “f” shaped curved groove, the carotid sulcus, for the accommodation of internal carotid artery surrounded by a plexus of sympathetic nerves; it is also related with the cavernous sinus and a series of structures within the sinus.
The carotid sulcus is more deep in the posterior part, because it is overhung medially by the petrosal process and laterally by a raised margin, the lingula.
Each greater wing projects laterally and then upward as a strong curved process from the side of the body. It presents three surfaces-cerebral (upper), lateral and orbital; two borders – posterior and squamosal; a tip and a triangular rough area medial to the tip, and a spine of sphenoid.
The cerebral (upper) surface is deeply concave, forms part of the floor of middle cranial fossa and accommodates the temporal lobe of the brain.
This surface presents the following foramina from before backwards arranged in a crescentic line:
1. Foramen rotundum:
It is a round canal extending forward from the middle cranial fossa to the pterygo-palatine fossa, and transmits the maxillary nerve.
2. Foramen ovale:
It is a fairly large oval aperture lying posterolateral to the foramen rotundum and opens into the infra-temporal fossa. The foramen ovale transmits—sensory part of mandibular nerve along with motor root of trigeminal nerve (before forming the mandibular nerve trunk in the infra-temporal fossa), accessory meningeal artery, lesser (superficial) petrosal nerve (occasional), emissary veins connecting cavernous sinus with pterygoid venous plexus, and sometimes the anterior trunk of middle meningeal sinus.
3. Foramen spinosum:
It lies close to and postero-lateral to oval foramen separated by a bar of bone, and transmits middle meningeal artery, nervous spinosus and occasionally the posterior trunk of middle meningeal sinus.
4. Canaliculus innominatus:
It is occasionally present in the bony bar between the foramina spinosum and ovale, and transmits the lesser petrosal nerve, which does not pass through the foramen ovale in such condition.
5. Emissary sphenoidal foramen:
Foramen Vesalii is occasionally found medial to foramen ovale, and gives passage to the emissary vein connecting the cavernous sinus to the pterygoid venous plexus.
The lateral surface of each greater wing is divided by a horizontal infra-temporal crest into upper and lower areas. The upper area forms a part of temporal fossa and gives origin to the temporalis muscle. The lower area including the infra-temporal crest forms the roof of the infra-temporal fossa and gives origin to the upper head of lateral pterygoid muscle (Fig. 1.11).
The orbital surface of the greater wing is smooth, quadrilateral and forms the posterior part of lateral wall of the orbit. Its upper and lateral margins are articular, join respectively with the orbital plate of frontal bone and the posteromedial margin of zygomatic bone. Lower and medial margins of the orbital surface are free and non-articular. The lower margin forms the posterolateral boundary of the inferior orbital fissure.
The medial margin forms the infero- lateral boundary of the superior orbital fissure which is a retort-shaped interval between greater and lesser wings of the sphenoid. A small tubercle of the medial margin gives attachment to a common tendinous ring, which extends across the superior orbital fissure and gains attachment around the upper and medial margins of the optic canal on the undersurface of the lesser wing. The tendinous ring gives origin to the four recti muscles of the eye ball and divides the superior fissure into three parts.
The posterior border of each greater wing is non-articular in the medial part and articular in the lateral part. The medial part forms the anterior boundary offoramen lacerum and presents the posterior opening of pterygoid canal transmitting pterygoid vessels and nerve. The foramen lacerum is bounded posteriorly by the apex of petrous temporal and is occupied by the anterior opening of carotid canal through which the internal carotid artery emerges and appears in the carotid sulcus after passing through the upper opening of foramen lacerum. (For further details see the Norma basalis). The lateral part of the posterior border articulates with the anterior border of petrous-temporal; along the line of articulation a sulcus tubae is formed in the exterior of the base of the skull for the lodgement of the cartilaginous part of the auditory tube.
The squamosal border of greater wing articulates with the antero-inferior margin of the squamous part of temporal bone.
The tip of greater wing is bevelled on the inner surface and overlaps for articulation with the sphenoidal angle of the corresponding parietal bone forming the region of pterion. The triangular rough area medial to the tip articulates with the frontal bone.
Spine of the sphenoid:
It is a small pointed process projecting downward from the junction of posterior and squamosal borders of greater wing. The spine provides the attachments to three ligaments, two muscles, and related to important structures on its medial and lateral sides (Fig. 1.12).
The ligaments are:
(i) Spheno-mandibular, extending from the spine to the lingula of mandibular foramen; it is developed from the fibrous envelope of Meckel’s cartilage of first branchial arch, cartilage cells being absorbed;
(ii) Anterior ligament of malleus, extends from the anterior process of malleus to the spine after passing through the medial end of petrotympanic fissure;
(iii) Pterygospinous, extends from the spine to a tubercle at the posterior border of the lateral pterygoid plate.
The muscles attached to the spine are:
(i) Tensor veli palatini, arising from anterior margin and adjoining medial surface;
(ii) Tensor tympani, arising from medial surface and the adjoining posterior margin.
