SHEEP BRAIN DISSECTION:  LAB 2

External Features:

 There are several systems for subdividing the brain.  The outline presented in TABLE 2 provides a very useful format for studying the brain as a set of divisions that emerged during evolution. The CNS is arranged in a stratified, or layered manner.  The strata were added as the nervous system evolved from a primitive neural tube to the elaborate structure we know as the human brain.  The lower, or caudal,  strata,  generally,  are involved with less complex neural activities than the higher, or more rostral, strata above them. 

Ventricular System:

 When in the skull, the external surface of the brain is awash in a bath of CSF.  There are also a number of CSF-filled cavities that are located on both the external surface of the brain and in its interior.  The fluid-filled cavities located on the external surface of the brain are called cisterns, while the fluid-filled cavities located internally are called ventricles.

When the dura mater was in place, it formed an enclosed space at the most ventro-caudal point of the cerebellum, just above its junction with the medulla and the pons.  This space is called the cisterna magna.  In the intact brain it is filled with cerebrospinal fluid.   The meninges form another enclosed space at the anterior limit of the cerebellum called the superior cistern.  Use the links  to the images that show where these two fluid filled chambers were located in the intact brain. (Click on the image for an enlarged view.)

 As discussed in the lecture and your textbook, the brain evolved from a primitive neural tube. As certain segments of the tube enlarged, the internal spaces in the tube followed suit.  This resulted in the creation of large spaces in the interior of the brain called ventricles.  The ventricles and their connecting passages are filled with the same CSF as that in the subarachnoid space.  The ventricles are connected to one another and to the subarachnoid space by apertures (openings or windows) called foramina (singular = foramen). Thus, the brain is cushioned by CSF that fills the ventricles within and the cisterns and subarachnoid space without.

 At the lateral junction of the cerebellum and the medulla notice the dark-brown, tufted material, the choroid plexus.  This material is a capillary bed, which, along with other tissue, is involved in the production of CSF.  We will see another choroid plexus in the lateral ventricles in Lab 3.

Important structures or features of the dorsal surface of the brain.

The dominant features of your sheep brain specimen are the two cerebral hemispheres and the exquisitely convoluted cerebellum.

or poles.  To see the four lobes of neocortex of the sheep identified, follow the link. The lobes of the brain are separated from one another by sulci, or fissures.   Two important sulci can be found in the anterior regions of the two hemispheres.  Together, these two sulci form a 'T.' The stem of the 'T' is made by the coronal sulcus, which runs parallel and lateral to the longitudinal fissure, the fissure that separates the two hemispheres.  (Note:  the coronal sulcus seems to be ill-named, because it runs perpendicular to the coronal plane of dissection.  Don't be confused by this.)  The coronal sulcus divides the frontal poles into approximately equal  left- and right-halves.  Follow  the coronal sulcus  caudally until it ends at the ansate sulcus, which forms the head of the 'T.'  The frontal lobe is separated from the parietal lobe by the ansate sulcus (called the central or Rolandic fissure in man).  The area anterior to the ansate sulcus is frontal lobe and the cortex posterior to the ansate sulcus is the parietal lobe. The gyrus immediately anterior  to the ansate sulcus is the precentral  gyrus.   The gyrus immediately posterior  to the ansate sulcus is known as the postcentral gyrus in the human brain.  Classically, the  precentral  gyrus is thought of as motor cortex, and the postcentral  gyrus as somatosensory cortex.  It is more  correct to think of the two as being predominantly motor and predominantly somatosensory, respectively, because other functions are also present.

Look at the dorsal aspect of the brain (and the spinal cord that extends from it).   The  dorso-medial sulcus marks the midline of the spinal cord. (Have the instructor or lab assistant point this out to you.) You should be able to see that the dorsal surface of the medulla, and the spinal cord, are marked by parallel, longitudinal striations formed by columns of fibers.  The most medial pair of these columns is the fasciculus gracilis; the more lateral pair of columns is the fasciculus cuneatus.  (Have the instructor or lab assistant point these out to you, if you are unclear about their locations.)  Fasciculus means tract. Collectively, these two fasciculi are also known as the dorsal columns.  The axons in these columns are ascending sensory fibers, carrying for the most part, light touch sensations from the body and limbs.  The fasciculus gracilis  conducts ipsilateral  information from the lower body and hind limbs, while the fasciculus cuneatus conducts ipsilateral information from the upper body and forelimbs. (Click on the image at the left for an expanded view.)


