Let's get into your head! Well, figuratively speaking :D The cranial cavity houses the brain, of course, but also includes vascular supply and venous drainage, cerebrospinal fluid, cranial nerves as they exit through various bony spaces, and meninges.

This post includes TWO downloadable drawings - one of the dural venous sinuses (and venous drainage for the whole head!), and one for the cerebral arterial circle (of Willis) plus cranial nerves and aneurysms.

Before we get into the cranial cavity itself, let's run through layers of the head - or the soft tissues on the outside of the skull. Then, we'll move through layers of bone forming the calvarium itself. Finally, we will look at layers of meninges that protect the brain.

Scalp

Do you know how helpful the scalp is? I mean, it covers our skull bones, for most of us it allows hair to cover our heads, AND it forms a handy mnenomic device for remembering the many layers of SCALP. How cool is that? Let's cover them in order:

Skin consisting of epidermis and dermis
Connective tissue represented by loose CT adipose in the hypodermis
Aponeurosis connects the two bellies of the occipitofrontalis muscle
Loose areolar CT allows the top 3 layers to glide on the bottommost layer
Pericranium/periosteum is a bit of CT over the cranial bones themselves

Layers of the calvarium

The calvarium of the cranial cavity is the bulbous part that protects the brain. The bones are "flat" (they all have a slight curve of them, of course, but they are typically described as the "flat bones" of the skull). The calvarium include the frontal, parietals, temporals, and occipital bones.

"Flat" bones of the cranium have three layers - basically an inside, middle, and outside.

Ectocranium - compact bone forming the outer table of the calvarium, covered by the scalp
Diploe - the middle layer of trabecular/spongy bone and red bone marrow
Endocranium - compact bone forming the inner table, includes grooves for veins and arteries

Let's talk a bit more about interesting features of the endocranial surface. The grooves for dural venous sinuses are quite large and the flow of blood within them is pretty important so I cover them in detail below. That section includes a discussion of grooves for a few arteries on the endocranial surface, too.

Additionally, small depressions on the endocranial surface can be seen on either side of the sagittal sinus - these are called granular foveolae - one of my favorite anatomy words EVER! Granular foveolae are small indentations that correspond to arachnoid granulations - tufts of arachnoid mater that allow cerebrospinal fluid to drain into the superior sagittal sinus. All of that is covered below but I mention it here because of the small bony indentations seen on the endocranium called granular foveolae <3

Cranial base internal fossae

If the calvarium and bones of the cranial vault are the roof of the cranial cavity, the base or floor would be bones like the ethmoid, sphenoid, basilar occiput, orbital plate of the frontal, and the mastoid, petrous, and tympanic portions of the temporal bone. The cranial base supports the brain from below, provides attachment points for the partitions of dura and many of the dural venous sinuses, and through it's many foramina and fissures provides routes by which blood supply can get into (and out of) the cranial cavity and for the cranial nerves to exit towards their peripheral targets. Get ready for it - there are 3 fossae that form the cranial base: anterior, middle, and posterior. Let's review them!

cranial fossae anterior middle posterior

Anterior cranial fossa

Located up front just as the name sounds, the anterior cranial fossa is formed by the orbital plate of the frontal and the lesser wings of the sphenoid bones. It includes bony features like the frontal crest and foramen cecum of the frontal, and the crista galli and cribriform plate of the ethmoid bone. The bony floor lies immediately beneath the inferior and anterior/rostal portions of the frontal lobe of the cerebrum. Contents of this fossa include the olfactory axons entering the cranial cavity through olfactory foramina of the cribriform plate of the ethmoid bone which synapse on the olfactory bulb of cranial nerve 1, the olfactory nerve. Olfactory tracts carry that information deeper into the forebrain so they too can also be seen in the anterior cranial fossa. The ridge of bone along the posterior free margin of the lesser wing of the sphenoid bone marks the end of the anterior cranial fossa.

Middle cranial fossa

The central or middle cranial fossa lies between the edge of lesser wing of sphenoid bone to the superior petrosal ridge of the petrous temporal bone. The floor of this space is formed by the greater wing of the sphenoid bone and the anterior surface of the petrous portion of the temporal bone. The space is occupied by the temporal lobes of the cerebrum. Centrally we find the body of the sphenoid bone with the sella turcica and many features related to the pituitary gland. On either side of that lies the cavernous (venous) sinus that we'll cover below. Intracranial portions of cranial nerves 2, 3, 4, 5, and 6 move through this fossa, as does portions of the internal carotid artery and anterior branches of the circle of Willis - major blood supply to the anterior brain and cranial cavity.

