Topography and Borders
The foramen magnum is located on the external base of the skull (basis cranii externa) and opens into the posterior cranial fossa (fossa cranii posterior).
The opening is surrounded on all sides by structures of the occipital bone:
- Basilar part (pars basilaris) — anterior boundary.
- Lateral parts (partes laterales) — paired structures located laterally.
- Occipital squama (squama occipitalis) — dorsal boundary.
- On the internal surface of the skull, immediately adjacent to the foramen magnum, runs the sulcus of the marginal sinus (sulcus sinus marginalis).
What Passes Through the Foramen Magnum?
The following anatomical structures pass through the foramen magnum:
- Medulla oblongata.
- Vertebral artery (a. vertebralis) — branches from the subclavian artery, ascends through the transverse foramina of the cervical vertebrae, and enters the cranial cavity through the foramen magnum.
- Spinal roots of the accessory nerve (radices spinales n. accessorii) — emerge from the lateral funiculus of the spinal cord at the C1–C6 segments, ascend, and enter the cranial cavity through the foramen magnum. Here they unite with cranial roots to form the common trunk of the accessory nerve (CN XI), which then exits the skull via the jugular foramen.
- Subarachnoid spaces of the brain and spinal cord — directly communicate with each other through the foramen magnum.
- Arteries and nerves.
Clinical Significance in Neurology and Oncology
The anatomical position of the foramen magnum makes it a critical site in several pathological processes:
- Herniation Syndrome: With a sharp increase in intracranial pressure (e.g., severe brain edema due to meningitis), portions of the brain can herniate into natural cranial openings, including the foramen magnum. Manifestations of herniation include generalized cerebral symptoms, focal neurological deficits, seizures, and hemodynamic instability.
- Tumor Involvement: Tumors of the cranial base, particularly those localized near the foramen magnum, can compress the spinal roots of the accessory nerve. This leads to flaccid paralysis of the sternocleidomastoid muscle and the upper part of the trapezius muscle.
Obstetric Significance
In obstetrics, the foramen magnum serves as an anatomical landmark for craniotomy of the after-coming head. The procedure is indicated when delivery of the after-coming head is obstructed in breech presentation of a dead fetus or in fetal hydrocephalus.
Access Technique:
- The assistant deflects the fetal trunk sharply posteriorly, and a vaginal speculum is inserted to protect the birth canal.
- An incision is made in the fetal scalp.
- Blunt dissection of the scalp from the cranial bones is performed until the foramen magnum is exposed.
- The perforator tip is placed directly against the foramen magnum, and the opening is enlarged by rotating the instrument 180° and then 360°, targeting a size of 3–4 cm.
- A large curette is introduced through the created access for excerebration—the destruction and removal of brain tissue—which reduces head volume and facilitates extraction.
Note: If the foramen magnum cannot be palpated, perforation is performed at the junction of the neck and occiput.