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Facial and Masticatory Muscles |
Text and Photos by Sarah Ogburn and Linda Evonne Brogdon Surface Anatomy Our opossum was an older female lab animal that had bred several times. Our specimen measured 185 mm from the tip of the nose to the base of the tail and 80 mm from the base of the tail to the tip of the tail. Its tail was naked and prehensile. There was a circular patch of lighter fur on the abdomen of the opossum just cranial to the base of the tail, where the nipples were arranged in a circular pattern. This patch is present instead of the pouch found in other opossums and marsupials. The feet were digitigrade and the hind feet were clearly prehensile with an opposable and divergent hallux. All the digits had claws except the great toe of the hind foot. Dissection First, we made an incision along the mid-dorsal line from the base of the skull to just cranial to the pelvis. Then we cut laterally from the mid-line cranial to the shoulder along the neck, and laterally from the mid-line cranial to the pelvis; thus exposing the animal's dorsal region. There were many segmental bundles of arteries, veins, and nerves traveling together from the intercostal spaces that, in life, supply blood to, drain blood from, and innervate the dermis and superficial fascia. These bundles had the appearance of white strands. The segmental arteries were extremely prominent and difficult to cut through. There were several deposits of adipose tissue just medial to the scapulae. The panniculus carnosus muscle was prominent, with visible fibers on the deep surface of the skin, and an observable insertion with the latissimus dorsi muscle on the humerus. The trapezius muscle was prominent, but its different segments were difficult to distinguish. The deltoid muscles were also observable, but difficult to distinguish from the trapezius muscles because of the small size of the animal. After carefully removing part of the panniculus carnosus muscle from the skin with scissors and leaving its insertion intact, we continued our transverse cuts to the ventral side of the opossum. We were careful to leave the "pouch" area intact, cutting cranially around it. We cut around the distal humerus, and removed the skin, thus exposing the entire ventral region from neck to pouch. The skin was difficult to remove as the animal was very small and the fascia was tough. The small scissors were essential in cutting through the fascia and segmental arteries and veins. The pectoantobrachialis muscle, the superficial pectoralis muscles, and deep pectoralis muscles were difficult to distinguish from one another; but the posterior pectoralis muscles were separate from the other pectoralis muscles. All of the pectoralis muscles were large, reflecting their role in locomotion. There were two very large sub-mandibular glands located on the ventral neck region. The clavicle was easily palpable. After examining all of the superficial muscles we cut the trapezius just caudal to the neck region and reflected it caudally, exposing the rhomboids. The rhomboids were very thin. We then began working to expose the deep shoulder muscles. We exposed the infraspinatus, supraspinatus, and subscapularis muscles with the probe. The deep shoulder muscles were more of a challenge to differentiate and identify. We cut and reflected latissimus dorsi, exposing the serratus ventralis thoracis muscle, which was easily visible. It was prominently serrated into muscular bundles and the entire muscle was fan-shaped. The atlanto-scapularis muscle was visible as a strap of muscle extending from the vertebral border of the scapula superficial to supraspinatus and infraspinatus, and inserting into the neck at the atlas. The serratus ventralis cervicus muscle was proximal and deep to the atlanto-scapularis muscle, originating from the transverse processes of the thoracic vertebrae and inserting on the ventro-medial side of the scapula. The serratus ventralis cervicus muscle corresponds to the levator scapulae muscle in other mammals. The omohyoid muscle was proximal and deep to the serratus ventralis cervicus muscle. The serratus ventralis cervicus muscle is much larger and wider than the strap-like omohyoid muscle. It is also more readily visible as it is more superficial. Function The marsupial shoulder is particularly interesting because of the role it plays in the infancy of the animal. Baby opossums are born after just 14 days of gestation [1]. The neonates must then crawl from the birth canal to their mother's nipples. This crawl requires strong shoulder muscles and a correspondingly strong brace for these muscles to attach to. The skull and brain must also be developed enough to make the journey; but other organs and structures are, by contrast, very underdeveloped at birth. Because of this, opossums are born with a shoulder brace composed of mesenchyme and a corresponding clavicular brace. This breast-shoulder brace is composed of the scapula, metacoracoid, procoracoid, first rib, sternal elements, and clavicle. The most important muscles for this climb are the pectoralis, triceps, and latissimus dorsi; these muscles are bulging, well developed and striated in the neonates. After birth, the breast-shoulder arch begins to dissolve and differentiate into separate bones, and the lower body begins to develop more fully. This method of locomotion at birth puts evolutionary constraints on the shoulder area of opossums and other marsupials [2]. Marsupials, therefore, are unlikely to evolve into forms that would require reconfiguration of the shoulder. Marsupials are unlikely to be dedicated cursorial, swimming, or flying animals. The shoulder muscles in Monodelphis are used for many activities, including holding the body upright, climbing, and handling food and objects. The shoulder has a wide range of motion to accommodate these activities, similar to the range of motion exhibited in the human shoulder [3]. Muscles' Origin, Insertion, and Function (adapted from sources [4, 5, 6] and our own observations)
References 2. Klima, M. 1987. "Early Development of the Shoulder Girdle and Sternum in Marsupials (Mammalia: Metatheria)." Pp. 1-91 in Beck, F.; Hild, W.; Kriz, W.; Ortmann, R.; Pauly, J.; Schiebler, T. (ed.) Advances in Anatomy, Embryology, and Cell Biology. 109. 3. Maas, S.; Baumann, K.; Halata, Z. 2001. Topography of Corpuscular Mechanoreceptors in the Shoulder Joint Region of Monodelphis domestica. The Anatomical Record. 263: 35-40. 4. Jenkins, F.; Weijs, W. 1979. The functional anatomy of the shoulder in the Virginia opossum (Didelphis virginiana). Journal of Zoology (London) 188: 379-410. 5. Dawson, T.; Finch, E.; Freedman, L.; Hume, I.; Renfree, M.; Temple-Smith, P. 1989. "Morphology and Physiology of the Metatheria." pp. 1-14 in Walton, D.; Richardson, B. (ed.) Fauna of Australia Volume 1B-Mammalia. Australian Government Publishing Service. Accessed 2004 at http://www.deh.goc.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume1b/17-ind.pdf 6. Ellsworth, A. 1976. The North American Opossum: An Anatomical Atlas.
Robert E. Krieger Publishing Company, New York. |
Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
