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| If you just print this out, you'll miss some of its function. As you review it online, you can put the cursor on parts of the brain figures and they will be identified for you. | |||||
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Sheep brain dissection instructions |
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Look at the exterior surface of the sheep brain. If you put your mouse on a part of the diagram, it will display the name. How is it different from the human brains figured in your book? Identify the cerebrum and its sulci and gyri. Find the frontal, temporal, parietal, and occipital lobes of the cerebrum. What is each lobe responsible for? Frontal lobe ___________________________________ Parietal lobe ___________________________________ Temporal lobe __________________________________ Occipital lobe ___________________________________ Behind the cerebrum, find the cerebellum. What is its function? _________________________________ Behind the cerebellum is the medulla oblongata, leading into the spinal cord. What does it control? __________________ |
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Lift the occipital lobe forward, away from the cerebellum, and look underneath to see the midbrain. There are four lumps on its dorsal surface; the superior colliculi and inferior colliculi. What do they do? Superior colliculi ___________________________________________ Inferior colliculi ____________________________________________ On the ventral surface of the sheep brain, you should be able to find the optic chiasm where nerve fibers from the medial half of each retina cross over to the other side of the brain. Anterior to the optic chiasm, the olfactory bulbs are attached to the ventral surfaces of the frontal lobes. Just behind the optic chiasm is the hypothalamus. You can tell it by finding the stump of the stalk that led from the hypothalamus to the pituitary gland, which has usually been removed. This stalk is called the infundibulum or hypophysis. |
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The hypothalamus is the bottom of the second segment of the brain. The walls of that segment are called the thalamus and the top is called the epithalamus or pineal, but you can't see them yet because the cerebrum is covering them. What is the function of the hypothalamus? ______________________________________________________ Why is the hypothalamus connected to the pituitary? _________________________________________ Behind the hypothalamus on the ventral surface of the brain, you can see the pons. Behind the pons is the ventral surface of the medulla oblongata. Before cutting into the brain, look for cranial nerves. The easiest to find are the optic nerves, which meet at the optic chiasm. Behind the optic chiasm, optic nerve fibers continue into the brain as the optic tracts. A tract is a group of nerve fibers running together through the brain tissue. The olfactory nerves connected the olfactory bulbs to sensory neurons in the sheep's nose. You won't see them, but if the olfactory bulbs are present, you will see the olfactory tracts. They are the stalks connecting the olfactory bulbs to the brain. The oculomotor nerves arise from the front of the pons and stretch forward toward eyes and the optic chiasm. They control the pupillary reflexes. Because they stretch so far, these nerves can be affected when the brain is pressed downwards because of swelling or bleeding within the skull. That is why a fixed pupil indicates increased pressure within the skull. Trochlear and Trigeminal nerves arise from the sides of the pons. The rest of the cranial nerves arise from the medulla, and can be very hard to distinguish. The abduscens nerves come from the anterior end of the medulla. The facial and vestibulocochlear nerves come from a little further back on each side. Behind them, the glossopharyngeal, vagus, and hypoglossal nerves arise from the sides of the medulla. The accessory nerves run posteriorly along each side of the medulla. You will probably not be able to find all of these on your sheep brain. |
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| Now you have reviewed the surface anatomy of the brain, take a brain that has been sectioned down the midsagittal plane. You should be able to identify the major areas of the brain in this view. | |
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You will notice that the ventral section of the brain is hollow. These hollows are called ventricles. There are 4 ventricles, indicated by blue shading on the drawing below. The first two are the lateral ventricles, which extend laterally into the cerebrum, one on each side. They both connect with the third ventricle, which lies on the midline of the brain and extends down into the hypothalamus. This ventricle drains posteriorly into the fourth ventricle, which is between the pons and the cerebellum. From this, the cavity continues down the spinal cord as the spinal canal. The ventricles are filled with cerebrospinal fluid. This fluid is created by networks of blood vessels inside the ventricles, shown in pink in the drawing below. Each network is called a choroid plexus. Look for them inside the ventricles. You've already identified the lobes of the cerebrum. Just below the cerebrum you will see the corpus callosum which connects the two cerebral hemispheres. The third ventricle fills the second segment of the brain. The floor of this segment is the hypothalamus, which is connected ventrally to the infundibulum and the pituitary. The walls of this segment are called the thalamus. Its roof is the epithalamus or pineal. |
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