Dissection of the Sheep Brain

Sheep brains in Carosafe®, a formalin analog, will be supplied to you for the following dissection exercises. The dissection must be carried out on the side directed in order not to interfere with the later procedures. Laboratory work should be supplemented by study of textbook, web and Sylvius.

I. Brain Membranes and Blood Vessels

To make the dissection easier, your specimen has had the brain membranes (meninges) removed already. In this lateral view and this ventral view, with meminges intact, the dura mater is obvious. The pia mater needs to be teased to retract into fissures (sulci), using this view of the external anatomy of the cerebral cortex as an example. The arachnoid, lying between these two membranes, is difficult to see.

Note the blood supply, especially the circle of Willis and the other vessels, seen in this ventral view. The hypophsis (pituitary) has probably already been removed in your brain to permit better observation of the circle of Willis, but it is intact in this brain. Find the basilar artery; the posterior communicating arteries; the posterior, middle, and anterior cerebral arteries; the internal carotid arteries; and the anterior communicating artery.

II. Cranial Nerves

Using ventral views 1, 2, 3, 4, & 5 locate the roots of the twelve pairs of cranial nerves. Each is listed below and its function identified. [Note, "motor" (below) includes parasympathetic output for nerves 3, 7, 9 and 10.]

III. Surface Anatomy of the Brain

Examine carefully the external form of the sheep brain. On the dorsal view, note the fissures (sulci) and convolutions (gyri); also identify the cerebellar vermis and hemispheres, the medulla and the longitudinal fissure. On the cerebral hemispheres identify the lateral fissure, the rhinal fissure and the suprasylvian fissure. Using ventral views 1, 2, 3, 4, & 5 identify the following structures:
  1. Olive
  2. Pons
  3. Cerebral peduncles
  4. Interpeduncular nucleus
  5. Mammillary bodies
  6. Diagonal band
  7. Amygdaloid nucleus
  8. Pyriform area
  9. Hippocampal gyrus
  10. Lateral and medial olfactory gyri and stria
  11. Trapezoid body
  12. Anterior perforated substance
IV. Brain Stem and Cortex

Observe the relations of the cerebral cortex to the cerebellar cortex. Note especially the difference in the number of convolutions on each structure. Use a scalpel to cut off a slice about one centimeter thick from the posterior pole of the left cerebral cortex and a similar slice from the left lateral border of the cerebellum. They should look like this. Compare the cut surfaces and observe the relations of the gray matter to the underlying white matter.

V. Rhombencephalon

The rhombencephalon (or hindbrain) is composed of the metencephalon (cerebellum and pons) and the myelencephaon (medulla oblongata). Note the attachment of the cerebellum to the medulla. Cut these attachments (cerebellar peduncles) on each side with the aid of a scalpel. Remove the cerebellum and put it aside for future study. These peduncles should be cut as high as possible, cutting into the substance of the cerebellum if necessary rather than into the structures of the medulla. Take care not to injure the delicate membranes beneath the cerebellum and forming the roof of the IVth ventricle. The view should look like this.

The IVth ventricle is sometimes called the cavity of the rhombencephalon. The cerebellum forms the roof of the ventricle but only for a short extent between the cerebellar peduncles. Behind, the roof is formed by a thin non-nervous membrane (tegmen) part of which is highly vascular and much folded. Note the choroid plexus of the IVth ventricle, composed capillary tufts and columnar epithelial tissue. The choroids plexi form a barrier between the blood and the cerebrospinal fluid, and are thought to be the source of the cerebrospinal fluid.

VI. Mid-sagittal view

Now cut the entire brain of the sheep into right and left halves. A long, thin knife or large steel spatula is best suited to this purpose. The incision should pass through the longitudinal fissure between the cerebral hemispheres to cut through the corpus callosum in the floor of this fissure, and then downward through the entire brain stem. Care should be taken to make this cut smooth and exactly in the median plane. It should be made with a single, long sweep of the knife. You may wish to locate the commissures as you cut through them. Examine carefully the cut surfaces of the brain and identify the structure brought into view. Refer to these photographs (1 and 2) of the median surface provided to you and locate the following structures on your sheep brain:

   

VII. Cerebellum

There are three cerebellar peduncles: brachium conjunctivum, brachium pontis, and corpus restiforme. Examine their cut surfaces on the dorsal aspect of the medulla. Separate the fibers of the three peduncles from each other on the cut surface. Then continue the separation of the conjunctivum and middle peduncles for approximately one centimeter downward along the dorso-lateral border of the medulla as they cross the spinal V tract superficially to continue into the cord as the dorsal spino-cerebellar tract. Dissect the fibers of the brachium pontis and the brachium conjunctivum. The latter can be followed to its decussation in the cerebral peduncle beneath the superior colleculus. Since you removed the cerebellum, you cannot do this tract dissection: With gray matter removed, the lateral aspect of the cerebellar peduncle is seen.