Follow the dorsal columns rostrally until they just begin to disappear beneath the cerebellum; you will find two small mounds.  These two swellings are the nucleus gracilis and the nucleus cuneatus.  (Remember from Lab 1 that nuclei are collections or clusters of cell bodies located within the CNS.) The axons in the dorsal columns synapse on cells located in the nucleus gracilis and nucleus cuneatus.  The axons of the cells residing in the two nuclei then exit the nucleus, decussate (cross the midline) and project to the contralateral thalamus where they synapse on cells in the ventroposterolateral nucleus (VPL).  The axons of the cells located in VPL project to somatosensory cortex (the post-central gyrus).  

Before you continue, stop to draw a schematic of the path that somatosensory information takes from spinal cord to postcentral gyrus.  Include the nucleus gracilis, nucleus cuneatus, VPL and postcentral gyrus in your drawing.

Important features of the ventral surface.

 In Lab 1, you located a large fiber structure, the cerebral peduncles, just anterior to the pons.  The oculomotor nerves can be seen exiting from them. Recall that the cell bodies that give rise to the axons in the cerebral peduncles are found in motor cortex and that they are called pyramidal cells.  The fibers in the cerebral peduncles continue to the spinal cord. They can be seen on the ventral surface of the medulla, where they are known as the pyramidal tract.  The axons continue to spinal cord where they split into three bundles; two of them are the lateral corticospinal tract and  the ventral (or anterior) corticospinal tract. 

Stop, now, and mentally trace the pyramidal tract from cerebral cortex to spinal cord.

The pyramidal motor system, one of two major motor systems in the body is in control of fine, discrete and voluntary motor activities such as writing, typing, or playing the piano. Other motor systems are concerned with gross motor movements, such as, dancing, walking, or waving goodbye.

 This may be a good time to restate conventions concerning names of tracts in the CNS.  If you keep the rule 'from-to' in mind, you will always be able to tell the site of origin and destination for a given tract.  The first name in the title indicates the site of origin of the tract, while the second name indicates the tract's destination.  The tract known as the corticospinal tract, according to the rule, originates from neurons whose cell bodies reside in the cortex and project their axons to the spinal cord.  Conversely, a tract called the spinothalamic tract originates from neurons in the spinal cord and ends, or synapses,  in the thalamus.

 At the anterior end of the ventral surface of the medulla, immediately caudal to the pons,  locate a band of transverse fibers called the trapezoid body.  The trapezoid body consists of fibers carrying information from the right ear to left auditory cortex and information from the left ear to right auditory cortex. The trapezoid body is to the auditory system what the optic chiasm is to the visual system.  Unlike somatosensory cortex, the auditory and visual cortices receive bilateral input, that is, each projection site receives information from both ears or both eyes, respectively.  (If it has not been stripped away, you will find the VIII cranial nerve, the vestibulo-cochlear or auditory nerve at the most lateral extent of the trapezoid body.

On the ventral surface of your sheep brain, locate the very prominent swelling between the trapezoid body and the cerebral peduncles, the pons.  Its name is derived from the Latin word, pons, which means 'bridge.'   The structure is aptly named, because of the great number of decussating fibers that cross the midline here and project to the cerebellum. In addition to the decussating fibers in the pons, there are "fibers of  passage," that is, fibers that merely pass through the pons on their way to some other target without synapsing in the pons.  Pyramidal tract fibers descending from motor cortex to their destination in the spinal cord are one example of these fibers of passage.

Immediately posterior to the the cerebral hemispheres, you find the cerebellum, a large, complex structure concerned with all levels of motor coordination. The cerebellum, which means 'little brain,'  also has an outer layer of cortex, that is, it is covered by a multilayered mantle of cells like the layer of cortex on the cerebral hemispheres. The cerebellar surface is characterized by intricate, extremely fine convolutions called folia.  The folia are analogous to the gyri of the cerebral hemispheres.  Like the cerebral hemispheres, the cerebellum has an inner core of white matter.  In the cerebellum this inner core of white matter is called the arbor vitae. (Click on the image below for a larger view.) The white-matter core consists of axons projecting to and from the cerebellar hemispheres, the spinal cord, sensory and motor cortices, and other regions of the brain.   The cerebellum sends information to the brain and spinal cord via axons that exit from the cerebellum.  In this way, we have information coming into the cerebellum that helps guide cerebellar control of our motor behavior.  Without an intact cerebellum, you would find it difficult to walk, maintain a sense of balance, or to perform a complex behavior, such as, hit a tennis ball with a tennis racket, an action that requires hand-eye coordination and timing. The cerebellum has other important functions; it is important for establishing skill memories and for the occurrence of classically conditioned responses.


