Cranial Cavity

#9 - Cranial Cavity

Your laboratory instructor is the only one allowed to saw, loosen and remove the calvaria (N7) from its tight attachment to the underlying dura mater and a wedge shaped area of the occipital bone. The instructors are the only personnel authorized to remove the brain and spinal cord and they used the following steps to remove these structures. First, the scalp is removed from the vertex to the level of the external ear. Using an autopsy saw, the calvaria is cut along a line about 2 cm. superior to the supraorbital ridges and the external occipital protuberance. The line lies just above the external auditory meatus. Care is taken not to saw all the way through both outer and inner tables of the calvaria, because the dura will also be severed. After a partial cut, a sharp blow with hammer and chisel cracks the inner table and permits the calvaria to be pried off the tightly attached dura. The calvaria is removed and the two laminae of compact bone (diploe) separated by a layer of spongy bone should be noted. Also, notice the markings on the inside of the calvaria made by blood vessels (N7), arachnoid granulations (879/N7), and the venous sinuses (N7).
The body is turned prone to complete the posterior sawing, after removal of the posterior neck muscles from the occipital region of the skull. A large triangular area of the occipital bone reaching the foramen magnum is sawed out to facilitate removal of the brain. The spinal cord is severed between the axis and the third cervical vertebra with a scalpel to facilitate subsequent removal of the brain.
After removal of the calvaria and posterior wedge, the coverings of the brain and spinal cord, the meninges (N94), can be seen. The outer layer of the meninges, the dura mater (875/N94), consists of two layers. The internal layer is true dura and is continuous with that of the spinal cord, whereas the external layer is endocranium (periosteum) and is continuous with the pericranium at the various foramina. In certain areas the true dura forms folds that extend into the interior of the cranial cavity as the falx cerebri (N97) and falx cerebelli (N97) and the tentorium cerebelli (N97).
The two layers of dura are separated at certain places to form intracranial venous sinuses. At this time locate and open the superior sagittal sinus (879/N94). Arachnoid granulations (N94) will be seen projecting into the sinus. Also note the entrances of superior cerebral veins (N94).
Two alternative approaches can be used to open and remove the dura mater and are outlined below:
1. Intact dural sac:
  1. The dura is cut around the entire perimeter where the calvaria has been sawed.
  2. The falx cerebri is cut at its attachment to the crista galli and the superior portion pulled back to the attachments along the transverse sinuses.
  3. The dura is cut along the length of the petrous temporal ridges toward the midline and then the attachments to the anterior and posterior clinoid processes are cut loose.
  4. The dura can then be removed mostly intact and its 3-D shape can be visualized.
2. Alternative dissection to visualize superior sagital sinus:
  1. A longitudinal cut on each side of superior sagittal sinus.
  2. A transverse cut on each side from center of the longitudinal cut laterally and inferiorly to just above ear. This should make four flaps which can be turned down over the edges of the cut skull bones.
  3. The dura is cut along the length of the petrous temporal ridges toward the midline and then the attachments to the anterior and posterior clinoid processes are cut loose.
To gain access to the cranial nerves in the posterior cranial fossa the tentorium cerebelli must be detached from the superior margin of the petrous temporal bone (N98). This is best accomplished using a scalpel. Because of the difficulty of retracting the cerebral hemispheres, some of the cutting is done blindly. By lowering the head over the edge of the tray or table, gravity will be an aid in retraction of the brain. Elevation of the temporal lobes from the middle cranial fossawill expose the anterior part of the tentorium. The tentorium is palpated to ascertain its point of attachment along the petrous temporal ridge and incised under visual control. Great care is exercised in carrying out the subsequent maneuvers, because the brains will be used subsequently in the neuroscience core. The scalpel blade is carried along the superior edge of the petrous temporal bone until the back of the skull is reached and then turned medially to complete the severance of the tentorium. After the tentorium has been severed from all its connections and the dural sac removed, the only structures holding the brain in the cranial cavity are the remaining cranial nerves and the carotid and vertebral arteries. Support for the brain with hands is essential. If too much tension is allowed on structures in the posterior cranial fossa the vertebral arteries will tear.
After the instructor has opened and removed the dural sac, observe the thin arachnoid mater (885/N96) covering the surface of the brain, and note that it does not extend down into the sulci or fissures, and thus forms the subarachnoid space (N94) deep to this membrane. Dipping into the sulci or fissures and tightly adherent to the surface of the brain and spinal cord is the pia mater (N96). Imagine these fissures filled with cerebrospinal fluid pushing the arachnoid mater out against the dura. The cerebral blood vessels (N96) lie in the subarachnoid space, but can easily be seen through it. The veins are more prominent than the arteries on the external surface of the brain; the arteries are readily seen only if they are injected. Observe some of the superior cerebral veins opening into the superior sagittal sinus. The hemispheres of the brain can be pulled laterally to observe the falx cerebri (878/N97), a fold of true dura that extends down into the longitudinal fissure. The falx can be traced posteriorly to the tentorium cerebelli (N98) which covers the superior surface of the cerebellum. The falx cerebri can be followed anteriorly to its attachment to the crista galli (842/N98) where it was cut.
Your instructor will demonstrate the cranial nerves as they are cut to remove the brain. They can be demonstrated either starting at the back and working forward or the body can be returned to the supine position and the brain removed by starting at the front. Reverse the following order if starting from the back.
