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Facial and Masticatory Muscles |
Photos and text by Alexandra Sardi and Janelle Cooper Dissection Notes: We began the dissection of the forelimb by removing the fascia of the brachium. Immediately visible were the rabbit’s proportionately huge triceps. The rabbit has an anconeus, medial head, lateral head and long head but no fourth head (1,2). While the heads were distinguishable, it was very difficult to separate them. The medial, lateral and long heads were the most superficial and visible. The anconeus was less easily identifiable, much smaller and deeper. Cranial to the triceps we found the biceps brachii, which had only one head and was also clearly visible. It was missing the short head. The brachialis, deep to the biceps was also visible but very small in comparison and hard to find. Caudal to the biceps and cranial to the triceps we also found the tensor fascia antibrachii, which was a thin but long muscle. We found the cleidobrachialis distal to the deltoideus. Discussion: The rabbit is a cursorial mammal but has a very unusual style of fast locomotion: half-bounding (1). In half-bounding, the two hind feet touch the ground almost in unison but the forefeet alternate (3). The time the forefeet spend on the ground is shorter at all speeds. This creates a gait in which the hind legs are much more important in providing the force than the forelimbs (3). Hence, we expected that the rabbit’s forelimbs would not be nearly as muscular as its hind limbs or the forelimbs of animals that gallop. This was the case in our rabbit. Rabbits do not use the forelimbs to provide propulsive force, so having less muscular forelimbs lightens the load and makes them faster runners (4). Rabbits have also been known to burrow for shelter from the heat or cold and also to find roots and moisture during droughts (3). This activity explains the comparatively large triceps that we found. The rabbit has a relatively short humerus, which allows the rabbit to apply more force when digging. The rabbit’s forelimbs were longer than those of the other burrowing animal in class, the beaver. This may be explained by the mode of locomotion. Beavers spend a lot of time swimming in streams and lakes but do not really run. Hence, they do not need their limbs to be light, like the rabbit does. This makes it possible for the beaver to have a much shorter and bulkier forelimb than the rabbit could ever get away with. Both the beaver and the rabbit have long, curved non-retractile claws, which aid in digging (3). However, the beaver has a wider forepaw, which scoops more like a shovel than the rabbit’s. The rabbit cannot supinate or pronate and it does not use its hands for procuring food or manipulating objects. It does not have use for pronating or supinating muscles that would add bulk to the arm and decrease speed. Muscles of the Brachium:
Adapted from McLaughlin, Charles A. [1] References: 1. McLaughlin, Charles and Chiasson, Robert, Laboratory Anatomy of the Rabbit (3rd Ed.), McGraw-Hill, New York : 1990 2. Popesko, P. , Rajtova, V., and Horak, J., A Colour Atlas of Anatomy of Small Laboratory Animals Volume One: Rabbit & Guinea Pig, Wolfe Publishing Ltd.: London, 1992. 3. Kardong, Kenneth V., Vertebrates: Comparative Anatomy, Function, Evolution. McGraw Hill, New York: 2002. 4. Kent, George C. and Carr, Robert K., Comparative Anatomy of the Vertebrates. McGraw-Hill, New York: 2001.
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Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