Posterior cranial fossa

Moving towards the back now, we find the posterior cranial fossa located from the superior petrosal ridge to the squamous occiput. Centrally, the anterior boundary is the dorsum sellae of the sphenoid bone. The floor and bony features of the posterior cranial fossa include the clivus, foramina magnum, jugular foramen, hypoglossal canal, and internal auditory meatus. The cerebellum, pons, and medulla occupy this space. Intracranial portions of cranial nerves 7, 8, 9, 10, 11, and 12 all move through this region. Blood supply to the posterior regions of the brain arrives by way of vertebral arteries. Posterior portions of the circle of Willis are also found here. The largest portions of dural venous sinuses terminate within this fossa to then exit the skull largely through the jugular foramen.

Cranial base & foramina

Several important relationships between vessels, nerves, and the bony housing of the cranial cavity are worth reviewing here. A good way to move through this material is to organize important foramina and fissures and what passes through them.

In the anterior cranial fossa we saw tiny holes in the cribriform plate of the ethmoid bone. These were how olfactory neurons pass axons into the cranial cavity.

Within the lesser wing of the sphenoid bone are large bony canals called the optic canal. The optic nerve (CN 2) and the ophthalmic artery use this canal to move from the middle cranial fossa to/from the orbit.

Between the lesser and greater wings of the sphenoid bone is the superior orbital fissure. This bony space connects the orbit and middle cranial fossa. Cranial nerves 3, 4, V1 (ophthalmic division of trigeminal), and 6 pass through the superior orbital fissure, as does the superior ophthalmic vein (a large venous connection between the superficial face and orbit and the cavernous sinus inside the cranial cavity).

Remember all those holes in the floor of the middle cranial fossa? The largest one is foramen ovale which allows CN V3, the mandibular division of the trigeminal nerve, to exit the cranial cavity. Just posterior and a bit lateral to that is the foramen spinosum. This is how the middle meningeal artery (a branch of the maxillary artery in the infratemporal fossa) enters the cranial cavity to put meningeal grooves on the endocranial surface of the calvarium. Anterior and medial to foramen ovale is foramen rotundum, which allows CN V2 (maxillary division of trigeminal) to move between the pterygopalatine fossa and the middle cranial fossa. Also recall that foramen lacerum is just adjacent to the sella turcica. Though this foramen is filled with cartilage in life, the large internal carotid artery passes superiorly above it, allowing the deep petrosal sympathetic nerve to leave the tunica adventitia of that vessel and pass through the cartilage inferiorly towards the pterygoid canal and into the pterygopalatine fossa.

In the posterior cranial fossa we see the internal auditory meatus. Here, cranial nerves 7 and 8 exit the cranial cavity. The facial nerve forms a few different branches while it is in the petrous temporal bone and exits to eventually innervate muscles of facial expression, taste from the anterior two-thirds of the tongue, and provide parasympathetic motor innervation to nearly all salivary and mucosal glands of the head. The vestibulocochlear nerve never leaves the petrous potion of the temporal bone because it's targets are the ear for hearing and balance.

Also in the posterior cranial fossa is the large jugular foramen. This allows cranial nerves 9, 10, and 11 to exit the cranial cavity. Tucked inside the medial wall of the foramen magnum is the hypoglossal canal, allowing cranial nerve 12 to exit the cranial cavity and head towards the tongue. Finally, the foramen magnum transmits the medulla oblongata, meninges, and vertebral arteries into/out of the cervical region.

Cranial nerves on the brain and exiting the cranial cavity

While we are here, let's review a bit of the cranial nerves.

