LABORATORY STUDY OF THE SHEEP BRAIN

WEEK 1 Structures

You are responsible for identifying the following structures in sheep brain specimens & corresponding structures in the human brain plates shown in red. Correct spelling matters.

 

A. Cerebral hemispheres - dorsal and lateral surfaces - Human Brain 7.1 7.2 7.3 7.6

 

1. The longitudinal fissure separates the two hemispheres. By gently pulling apart the two hemispheres, it will be seen that this fissure completely separates the hemispheres at the anterior and posterior ends, but that at its central part the two hemispheres are joined by the corpus callosum, a great transverse band of commissural fibers.

2. The cerebral cortex is the thin layers of gray matter forming the surface of the hemisphere.

3 The cortex is thrown into convolutions or folds called gyri (singular = gyrus) separated from one another by grooves call sulci (singular = sulcus). Some of the more important grooves are called fissures.

4. The cruciate fissure runs laterally from the longitudinal fissure so as to divide the hemisphere into an anterior third and a posterior two-thirds. (This fissure is analogous to the central sulcus of the human brain.) The cruciate / central fissure marks the posterior boundary of the frontal lobe and the anterior boundary of the temporal lobe.

5. The superior frontal sulcus runs forward from about the middle of the cruciate fissure to the anterior tip of the frontal lobe.

6. The lateral fissure or fissure of Sylvius begins on the dorsal surface about half an inch behind the lateral extremity of the cruciate fissure. It runs vertically downward and then divides into a short posterior ramus (branch) and a longer anterior ramus. (In the human brain, the enormous development of the hemispheres, particularly of the frontal lobes, had resulted in the lateral sulcus (fissure) assuming a more horizontal direction.)

7. The suprasylvian sulcus arches over the upper extremity of the lateral fissure, its posterior limb extending backward toward the occipital pole.

8. The rhinal fissure marks the horizontal boundary between the lateral and the ventral surfaces the human correlate is the superior temporal sulcus.

9. For convenience of description, five lobes or areas of the cerebral cortex are recognized: the frontal, parietal, occipital, temporal, and insular.

10. The frontal lobe is the part of the hemisphere anterior to the cruciate fissure.

11 The parietal lobe includes about 1/2 of the dorsal surface behind the cruciate fissure.

12. The occipital lobe occupies the posterior end of the hemisphere.

13. The temporal lobe is separated from the parietal lobe above by the posterior two thirds of the suprasylvian sulcus.

14. The insula lobe is the area enclosed between the diverging anterior and posterior rami of the lateral fissure and the rhinal fissure. Gently open up the rami of the lateral fissure near their junction; it will be seen that the insula is in fact a separate lobe, separated from the rest of the hemisphere by the deep rami of the lateral fissure. (In man, the insula becomes completely covered by the adjacent folds of the frontal and temporal lobes, which constitute the operculum.)

15. Some specially well-marked and constant gyri seen on the surfaces are as follows:

a. Superior frontal gyrus, the area between the longitudinal fissure and the superior frontal sulcus. It is the motor area of the sheep brain.

b. Medial frontal gyrus, lateral to the superior frontal sulcus.

c. Inferior frontal gyrus, dorsal to the anterior ramus of the lateral fissure.

d.   Orbital gyrus, in front of the insula, between the anterior ramus of the lateral fissure and the rhinal fissure.

16. Specific human gyri: Human Brain 7.3 & 12.1

a.    Precentral Gyrus - frontal lobe gryus located posterior to the superior, middle, and inferior frontal gyri and anterior to the central sulcus.

b.   Postcentral Gyrus - parietal lobe gryus located posterior to the central sulcus

c.    Angular Gyrus - parietal lobe gryus located on the border with the temporal lobe

d.   Wernicke's area - dorsal-most area in the left temporal lobe posterior to primary auditory cortex and anterior to the angular gyrus

e.    Broca's area - left frontal lobe adjacent and anterior to the ventral motor cortex (superior frontal gyrus)

f.     Superior temporal gyrus, the area located inferior to the lateral fissure. It is the primary auditory area of the brain.

g.    Middle temporal gyrus, located inferior to the superior temporal gyrus.

h.    Inferior temporal gyrus, located inferior to the middle temporal gyrus.

 

B. Cerebellum - dorsal and lateral surfaces. Human Brain 6.2

 

1. The cerebellum of the sheep lies behind the cerebral hemispheres. (The human cerebellum lies beneath the posterior parts of the cerebral hemispheres; this change in position is due to the greater development of the cephalic flexure and to the enormous growth of the cerebral hemispheres.

