# Foramen Magnum: Anatomy, Contents, and Comparative Notes The foramen magnum is the large opening in the occipital bone at the base of the skull through which the medulla oblongata, its meninges, the vertebral arteries, and several other structures pass to connect the cranial cavity with the vertebral canal. It is the boundary between the brainstem above and the spinal cord below, and its size, shape, and position differ across species in ways that matter for anatomy, evolution, and clinical diagnosis. This single opening sits at the crossroads of the central nervous system, the vertebral blood supply, and the bony architecture of the skull base. Because almost every structure that enters or leaves the posterior cranial fossa must pass through it, the foramen magnum is a landmark for neurosurgeons, radiologists, veterinarians, and paleoanthropologists alike. Its position relative to the rest of the skull has been used for decades to infer posture and locomotion in fossil hominins, and its dimensions are now measured routinely in dogs and cats to screen for malformations that compress the brainstem. ## What the Foramen Magnum Is The term "foramen" is Latin for an opening or hole, and "magnum" means large. The foramen magnum is therefore literally the "large hole," and it is the largest of the many foramina in the human skull. It sits in the occipital bone, the bone that forms the back and base of the cranium, and it is bordered by the occipital condyles on either side, the basilar part of the occipital bone in front, and the squamous part behind. The opening is roughly oval in humans, slightly wider than it is long in many individuals, and it transmits the lowest part of the brainstem as that structure transitions into the spinal cord. The junction between the medulla oblongata and the spinal cord is conventionally placed at the level of the foramen magnum, which makes the opening an anatomical landmark rather than a simple hole. ## Why the Foramen Magnum Matters The foramen magnum is the only route by which the central nervous system communicates between the skull and the spine. Anything that narrows it, whether a congenital malformation, a tumor, a bone thickening, or a traumatic displacement, can compress the medulla and the upper cervical cord. That compression can interfere with breathing, heart rate, and motor function because the medulla contains the nuclei that control those involuntary activities. In [veterinary medicine](/blog/careers/veterinary-medicine-careers-from-clinical-practice-to-public-health), the size of the foramen magnum is a screening measurement for Chiari-like malformation in small breeds, especially Cavalier King Charles Spaniels. In human medicine, it is the reference point for defining Chiari malformation type I, in which the cerebellar tonsils descend below the opening. In paleoanthropology, its position relative to the carotid foramina and other landmarks is used to distinguish bipedal hominins from quadrupedal apes [1]. ## Anatomy of the Foramen Magnum

### Bony Borders The foramen magnum is enclosed entirely by the occipital bone. Four parts of that bone contribute to its rim: - The basilar part (also called the basioccipital) forms the anterior border. - The two lateral parts (exoccipitals) form the sides and carry the occipital condyles. - The squamous part forms the posterior border. The occipital condyles are two kidney-shaped articular surfaces that sit on either side of the foramen and articulate with the first cervical vertebra, the atlas. Their size and orientation determine how the head sits on the neck and how much rotation and flexion the skull can perform. ### Shape and Dimensions in Humans In adult humans, the foramen magnum is typically oval, with the long axis running front to back or slightly wider than long depending on the individual. The area of the opening in modern humans is commonly reported in the range of roughly 600 to 800 mm², though this varies with sex, body size, and population. The occipital condyles in humans are approximately 20 to 25 mm long and 10 to 15 mm wide, and they are oriented so that the articular surfaces face downward and slightly outward. These numbers matter because they set the baseline against which other species and other breeds are compared. A foramen magnum that is significantly smaller than expected for the body size of the animal raises concern for stenosis, a narrowing that can compress the neural tissue passing through it. ### Position Within the Skull The position of the foramen magnum relative to the rest of the skull is described as anterior (forward, toward the face) or posterior (backward, toward the occiput). In humans, the foramen is relatively anterior and sits near the center of the skull base, which places the head in a balanced position on top of the vertebral column. In chimpanzees and other nonhuman apes, the foramen is more posterior, which reflects a more horizontal orientation of the head and a quadrupedal posture. A study comparing 16 modern human crania with 19 chimpanzee crania found that the distance from the anterior border of the foramen magnum to a chord connecting the carotid foramina was significantly smaller in humans, even after controlling for total cranial length [1]. That