Head and Neck Lab 2A

Three further tasks are involved in removing the brain:

1. Removal of a wedge of bone from the occipital region of the skull.  This will expose the posterior part of the cerebellum, and will facilitate separation of the calvaria from the dura mater.

2. Removal of the calvaria (the bones of the top of the skull) to expose the meninges and the cerebrum of the brain.

3. Removal of the dural cap from the brain (if you have a fully intact dura call a TA over to try to remove it in one piece!)

Recall that there are two layers of dura mater, an outer periosteal dura (= endosteum, or endosteal dura)and an inner meningeal dura.  The periosteal dura is essentially the periosteum that lines the inside of the skull.  

The arteries you see embedded in the dura are branches of the middle meningeal a.  The middle meningeal a. is a branch of the maxillary a. (which is a branch of the external carotid a.) that passes through the foramen spinosum to enter the skull.  The middle mengingeal a. supplies blood to the dura mater and to the bone of the calvaria, but it does not supply blood to the brain.  

The two layers of dura separate along the superior median plane to form the superior sagittal sinus, one of the main dural venous sinuses that receive venous blood from cerebral veins and drain it away from the brain and ultimately back into the systemic venous circulation.  Make an incision along the superior sagittal sinus to view the inside of this space, which is roughly triangular in cross section.  

Within the superior sagittal sinus, and along its margin, you’ll see small lumps of tissue that look like tiny heads of broccoli.  These are arachnoid granulations, which are places where the arachnoid mater pierces the meningeal dura and allows cerebrospinal fluid (CSF) to drain from the subarachnoid space around the brain into the superior sagittal sinus.  

So the superior sagittal sinus not only drains venous blood away from the brain, but also drains CSF from the subarachnoid space.

The potential space between the inside of the calvaria and the dura mater is the epidural space.

Look at the inner surface of the removed calvaria to see impressions from some of the structures you just identified: impressions from the middle meningeal aa., impressions from arachnoid granulations, and a median impression from the superior sagittal sinus.  The median impression is harder to see than the others, and doesn’t show up in the photo.

In life the arachnoid mater is pushed up against the meningeal dura by CSF, and the potential space between the arachnoid mater and the dura mater is the subdural space.

Grab a bit of the arachnoid with your forceps and pull if off the surface of the brain.  It offers a bit of resistance because the arachnoid mater is connected to the pia mater by thousands of tiny arachnoid trabeculae.  Continue to pull off the arachnoid mater until you’ve cleared a patch of the brain.  

The pia mater lies directly on the brain and follows all the hills (gyri, singular gyrus) and valleys (sulci, singular sulcus).  You cannot pull the pia mater off the brain.

In life there is an actual space between the arachnoid mater and the pia mater called the subarachnoid space.  This space is filled with CSF, and that CSF pushes the arachnoid mater up against the dura mater and up off of the pia mater and the brain, such that the brain is surrounded by CSF and is not in contact with any of the bone of the skull.  The brain floats in the CSF, tethered to the arachnoid mater by the arachnoid trabeculae.

Dural folds:

The dural folds are double folds of meningeal dura that run perpendicular to the periosteal dura.  They compartmentalize the cranial cavity and thereby help stabilize the brain.  The main dural sinuses are formed at the base of these dural folds, where the periosteal and meningeal dura split.

The falx cerebri is the largest of the dural folds.  It separates the two cerebral hemispheres, from the median plane of the calvaria to the corpus callosum of the brain.  The superior sagittal sinus runs along the base of the falx cerebri (where it attaches to the skull), and in the inferior margin of the falx cerebri is the smaller inferior sagittal sinus.  Anteriorly the falx cerebri attaches to the crista galli of the ethmoid bone, and posteriorly it attaches to another large dural fold.

The tentorium cerebebelli is the large dural fold that the falx cerebri attaches to posteriorly.  The tentorium runs more or less at a right angle to the falx cerebri, and separates the occipital lobe of the cerebrum from the cerebellum.  It forms a tent over the cerebellum, hence the name.  At the base of the tentorium (where it attaches to the skull) are the paired right and left transverse dural sinuses.  Anteriorly the two sides of the tentorium attach to the petrous part of the temporal bone.  

