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1. Identify the anterior, middle, and posterior cranial fossae. What are the pertinent foramina/fossae/canals in each, and what do they transmit?

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Boundaries of the cranial fossae:

Anterior cranial fossa

Middle cranial fossa

Posterior cranial fossa

2. What are the three types of cranial meninges? What is the difference between cranial and spinal dura mater? Where are dural infoldings/partitions located?

• Pia mater ("delicate mother")

  • is a delicate investment that is closely applied to the brain and dips into fissures and sulci.

  • enmeshes blood vessels on the surface of the brain.

• Arachnoid mater ("spidery mother")

  • is a filmy, transparent, layer that is connected to the pia mater via trabeculae.

  • is separated from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid (CSF). The subarachnoid space may contain blood after hemorrhage of a cerebral artery (formation of subarachnoid hematoma).

  • projects into the superior sagittal sinus to form arachnoid villi, which serve as sites where CSF diffuses into the blood.

    • Arachnoid granulations

      • are tuft-like collections of highly folded arachnoid that project into the superior sagittal sinus and its lateral lacunae (lateral extensions of the superior sagittal sinus).

      • release CSF into the superior sagittal sinus and often produce erosion or pitting (granular foveolae) of the inner surface of the calvaria.

• Dura mater ("tough mother")

  • outermost covering

  • two layers in the cranium (one in the spinal canal)

    • periosteal dura - lines the cranial cavity

    • meningeal layer - inner layer that is sometimes separated from the periosteal layer, forming dural venous sinuses and partitions

The tentorial notch is a distinct curved notch in the tentorium cerebelli. The notch also separates the tentorium cerebelli from the diaphragma sellae.

3. What are the two main arteries that supply the brain? What is the clinical importance of the cerebral arterial circle (circle of Willis)? What arteries make up the arterial circle?

• Internal carotid a. - gives rise to the:

  • Ophthalmic a,

  • Posterior communicating a,

  • Anterior cerebral a, and

  • Middle cerebral a.

• Vertebral a. - gives rise to the:

  • Anterior spinal a. - paired branches that unite in the midline.

  • Posterior inferior cerebellar aa., from which arise the

    • Posterior spinal arteries

  • Basilar a. - formed by union of the vertebral arteries, it gives rise to:

    • Anterior inferior cerebellar aa.,

    • Superior cerebellar aa., and bifurcates into the

    • Posterior cerebral aa.

Cerebral arterial circle (Circle of Willis) - forms an important means of collateral circulation in case of obstruction. The circle itself is an example of collateral circulation, but branches of the circle are end arteries and there is little collateral circulation in the brain itself. Formed by the union of the anterior cerebral, anterior communicating, posterior communicating, and posterior cerebral arteries.

4. Map out the dural venous sinuses. Where do the majority of the sinuses eventually drain?

The cranial dura mater is composed of two, fused layers: meningeal (inner) and endosteal (outer). In certain areas, the layers separate, forming dural venous sinuses, allowing drainage of blood from the brain.

The superior sagittal sinus runs the length of the superior falx cerebri, the dural partition that runs between the cerebral hemispheres in the longitudinal fissure. It drains into the confluence of sinuses.

The confluence of sinuses is an important landmark in understanding dural venous sinuses. This is the junction of three sinuses: superior sagittal, straight, and occipital, that then flow into the bilaterally paired transverse sinuses. It is typically located superolateral to the internal occipital protuberance.

The inferior sagittal sinus is located on the inferior border of the falx cerebri, and continues inferiorly, uniting with the great cerebral v., as the straight sinus. The straight sinus is located at the meeting point of the falx cerebri and tentorium cerebelli and drains into either the confluence of sinuses, or left transverse sinus. The occipital sinus is difficult to locate in dissection, but can be identified in the falx cerebelli, draining into the confluence of sinuses.

The transverse sinuses extend laterally along the occipital bone (as indicated by marked indentations in the bone). At the level of the petrous part of the temporal bone, these sinuses are known as sigmoid sinuses. The sigmoid sinuses become the internal jugular veins (after uniting with the inferior petrosal sinuses) at the level of the jugular foramina.

The superior and inferior petrosal sinuses can be located superior and inferior to the petrous part of the temporal bone, respectively. The superior petrosal sinus drains into the proximal sigmoid sinus. The inferior petrosal sinus drains into the distal sigmoid sinus to form the internal jugular vein.

The cavernous sinuses are relatively expansive sinuses lateral to the sella turcica, and extending between the superior orbital fissures (anteriorly) and the apices of the petrous part of the temporal bones (posteriorly). Tributaries of these sinuses include ophthalmic veins, sphenoparietal sinuses, and superficial middle cerebral veins. The cavernous sinuses drain into the petrosal sinuses and emissary veins. Many structures traverse the cavernous sinus, including the internal carotid aa., abducens n. (CN VI), and carotid plexus of sympathetic nn. The oculomotor n. (CN III), trochlear n. (CN IV), and the ophthalmic (V1) and maxillary (V2) divisions of the trigeminal n. (CN V) are located in the lateral walls of the cavernous sinuses. Cavernous sinus thromboses may endanger these structures.

5. Understand the basics of the cranial nerves. Name, number, sensory (afferent)/ motor (efferent)/both, and what general structures do they innervate?

6. What is cerebrospinal fluid (CSF)? What is CSF’s functions, and where is it located? Describe the ventricular system.

CSF is formed in the brain from arterial blood supplying the choroid plexuses of the ventricles, and drains via arachnoid granulations projecting into the superior sagittal sinus.

• Parts of the ventricular system

  • The cerebral hemispheres are hollow, each containing a lateral ventricle. The ventricles contain secretory tissue, choroid plexuses, which convert arterial blood into cerebrospinal fluid (CSF). The lateral ventricles communicate with the midline third ventricle by way of the interventricular foramen. A thin membrane with an attached choroid plexus roofs the third ventricle. In the midbrain, the narrow cerebral aqueduct connects the third and fourth ventricles.

  • The fourth ventricle lies between the pons, cerebellum, and the medulla. It communicates with the cerebral aqueduct, the central canal of the spinal cord, and the subarachnoid space. The roof of the fourth ventricle also has a choroid plexus. The roof of the fourth ventricle is perforated by a small median aperture and two lateral apertures that allow cerebrospinal fluid to exit the ventricular system and bathe the brain and spinal cord.

• The flow of CSF from production to reabsorption:

  • CSF is secreted (produced) by the choroidal epithelial cells of the choroid plexuses in the lateral, third, and fourth ventricles.

  • CSF leaves the lateral ventricles through the interventricular foramen and enters the third ventricle. CSF then passes through the cerebral aqueduct into the fourth ventricle. CSF leaves this ventricle through its median and lateral apertures and enters the subarachnoid space, which is continuous around the spinal cord and brain. The arachnoid forms borders of cisterns (spaces around the brain), filled with CSF.

• Reabsorption of CSF (reabsorption into the venous system) - the main site of CSF absorption (reabsorption) into the venous system is through arachnoid granulations. The subarachnoid space containing CSF extends into the arachnoid granulations, which in turn project upward through the dura into the superior sagittal sinus and lateral projections from it called lateral lacunae.