Olfactory nerve: special sense of smell (SVA) consisting of bipolar neurons in the olfactory mucosa of the nasal cavity. Axons pass superiorly through the cribriform plate of ethmoid bone to synapse within the olfactory bulb. The olfactory tract takes fibers into limbic structures, thalamus, and cortex.
Optic nerve: a nerve for the special sense of vision (SSA). Bipolar neurons lie in the retina. Those axons form the optic nerve. This nerve travels in the optic canal within the lesser wing of sphenoid bone. Left and right optic nerves form the optic chiasm. From there, optic tracts pass to lateral geniculate bodies of thalamus, then optic radiations, then to the visual cortex of occipital lobe.
Oculomotor nerve: this one provides motor fibers to skeletal muscles that move the eye (GSE) and smooth muscles in the orbit (GVE parasympathetic). It exits the midbrain at the interpeduncular fossa. Then, it passes along the wall of the cavernous sinus (within it). Within the superior orbital fissure it forms superior and inferior divisions that reach muscles on the upper and lower eye. Motor fibers to skeletal muscles (GSE fibers) reach the superior rectus, inferior rectus, medial rectus, and inferior oblique muscles. Motor fibers to smooth muscle of the eye (GVE fibers) reach the pupillary constrictor muscle and the ciliary muscle.
Trochlear nerve: a small nerve that provides motor innervation to the to superior oblique muscle (GSE). It arises from the contralateral trochlear nucleus, decussates in the midbrain, then emerges on the dorsal surface caudal to inferior colliculus. It passes through the subarachnoid space, cavernous sinus, then the superior orbital fissure. It courses superior to (and outside of) the common tendinous ring towards the superior oblique muscle near it's origin on the sphenoid.
Trigeminal nerve: provides general sense (GSA) from the superficial and deep faces, including most cavities. It also provides motor innervation (SVE) to muscles of mastication (arch 1 derivatives). The TRIgeminal has three parts: ophthalmic (V1), maxillary (V2), and mandibular (V3) nerves. CN V1 and V2 are entirely sensory. CN V3 is a mix of motor and sensory fibers. Each division exits the cranial cavity independently. Branches move through the superior orbital fissure (V1), foramen rotundum (V2), and foramen ovale (V3).
Abducens nerve: motor to lateral rectus muscle (GSE). The abducens nucleus is in the cadual pons. It exits at the inferior pontine sulcus, pierces the dura mater over the clivus to pass through the cavernous sinus, and accesses the orbit via the superior orbital fissure then common tendinous ring. It dives into lateral rectus from the medial side.
Facial nerve: with nuclei in the pons, the facial nerve exits the brain stem at the pontomedullary junction. It enters the internal auditory meatus and branches into several named nerves (chorda tympani n., greater petrosal n., and then just the "facial n." The facial nerve proper exits the cranial cavity by way of the stylomastoid foramen. It has motor targets in the muscles of facial expression (SVE to arch 2 derivatives). Greater petrosal nerve and chorda tympani nerve both carry pre-ganglionic parasympathetic (GVE) innervation to most salivary and mucosal glands of the face. Greater petrosal nerve exits the petrous portion of the temporal bone, puts a groove of the same name on the floor of the middle cranial fossa, and heads towards the pterygoid canal and pterygopalatine fossa. Chorda tympani nerve exits the petrous portion of the temporal bone at the petrotympanic fissure. In addition to parasympathetic motor fibers, it also includes sensory targets in the form of special sense (SVA) from the anterior two-thirds of the tongue. The facial nerve includes a small bit of general sense (GSA) from the external ear.
Vestibulocochlear nerve: provides the special sense of hearing and our sense of balance (SSA fibers. It emerges from the brain at the cerebellar-pontine angle. From there it quickly enters the internal auditory meatus. Structures for balance include the vestibular ganglion, hair cells in cristae ampullaris, hair cells in maculae of utricle and saccule, and the flocculonodular lobe of the cerebellum. Structures related to hearing include the spiral ganglion and hair cells in the organ of Corti.
Glossopharyngeal nerve: this nerve includes many types of fibers to and from arch 3 derivatives of embryological development. It exits the medulla at the postolivary sulcus. It then exits the cranial cavity at the jugular foramen. CN 9 includes fibers for general sense (GSA & GVA) from the posterior one-third of the tongue, the auditory tube, tonsils, oropharynx, and tympanic cavity. It also listens to baro- and chemoreceptors (GVA fibers) in the carotid body and sinus. There is more though! The glossopharyngeal also provides for the special sense of taste (SVA fibers) from the posterior one-thrid of the tongue. And, it helps with salivation from the parotid gland (GVE fibers) and swallowing (SVE fibers) via stylopharyngeus muscle.
Vagus nerve: (love this one!) exits the medulla at the postolivary sulcus. It exits the skull at the jugular foramen. Targets of the vagus nerve in the neck are derivative of arches 4 and 6 of a developing embryo. Fibers include general sense (GSA) from the posterior cranial fossa dura mater, the external auditory meatus, and the tympanic membrane. General sense (GVA fibers) comes from mucous membranes of the pharynx, larynx, and viscera in the thorax and abdomen (up to the splenic flexure). Taste buds near epiglottis (SVA) are also innervated through the vagus nerve. Motor (SVE fibers) go to laryngeal muscles, pharyngeal constrictors, and most soft palate muscles. Lastly, the vagus nerve is the main parasympathetic nerve below neck (GVE fibers), with targets like the heart, lungs, and most of the intestines.
Accessory nerve: this nerve exits the medulla at the postolivary sulcus, then exits the skull at the jugular foramen. It includes motor (SVE) fibers to the sternocleidomastoid and trapezius muscles that move the head and shoulder.
Hypoglossal nerve: exits the medulla in the preolivary sulcus, then exits the skull at the hypoglossal canal. CN 12 includes motor (GSE) fibers to intrinsic and extrinsic muscles of the tongue. All it's muscles end in -glossus except palatoglossus muscles (which is CN 10, instead). That includes extrinsic muscles like genioglossus, styloglossus, and hyoglossus muscles.

