Note: The calvaria is a component of the cranium and is often referred to as the roof of the cranial vault (neurocranium). The cranial base (basicranium) is the other part of the neurocranium. The remainder of the cranium is the face (viscerocranium). In other words:

skull = cranium + mandible,

cranium = neurocranium + viscerocranium,

neurocranium = calvaria + basicranium.

Note: There are 5 tissue layers that cover the calvaria, and are illustrated by the mnemonic SCALP (superficial to deep): Skin, dense subcutaneous Connective tissue, epicranial Aponeurosis, Loose connective tissue, and Pericranium. The anatomical scalp is composed of the three most superficial layers. The loose connective tissue, composed mostly of areolar tissue, is considered the ‘danger zone’ of the scalp, as infections, blood, etc. may quickly spread through this zone and pass into the cranial cavity via valveless emissary vv.

2.) Reflect the soft tissue (e.g. temporalis m., occipitofrontalis m., and associated fascia) laterally to fully expose the skull. Take precaution in soft tissue reflection near the supra-orbital margin to best preserve supra-orbital neurovasculature, as it is transmitted via the supra-orbital notch/foramen.

Photo 2. Procedural: preparing for the craniotomy

3.) With an autopsy (Stryker) saw, complete a circumferential cut around the exposed bone of the calvaria (superior to both the supra-orbital margin and the external occipital protuberance). Cut through the bone in order to completely remove the calvaria, but preserve the meninges and brain. Propping the head and neck on a wooden block may stabilize this area.

Note: When using an autopsy saw to cut through the calvaria, take caution to not apply too much force, or cut too deeply. Try to cut just through the calvaria, and if there are any areas of bone remaining, you can use a chisel to break these final attachments.

Photo 3. Procedural: craniotomy

Observe the dura mater and superior sagittal sinus

Find these structures:

• Meninges

  • Dura mater

  • Arachnoid mater

    • Arachnoid trabeculae

    • Subarachnoid space

    • Arachnoid granulations

  • Pia mater

• Superior sagittal sinus

4.) Identify the dura mater covering the brain. Portions of dura mater may adhere to the endosteum of the calvaria.

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

Photo 4. Brain, with dura mater

5.) Locate the superior sagittal sinus between the cerebral hemispheres. Identify arachnoid mater (and specializations) and pia mater.

Note: 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.

Photo 5. Superior sagittal sinus

Note: The arachnoid and pia mater are often indistinguishable in dissection and are connected by arachnoid trabeculae. During life, these layers are separated by the subarachnoid space, which is filled with cerebrospinal fluid (CSF).

Photo 6. Pia mater

Note: Arachnoid granulations are projections of arachnoid mater into the dural venous sinuses and their lateral projections, lacunae. These are most obviously associated with the superior sagittal sinus, but also located with the superior petrosal, transverse, and straight dural venous sinuses. These granulations play a role in transfer of CSF into the venous system. These granulations (particularly when enlarged) are closely associated with bone and form pits, known as granular foveolae.

Photo 7. Granular foveolae

Remove the brain from the cranium, and observe structures and neurovasculature

Find these structures:

• Cranial nerves

  • Olfactory nn. (CN I)

  • Optic n. (CN II)

  • Oculomotor n. (CN III)

  • Trochlear n. (CN IV)

  • Trigeminal n. (CN V)

  • Abducens n. (CN VI)

  • Facial n. (CN VII)

  • Vestibulocochlear n. (CN VIII)

  • Glossopharyngeal n. (CN IX)

  • Vagus n. (CN X)

  • Accessory n. (CN XI)

  • Hypoglossal n. (CN XII)

• Arteries

  • Vertebral a.

    • Anterior spinal a.

    • Posterior inferior cerebellar a.

      • Posterior spinal a.

  • Basilar a.

    • Anterior inferior cerebellar a.

    • Superior cerebellar a.

    • Posterior cerebral a.

  • Internal carotid a.

    • Middle cerebral a.

    • Anterior cerebral a.

      • Anterior communicating a.

    • Posterior communicating a.

  • Cerebral arterial circle (of Willis)

6.) Lift the brain as superiorly as possible without doing damage. Carefully sever the cranial nerves and vasculature where they enter the basicranium using scissors. It may be necessary to incise some of the dural partitions, but try to maintain as many as possible. In particular, the tentorium cerebelli may need to be incised (with scissors) unilaterally to aid in removal of the cerebellum and brainstem. Cut (with scissors) the brainstem as distally as possible. Remove the brain.