Relations of the spine:
Medially, cartilaginous part of auditory tube and chorda tympani nerve; Laterally, auriculo-temporal nerve.
Each wing is a triangular plate of bone projecting laterally from the upper and anterior part of the side of the body, and connected to the latter by anterior and posterior roots which enclose between them and the sphenoidal body the optic canal. The optic canal transmits optic nerve and ophthalmic artery, infero-lateral to the nerve.
The lesser wing presents anterior and posterior borders, and upper and lower surfaces.
The anterior border articulates with the posterior margin of the orbital plate of frontal bone. The posterior border is free and concave, forms the junction of the floor between anterior and middle cranial fossae, and projects into the stem of the lateral sulcus of cerebral hemisphere. The spheno-parietal sinus lies along the undersurface of the posterior border. Medial end of each posterior border projects backward as a prominent anterior clinoid process, which gives attachments to the following:
(i) Anterior end of the free margin of the tentorium cerebelli;
(ii) Carotido-clinoid ligament, connecting with middle clinoid process;
(iii) Inter-clinoid ligament, extending between anterior and posterior clinoid processes.
The upper surface of lesser wing forms a part of the floor of anterior cranial fossa and is related to the orbital surface of the frontal lobe. The lower surface forms posterior part of the roof of orbit and presents the anterior opening of optic canal.
Just outside the upper and medial margins of the optic canal, it gives attachment to a part of common tendinous ring for the origins of the four recti muscles of eye ball. Outside the tendinous ring the lower surface provides separate origins to levator palpabrae superioris and more medially the superior oblique muscle of eye ball.
Superior orbital fissure:
It is a retort-shaped cleft between the lesser and greater wings. (For further details see the interior of the base of skull).
These extend vertically downwards from the junctions of the root of greater wings and the body. Each process consists of lateral and medial pterygoid plates which are separated behind from each other by a pterygoid fossa. Anterior surface of both plates is continuous and forms the posterior wall of pterygopalatine fossa which presents in the upper part the following three foramina, from lateral to medial side with downward inclination:
(i) Foramen rotundum;
(ii) Anterior opening of pterygoid canal;
(iii) Palatino-vaginal or pharyngeal canal (See Fig. 1.9).
A triangular pterygoid fissure affects the lower part of anterior surface, which fits in the articulated skull with the margins of pyramidal process of palatine bone.
The lateral pterygoid plate is quadrilateral and presents anterior and posterior margins, lateral and medial surfaces. Anterior margin is free and forms posterior boundary of pterygomaxillary fissure, through which infra-temporal fossa communicates with pterygopalatine fossa. Posterior margin is also free and presents a small process near its centre for the attachment of pterygo-spinous ligament from the spine of sphenoid. Lateral surface of the lateral plate forms the medial wall of infra-temporal fossa and provides origin to the lower head of lateral pterygoid muscle (See Fig. 1.11). Medial surface of the plate gives principal origin to medial pterygoid muscle. Therefore, the lateral pterygoid is a muscle plate.
The medial pterygoid plate is more elongated and presents anterior and posterior margins, lateral and medial surfaces. Anterior margin articulates with the posterior border of perpendicular plate of palatine bone. Posterior margin is free and presents in the lower part a hook-like process, the pterygoid hamulus, around the lateral side of which the tendon of tensor veli palatini turns abruptly medially before reaching the soft palate as the palatine aponeurosis.
The tip of the hamulus gives attachment to the pterygomandibular raphe; the latter is attached below to the mandible behind the last molar tooth, and intervenes between the origins of buccinator in front and superior constrictor muscle of pharynx behind.
Close to the hamulus, the lower part of posterior border gives attachment to the upper part of superior constrictor muscle of pharynx along with bucco-pharyngeal and pharyngo-basilar fasciae. Traced above, the posterior border of medial plate forms a scaphoid fossa which provides principal origin to tensor veli palatini. A triangular projection of bone, processus tuberius, affects the middle of the posterior border and the notched area above the process lodges the medial end of the cartilaginous part of auditory tube, before the latter opens into the lateral wall of naso-pharynx. Lateral surface of medial plate is related to the tensor veli palatini, and its medial surface is covered by the muco- periosteum of the nasal cavity.
The tuberculum sellae divides the bone into pre – and post-sphenoidal parts; both parts are fused in the 7th or 8th months of intrauterine life.
Pre-sphenoidal part includes anterior part of the body and both lesser wings, and is ossified from six centres in cartilage: One for each lesser wing; two centres for the anterior part of the body, and one for each sphenoidal concha.
Post-sphenoidal part is ossified from eight centres: one for each greater wing; the root of greater wing including pterygoid canal and foramen rotundum is ossified in cartilage; rest of the greater wing and lateral pterygoid plate are ossified from the same centre in membrane; two centres appear in cartilage for the posterior part of the body; one centre for each medial pterygoid plate in membrane, except the hamulus which is ossified in cartilage; one for each lingula in cartilage.
At birth, the sphenoid consists of three pieces. The body and lesser wings form one piece, the greater wing and pterygoid process on each side forming other two pieces. During the first year the pieces unite to form a single bone.