Cut the cerebellum into two halves along the medial sagittal plane. The cerebellar gray and white matter seen in the median section of the vermis has the appearance of arbor vitae.

VIII. Dissection of the Tracts

Pyramidal tract

The dissection of the cerebrum outlined below is to be carried out on the right half of the sheep brain. Locate the pyramid on the ventral surface just caudal to the pons. Strip the pons fibers back from the cut median surface for a short distance. Teasing them, you can see the direction the axons go. Expose the longitudinal pyramidal fibers lying just dorsal to the pontine fibers. Tease them and you see that they go at right angles to the pons fibers. In the sheep, the pyramidal fibers interdigitate with fibers of the trapezoid body which thus must be destroyed to expose the pyramidal tract. By careful teasing, the pyramidal tract can be followed spinalward only as far as the decussation. The fibers cannot be followed any further. The pyramidal tract can be dissected cephalad from the pons through the cerebral peduncle. This dissection should be deferred until later.

Association tracts

The arrangement of fibers in the subcortical white matter can be examined by careful teasing. Only some of these fibers are to be studied at this time. The dissection outlined in this section should not be carried farther than indicated.

Refer to this figure. Select a region along the dorsal border of the medial surface of the hemisphere. Scrape away the gray matter covering two adjacent gyri to expose the short associational (arcuate) fibers connecting these gyri. Further teasing can reveal similar fibers lying deep in the white matter, which connect the more remote gyri. Corona radiata is the name given to those fibers running between the cortex and the brain stem via the internal capsule of the corpus striatum. These include the thalamo-cortical sensory projection fibers, other thalamo-cortical connections and the cortical efferent tracts (the corticospinal, corticobulbar and corticopontile). These fibers diverge from the upper border of the internal capsule like the rays of a crown, hence "corona radiata."

The cingulum is a long associational tract close to the medial cortical surface of the hemisphere. It runs parallel with the dorsal surface of the corpus callosum in part of its course, beginning anteriorly in the gyrus subcallosus beneath the head of the corpus callosum. It arches upward at the genu of the corpus callosum, passes around the splenium of the callosum posteriorly and then bends downward, forward and lateral-ward to the region of the hippocampal gyrus. Begin its dissection at about the middle of the corpus callosum, following it both anteriorly and posteriorly.
The corpus callosum are fibers that connect the neocortex of one hemisphere with that of the opposite hemisphere. Tease the callosal fibers a small distance and note that the texture tells you the direction the axons go.

IX. Mesencephalon and Prosencephalon

The mesencephalon includes all of the structures of the midbrain, such as the superior colliculus, inferior colliculus, cerebral peduncles, and the interpeduncular nucleus. The prosencephalon is divided into the diencephalon and the telencephalon. The thalamus and hypothalamus (including the mammillary bodies) are the major parts of the diencephalon, and the cerebral cortex, corpus striatum (internal capsule, caudate nucleus, putamen and globus pallidus) and rhinencephalon form the telencephalon.

Rhinencephalon and Limbic System

In older terminologies the rhinencephalon is often referred to as smell brain. Although the olfactory bulb can be shown to have connections with rhinencephalon, this is true for only some of its parts. More recently the rhinencephalon has been divided into three interconnected parts: 1. Primary olfactory structures directly related to the olfactory bulb. 2. Second system receiving fibers from the first consisting primarily of septal region and amygdaloid complex. 3. The third system consists of cingulate and entorhinal cortex and the hypothalamus. These appear to be remotely, if at all, related to olfactory afferents. Both the second and third systems send efferents to the hypothalamus. The limbic system consists of the second and third systems in addition to two diencephalic structures, the anterior thalamus and the mammillary bodies. The structures of the rhinencephalon are relatively more developed in the sheep, so that dissection is easier than in the human brain. The primary olfactory neurons give rise to fibers that terminate in the olfactory bulb, the primary olfactory center of the brain. From here to the neurons of the second order (mitral cells), give rise to axons forming the olfactory tracts or striae (lateral and medial) terminating in secondary olfactory centers in the basal parts of the cerebral hemisphere.