Photo 8. Procedural: cutting neurovasculature

7.) Focus on the inferior portion of the brain and brainstem. Remove any meninges that may occlude the view of neurovasculature in this area. Locate the arteries on the inferior portions of the brain and anterior surface of the brainstem.

Note: The vertebral a., a branch of the 1st part of the subclavian a., ascends toward the brain through the transverse foramina of C6-C1 and the foramen magnum of the occipital bone. The anterior spinal a. is a small branch of combined contributions from the vertebral aa., which is located anteriorly in the midline of the medulla. The much larger posterior inferior cerebellar artery (PICA) is typically located at the level where the vertebral arteries join to form the basilar artery. This artery can be seen descending posterior to the glossopharyngeal and vagus nn. and will anastomose with the anterior inferior cerebellar a. Typically the posterior spinal aa. branch from the posterior inferior cerebellar aa., but they can arise independently from the vertebral aa.

Photo 9. Vertebral arteries and branches

Note: Vertebral aa. join at the junction of pons and medulla to form the basilar a., which runs along the midline of pons. The basilar a. has three main branches: anterior inferior cerebellar, superior cerebellar, and posterior cerebral (the terminal branches). The anterior inferior cerebellar aa. (AICA) are ventrally associated with abducens (CN VI), facial (CN VII), and vestibulocochlear nn. (CN VIII) , and form anastomoses with posterior inferior cerebellar aa. (PICA). The oculomotor n. (CN III) is located in a vulnerable position between the superior cerebellar and posterior cerebral aa.

Photo 10. Basilar a. and branches

Note: The internal carotid aa. (ICAs) are the major source of blood supply to the forebrain and may form anastomoses with branches of the basilar a. in the cerebral arterial circle (of Willis). The ICA is a branch of the common carotid artery and enters the skull through the carotid canal of the temporal bone.

Note: The ICA has three main branches in this area: middle cerebral a., anterior cerebral a., and posterior communicating a. The anterior (smaller) and middle (larger) cerebral aa. are the terminal branches of ICA. The middle cerebral a. is located in the lateral fissure of the brain. The anterior cerebral a. loops around the optic chiasm, moves anterolateral to the longitudinal fissure, and gives off a branch to the contralateral anterior cerebral a., the anterior communicating a.

Note: The cerebral arterial circle (of Willis) is a prominent anastomosis of the internal carotid aa. (anterior cerebral, anterior and posterior communicating aa.) and basilar a. (posterior cerebral aa.) branches, surrounding the infundibulum and optic chiasm. It is not uncommon for the cerebral arterial circle to be incomplete, i.e. missing one or more of the communicating branches.

Photo 11. Cerebral arterial circle (of Willis)

8.) Locate the cranial nerves (or components of the cranial nerves) from their origins on the brain and brainstem.

Note: Olfactory nn. (CN I), optic n. (CN II), and accessory n. (CN XI) are the only cranial nerves not originating from cranial nerve nuclei of the brainstem (midbrain, pons, and medulla oblongata). The olfactory bulb and tracts, which will be the only portions of the olfactory nn. visible today, are located on the inferior portions of the frontal lobes, lateral to the anterior cerebral aa. The optic nn. and chiasm can be located medial to the anterior cerebral aa. and near the internal carotid aa. The accessory n. rootlets can be seen ascending the spinal cord and closely associated with the medulla oblongata inferior to the vagus n.

Photo 12. Base of brain: olfactory tract, optic n., accessory n.

Note: The oculomotor (CN III) and trochlear (CN IV) nn. originate on the midbrain and can be located between the posterior cerebral a. and superior cerebellar a. Oculomotor n. is larger and oriented more in the midline. Trochlear n. is the smallest cranial nerve, and is the only cranial nerve that exits the brainstem dorsally.

Photo 13. Oculomotor n. and arterial associations

Note: Four cranial nerves originate on the pons: trigeminal (CN V), abducens (CN VI), facial (CN VII), and vestibulocochlear (CN VIII) nn. The trigeminal n. is the largest cranial nerve, and can be identified as a large trunk on the lateral sides of the pons. Abducens, facial, and vestibulocochlear (medial to lateral) are all located on the inferior border of pons.

Note: Glossopharyngeal (CN IX), vagus (CN X), and hypoglossal (CN XII) nn. originate on the medulla oblongata. Glossopharyngeal and vagus nn. are located on the lateral sides of the medulla, with the smaller glossopharyngeal more superiorly positioned. The hypoglossal n. rootlets are more medially and inferiorly positioned on the medulla.

