Foramen magnum

Occipital bone

• The occipital bone surrounds the oval-shaped foramen magnum, which is wider posteriorly than anteriorly. The narrower anterior part sits above the odontoid process and it encroached on from laterally by the occipital condyles.

• The wider posterior part transmits the medulla.

• The upper margin of the occipital bone articulates with the parietal bone at the lambdoid suture, and its lower margin articulates with the mastoid portion of the temporal bone at the occipitomastoid suture

• Divided into
Squamosal part located above and behind the foramen magnum
• Internally concave.
• The internal surface is divided into four unequal fossae by the sulcus of the
• Superior sagittal sinus,
• Internal occipital crest
• The internal occipital crest bifurcates above the foramen magnum to form a V-shaped ridge between the limbs of which is the vermian fossa.
• Transverse sinuses.
• Externally convex
• Has several prominences.
• External occipital protuberance (inion),
• Largest prominence
• Is situated at the central part of the external surface.
• The superior nuchal line radiates laterally from the protuberance.
• A vertical ridge, the external occipital crest, descends from the external occipital protuberance to the midpoint of the posterior margin of the foramen magnum.
• The inferior nuchal lines run laterally on both sides from the midpoint of the crest.
• The internal surface of the squamous part is concave and has a prominence, the internal occipital protuberance, near its center.
Basal (clival) part situated in front of the foramen magnum
• A thick quadrangular plate of bone that extends forward and upward to join the sphenoid bone just below the dorsum sellae.
• The superior surface of the clivus slopes upward from the foramen magnum and is concave from side to side.
• The clivus is separated on each side from the petrous part of the temporal bone by the petroclival fissure that ends posteriorly at the jugular foramen.
• The occipitomastoid suture extends posterolateral from the jugular foramen.
• On the inferior surface of the basilar part, a small elevation, the pharyngeal tubercle, gives attachment to the fibrous raphe of the pharynx.
Paired condylar parts located lateral to the foramen magnum
• Situated lateral to the foramen magnum on the external surface.
• The alar tubercle, which gives attachment to the alar ligament, is situated on the medial side of each condyle. The hypoglossal canal is situated above the condyle.
• The condylar fossa, which may be converted into a foramen for the passage of an emissary vein, is located behind the condyle.
• The jugular process of the occipital bone extends laterally from the posterior half of the condyle and articulates with the jugular surface of the temporal bone.
• The sulcus of the sigmoid sinus crosses the superior surface of the jugular process. The jugular foramen is bordered posteriorly by the jugular process of the occipital bone and anteriorly by the jugular fossa of the petrous temporal bone.
• The jugular tubercle lies on the internal surface above the hypoglossal canal.

 

Foramen magnum

• Abbreviations
• A., artery; Bas., basilar; Cer.Med., cerebellomedullary; CN, cranial nerve; Cond., condyle; Dent., dentate; Fiss., fissure; Hypogl., hypoglossal; Lig., ligament; Men., meningeal; Occip., occipital; P.I.C.A., posteroinferior cerebellar artery; Post., posterior; Sp., spinal; Vert., vertebral

• a, anterior tubercle

• b, dens

• c, occipital condyle

• d, vertebral artery (VA)

• e, posterior ramus of C1

• f, nerve XI

• g, medulla

• h, dura

• i, dentate ligament (DL)

• j, membrane tectoria

• k, foramina transversaria

• l, transverse process

• m, anterior ramus of C1

• n, transverse band of cruciform ligament

 

• A suboccipital craniectomy and upper cervical laminectomy exposes the dura.

• The vertebral arteries pass medially across the upper surface of the atlas where they give off the posterior meningeal arteries that ascend to supply the dura on the posterior aspect of the foramen magnum and posterior fossa.

• Insert, upper right. The upper margin of the left half of the arch of the atlas forms an osseous ring around the vertebral artery just proximal to where it enters the dura.

 

• Enlarged view of another foramen magnum after opening the dura.

• The right PICA arises outside the dura and penetrates the dura with the vertebral artery.

• The rostral end of the dentate ligament passes between the vertebral artery and the PICA to insert into the dura along the lateral margin of the foramen magnum.