Whew! That was a lot, even for me!

Meningeal layers

Meninges come in three layers - again, and outside, middle, and inside layers. Anatomists are predicable and love the rule of threes, huh?! :D All kidding aside, these layers surround, support, and protect the brain. All three layers of meninges extend into the spinal canal, too.

Dura mater

Dura mater is the outermost layer of meninges proper. The name dura mater translates to "tough mother" and it is no joke! The dura mater is super tough and protective. It is made of fibrous connective tissue with no elasticity. The dura mater may also be called the pachymeninx which means “thick membrane.”

The dura mater is itself made of two layers - periosteal (or external) and meningeal (or internal). In most places these two layers of dura mater are fused. However, the dura mater forms partitions inside the cranial cavity (falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sella - covered below!). The periosteal/external layer of dura mater is always adhered to the inside aspect of the cranial bones; the meningeal/internal layer peels off to form partitions. The space between external and internal dural layers is filled with venous blood, collectively forming dural venous sinuses (also covered below!). The meningeal layer of dura extends into the vertebral canal to surround the spinal cord (the periosteal layer stays in the cranium since vertebrae have their own periosteum).

Before we leave this layer, let's briefly discuss spaces on either side of the dura mater. The extradural/epidural space could develop between dura mater and endocranial surface of bone but it isn’t supposed to be there; if it is, it is pathological (ie: an epidural hematoma caused by broken arteries lining the endocranial surface, usually branches of the middle meningeal artery). The subdural space is technically between the dura and arachnoid mater layers. This is a "potential" space meaning it shouldn't really have anything in it since the arachnoid is supposed to be pushed up against the dura by arachnoid trabeculae (akin to fibroblasts) and cerebrospinal fluid. If blood or other fluid is within the subdural space it is indicative of head trauma or pathology, or you are looking at meninges in a cadaver since CSF cannot support the arachnoid so it sinks down onto the brain itself.

Arachnoid mater

Arachnoid mater forms the middle layer of meninges. The layer is named for the "web like" look of arachnoid trabeculae that support the arachnoid mater. The tissue itself is avascular, composed of fibroblasts, collagen, and some elastic fibers. The subarachnoid space is filled with cerebrospinal fluid (CSF) which also pushes the arachnoid mater itself up against the dura mater (meningeal layer, if we're being specific). Parallel to the sagittal suture the arachnoid mater forms tufts that project into the superior sagittal (dural venous) sinus located there. These tufts are called arachnoid granulations - they push against the endocranial layer of bone to form arachnoid foveolae. Arachnoid granulations allow CSF to flow back into the venous system, conserving that fluid and allowing fresh CSF to flow through the subarachnoid space.

Pia mater

Pia mater is the "delicate mother" that lies immediately on the surface of the cortex of the brain. It consists of a single layer of fibroblasts externally which are supported internally by the feet processes of astrocytes (one of the glial cells of the brain). One cannot dissect the pia mater away from the brain the way that the dura or arachnoid mater can be lifted. The pia contributes to the blood-brain barrier, protecting the neurons of the cerebral gray matter cortex from blood. Pia follows all the gyri and sulci of the brain, and forms periarterial sheaths along cerebral vasculature as vessels enter and exit brain tissue itself.