Photo 14. Cranial nerves brainstem model

Photo 15. Cranial nerves brainstem model

Identify the dural partitions and dural venous sinuses

Find these structures:

• Dura mater

  • Falx cerebri

  • Diaphragma sellae

  • Tentorium cerebelli

    • Tentorial notch

  • Falx cerebelli

• Dural venous sinuses

  • Superior sagittal sinus

  • Inferior sagittal sinus

  • Great cerebral v.

  • Straight sinus

  • Occipital sinus

  • Confluence of sinuses

  • Transverse sinus

  • Superior petrosal sinus

  • Sigmoid sinus

  • Inferior petrosal sinus

  • Cavernous sinus

9.) Locate the dural partitions. If dural partitions were removed during an earlier step, look at other donors or plastinated specimens.

Note: Dural partitions are elaborations of the meningeal layer of cranial dura mater. There are four partitions: falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sellae.

Note: The falx cerebri is located in the longitudinal fissure between the cerebral hemispheres, while the falx cerebelli is positioned between the cerebellar hemispheres. Tentorium cerebelli is located between the occipital lobes of the cerebrum and the cerebellum. The forebrain is superior to the tentorium, while the hindbrain is inferior. The midbrain is located at the level of the tentorial notch, which is a distinct curved notch in the tentorium cerebelli. This notch also separates the tentorium cerebelli from the diaphragma sellae, which covers the sella turcica of the sphenoid bone and pituitary gland.

Photo 16. Dural partitions: falx cerebri & tentorium cerebelli

Photo 17. Dural partitions: Diaphragmatic sellae & tentorium cerebelli

10.) Locate the dural venous sinuses within the dural partitions. Recall that the superior sagittal sinus was located earlier in this dissection, and portions may still be visible (particularly near the confluence of sinuses).

Note: The confluence of sinuses is an important landmark in understanding dural venous sinuses. This is the junction of five sinuses: superior sagittal, straight, transverse (bilaterally), and occipital. It is typically located superolateral to the internal occipital protuberance.

Note: 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.

Photo 18. Dural venous sinuses, superolateral view

Note: 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.

Photo 19. Dural venous sinuses

Note: 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.

Note: 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 vv., sphenoparietal sinuses, and superficial middle cerebral vv., and the cavernous sinuses drain into the petrosal sinuses and emissary vv. Many structures traverse the cavernous sinus, including the internal carotid a., 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.

Photo 20. Dural venous sinuses

Note: The valveless emissary vv. are an important connection between veins external to the cranium and the dural venous sinuses. Flow of blood is typically away from the brain. There are numerous emissary veins, but the largest include frontal, parietal, mastoid, and posterior condylar emissary veins.

Locate the cranial nerves exiting the skull

Find these structures:

• Internal carotid a.

• Cranial nerves

  • Olfactory nn. (CN I)

  • Optic n. (CN II)

  • Oculomotor n. (CN III)

  • Trochlear n. (CN IV)

  • Trigeminal n. (CN V)

  • Abducens n. (CN VI)

  • Facial n. (CN VII)

  • Vestibulocochlear n. (CN VIII)

  • Glossopharyngeal n. (CN IX)

  • Vagus n. (CN X)

  • Accessory n. (CN XI)

  • Hypoglossal n. (CN XII)

11.) Locate the cranial nerves in association with the cranial base. For a clearer view, dura can be removed unilaterally.

Note: The hypoglossal n. (CN XII) exits through the hypoglossal canal of the occipital bone. This canal is just superolateral to the foramen magnum.

Photo 21. Hypoglossal n. (CN XII)

Note: The glossopharyngeal n. (CN IX), vagus n. (CN X), and accessory n. (CN XI) exit the cranial cavity through the jugular foramen. The accessory n. is distinguishable because it ascends through the foramen magnum to exit the cranial cavity through the jugular foramen. CN IX is smaller than CN X.

Photo 22. Contents of jugular foramen

Note: Facial n. (CN VII) and vestibulocochlear n. (CN VIII) exit the cranial cavity enter the internal acoustic meatus of the temporal bone. Vestibulocochlear n. is the larger of the two nerves.

Note: Abducens n. (CN VI) may be found on the clivus of the occipital bone, but it exits through the superior orbital fissure after traversing the cavernous sinus on its way to the lateral rectus m.

Photo 23. Contents of internal acoustic meatus and abducens n. (CN VI)

Note: The roots of the trigeminal n. and the smaller trochlear n. can be found in association with dura on the petrous part of the temporal bone.

Note: The oculomotor n. (CN III) is located on the lateral wall of the sella turcica. The pituitary gland sits in the hypophysial fossa with the internal carotid a. superolateral to the gland. The optic n. is just superior to the ICA.

Photo 24. Neurovasculature associated with sella turcica