Smaller dural folds include the falx cerebelli (separates the left and right lobes of the cerebellum) and the diaphragma sellae (forms the roof of the sella turcica, which holds the pituitary gland).

Dural venous sinuses:

We’ve already touched on the superior sagittal sinus and the inferior sagittal sinus.

The inferior sagittal sinus runs posteriorly in the inferior margin of the falx cerebri.  Along the line where the falx cerebri makes contact with the tentorium cerebelli is another dural sinus, the straight sinus.  The straight sinus receives venous blood from the inferior sagittal sinus and is basically a continuation of the inferior sagittal sinus.

The superior sagittal sinus and the straight sinus eventually meet at a point called the confluens (= confluence) of the sinuses.  Look at the inner surface of the wedge of occipital bone you cut out of the cadaver’s skull, and you should see impressions of the superior sagittal sinus, the confluens of the sinuses, and the transverse sinuses.

From the confluens of the sinuses blood flows into the right and left transverse sinuses (which are at the base of the tentorium cerebelli).  

The left and right transverse sinuses lead to the left and right sigmoid sinuses (which are somewhat S-shaped).  

Finally, the sigmoid sinuses lead to the jugular foramina, where the internal jugular veins form.

Smaller dural sinuses that are not visible at this time include the superior petrosal sinus and the cavernous sinus.  We’ll come back to the cavernous sinus.

Brain removal:

To remove the brain in one piece there are 2 cuts that must be made to the dural infoldings.

1. Find the confluens of the sinuses; the place where the falx cerebri intersects the tentorium cerebellil. Cut through the tentorium just lateral to the straight sinus. Be sure to cut through the entire depth of the tentorium on left and right sides of the falx cerebri.

2. Be sure that the anterior edge of the falx cerebri is cut away from its attachment to the crista galli of the ethmoid bone.  You should now have all of the free dura in hand.  At this point the only thing connecting the brain to the skull should be the cranial nn. and the internal carotid and vertebral aa.  

3. Turn the cadaver supine, if you have not already done so. As you remove the brain you will cut through the cranial nn. and the internal carotid and vertebral aa.  Take your time with this and make sure everyone gets a chance to see the structures as you cut through them.  As you cut through nerves and arteries cut them in the middle as much as possible, so that you have a visible stump attached to the brain and one attached to the base of the skull.  Refer to the figure to help you determine which nerves you’re cutting through.

4. Gently pull the frontal lobes of the cerebrum up away from the base of the skull.  On either side of the crista galli you’ll see the olfactory bulbs sitting on the cribriform plates and the olfactory nerve tracts leading away from them to the brain.  Use a blunt instrument to gently peel the bulbs away from the cribriform plates. They are more fragile than peripheral nerves!  The olfactory bulbs and tracts should stay attached to the brain.

5. Next you’ll see the optic tracts and the optic chiasm where the nerve tracts cross.  Cut the optic tracts.

6. Next in line are the oculomotor nn., and the small trochlear nn. and abducent nn.  Also in this area you’ll see the internal carotid aa. Cut through these structures. 

7. The trigeminal nn. are fairly robust.  You will cut through them after they exit the brain but before they divide into V1, V2, and V3 (this happens within the cavernous sinus, which you haven’t opened yet).

8. The facial nn. and vestibulocochlear nn. exit the skull together.  Cut them.

9. The glossopharyngeal nn., vagus nn., and accessory nn. all leave the skull together through the jugular canal.  Cut through the middle of those 3 nerves.

10. Finally, the hypoglossal nn. exit the skull close to the foramen magnum.  Cut through this last cranial nerve.

11. The only remaining structures holding the brain to the rest of the body at this point are the vertebal aa. and the spinal cord.  Once you cut through them you’ll have the brain in hand.

Arterial supply of the brain and the Circle of Willis:

You may need to peel away the arachnoid mater from the base of the brain to see the arterial supply and the Circle of Willis.

