The brain, like the spinal cord, is enclosed in three layers of meninges:dura, arachnoid, and piamaters. However, unlike the spinal dura, the dura mater of the brain consists of two layers. The outer dura is the periosteum of the inside of the skull, and is continuous (through the sutural membranes and around the margins of the foramina) with the periosteum on the skull's outer surface. The inner layer of dura is the covering of the brain.
Bilayered folds of the dura extend downward between the two cerebral hemispheres (the falx cerebri), and then stretch transversely across between the cerebrum and cerebellum (the tentorium cerebelli). These folds form a fibrous "skeleton" that lends internal support to the brain and helps preserve its shape.
You will remove the skull cap (calvarium), attempting to leave behind both the periosteal and the meningeal dura. This requires care; at the same time, some force is needed to separate the periosteal layer from the inner surface of the skull. Before you can saw open the skull you must:
Remove any remnants of the scalp.
Examine the origin of the temporalis muscle.
Clear away its fibers on both sides.
NOTE: ALL tissue must be removed from the skull or the saw will not cut through the bone very well.
Tie a string around the greatest circumference of the skull cap, at a level just above the eyebrows and above the external occipital protuberance.
Mark this plane with a Sharpie, and then saw through the skull cap following the line all the way around.
Place a sharp-edged chisel into the saw cut around the skull
Begin prying the skull cap up, and insinuating the chisel point between the bone and the periosteum on the inner table (i.e. the inner surface layer of compact bone) of the skull to help peel the periosteum from the skull.
NOTE: Get an instructor to demonstrate if you are unsure!
Continue freeing the periosteal dura from the inner table all around the saw cut.
Eventually, you will free the entire skull cap, leaving behind the cerebral hemispheres complete with their covering of meninges. Sometimes, the adhesion of the dura to the inside of the calvarium cannot be easily freed, and the dura may tear away during the process of lifting the skull cap. Work patiently, but be persistent and try to leave as much dura covering the brain as possible.
2. FALX CEREBRI AND SAGITTAL SINUSES
Make an incision through the dura on either side, parallel to the superior sagittal sinus, from as far forward to as far backward as you can.
Reflect the dura, exposing the cerebral hemispheres.
Examine the piece of dura that you left attached between the hemispheres.
Gently pull apart the two hemispheres, severing cerebral veins as necessary.
Look for the lower free edge of the falx cerebri, containing the inferior sagittal sinus.
Open up the superior sagittal sinus in the midline, and display the arachnoid granulations. These return cerebrospinal fluid to the venous blood.
The veins of the brain empty into blood-filled spaces called the cranial venous sinuses, which are essentially endothelial lined spaces between the two layers of the dura mater. You have already seen several of these sinuses in the process of removing the cerebral hemispheres. Between the cerebral hemispheres, the fold of dura called the falx cerebri encloses the superior and inferior sagittal sinuses.
3. TENTORIUM CEREBELLI AND TRANVERSE SINUSES
Between the cerebral hemispheres and the cerebellum, find the dural fold called the tentorium cerebelli that encloses the superior petrosal and the transverse sinuses.
The superior petrosal and tranverse sinus drains into the sigmoid sinus, which empties through the jugular foramen into the internal jugular vein.
4. REMOVING THE BRAIN, CEREBELLUM AND BRAINSTEM INTACT
HINT:Depending on the state of preservation and fixation of the cadaver, the cranial contents may be semi-liquid; if so, they should be scooped out and discarded. If not, proceed with the dissection:
Compare the dorsal and ventral surfaces of the forebrain with the atlas figures, noting the sulci and gyri.
On the ventral surface, locate the optic tracts, optic nerves (CN II), and optic chiasma.
Find the stalk of the hypophysis.
Use your hand to gently lift the frontal lobes
Use scissors to cut the cerebral falx where it is attached to the crista galli.
Use scissors to cut the cerebral veins where they enter the superior sagittal sinus so that the veins will remain on the surface of the brain.
Grasp the cerebral falx near the crista galli, and pull it slightly superiorly and posteriorly from between the cerebral hemispheres to make sure it is free at the anterior end. At its posterior end, the cerebral falx will remain attached to the cerebellar tentorium.
On the right side, gently lift the occipital lobe (posterior part of brain), and observe the cerebellar tentorium.
Beginning anteriorly, use a scalpel to cut along the peripheral margin of the cerebellar tentorium as close to the bone as possible:
Sever the cerebellar tentorium from the posterior clinoid process and then from the superior border of the petrous part of the temporal bone (i.e., along the superior petrosal sinus).
The cut should continue at least to the posterolateral end of the superior border of the petrous part of the temporal bone, near the intersection of the tranverse and sigmoid sinuses (see figure below).
Repeat this cut on the left side.
Pull the cerebral falx and cerebellar tentorium posteriorly from between the cerebral hemispheres and cerebellum.
