Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Brachium of the Opossum (Monodelphis domestica)

Text and photos by Sarah Ogburn and Linda Evonne Brogdon

Overview:

Monodelphis domestica is primarily a terrestrial quadruped with a fairly generalized form of locomotion.  However, most of its ancestors and closest relatives are primarily arboreal, and M. domestica has retained arboreal behavior and anatomical adaptations.  Its hands and feet have five digits and are prehensile.  These are adaptations for grasping branches and are used in manipulating objects and food.  However, the proportions of its hands are reduced from very long grasping digits used for navigating terminal branches of trees to a more terrestrial, shorter digit form [1].

Opossums exhibit a parasagittal crouched posture that is probably similar to that of the earliest therians.  The anatomy of the forelimb is, therefore, different from the more modified, fully parasagittal postures of other mammals. [2, 3].  It uses walking gaits at slow speeds and trots at higher speeds.  However, unlike many marsupials which use walking gaits preferentially, M. domestica uses trotting gaits more often for steady-speed locomotion [2].

Dissection:

We began our dissection by continuing to pull the skin back to the proximal antebrachium in order to reveal the entire brachial region.  We cut the excess skin off using the scissors.  The brachial plexus was easily observable.  Because it was superficial to the brachium muscles, we cut and reflected it to make the flexor muscles more visible. Then we used the tweezers to separate a few last segments of the deep fascia investing the brachial muscles.  Next, we used the probes to separate the different muscles that we were looking for.  The extensor muscles covered the lateral and posterior part of the brachium.  The triceps were extremely prominent, and all attached to the olecranon process. The lateral and scapular heads were readily visible, while the medial head was located deep to the brachial plexus on the medial side of the arm.  The dorso-epitrochlearis muscle was somewhat difficult to differentiate from the lateral head of the triceps.  The flexor muscles were more anteromedial and more strap-like.  We took some time to move around the superficial biceps brachii muscles in order to expose the underlying brachialis muscle.  Coracobrachialis was difficult to find and was deep to biceps brachii. Monodelphis domestica lacks two extensors that are present in many other mammals--tensor fascia antebrachii and anconeus. 

The triceps muscles in M. domestica are the largest muscles in the entire shoulder and forelimb region.  They have several functions during locomotion. Firstly, they resist the flexion of the elbow while the limb is flexed and in contact with the ground. This is important in shock absorption while moving.  The triceps muscles are also very important in resisting the different directional forces that exert torque on the body while moving. The triceps are also a principal actor in the extension of the elbow. The triceps in M. domestica are situated on the lateral side of the forelimb as opposed to the more dorsal placement in many other mammals. This is because opossums have a parasagittal posture versus the fully parasagittal posture in many mammals. The limbs are not positioned as close to or under the body [5].  We examined the Rattus norvegicus as comparitive material.  We found that the rat’s triceps muscles were positioned far more dorsally than the opossum’s. 

The biceps brachii and brachialis muscles are the principal flexors of the elbow joint.  The biceps brachii muscle is located on the anterior side of the arm and can be seen from the medial view.  The coracobrachialis muscle is very small and deep to the biceps brachii muscle.  The brachialis muscle can be seen from the lateral side of the brachium, between the triceps muscle and the biceps brachii muscle. The biceps brachii muscles were visible on the anterior side of the brachium in the rat and were visible from the lateral view.  In the opossum, the biceps muscles are not at all visible from a lateral view, as they are positioned far more anterior-medially.  Also, the biceps brachii muscles in the rat were larger relative to the triceps than the biceps brachii muscles in the opossum.

Muscles’ Origin, Insertion, and Function [3, 4, 5] and our observations:

References:

1. Lemelin, P. 1999. Morphological correlates of substrate use in didelphid marsupials: implications for primate origins. Journal of Zoology (London) 247: 165-175.

2. Parchman, A.: Reilly, S.; Biknevicius, A. 2003. Whole-body mechanics and gaits in the gray short-tailed opossum Monodelphis domestica: integrating patterns of locomotion in a semi-erect mammal. The Journal of Experimental Biology. 206: 1379-1388.

3. Jenkins, F.; Weijs, W. 1979. The functional anatomy of the shoulder in the Virginia opossum (Didelphis virginiana). Journal of Zoology (London) 188: 379-410.

4. 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

5. Ellsworth, A. 1976. The North American Opossum: An Anatomical Atlas. Robert E. Krieger Publishing Company, New York.