Nestled between the cerebellum and the cerebral hemispheres are two prominent elevations sitting symmetrically on either side of the midline.  You may have to pull your cerebellum gently and caudally to reveal them. Collectively, these four structures are called the corpora quadrigemina ('bodies of four twins'), but it is easier to remember them in their pairwise configurations: the caudal and smaller pair, the inferior colliculi, are part of the auditory system, while the larger, anterior pair, the superior colliculi,  are part of the visual system.  This region of the midbrain is also called the tectum ('roof'), because the colliculi ('little hills') form the roof, or upper boundary of the Aqueduct of Sylvius. Look at the figure connected to this link to see the relationship between the colliculi (tectum) and the aqueduct of sylvius. The figure will clarify the location of the colliculi. If you are unable to see any of the structures named in this paragraph clearly and easily, seek help from the instructor, or lab assistant.

On the ventral surface of the brain, at the midline just anterior to the oculomotor nerve, locate the small, but distinct, tissue that looks like the tip of a tongue.  These are the mammillary bodies,  which mark the caudal limit of the hypothalamus.  Cells in the mammillary bodies are particularly vulnerable to alcohol.  Autopsies have shown significant destruction of the mammillary bodies in chronic alcoholics suffering from a severe memory disorder known as Korsakoff's syndrome.  Some neurologists believe that the mammillary bodies are involved in memory processes.

 While the mammillary bodies form the caudal limit of the hypothalamus, its anterior border is marked by the optic chiasm.  The  lateral boundaries of the hypothalamus are rimmed by the medial edges of the cerebral peduncles.  The general outline of the hypothalamus from the ventral aspect, thus, assumes a diamond-like configuration.  Although the hypothalamus is not a very large structure, it is quite complex.  The hypothalamus contains many different nuclei that are concerned with regulation of temperature, hunger and satiety, sexual behavior, and, perhaps, even sexual preference.

The last structures to concern us are evolutionarily older, archi-cortex.  On the ventral aspect of the brain, notice the moderately large, relatively smooth masses of cortical tissue just lateral to the cerebral peduncles.   Follow the tissue from its most caudal limit near the lateral-most partof the pons to its most anterior limit near the olfactory bulbs.  This mass of tissue, the rhinencephalon or 'smell brain,' is easily visible in the sheep brain, but it is hidden from external view by the temporal lobe in human brain.  One important structure located in the rhinencephalon is the hippocampal gyrus, a structure that is exremely important for development and maintenance of memories. It should not surprise you to learn that loss of cells in the hippocampus is one of the characteristics of Alzheimer's patients, individuals who suffer severe memory impairments.

The hippocampus is critical to the functioning of our declarative memory processes, among other behaviors.   Declarative memory processes are those processes concerned with our memory for facts, for example, the capital of the state of California, or the name of the structure that is intimately involved in the development of memory (hippocampus).

At the rostral end of the rhinencephalon,  at the place where the optic tracts disappear just medial to the rhinencephalon, notice the small but distinct mound.   This is the amygdala, a nucleus that has been associated with certain emotions,  such as aggression and fear.  Recent research has shown that the amygdala is important for emotional learning.   Our emotional memory system is distinct from our declarative memory system.  For example, you may have been in a terrible car accident that was immediately preceded by the blaring of a car horn.  Later, you may find that you become tense and anxious when you hear car horns.  Your memory for the details of the accident, for example, when and where the accident occurred, who else was in the car, or what kind of car struck you are declarative memories that are dependent upon the hippocampus.  The emotional memories, fear and anxiety associated with the accident, are activated via the amygdala, which plays an essential role in modulating conditioned emotional responses. The amygdala was recently found to play a role in psychological drug dependence. More about that in lecture.

 This completes the second section of the dissection.  Now would be a good time to review the structures you observed in the first dissection.  Once again, it would be helpful to make an index card for each term, or structure that appears in the list of important terms and structures.  Make certain that you can either define, identify,  or locate  each item, as appropriate.

 You will find that it is not very effective to study only the figures, because the practicum will require you to be able to identify structures on actual tissue.  Use your brain specimen as you study.  I encourage you to study in groups.  Point out the structures listed in the dissection guides for one another, can you name them without resorting to the guide?  Can you specify what functions the structures support, etc?

Ventral View with Pop-up Labels
Dorsal View with Pop-up Labels
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