relationship is one of the tools used to assess fragmentary fossils, where the foramen magnum itself may be missing but the carotid foramina are preserved. ## Contents of the Foramen Magnum The structures that pass through the foramen magnum can be grouped into neural, vascular, and ligamentous categories. The table below summarizes each structure, its course, and the clinical consequence of injury or compression. | Structure | Course through the foramen | Clinical consequence | |--|--|--| | Medulla oblongata with meninges | Descends from the posterior cranial fossa into the vertebral canal, enclosed by dura, arachnoid, and pia mater | Compression causes respiratory depression, cardiac instability, and long-tract motor and sensory deficits | | Vertebral arteries (paired) | Ascend through the foramen, pierce the dura, and join to form the basilar artery | Narrowing or dissection reduces posterior circulation and can cause brainstem stroke | | Anterior spinal artery | Single vessel formed near the foramen from branches of the vertebral arteries, runs along the anterior median fissure of the cord | Occlusion causes anterior cord syndrome with loss of motor function and pain and temperature sensation | | Posterior spinal arteries (paired) | Arise near the foramen and descend along the posterior cord | Occlusion contributes to posterior column ischemia and loss of proprioception | | Spinal accessory nerve rootlets (CN XI) | Cranial rootlets exit near the foramen, spinal rootlets ascend through it from the upper cervical cord | Injury causes weakness of the sternocleidomastoid and trapezius muscles | | Tectorial membrane | Continuation of the posterior longitudinal ligament, passes upward through the foramen to attach to the clivus | Disruption in trauma destabilizes the craniocervical junction | | Alar ligaments (paired) | Run from the dens of the axis to the lateral margins of the foramen magnum | Injury allows excessive rotation and flexion of the head | ### Neural Contents The medulla oblongata is the lowest part of the brainstem and the most important structure to pass through the foramen. It carries the corticospinal tracts that control voluntary movement, the dorsal column pathways that carry vibration and proprioception, and the nuclei for cranial nerves IX, X, XI, and XII. The medulla is surrounded by the three meninges, and the dura mater becomes continuous with the spinal dura at the level of the foramen. The spinal accessory nerve, cranial nerve XI, has a peculiar anatomy. Its cranial rootlets emerge from the medulla and its spinal rootlets arise from the upper five or six segments of the cervical cord. The spinal rootlets ascend through the foramen magnum before joining the cranial rootlets, which is why the nerve is described as passing through the opening even though its cell bodies lie in the spinal cord. ### Vascular Contents The two vertebral arteries ascend through the neck, enter the skull through the foramen magnum, and merge at the lower border of the pons to form the basilar artery. Before they merge, they give off the anterior spinal artery and the two posterior spinal arteries, which supply the spinal cord. The anterior spinal artery is formed by the union of small branches from each vertebral artery, while the posterior spinal arteries usually arise as branches of the posterior inferior cerebellar arteries or directly from the vertebral arteries. This arrangement means that the blood supply to the upper spinal cord depends on vessels that pass through the foramen magnum. Compression or dissection of the vertebral arteries at this level can therefore cause ischemia in both the brainstem and the cervical cord. ### Ligamentous Contents The tectorial membrane is a broad band of fibrous tissue that covers the dens and the alar ligaments. It is the upward continuation of the posterior longitudinal ligament and attaches to the basilar part of the occipital bone above the foramen magnum. The alar ligaments are two short, strong bands that run from the sides of the dens to the lateral margins of the foramen magnum. Together with the transverse ligament of the atlas, they limit rotation and flexion of the head on the neck. Because these ligaments pass through or attach at the margins of the foramen, they are part of the craniocervical junction that stabilizes the skull on the spine. Disruption of the tectorial membrane or the alar ligaments in trauma can allow the occipital condyles to slide on the atlas, which risks compression of the medulla. ## How the Foramen Magnum Is Examined ### Imaging in Humans In clinical practice, the foramen magnum is assessed with computed tomography (CT) and magnetic resonance imaging (MRI). CT shows the bony margins and the occipital condyles in detail, which is useful for trauma and for measuring the dimensions of the opening. MRI shows the medulla, the cerebellar tonsils, and the ligaments, which is essential for diagnosing Chiari malformation and for evaluating soft tissue compression. Chiari malformation type I is defined historically by descent of the cerebellar tonsils more than 3 to 5 mm below the foramen magnum [2]. That threshold has been questioned because the degree of tonsillar descent does not always correlate with symptom severity. Research