NOTE: The tentorium cerebelli covers the cerebellum and holds it in the posterior cranial fossa. If you do not pull the falx and the cerebellar tentorium away, the brain may tear from the cerebellum and brainstem when you try to pull it out.
If the tentorium is particuarly robust, you may need to extend the posteriorlateral cut along the margin of the tentorium into the region of the transverse sinuses as indicated by the green dotted lines in the figure below.
With the dural infoldings detached, the brain may be gently raised to expose the cranial nerves and vessels that are located on its inferior surface.
Use your fingers to gently elevate the frontal lobe.
Use a probe to lift the olfactory bulb from the cribriform plate on each side of the crista galli. As you do this, you will tear the small nerve rootlets leaving the olfactory bulb and piercing through the cribiform plate. If the bulb does not lift up easily, you might need to use a scalpel to sever these small rootlets.
Use a scalpel to cut the following structures bilaterally in order:
internal carotid artery, and
oculomotor nerve (CN III).
Cut the stalk of the pituitary gland at the midline.
Raise the brain slightly higher, and cut the following structures bilaterally in order:
trochlear nerve (CN IV),
trigeminal nerve (CN V), and
abducens nerve (CN VI).
NOTE: You may want to ask one of your instructors to assist you at this point. These nerves can be small and easy to miss. You should make sure that as you cut each cranial nerve, you leave enough on the brain and enough in the skull to identify the nerves in both locations.
Elevate the cerebrum and brainstem still further and cut the following structures bilaterally:
Facial (CN VII) and vestibulocochlear nerves (CN VIII) near the internal acoustic meatus
Glossopharyngeal (CN IX), vagus (CN X), and accessory nerves(CN XI) near the jugular foramen
Hypoglossal nerve (CN XII) near the hypoglossal canal
HINT - Of these nerves, only CN XII (hypoglossal) courses by itself as it exits the cranial fossa and therefore is clearly recognizable. CN VII and VIII exit together and therefore are not clearly identifiable, per se, but you should be aware of where they are found in the cranial fossa --the same goes for CN IX, X, and XI.
Sever the two vertebral arteries where they enter the skull through the foramen magnum
Use a scalpel to cut the cervical spinal cord as low in the foramen magnum (or cervical vertebral canal) as you can reach by extending the scalpel deep into the opening of the foramen magnum.
Support the brain with the palm of one hand under the frontal lobes and your fingers extending down the ventral surface of the brainstem.
Insert the tip of your middle finger into the cut that was made across the cervical spinal cord to support the brainstem and cerebellum.
Roll the brain, brainstem, and cerebellum posteriorly and out of the cranial cavity in one piece.
The brain should be stored with the cadaver and kept moist.
5. SINUSES AND DIAPHRAGMA SELLAE
Examine the dura left behind in the cranial cavity
Locate the right and left transverse sinuses in the posterior-lateral edges of the tentorium cerebelli.
At the junction of the falx cerebri and the tentorium cerebelli (which you cut), trace the straight sinus.
Determine whether the straight sinus empties into the right or left transverse sinus (usually, it is the left).
Locate the posterior and anterior clinoid processes
Identify the attached diaphragma sellae.
The potential space through which the diaphragma sellae runs is expanded anteriorly and medially to form the cavernous sinus.
FUNCTIONAL ANATOMY: The cavernous sinus, a space filled with trabeculae of connective tissue, is named for a fancied resemblance to the corpora cavernous of the penis. Venous blood drains into it from the veins of the orbit and some of the superficial veins of the brain. It connects with the sigmoid sinus through inferior and superior petrosal sinuses.
6. THE CRANIAL NERVES
With the brain removed, locate the twelve cranial nerves on the base of the brain.
Locate the corresponding exits of the cranial nerves in the floor of the cranial fossa.
The steps below will help you to orient yourself to additional features. If the brain has been discarded you will still be able to find evidence of the majority of these structures from the internal surface of the braincase.
You previously severed the falx cerebri's attachment to the crista galli between the two olfactory bulbs. Strip away the dura mater lying on the floor of the anterior cranial fossa.
Remove the diaphragma sellae and the dura covering the cavernous sinuses.
Note: this figure is used here to illustrate the diaphragma sellae. Since your cadaver's brain will have been cut at the level of the medulla/spinal cord (i.e. at the foramen magnum) for removal, you will NOT see the midbrain, cerebellum, tentorium cerebelli, or vessels (except for the internal carotid) as shown in this figure.
Identify the oculomotor (III), trochlear (IV), opthalmic (V1) and maxillary (V2) nerves running along the lateral wall of the cavernous sinus and the abducens nerve (VI) and internal carotid artery runnin within the sinus.
The spatial arrangement of these structures is such that aneurisms of the carotid artery within the sinus (a common location for this to occur) will impinge on the abducens nerve first, thus leading to a lateral gaze palsy. Similarly, infections spreading into the cavernous sinus will affect the internal carotid artery and abducens nerve before impacting on the more lateral structures.
Identify the optic nerves passing into the optic canals.
