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

Hip and Thigh of the Opossum (Monodelphis domestica)

Text and photos by Sarah Ogburn and Linda Brogdon

Dissection:

We removed the skin from the legs using small scissors and a probe.  The skin came off fairly easily.  Once the skin was removed from the hindquarters, it was necessary to clean off fat and fascia that was covering the thigh.  Once this was removed, the muscles were clearly visible and well differentiated.  The pyramidalis muscle extended from the palpable epipubic bone to the linea alba. We dissected the lateral thigh, beginning with the most anterior muscles and working our way caudally.  The long, strap-like sartorius muscle is the most anterior muscle.  Caudal and proximal to sartorius, we located the large, fan-shaped gluteus maximus muscle.  The prominent rectus femoris muscle and the vastus externus muscle are distal to the gluteus maximus muscle and are both very large.  The vastus externus muscle is deep to the rectus femoris muscle.  Caudal to these muscles, we located the femorococcygeus muscle.  The biceps femoris, crurococcygeus, and semitendinosus muscles are difficult to differentiate from one another and have many fibers that run together.  The biceps femoris muscle is the largest and most anterior. The crurococcygeus muscle is caudal to it and smaller.  A slip of semitendinosus is visible in lateral view.  It is caudal to the crurococcygeus muscle.

Next we examined the medial side of the thigh.  The sartorius muscle is the most anterior muscle in this view.  Caudal to the sartorius muscle, we located the large belly of the rectus femoris muscle.  This muscle is much more visible in the medial view.  The prominent vastus internus muscle is caudal to and superficial to it.  The thin adductor longus muscle is caudal to the vastus internus muscle.  This muscle was torn during our dissection.  The adductor magnus muscle is larger than and caudal to the adductor longus muscle.  Caudal to the adductor muscles, we located the large semitendinosus muscle, which is more visible from the medial side of the thigh than the lateral side.  A very small slip of the semimembranosus muscle is visible just caudal to the semitendinosus.  Superficial to the semitendinosus and semimembranosus muscles is the large gracilis muscle. 

To observe the deep muscles of the hip we cut and reflected the gluteus maximus and femorococcygeus muscles.  The most anterior muscle is the fan-shaped gluteus medius muscle.  The piriformis muscle makes up the posterior edge of the fan shape created by gluteus medius.  These two muscles are only distinguishable by the orientation of the muscle fibers.  The fibers of gluteus medius run more posterio-medially, while the fibers of piriformis are oriented more medio-laterally.  The rest of the deep muscles of the thigh and pelvis are relatively small.  We examined these muscles from the lateral view.  A small part of the gemellus muscles can be seen posterior to and deep to the piriformis muscle.  We could not differentiate the gemellus superior muscle from gemellus inferior muscle.  Posterior to the gemellus muscles, we located the longer and larger obturator externus muscle.  The strap-like quadratus femoris muscle is posterior to the obturator internus muscle.  It originated from the ischium and inserted on the posterior surface of the femur.  Medial and posterior to the quadratus femoris muscles is the obturator externus muscle.

Function:

The glutei muscles extend and abduct the thigh but also play a role in the lateral rotation of the pelvis.  The glutei are similar in function to the supraspinatus muscle of the forelimb.  Both extend and abduct, have short lever arms, extensive origins, and help to stabilize the joints.  The hamstring muscles also act as extensors of the thigh, and are more powerful than the glutei muscles in this capacity because they must overcome the inertia of the limb at the beginning of leg movement.  The glutei function as fast—rather than powerful—extensors.  The adductor muscles are important in arboreal locomotion because they help to stabilize the foot against the medially positioned branches.  The pectoralis muscles of the forelimb operate similarly [1].  The crurococcygeus and femorococcygeus muscles move the hind-legs posteriorly when the animal is hanging by the tail and can flex the tail laterally.  The ability to move the hind-limbs posteriorly while suspended by the tail allows the animal to resume a quadrupedal stance after hanging [2].

During locomotion, the position and movement of the hind limb is different from those of many other mammals.  The opossum exhibits a not-quite-parasagittal posture, and the femur is accordingly held at a 40-degree angle (in the species Didelphis marsupialis, the same pattern holds for Monodelphis) from the body.  This contrasts with an angle of about 10-15 degrees in the cat and 25 degrees in the rat.  In the opossum the femur is raised above the hip level during locomotion, unlike the cat whose femur only comes within 35 degrees of the horizontal.  The rat elevates the femur above the hip, but not to the degree seen in the opossum.  The patterns of movement seen in the opossum and the rat are probably close to that of ancestral mammals [3].

Muscles’ Origin, Insertion, and Function (from our observations and sources [1], [2] and [4])

We used figures from sources [1] and [2] to help us with our dissection.

References:

1. 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.

2. Elftman, H.O. 1929.  Functional adaptations of the pelvis in marsupials. Bulletin American Museum of Natural History 58: 189-232.

3. Jenkins, F. 1971. Limb posture and locomotion on the Virginia opossum (Didelphis marsupialis) and in other non-cursorial mammals.  Journal of Zoology, London 165: 303-315.

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