The Dissection of the Sheep Brain

Lab Participant Names:

The Dissection of the Sheep Brain

I. Objective: Locate, identify & state the functions of all the structures in the sheep’s brain.

II. Materials: sheep brain, dissecting tray & instruments per 2 students; disposable gloves, Textbook & notes; Pen, external anatomy diagram worksheet

III. Instructions: Obtain a sheep brain, all other materials & return to your workstation.

External Anatomy:

1. Turn you sheep brain so that you are viewing its’ left lateral aspect. Compare the various areas of the sheep brain to your textbook (pg. 216).

2. Place the ventral surface of the sheep brain down on the dissecting tray & observe the fragments of the dura mater. Feel its consistency & notice its toughness.

3. Examine the superior surface of the brain. Notice that, like the human brain, its surface is thrown into convolutions. Identify the arachnoid mater, which appears on the brain surface as a delicate “cottony” material spanning the fissures.

4. Lay the brain flat on the dissecting tray so as to view the external surface. Identify the external structures indicated below on the brain. Then label the external anatomy diagram with all the structures.

Cerebellum, cerebral hemisphere, spinal cord, medulla oblongata, temporal lobe, parietal lobe, frontal lobe, occipital lobe

5. Carefully examine the cerebellum. Notice that, in contrast to the human cerebellum, it is not divided longitudinally, & that its fissures are oriented differently.

6. To expose the dorsal surface of the midbrain, gently force the cerebrum & cerebellum apart. Identify the corpora quadrigemina, four rounded prominences on the dorsal midbrain surface. What is the function of the corpora quadrigemina? ______

______

Also locate the pineal body, which appears as a small oval protrusion in the midline just anterior to the corpora quadrigemina.

7. Observe the convolution & fissures on the surface of the brain. Why does the brain have these sulci & gyri? ______

______

8. What is the general name for the 3 protective membranes that cover the brain? ______

9. Which one of these membranes might be found closely adhering to the sheep’s brain? ______
10. Complete the table for the major external brain regions.

Ventral Structures:

Pg. 230 in your textbook or wksht 14 illustrates the important features of the ventral surface of the brain. Turn the brain over so that its ventral surface is up.

1. Look for the club-like Olfactory Bulbs on the inferior surface of the frontal lobes of the cerebral hemispheres.

a. How do you suppose the size of these olfactory bulbs compare w/those of humans? ______

1. Is the sense of smell more important as a protective & a food-getting sense in sheep or in humans? ______

2. The Optic Nerve (II) carries sensory impulses concerned w/ ______from the retina of the eye. Identify the optic nerves, the optic chiasma (the point where some of the fibers of ea. optic nerve cross over to the opposite side), & the optic tracts, which continue from the optic chiasma.

3. Posterior to the optic chiasma, identify the stalk of the pituitary gland (Note: the pituitary gland is usually difficult to see on the sheep brain) & then the mammillary body (relay sensations of smell). . Notice that the sheep’s mammillary body is a single rounded eminence. In humans it is a double structure.

4. Identify the cerebral peduncles (which convey ascending & descending impulses in the midbrain)on the ventral aspect of the midbrain, just posterior to the mammillary body. Also identify the large oculomotor nerves (III), which arise from the ventral midbrain surface, & the tiny trochlear nerves (IV), seen at the midbrain-pons junction. These cranial nerves provide motor fibers to the extrinsic muscles of the eyeball.

5. Return to the junction of the pons & midbrain & proceed posteriorly to identify the following cranial nerves all arising from the pons:

*Trigeminal nerves (V), which are involved in ______& sensations of the head & face.

*Abducens nerves (VI), which abduct the eye (& thus work in conjunction w/cranial nerves III & IV).

*Facial nerves (VII), large nerves involved in ______sensation, gland function (salivary & lacrimal glands), & facial expressions.

6. Continue posteriorly to identify the following:

*Vestibulocochlear nerves (VIII), purely ______nerves that are involved w/hearing & equilibrium.

*Glossopharyngeal nerves (IX), which contain motor fibers innervating throat structures & sensory fibers transmitting taste stimuli.

*Vagus nerves (X), often called “wanderers”, which serve many organs of the head, thorax, & abdominal cavity.

*Accessory nerves (XI), which serve muscles of the neck, larynx, & shoulder; notice that the accessory nerves arise from both the medulla & the spinal cord.

*Hypoglossal nerves (XII), which stimulate tongue & neck muscles.

Internal Anatomy Instructions:

1. The internal structure of the brain can only be examined after further dissection. Position the brain ventral side down & make a cut completely through it in a superior to inferior direction. Cut through the longitudinal fissure, corpus callosum, & midline of the cerebellum. See fig. 7.17 a & b on pg. 222 in your textbook for reference while you work.

2. A thin nervous tissue membrane immediately ventral to the corpus callosum separates the lateral ventricles from each other. Pierce this membrane & probe the cavity of the lateral ventricle.

3. Identify the thalamus, which forms the walls of the 3rd ventricle. The intermediate mass spanning the ventricular cavity appears as a round protrusion of the thalamus wall.

4. The hypothalamus forms the floor of the 3rd ventricle. Identify the optic chiasma, stalk of the pituitary, & mammilary body on its exterior surface. You can see the pineal body at the posterior end of the 3rd ventricle.

5. Locate the midbrain by identifying the corpora quadrigemina that form its dorsal roof. Follow the cerebral aqueduct (connects the 3rd ventricle to the 4th) through the midbrain tissue to the 4th ventricle. Identify the cerebral peduncles, which form its anterior walls.

6. Identify the ponsmedulla, anterior to the fourth ventricle. The medulla continues into the spinal cord w/out any obvious anatomical change, but the point at which the 4th ventricle narrows to a small canal is generally accepted as the beginning of the spinal cord.

7. Identify the cerebellum posterior to the 4th ventricle & notice the internal treelike arrangement of its white matter called the arbor vitae.

8. You will be responsible for knowing the location of the following structures. Label the internal anatomy diagram worksheet w/all the structures.

Corpus callosum, medulla oblongata, pituitary gland, ventricle, pons, sulci, gyri, cerebral cortex, thalamus, hypothalamus, cerebellum, pineal gland.

9. Complete the table for the major internal brain structures.

Instructions – Pin Labeling / Worksheet Completion:

1. Obtain FIVE colored pins. Choose any FIVE structures, internal or external, & gently place a pin in each region of the sheep brain. Be very accurate in your placement.

2. Bring your completed worksheets/diagrams & your sheep brain w/pins inserted to your teacher for a group evaluation. You will be evaluated on correct placement of your pins & the ability to state the function of each part (without use of your worksheets). Your teacher will also ask you to identify THREE other structures & functions.

3. After your evaluation, wash off the pins, return the brain to the bin, wash all equipment including the dissection tray & work station & then wash your hands thoroughly.

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