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

Antebrachium of the Opossum (Monodelphis domestica)

Photos and text by Sarah Ogburn and Linda Brogdon

Overview :

Monodelphis domestica has five digits on its hand.  Each digit has a claw.  The forearm measured 21 mm.  The brachium measured 25mm, and the entire arm length (including the manus) was 65mm.  The manus measured 11mm from the wrist to the longest digit.  The digits measured as follows: 1: 5mm; 2: 6mm; 3: 7.5mm, 4: 7mm, 5: 6mm.

M. domestica has gaits very near to those of a generalized quadruped vertebrate.  It uses walking gaits at slow speeds and trots at higher speeds.  Unlike many marsupials which prefer walking gaits, M. domestica uses trotting gaits more often for steady-speed locomotion. Interestingly, it appears that M. domestica uses spring mechanics while trotting.  Spring mechanics allow animals to store elastic energy in the ligaments and tendons, thereby reducing the amount of work muscles must do.  However, M. domestica does not seem to have the anatomical capability of utilizing spring mechanics, as it stands semi-erect and its tendons and ligaments are short and thick [1].  The flexor carpi radialis muscle may be an exception to this generalization [2].

The manus of M. domestica is prehensile and the palmar surface is covered with fleshy protuberances. These protuberances assist in the grasping of small objects, food, and branches.  However, M. domestica is less adapted for arboreal life (especially in regards to grasping terminal branches) than many other opossums.  Animals that must grasp branches of small diameter have hands with long digits relative to the metacarpals.  Opossum genera such as Marmosa and Caluromys are primarily arboreal and exhibit this adaptation.  Animals that are primarily terrestrial or grasp larger branches have no need for long digits.  The opossum genera Didelphis, Philander, and Monodelphis are primarily terrestrial and tend to utilizes larger branches when locomoting in trees.  These genera do not exhibit the degree of digit elongation seen in the more arboreal opossums [3].

First we removed the remaining skin from the antebrachium using the small scissors and probe.  We had to work slowly and carefully, as the fascia was extremely tough and the muscles in the area were very delicate.  We left the dorsal carpal ligament and transverse carpal ligament retinacula intact. Skinning the hand was extremely difficult; the fleshy pads on the palmar surface of the hand were very difficult to separate from the underlying fascia.  We used a microscope (8x) to locate and separate the extensors, first locating the separate tendons and working proximally with a needle to separate the muscles.  We repeated the process for the flexors.  Under the microscope, all the muscles were fairly easy to identify.  The supinator muscle, brachioradialis muscle, and extensor carpi radialis longus and brevis muscles were present in a large bundle on the radial (thumb side) of the forearm and the tendon of abductor pollicis longus muscle passed over this bundle.  We moved laterally (towards the ulna) and found the extensor digitorum communis muscle (which diverges into four tendons of insertion) and the extensor digitorum lateralis muscle.  The extensor carpi ulnaris muscle was prominent and lateral (ulnar) to these muscles.  The extensor indicis proprius muscle was the most lateral (ulnar) muscle and its tendon ran deep and medial to the extensor carpi ulnaris muscle.  The flexors were more difficult to separate, as they seemed to be more disturbed during the skinning process.  The pronator teres muscle was the most medial (radial) muscle.  Lateral to it was the flexor carpi radialis muscle with a very long tendon of insertion.  The flexor digitorum profundus muscle was visible just lateral to the flexor carpi radialis muscle and deep to the flexor digitorum superficialis muscle.  The flexor digitorum superficialis muscle was prominent and its tendon was very wide.  The palmaris longus and flexor carpi ulnaris muscles were the most lateral (ulnar) muscles and were connected to each other proximal to the tendons. 

The manus was extremely small and difficult to dissect.  The abductor pollicis brevis muscle was prominent on the medial (radial) side of the hand.  The abductor digiti minimi muscle was visible on the lateral (ulnar) side of the hand. The lumbrical muscles were visible as small white cords attached to digits 2-4.

The two main groups of muscles in the forearm are the extensors and the flexors.  The extensor group originates from the lateral epicondyle of the humerus and serves to extend the wrist and fingers, depending on the tendons of insertion.  Within this group, the muscle bundle containing the supinator, brachioradialis, and extensor carpi radialis longus and brevis muscles is very large.  These muscles supinate the forearm and are active in manipulating objects, food, and branches.

The flexor group originates from the medial epicondyle of the humerus and serves to flex the wrist and fingers.  While M. domestica does not seem to have the anatomical capability for utilizing spring mechanics (even though it appears to use spring mechanics in locomotion), researchers have taken special note of the very long tendon of insertion of the flexor carpi radialis muscle.  This long tendon may be important in producing tension in the muscle and may allow the flexor carpi radialis muscle to store elastic energy [2].

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

References:

1. 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.

2. McConathy, D.; Giddings, C.; Gonyea, W. 1983. Structure-function relationships of the flexor carpi radialis muscle compared among four species of mammals. Journal of Morphology 175: 279-292.

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

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

5. Stein, B. 1981. Comparative Limb Myology of Two Opossums, Didelphis and Chironectes.  Journal of Morphology 169: 113-140.