The posterior circulation of the brain is supplied by the two vertebral aa. that enter the skull through the foramen magnum.  

The first branches of the vertebral aa. are the posterior inferior cerebellar aa. (PICA).

The two vertebral aa. join to form a single basilar a.

The first branches of the basilar a. are the anterior inferior cerebellar aa. (AICA).

The labyrinthine aa. branch from either the basilar a. or the AICA.   Each labyrinthine aa. travels with the facial n and vestibulocochlear n. into the internal auditory meatus.  The labyrinthine a. and AICA are separated by the abducent n.

The basilar a. gives off a series of small pontine aa. as it travels anteriorly.  

The basilar a. gives off a left and right superior cerebellar a., and then splits into the left and right posterior cerebral a.  The superior cerebellar aa. and the posterior cerebral aa. are separated by the oculomotor n.  The posterior cerebral aa. give off small branches, the posterior communicating aa. that connect the posterior circulation to the anterior circulation of the brain.  

The anterior circulation of the brain is supplied by the two internal carotid aa. that enter the skull through the carotid canals.  

After exiting the carotid canals the internal carotid aa. travel between the two layers of dura in the cavernous sinuses on either side of the sella turcica of the sphenoid bone.  Within or just beyond the cavernous sinus the internal carotid a. gives off an ophthalmic a. that travels into the orbit through the optic canal.  

The main part of each internal carotid a. runs deep in the lateral sulcus as the middle cerebral a.  You’ll need to separate the temporal and parietal lobes of the brain by gently tearing through the arachnoid mater that lies over the lateral sulcus to get a full view of the middle cerebral a. 

The main anterior branch of the internal carotid a. is the anterior cerebral a., which travels anteriorly and medially to run in the longitudinal fissure of the cerebrum.  Gently separate the left and right hemispheres of the cerebrum along the longitudinal fissure and follow the anterior cerebral aa. as they run along the surface of the corpus callosum.  

The two anterior cerebral aa. are connected to one another just anterior to the optic chiasm by the anterior communicating a.  Sometimes the anterior communicating a. is just a region where the two anterior cerebral aa. are connected to one another.  

As stated earlier, the posterior communicating aa. connect the middle cerebral and posterior cerebral aa., and thus connect the anterior and posterior circulations of the brain.  

The significance of the Circle of Willis is that it allows for multiple possible alternative paths of blood flow to the same regions of the brain (anastomoses).  If the circle is complete (of course there’s a good deal of variation in the completeness of the circle), blood can move between the anterior and posterior circulations, and/or between the left and right sides of the circulation.  So for example the effect of a slow occlusion of the internal carotid aa. (as occurs with atherosclerosis of those arteries) can be mitigated by increased flow through the posterior communicating a. to the middle cerebral a.

Storing the brain:

When you’ve finished examining the brain for the day, place it back in the calvaria, cover the brain with a few wet paper towels, and then put the calvaria+brain+towels into a plastic bag to keep it from drying out.  

Placing the brain back in the calvaria will prevent it from getting a flat spot.

Start by reviewing the stumps of the cranial nn. on the brain.  Some are large and obvious, but others are small, or are made up of many small cranial nervlets on the brain. You may as well go in order from I to XII.

CN I, Olfactory tract and bulb - lie on the inferior surface of the frontal lobe of the cerebrum.  The actual olfactory n. is made up of many small olfactory nervelets that pass through the cribriform plate and synapse with secondary sensory neurons that make up the optic bulb.  The olfactory tract is the axons of those secondary sensory neurons.

CN II, Optic tract - cross at the optic chiasm.  These are the axons of third-order sensory neurons.  The first-order neurons of the optic system are in the retina.

CN III, Oculomotor n. - emerge from the brainstem between the posterior cerebral and superior cerebellar aa.

CN IV, Trochlear n. - the only CN that exits the brainstem from its dorsal surface.

CN V, Trigeminal n. - a large CN that exits just lateral to the pons.

CN VI, Abducent n. - small, exits the brainstem close to the midline at the pontomedullary junction.

