Sheep Brain Dissection

Name(s)______

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 found on web

Sheep brain dissection guide which can be found here http://mcdanatomyandphysiology.wikispaces.com/Nervous+System Look for the sheep brain dissection pdf

The guide should be used and referenced during the entire dissection. Each member of the group should read the directions as the brain is being dissected. This form should be filled out and saved as “period_one group name_sheep brain” Drop it in the student drop folder when complete.

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.

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 3 visible parts on this side? Use handout pg. 1. 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 so that all of the dura comes off. 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 side. 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?

7. What does this indicate about the mental abilities of a sheep when compared to those of a human? Why? Hint: think about the amount of wrinkles.

D. THE CEREBRUM

8. Use the diagram and your sheep brain 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.

13. What does the cerebellum control in humans? ______14. Describe the exterior of the cerebellum of the sheep brain (3 observations) 1. 2. 3.

F. SPINAL CORD

15. 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.

16. Describe the appearance of the spinal cord.

17. How is the position of the spinal cord in the sheep different than in the human brain? Why?

G. CORPORA QUADRIGEMINA

18. With your hands, bend the specimen so you separate the cerebrum from the cerebellum along the transverse fissure. 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 and pineal body as you locate them.

Inferior colliculi______Superior colliculi______Pineal body______

20. Describe the colliculi you just located. H. STRUCTURES OF THE HYPOTHALAMUS

21. 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. Check here when you have located the mamillary body. ______

I. PONS

23. Locate the pons.

24. Describe the pons of the sheep brain.

J. MEDULLA

25. Find the medulla oblongata, posterior to the pons. This structure is more commonly called the medulla.

26. What does the medulla do in the human brain?

K. PREPARATION OF THE SAGITTAL SECTION

27. 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.

28. 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. Refer to the Sagittal Section of Sheep Brain diagram.

29. Describe the corpus callosum. What is its job in the human brain?

M. INTERNAL STRUCTURES OF THE BRAIN 30. Examine either half of the brain. Locate the corpus callosum. Follow this fibrous band around posteriorly. The fibrous band inferior to the corpus callosum is the fornix. Between the corpus callosum and fornix is an opening. This opening is the lateral ventricle.

31. 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. The loose, feathery tissue inside the lateral ventricle is the hippocampus.

32. Check off the fornix, hippocampus, and lateral ventricle as you locate them.

Fornix______Hippocampus______Lateral ventricle______

33. 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.

34. Directly inferior to the thalamus and the cavity of the third ventricle is the hypothalamus. If your brain is well preserved, the hypothalamus will be a distinct V- shaped structure. The pituitary gland was removed with the dura mater and would be located inferior to the hypothalamus.