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DISSECTION OF THE SHEEP BRAIN
I. EXTERNAL VIEWS
A. DORSAL and LATERAL VIEWS In life the brain is covered by three membranes: the meninges. In the specimens you receive, most of the dura mater has been removed, but the thin pia mater remains intact. One specimen located in the front of the lab, however, will have the majority of the meninges intact. Please make a point to see this specimen to examine the tough, outer dura mater and the subarachnoid space. On your specimen note how the arachnoid space fails to extend into the depths of the fissures covering the forebrain. In contrast, the innermost of the three meninges, the delicate pia mater, which can be clearly seen only through a microscope, is directly applied to the surface of the brain and extends into every nook and cranny. Examine the brain from the dorsal and lateral surfaces ( Figures A and B ). Visible are the two cerebral hemispheres, the cerebellum and the caudal tip of the medulla. In animals, such as man, in which the hemispheres are better developed they cover the cerebellum dorsally and block it from view. The right and left cerebral hemispheres are separated by the longitudinal fissure which, in life, is filled by a large blood vessel. If the walls of the fissure are very gently separated, the corpus callosum may be seen in its depths. (It may be necessary to remove some of the arachnoid in this region with forceps.) The surface of the sheep cerebrum is covered by numerous gyri and sulci whose pattern is rather different than that of man. Brains, such as those of man and the sheep, which display convolutions are said to be gyrencephalic. In contrast, the cerebrum of many small animals such as the rat is smooth and is said to be lissencephalic (smooth). Identify the frontal, temporal and occipital lobes and poles. The various possible views of an anatomical structure are referred to as dorsal (Latin: dorsum = back) for a top view; ventral (Latin: venter = belly) for a view from the underside; rostral (Latin: rostrum = beak) for a front view; caudal (Latin: cauda = tail) for a rear view; lateral for a side view and medial for a view from the midline. The same terms are applied to regions within a structure, e.g. the medial hypothalamus, or the caudal part of the midbrain. Note the gross divisions of the brain which are visible in a dorsal view. The brainstem is completely covered by the cerebellum and cerebrum except for the caudal part of the medulla. The cerebellum consists of a medially placed vermis (It looks a bit like a worm. Latin: vermis = worm) and two laterally placed hemispheres. The entire outer surface of the cerebellum consists of the cerebellar cortex (Latin: cortex = bark) which is folded up into a series of narrow ridges called folia (singular, folium). The cerebrum is divided into left and right halves, the cerebral hemispheres, by the medial longitudinal fissure. As in the cerebellum, the outer surface consists of cortex, the cerebral cortex, folded into a series of ridges. In the cerebrum, these ridges are called gyri (singular gyrus) and the furrows separating them are called either sulci (singular, sulcus) or fissures. The sulci are filled with many small blood vessels and with the pia and arachnoid membranes. If these are pulled out carefully with fine forceps the depths of the sulci can be seen. If the cerebral hemispheres are spread slightly and the blood vessels and pia are removed, it is possible to see a heavy band of white fibers connecting the hemispheres. This is the corpus callosum. Gently spread apart the cerebellum and the cerebrum. Pick out the pia and blood vessels to expose four rounded mounds, the superior and inferior colliculi (see Figure C ). In sheep, the superior colliculi are much larger and better developed than the inferior colliculi. When examining the brain notice that some structures (the optic nerves are a good example) are glistening white while others are of a dull grey or ivory color (the cortex is a good example). These color differences are the basis of the terms "grey matter" and "white matter". White matter
owes its color to the presence of large numbers of myelinated (myelin covered) nerve fibers while grey matter consists of nerve cell bodies and fine unmyelinated fibers. The cerebellum is separated from the cerebrum by the transverse fissure and is covered by many small ridges known as folia. The cerebellum is divided into a median, wormlike vermis and two lateral cerebellar hemispheres. If the arachnoid is cleaned out from the transverse fissure and the cerebellum gently retracted, the superior and inferior colliculi may be seen. These structures form the roof, tectum, of the midbrain. Just in front of the superior colliculus and on the midline, observe the mysterious pineal gland, thought by Descartes to be the seat of the soul.