using phase-contrast MRI has shown that cardiac-related spinal cord tissue motion at the foramen magnum is significantly greater in patients with Chiari malformation type I than in healthy controls, and that this motion decreases after decompression surgery [2]. That finding supports the use of dynamic imaging in addition to static measurements. ### Imaging in Animals In veterinary medicine, the foramen magnum is measured on CT or radiographs to screen for Chiari-like malformation and for occipital dysplasia. The measurements typically include the height and width of the opening, the dimensions of the occipital condyles, and the presence of a dorsal notch in the bony rim. A dorsal notch is a gap in the dorsal margin of the foramen magnum that is covered by fibrous tissue. It is common in some brachycephalic breeds and is considered a form of occipital dysplasia. In Pekingese dogs, a study of 75 adult and 5 juvenile skulls found a dorsal notch in all but two skulls, with a mean foramen magnum area of 138.1 ± 26.1 mm², a mean total height of 15.0 ± 2.9 mm, and a mean maximal width of 13.3 ± 1.1 mm [3]. The fibrous membrane covering the notch prevents prolapse of the cerebellum or brainstem through the enlarged opening. ## Comparative Notes: Humans, Dogs, and Horses ### The Human Condition In humans, the foramen magnum is relatively anterior and the occipital condyles are oriented to support an upright head. The opening is roughly 600 to 800 mm² in area, and the medulla passes through it vertically. The dens of the axis sits just below the foramen, separated from the medulla by the transverse ligament, the tectorial membrane, and the meninges. ### The Canine Skull Base In dogs, the foramen magnum is relatively larger and more ventrally placed than in humans. The skull base is flatter, the occipital condyles are more elongated, and the opening sits closer to the ventral surface of the skull. This reflects the quadrupedal posture of dogs, in which the head is held forward rather than balanced on top of the spine. Brachycephalic breeds such as the Cavalier King Charles Spaniel, the Pekingese, and the French Bulldog have a shortened skull base and a foramen magnum that is often smaller and more dorsally notched than in mesaticephalic breeds. In Cavalier King Charles Spaniels, Chiari-like malformation is common and is characterized by overcrowding of the caudal fossa, descent of the cerebellar tonsils, and syringomyelia. The foramen magnum in these dogs may be narrowed, and the dorsal notch may be covered by a fibrous membrane that contributes to compression. The Pekingese data illustrate the range of normal variation in a brachycephalic breed. A mean area of 138.1 mm² is far smaller than the human range, but it is appropriate for a dog of that size [3]. The clinical concern is not the absolute size but whether the opening is large enough for the neural tissue it must transmit. ### The Equine Skull Base In horses, the foramen magnum is also relatively large and ventrally placed. The equine skull base is long and the occipital condyles are broad, which reflects the heavy head and the strong nuchal musculature needed to support it. The dens of the axis sits just rostral to the foramen, and the craniocervical junction is stabilized by the same ligamentous complex seen in other mammals. The equine foramen magnum is a landmark for cerebrospinal fluid collection and for diagnosing conditions such as occipitoatlantoaxial malformation. Because the opening is large, the medulla and the upper cervical cord are relatively accessible for imaging, but the thickness of the surrounding bone and muscle makes radiography challenging. ### The Odontoid Process and Dens In all mammals, the dens (odontoid process) of the second cervical vertebra, the axis, sits just rostral to the foramen magnum. The dens projects upward into the ring of the atlas and is held in place by the transverse ligament. The tectorial membrane covers the dens and continues upward to the clivus, and the alar ligaments connect the dens to the margins of the foramen. This arrangement means that the dens is separated from the medulla by a thin layer of ligament and meninges. In humans, fracture or dislocation of the dens can compress the medulla directly, which is why injuries to the upper cervical spine are treated as emergencies. In dogs and horses, congenital malformations of the dens can cause similar compression. ## Species Differences in Foramen Magnum Shape and Size The shape and size of the foramen magnum vary widely across mammals. Comparative studies have used geometric morphometrics to quantify these differences and to relate them to phylogeny, locomotion, and brain size. A study of five mammalian orders, including the straw-colored fruit bat (Eidolon helvum), the free-tailed bat (Tadarida brachyptera), the white-bellied pangolin (Phataginus tricuspis), the African four-toed hedgehog (Atelerix albiventris), and the domestic dog (Canis familiaris), found that size overrode shape in the principal component analysis and that the dog showed the highest variation in foramen magnum evolution [4]. That finding suggests that domestication and breed formation have produced more shape diversity in the canine foramen magnum than is seen