Foramina and fissures of the skull

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Foramina/fissures of the anterior cranial fossa

There are only two foramina to mention in the anterior cranial fossa, but one is a group of foramina associated with the first of the cranial nerves we will be mentioning.

• Foramen caecum (or cecum): Is the most anterior of the holes in the floor of the skull. It lies in the frontal bone, just anterior to the ethmoid bone. It allows the passage of an emissary vein that comes from the nasal cavity and drains into the superior sagittal sinus, part of the venous drainage system associated with the brain. In general, emissary veins are veins that drain blood from outside the cranial cavity (in this case from the nose) to inside the cranial cavity via foramina in the skull. Interestingly, this particular emissary vein can be a route for infection to pass from the nose into the cranial cavity!

• Olfactory foramina in the cribriform plate: These holes make up a very important part of the pathway of the first cranial nerve (CNI), the olfactory nerve. Nerve endings in the top of our nose, responsible for our sense of smell, pass through these holes in the cribriform plate of the ethmoid bone. These nerve endings then combine within the cranial cavity on top of the cribriform plate to form the olfactory bulb, and then the olfactory tract which will relay information about what we are smelling to our brain.

Foramina/Fissures of the Middle Cranial Fossa

While there are more holes associated with the middle cranial fossa than either the anterior or posterior cranial fossae, thankfully there is also a mnemonic that may help you remember what the main ones are! Thinking about countries where the winters can be really cold, reminds me of this mnemonic: ' O so frosty!' (Just remember the last two letters aren’t used for any holes) - Note that these follow the progression (anterior to posterior). 

• Optic canal (O): Lies within the lesser wing of the sphenoid just anterior and medial to the anterior clinoid process of the sphenoid bone. The optic canal allows two very important structures to pass to and from the eye. The ophthalmic artery, which supplies blood to the retina of the eye, and the second cranial nerve (CNII), the optic nerve with its dural sheath, which sends visual information from the eye to the brain to be interpreted. To remember these structures easily, use the mnemonic "Only One Drink" (Ophthalmic artery, Optic nerve, Dural sheath).

• Superior orbital fissure (SOF): This crack in the middle cranial fossa, that lies just posterior and lateral to the optic canal, can be better appreciated if viewed from the front, as it lies at the back of the eye orbit or socket. As mentioned earlier, a fissure is usually located between two structures, and in this case, the superior orbital fissure is located between the lesser and greater wings of the sphenoid. Superior to the fissure in the orbit is the previously mentioned optic canal. Because it also gives passageway to and from the orbit, the superior orbital fissure also allows important structures to pass to and from the eye. The third cranial nerve (CNIII), the oculomotor nerve, provides nervous input to four out of the six muscles that move the eyeball. The fourth cranial nerve (CNIV), the trochlear nerve, and the sixth cranial nerve (CNVI), the abducent nerve, provide nervous input to the other two muscles that move the eyeball. Also passing through the superior orbital fissure, on its way through the orbit carrying sensory information from the forehead and parts of the face near the eyes, is the fifth cranial nerve (CNV), the trigeminal nerve. More specifically, the first division of the trigeminal nerve known as the ophthalmic nerve, typically written as CNV1. The final structures to pass through the superior orbital fissure are the ophthalmic veins.

• Foramen rotundum (R): This small round hole lies posterior to the optic canal and the superior orbital fissure on the floor of the skull. It allows the second division of the trigeminal nerve (CNV2), the maxillary nerve, to enter the skull carrying sensory information coming from areas of the face associated with the region under the eyes, and the upper lip, or the skin overlying the maxilla bone. To always have in mind which structure traverses the forament rotundum, remember this sentence "Foramen rotundum is MAXimally rounded".
• Foramen ovale (O): This oval shaped hole lies posterior and lateral to the foramen rotundum. The foramen ovale allows passage of the final division of the trigeminal nerve, the mandibular nerve (CNV3). Not surprisingly perhaps, the mandibular nerve enters the skull through the foramen ovale bringing sensory information from the face and skin that overlies the mandible, or lower jaw bone. The foramen ovale occasionally also allows passage of the lesser petrosal nerve (a branch of the ninth cranial nerve (CNIX), the glossopharyngeal) which will innervate the parotid gland, as well as the emissary vein and accessory meningeal branch of the maxillary artery. A useful mnemonic to remember these structures is "MALE" (Mandibular branch of trigeminal nerve, Accessory meningeal branch of maxillary artery, Lateral petrosal nerve, Emissary vein).

• Foramen spinosum (S): This foramen is the smallest and most lateral of those found in the middle cranial fossa, and lies posterior and lateral to the foramen ovale. It allows the middle meningeal artery to enter the skull. The middle meningeal artery (a branch of the maxillary artery, which is in turn a branch of the external carotid artery) supplies blood to the dura mater of the meninges that surrounds the brain.