The instructor will insert fingers between the frontal lobe and the floor of the anterior cranial fossa to gently raise the frontal lobe to display the olfactory bulb as it lies on the cribriform plate (842/N9). If possible, identify the olfactory tract extending posteriorly from the bulb to the inferior surface of the hemisphere. The bulbs can be teased away from the cribiform plate so that bulbs and tracts remain with the brain. Additional elevation of the two frontal lobes will display the optic nerves and chiasm (N101) in the midline about 2 ½ inches posterior to the crista galli. With a small pair of scissors the optic nerves are cut leaving the optic chiasm on the brain. (It is best to sever all cranial nerves with a scissors; pressure on them even with a sharp scalpel is likely to tear them from the brain). Immediately posterior to each optic nerve are the internal carotid arteries (893/N132). In the midline the very slender infundibulum (N100) (stalk of the hypophysis) is seen passing from the floor of the brain to the gland that lies under the diaphragma sellae (878/N98) in the hypophyseal fossa (N9). The infundibulum is cut if it still remains intact. Next, the oculomotor nerves (N98) pass from the inferior surface of the brain stem to enter the dura mater slightly lateral and posterior to the internal carotid arteries. Observe that near the ventral surface of the brain stem this nerve passes between two arteries. The anterior of these arteries is the posterior cerebral artery (N132) and the posterior one is the superior cerebellar artery (N132). Both are branches of the basilar artery (N132). The oculomotor nerves are cut about one-fourth of an inch from their entrance into the dura. The very slender trochlear nerve (N108) can occasionally be seen as it passes from the side of the brain stem into the medial free border of tentorium cerebelli (N98) about one-half inch posterior to the point where the oculomotor nerve enters the dura mater. If found, the trochlear nerves are cut with a pair of scissors, leaving about one-half inch of them attached to the dura. (The trochlear nerve is more difficult to find than the others). Identify the larger trigeminal nerves (N108) more laterally and the slender abducens nerves (N108) to either side of the midline. They are severed leaving short stubs projecting from the dura. The facial (N108) and vestibulocochlear nerves (N108) are identified entering the internal acoustic meatus (N98) on the posterior surface of the petrous temporal bone. Inferior to the internal acoustic meatus the glossopharyngeal, vagus and spinal accessory nerves (CN IX, X & XI, N108) are seen entering the jugular foramen (N98). The spinal part of the spinal accessory nerves (N98) emerge from the cervical level of the spinal cord and enter the cranial cavity through the foramen magnum (N8). They may be left intact. Finally, the hypoglossal nerves (N98) and the vertebral arteries (894/N132) are located near the foramen magnum and cut. The brain is now unattached and can be placed in the calvaria for observation of the arterial pattern of blood supply around the cerebral arterial circle (Circle of Willis) (N132) as well as the site of emergence of the cranial nerves.
With a scalpel or scissors open the following venous sinuses (879/N98) in the dura inside the cranial cavity. Clotted blood may be found in them.
1. Transverse sinus (N98).
2. Sigmoid sinus (N98).There are other sinuses of minor significance. The opening of the important cavernous sinus is described in Q below.
Does your cadaver have a confluence of sinuses?
Observe as many of the following arteries (N132) on the base of the brain as possible. Cerebral arteries will be studied in the Neuroscience Core 4, but it is advantageous to start the process now with simple observation.
1. Vertebral.
2. Anterior spinal.
3. Basilar and its pontine and cerebellar branches.
4. Posterior cerebral.
5. Superior cerebellar.
6. Posterior communicating.
7. Internal carotid.
8. Anterior cerebral.
9. Middle cerebral.
10. Anterior communicating.
The cerebral arterial circle (Circle of Willis) (N133) is an anastomotic circle that protects the brain in case of occlusion of major vessels. Occlusion of terminal branches will usually lead to deficits. Aneurysm (abnormal bulging of the arterial wall) is a frequent occurrence in the vessels of the cerebral arterial circle.
Study of the specific anatomy of the emergence of the cranial nerves from the brain will occur in the Neuroscience Core. Since this is the only opportunity to relate the brain to the skull, it would be well to give brief consideration to the general area of the brain giving rise to each of the cranial nerves (N108).
Identify the middle meningeal artery (883/N95) in the flaps of dura as well as in the middle cranial fossa. The artery lies in the dura (some describe it between true dura and periosteal dura) and is more important as a supply for bone than meninges.
Identify the cranial nerve stubs in the cranial fossae. Associate each with its proper fossa and the foramen it uses to exit the cranial cavity.
Carefully remove the true dura from the lateral surface of the trigeminal nerve (fifth cranial nerve) and display the trigeminal (semilunar) ganglion (N98) located in a cave of dura. In other words, it lies between true and periosteal dura. Then follow the ophthalmic and maxillary divisions (N98) of this nerve anteriorly, to the superior orbital fissure and foramen rotundum, respectively, and the mandibular division (N98) inferiorly to the foramen ovale. Carefully lift and reflect the trigeminal ganglion laterally and locate the small motor root of the fifth nerve passing the ganglion on its medial side. It passes through the foramen ovale as part of the mandibular division. The motor root has no direct connection with the ganglion.
Cut the diaphragma sellae (N98) that covers the hypophysis (N98) and observe this gland. The structures such as the optic chiasm that are liable to compression when the hypophysis enlarges should be visualized.
Locate the cavernous sinus (880/N98) situated just lateral to the sella turcica. It lies between true and periosteal dura. It receives venous drainage from the orbit via the ophthalmic veins that have connections with deep veins of the face and drains into the internal jugular vein via the petrosal sinuses. Because of communication with the part of the face frequently involved with infection (acne), this sinus is of greater importance clinically than most.
Follow the oculomotor and trochlear nerves (N98) as they course in the lateral wall (true dura) of the cavernous sinus. Then follow the abducens nerve (N98) into the middle of the sinus. Note that the ophthalmic division of the trigeminal (N98) is also in the lateral wall. Some authors also put the maxillary division (N98) in the wall as well.
Locate the internal carotid artery (N98) passing through the cavernous sinus in its sinuous route through the skull to the base of the brain. The abducens nerve is in contact with the artery.