in the other species studied. A separate study of the African four-toed hedgehog reported a mean foramen magnum height of 0.51 ± 0.05 cm and a mean width of 0.64 ± 0.04 cm, with occipital condylar and interparacondylar widths of 1.00 ± 0.12 cm and 1.62 ± 0.07 cm [5]. The foramen magnum index, defined as the ratio of width to height, was 83.4 ± 5.51 in males and 76.3 ± 6.37 in females, and dorsal notches were observed in three distinct types [5]. In camels, a study of 30 adult heads found a mean foramen magnum height and width of 4.04 ± 0.15 cm and 3.70 ± 0.16 cm in males and 3.65 ± 0.27 cm and 3.45 ± 0.21 cm in females, with a foramen magnum index over 100 [6]. The dorsal border of the foramen presented either a smoothly curved bony margin, a small ventrally directed median bony protrusion, or a dorsal notch [6]. These morphological categories are used to classify the foramen magnum in comparative anatomy. In cats, a CT study of 102 mixed-breed heads found a mean maximum length of the foramen magnum of 11.18 ± 0.84 mm in males and 10.63 ± 0.72 mm in females, and a mean maximum internal width of 14.43 ± 0.72 mm in males and 13.75 ± 1.01 mm in females [7]. The differences between sexes were statistically significant, which is relevant for forensic and archaeological identification [7]. ## Clinical Relevance ### Chiari Malformation and Chiari-Like Malformation Chiari malformation type I in humans is defined by descent of the cerebellar tonsils below the foramen magnum. The condition can cause headaches, neck pain, balance problems, and, in severe cases, syringomyelia, a fluid-filled cavity in the spinal cord. The foramen magnum is the reference point for the diagnosis, and decompression surgery aims to enlarge the opening and restore normal cerebrospinal fluid flow [2]. Chiari-like malformation in dogs is the analogous condition. It is most common in Cavalier King Charles Spaniels and other toy breeds, and it is diagnosed by measuring the descent of the cerebellar tonsils and the dimensions of the foramen magnum on MRI. The condition is often accompanied by syringomyelia, which causes scratching, neck pain, and abnormal sensation. ### Foramen Magnum Stenosis Foramen magnum stenosis is a narrowing of the opening that compresses the medulla and the upper cervical cord. In humans, it can be congenital or acquired, and it is a feature of some skeletal dysplasias such as achondroplasia. In animals, it has been documented in captive lions, where a study of 575 skulls found that captive lions had a significantly smaller foramen magnum height (17.36 ± 3.20 mm) than wild lions (19.77 ± 2.11 mm), with greater variability in the captive group [8]. No such difference was found between wild and captive tigers, which suggests that the effect in lions is related to captivity-specific factors rather than to captivity in general [8]. ### Trauma and Instability The foramen magnum is at the center of the craniocervical junction, and injuries that disrupt the tectorial membrane or the alar ligaments can allow the occipital condyles to slide on the atlas. This instability can compress the medulla and is a surgical emergency. In humans, the condition is called craniocervical instability or atlanto-occipital dislocation, and it is often fatal if not treated promptly. ## Common Mistakes and Limitations **Confusing the foramen magnum with the foramen ovale or foramen lacerum.** The foramen ovale transmits the mandibular division of the trigeminal nerve, and the foramen lacerum is a gap between the sphenoid and temporal bones. Neither is related to the foramen magnum, and mixing them up is a common error in exam settings. **Assuming the foramen magnum is the same shape in all breeds.** The opening varies from ovoid to rectangular to dorsally notched, and the shape can differ between males and females of the same species [7]. A single reference range does not apply to all animals. **Treating tonsillar descent as the only criterion for Chiari malformation.** The 3 to 5 mm threshold is a historical definition, and research has shown that dynamic measurements such as spinal cord motion at the foramen magnum can be more informative in some patients [2]. **Overlooking the dorsal notch.** In brachycephalic breeds, the dorsal notch is covered by fibrous tissue and may not be visible on plain radiographs. CT or MRI is needed to assess the full extent of the opening and the soft tissue covering it [3]. **Assuming that a small foramen magnum always causes symptoms.** Many animals and humans with a small opening are asymptomatic, and the clinical significance depends on the size of the neural structures passing through it, the presence of syringomyelia, and the dynamics of cerebrospinal fluid flow. **Using the foramen magnum alone to infer locomotion.