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Facial and Masticatory Muscles |
Photos and text by Sarah Ogburn and Linda Brogdon Dissection: We removed all the muscles covering the brain case with small scissors and scalpel. Then we used the one blade of the scissors to perforate the brain case just superior to the foramen magnum. We cut laterally on both sides to expose the brain. It was necessary to cut fairly far rostrally to expose the olfactory bulbs. Once the brain was exposed, we used the probe to begin lifting and separating it from the brain case. We worked from the rostral caudally. It was necessary to sever the infundibulum and leave the pituitary gland in the brain case. Once the brain was nearly free, we cut the spinal cord to fully remove the brain. Circulation of the brain:Much of the circulation of the brain was impossible to find and identify as our specimen is not latex-injected and is extremely small. However, there is extensive literature on the subject. Marsupials have a unique arrangement of the vessels carrying blood to and from the head. In placental mammals and monotremes, the arteries branch into anastomosing capillary networks and the blood then passes into fine branches of the veins. This is how most of marsupial circulation works, except for circulation inside the brain. Vessels in the brain form pairs consisting of an artery and a vein that branch together and are enclosed in a single membrane. At the finest branching, the ends meet and form a terminal end loop. If a single capillary is severed or damaged, some nerves can be deprived of their only blood supply. Only a few reptiles and amphibians have this arrangement [1]. The vertebral veins, the maxillary veins, and the internal jugular veins all drain the brain. However, the internal jugular is greatly reduced and the other veins are responsible for the bulk of drainage [1]. The internal carotid arteries and the vertebral arteries are responsible for supplying the brain with blood. The vertebral arteries supply most of the blood and the internal carotid arteries are very small. Forebrain:Olfactory bulbs: The olfactory bulbs are the most rostrally placed structures of the brain. They appeared to be damaged when we opened the brain case. We are not sure if this damage occurred before or during our dissection. They were barely intact, but their general size could be estimated. They are prominent, but not as prominent relative to body size as the rat’s olfactory bulbs or those of other marsupials. Most relatives of Monodelphis domestica, especially Didelphis, have extremely large olfactory bulbs [2]. Cerebral hemispheres: The cerebral hemispheres are roughly almond shaped and located near the rostral end of the brain, caudal to the olfactory bulbs. The hemispheres are completely smooth without any visible sulci. Although smaller in size, they greatly resembled the cerebral hemispheres of the rat. The cerebral hemispheres are responsible for memory, higher motor control, and sensory functions. Areas controlling the sense of smell are located near the rostral end of the hemispheres; areas for hearing are located more caudally and ventrally. The opossum has large areas of the brain devoted to the control of the upper lip and vibrissae, control of the ear, and control of the hands. This is expected given the opossum’s reliance on these parts of the body [3] Corpus callosum: The corpus callosum is absent in all marsupials. In polyprotodont marsupials like M. domestica, two bundles of fibers connect the cerebral hemispheres instead. The dorsal bundle connects the hippocampi of the two sides and is called the dorsal commissure. The anterior commissure connects the olfactory bulbs, amygdalar regions, basal ganglia, and cerebral cortex. [2] Infundibulum: The infundibulum is the stalk of the pituitary gland. It is located on the ventral surface of the brain and is extremely small. It was most visible in sagittal section. The infundibulum is located near the middle of the ventral aspect of the brain as viewed in sagittal section, just rostral to the pons. Hindbrain:Pons: The pons is extremely small in opossums. It is located just caudal to the hypophysis and rostral to the medulla on the ventral side of the brain. The pons is responsible for controlling respiratory function. Cerebellum: The cerebellum of marsupials has the same basic organization as in placentals. Variations are usually due to body size and muscular organization. The cerebellum consists of parafloccular lobes, hemispheres, and a centrally located vermis. The vermis is subdivided into lobules. Opossums have enlarged lobules I and II; this enlargement relates to a well-developed tail musculature and prehensibility. [2]. The cerebellum is also responsible for equilibrium of the body in space, fine movement, and overall coordination of muscle activity. Cranial Nerves:The cranial nerves emerge from foramina or fissures in the base of the cranium. The sheaths that surround the cranial meninges also cover all twelve cranial nerves. We were unable to locate many of the cranial nerves due to their extremely small size. Using a dissecting microscope at q magnification of 10x, we were able to locate the optic and oculomotor nerves. Optic nerve: The optic nerve was one of the only nerves we were able to locate. It was located on the ventral side of the brain and exits close to the midline just caudal to the cerebral hemispheres. The optic nerve is responsible for carrying visual impulses from the eyes to the brain. Oculomotor nerve: The oculomotor nerve was the only other nerve (along with the optic nerve) that we were able to locate. It is located in the midbrain. It was seen exiting the brain from in between the hypophysis and the pons on the ventral surface of the brain. It is responsible for innervating the muscles that move the eye and surrounding structures.Like most other spinal nerves, the cranial nerves contain sensory fibers, motor, or a combination of both. The type of innervation of each nerve is classified into three parts: general versus special, visceral versus somatic, and efferent versus afferent. Special innervation refers to anything concerning the major senses: vision, smell, taste, and hearing, while general covers everything else. Visceral classification is reserved for structures that cannot be willingly controlled such as blood vessels and organs, whereas structures classified as somatic, such as striated muscle, can be. Afferent simply refers to sensory innervation and efferent is motor. For example, the olfactory nerve (CN I), which is special visceral afferent (SVA), controls a special sense e.g., smell, and provides sensory innervation to structures not under conscious control of the subject e.g., olfactory epithelium.
Eye: The eye of M. domestica is quite large relative to body size. The retina is much like that of a reptile or a bird’s, containing single and double cones and rods. Opossums have color vision and the presence of oil droplets within the cones has been detected [4]. It is hypothesized that these droplets heighten contrast of colored object. The opossum also has an extremely specialized tapetum lucidum (a reflective layer of cells behind the retina that reflects light back to the rods, aiding in nocturnal vision). The walls of the tapetum are thick, and the tapetum has been characterized as “superior” to that of many placental mammals. [1] Opossums have an orange eye-shine. Most marsupials have an avascular retina, with no retinal vessels visible. Opossums are the one exception however. In the retina there are pairs of arteries and veins that are joined by capillary loops similar to those seen in the brain. [5] References: We used figures from sources [6] and [7] for our dissection. 1. Dom, R.; Fisher, B.; Martin, G. 1970. The venous system of the head and neck of the opossum (Didelphis virginiana). Journal of Morphology 132: 487-496 2. Johnson, J. 1977. Central nervous system of marsupials. pp. 159-278 in Hunsaker, D. (ed.) The Biology of Marsupials. 3. Lende, R. 1963. Motor representation in the cerebral cortex of the opossum. Journal of Comparative Neurology. 4. Friedman, H. 1967. Colour vision in the Virginia opossum. Nature: 213: 835-836. 5. Barbour, R. 1977.Anatomy of marsupials. pp. 237-272 in Stonehouse, B. and Gilmore, D. (ed.s.) The Biology of Marsupials. 6. Loo, Y.T. 1930. The forebrain of the opossum, Didelphis virginiana. Journal of Comparitive Neurology 51 (1): 13-64 7. Voris, H. and Hoerr, N. 1932. The hindbrain of the opossum, Didelphis virginiana. Journal of Comparative Neurology 54 (2): 277-356
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Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