• The accessory nerve ascends posterior to both the PICA and the vertebral artery.

• The vertebral artery gives rise to a posterior spinal artery that passes along the posterolateral aspect of the spinal cord and medulla.

• The hypoglossal rootlets are stretched over the posterior aspect of the vertebral artery.

• After VA penetrate dura the anterior to posterior orientation

• Anterior to posterior
• VA
• Dentate ligament
• CN11
• CN1/2 roots

• The right tonsil has been retracted to expose the caudal end of the fourth ventricle, which is located above the foramen magnum.

• The right PICA ascends through the foramen magnum and along the posterior margin of the medulla to reach the cerebellomedullary fissure.

 

• Another specimen.

• The rostral end of the dentate ligament
• Passes between the posterior spinal artery and vertebral artery
• Attaches to the dura at the level of the foramen magnum.

• The accessory nerve ascends behind the posterior spinal artery.

• The C1 nerve root receives a contribution from the accessory nerve and passes through the dura with the vertebral artery and courses along the lower margin of the artery.

• The posterior spinal artery arises inside the dura and passes between the dentate ligament and accessory nerve and gives rise to ascending branches to the medulla and descending branches to the spinal cord.

• The vertebral arteries ascend in front of the brainstem and give rise to the anterior spinal artery.

 

• The C1 ventral roots penetrate the dura with the vertebral artery. The hypoglossal rootlets pass behind the vertebral arteries.

 

Anterior to posterior stepwise dissection

• Stepwise dissection of a cross section showing the relationship of the foramen magnum and clivus to the nasal and oral cavities, pharynx, and infratemporal fossa.

• Abbreviations
• A., artery; A.I.C.A., anteroinferior cerebellar artery; Ant., anterior; Atl., atlanto-; Cap., capitis; Car., carotid; CN, cranial nerve; Eust., eustachian; For., foramen; Infratemp., infratemporal; Int., internal; Jug., jugular; Lat., lateral; Lig., ligament; Long., longus; M., muscle; Mandib., mandibular; Max., maxillary; Med., medial; Memb., membrane; Occip., occipital; Pteryg., pterygoid; Rec., rectus; Sp., spinal; Sphen., sphenoid; Trans., transverse; Vert., vertebral.

• The soft palate, which has been preserved, is located at the level of the foramen magnum. The infratemporal fossa, located below the greater sphenoid wing and middle cranial fossa, contains the pterygoid muscles, maxillary artery, mandibular nerve branches, and the pterygoid venous plexus and opens posteriorly into the area around the carotid sheath, as shown on the left side.

 

• Enlarged view. The soft palate has been divided in the midline and the leaves reflected laterally.

• The atlanto-occipital joints and the foramen magnum are located at approximately the level of the hard palate.

• The anterior arch of C1 and the dens are located behind the oropharynx, and

• the clivus is located behind the nasopharynx and sphenoid sinus.

• The prominence over the longus capitis and the anterior arch of C1 are seen through the pharyngeal mucosa.

 

• The mucosa lining the posterior pharyngeal wall has been reflected to the right, exposing the longus capitis that attaches to the clivus and the part of the longus colli that attaches to the anterior arch of C1.

• The eustachian tube has been divided.

• The rectus capitis anterior extends from the transverse process of C1, posterolateral to the longus capitis, to attach to the occipital bone in front of the occipital condyle.

 

• The clivus and anterior arch of C1 have been removed.

• The dura has been opened to expose the vertebral and basilar artery. The dens has been preserved.

• The structures in the right infratemporal fossa and part of the right carotid artery and mandible have been removed to expose the right vertebral artery ascending between the C2 and C1 transverse processes.

 

• Enlarged view of the step between C and D. The anterior arch of C1 has been removed to expose the odontoid process and the lower part of the clivus. The left longus coli and longus capitis have been reflected out of the exposure. The atlanto-occipital joint is exposed at the level of the odontoid apex. The transverse part of the cruciform ligament, also called the transverse ligament, extends across the back of the dens and attaches to a tubercle on the medial side of each lateral mass of the axis. The tectorial membrane, a cephalic extension of the posterior longitudinal ligament, lines the posterior clival surface. The alar ligaments attach to the lateral edges of the foramen magnum.