Leptomeninges

One final note about terminology - together the arachnoid and pia mater may be referred to as leptomeninx or “slender membrane.” These layers are derived from one layer in a developing embryo (from neural crest mesenchyme); they are parietal and visceral components of the same embryonic layer. Arachnoid trabeculae connect the layers in life (these are flattened, irregular-shaped fibroblasts).

Partitions of dura mater

Let's return to the idea of a two-layered dura mater that forms several important structures within the cranial cavity - partitions and venous sinuses. Recall that the periosteal/external layer of dura mater never leaves the endocranial surface of bone. But, the meningeal/internal layer of dura peels off the periosteal/external layer to create both the partitions and the dural venous spaces.

The falx cerebri is a sickle-shaped partition of dura that lies between left and right cerebral hemispheres. Thus, this midline structure lies in the longitudinal cerebral fissure. Anteriorly it is attached to frontal crest and crista galli of the ethmoid bone. Posteriorly it attaches to the internal occipital protuberance. Both the superior and inferior dural venous sinuses are found within this fold of dura mater.

The falx cerebri fans out to become the tentorium cerebelli which attaches to the superior petrosal ridges, clinoid processes of the sphenoid bone, and the groove for transverse sinus of the occipital bone. This partition of dura separates occipital cerebral lobes from the cerebellum. The area above it is known as the "supratentorial" space while that below it is the "infratentorial" space. A concave opening in the anterior portion of the tentorium cerebelli, known as the tentorial notch, allows the brain stem to pass through. This partition creates the superior petrosal sinus as well as the transverse dural venous sinus.

A small partition of dura is found in the midline of the posterior cranial fossa, between the left and right hemispheres of the cerebellum - thus, it is called the falx cerebelli. It is attached to and runs along the internal occipital crest. The occipital dural venous sinus runs within this partition.

The smallest partition of dura mater is the diaphragma sellae, an opening over sella turcica of the sphenoid bone. This circular opening in the dura mater is suspended between the four clinoid processes of the sella turcica. It forms a partial roof over the hypophysial/pituitary fossa, allowing the infundibular stalk and hypophysial venous plexus to pass through.

Spaces of the meninges

The extradural or epidural “space” lies between the periosteal dura mater and endocranial surface of bone. It is only present pathologically (eg: torn meningeal vessels). This space is not continuous with the spinal epidural space because the periosteal dura stays in the cranium (vertebrae have their own periosteal layer).

The subdural “space” lies between the meningeal dura and arachnoid mater layers. Normally this contains very small amounts of extracellular fluid but otherwise is not a natural space.

As discussed above, the subarachnoid space lies between arachnoid and pia mater layers. It is a real space that contains CSF, trabecular cells, and cerebral vessels.

Hematomas

Hematomas are pathological collections of blood where it should not be. In the cranial cavity hematomas form between layers of meninges or within the brain tissue itself.

Epidural hematomas result from arterial (high pressure) blood, usually from middle meningeal artery and branches.
Subdural hematomas result from venous (low pressure) blood, usually from torn bridging veins connecting to the superior sagittal dural venous sinus.
Intracerebral hematomas result from vascular breaks within brain tissue itself.

Dural venous sinuses (DVS)

Like the meninges we discussed above, one of the important concepts to understand inside the cranial cavity are the dural venous sinuses and the flow of blood through them. Recall that the meningeal layer of dura mater reflects away from the outer layer, leaving spaces called dural venous sinuses. Many of these sinuses create grooves of the same name on the endocranial surface of the cranium.

Dural venous sinuses contain venous drainage from the brain and cranium itself, most of which ends up in the internal jugular vein. Thus, the direction of blood flow tends to flow posteriorly and inferiorly. Let's review the major dural venous sinuses before we summarize the flow of blood.

The superior sagittal sinus lies in the superior border of the falx cerebri. It is a major drainage point for superior cerebral veins and AND cerebrospinal fluid. Above, we reviewed arachnoid mater and the arachnoid granulations that form along the superior sagittal sinus, leaving granular foveolae depressions on the endocranial surface of the calvarium. The arachnoid granulations are tufts of arachnoid mater. Since cerebrospinal fluid continues to be produced at a constant rate, pressure pushes excess CSF to drain through the arachnoid granulations into the superior sagittal venous sinus. Venous blood drains into the superior sagittal sinus from superior cerebral veins and emissary veins of the scalp. Blood (&CSF) flows to drain posteriorly into the confluence of sinus.