Cranial Fossae:

Now we’ll turn to the inside of the skull base, and look for the CN stubs where they enter the dura mater and where they exit the skull.  The base of the skull is divided by bony landmarks into 3 cranial fossae, and each of those fossae contain some number of foramina that the cranial nerves exit.

It’s important to keep in mind that where the CNs pierce the dura (where they disappear from sight) is not necessarily where they exit the skull. Sometimes the CN’s run between the two layers of dura for some distance before they reach the foramina they exit the skull from.  You should be able to identify the foraminae of the skull both in a dry skull and in the cadaver’s skull.

Now review the definitions of the cranial fossae. You may want to use the dried skulls that are available in the blue lab:

The anterior cranial fossa is formed mostly by an extension of the frontal bone, the orbital plate, and is bordered posteriorly by the lesser wing of the sphenoid bone.  In the midline, part of the ethmoid bone sits within the frontal bone.  The cristal galli (attachment site for falx cerebri) and the cribriform plate (openings for olfactory nervelets) are part of the ethmoid bone.  Therefore CN I is the only CN that passes through the anterior cranial fossa, specifically through the holes in the cribriform plate.

The middle cranial fossa contains many of the cranial foramina, which are concentrated in the greater wing of the sphenoid bone.  The middle fossa consists of the greater wing of the sphendoid bone the squamous part of the temporal bone, and the anterior region of the petrous part of the temporal bone.

The posterior cranial fossa consists of the posterior part of the petrous part of the temporal bone and the occipital bone. 

One student should now work on peeling the dura from the middle and posterior cranial fossae while the other starts on the orbit. It is hard to peel the dura off, so do your best to reveal the following nerves and foramina.

Just medial to the anterior clinoid process of the lesser wing of the sphenoid you’ll find the optic canal, which allows passage of the optic nerve tract and the ophthalmic a. into the orbit.

Just inferior to the greater wing of the sphenoid, sitting between the greater and lesser wings, is an elongate slit, the superior orbital fissure.  The superior orbital fissure allows passage of CN III, CN IV, CN V1, and CN VI into the orbit.  The ophthalmic v., and sympathetic neurons also pass through the superior orbital fissure.  All of these nerves pass through the cavernous sinus posterior to the superior orbital fissure.  Carefully peel the dura off the cavernous sinus to see the

Just inferior to the superior orbital fissure is the foramen rotundum, which allows CN V2 to pass out to the upper jaw (maxilla, thus the maxillary n.).  CN V2 will be visible in the opened cavernous sinus.

Posterior and lateral to the foramen rotundum is the foramen ovale, which allows passage of CN V3, also visible in the opened cavernous sinus.  

Lateral to the foramen ovale is the foramen spinosum, which allows passage of the middle meningeal a. into the skull.  The foramen spinosum is the origin point of the skull impressions left by the middle mengingeal a. 

In a dry skull, look just lateral to the body of the sphenoid bone and you’ll see the openings into the carotid canal, which allow the internal carotid a. to enter the cranial cavity.  

Between the temporal and occipital bone is an irregularly shaped “foramen” called the foramen lacerum.  In life the foramen lacerum is covered by heavy connective tissue, so it is only a foramen in the dry skull when that connective tissue has been removed.

The posterior cranial fossa consists of the posterior part of the petrous part of the temporal bone, and the occipital bone.  The most obvious feature of the posterior fossa is the foramen magnum.  The spinal cord, vertebral aa., and CN XI pass through the foramen magnum.  Recall that CN XI passes through the foramen magnum to enter the cranial cavity, not to exit the cranial cavity.

Just anterior to the foramen magnum are the paired hypoglossal canals that CN XII passes through on its way to the tongue.

The jugular foramen sits between the occipital and temporal bones.  This is where the sigmoid sinuses become the internal jugular vv., and also where CN IX, X, and XI pass out of the skull.

Finally, in the petrous part of the temporal bone is the internal auditory (= internal acoustic) meatus, where CN VII and CN VIII pass out of the cranial cavity and into the temporal bone.