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

Shank and Foot of the Opossum (Monodelphis domestica)

Photos and text by Sarah Ogburn and Linda Brogdon

Appearance:

The opossum has a highly specialized hind foot with five digits.  It has a fully prehensile hallux that diverges 50-55 degrees from the central axis of the foot. Each digit has claws and the claws of the hind feet are longer than those of the forefeet.  The foot has well developed plantar pads to increase friction over the substrate. Among Didelphidae, the more terrestrial monodelphids have comparatively small plantar pads [1].  The digits of the foot are also not as elongate as they are in more arboreal opossums [2]. 

We removed the skin covering the lower leg and foot by using the probe to cut through the fascia.  We were able to peel the skin off, unlike in the dissection of the forearm and hand.  Once we had removed the skin, it was fairly easy to identify the muscles using the dissecting microscope at a magnification of 8X. 

We began by identifying the extensors and peroneal muscle groups of the shank.  These muscles are located on the lateral side of the shank.  The largest and most anterior muscle is the tibialis anterior muscle.  The tendon of the extensor hallucis longus muscle is visible at the distal end of the shank, posterior to and deep to the tibialis anterior muscle.  It is a small strap-like muscle.  There is an empty space between the tibialis anterior muscle and the extensor digitorum longus muscle through which the tibia is visible.  The extensor digitorum longus muscle is fairly large and long.  Near the distal end of the shank and posterior to the extensor digitorum longus muscle, the tendon of the extensor digitorum brevis muscle is visible.  Posterior to the extensor digitorum brevis muscle are the peroneus brevis and peroneus longus muscles.  We were unable to differentiate these muscles near the proximal end of the shank, but their tendons of insertion were clearly separate.  The peroneus brevis muscle is anterior to the peroneus longus muscle. We could not locate the other peroneal muscle—peroneus tertius.  Our sources say that it originates from the head of the fibula and inserts on the distal phalanx of the fifth digit.  The gastrocnemius externus muscle is also visible from the lateral view and is the most posterior muscle.

After identifying the extensors, we rotated the shank to the posterior view to identify the flexors.  We began with the most lateral muscles.  The most lateral muscle is the fused gastrocnemius externus and soleus muscle.  This is the largest muscle of the shank.  The plantaris muscle is located deep to and in between the gastrocnemius externus muscle and the gastrocnemius internus muscle. Plantaris is a fairly large muscle.  Medial to the plantaris muscle is the large gastrocnemius internus muscle.  It is smaller than the gastrocnemius externus muscle and has a very long tendon of insertion.  Deep to the gastrocnemius and plantaris muscles is the wide flexor digitorum longus muscle, which is only visible medial to the gastrocnemius internus muscle.  We could not differentiate the flexor digitorum brevis muscle from the flexor digitorum longus muscle, but our source says that it originates from the common tendon of the flexor digitorum longus muscle and inserts on the middle phalanges of digits III-V.  Pulling the tibialis posterior muscle away from the shank exposed the flexor hallucis longus muscle. It was long and thin and medial to the flexor digitorum longus muscle.  The most medial muscle is the tibialis posterior muscle. 

Most of the muscles of the shank are very long with relatively short tendons of insertion.  This contrasts sharply with the muscles of the rat, which were concentrated proximally with very long tendons of insertion.  This difference is presumably phylogenetic. 

A variety of different climbing mammals have the ability to reverse the posture of their hind feet in order to hang upside down or to descend headfirst from trees.  Squirrels, ringtails, bearcats, margays, kinkajous, coatis, tree shrews, some prosimians, and some marsupials have this capability.  However, the joints at which this rotation occurs differ between metatherian mammals and eutherian mammals.  In eutherians, three joints and movements are involved in hind foot reversal: plantar flexion at the crurotalar joint, inversion at the subtalar joint, and supination at the transverse tarsal joint.  Animals that can reverse their hind feet simply exhibit a high degree of mobility at these joints, rather than any other specialization.  Hind foot reversal in metatherian mammals is accomplished by different means.  In eutherians, the malleoli of the tibia and fibula restrict any abduction or adduction of the foot—permitting primarily dorsiflexion and plantarflexion.  In metatherians that can reverse their hind feet, the malleoli do not extend distally and to envelop the astragalus as they do in eutherians.  Rather, they have a bicondylar distal tibia and broad, shallow astragalotibial and astragalofibular facets that allow for the abduction of the foot a further 90 degrees from the normal foot posture, which diverges 30-35 degrees [1].  The peroneus brevis muscle facilitates this abduction. Plantarflexion at the cruroastragalar and subastragalar joints, coupled with supination at the transverse tarsal joint aids in the reversal of the foot in opossums.  The flexor digitorum longus and brevis muscles supinate the foot at the transverse tarsal joint.  In less arboreal opossums—such as Monodelphis, the medial malleolus of the tibia is slightly more prominent and the astragalotibial facets are less shallow than in more arboreal opossums.  This gives monodelphids and other more terrestrial opossums a reduced range of motion in the cruroastragalar joint [3].

The opossum is born with extremely well developed forelimbs for propelling itself to its mother’s nipples.  The hindlimbs, however, are extremely undeveloped, paddle-like nubs at birth.  The hindlimbs develop rapidly in the second and third weeks after birth in order to “catch up” to the forelimbs.  By the fourth week after birth, young opossums can detach and walk—the hindlimbs can partially support the body weight and the ankle and knee joints can be flexed.  By five weeks, the movements of the hindlimbs become coordinated with those of the forelimbs, and by seven weeks adult locomotor movements are achieved [4]. 

Muscle Origins, Insertions, and Functions (from our observations and sources [3] and [5])

References:

We used source [4] to help with our dissection.

1. Jenkins, F. and McClearn, D. 1984. Mechanisms of hind foot reversal in climbing mammals. Journal of Morphology 182: 197-219.

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

3. Argot, C. 2002. Functional-adaptive analysis of the hindlimb anatomy of extant marsupials and the paleobiology of the Paleocene marsupials Mayulestes ferox and Pucadelphys andinus. Journal of Morphology 253: 76-108.

4. Martin, K. and Mackay, S. 2003. Postnatal development of the fore- and hindlimbs in the grey short-tailed opossum, Monodelphis domestica.

5. Stein, B. 1981. Comparative limb myology of two opossums, Didelphis and Chironectes.  Journal of Morphology 169: 113-140.