If possible, identify the greater and lesser petrosal nerves (both of these are difficult to find, but of the two, the greater petrosal is larger and therefore easier to see exiting the petrous portion of the temporal bone and running anteriorly to go to the pterygopalatine ganglion.
The greater and lesser petrosal nerves carry parasympathetics from the facial (VII) and glossopharyngeal (IX)nerves, respectively, to the pterygopalatine and otic ganglia. These ganglia are attached to the maxillary(V2) and mandibular (V3) nerves, respectively, just outside the skull.
Note: since your cadaver's brain should have been cut at the level of the medulla/spinal cord (i.e. at the foramen magnum) for removal, you will NOT see the midbrain as shown in this figure. However, you should see the cranial nerves illustrated once you strip away the dura as shown.
7. THE CIRCLE OF WILLIS
Examine the base of the brain and trace the anastomotic "circle of Willis" surrounding the infundibular stalk and optic chiasm.
Trace the vertebral arterial supply into the circle of Willis by locating the two vertebral arteries on the ventral surface of the spinal cord.
Trace the vertebral arteries up to the point where they join to form the basilar artery on the ventral surface of the brainstem.
Continue following the basilar artery until you reach its junction with the circle of Willis.
The vertebral arteries supply not only the hindbrain but also the posterior ends of the cerebral hemispheres via the posterior cerebral arteries.
Review the course taken by the internal carotid artery through the petrous temporal bone.
Herniation of brain and/or cerebellum with increased intracranial pressure:
When the volume of tissue and fluid inside the skull increases beyond the limit permitted by compression of veins and displacement of CSF, intracranial pressure rises. The cranial vault is subdivided by rigid dural folds (falx and tentorium), and a focal expansion of the brain displaces it in relation to these partitions. If the expansion is sufficiently large, herniation occurs. Herniation often leads to "pinching" and vascular compromise of the compressed tissue, producing infarction, additional swelling, and further herniation. There are three main types of herniation:
Subfalcine (cingulate) herniation occurs when unilateral or asymmetric expansion of a cerebral hemisphere displaces the cingulate gyrus under the edge of the cerebral falx. The herniated tissue often compresses both the left and right anterior cerebral arteries leading to symptoms associated with bilateral ischemic injury to the medial aspects of the cerebral hemispheres (weakness and/or paresthesia of the legs).
Transtentorial (uncinate)herniation occurs when the medial aspect of the temporal lobe (the uncus) is compressed against the free margin of the cerebellar tentorium. As the temporal lobe is displaced downward, the oculomotor nerve may be compressed, resulting in pupillary dilation and impaired ocular movements on the side of the lesion ("blown pupil"). The posterior cerebral artery may also be compressed, resulting in ischemic injury to tissue supplied by that vessel, including the primary visual cortex.
Tonsillarherniation refers to displacement of the cerebellar tonsils through the foramen magnum. This type of herniation is life-threatening, because it causes brain stem compression and compromises vital respiratory and cardiac centers in the medulla.
Normally the dura is fused with the periosteum on the internal surface of the skull. Dural arteries, most importantly the middle meningeal artery, are vulnerable to injury, particularly with temporal skull fractures in which the fracture lines cross the course of the vessel. In children, in whom the skull is deformable, a temporary displacement of the skull bones leading to laceration of a vessel can occur in the absence of a skull fracture.
Once a vessel has been torn, the extravasation of blood under arterial pressure can cause the dura to separate from the inner surface of the skull (i.e. the bleeding is “epidural”). The expanding hematoma has a smooth inner contour that compresses the brain surface. Patients may be lucid for several hours following the initial injury before the onset of neurologic signs that then tend to progress rapidly. Treatment often requires a craniotomy (creating an opening in the skull) to relieve intracranial pressure followed by repair of the torn meningeal vessel(s).
Between the inner surface of the dura mater and the arachnoid mater layer lies a potential “subdural space.” Bridging veins travel from the convexities of the cerebral hemispheres through the subarachnoid space and the subdural space to empty into the dural venous sinuses. The brain, floating freely bathed in CSF, can move within the skull, but the venous sinuses are fixed, and the displacement of the brain that occurs in trauma such as a fall can tear the bridging veins at the point where they penetrate the dura resulting in bleeding into the potential space between the dura and arachnoid mater (i.e. “subdural”). In elderly individuals with brain atrophy, the bridging veins are stretched out and the brain has additional space for movement, hence the increased rate of subdural hematomas in these patients, even after relatively minor head trauma. Infants are also particularly susceptible to subdural hematomas because their bridging veins are thin-walled.
Subdural hematomas are most common over the lateral aspects of the cerebral hemispheres, and symptoms most often manifest within 48 hours of injury and tend to progress slowly. Neurologic signs commonly observed are attributable to the pressure exerted on the adjacent brain. There may be focal signs, but often the clinical manifestations are nonlocalizing and include headache and confusion. The treatment of subdural hematomas is to remove the blood and any clots that have started to form.