B. VENTRAL VIEW
A ventral view reveals the underside of the brainstem and nearly all the cranial nerves as well as the pyriform lobe ( Figure D ). In most of the specimens, the pituitary gland has been removed to reveal the hypothalamus. Also, make a point to see the specimen at the front of the lab which has dye injected into the vascular system and examine the Circle Of Willis. On your specimen note that the floor of the midbrain consists of the two cerebral peduncles (Latin: pedunculus = stem) separated by an interpeduncular space. These peduncles disappear under the surface of the pons (Latin: pontis = bridge) but some of the fibers which they contain re-emerge as the pyramidal tracts which appear on the medial surface of the medulla. Axons which run in the pyramidal tracts originate mainly from cells in the rostral neocortex, run successively through the corona radiata and internal capsule which lies under the cortex, then through the cerebral peduncle and pyramidal tract before finally terminating in the spinal cord. In the medulla, just above the spinal cord, many of the axons of these corticospinal neurons cross to the opposite side of the nervous system. That is, axons from the left cortex run mainly to the right side of the spinal cord while axons from the right cortex run mainly to the left side of the spinal cord. Corticospinal cells play a role in the control of movement, permitting the neocortex of each cerebral hemisphere to exert a direct effect on the activity of spinal neurons which run to muscles on the opposite side of the body. Some bone and dura probably remain under the hypothalamus. Remove this, being very careful not to damage the brain. Parts of all of the major divisions of the brain are visible in ventral view. Starting caudally, the medulla can be seen. Note the presence of a shallow median fissure running along the ventral surface of the medulla and ending abruptly at the caudal border of the pons. This is the ventral median fissure. Next to it, on either side, lie two longitudinal ridges, the pyramids, through which the pyramidal (corticospinal) tract passes. Lateral to the pyramids may be seen two indistinct rounded bumps, the olives. These bumps overly the inferior olivary nuclei, one of the most important sites of cerebellar afferents. (The olives are much more prominent in man.) A number of cranial nerves (VI-XII) can be seen exiting from the medulla. Immediately rostral to the olives, and just behind the pons, a narrow transverse band can be seen crossing the midline. This is the trapezoid body and is formed by decussating auditory fibers. In primates, including Man, the trapezoid body is present, but is buried under the greatly enlarged pons found in these animals. The pons (bridge) is a transversely elongated mass which turns upward on either side towards the cerebellum. The pontine nuclei form a major way station between the cerebral cortex and the cerebellum, pontocerebellar fibers entering the cerebellum through the middle cerebellar peduncle which can be seen extending laterally from the pons into the cerebellum. Exiting through the middle peduncle is the trigeminal nerve (Cranial nerve V). The largest of the cranial nerves, the trigeminal is the main sensory nerve of the face. Just in front of the pons is the midbrain. In ventral view one can see the cerebral peduncles (crus cerebri), two massive fiber bundles containing corticospinal, corticopontine and corti- cobulbar fibers. These fibers are continuous rostrally with the internal capsule and caudally with the pyramids. The depression between the peduncles is called the interpeduncular fossa. Cranial nerve III, the oculomotor nerve, can be seen exiting from the sides of this fossa.
