1 Name__________________ Date___________Hour_____ Sheep Brain Dissection Introduction: For the past two weeks we have examined the anatomy of the human brain. Since the basic plan of all mammalian brains is the same, we can expand our understanding of the structure of the human brain by studying the structure of the brain of some other mammal. Because of its size and relative availability, the sheep brain is often used for comparative dissection. The sheep brain is somewhat smaller than the human brain but the structures are essentially the same and the sheep brain is quite easy to work with. In this activity, we will dissect the sheep brain and examine those structures common to both the sheep and the human brain. Materials Dissecting tray Dissecting kit Sheep brain Sheep dissection brain pictures (Packet) Procedure A. MENINGES 1. Examine the surface of the brain. You may be able to locate a tough sac-like protective layer around the brain. This is the dura mater (literally, “tough mother”), the outermost of the meninges, the three protective connective tissue membranes that surround the brain and the spinal cord. It is possible that the dura mater has been removed. If the dura mater is still present, follow the directions in part B to carefully remove it. 2. Regardless of whether you have found a dura mater, you probably will not be able to locate the second meningeal layer, the arachnoid layer, or arachnoid membrane. The arachnoid layer is sometimes called the arachnoid matter (literally, “spider mother”). This web-like structure does not withstand preservation well. (If you notice tufts of cottony tissue across certain brain fissures, however, you may have located remnants of the arachnoid mater.) 3. You should be able to locate the inner layer, the pia mater (literally, “gentle mother”), which tightly adheres to the sulci and gyri of the brain. Gently separate at least part of the pia mater from the surface of the brain, by lifting it up with your forceps. 4. Describe the meningeal layers you were able to locate. 2 B. BRAIN DISSECTION Examine the sheep brain. Hold the brain so that the superior side is up. Notice the thin outer covering on the brain. What is this called?__________________________________ Make 3 observations about this outer covering. 1. 2. 3. Hold the brain so that the inferior side is up. What are the visible parts on this side? 1. 2. 3. You are now going to carefully take the dura mater off the brain. Hold the brain so the superior side is facing up. Find the opening located near the frontal lobe. Using your scissors cut the dura mater from front to back, off to one side of the brain. Now cut up the other side. Peel back and cut off the dura mater. Describe the texture of the cerebral cortex (3 observations) 1. 2. 3. What are the valleys called?______________ What are the ridges called?_______________ What is the purpose of all the valleys and ridges?______________________________ ______________________________________________________________________ Turn your brain so the inferior side is up. You are now going to remove the dura mater on that side. Cut between the optic nerve and the pituitary gland. Carefully cut around the brain stem and cerebellum. Since this tissue is very fragile, it is important to use care at this point. C. GENERAL COMPARISON 5. Turn your sheep brain so that you are viewing its left lateral aspect. Compare the various areas of the sheep brain (cerebrum, brain stem, cerebellum) to the photo of the human brain in the sheep dissection brain pictures. 6. Relatively speaking, which of these structures is obviously larger in humans? 3 7. What does this indicate about the mental abilities of a sheep when compared to those of a human? D. THE CEREBRUM Diagram I 8. Use the diagram to the right and the Intact Sheep Brain (ventral and Dorsal view) and the Dorsal Midbrain Structures of the Sheep Brain pictures on the Brain pictures to help you identify the following structures. 9. Begin with your sheep brain ventral surface down in your dissecting pan. The most prominent feature of the dorsal surface of the brain is the cerebrum. The longitudinal fissure divides the cerebrum into the right and left cerebral hemispheres. Note the numerous sulci and gyri forming the surface of the brain. You are actually looking at the cerebral cortex, the gray matter of the brain where most voluntary thought processes take place. 10. Observe that each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal and occipital. These lobes are directly inferior to the corresponding structures in the cranial bones. Identify these lobes on your sheep brain. Check them off as you find them. Frontal______ Parietal______ Temporal______ Occipital______ 11. How do these lobes compare with the human cerebral lobes? E. CEREBELLUM 12. Locate the cerebellum by finding the transverse fissure posterior to the cerebrum that separates it from the cerebellum. Note that the sulci and gyri of the cerebellum seem to be much more tightly packed that the corresponding structures in the cerebrum. 4 13. Note the protruding central area of the cerebellum, the vermis. (This midline structure is also present in humans. In humans, however, the vermis is reduced in size and the cerebellum is divided into hemispheres separated by the flax cerebelli.) 14. What does the cerebellum control in humans? _______________________________________ 15. Describe the exterior of the cerebellum (3 observations) 1. 2. 3. F. SPINAL CORD 16. Locate the spinal cord, inferior to the cerebellum. Depending on the condition of the brain and how much of the spinal cord is present, you may be able to find remnants of one or more of the cervical (spinal) nerves. 17. Describe the appearance of the spinal cord. G. CORPORA QUADRIGEMINA 18. With your hands, bend the specimen so you separate the cerebrum from the cerebellum along the transverse fissure. Identify the corpora quadringemins, the four prominent swellings immediately anterior to the cerebellum. The paired swellings closest to the cerebellum are the inferior colliculi, which are involved with auditory reflexes. Centrally located at the anterior end of the superior colliculi , which are involved with visual reflexes. Centrally located at the anterior end of the superior colliculi is the pineal body, the endocrine gland which secretes the hormone megaton. Use the dorsal Midbrain Structures of the Sheep Brain picture to help you identify these parts. 