- List and describe the principal structures of the sheep brain
- Identify important parts of the sheep brain in a preserved specimen
Materials: Dissection tools, lab glasses, lab gloves, preserved specimen
External Sheep Brain
The sheep brain is quite similar to the human brain except for proportion. The sheep has a smaller cerebrum. Also the sheep brain is oriented anterior to posterior whereas the human brain is superior to inferior.
- The tough outer covering of the sheep brain is the dura mater, one of three meninges (membranes) that cover the brain. You will need to remove the dura mater to see most of the structures of the brain. Remove the dura mater while leaving other structures intact.
- The most prominent feature of the brain is the cerebrum - which is divided into nearly symmetrical left and right hemispheres by a deep longitudinal fissure.
- The surface of the cerebrum is covered with large folds of tissue called gyri. The grooves between the gyri are sulci. The deeper sulci are often termed fissures. The fissures are used as landmarks to divide the surface of the cerebrum (the cerebral cortex) into regions:
frontal lobes / parietal lobes / occipital lobes / temporal lobes * Locate each of the lobes of the brain.
- The smaller, rounded structure at the back of the brain is the cerebellum. The cerebellum has smaller gyri that are roughly parallel to one another. Compare the gyri of the cerebellum to that of the cerebrum. Removing the dura mater from the cerebellum can be tricky business. Look for areas on the side of the brain that you can snip to peel the dura mater off.
- Turn the brain over so that the cerebrum is down. The most prominent structure visible on the ventral side of the brain is the optic chiasma, where the two optic nerves cross over each other and form an “X” shape. Locate the optic chiasma.
- The pituitary gland is a large round structure under the chiasma. If you removed this area with the dura mater, you may need to replace it to see the chiasma and pituitary gland.
- Toward the front of the brain are two prominent round structures; find the olfactory bulbs.
- Toward the back of the brain, are bulges that indicate the midbrain, the pons, and the medulla.
- Just behind the optic chiasma is a raised area or bump that indicates the infundibulum (also known as the pituitary stalk). This is where the pituitary was attached to (which was probably removed with the dura mater).
- Oculomotor nerves may be visible to each side of the pituitary gland (or stalk). Or in some cases you may find them stuck to the dura mater that you removed with the pituitary gland.
- Carefully bend the cerebellum to get an inside glimpse of the brain. The bumps you see (kind of resemble a “butt”) are the superior colliculi. The smaller ones underneath are inferior colliculi.
- If you gently push those structures down, you can see the tiny nub of the pineal gland.
Internal Sheep Brain
- Use a knife or long-bladed scalpel to cut the specimen along the longitudinal fissure. This will allow you to separate the brain into the left and the right hemisphere. Lay one side of the brain on your tray to locate the structures visible on the inside. You should also cut through the cerebellum.
- The corpus callosum had been connecting the two cerebral hemispheres and can now be clearly seen in the brain section.
- The tiny space within the corpus callosum (which hold cerebrospinal fluid) is called the lateral ventricle. Underneath it, you can find the third ventricle. There are other ventricles within the brain, but those are the easiest to locate in a preserved specimen. The white area between those two ventricles is the fornix. The fourth ventricle is the space under the cerebellum.
- Inferior to the corpus callosum is a round structure known as the thalamus. It seems it almost perfectly centered. Just behind the thalamus is the pineal body (gland). The hypothalamus is also round shaped but is lower and toward the front of the brain.
- The pons, medulla, cerebellum and spinal cord are also visible in the side view of the brain. Gently separate the cerebellum at the transverse fissure, which separates it from the cerebrum.
- Within the cerebellum, locate the arbor vitae, named such because the white lines resemble a tree.
- Use a scalpel to cut a cross section of the cerebrum in the occipital lobe area. You should be able to see the color and texture differences of the white matter and the gray matter.
Sheep Brain Dissection Analysis
Match the structure to the description.
LABEL THE BRAIN (The words above may be helpful, but write them out and do not use the letters. This will help you remember the structures.)