** A study of rodents, strepsirrhine primates, and marsupials found no relationship between locomotor pattern and foramen magnum angle in most groups, and that relative brain size and auditory bullae size can influence the angle [9]. The position of the foramen magnum is one clue among many, not a standalone indicator. ## Quick Review - The foramen magnum is the large opening in the occipital bone that connects the cranial cavity to the vertebral canal. - It transmits the medulla oblongata with its meninges, the vertebral arteries, the anterior and posterior spinal arteries, the spinal accessory nerve rootlets, the tectorial membrane, and the alar ligaments. - In humans, the area is roughly 600 to 800 mm², and the occipital condyles are about 20 to 25 mm long. - In dogs and horses, the foramen is relatively larger and more ventrally placed than in humans, reflecting quadrupedal posture. - The dens of the axis sits just rostral to the foramen, separated from the medulla by the transverse ligament, tectorial membrane, and meninges. - Chiari malformation type I is defined by descent of the cerebellar tonsils below the foramen magnum, and Chiari-like malformation is the canine equivalent. - Brachycephalic breeds such as the Cavalier King Charles Spaniel and Pekingese often have a smaller, dorsally notched foramen magnum. ## Frequently Asked Questions ### What passes through the foramen magnum? The medulla oblongata with its meninges, the vertebral arteries, the anterior and posterior spinal arteries, the spinal accessory nerve rootlets, the tectorial membrane, and the alar ligaments pass through or attach at the foramen magnum. ### How large is the foramen magnum in humans? The area of the foramen magnum in adult humans is commonly reported in the range of roughly 600 to 800 mm², with the occipital condyles measuring about 20 to 25 mm long and 10 to 15 mm wide. ### How does the foramen magnum differ in dogs and horses? In dogs and horses, the foramen magnum is relatively larger and more ventrally placed than in humans, and the skull base is flatter. Brachycephalic dog breeds often have a smaller opening with a dorsal notch. ### What is Chiari malformation? Chiari malformation type I is a condition in which the cerebellar tonsils descend more than 3 to 5 mm below the foramen magnum, potentially causing headaches, neck pain, and syringomyelia. ### Where is the dens relative to the foramen magnum? The dens, or odontoid process, of the second cervical vertebra sits just rostral to the foramen magnum, separated from the medulla by the transverse ligament, the tectorial membrane, and the meninges. ### Why is the foramen magnum important in paleoanthropology? The position of the foramen magnum relative to the carotid foramina is used to distinguish bipedal hominins from quadrupedal apes, because a more anterior foramen magnum is associated with an upright posture. ## Related Articles - [Comparative Anatomy of the Mammalian Kidney](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/comparative-anatomy-mammalian-kidney) - [Comparative Anatomy of the Mammalian Heart](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/comparative-anatomy-mammalian-heart) - [Veterinary Anatomy Book Selection: A Comparative Guide for NAVLE](/knowledge/veterinary-medicine/navle-exam-prep/veterinary-anatomy-book-selection-a-comparative-guide-for-navle) - [Comparative Anatomy of the Ruminant and Equine Eye](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/comparative-anatomy-ruminant-equine-eye) - [Comparative Anatomy of the Avian Skeleton: Flight and Locomotion](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/comparative-anatomy-avian-skeleton-flight-locomotion) - [Comparative Anatomy of the Avian Respiratory System](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/comparative-anatomy-avian-respiratory-system) - [Jugular Foramen: Anatomy, Contents, and Clinical Notes](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/jugular-foramen-anatomy-contents-and-clinical-notes) - [Obturator Foramen: Anatomy, Borders, and Contents](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/obturator-foramen-anatomy-borders-and-contents) ## Sources 1. [Brief communication: foramen magnum-carotid foramina relationship: is it useful for species designation?](https://pubmed.ncbi.nlm.nih.gov/10564576/) 2. [Cardiac-Related Spinal Cord Tissue Motion at the Foramen Magnum is Increased in Patients with Type I Chiari Malformation and Decreases Postdecompression Surgery.](https://pubmed.ncbi.nlm.nih.gov/29733988/) 3. [Morphometric analysis of the foramen magnum in Pekingese dogs.](https://pubmed.ncbi.nlm.nih.gov/8141494/) 4. [Probable developmental, neurological and evolutionary trend in complex morphological structures: foramen magnum outlines analytic comparisons in four orders; Chiroptera: (Eidolon helvum), (Tadarida brachyptera), Pholidota (Phataginus tricuspis), Eulipotyphla (Atelerix albiventris) and Carnivora (Canis familiaris).](https://pubmed.ncbi.nlm.nih.gov/40399809/) 5. [Morphometrics of foramen magnum in African four-toed hedgehog (Atelerix albiventris).](https://pubmed.ncbi.nlm.nih.gov/26050805/) 6. [Morphological analysis and osteometry of the foramen magnum of the one-humped camel (Camelus dromedarius).](https://pubmed.ncbi.nlm.nih.gov/22783991/) 7. [Detection of sexual dimorphism of the foramen magnum in cats using computed tomography.](https://pubmed.ncbi.nlm.nih.gov/36999680/) 8. [Comparative skull analysis suggests species-specific captivity-related malformation in lions (Panthera leo).](https://pubmed.ncbi.nlm.nih.gov/24718586/) 9. [Locomotor pattern fails to predict foramen magnum angle in rodents, strepsirrhine primates, and marsupials.](https://pubmed.ncbi.nlm.nih.gov/27178457/)