Structures that ascend and descend through foramen magnum
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Structures Passing Through the Foramen Magnum
The foramen magnum is the largest foramen in the skull, situated in the deepest part of the posterior cranial fossa. It is bounded anteriorly by the basilar part of the occipital bone, laterally by the lateral parts of the occipital bone, and posteriorly by the squamous occipital bone.
Structures That Ascend Through the Foramen Magnum (from spinal canal into cranium)
| Structure | Details |
|---|---|
| Vertebral arteries (×2) | Ascend through the transverse foramina of C6-C1, then enter the cranial cavity through the foramen magnum. Each gives off a small meningeal branch on entry. |
| Spinal roots of CN XI (Accessory nerve) | The spinal component arises from C1-C5/C6 cord, passes through the lateral funiculus, ascends through the foramen magnum into the cranial cavity, crosses the occipital bone, and then exits through the jugular foramen. |
| Anterior spinal artery | Forms just after the vertebral arteries enter the cranium (from branches of each vertebral artery uniting), then descends. So technically it is formed intracranially from ascending vessels. |
| Posterior spinal arteries (×2) | Similarly originate from the vertebral arteries (or PICA) just after they ascend through the foramen magnum. |
| Emissary veins | Small veins connecting the internal vertebral venous plexus with intracranial sinuses may traverse the foramen. |
Structures That Descend Through the Foramen Magnum (from cranium into spinal canal)
| Structure | Details |
|---|---|
| Medulla oblongata / Brainstem | The brainstem is continuous with the spinal cord at the foramen magnum - the medulla descends to become the spinal cord at this junction. |
| Anterior spinal artery | Forms intracranially, then descends in the anterior median fissure of the spinal cord. |
| Posterior spinal arteries (×2) | Pass posteriorly around the medulla then descend on the posterior surface of the spinal cord. |
| Spinal meninges (dura, arachnoid, pia) | The cranial meninges are continuous with the spinal meninges at the foramen magnum - the dura attaches to the margins and projects down as the spinal dura (theca). |
| Tonsils of cerebellum (pathological) | In Chiari malformation or raised ICP, the cerebellar tonsils may herniate downward through the foramen magnum. |
Structures Passing Both Ways / Neither Strictly
| Structure | Details |
|---|---|
| Spinal cord / Brainstem junction | The medulla-cord continuum - structurally present at this opening |
| Meninges (three layers) | Continuous at this junction without strict directionality |
| Subarachnoid space / CSF | Freely communicates through the foramen between cranial and spinal compartments |
| Ligaments | The membrana tectoria, apical ligament, alar ligaments, and anterior/posterior atlanto-occipital membranes attach to the margins of the foramen magnum |
Memory Aid
"2 V's, 1 M, 1 N, 3 S's"
- 2 Vertebral arteries (ascending)
- 1 Medulla (descending as cord)
- 1 Nerve - CN XI spinal roots (ascending)
- 3 Spinal structures - anterior spinal artery (×1) + posterior spinal arteries (×2), formed from vertebral arteries just inside the foramen
Sources: Gray's Anatomy for Students (structures at foramen magnum, vertebral arteries); Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol 2 (posterior skull base anatomy); Localization in Clinical Neurology 8e (CN XI spinal roots ascending through foramen magnum)
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