One way to learn more about how something works is to look inside it. Today you will dissect, or take apart, a sheep’s brain. This guide will help you explore the structure of the brain. Your booklet will give you more information about what the different parts of the brain do, and it has space for you to write or draw what you see during the dissection.

Be sure to wear gloves, and to wash your hands thoroughly afterward to wash away any chemicals or tissue that may have gotten on your hands.

Anatomical Directions

Similar to using north, south, east, and west to describe relative locations of places on a map, when you are looking at anatomical structures there are terms that are used to help describe the relative location of the brain structures. These terms refer to the orientation of the brain in the body, so they are the same no matter how you hold or rotate the brain.

• Anterior (rostral or “towards the nose”) means towards the front.• Posterior (caudal or “towards the tail”) means towards the back.• Lateral means towards the side.• Medial means towards the middle.• Dorsal (“towards the back”) means on top• Ventral (“towards the stomach”) means on the bottom.

Brain Exploration Guide

Examine the exterior (outside surfaces) of the brain before you begin to cut into it. You can see several structures just by looking at the outside!

Examine the dorsal (top) surface of the brain. In a live animal, nerves that end in every part of the body join together in a bundle called the spinal cord. You can see a bit of the spinal cord sticking out of the posterior (back) side of the brain. The spinal cord is continuous with a region of the brain, the brainstem, which begins where the spinal cord thickens just before meeting the rest of the brain. The smaller wrinkly bulge just anterior (in front of) the brainstem is the cerebellum, which contributes to motor control and balance. The rest of the brain surface, with larger wrinkles, is the cerebral cortex, which you will learn about in more detail later in this guide. The indented line running through the median (middle) of the brain is the longitudinal fissure, which divides the cerebrum into two cerebral hemispheres.

Look carefully at the ventral surface (underside) of the brain. From this side, you can see the brain stem and the beginnings of the spinal cord better. You may also be able to see the X where the optic nerves carrying information from each eye at the anterior of the head join, and then split again into the optic tracts that carry information on each side of the visual field to the posterior of the brain. This joining place is called the optic chiasm. Most of each optic nerve has been cut away to remove the brain from the head, but the stumps remain to form the X. At the anterior of the brain, you may also be able to see the olfactory bulbs—two paler, oval shapes at the very front of the ventral surface.

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Looking at the lateral (side) view is a good way to begin to understand the cerebral cortex better. All of the large-wrinkled surface in the anterior portion of the brain is the cerebral cortex.

• The most anterior portion on each side are the frontal lobes, which is involved in the mental processes of attention, planning, and motivation.

• Just posterior to the frontal lobe on the sides of the brain are the temporal lobes, which are involved in processing different types of memories. In humans, the left temporal lobe also helps with language processing.

• The parietal lobes, posterior to the temporal lobe and superior to (above) the temporal lobes, process information about the different senses and help with motor planning for muscles all over the body.

• The occipital lobes, the most posterior regions of the cerebral cortex, are mainly involved in processing visual information carried there by the optic tracts.

Looking at the dorsal surface of the brain and gently bending it slightly with your hands, you can look underneath the occipital lobes and see part of the brainstem. The pair of larger lumps are the superior colliculi, and the two smaller lumps are the inferior colliculi. Both are involved in sensory processing—each inferior colliculus receives information from the ears, and each superior colliculus receives information from the eyes, as well as other sensory structures.

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Before you begin to cut your brain, find another pair of students to work with. There are two main types of cuts to make in the brain that we will look at today—sagittal cuts (separating the left and right halves of the brain) and coronal cuts (separating the front and back halves of the brain). Pick which of your two pairs will make the sagittal cuts on one brain, and which will make the coronal cuts on the other brain—that way, all four of you will get to see each type of cut.

Using your butter knife, make the sagittal cut down the middle of the brain, along the longitudinal fissure. The brain tissue is soft enough to cut, but it will still offer some resistance. Gently saw the knife to help break the connections between the two halves of the brain. Keep working until the two halves are separated.

Examine the inside surface of the brain that you have revealed.

• Notice some holes or cavities inside the brain. These are ventricles, which allow cerebrospinal fluid to flow through, support and cushion the brain.

• The diagonal white structure near the ventricles is the corpus callosum, which allows the left and right halves of the brain to communicate.

• How do the insides of other structures (the spinal cord, the superior and inferior colliculi, the cerebellum, the optic chiasm) compare to how they looked on the surface?

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On one of the halves of your brain, make a second sagittal cut (front to back), this one lateral (to the side) of the first. The cut shown below was made in the area of the temporal lobe. This will reveal a cross-section of the cerebral cortex.

• How do the wrinkles on the surface relate to what you can see inside?• The gray material, or gray matter near the surface is composed of the cell bodies of many thousands

of neurons, while the white material or white matter is made of the extended processes (dendrite and axons) and connections between those neurons. This area is white because of myelin. Special support (glial) cells wrap around the processes of neurons, acting like the insulation on an electrical cord.

With your other pair’s brain, you will perform coronal cuts. Start near the anterior end (front) of the brain, and cut off just the front portion. You will see a cross-section of the frontal lobe.

• How does it compare to the cross-section of the temporal lobe you saw in the other brain?

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Continue to make coronal cuts, moving back toward the posterior end of the brain (like slicing a loaf of bread), PAUSING after each cut to examine the new surface of you have revealed.

• Do you notice ventricles in these cuts as well? Can you see the white corpus callosum connecting the left and ride sides of the brain in these cuts as well?

• Can you distinguish midbrain structures as they begin to appear inside the cerebral cortex?

Keep cutting and examining each cut! If you see other structures and are curious what they are, just ask for help. Try to remember them and draw or describe them in your booklet as soon as you wash

your hands after the dissection, so you can ask to learn more about them later.

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