Speaking of, the confluence of sinus sits near the internal occipital protuberance, posteriorly. The confluence receives venous blood from the superior sagittal, straight, and occipital dural venous sinus. Blood then flows out of the confluence through both transverse sinuses, into the sigmoid sinuses, and finally out of the cranial cavity through the jugular foramen as the internal jugular vein.

A very important venous space is the cavernous sinus that sits on either side of the sella turcica of the sphenoid bone. The cavernous sinus includes venous blood of course, but several neurovascular structures move through it too - the internal carotid artery and sympathetic plexus, cranial nerves 3, 4, 6, and V1 (ophthalmic nerve). The cavernous sinuses are formed as the meningeal dura on the floor of the middle cranial fossa reflects superiorly towards the lesser wing of the sphenoid bone. Left and right cavernous sinuses are connected in the midline by the intercavernous sinus. The cavernous sinus is an important connection between veins of the face and orbit, the cranial cavity, and the deep face. It receives venous blood from the sphenoparietal, superior and inferior ophthalmic veins, the hypophysial plexus, and superficial middle cerebral v. The cavernous sinus itself drains into the superior and inferior petrosal sinuses, basilar, and pterygoid venous plexuses in the infratemporal fossa.

The inferior sagittal sinus runs along the inferior margin of the falx cerebri, mirroring the superior sagittal sinus. It flows into the straight sinus.

The straight sinus runs in the “seam” between the falx cerebri and tentorium cerebelli partitions of dura mater. The straight sinus receives the inferior sagittal sinus and the great cerebral vein (of Galen). The straight sinus carries blood to the confluence of sinus (which drains into the transverse then sigmoid sinuses to form the internal jugular vein outside of the cranial cavity).

The transverse sinus extends laterally from the confluence of sinus. It courses within the groove of the same name on the endocranial surface of the occiput, along the posterolateral margin of the tentorium cerebelli. The superior petrosal sinus joins the transverse sinus just as it bends inferiorly to become the sigmoid sinus.

The sigmoid sinus is an S-shaped extension of the transverse sinus. It too forms wide grooves of the same name in the posterior cranial fossa. The sigmoid sinus ends at the jugular foramen. The inferior petrosal sinus joins the sigmoid just superior to the jugular foramen and formation of the internal jugular vein.

The superior petrosal sinus runs along the superior petrosal ridge of the temporal bone within the attachment of the tentorium cerebelli. It creates a bony groove along its course. The superior petrosal sinus carries blood from the cavernous sinus to the transverse sinus (then sigmoid sinus and into the internal jugular vein).

The inferior petrosal sinus occupies a groove created by the junction of the petrous temporal bone (medial side) and the basilar occiput. The inferior petrosal sinus carries blood from the cavernous sinus to the internal jugular vein right at the jugular foramen. It also communicates with basilar and vertebral venous plexuses.

Here is a good place to mention many small emissary veins that are technically not dural venous sinuses themselves but they do form connections between the dural venous sinuses and veins of the scalp and cranial vault. Frontal emissary veins at the foramen cecum connects the superior sagittal sinus to mucosal veins of the paranasal (air) sinuses. Parietal emissary veins connect the superior sagittal sinus to scalp veins. Mastoid emissary veins connect the sigmoid sinus to occipital or posterior auricular veins. Posterior condylar emissary veins connect sigmoid and suboccipital venous plexuses.

Download the dural venous sinus drawing

CSF cerebrospinal fluid

Now that we know where cerebrospinal fluid drains (arachnoid granulations into the superior sagittal sinus), let's talk about what CSF is, where it is produced, and how it flows. CSF protects the brain as a cushion, preventing the weight of the brain from compressing nerves and vessels in the cranial base. It is similar to blood plasma, but with less protein.

Cerebrospinal fluid is produced by ependymal cells of the choroid plexus that lines all ventricles and the central canal of the spinal cord. It is produced at a rate of ~400-500 mL/day.

As we've seen above, arachnoid granulations project into dural sinuses to return CSF to blood.