2. The cerebellar cortex is the outer gray layer of the cerebellum.

3. The surface of the cerebellum is divided by transverse sulci into folia (singular folium).

4. The vermis is the ridge that forms an almost complete medial circle around the cerebellum.

5. The cerebellar hemispheres are the lateral masses on either side of the vermis.

6. The flocculus is the small, most ventrally located lobule of the cerebellar hemisphere and overhangs the brainstem. The flocculi of the two sides, together with a median portion, not visible at present, called the nodule, constitute the flocculonodular lobe. The remainder of the cerebellum is called the corpus cerebelli, and is divided into an anterior and posterior lobe.

7. The anterior lobe is the most rostral part of the cerebellum. It may be seen by gently pulling the cerebellum away from the cerebrum. The folia run continuously through vermis and lateral portions, so that there is no apparent distinction between vermis and hemispheres other than that the vermis is a slightly raised structure.

8. The fissure prima is the transverse fissure that separates the anterior cerebellar lobe from the posterior cerebellar lobe.

9. The posterior lobe is the remainder of the corpus cerebelli.

 

 

C. Rhinencephalon - The following structures, located on the ventral aspect of the hemispheres, constitute the olfactory lobe, the portion of the rhinencephalon visible without dissection. They are separated by the rhinal fissure from the neopallium, the non-oilfactory cortex, which in the mammals more and more submerges the olfactory structures. Human Brain 7.1 8.1

1. The olfactory bulb is the flattened ovoid body at the rostral end of the hemisphere. It is the primary olfactory center; numerous fine filaments of the olfactory nerve (the axons of bipolar receptor cells in the nasal mucous membrane) enter its ventral side.

2. The olfactory bulb is connected to the hemisphere by a triangular enlargement, the olfactory trigone, from which diverge the medial and lateral olfactory gyri.

3. The medial olfactory gyrus extends to the medial surface of the hemisphere.

4. The lateral olfactory gyrus runs caudally and slightly laterally, separate from the lateral surface of the hemisphere by the rhinal fissure. On its medial border there is a white band of fibers, the lateral olfactory stria.

5. The anterior perforated substance is the rhomboid area bounded rostrally by the olfactory trigone and caudally by the optic tract.

6. The hippocampal gyrus is the smooth rounded termination of the temporal lobe between the brain stem medially and the rhinal fissure lateral1y. This gyrus and the lateral olfactory gyrus with which it is continuous rostrally constitute the pyriform area.

 

D. Brain Stem - ventral aspect. Human Brain 6.1 6.2

 

1. The optic chiasm is the prominent white cross at the caudal limit of the longitudinal fissure. The optic nerves, whose cut ends are seen, here meet and about 80% of the fibers of each nerve cross to the opposite side, forming the chiasm. (In lower vetebrates, the decussation is complete; in man, the fibers from the medial halves of the retina cross. Beyond the chiasm, the visual pathway continues as the optic tracts, which pass upward and disappear beneath the hippocampal gyrus.)

2. The infundibulum is an elevation of gray matter immediately caudal to the optic chiasm. It is the stalk of the pituitary body or hypophisis. (The pituitary body has been removed from most of the specimens. )The infundibulum is hollow; its cavity communicates with the third ventricle (the cavity of the diencephalon).

3. The tuber cinereum is the triangular eminence of gray matter from which the infundibulum projects.

4. The mammillary bodies form the rounded, usually bibbed, mass at the caudal extremity of the tuber cinereum.

5 The above four structures connect to the hypothalamus forming the floor of the third ventricle and hence of the diencephalon.

6. The interpeduncular fossa is the deep triangular depression caudal to the mammillary bodies. The floor of this fossa is the posterior perforated substance.

7. The cerebral peduncles are the large rope-like strands of white matter which emerge from the rostral border of the pons, diverge laterally on either side of the interpeduncular fossa and mammillary bodies, and disappear under the optic tracts into the cerebral hemispheres. The peduncles that form the ventral part of the mesencephalon, include almost all of the fiber tracts connecting the hemispheres with lower levels.

8. The pons, as seen from the ventral surface, is a broad transverse band of fibers arching around the caudal ends of the cerebral peduncles. On each side the fibers are gathered into a rounded bundle that enters the cerebellum as its middle peduncle (brachium pontis).

9. The trapezoid body is another, smaller and often indistinct, transverse band just caudal to the pons.

10. The medulla oblongata is the club-shaped enlargement between the trapezoid body and the spinal cord.

11. The ventral median fissure is the groove along the ventral midline of the medulla.

12. The pyramids are the longitudinal elevations on either side of the ventral median fissure. The elevations are caused by the subjacent pyramidal or corticospinal tracts.

13. The olive is a slight prominence lateral to the pyramid and just caudal to the trapezoid body. The prominence is caused by a subjacent mass of gray matter, the inferior olivary nucleus.

 

 

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