Tentorium cerebelli

(iii) The optic nerves and internal carotid arteries as they pierce the dura, medial to the anterior clinoid processes [Fig. 8.6].

Crista galli

Cribriform plate of ethmoid Orbital plate of frontal bone

Body of sphenoid Lesser wing of sphenoid

Anterior clinoid process Posterior clinoid process

Temporal bone, squamous part Temporal bone, petrous part Grooves for middle meningeal veins on greater wing of sphenoid

Groove for superior petrosal sinus

Groove for sigmoid sinus

Groove for transverse sinus

Groove for inferior petrosal sinus Hypoglossal canal, position of

Internal occipital Protuberance Hypophysial fossa

Internal acoustic meatus Jugular foramen (hidden)

Parietal bone Foramen lacerum

Foramen spinosum Foramen ovale

Optic canal Diploic spaces

Frontal sinus (air)

Groove for greater petrosal nerve

Fig. 8.7 Internal surface of the base of the skull.

Superior sagittal sinus

Falx cerebri

Straight sinus

Foramen magnum Cerebellar fossa

Transverse sinus Tentorium

cerebelli Cerebral fossa

Fig. 8.8 The floor of the cranial cavity after removal of the brain, but with the arteries at the base of the brain in situ.

(iv) The great cerebral vein at the junction of the free edges of the falx and tentorium [Fig. 8.6].

This wide, sloping fold of the dura has a free, curved margin surrounding the midbrain, and

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Structures seen after removal of the cerebrum

At the attached margin, the two layers of the du

ral fold diverge to enclose: (1) the wide transverse sinus between the internal occipital protuberance and the base of the petrous temporal bone; and (2) the superior petrosal sinus on the superior margin of the petrous temporal bone [Figs. 8.2B, 8.8, 8.9].

At the apex of the petrous temporal bone, the free and attached margins of the tentorium cross DISSECTION 8.5 Removal of the brain

Objective

I. To remove the brain in one piece.

Instructions

Fig. 8.5 is a sagittal section of the head which shows the relationship of the main parts of the brain to the dural folds and the base of the brain. Identify the cerebrum (largely hidden by the falx cerebri), cerebellum, pons, and medulla on this section prior to starting the dissection.

1. Detach the falx cerebri from the crista galli, and pull the falx up posteriorly from between the hemispheres.

2. Place a block under the shoulders to allow the head to fall back. This makes the frontal lobes fall away from the anterior cranial fossae and exposes the olfactory bulbs. Elevate the olfactory bulbs gently from the cribriform plate of the ethmoid, tearing the olfactory nerves which pierce them.

3. The large optic nerves then come into view and should be divided close to the optic canal [Fig. 8.6].

4. Step 3 exposes the internal carotid arteries, with the infundibulum passing vertically to the hypo- physis between them. Divide all three structures [Fig. 8.6].

5. Allow the brain to fall backwards as each structure is divided, but support it posteriorly so as to avoid damaging it on the skull.

6. Posterior to the infundibulum, identify the dorsum sellae with a posterior clinoid process at each lateral extremity [Fig. 8.6] and the oculomotor nerves passing forwards, one on each side of it [Fig. 8.6].

7. Lateral to each oculomotor nerve is the free edge of the tentorium cerebelli. Turn the margin of the tentorium laterally, and divide the slender trochlear nerve which lies just under its free margin [Fig. 8.2B].

8. Turn the head forcibly to one side, and raise the posterior part of the cerebral hemisphere from the tentorium with your fingers. Divide the tentorium along its attachment to the petrous temporal bone [Figs. 8.6, 8.9], taking care not to injure the cerebellum beneath.

9. Then turn the head to the opposite side, and re- peat the procedure on the other side.

10. Let the brain fall backwards, so as to draw the brainstem away from the anterior wall of the posterior cranial fossa, and bring the lower cranial nerves into view.

11. Divide the oculomotor nerves and, allowing the brain to fall first to one side and then to the other, identify and divide the trigeminal, abducens, facial, and vestibulocochlear nerves. The last two enter the internal acoustic meatus [Figs. 8.2B, 8.9].

12. Identify and cut the nerves passing to the jugular foramen [Figs. 8.2B, 8.10], but leave the accessory nerve on one side by detaching it from the brain.

13. Look for the hypoglossal nerves, which are deeper and more medial, and cut them also [Fig. 8.10].

14. Press the pons [Fig. 8.5] further posteriorly, and identify the two vertebral arteries ascending to form the basilar artery on its surface.

15. Pass a knife into the vertebral canal in front of the medulla oblongata, and cut the spinal cord and the vertebral arteries.

16. Remove the brain from the cranial cavity, dividing any roots of the accessory nerve still attached to the spinal cord.

17. Removal of the brain in this way, without complete separation of the tentorium from the skull, ruptures the great cerebral vein. Find the cut end of the great cerebral vein where it enters the straight sinus at the junction of the free margins of the falx and tentorium.

a fixed margin attached to the skull at the edge of the posterior cranial fossa. The free margin of the tentorium extends almost horizontally backwards from each anterior clinoid process, forming the tentorial notch. At the notch, the posterior cranial fossa continues with the su

pratentorial compartments of the cranial cavity [Fig. 8.6].

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The cranial cavity

which unites these two margins, anteromedial to the crossing, forms the roof of the hypophysial fossa and the lateral wall of the cavernous sinus [Fig. 8.6].

each other. The free margin passes forwards to the anterior clinoid process above the attached margin. The attached margin passes medially to the posterior clinoid process. The dura mater

Anterior communicating A.

Infundibulum

Abducens N.

Trigeminal N.

Trochlear N.

Facial and vestibulocochlear Nn.

Glossopharyngeal N.

Vagus N.

Accessory N.

Hypoglossal N.

Vertebral A.

Basilar A.

Superior cerebellar A.

Posterior cerebral A.

Oculomotor N.

Posterior communicating A.

Internal carotid A.

Optic N.

Anterior cerebral A.

Fig. 8.9 Oblique section through the posterior cranial fossa. The brain and spinal cord have been removed.

Fig. 8.10 Coronal section through the skull at the level of the foramen magnum. The brain has been removed to show the fossae which lodge the occipital lobes of the hemispheres and the cerebellum.

Sensory root, trigeminal N.

Facial N.

Cut edge of tentorium Nervus intermedius

Vestibulocochlear N.

Tranverse sinus