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Facial and Masticatory Muscles |
We believe that our rat was euthanized through a blow to the head and dorsal neck region. Our rat’s brain was completely destroyed before we even skinned the head. Once the muscles were removed, we discovered that the thin cranium was cracked and the brain had turned into mush. In fact, the coronal suture ventral to the interparietal bone and dorsal to the parietal bones had separated. Unfortunately, this meant that we were unable to observe, study, or dissect the brain of our animal. As a result, the material in this lab is based on our research and not on our dissection experience. The cerebral cortex is smooth (lissencephalic) rather than highly folded (gyrencephalic) [1]. An advantage of having a highly folded and wrinkled cortex is that the folds allow more gray matter per ounce of white matter [2]. The rabbit and beaver are also lissencephalic mammals while the cat and fetal pig are gyrencephalic. This may mean that rats and other lissencephalic animals require fewer cell bodies (gray matter) than axons (white matter) compared to gyrencephalic animals. The forebrain is the frontal division of the brain, which contains the right and left cerebral hemispheres, the thalamus, and the hypothalamus. The forebrain takes up more than half of the cerebral space, while the cerebellum and the olfactory bulbs each take approximately 25 percent of the space. The olfactory bulbs are gigantic compared to those of the cat, pig, opossum, and the rabbit. While the rabbit has large olfactory bulbs, they are much smaller relative to the rest of the brain and make up only about 12.5 percent of its brain space. The beaver is the only mammal in class that has olfactory bulbs comparable to those of the rat. The rat and the beaver, both in the order Rodentia, have poor vision and must rely on their olfactory and auditory senses in order to move around and orient themselves. Their poor vision is reflected by the size of their optic nerves, which are relatively small [3]. The cerebellum consists of three lobes in the median portion (anterior, central and posterior lobes), of three lobes on lateral to the median portion (lunate, ansiform, and paramedian lobes), and two prominent parafloccular lobes that are found deep and lateral to the median and lateral portions of the cerebellum [1]. The parafloccular lobe is associated with coordination and muscle tone [4]. Aside from the rat, the beaver and the rabbit also have large parafloccular lobes, but the rabbit’s lobes are much larger than those of either the rat or the beaver. The cranial nerves of the rat follow the normal mammalian pattern. Information on the cranial nerves and their function can be found in most mammalian comparative anatomy textbooks or on this Web site http://info.med.yale.edu/caim/cnerves/ [4]. References: [1] Ask Jeeves. Glossary of terms. Retrieved on April 20, 2004 at http://web.ask.com/ [2] Cartmill, Matt et al. Human Structure. Cambridge: Harvard University Press, 1987. [3] Brain Museum. Comparative Mammalian Brain Collections. Retrieved on April 21, 2004 at http://www.brainmuseum.org/index.html [4] Yale University School of Medicine. Cranial Nerves. Retrieved on
April 20,
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Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