19. Check off the colliculi as you locate them. Inferior colliculi_______ Superior colliculi________ 20. How do the colliculi and pineal body of the sheep compare with the colliculi and pineal body of the human? 5 H. STRUCTURES OF THE HYPOTHALAMUS 21. Observe the structure that seems to originate under the optic chiasma, the infundibulum, or the stalk of the pituitary gland (see diagram II). In actuality, the infundibulum arises from the hypothalamus. Some specimens may still have the pituitary gland attached to the infundibulum. (we will examine the hypothalamus further when we study the sagittal anatomy of the brain.) If you sheep brain is overly preserved, you may find the infundibulum a bit mushy and thus easily destroyed. Posterior to the infundibulum is the single-lobed mammillary body. In humans the mammillary body is doubled-lobed. The mammillary body controls the feeding reflexes. 22. How do these structures compare with the corresponding human structures? I. CEREBRAL PEDUNCLES 23. Find the cerebral peduncles, posterior to the mammillary body; these are fibrous tracts connecting the cerebrum and the medulla. 24. Describe the cerebral peduncles of the sheep brain. J. PONS 25. Locate the pons, posterior to the cerebral peduncles. The trochlear nerves, cranial nerve IV, can be found at the junction of the midbrain and the pons. 26. Describe the pons of the sheep brain. K. MEDULLA 27. Find the medulla oblongata, posterior to the pons. This structure is more commonly called the medulla. 28. What does the medulla do in the human brain? 6 L. PREPARATION OF THE SAGITTAL SECTION 29. To explore the internal structures of the brain, it will be necessary to separate the brain into left and right halves. Take the knife and carefully cut down the longitudinal fissures. Refer to diagram II and the brain packet to identify the structures described. Diagram II 7 30. Manually move the left and right cerebral hemispheres apart from each other. Note the thick fibrous tissue holding the hemispheres together. This is the corpus callosum, the nervous tissue that functions as a communicating band between the cerebral hemispheres. 31. Describe the corpus callosum. What is its job in the human brain? M. INTERNAL STRUCTURES OF THE BRAIN 32. Examine either half of the brain. Locate the corpus callosum. Follow this fibrous band around posteriorly. The posterior end of the corpus callosum is known as the spleniuium of the corpus callosum. The fibrous band inferior to the corpus callosum is the fornix. Between the corpus callosum and fornix is an opening. If your incision was good, this opening is covered by a thin membrane known as the septum pellucidum. If you destroyed the septum pellucidum, note where it should be. 33. The area behind the septum pellucidum is the lateral ventricle. Put you blunt probe into the ventricle and get a feel for the size of this cavity. The brain has a lateral ventricle in each cerebral hemisphere. 34. Inferior to the fornix you will find a large round area known as the intermediate mass of the thalamus. Run your probe along the underside of the thalamus and see if you can locate a cavity. This is the third ventricle. Refer to diagrams II and page 2 of the packet to help you visualize this third ventricle. The third ventricle is sometimes difficult to recognize as a cavity in preserved brains. Between the lateral ventricles and also communicating with this third ventricle is the interventricular foramen, also known as the foramen of Munro. You will probably not be able to locate this foramen. 35. Relocate on the sagittal section those structures you found when you examined the dorsal and ventral views of the brain. Note particularly the hypothalamus, which is directly inferior to the thalamus and the cavity of the third ventricle. If your brain is well preserved, the hypothalamus will be a distinct V-shaped structure. The infundibulum extends from the hypothalamus. The pituitary gland is the knob-like structure at the end of the infundibulum. 8 Questions: 1. Examine Sheep Brain Picture #1 in your packet. Match the parts below with the correct letter from the diagram. __________Cerebellum __________Cerebrum __________Gyri __________Longitudinal fissure __________Medulla oblongata __________Sulci 2. Examine Sheep Brain Picture #2 in your packet. Match the parts below with the correct letter from the diagram. __________Olfactory bulb __________Optic nerve __________Optic chiasma ___________Optic tract __________Mammillary body ___________Pons __________Medulla oblongata __________Spinal cord __________Cerebrum __________Cerebellum 3. Examine Sheep Brain Picture #3 in the packet. Match the parts below with the correct letter from the diagram. __________Pineal body __________Cerebellum __________Fornix __________Corpus callosum __________Olfactory bulb __________Thalamus __________Pons __________Medulla oblongata 9 4. Examine Sheep Brain Picture #4 in the packet. Match the parts below with the correct letter from the diagram. __________Spinal cord __________Cerebellum __________Cerebrum 5. Examine Sheep Brain Picture #5 in the packet. Match the parts below with the correct letter from the diagram. __________Cerebral peduncle __________Medulla oblongata __________Mammillary body _________Spinal cord __________Olfactory bulb __________Pons 6. Examine Sheep Brain Picture #6 in the packet. Match the parts below with the correct letter from the diagram. __________Cerebellum __________Cerebral Hemisphere __________Medulla oblongata __________Corpus callosum __________Pineal body __________Pons __________Thalamus __________Spinal cord 7. Identify the part of the meninges that is being described below. Use the key provided to indicate your answers. A = Arachnoid P = Pia mater D = Dura mater __________Outer most meninges __________Provides support to brain __________Delicate connective tissue __________Middle layer __________Directly covers & is attached __________Inner most layer to the surface of the brain.