There are two other foramina in the middle cranial fossa, not covered by the mnemonic mentioned above. They are the:

• Foramen lacerum: Is located posterior and medial to the foramen ovale, and may be filled with cartilage. It does, however, still allow passage of the greater petrosal nerve, a branch of the seventh cranial nerve (CNVII), the facial nerve. It provides innervation to glands located above the level of the mouth, for example the lacrimal gland near the eye which is associated with tear production.
• Carotid canal: Lies immediately posterior and lateral to the foramen lacerum. This hole allows the internal carotid artery, one of the major blood supplies to the brain, to enter the skull. Branches of the internal carotid also supply blood to the eye, the contents of the orbit, and the forehead.

Foramina/fissures of the posterior cranial fossa

Within the posterior cranial fossa, there are four main holes in the floor of the skull. They will be listed in order of the cranial nerves that enter and exit through them, but you may want to remember their location by visualizing where they are in relationship to the largest one, the foramen magnum.

• Internal acoustic meatus (meatus = passage): Other than the foramen magnum, it is the most anterior of the holes in the posterior cranial fossa. Interestingly, this is one end of a passageway that houses our hearing and balance apparatus. On the other end of this passageway is the external acoustic meatus, or the hole for our ear! There are two cranial nerves that enter the skull through the internal acoustic meatus, the seventh (CNVII), the facial nerve, and the eighth (CNVIII), the vestibulocochlear nerve. The facial nerve has a wide range of functions, including providing nervous input to the muscles of facial expression and taste sensation from the tongue, among others. The vestibulocochlear nerve brings sensory information from our cochlea (part of our hearing apparatus) and from the vestibular canal system contained within the ear.

• Jugular foramen: The jugular foramen is also an easier one to locate as its shape is more consistent with a fissure than it is with some of the rounder holes we have been talking about. It allows passage for the ninth (CNIX), tenth (CNX), and eleventh (CNXI) cranial nerves, the glossopharyngeal, the vagus, and the spinal accessory nerve respectively. The function of CNIX is suggested by its name. Glossa is Greek for tongue, and the pharynx is the superior extension of the esophagus and trachea that lies behind the oral and nasal cavities. Its function is then related to the innervation of structures of the tongue and pharynx. The vagus nerve (from the Latin word for vague), so named because its course is vague and wandering as it innervates many of the organs in the abdominal cavity. The spinal accessory nerve (sometimes called simply the accessory nerve) is unique in that it is a cranial nerve that actually arises from the upper segments of the cervical portion of the spinal cord. It then ascends into the skull via the foramen magnum, and then back out through the jugular foramen to supply two muscles in the neck, the sternocleidomastoid and the trapezius. Finally, the jugular foramen allows the vein for which it is named to exit the neck. The internal jugular vein drains much of the blood from the head and parts of the neck down into the brachiocephalic vein and finally to the heart via the superior vena cava.

• Hypoglossal canal: This hole is the one closest to the foramen magnum. It allows passage of the twelfth cranial nerve (CNXII), the hypoglossal nerve. As with the glossopharyngeal, the hypoglossal nerve is associated with the tongue, and provides innervation to all but one of the muscles of the tongue.
• Foramen magnum: You can’t miss this one! The name in Latin means “great hole” and it has to be relatively large to allow the lower end of the brainstem and the upper end of the spinal cord to pass through. Importantly, the foramen magnum also allows the vertebral arteries to enter the skull. These arteries travel up each side of the spinal column and enter the skull to provide another important supply of blood to the brain.

Try out our quiz to test your knowledge on the foramina and fissures of the skull:

Anterior cranial fossa

• Foramen caecum/cecum: Emissary veins from the nasal cavity
• Olfactory foramina: Olfactory nerve endings (CNI)

Middle Cranial Fossa

• Optic canal: Optic nerve (CNII), ophthalmic artery
• Superior orbital fissure: Oculomotor nerve (CNIII), trochlear nerve (CNIV), ophthalmic division of the trigeminal nerve (CNV1), abducent nerve (CNVI), ophthalmic veins
• Foramen rotundum: Maxillary division of the trigeminal nerve (CNV2)
• Foramen ovale: Mandibular division of the trigeminal nerve (CNV3) (variably  the lesser petrosal nerve)
• Foramen spinosum: Middle meningeal artery
• Foramen lacerum: Greater petrosal nerve
• Carotid canal: internal carotid artery

Posterior Cranial Fossa

• Internal acoustic foramen: Facial nerve (CNVII), Vestibulocochlear nerve (CNVIII)
• Jugular foramen: Glossopharyngeal nerve (CNIX), vagus nerve (CNX), descending portion of the spinal accessory nerve (CNXI), internal jugular vein
• Hypoglossal canal: Hypoglossal nerve (CNXII)
• Foramen magnum: Brainstem/spinal cord, vertebral arteries, ascending portion of the spinal accessory nerve (CNXI)