1. The sheep brain is enclosed in a tough outer covering called the dura mater. You can still see some structures on the brain before you remove the dura mater. Take special note of the pituitary gland and the optic chiasma. These two structures will likely be pulled off when you remove the dura mater.Brain with Dura Mater Intact
Removal of the Dura Mater
2. This image shows the ventral surface of the sheep's brain with most of the dura mater removed. The pituitary gland and the optic chiasma are still intact.
(A = pituitary gland, B = optic chiasma, C = olfactory bulb)
3. On this image, the dura matter has been completely removed, you can still see the optic chiasma but the pituitary gland is missing. The infundibulum (pituitary stalk) is now visible in the center. Careful dissection also reveals two other large nerves: the oculomotor nerves (C.2). Often these two nerves are removed with the dura mater, but in this image they are still intact.
4. If you flip the brain over to the other side, you can see the cerebellum, it will be loosely attached to the cerebrum in most cases. If you did not carefully remove the dura mater you may have accidentally pulled the entire cerebellum away from the brain. The lobes of the brain are visible, as well as the transverse fissure, which separates the cerebrum from the cerebellum. The convolutions of the brain are also visible as bumps (gyri) and grooves (sulci).
5. The gap between the cerebrum and the cerebellum at the transverse fissure can reveal some internal parts of the brain. In this image, a student is bending the cerebellum down to show the superior and inferior colliculi. Just behind the colliculi, the pineal gland is just barely visible.
6. Using a scalpel and the longitudinal fissure as a guide, the brain is separated into the left and the right hemispheres. Sharp scalpels work best for this procedure. Always leave the specimen in the dissecting tray when cutting it, do NOT hold it in your hand!
If you are very careful, you will cleanly cut the brain into two halves and can see the internal structures, the most visible of them being the corpus callosum, which divides the left and right hemispheres. The cerebrum will still be visible as a wrinkled structrure, and you can even locate the "bumps" of the superior and inferior colliculi. Remember, you located those structures by pulling down the cerebellum.
The cerebellum, when cut will have a very distinct tree-like white area within it. This is called the arbor vitae, or the tree of life.
7. In the image below, a probe indicates the location of the lateral ventricle.
8. Once the brain is cut this way, the colliculi can also be seen from the inside and the pineal gland is revealed only if you made a very careful incision.
On this image, the pineal is pinned in yellow and the pin continues on to where the colliculi have been bisected.
9. Other major structures are visible, here the probe indicates the arbor vitae (tree of life) found within the cerebellum. The fissure between the cerebrum and the cerebellum is called the transverse fissue. The cerebellum only loosely connects to the rest of the brain when the dura is removed.
10. This brain is pinned to show the pineal gland, thalamus and lateral ventricle.
11.The image below shows a cleanly separated brain with the major internal structures visible and labeled.
12. Finally, a section of the brain is cut to examine the difference between white matter and gray matter.