Foramen magnum

Topography and boundaries

The foramen magnum is located at the external base of the skull (basis cranii externa) and connects with the posterior cranial fossa (fossa cranii posterior).

The opening is bounded on all sides by structures of the occipital bone:

• Basilar part (pars basilaris) — the anterior part.
• Lateral parts (partes laterales) — paired structures located on the sides.
• Occipital squama (squama occipitalis) — the dorsal part.

On the internal surface of the skull, in close proximity 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 opening:

• Medulla oblongata.
• Vertebral artery (a. vertebralis) — originates from the subclavian artery, ascends in the canal of the transverse processes 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 level of segments C1–C6, ascend, and enter the cranial cavity through the foramen magnum. Here they join the cranial roots to form the common trunk of the accessory nerve (CN XI), which then leaves the skull through 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 location of the opening determines its involvement in a number of pathological processes:

• Displacement syndrome (herniation): With a sharp increase in intracranial pressure, for example, in brain edema and swelling against the background of severe meningococcal infection, parts of the brain herniate into the natural openings of the skull, including the foramen magnum. Manifestations of displacement syndrome include general cerebral symptoms, focal neurological signs, seizures, and hemodynamic disturbances.
• Tumor lesions: Tumors of the skull base, especially those localized in the region of the foramen magnum, can cause damage to the spinal roots of the accessory nerve. This leads to flaccid paralysis of the entire sternocleidomastoid muscle and the upper part of the trapezius muscle.

Significance in obstetrics

In obstetrics, the foramen magnum is an anatomical landmark for craniotomy of the after-coming head. The procedure is indicated when delivery of the after-coming head is difficult in breech presentation of a dead fetus or in fetal hydrocephalus.

Access technique:

1. The assistant deflects the fetal torso sharply posteriorly, and a vaginal speculum is inserted into the vagina to protect the birth canal.
2. Incision of the fetal scalp is performed.
3. Blunt separation of the scalp from the skull bones is carried out until the foramen magnum is identified.
4. The perforator spear is placed directly against the foramen magnum, after which the opening is enlarged: the instrument is rotated first 180°, then 360°; the target opening size is 3–4 cm.
5. Through the formed access, a large curette is introduced for exenteration — destruction and removal of brain tissue, which leads to a decrease in head volume and facilitates its extraction.

Note: If the foramen magnum cannot be located, perforation is performed at the transition of the neck to the occiput.

Class and exam questions

What else to study on this topic

• Posterior cranial fossa (fossa cranii posterior)
• Accessory nerve (n. accessorius, CN XI)
• External cranial base (basis cranii externa)
• Blood supply to the brain (vertebrobasilar system)

Other topics in the "Anatomical Structures" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.