Cisterns and flow of CSF

Even though CSF is produced in all ventricles of the brain and central spinal cord, the largest collection of ependymal cells that produce CSF are found in the lateral ventricles (they are the biggest, after all) so it makes sense to start there. CSF is always flowing though, since it is produced at a regular rate. Nonetheless, let's start in the lateral ventricles and track the flow of CSF.

From the lateral ventricles CSF flows through the interventricular foramen (foramen of Monroe) into the 3rd ventricle. From there it flows through the cerebral aqueduct into the 4th ventricle. Then, it flows out through either one median aperture (foramen of Magendie) or two lateral apertures (foramina of Luschka). It can also flow into the central canal of the spinal cord and cisterns within the subarachnoid space.

Cisterns are dilations within the subarachnoid space. There are several of them, named for the regions of brain they are nearest to. For example, the cerebellomedullary cistern (the largest cistern) is between the cerebellum and medulla oblongata. It includes posterior (cisterna magna) and lateral portions. A pontocerebellar cistern lies ventral to pons and is continuous with the spinal subarachnoid space inferiorly. An interpeduncular cistern lies between cerebral peduncles of the midbrain. Similarly, the chiasmatic cistern is found inferior and anterior to the optic chiasm. A quadrigeminal cistern lies posterior to the corpora quadrigemina (also known as the superior cistern or the cistern of the great cerebral vein of Galen). An ambient cistern lies along the lateral aspect of the midbrain, continuous with the quadrigeminal cistern posteriorly. Finally, a lumbar cistern surrounds the cauda equina within the central canal of the vertebral column, near the L2-4 levels.

Blood supply to/from dura mater

The dura is supplied with blood from nearby arteries to the calvarium. Venous drainage of the dura parallels arterial supply.

The main artery that supplies dura mater is the middle meningeal artery, a branch of the first part of the maxillary artery. The middle meningeal artery enter the cranial cavity via the foramen spinosum, a hole in the greater wing of the sphenoid bone, in the floor of the middle cranial fossa. Meningeal grooves chart the course of this artery along the endocranial wall. A frontal branch passes near pterion then towards vertex - this is the branch that most often leads to an epidural hematoma if ruptured. A parietal branch passes posteriorly. The middle meningeal vein drains into the pterygoid venous plexus in the infratemporal fossa.

Other arteries that supply the dura include the ophthalmic, occipital, and vertebral arteries.

Innervation of dura mater

Nerve fibers innervate the dura mater, just as it receives blood supply and venous drainage. These are nociceptors that transmit pain to nerve nearby nerves. As with the superficial and deep face regions, the trigeminal nerve is the main nerve we're concerned with here, though it is not the only one, as we'll see.

Regions of the anterior and middle cranial fossae dura mater, plus the supratentorial dura mater, are innervated by anterior meningeal branches from the anterior ethmoidal nerve (a branch of the nasociliary nerve from CN V1 the ophthalmic division of trigeminal nerve). CN V1 has the largest distribution of sensory fibers to the dura, though CN V2 and V3 also have meningeal branches to the anterior and middle cranial fossae. Meningeal nerves also run in the periarterial plexuses of the middle meningeal artery.

In the posterior cranial fossa and infratentorial dura mater (the inferior surface of the tentorium cerebelli), cervical spinal nerves from C2 and 3 use CN 10 and 12 to climb into the cranial cavity and innervate the dura there. CN 10 and 12 (and some include 11) also have their own meningeal nerves.

The density of sensory innervation is greater near the superior sagittal sinus, in the tentorium cerebelli, and along vessels supplying or draining the dura.

Dural pain is referred as a headache, arising in the regions corresponding to cutaneous innervation of the same nerve (ie face and mucosa, or upper cervical spine).

Blood supply to the brain and cranial cavity

Blood supply to the cranial cavity and brain itself is kind of a big deal - and the arteries that contribute to brain blood supply are themselves quite large. Generally speaking, we can conceptualize anterior (internal carotid arteries) and posterior (vertebral arteries) contributions to blood supply in the cranial cavity and brain. They will connect through the cerebral circle (also known as the circle of Willis). Terminal branches of these vessels run in the subarachnoid space.

Internal carotid artery

The internal carotid arteries (ICA) are branches of the common carotid arteries that form in the neck around the C4 vertebral level. The internal carotid arteries have no branches in the neck - they head straight for the carotid canal in the petrous portion of the temporal bone to get into the cranial cavity. Thus, the ICA has four portions: cervical, petrosal, cavernous, and cerebral.