 

• Enlarged view of the exposure shown in D.

 

• Exposure after opening of the clivus. Both vertebral and anteroinferior cerebellar arteries (AICAs) and the anterior spinal artery are exposed.

 

 

Posterior to anterior stepwise dissection.

• Abbreviation
• A., artery; A.I.C.A., anteroinferior cerebellar artery; Ant., anterior; Asc., ascending; Atl., atlanto-; Bas., basilar; Br., branch; Bridg., bridging; Cap., capitis; Car., carotid; CN, cranial nerve; Cruc., cruciform; Dent., dentate; Desc., descending; Flocc., flocculus; For., foramen; Horiz., horizontal; Inf., inferior; Int., internal; Jug., jugular; Lev., levator; Lig., ligament; Longiss., longissimus; M., muscle; Maj., major; Med., median, medullary; Memb., membrane; Men., meningeal; Min., minor; Obl., oblique; Occip., occipital; P.I.C.A., posteroinferior cerebellar artery; Post., posterior; Proc., process; Rec., rectus; Scap., scapulae; Semispin., semispinalis; Sp., spinal; Spin., spinalis; Splen., splenius; Sternocleidomast., sternocleidomastoid; Sup., superior; Trans., transverse; V., vein; Vent., ventricle; Vert., vertebral.

• The right sternocleidomastoid and trapezius have been reflected to expose the splenius capitis.

• The left splenius capitis has been removed to expose the underlying semispinalis and longissimus capitis.

 

• Both semispinalis capitis muscles have been reflected laterally to expose the suboccipital triangles bilaterally.

 

• The muscles forming the left suboccipital triangle have been removed.

• The vertebral artery ascends slightly lateral from the transverse process of C2 to reach the transverse process of C1 and turns medially behind the superior facet of C1 to reach the upper surface of the posterior arch of C1.

• The C2 ganglion is located between the posterior arch of C1 and the lamina of C2.

• The dorsal ramus of C2 produces a medial branch that forms the majority of the greater occipital nerve.

 

• The muscles forming both suboccipital triangles have been removed.

• The rectus capitis posterior minor, which extends from the posterior arch of C1 to the occipital bone below the inferior nuchal line, has been preserved.

• The vertebral arteries cross the posterior arch of the atlas and penetrate the posterior atlanto-occipital membrane to reach the dura.

 

 

• The cerebellar tonsils, the foramen of Magendie, and lower part of the fourth ventricle are situated above the foramen magnum.

• The vertebral artery penetrates the dura below the foramen magnum and ascends through the foramen in front of the dentate ligament and accessory nerves.

• The glossopharyngeal, vagus, and accessory nerves pass through the jugular foramen, which is located lateral to the anterior half of the foramen magnum.

 

• The cerebellum has been removed.

• The vertebral arteries pass through the foramen magnum to reach the front of the medulla.

 

• Enlarged view of the left half of the foramen magnum.

• The vertebral artery passes behind and below the atlanto-occipital joint, penetrates the dura, and passes in front of the dentate ligament and accessory nerve.

• The rostral end of the dentate ligament attaches to the dura at the level of the foramen magnum.

• The C1 nerve penetrates the dura with the vertebral artery.

• The hypoglossal nerve passes behind the vertebral artery and enters the hypoglossal canal.
• The hypoglossal nerve is separated into several bundles as it penetrates the dura.

• The posterior spinal artery arises as the vertebral artery enters the dura and gives rise to ascending and descending branches.

 

• A longitudinal strip of the medulla and floor of the fourth ventricle has been removed to expose the vertebrobasilar junction, the origin of the anterior spinal artery, and the median anterior medullary and median anterior spinal veins.

 

• The right half of the medulla has been removed.

• The anterior spinal artery arises predominantly from the left vertebral artery, but has a small contribution from the right vertebral artery.

• Two bundles of right hypoglossal rootlets penetrate the dura.

• The medulla has been removed to expose the vertebral and anterior spinal arteries. The C1 nerve roots penetrate the dura with the vertebral artery.