can be found emerging just behind the inferior colliculus. V. Trigeminal Nerve The trigeminal nerve (tri + Latin: geminus = twin) is a mixed nerve which means that it contains both sensory and motor fibers. The main portion, which is split into three divisions, carries sensory information from the skin, muscles and other structures of the head such as the bones and teeth. A dentist can reduce or abolish the pain produced by drilling into teeth by injecting a local anesthetic into the vicinity of branches of the Vth nerve. Local anesthetics are drugs that block the passage of nerve impulses. After injection, drilling can still set up nerve impulses at the ends of the nerve fibers but because these impulses cannot reach the brain, no pain is felt. In addition to the sensory branches the trigeminal nerve includes a motor nerve which runs to the jaw muscles to control biting and chewing. VI. Abducens The abducens is a pure motor nerve which together with the oculomotor and trochlear nerves, controls the movements of the eye. VII. Facial The facial nerve is a mixed nerve, carrying sensory fibers for taste from the tongue as well as motor fibers to the muscles of the face. Movements such as winking or smiling depend upon an intact facial nerve. Other motor fibers run to the salivary glands and tear glands to produce, respectively, salivation and the secretion of tears. VIII. Vestibulo-cochlear Nerve The VIlIth nerve is a sensory nerve consisting of a cochlear division and a vestibular division, which run from the cochlea and the vestibular apparatus, respectively. The cochlea is a coiled spiral structure (Latin: cochlea = snail shell) which contains receptor cells that are sensitive to vibrations set up by sounds acting on the eardrum. Thus, the cochlear nerve is concerned with hearing. The vestibular apparatus is a complex system of liquid filled sacs and tubes which contain receptors sensitive to gravity and acceleration. These receptors, via the cells of the vestibular nerve inform the brain of body position and movement. If the vestibular nerve (or the vestibular apparatus) is destroyed, the sense of balance is impaired. It becomes difficult to maintain an erect posture, especially in the dark or when the eyes are closed. IX. Glossopharyngeal Nerve The glossopharyngeal nerve (Greek: glossa = tongue + pharynx) is a mixed nerve carrying taste-sensitive fibers from the tongue, other sensory fibers from the pharynx, plus motor fibers to a muscle in the throat and to the salivary glands. X. Vagus The vagus nerve (Latin: vagus = wandering) is a mixed nerve which innervates many of the viscera or internal organs such as the heart, lungs and stomach. By means of sensory vagal fibers the brain is kept informed of the state of these organs and by means of motor vagal fibers it can control their activity. The vagus also contains sensory and motor fibers that run to the pharynx, palate and nearby structures. XI. Accessory Spinal Nerve The accessory nerve is a motor nerve carrying fibers that originate in the medulla and others that originate in the spinal cord. These fibers control muscles in the neck and shoulder. XII. Hypoglossal The hypoglossal is a motor nerve controlling the muscles of the tongue.
II. INTERNAL VIEWS
A. MIDSAGITTAL SECTION
In addition to your full brain you should also have a specimen which has been cut along the midline. If not, cut your brain into right and left halves to expose the medial surfaces. To do this, place the brain on a table, ventral surface downward. Place a knife with a long thin blade in the medial longitudinal fissure. Then, with a single smooth stroke, cut the brain in half. Figure E shows the main structures visible in a medial view of the sheep brain. Identify, on the median surface, the medulla, cerebellum, pons, colliculi, pineal gland, hypothalamus and corpus callosum. Separating the cerebellum from the brainstem is the fourth ventricle. Caudally, the roof of the ventricle is composed of a reddish or brownish material. This is the posterior choroid plexus and it is one of the sites responsible for the formation of cerebrospinal fluid. Observe that the cerebellum is composed of a thin gray cerebellar cortex and of underlying white matter which resembles somewhat a leaf of a tree (the arbor vitae.) Deep within the white matter of the cerebellum are buried three (on each side) deep cerebellar nuclei. Almost all of the output of the cerebellum arises from these structures which cannot be visualized in sagittal sections. The thalamus can be seen immediately above the hypothalamus, from which it is separated by a shallow grove on the surface of the third ventricle. The third ventricle, which separates the right and left halves of the thalamus and hypothalamus, is so thin that its location may not be obvious, but its walls can be identified by their shiny appearance, as contrasted to the duller surface of freshly cut brain. The thalamus has grown so large that the right and left sides meet and fuse across the midline forming an ovoid structure, the massa intermedia which obliterates much of the central portion of the third ventricle. The roof of the third ventricle is formed by a very thin band of tissue which thickens on the anterodorsal side of the massa intermedia to form the reddish anterior choroid plexus, a major site of CSF production. Rostrally the choroid plexus disappears from view as it bends into the lateral ventricles forming, as it does so, the posterior border of the interventricular foramen of Monroe. On the dorsocaudal border of the thalamus, just