Dissection - SHEEP BRAIN DISSECTION GUIDE NEUROANATOMICAL...

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SHEEP BRAIN DISSECTION GUIDE NEUROANATOMICAL TERMS OF REFERENCE AXES: Animals can be divided into three principal axes --nose to tail, back to belly, and center to side. Nose to tail is typically called rostral-caudal (or anterior-posterior ). The belly is labeled ventral and the back is labeled dorsal . The center to side axis is referred to as medial to lateral . These axes are used to describe position RELATIVE to a reference point: thus, the eyes are rostral to the ears, but caudal to the nose. PLANES : It is conventional when studying the structure of the nervous system to examine sections that have been cut in the three planes that are perpendicular to the three principal axes. Cross-sections cut perpendicular to the rostral-caudal axis are coronal sections (i.e. in a coronal plane); those cut perpendicular to the dorso-ventral axis are horizontal , and those cut perpendicular to the medio-lateral axis are sagittal sections. If a sagittal section is made at the midline, it is called a mid-sagittal section; if it is cut lateral to the midline, it is called a parasagittal section. It is also useful to know that most fiber pathways (except those with special names like "fornix") are named by joining together first the name of the origin then the name of the destination of the pathway --e.g. corticospinal tract from the cortex to the spinal cord. It is customary to end the origin name in "o" and make the ending adjectival --e.g. pontocerebellar fibers from the pons to the cerebellum. One source of confusion arises because many neuroanatomy textbooks are written with specific reference to the human brain. Because humans walk upright, we can consider that the cerebrum of the human brain is at approximately a 90 angle to the brainstem and spinal cord. This is in contrast to other species in which the brain and spinal cord form essentially a straight-line system. DISSECTION OF THE SHEEP BRAIN Sheep or cow brains are often used to demonstrate mammalian neuroanatomy because they are large and can be obtained easily. The brains are soaked in a preservative formaldehyde solution and should be rinsed thoroughly in cold running water prior to dissection. A thin, protective glove is recommended when handling formaldehyde-fixed specimens. Keep fumes, fingers and spray away from eyes. If fluid does come into contact with eyes, flush immediately with cold water. Contact lenses should not be worn. Avoid direct inhalation of noxious fumes. During dissection the specimens should be kept constantly wet to prevent hardening and discoloration. I. SUPERFICIAL ASPECTS OF THE BRAIN A. DORSAL SURFACE . The brain may be covered by a gray, tough membrane, the dura mater , which is the outermost layer of the three protective layers ( meninges ) of connective tissue, that surround the brain and spinal cord. Remove the dura by peeling it gently away from the brain with forceps so that the cortical surface is clearly visible. Note that the cerebral cortex (outer layer or "bark" of the cerebrum) has
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Dissection - SHEEP BRAIN DISSECTION GUIDE NEUROANATOMICAL...

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