Cervical ICA starts when the vessel branches
from the common carotid a. ~C4 vertebral level. Gray rami communicantes from the sympathetic chain join the ICA and course within the tunica adventitia of this vessel. These fibers coalesce as the “internal carotid nerve” as the vessel enters the carotid canal.
Petrosal ICA is within the carotid canal of the petrous temporal bone. It courses anteromedially within the bony canal.
Cavernous ICA is the portion that exits the carotid canal and bends anteriorly then superiorly within the cavernous sinus. This portion courses superior to foramen lacerum (closed by cartilage) and creates the “carotid groove” on the side of the sphenoid body.
Cerebral ICA enters the middle cranial fossa and bends again, 180° posteriorly, and forms terminal branches (anterior & middle cerebral arteries). The bends of the petrous and cavernous portions of the ICA are called the “carotid siphon.” There is a small amount of heat exchange between the warm ICA blood and the venous blood in the cavernous sinus. This difference is not significant for humans, but it is so for animals that run fast or long distances (e.g.: horses, cheetahs).

Vertebral arteries

Left and right vertebral arteries contribute to posterior supply of the brain and cranial cavity. Vertebral arteries form in root of the neck as the first branch of the subclavian artery. Asymmetry is common, and the left side is usually larger than the right. Like the ICA, vertebral arteries are quite long so they have different segments:

Cervical vertebral arteries course within the transverse foramina of cervical vertebrae from C6 to C1 levels.
Atlantic portions of the vertebral arteries are related to the superior aspect of C1. This section pierces dura and arachnoid maters to pass through the foramen magnum into the cranial cavity.
Intracranial portions of left and right vertebral arteries unite at near the caudal pons to form the basilar artery.
Basilar artery (technically it's own vessel and no longer a vertebral artery) itself terminates as posterior cerebral arteries.

Cerebral arteries

The cerebral cortex and deeper brain tissues are supplied by cerebral arteries coursing in the subarachnoid space:

Anterior cerebral artery comes from the ICA. It supplies medial and superior portions of the cerebral hemispheres, except for the occipital lobe. Left and right anterior cerebral arteries are connected by the anterior communicating artery.
Middle cerebral artery is a large branch of the ICA. It supplies most of the lateral surface of cerebral hemispheres and the temporal pole.
Posterior cerebral artery comes from the basilar artery. It supplies the inferior cerebrum and occipital pole. The posterior communicating artery connects the posterior cerebral artery to the internal carotid artery, completing the cerebral arterial circle (of Willis).

Circle of Willis (cerebral arterial circle)

Blood supply to the brain is so important that vessels form regular anastomoses so that the brain gets the oxygen it needs. Technically speaking, the circle of Willis (cerebral arterial circle) lies only within the interpeduncular fossa around the midbrain but it is formed by several arteries feeding into it and connecting anterior (ICA) and posterior (vertebro-basilar) artery branches.

From anterior to posterior, the circle is formed by:

Anterior communicating a.
Anterior cerebral a.
Internal carotid a.
Posterior communicating a.
Posterior cerebral a.

To review, posterior communicating artery is found between the ICA and posterior cerebral arteries. It connects anterior and posterior circulations of the brain. Similarly, the anterior communicating artery is found between right and left anterior cerebral arteries. It forms the arterial connections necessary to complete the circle of Willis (cerebral arterial circle).

Download the arterial circle drawing

Venous drainage of the brain

Many valveless cerebral and cerebellar veins drain into adjacent dural venous sinuses (DVS), most of which drain into the internal jugular vein at the jugular foramen.

Superior cerebral veins drain into the superior sagittal dural venous sinus
Inferior and superficial middle cerebral veins drain into the straight, transverse, and superior petrosal dural venous sinuses
Great cerebral vein (of Galen) meets the inferior sagittal dural venous sinus to drain into the straight dural venous sinus
Superior and inferior cerebellar veins drain into transverse and sigmoid dural venous sinuses

Summary

Well, that was fun! What a whirlwind tour through things that live in your head (and mine, too!). From arteries, veins, special veins called dural venous sinuses, to cranial nerves and how they exit the cranial cavity, meninges and their spaces, plus TWO downloadable PDF drawings - I hope we covered what you needed.

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Cranial cavity