in front of the pineal gland, you can see a small ovoid nucleus, the habenula. This structure receives a large input from the septum and hypothalamus through a fiber bundle, the stria medullaris which can be seen extending rostrally from the habenula along the dorsal border of the thalamus. The habenula, in turn, projects to the midbrain through the fasciculus retroflexus which can be identified, and traced (if you're careful and lucky); by scraping away the tissue located ventral to the habenula. The three structures mentioned in this paragraph are among the most conservative found in the vertebrate brain, looking almost the same in fish and humans. Almost nothing is known however as to the function of this enigmatic system. The fornix is an elongated band of fibers lying in the midline ventral to the corpus callosum from which it is separated by the septum. The septum is very thin caudally and is easily torn. If your section is not exactly on the midline the septum will be seen on one side only, the lateral ventricle being exposed on the other. The fornix originates in the hippocampus and projects to the septum and the hypothalamus, especially the mamillary bodies. (The fornix also contains hippocampal afferents, notably those originating in the septum.) The fornix lies in the floor of the lateral ventricle until it reaches the foramen of Monroe where it turns ventrally and caudally and disappears into the substance of the hypothalamus. It can be traced caudally through the hypothalamus by very carefully scraping away the hypothalamic gray matter next to the ventricle (try this on one side only!). After you have traced the fornix back to the mamillary body, try to scrape away the superficial gray matter dorsal and anterior to the mamillary body. If you are very careful, and lucky, you will be able to identify the mamillothalamic tract, a major fiber bundle which conveys information from the mamillary body to the anterior thalamic region. The anterior
stopping as soon as you come to white matter of the internal capsule. (Great care is needed!!) Deeper in the brain the internal capsule passes lateral to the thalamus separating it from the globus pallidus. Many fibers in the capsule terminate (or originate) in the thalamus, but others extend into the hind- brain or even into the spinal cord, passing first through the cerebral peduncles and then the pyramids. Attempt to demonstrate the relation of the capsule to the thalamus and its continuity with the cerebral peduncle by carefully scraping away the thalamus beginning at its dorsal border, where you have already exposed the internal capsule. Finally, cut away the cerebellum from the hindbrain doing as little damage to the brain stem as possible. Examine the floor of the fourth ventricle. You can see a number of bumps and ridges which overlie various cranial nerve nuclei. These structures all have names.
C. THE HIPPOCAMPAL FORMATION AND DORSAL BRAINSTEM
The dissection shown in Figure C was produced by removal of the cerebellum and the caudal half of the right neocortex and corpus callosum. To remove the cerebellum, gently pull the caudal end of the medulla ventralward to open the IVth ventricle. The floor of the IVth ventricle is an elongated hollow partly filled by a rounded structure (part of the cerebellum) medial to the laterally placed cerebellar peduncles. These peduncles can be cut through with a scalpel, taking care not to damage the medulla and pons. The cerebellar peduncles contain nerve fibers connecting the cerebellum with the spinal cord, brainstem and thalamus. The neocortex is best removed by scraping it away with a thin piece of wood. The soft grey matter of the cortex can be removed rapidly, but proceed more cautiously when the deeper white matter is reached. Below the white matter lies the lateral ventricle which can be pulled open to fa- cilitate removal of the remaining overlying neocortex. This will expose the hippocampal formation, a type of cortex shaped somewhat like a ram's horn. Note that the lateral ventricle ends as a pocket not far above the surface of the pyriform lobe and adjacent to the ventral tip of the hippocampal formation. Removal of the caudal neocortex exposes the superior (rostral) colliculus, the inferior (caudal) colliculus and the pineal gland as well as the hippocampal formation (Latin: colliculus little mound). The terms superior and inferior are survivors from an old system used to name parts of the body. A "superior" structure is one which has a higher position than an “inferior” structure in a human standing erect.
D. TRANSVERSE SECTIONS
Make transverse sections through the as of yet untouched hemisphere at the following levels: (1) olfactory tubercle, (2) optic chiasm, (3) caudal hypothalamus, (4) superior colliculus, (5) pons and (6) fourth ventricle. Find as many of the structures described above as you can on these sections, and identify as many structures on each section as you can. On the first 2 sections be careful to identify the putamen and the caudate and study their relation to the internal capsule. On section 2 or 3 (depending on the exact level of your cut) observe the fornix passing through the substance of the hypothalamus. In this section it may be possible to observe the fornix twice, owing to its arched shape. Can you find the amygdala? In section 4, attempt to find the substantia nigra located dorsal to the cerebral peduncle. (This structure is not pigmented in the sheep.) You may also be able to observe the central gray substance surrounding the cerebral aqueduct.
