Lab 2B
Brain removal, meninges, and associated structures

• Remove the calvaria and a wedge of bone from the back of the skull to expose the brain

• Dissect the meninges, the spaces between them, the dural folds and dural venous sinuses.

• Remove the brain from the skull

• Observe the superficial features of the brain: gyri and sulci, cranial nerve roots, the anterior and posterior circulation and the circle of Willis.

3 main tasks are involved in removing the brain

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

2. Cervical laminectomy (completed last lab by Group A)

3. Removal of a wedge of bone from the occipital region of the skull. This will expose the posterior part of the cerebellum, and will connect the opening you created in the calvaria with the opening created in the spinal column.

The cervical laminectomy was done in an earlier lab, so today you’ll remove the calvaria and the occipital wedge of bone

Removing the calvaria:

Plan your cut. It’s important to cut low enough on the skull that the hole you make in the top of the skull is large enough for the brain to pass through. Follow the dashed line shown in figure 2.8, which passes just superior to the eyebrows and the ears, and continue that line around the back of the skull. Cut through any remaining soft tissue along the cut line with a scalpel before cutting with the saw.

Begin cutting with an electric bone saw. Let the saw do the work, you shouldn’t be pushing the saw against the bone with a lot of force. Ideally you’d like to cut about halfway through the depth of the bone all the way around your line, and then break the remaining bone by chiseling around the line. This will minimize the potential for damage to the underlying dura and the brain. However the reality of the situation is that you are likely to cut all the way through the bone with the bone saw in some regions. If you feel the bone saw “drop” it means you’ve cut all the way through the bone, so don’t cut any deeper.

Chisel through any remaining bone not cut by the bone saw.

Even having cut through all the bone, the calvaria are not going to just pop off. The outer layer of the dura mater is tightly adhered to the deep surface of the calvaria and the rest of the inside of the skull. Now you need to separate that dura from the inside of the calvaria in order to remove the calvaria. This next step will take a lot of work.

The goal is to separate the skull bones from the dura and leave the dura attached to the cadaver as you peel the calvaria off. Sometimes that is not possible — a cap of dura comes up with the calvaria. That’s okay; just peel the dura off the calvaria once removed, so you can study the dura.

Removal of a wedge of occipital bone:

Removing this wedge of occipital bone will allow you to see the posterior part of the cerebrum, the cerebellum, and structures of the brainstem. It allows you to demonstrate the continuity of the brain and the spinal cord. And it’s necessary to remove the brain in one piece.

Use the electric bone saw to make the saw cuts shown in the figure to the right. The points labeled A are the edge of the skull that remains after removal of the calvaria. The points labeled B are the edge of the foramen magnum.

Make sure the wedge you cut out is wide enough between points A to get a clear view of the posterior brain structures. Ask a member of the teaching staff if you’re unsure.

If you were able to separate the dura mater from the inside of the skull you should have a view something like this figure. 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.

Now incise the dura mater along the dashed lines shown in the figure. This will allow us to see the arachnoid and pia mater. Along the lines where you cut the dura, the two dural layers are fused together. They only separate where they form the dural sinuses. You can demonstrate this by continuing the vertical incision up through the superior sagittal sinus, where the single fused layer splits into the separate meningeal and dural layers to form the sinus.

Lift up the flaps of dura you’ve created and you’ll see the arachnoid mater covering the surface of the brain. It appears as a thin membrane, and just beneath it you can see cerebral aa. and vv.

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.
   
   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 brain. Refer to the figure to help you determine which nerves you’re cutting through.

3. 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.

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

5. 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.

6. 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).

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

8. 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.

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

10. 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.

It is VERY IMPORTANT that you cover the skull base with wet paper towels after the brain is removed. The next group will be peeling the dura off the skull base in the next lab, and if the dura dries out it is nearly impossible to peel the dura off. Cover it now and wet it down good, wet it real good.

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.