Other tracts of the rhinencephalon usually named by hyphenated compound words of which the second member designates the center into which the tract discharges. For example (refer to this figure) [Also note that, if your cut was a bit off of mid-sagittal, these tracts may already be visible.]:

Mammillo-thalamic tract . This tract runs from the mammillary body forward and dorsalward to the anterior nucleus of the thalamus. It can be readily dissected by scrapping off the ependyma of the third ventricle, beginning in the region of the mammillary body.

Habenulo-pendunclar tract . This tract also can be readily dissected (although watch out, it is easy to scrape right through it). It runs from the habenula into the ventral part of the cerebral peduncle immediately behind the mammillary body.

The fornix can also be followed down toward the mammillary bodies.

The hippocampus and fornix.

The hippocampus (archipallium) is located in the floor of the posterior horn of the lateral ventricle, together with the fimbria and hippocampal commissure. Refer to this figure. Now cut through the splenium of the corpus callosum to the tip of the hippocampal gyrus. This is one of the 2 cuts you would make to cut out a pie wedge of the cerebral cortex to reveal the hippocampus. Peel apart this opening to reveal the hippocampus (refer to this figure). Note that the hippocampus looks white. This is because it is covered by the fimbria, fibers that form the fornix. The column of the fornix thus is the efferent projection tract from the hippocampus to the mammillary body (and habenula).
Teasing the fibers of the fimbria, you can tell by the texture what direction they go. Here is another view of the hippocampus; Note that, in this view, there is a nick through the fimbria revealing the gray matter of the hippocampus.

Cut through the genu of the corpus callosum forward and downward towards the olfactory bulb, to open up the anterior horn of the lateral ventricle. In the sheep, unlike man, this is directly continuous with the ventricle of the olfactory bulb.

The ventricular wall can be torn away along the border between the head of the caudate nucleus and the hippocampus by careful pulling. Refer to this figure. This reveals the internal capsule fibers passing downward and backward, lateral to the caudate nucleus. You can tell, by the texture of the tearing, the direction of the axons.

XI. Optic System

With trimming, caudate and hippocampus are clear in this figure. Remove the hippocampus and identify lateral and medial geniculate bodies (refer to this figure). You can also see the inferior (auditory) and superior (visual) colliculi on the lateral surface of the thalamus and midbrain. Follow the optic tract from the chiasm to the lateral geniculate body.

XII. Further Dissection of the Pyramidal Tract

As the last step of the dissection, careful tearing down of the fibers will permit you to follow out some of the internal capsule fibers into the regions of the thalamus, midbrain, and medulla oblongata, especially the cortico-spinal (pyramidal) tract. Although functionally a motor and thus a descending tract, it can more easily be traced from the medulla upward to the higher centers. It appears as an eminence (pyramid) on the ventral surface of the medulla below the pons near the midline. Its fibers interlace with those of the pons, where the cortico-spinal tract was located earlier. It can then be followed along the ventral surface of the mesencephalon through the cerebral peduncle into the internal capsule. The cell bodies for these fibers lie in the superior frontal gyrus.

XIII. Dissection of the Left Half

The left half of the specimen may be cut into a series of transverse or longitudinal slices.

A suggested method of making these sections is first to cut a "horizontal" section through the neocortex, about three millimeters dorsal to the corpus callosum and in the same plane as this structure. This section will actually slope ventrally toward the frontal cortex, as does the corpus callosum. Then cut laterally through both the genu and splenium of the corpus callosum to the extremes of the anterior and inferior horns of the lateral ventricle, respectively. After removing the corpus callosum, extend the cuts anteriorly and posteriorly to expose the head of the caudate nucleus, and the hippocampus. Refer to this figure. Observe the relations of the lateral ventricle, the hippocampus, and the head of the caudate nucleus to one another.

Another section can be made in the same plane, but at a somewhat smaller angle from the horizontal, immediately superior to the pineal body, through the hippocampus and the head of the caudate. Be careful to avoid cutting through the dorsal surface of the thalamus. Refer to this figure. Note the relation of the internal capsule to the adjacent structures. The basal ganglia are made up of caudate, putamen and globus pallidus. Caudate + putamen = striatum (striated because strands of internal capsule make it look striated), clearly seen in this section. Putamen + globus pallidus = lentiform nucleus [lens shaped], clearly seen in this section. This section is also excellent for seeing the hippocampus overlying the lateral geniculate of the thalamus.

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This page was last updated 2/1/06